Merge pull request #154 from 0xPolygonMiden/next

Tracking PR for v0.6.0 release

CHANGELOG.md

@@ -1,3 +1,17 @@
+## 0.6.0 (2023-06-25)
+
+* [BREAKING] Added support for recording capabilities for `MerkleStore` (#162).
+* [BREAKING] Refactored Merkle struct APIs to use `RpoDigest` instead of `Word` (#157).
+* Added initial implementation of `PartialMerkleTree` (#156).
+
+## 0.5.0 (2023-05-26)
+
+* Implemented `TieredSmt` (#152, #153).
+* Implemented ability to extract a subset of a `MerkleStore` (#151).
+* Cleaned up `SimpleSmt` interface (#149).
+* Decoupled hashing and padding of peaks in `Mmr` (#148).
+* Added `inner_nodes()` to `MerkleStore` (#146).
+
 ## 0.4.0 (2023-04-21)
 
 - Exported `MmrProof` from the crate (#137).

Cargo.toml

@@ -1,12 +1,12 @@
 [package]
 name = "miden-crypto"
-version = "0.4.0"
+version = "0.6.0"
 description = "Miden Cryptographic primitives"
 authors = ["miden contributors"]
 readme = "README.md"
 license = "MIT"
 repository = "https://github.com/0xPolygonMiden/crypto"
-documentation = "https://docs.rs/miden-crypto/0.4.0"
+documentation = "https://docs.rs/miden-crypto/0.6.0"
 categories = ["cryptography", "no-std"]
 keywords = ["miden", "crypto", "hash", "merkle"]
 edition = "2021"
@@ -35,6 +35,6 @@ winter_math = { version = "0.6", package = "winter-math", default-features = fal
 winter_utils = { version = "0.6", package = "winter-utils", default-features = false }
 
 [dev-dependencies]
-criterion = { version = "0.4", features = ["html_reports"] }
+criterion = { version = "0.5", features = ["html_reports"] }
 proptest = "1.1.0"
 rand_utils = { version = "0.6", package = "winter-rand-utils" }

README.md

@@ -12,17 +12,16 @@ For performance benchmarks of these hash functions and their comparison to other
 ## Merkle
 [Merkle module](./src/merkle/) provides a set of data structures related to Merkle trees. All these data structures are implemented using the RPO hash function described above. The data structures are:
 
-* `MerkleTree`: a regular fully-balanced binary Merkle tree. The depth of this tree can be at most 64.
-* `SimpleSmt`: a Sparse Merkle Tree, mapping 64-bit keys to 4-element leaf values.
-* `MerklePathSet`: a collection of Merkle authentication paths all resolving to the same root. The length of the paths can be at most 64.
-* `MerkleStore`: a collection of Merkle trees of different heights designed to efficiently store trees with common subtrees.
 * `Mmr`: a Merkle mountain range structure designed to function as an append-only log.
+* `MerkleTree`: a regular fully-balanced binary Merkle tree. The depth of this tree can be at most 64.
+* `MerklePathSet`: a collection of Merkle authentication paths all resolving to the same root. The length of the paths can be at most 64.
+* `MerkleStore`: a collection of Merkle trees of different heights designed to efficiently store trees with common subtrees. When instantiated with `RecordingMap`, a Merkle store records all accesses to the original data.
+* `PartialMerkleTree`: a partial view of a Merkle tree where some sub-trees may not be known. This is similar to a collection of Merkle paths all resolving to the same root. The length of the paths can be at most 64.
+* `SimpleSmt`: a Sparse Merkle Tree (with no compaction), mapping 64-bit keys to 4-element values.
+* `TieredSmt`: a Sparse Merkle tree (with compaction), mapping 4-element keys to 4-element values.
 
 The module also contains additional supporting components such as `NodeIndex`, `MerklePath`,  and `MerkleError`  to assist with tree indexation, opening proofs, and reporting inconsistent arguments/state.
 
-## Extra
-[Root module](./src/lib.rs) provides a set of constants, types, aliases, and utils required to use the primitives of this library.
-
 ## Crate features
 This crate can be compiled with the following features:
 

benches/smt.rs

@@ -18,8 +18,8 @@ fn smt_rpo(c: &mut Criterion) {
                     (i, word)
                 })
                 .collect();
-            let tree = SimpleSmt::new(depth).unwrap().with_leaves(entries).unwrap();
-            trees.push(tree);
+            let tree = SimpleSmt::with_leaves(depth, entries).unwrap();
+            trees.push((tree, count));
         }
     }
 
@@ -29,10 +29,9 @@ fn smt_rpo(c: &mut Criterion) {
 
     let mut insert = c.benchmark_group(format!("smt update_leaf"));
 
-    for tree in trees.iter_mut() {
+    for (tree, count) in trees.iter_mut() {
         let depth = tree.depth();
-        let count = tree.leaves_count() as u64;
-        let key = count >> 2;
+        let key = *count >> 2;
         insert.bench_with_input(
             format!("simple smt(depth:{depth},count:{count})"),
             &(key, leaf),
@@ -48,10 +47,9 @@ fn smt_rpo(c: &mut Criterion) {
 
     let mut path = c.benchmark_group(format!("smt get_leaf_path"));
 
-    for tree in trees.iter_mut() {
+    for (tree, count) in trees.iter_mut() {
         let depth = tree.depth();
-        let count = tree.leaves_count() as u64;
-        let key = count >> 2;
+        let key = *count >> 2;
         path.bench_with_input(
             format!("simple smt(depth:{depth},count:{count})"),
             &key,

benches/store.rs

@@ -1,5 +1,5 @@
 use criterion::{black_box, criterion_group, criterion_main, BatchSize, BenchmarkId, Criterion};
-use miden_crypto::merkle::{MerkleStore, MerkleTree, NodeIndex, SimpleSmt};
+use miden_crypto::merkle::{DefaultMerkleStore as MerkleStore, MerkleTree, NodeIndex, SimpleSmt};
 use miden_crypto::Word;
 use miden_crypto::{hash::rpo::RpoDigest, Felt};
 use rand_utils::{rand_array, rand_value};
@@ -104,10 +104,7 @@ fn get_leaf_simplesmt(c: &mut Criterion) {
             .enumerate()
             .map(|(c, v)| (c.try_into().unwrap(), v.into()))
             .collect::<Vec<(u64, Word)>>();
-        let smt = SimpleSmt::new(SimpleSmt::MAX_DEPTH)
-            .unwrap()
-            .with_leaves(smt_leaves.clone())
-            .unwrap();
+        let smt = SimpleSmt::with_leaves(SimpleSmt::MAX_DEPTH, smt_leaves.clone()).unwrap();
         let store = MerkleStore::from(&smt);
         let depth = smt.depth();
         let root = smt.root();
@@ -215,10 +212,7 @@ fn get_node_simplesmt(c: &mut Criterion) {
             .enumerate()
             .map(|(c, v)| (c.try_into().unwrap(), v.into()))
             .collect::<Vec<(u64, Word)>>();
-        let smt = SimpleSmt::new(SimpleSmt::MAX_DEPTH)
-            .unwrap()
-            .with_leaves(smt_leaves.clone())
-            .unwrap();
+        let smt = SimpleSmt::with_leaves(SimpleSmt::MAX_DEPTH, smt_leaves.clone()).unwrap();
         let store = MerkleStore::from(&smt);
         let root = smt.root();
         let half_depth = smt.depth() / 2;
@@ -292,10 +286,7 @@ fn get_leaf_path_simplesmt(c: &mut Criterion) {
             .enumerate()
             .map(|(c, v)| (c.try_into().unwrap(), v.into()))
             .collect::<Vec<(u64, Word)>>();
-        let smt = SimpleSmt::new(SimpleSmt::MAX_DEPTH)
-            .unwrap()
-            .with_leaves(smt_leaves.clone())
-            .unwrap();
+        let smt = SimpleSmt::with_leaves(SimpleSmt::MAX_DEPTH, smt_leaves.clone()).unwrap();
         let store = MerkleStore::from(&smt);
         let depth = smt.depth();
         let root = smt.root();
@@ -361,7 +352,7 @@ fn new(c: &mut Criterion) {
                         .map(|(c, v)| (c.try_into().unwrap(), v.into()))
                         .collect::<Vec<(u64, Word)>>()
                 },
-                |l| black_box(SimpleSmt::new(SimpleSmt::MAX_DEPTH).unwrap().with_leaves(l)),
+                |l| black_box(SimpleSmt::with_leaves(SimpleSmt::MAX_DEPTH, l)),
                 BatchSize::SmallInput,
             )
         });
@@ -376,7 +367,7 @@ fn new(c: &mut Criterion) {
                         .collect::<Vec<(u64, Word)>>()
                 },
                 |l| {
-                    let smt = SimpleSmt::new(SimpleSmt::MAX_DEPTH).unwrap().with_leaves(l).unwrap();
+                    let smt = SimpleSmt::with_leaves(SimpleSmt::MAX_DEPTH, l).unwrap();
                     black_box(MerkleStore::from(&smt));
                 },
                 BatchSize::SmallInput,
@@ -418,7 +409,7 @@ fn update_leaf_merkletree(c: &mut Criterion) {
                     // The MerkleTree automatically updates its internal root, the Store maintains
                     // the old root and adds the new one. Here we update the root to have a fair
                     // comparison
-                    store_root = store.set_node(root, index, value).unwrap().root;
+                    store_root = store.set_node(root, index, value.into()).unwrap().root;
                     black_box(store_root)
                 },
                 BatchSize::SmallInput,
@@ -442,10 +433,7 @@ fn update_leaf_simplesmt(c: &mut Criterion) {
             .enumerate()
             .map(|(c, v)| (c.try_into().unwrap(), v.into()))
             .collect::<Vec<(u64, Word)>>();
-        let mut smt = SimpleSmt::new(SimpleSmt::MAX_DEPTH)
-            .unwrap()
-            .with_leaves(smt_leaves.clone())
-            .unwrap();
+        let mut smt = SimpleSmt::with_leaves(SimpleSmt::MAX_DEPTH, smt_leaves.clone()).unwrap();
         let mut store = MerkleStore::from(&smt);
         let depth = smt.depth();
         let root = smt.root();
@@ -467,7 +455,7 @@ fn update_leaf_simplesmt(c: &mut Criterion) {
                     // The MerkleTree automatically updates its internal root, the Store maintains
                     // the old root and adds the new one. Here we update the root to have a fair
                     // comparison
-                    store_root = store.set_node(root, index, value).unwrap().root;
+                    store_root = store.set_node(root, index, value.into()).unwrap().root;
                     black_box(store_root)
                 },
                 BatchSize::SmallInput,

src/hash/rpo/digest.rs

@@ -2,7 +2,7 @@ use super::{Digest, Felt, StarkField, DIGEST_SIZE, ZERO};
 use crate::utils::{
     string::String, ByteReader, ByteWriter, Deserializable, DeserializationError, Serializable,
 };
-use core::{cmp::Ordering, ops::Deref};
+use core::{cmp::Ordering, fmt::Display, ops::Deref};
 
 // DIGEST TRAIT IMPLEMENTATIONS
 // ================================================================================================
@@ -85,6 +85,28 @@ impl From<RpoDigest> for [Felt; DIGEST_SIZE] {
     }
 }
 
+impl From<&RpoDigest> for [u64; DIGEST_SIZE] {
+    fn from(value: &RpoDigest) -> Self {
+        [
+            value.0[0].as_int(),
+            value.0[1].as_int(),
+            value.0[2].as_int(),
+            value.0[3].as_int(),
+        ]
+    }
+}
+
+impl From<RpoDigest> for [u64; DIGEST_SIZE] {
+    fn from(value: RpoDigest) -> Self {
+        [
+            value.0[0].as_int(),
+            value.0[1].as_int(),
+            value.0[2].as_int(),
+            value.0[3].as_int(),
+        ]
+    }
+}
+
 impl From<&RpoDigest> for [u8; 32] {
     fn from(value: &RpoDigest) -> Self {
         value.as_bytes()
@@ -134,6 +156,15 @@ impl PartialOrd for RpoDigest {
     }
 }
 
+impl Display for RpoDigest {
+    fn fmt(&self, f: &mut core::fmt::Formatter<'_>) -> core::fmt::Result {
+        for byte in self.as_bytes() {
+            write!(f, "{byte:02x}")?;
+        }
+        Ok(())
+    }
+}
+
 // TESTS
 // ================================================================================================
 

src/hash/rpo/tests.rs

@@ -2,7 +2,10 @@ use super::{
     Felt, FieldElement, Hasher, Rpo256, RpoDigest, StarkField, ALPHA, INV_ALPHA, ONE, STATE_WIDTH,
     ZERO,
 };
-use crate::utils::collections::{BTreeSet, Vec};
+use crate::{
+    utils::collections::{BTreeSet, Vec},
+    Word,
+};
 use core::convert::TryInto;
 use proptest::prelude::*;
 use rand_utils::rand_value;
@@ -203,7 +206,7 @@ fn sponge_bytes_with_remainder_length_wont_panic() {
     // size.
     //
     // this is a preliminary test to the fuzzy-stress of proptest.
-    Rpo256::hash(&vec![0; 113]);
+    Rpo256::hash(&[0; 113]);
 }
 
 #[test]
@@ -227,12 +230,12 @@ fn sponge_zeroes_collision() {
 
 proptest! {
     #[test]
-    fn rpo256_wont_panic_with_arbitrary_input(ref vec in any::<Vec<u8>>()) {
-        Rpo256::hash(&vec);
+    fn rpo256_wont_panic_with_arbitrary_input(ref bytes in any::<Vec<u8>>()) {
+        Rpo256::hash(bytes);
     }
 }
 
-const EXPECTED: [[Felt; 4]; 19] = [
+const EXPECTED: [Word; 19] = [
     [
         Felt::new(1502364727743950833),
         Felt::new(5880949717274681448),

src/merkle/empty_roots.rs

@@ -1,6 +1,12 @@
-use super::{Felt, RpoDigest, WORD_SIZE, ZERO};
+use super::{Felt, RpoDigest, Word, WORD_SIZE, ZERO};
 use core::slice;
 
+// CONSTANTS
+// ================================================================================================
+
+/// A word consisting of 4 ZERO elements.
+pub const EMPTY_WORD: Word = [ZERO; WORD_SIZE];
+
 // EMPTY NODES SUBTREES
 // ================================================================================================
 
@@ -1570,7 +1576,7 @@ fn all_depths_opens_to_zero() {
         assert_eq!(depth as usize + 1, subtree.len());
 
         // assert the opening is zero
-        let initial = RpoDigest::new([ZERO; WORD_SIZE]);
+        let initial = RpoDigest::new(EMPTY_WORD);
         assert_eq!(initial, subtree.remove(0));
 
         // compute every node of the path manually and compare with the output

src/merkle/index.rs

@@ -1,4 +1,5 @@
 use super::{Felt, MerkleError, RpoDigest, StarkField};
+use core::fmt::Display;
 
 // NODE INDEX
 // ================================================================================================
@@ -40,6 +41,12 @@ impl NodeIndex {
         }
     }
 
+    /// Creates a new node index without checking its validity.
+    pub const fn new_unchecked(depth: u8, value: u64) -> Self {
+        debug_assert!((64 - value.leading_zeros()) <= depth as u32);
+        Self { depth, value }
+    }
+
     /// Creates a new node index for testing purposes.
     ///
     /// # Panics
@@ -67,12 +74,26 @@ impl NodeIndex {
         Self { depth: 0, value: 0 }
     }
 
-    /// Computes the value of the sibling of the current node.
-    pub fn sibling(mut self) -> Self {
+    /// Computes sibling index of the current node.
+    pub const fn sibling(mut self) -> Self {
         self.value ^= 1;
         self
     }
 
+    /// Returns left child index of the current node.
+    pub const fn left_child(mut self) -> Self {
+        self.depth += 1;
+        self.value <<= 1;
+        self
+    }
+
+    /// Returns right child index of the current node.
+    pub const fn right_child(mut self) -> Self {
+        self.depth += 1;
+        self.value = (self.value << 1) + 1;
+        self
+    }
+
     // PROVIDERS
     // --------------------------------------------------------------------------------------------
 
@@ -117,11 +138,26 @@ impl NodeIndex {
     // STATE MUTATORS
     // --------------------------------------------------------------------------------------------
 
-    /// Traverse one level towards the root, decrementing the depth by `1`.
-    pub fn move_up(&mut self) -> &mut Self {
+    /// Traverses one level towards the root, decrementing the depth by `1`.
+    pub fn move_up(&mut self) {
         self.depth = self.depth.saturating_sub(1);
         self.value >>= 1;
-        self
+    }
+
+    /// Traverses towards the root until the specified depth is reached.
+    ///
+    /// Assumes that the specified depth is smaller than the current depth.
+    pub fn move_up_to(&mut self, depth: u8) {
+        debug_assert!(depth < self.depth);
+        let delta = self.depth.saturating_sub(depth);
+        self.depth = self.depth.saturating_sub(delta);
+        self.value >>= delta as u32;
+    }
+}
+
+impl Display for NodeIndex {
+    fn fmt(&self, f: &mut core::fmt::Formatter<'_>) -> core::fmt::Result {
+        write!(f, "depth={}, value={}", self.depth, self.value)
     }
 }
 
@@ -154,7 +190,7 @@ mod tests {
             if value > (1 << depth) { // round up
                 depth += 1;
             }
-            NodeIndex::new(depth, value.into()).unwrap()
+            NodeIndex::new(depth, value).unwrap()
         }
     }
 

src/merkle/merkle_tree.rs

@@ -1,11 +1,6 @@
-use super::{
-    Felt, InnerNodeInfo, MerkleError, MerklePath, NodeIndex, Rpo256, RpoDigest, Vec, Word,
-};
-use crate::{
-    utils::{string::String, uninit_vector, word_to_hex},
-    FieldElement,
-};
-use core::{fmt, slice};
+use super::{InnerNodeInfo, MerkleError, MerklePath, NodeIndex, Rpo256, RpoDigest, Vec, Word};
+use crate::utils::{string::String, uninit_vector, word_to_hex};
+use core::{fmt, ops::Deref, slice};
 use winter_math::log2;
 
 // MERKLE TREE
@@ -14,7 +9,7 @@ use winter_math::log2;
 /// A fully-balanced binary Merkle tree (i.e., a tree where the number of leaves is a power of two).
 #[derive(Debug, Clone, PartialEq, Eq)]
 pub struct MerkleTree {
-    nodes: Vec<Word>,
+    nodes: Vec<RpoDigest>,
 }
 
 impl MerkleTree {
@@ -34,10 +29,12 @@ impl MerkleTree {
 
         // create un-initialized vector to hold all tree nodes
         let mut nodes = unsafe { uninit_vector(2 * n) };
-        nodes[0] = [Felt::ZERO; 4];
+        nodes[0] = RpoDigest::default();
 
         // copy leaves into the second part of the nodes vector
-        nodes[n..].copy_from_slice(&leaves);
+        nodes[n..].iter_mut().zip(leaves).for_each(|(node, leaf)| {
+            *node = RpoDigest::from(leaf);
+        });
 
         // re-interpret nodes as an array of two nodes fused together
         // Safety: `nodes` will never move here as it is not bound to an external lifetime (i.e.
@@ -47,7 +44,7 @@ impl MerkleTree {
 
         // calculate all internal tree nodes
         for i in (1..n).rev() {
-            nodes[i] = Rpo256::merge(&pairs[i]).into();
+            nodes[i] = Rpo256::merge(&pairs[i]);
         }
 
         Ok(Self { nodes })
@@ -57,7 +54,7 @@ impl MerkleTree {
     // --------------------------------------------------------------------------------------------
 
     /// Returns the root of this Merkle tree.
-    pub fn root(&self) -> Word {
+    pub fn root(&self) -> RpoDigest {
         self.nodes[1]
     }
 
@@ -74,7 +71,7 @@ impl MerkleTree {
     /// Returns an error if:
     /// * The specified depth is greater than the depth of the tree.
     /// * The specified index is not valid for the specified depth.
-    pub fn get_node(&self, index: NodeIndex) -> Result<Word, MerkleError> {
+    pub fn get_node(&self, index: NodeIndex) -> Result<RpoDigest, MerkleError> {
         if index.is_root() {
             return Err(MerkleError::DepthTooSmall(index.depth()));
         } else if index.depth() > self.depth() {
@@ -114,6 +111,32 @@ impl MerkleTree {
         Ok(path.into())
     }
 
+    // ITERATORS
+    // --------------------------------------------------------------------------------------------
+
+    /// Returns an iterator over the leaves of this [MerkleTree].
+    pub fn leaves(&self) -> impl Iterator<Item = (u64, &Word)> {
+        let leaves_start = self.nodes.len() / 2;
+        self.nodes
+            .iter()
+            .skip(leaves_start)
+            .enumerate()
+            .map(|(i, v)| (i as u64, v.deref()))
+    }
+
+    /// Returns n iterator over every inner node of this [MerkleTree].
+    ///
+    /// The iterator order is unspecified.
+    pub fn inner_nodes(&self) -> InnerNodeIterator {
+        InnerNodeIterator {
+            nodes: &self.nodes,
+            index: 1, // index 0 is just padding, start at 1
+        }
+    }
+
+    // STATE MUTATORS
+    // --------------------------------------------------------------------------------------------
+
     /// Replaces the leaf at the specified index with the provided value.
     ///
     /// # Errors
@@ -137,28 +160,18 @@ impl MerkleTree {
 
         // update the current node
         let pos = index.to_scalar_index() as usize;
-        self.nodes[pos] = value;
+        self.nodes[pos] = value.into();
 
         // traverse to the root, updating each node with the merged values of its parents
         for _ in 0..index.depth() {
             index.move_up();
             let pos = index.to_scalar_index() as usize;
-            let value = Rpo256::merge(&pairs[pos]).into();
+            let value = Rpo256::merge(&pairs[pos]);
             self.nodes[pos] = value;
         }
 
         Ok(())
     }
-
-    /// Returns n iterator over every inner node of this [MerkleTree].
-    ///
-    /// The iterator order is unspecified.
-    pub fn inner_nodes(&self) -> InnerNodeIterator<'_> {
-        InnerNodeIterator {
-            nodes: &self.nodes,
-            index: 1, // index 0 is just padding, start at 1
-        }
-    }
 }
 
 // ITERATORS
@@ -168,7 +181,7 @@ impl MerkleTree {
 ///
 /// Use this to extract the data of the tree, there is no guarantee on the order of the elements.
 pub struct InnerNodeIterator<'a> {
-    nodes: &'a Vec<Word>,
+    nodes: &'a Vec<RpoDigest>,
     index: usize,
 }
 
@@ -246,13 +259,17 @@ pub fn path_to_text(path: &MerklePath) -> Result<String, fmt::Error> {
 #[cfg(test)]
 mod tests {
     use super::*;
-    use crate::merkle::{int_to_node, InnerNodeInfo};
+    use crate::{
+        merkle::{digests_to_words, int_to_leaf, int_to_node, InnerNodeInfo},
+        Felt, Word, WORD_SIZE,
+    };
     use core::mem::size_of;
     use proptest::prelude::*;
 
-    const LEAVES4: [Word; 4] = [int_to_node(1), int_to_node(2), int_to_node(3), int_to_node(4)];
+    const LEAVES4: [RpoDigest; WORD_SIZE] =
+        [int_to_node(1), int_to_node(2), int_to_node(3), int_to_node(4)];
 
-    const LEAVES8: [Word; 8] = [
+    const LEAVES8: [RpoDigest; 8] = [
         int_to_node(1),
         int_to_node(2),
         int_to_node(3),
@@ -265,7 +282,7 @@ mod tests {
 
     #[test]
     fn build_merkle_tree() {
-        let tree = super::MerkleTree::new(LEAVES4.to_vec()).unwrap();
+        let tree = super::MerkleTree::new(digests_to_words(&LEAVES4)).unwrap();
         assert_eq!(8, tree.nodes.len());
 
         // leaves were copied correctly
@@ -284,7 +301,7 @@ mod tests {
 
     #[test]
     fn get_leaf() {
-        let tree = super::MerkleTree::new(LEAVES4.to_vec()).unwrap();
+        let tree = super::MerkleTree::new(digests_to_words(&LEAVES4)).unwrap();
 
         // check depth 2
         assert_eq!(LEAVES4[0], tree.get_node(NodeIndex::make(2, 0)).unwrap());
@@ -301,7 +318,7 @@ mod tests {
 
     #[test]
     fn get_path() {
-        let tree = super::MerkleTree::new(LEAVES4.to_vec()).unwrap();
+        let tree = super::MerkleTree::new(digests_to_words(&LEAVES4)).unwrap();
 
         let (_, node2, node3) = compute_internal_nodes();
 
@@ -318,12 +335,12 @@ mod tests {
 
     #[test]
     fn update_leaf() {
-        let mut tree = super::MerkleTree::new(LEAVES8.to_vec()).unwrap();
+        let mut tree = super::MerkleTree::new(digests_to_words(&LEAVES8)).unwrap();
 
         // update one leaf
         let value = 3;
-        let new_node = int_to_node(9);
-        let mut expected_leaves = LEAVES8.to_vec();
+        let new_node = int_to_leaf(9);
+        let mut expected_leaves = digests_to_words(&LEAVES8);
         expected_leaves[value as usize] = new_node;
         let expected_tree = super::MerkleTree::new(expected_leaves.clone()).unwrap();
 
@@ -332,7 +349,7 @@ mod tests {
 
         // update another leaf
         let value = 6;
-        let new_node = int_to_node(10);
+        let new_node = int_to_leaf(10);
         expected_leaves[value as usize] = new_node;
         let expected_tree = super::MerkleTree::new(expected_leaves.clone()).unwrap();
 
@@ -342,7 +359,7 @@ mod tests {
 
     #[test]
     fn nodes() -> Result<(), MerkleError> {
-        let tree = super::MerkleTree::new(LEAVES4.to_vec()).unwrap();
+        let tree = super::MerkleTree::new(digests_to_words(&LEAVES4)).unwrap();
         let root = tree.root();
         let l1n0 = tree.get_node(NodeIndex::make(1, 0))?;
         let l1n1 = tree.get_node(NodeIndex::make(1, 1))?;
@@ -391,8 +408,8 @@ mod tests {
             let digest = RpoDigest::from(word);
 
             // assert the addresses are different
-            let word_ptr = (&word).as_ptr() as *const u8;
-            let digest_ptr = (&digest).as_ptr() as *const u8;
+            let word_ptr = word.as_ptr() as *const u8;
+            let digest_ptr = digest.as_ptr() as *const u8;
             assert_ne!(word_ptr, digest_ptr);
 
             // compare the bytes representation
@@ -405,11 +422,13 @@ mod tests {
     // HELPER FUNCTIONS
     // --------------------------------------------------------------------------------------------
 
-    fn compute_internal_nodes() -> (Word, Word, Word) {
-        let node2 = Rpo256::hash_elements(&[LEAVES4[0], LEAVES4[1]].concat());
-        let node3 = Rpo256::hash_elements(&[LEAVES4[2], LEAVES4[3]].concat());
+    fn compute_internal_nodes() -> (RpoDigest, RpoDigest, RpoDigest) {
+        let node2 =
+            Rpo256::hash_elements(&[Word::from(LEAVES4[0]), Word::from(LEAVES4[1])].concat());
+        let node3 =
+            Rpo256::hash_elements(&[Word::from(LEAVES4[2]), Word::from(LEAVES4[3])].concat());
         let root = Rpo256::merge(&[node2, node3]);
 
-        (root.into(), node2.into(), node3.into())
+        (root, node2, node3)
     }
 }

src/merkle/mmr/accumulator.rs

@@ -1,7 +1,6 @@
 use super::{
-    super::Vec,
-    super::{WORD_SIZE, ZERO},
-    MmrProof, Rpo256, Word,
+    super::{RpoDigest, Vec, ZERO},
+    Felt, MmrProof, Rpo256, Word,
 };
 
 #[derive(Debug, Clone, PartialEq)]
@@ -29,31 +28,63 @@ pub struct MmrPeaks {
     /// leaves, starting from the peak with most children, to the one with least.
     ///
     /// Invariant: The length of `peaks` must be equal to the number of true bits in `num_leaves`.
-    pub peaks: Vec<Word>,
+    pub peaks: Vec<RpoDigest>,
 }
 
 impl MmrPeaks {
     /// Hashes the peaks.
     ///
-    /// The hashing is optimized to work with the Miden VM, the procedure will:
-    ///
-    /// - Pad the peaks with ZERO to an even number of words, this removes the need to handle RPO padding.
-    /// - Pad the peaks to a minimum length of 16 words, which reduces the constant cost of
-    ///   hashing.
+    /// The procedure will:
+    /// - Flatten and pad the peaks to a vector of Felts.
+    /// - Hash the vector of Felts.
     pub fn hash_peaks(&self) -> Word {
-        let mut copy = self.peaks.clone();
-
-        if copy.len() < 16 {
-            copy.resize(16, [ZERO; WORD_SIZE])
-        } else if copy.len() % 2 == 1 {
-            copy.push([ZERO; WORD_SIZE])
-        }
-
-        Rpo256::hash_elements(&copy.as_slice().concat()).into()
+        Rpo256::hash_elements(&self.flatten_and_pad_peaks()).into()
     }
 
-    pub fn verify(&self, value: Word, opening: MmrProof) -> bool {
+    pub fn verify(&self, value: RpoDigest, opening: MmrProof) -> bool {
         let root = &self.peaks[opening.peak_index()];
         opening.merkle_path.verify(opening.relative_pos() as u64, value, root)
     }
+
+    /// Flattens and pads the peaks to make hashing inside of the Miden VM easier.
+    ///
+    /// The procedure will:
+    /// - Flatten the vector of Words into a vector of Felts.
+    /// - Pad the peaks with ZERO to an even number of words, this removes the need to handle RPO
+    ///   padding.
+    /// - Pad the peaks to a minimum length of 16 words, which reduces the constant cost of
+    ///   hashing.
+    pub fn flatten_and_pad_peaks(&self) -> Vec<Felt> {
+        let num_peaks = self.peaks.len();
+
+        // To achieve the padding rules above we calculate the length of the final vector.
+        // This is calculated as the number of field elements. Each peak is 4 field elements.
+        // The length is calculated as follows:
+        // - If there are less than 16 peaks, the data is padded to 16 peaks and as such requires
+        //   64 field elements.
+        // - If there are more than 16 peaks and the number of peaks is odd, the data is padded to
+        //   an even number of peaks and as such requires `(num_peaks + 1) * 4` field elements.
+        // - If there are more than 16 peaks and the number of peaks is even, the data is not padded
+        //   and as such requires `num_peaks * 4` field elements.
+        let len = if num_peaks < 16 {
+            64
+        } else if num_peaks % 2 == 1 {
+            (num_peaks + 1) * 4
+        } else {
+            num_peaks * 4
+        };
+
+        let mut elements = Vec::with_capacity(len);
+        elements.extend_from_slice(
+            &self
+                .peaks
+                .as_slice()
+                .iter()
+                .map(|digest| digest.into())
+                .collect::<Vec<Word>>()
+                .concat(),
+        );
+        elements.resize(len, ZERO);
+        elements
+    }
 }

src/merkle/mmr/full.rs

@@ -10,10 +10,10 @@
 //! depths, i.e. as part of adding adding a new element to the forest the trees with same depth are
 //! merged, creating a new tree with depth d+1, this process is continued until the property is
 //! restabilished.
-use super::bit::TrueBitPositionIterator;
 use super::{
-    super::{InnerNodeInfo, MerklePath, Vec},
-    MmrPeaks, MmrProof, Rpo256, Word,
+    super::{InnerNodeInfo, MerklePath, RpoDigest, Vec},
+    bit::TrueBitPositionIterator,
+    MmrPeaks, MmrProof, Rpo256,
 };
 use core::fmt::{Display, Formatter};
 
@@ -28,6 +28,7 @@ use std::error::Error;
 ///
 /// Since this is a full representation of the MMR, elements are never removed and the MMR will
 /// grow roughly `O(2n)` in number of leaf elements.
+#[derive(Debug, Clone)]
 pub struct Mmr {
     /// Refer to the `forest` method documentation for details of the semantics of this value.
     pub(super) forest: usize,
@@ -38,7 +39,7 @@ pub struct Mmr {
     /// the elements of every tree in the forest to be stored in the same sequential buffer. It
     /// also means new elements can be added to the forest, and merging of trees is very cheap with
     /// no need to copy elements.
-    pub(super) nodes: Vec<Word>,
+    pub(super) nodes: Vec<RpoDigest>,
 }
 
 #[derive(Debug, PartialEq, Eq, Copy, Clone)]
@@ -129,7 +130,7 @@ impl Mmr {
     /// Note: The leaf position is the 0-indexed number corresponding to the order the leaves were
     /// added, this corresponds to the MMR size _prior_ to adding the element. So the 1st element
     /// has position 0, the second position 1, and so on.
-    pub fn get(&self, pos: usize) -> Result<Word, MmrError> {
+    pub fn get(&self, pos: usize) -> Result<RpoDigest, MmrError> {
         // find the target tree responsible for the MMR position
         let tree_bit =
             leaf_to_corresponding_tree(pos, self.forest).ok_or(MmrError::InvalidPosition(pos))?;
@@ -153,7 +154,7 @@ impl Mmr {
     }
 
     /// Adds a new element to the MMR.
-    pub fn add(&mut self, el: Word) {
+    pub fn add(&mut self, el: RpoDigest) {
         // Note: every node is also a tree of size 1, adding an element to the forest creates a new
         // rooted-tree of size 1. This may temporarily break the invariant that every tree in the
         // forest has different sizes, the loop below will eagerly merge trees of same size and
@@ -164,7 +165,7 @@ impl Mmr {
         let mut right = el;
         let mut left_tree = 1;
         while self.forest & left_tree != 0 {
-            right = *Rpo256::merge(&[self.nodes[left_offset].into(), right.into()]);
+            right = Rpo256::merge(&[self.nodes[left_offset], right]);
             self.nodes.push(right);
 
             left_offset = left_offset.saturating_sub(nodes_in_forest(left_tree));
@@ -176,7 +177,7 @@ impl Mmr {
 
     /// Returns an accumulator representing the current state of the MMR.
     pub fn accumulator(&self) -> MmrPeaks {
-        let peaks: Vec<Word> = TrueBitPositionIterator::new(self.forest)
+        let peaks: Vec<RpoDigest> = TrueBitPositionIterator::new(self.forest)
             .rev()
             .map(|bit| nodes_in_forest(1 << bit))
             .scan(0, |offset, el| {
@@ -212,7 +213,7 @@ impl Mmr {
         relative_pos: usize,
         index_offset: usize,
         mut index: usize,
-    ) -> (Word, Vec<Word>) {
+    ) -> (RpoDigest, Vec<RpoDigest>) {
         // collect the Merkle path
         let mut tree_depth = tree_bit as usize;
         let mut path = Vec::with_capacity(tree_depth + 1);
@@ -247,7 +248,7 @@ impl Mmr {
 
 impl<T> From<T> for Mmr
 where
-    T: IntoIterator<Item = Word>,
+    T: IntoIterator<Item = RpoDigest>,
 {
     fn from(values: T) -> Self {
         let mut mmr = Mmr::new();

src/merkle/mmr/mod.rs

@@ -6,7 +6,7 @@ mod proof;
 #[cfg(test)]
 mod tests;
 
-use super::{Rpo256, Word};
+use super::{Felt, Rpo256, Word};
 
 // REEXPORTS
 // ================================================================================================

src/merkle/mmr/tests.rs

@@ -1,10 +1,14 @@
-use super::bit::TrueBitPositionIterator;
-use super::full::{high_bitmask, leaf_to_corresponding_tree, nodes_in_forest};
 use super::{
-    super::{InnerNodeInfo, Vec, WORD_SIZE, ZERO},
-    Mmr, MmrPeaks, Rpo256, Word,
+    super::{InnerNodeInfo, Vec},
+    bit::TrueBitPositionIterator,
+    full::{high_bitmask, leaf_to_corresponding_tree, nodes_in_forest},
+    Mmr, MmrPeaks, Rpo256,
+};
+use crate::{
+    hash::rpo::RpoDigest,
+    merkle::{int_to_node, MerklePath},
+    Felt, Word,
 };
-use crate::merkle::{int_to_node, MerklePath};
 
 #[test]
 fn test_position_equal_or_higher_than_leafs_is_never_contained() {
@@ -99,7 +103,7 @@ fn test_nodes_in_forest_single_bit() {
     }
 }
 
-const LEAVES: [Word; 7] = [
+const LEAVES: [RpoDigest; 7] = [
     int_to_node(0),
     int_to_node(1),
     int_to_node(2),
@@ -114,14 +118,14 @@ fn test_mmr_simple() {
     let mut postorder = Vec::new();
     postorder.push(LEAVES[0]);
     postorder.push(LEAVES[1]);
-    postorder.push(*Rpo256::hash_elements(&[LEAVES[0], LEAVES[1]].concat()));
+    postorder.push(Rpo256::merge(&[LEAVES[0], LEAVES[1]]));
     postorder.push(LEAVES[2]);
     postorder.push(LEAVES[3]);
-    postorder.push(*Rpo256::hash_elements(&[LEAVES[2], LEAVES[3]].concat()));
-    postorder.push(*Rpo256::hash_elements(&[postorder[2], postorder[5]].concat()));
+    postorder.push(Rpo256::merge(&[LEAVES[2], LEAVES[3]]));
+    postorder.push(Rpo256::merge(&[postorder[2], postorder[5]]));
     postorder.push(LEAVES[4]);
     postorder.push(LEAVES[5]);
-    postorder.push(*Rpo256::hash_elements(&[LEAVES[4], LEAVES[5]].concat()));
+    postorder.push(Rpo256::merge(&[LEAVES[4], LEAVES[5]]));
     postorder.push(LEAVES[6]);
 
     let mut mmr = Mmr::new();
@@ -195,8 +199,8 @@ fn test_mmr_simple() {
 #[test]
 fn test_mmr_open() {
     let mmr: Mmr = LEAVES.into();
-    let h01: Word = Rpo256::hash_elements(&LEAVES[0..2].concat()).into();
-    let h23: Word = Rpo256::hash_elements(&LEAVES[2..4].concat()).into();
+    let h01 = Rpo256::merge(&[LEAVES[0], LEAVES[1]]);
+    let h23 = Rpo256::merge(&[LEAVES[2], LEAVES[3]]);
 
     // node at pos 7 is the root
     assert!(mmr.open(7).is_err(), "Element 7 is not in the tree, result should be None");
@@ -214,7 +218,7 @@ fn test_mmr_open() {
         "MmrProof should be valid for the current accumulator."
     );
 
-    // nodes 4,5 are detph 1
+    // nodes 4,5 are depth 1
     let root_to_path = MerklePath::new(vec![LEAVES[4]]);
     let opening = mmr
         .open(5)
@@ -361,10 +365,10 @@ fn test_mmr_inner_nodes() {
     let mmr: Mmr = LEAVES.into();
     let nodes: Vec<InnerNodeInfo> = mmr.inner_nodes().collect();
 
-    let h01 = *Rpo256::hash_elements(&[LEAVES[0], LEAVES[1]].concat());
-    let h23 = *Rpo256::hash_elements(&[LEAVES[2], LEAVES[3]].concat());
-    let h0123 = *Rpo256::hash_elements(&[h01, h23].concat());
-    let h45 = *Rpo256::hash_elements(&[LEAVES[4], LEAVES[5]].concat());
+    let h01 = Rpo256::merge(&[LEAVES[0], LEAVES[1]]);
+    let h23 = Rpo256::merge(&[LEAVES[2], LEAVES[3]]);
+    let h0123 = Rpo256::merge(&[h01, h23]);
+    let h45 = Rpo256::merge(&[LEAVES[4], LEAVES[5]]);
     let postorder = vec![
         InnerNodeInfo {
             value: h01,
@@ -396,17 +400,20 @@ fn test_mmr_hash_peaks() {
     let mmr: Mmr = LEAVES.into();
     let peaks = mmr.accumulator();
 
-    let first_peak = *Rpo256::merge(&[
-        Rpo256::hash_elements(&[LEAVES[0], LEAVES[1]].concat()),
-        Rpo256::hash_elements(&[LEAVES[2], LEAVES[3]].concat()),
+    let first_peak = Rpo256::merge(&[
+        Rpo256::merge(&[LEAVES[0], LEAVES[1]]),
+        Rpo256::merge(&[LEAVES[2], LEAVES[3]]),
     ]);
-    let second_peak = *Rpo256::hash_elements(&[LEAVES[4], LEAVES[5]].concat());
+    let second_peak = Rpo256::merge(&[LEAVES[4], LEAVES[5]]);
     let third_peak = LEAVES[6];
 
     // minimum length is 16
     let mut expected_peaks = [first_peak, second_peak, third_peak].to_vec();
-    expected_peaks.resize(16, [ZERO; WORD_SIZE]);
-    assert_eq!(peaks.hash_peaks(), *Rpo256::hash_elements(&expected_peaks.as_slice().concat()));
+    expected_peaks.resize(16, RpoDigest::default());
+    assert_eq!(
+        peaks.hash_peaks(),
+        *Rpo256::hash_elements(&digests_to_elements(&expected_peaks))
+    );
 }
 
 #[test]
@@ -422,29 +429,29 @@ fn test_mmr_peaks_hash_less_than_16() {
 
         // minimum length is 16
         let mut expected_peaks = peaks.clone();
-        expected_peaks.resize(16, [ZERO; WORD_SIZE]);
+        expected_peaks.resize(16, RpoDigest::default());
         assert_eq!(
             accumulator.hash_peaks(),
-            *Rpo256::hash_elements(&expected_peaks.as_slice().concat())
+            *Rpo256::hash_elements(&digests_to_elements(&expected_peaks))
         );
     }
 }
 
 #[test]
 fn test_mmr_peaks_hash_odd() {
-    let peaks: Vec<_> = (0..=17).map(|i| int_to_node(i)).collect();
+    let peaks: Vec<_> = (0..=17).map(int_to_node).collect();
 
     let accumulator = MmrPeaks {
         num_leaves: (1 << peaks.len()) - 1,
         peaks: peaks.clone(),
     };
 
-    // odd length bigger than 16 is padded to the next even nubmer
-    let mut expected_peaks = peaks.clone();
-    expected_peaks.resize(18, [ZERO; WORD_SIZE]);
+    // odd length bigger than 16 is padded to the next even number
+    let mut expected_peaks = peaks;
+    expected_peaks.resize(18, RpoDigest::default());
     assert_eq!(
         accumulator.hash_peaks(),
-        *Rpo256::hash_elements(&expected_peaks.as_slice().concat())
+        *Rpo256::hash_elements(&digests_to_elements(&expected_peaks))
     );
 }
 
@@ -476,3 +483,10 @@ mod property_tests {
         }
     }
 }
+
+// HELPER FUNCTIONS
+// ================================================================================================
+
+fn digests_to_elements(digests: &[RpoDigest]) -> Vec<Felt> {
+    digests.iter().flat_map(Word::from).collect()
+}

src/merkle/mod.rs

@@ -1,6 +1,6 @@
 use super::{
     hash::rpo::{Rpo256, RpoDigest},
-    utils::collections::{vec, BTreeMap, Vec},
+    utils::collections::{vec, BTreeMap, BTreeSet, KvMap, RecordingMap, Vec},
     Felt, StarkField, Word, WORD_SIZE, ZERO,
 };
 use core::fmt;
@@ -26,31 +26,39 @@ pub use path_set::MerklePathSet;
 mod simple_smt;
 pub use simple_smt::SimpleSmt;
 
+mod tiered_smt;
+pub use tiered_smt::TieredSmt;
+
 mod mmr;
 pub use mmr::{Mmr, MmrPeaks, MmrProof};
 
 mod store;
-pub use store::MerkleStore;
+pub use store::{DefaultMerkleStore, MerkleStore, RecordingMerkleStore, StoreNode};
 
 mod node;
 pub use node::InnerNodeInfo;
 
+mod partial_mt;
+pub use partial_mt::PartialMerkleTree;
+
 // ERRORS
 // ================================================================================================
 
 #[derive(Clone, Debug, PartialEq, Eq)]
 pub enum MerkleError {
-    ConflictingRoots(Vec<Word>),
+    ConflictingRoots(Vec<RpoDigest>),
     DepthTooSmall(u8),
     DepthTooBig(u64),
-    NodeNotInStore(Word, NodeIndex),
-    NumLeavesNotPowerOfTwo(usize),
+    DuplicateValuesForIndex(u64),
+    DuplicateValuesForKey(RpoDigest),
     InvalidIndex { depth: u8, value: u64 },
     InvalidDepth { expected: u8, provided: u8 },
     InvalidPath(MerklePath),
-    InvalidEntriesCount(usize, usize),
-    NodeNotInSet(u64),
-    RootNotInStore(Word),
+    InvalidNumEntries(usize, usize),
+    NodeNotInSet(NodeIndex),
+    NodeNotInStore(RpoDigest, NodeIndex),
+    NumLeavesNotPowerOfTwo(usize),
+    RootNotInStore(RpoDigest),
 }
 
 impl fmt::Display for MerkleError {
@@ -60,9 +68,8 @@ impl fmt::Display for MerkleError {
             ConflictingRoots(roots) => write!(f, "the merkle paths roots do not match {roots:?}"),
             DepthTooSmall(depth) => write!(f, "the provided depth {depth} is too small"),
             DepthTooBig(depth) => write!(f, "the provided depth {depth} is too big"),
-            NumLeavesNotPowerOfTwo(leaves) => {
-                write!(f, "the leaves count {leaves} is not a power of 2")
-            }
+            DuplicateValuesForIndex(key) => write!(f, "multiple values provided for key {key}"),
+            DuplicateValuesForKey(key) => write!(f, "multiple values provided for key {key}"),
             InvalidIndex{ depth, value} => write!(
                 f,
                 "the index value {value} is not valid for the depth {depth}"
@@ -72,9 +79,12 @@ impl fmt::Display for MerkleError {
                 "the provided depth {provided} is not valid for {expected}"
             ),
             InvalidPath(_path) => write!(f, "the provided path is not valid"),
-            InvalidEntriesCount(max, provided) => write!(f, "the provided number of entries is {provided}, but the maximum for the given depth is {max}"),
-            NodeNotInSet(index) => write!(f, "the node indexed by {index} is not in the set"),
-            NodeNotInStore(hash, index) => write!(f, "the node {:?} indexed by {} and depth {} is not in the store", hash, index.value(), index.depth(),),
+            InvalidNumEntries(max, provided) => write!(f, "the provided number of entries is {provided}, but the maximum for the given depth is {max}"),
+            NodeNotInSet(index) => write!(f, "the node with index ({index}) is not in the set"),
+            NodeNotInStore(hash, index) => write!(f, "the node {hash:?} with index ({index}) is not in the store"),
+            NumLeavesNotPowerOfTwo(leaves) => {
+                write!(f, "the leaves count {leaves} is not a power of 2")
+            }
             RootNotInStore(root) => write!(f, "the root {:?} is not in the store", root),
         }
     }
@@ -87,6 +97,16 @@ impl std::error::Error for MerkleError {}
 // ================================================================================================
 
 #[cfg(test)]
-const fn int_to_node(value: u64) -> Word {
+const fn int_to_node(value: u64) -> RpoDigest {
+    RpoDigest::new([Felt::new(value), ZERO, ZERO, ZERO])
+}
+
+#[cfg(test)]
+const fn int_to_leaf(value: u64) -> Word {
     [Felt::new(value), ZERO, ZERO, ZERO]
 }
+
+#[cfg(test)]
+fn digests_to_words(digests: &[RpoDigest]) -> Vec<Word> {
+    digests.iter().map(|d| d.into()).collect()
+}

src/merkle/node.rs

@@ -1,9 +1,9 @@
-use super::Word;
+use crate::hash::rpo::RpoDigest;
 
 /// Representation of a node with two children used for iterating over containers.
 #[derive(Debug, Clone, PartialEq, Eq)]
 pub struct InnerNodeInfo {
-    pub value: Word,
-    pub left: Word,
-    pub right: Word,
+    pub value: RpoDigest,
+    pub left: RpoDigest,
+    pub right: RpoDigest,
 }

src/merkle/partial_mt/mod.rs

@@ -0,0 +1,329 @@
+use super::{
+    BTreeMap, BTreeSet, MerkleError, MerklePath, NodeIndex, Rpo256, RpoDigest, ValuePath, Vec, ZERO,
+};
+use crate::utils::{format, string::String, word_to_hex};
+use core::fmt;
+
+#[cfg(test)]
+mod tests;
+
+// CONSTANTS
+// ================================================================================================
+
+/// Index of the root node.
+const ROOT_INDEX: NodeIndex = NodeIndex::root();
+
+/// An RpoDigest consisting of 4 ZERO elements.
+const EMPTY_DIGEST: RpoDigest = RpoDigest::new([ZERO; 4]);
+
+// PARTIAL MERKLE TREE
+// ================================================================================================
+
+/// A partial Merkle tree with NodeIndex keys and 4-element RpoDigest leaf values. Partial Merkle
+/// Tree allows to create Merkle Tree by providing Merkle paths of different lengths.
+///
+/// The root of the tree is recomputed on each new leaf update.
+#[derive(Debug, Clone, PartialEq, Eq)]
+pub struct PartialMerkleTree {
+    max_depth: u8,
+    nodes: BTreeMap<NodeIndex, RpoDigest>,
+    leaves: BTreeSet<NodeIndex>,
+}
+
+impl Default for PartialMerkleTree {
+    fn default() -> Self {
+        Self::new()
+    }
+}
+
+impl PartialMerkleTree {
+    // CONSTANTS
+    // --------------------------------------------------------------------------------------------
+
+    /// Minimum supported depth.
+    pub const MIN_DEPTH: u8 = 1;
+
+    /// Maximum supported depth.
+    pub const MAX_DEPTH: u8 = 64;
+
+    // CONSTRUCTORS
+    // --------------------------------------------------------------------------------------------
+
+    /// Returns a new empty [PartialMerkleTree].
+    pub fn new() -> Self {
+        PartialMerkleTree {
+            max_depth: 0,
+            nodes: BTreeMap::new(),
+            leaves: BTreeSet::new(),
+        }
+    }
+
+    /// Appends the provided paths iterator into the set.
+    ///
+    /// Analogous to [Self::add_path].
+    pub fn with_paths<I>(paths: I) -> Result<Self, MerkleError>
+    where
+        I: IntoIterator<Item = (u64, RpoDigest, MerklePath)>,
+    {
+        // create an empty tree
+        let tree = PartialMerkleTree::new();
+
+        paths.into_iter().try_fold(tree, |mut tree, (index, value, path)| {
+            tree.add_path(index, value, path)?;
+            Ok(tree)
+        })
+    }
+
+    // PUBLIC ACCESSORS
+    // --------------------------------------------------------------------------------------------
+
+    /// Returns the root of this Merkle tree.
+    pub fn root(&self) -> RpoDigest {
+        self.nodes.get(&ROOT_INDEX).cloned().unwrap_or(EMPTY_DIGEST)
+    }
+
+    /// Returns the depth of this Merkle tree.
+    pub fn max_depth(&self) -> u8 {
+        self.max_depth
+    }
+
+    /// Returns a node at the specified NodeIndex.
+    ///
+    /// # Errors
+    /// Returns an error if the specified NodeIndex is not contained in the nodes map.
+    pub fn get_node(&self, index: NodeIndex) -> Result<RpoDigest, MerkleError> {
+        self.nodes.get(&index).ok_or(MerkleError::NodeNotInSet(index)).map(|hash| *hash)
+    }
+
+    /// Returns true if provided index contains in the leaves set, false otherwise.
+    pub fn is_leaf(&self, index: NodeIndex) -> bool {
+        self.leaves.contains(&index)
+    }
+
+    /// Returns a vector of paths from every leaf to the root.
+    pub fn paths(&self) -> Vec<(NodeIndex, ValuePath)> {
+        let mut paths = Vec::new();
+        self.leaves.iter().for_each(|&leaf| {
+            paths.push((
+                leaf,
+                ValuePath {
+                    value: self.get_node(leaf).expect("Failed to get leaf node"),
+                    path: self.get_path(leaf).expect("Failed to get path"),
+                },
+            ));
+        });
+        paths
+    }
+
+    /// Returns a Merkle path from the node at the specified index to the root.
+    ///
+    /// The node itself is not included in the path.
+    ///
+    /// # Errors
+    /// Returns an error if:
+    /// - the specified index has depth set to 0 or the depth is greater than the depth of this
+    /// Merkle tree.
+    /// - the specified index is not contained in the nodes map.
+    pub fn get_path(&self, mut index: NodeIndex) -> Result<MerklePath, MerkleError> {
+        if index.is_root() {
+            return Err(MerkleError::DepthTooSmall(index.depth()));
+        } else if index.depth() > self.max_depth() {
+            return Err(MerkleError::DepthTooBig(index.depth() as u64));
+        }
+
+        if !self.nodes.contains_key(&index) {
+            return Err(MerkleError::NodeNotInSet(index));
+        }
+
+        let mut path = Vec::new();
+        for _ in 0..index.depth() {
+            let sibling_index = index.sibling();
+            index.move_up();
+            let sibling =
+                self.nodes.get(&sibling_index).cloned().expect("Sibling node not in the map");
+            path.push(sibling);
+        }
+        Ok(MerklePath::new(path))
+    }
+
+    // ITERATORS
+    // --------------------------------------------------------------------------------------------
+
+    /// Returns an iterator over the leaves of this [PartialMerkleTree].
+    pub fn leaves(&self) -> impl Iterator<Item = (NodeIndex, RpoDigest)> + '_ {
+        self.leaves.iter().map(|&leaf| {
+            (
+                leaf,
+                self.get_node(leaf)
+                    .unwrap_or_else(|_| panic!("Leaf with {leaf} is not in the nodes map")),
+            )
+        })
+    }
+
+    // STATE MUTATORS
+    // --------------------------------------------------------------------------------------------
+
+    /// Adds the nodes of the specified Merkle path to this [PartialMerkleTree]. The `index_value`
+    /// and `value` parameters specify the leaf node at which the path starts.
+    ///
+    /// # Errors
+    /// Returns an error if:
+    /// - The depth of the specified node_index is greater than 64 or smaller than 1.
+    /// - The specified path is not consistent with other paths in the set (i.e., resolves to a
+    ///   different root).
+    pub fn add_path(
+        &mut self,
+        index_value: u64,
+        value: RpoDigest,
+        path: MerklePath,
+    ) -> Result<(), MerkleError> {
+        let index_value = NodeIndex::new(path.len() as u8, index_value)?;
+
+        Self::check_depth(index_value.depth())?;
+        self.update_depth(index_value.depth());
+
+        // add provided node and its sibling to the leaves set
+        self.leaves.insert(index_value);
+        let sibling_node_index = index_value.sibling();
+        self.leaves.insert(sibling_node_index);
+
+        // add provided node and its sibling to the nodes map
+        self.nodes.insert(index_value, value);
+        self.nodes.insert(sibling_node_index, path[0]);
+
+        // traverse to the root, updating the nodes
+        let mut index_value = index_value;
+        let node = Rpo256::merge(&index_value.build_node(value, path[0]));
+        let root = path.iter().skip(1).copied().fold(node, |node, hash| {
+            index_value.move_up();
+            // insert calculated node to the nodes map
+            self.nodes.insert(index_value, node);
+
+            // if the calculated node was a leaf, remove it from leaves set.
+            self.leaves.remove(&index_value);
+
+            let sibling_node = index_value.sibling();
+
+            // Insert node from Merkle path to the nodes map. This sibling node becomes a leaf only
+            // if it is a new node (it wasn't in nodes map).
+            // Node can be in 3 states: internal node, leaf of the tree and not a tree node at all.
+            // - Internal node can only stay in this state -- addition of a new path can't make it
+            // a leaf or remove it from the tree.
+            // - Leaf node can stay in the same state (remain a leaf) or can become an internal
+            // node. In the first case we don't need to do anything, and the second case is handled
+            // by the call of `self.leaves.remove(&index_value);`
+            // - New node can be a calculated node or a "sibling" node from a Merkle Path:
+            // --- Calculated node, obviously, never can be a leaf.
+            // --- Sibling node can be only a leaf, because otherwise it is not a new node.
+            if self.nodes.insert(sibling_node, hash).is_none() {
+                self.leaves.insert(sibling_node);
+            }
+
+            Rpo256::merge(&index_value.build_node(node, hash))
+        });
+
+        // if the path set is empty (the root is all ZEROs), set the root to the root of the added
+        // path; otherwise, the root of the added path must be identical to the current root
+        if self.root() == EMPTY_DIGEST {
+            self.nodes.insert(ROOT_INDEX, root);
+        } else if self.root() != root {
+            return Err(MerkleError::ConflictingRoots([self.root(), root].to_vec()));
+        }
+
+        Ok(())
+    }
+
+    /// Updates value of the leaf at the specified index returning the old leaf value.
+    ///
+    /// This also recomputes all hashes between the leaf and the root, updating the root itself.
+    ///
+    /// # Errors
+    /// Returns an error if:
+    /// - The depth of the specified node_index is greater than 64 or smaller than 1.
+    /// - The specified node index is not corresponding to the leaf.
+    pub fn update_leaf(
+        &mut self,
+        node_index: NodeIndex,
+        value: RpoDigest,
+    ) -> Result<RpoDigest, MerkleError> {
+        // check correctness of the depth and update it
+        Self::check_depth(node_index.depth())?;
+        self.update_depth(node_index.depth());
+
+        // insert NodeIndex to the leaves Set
+        self.leaves.insert(node_index);
+
+        // add node value to the nodes Map
+        let old_value = self
+            .nodes
+            .insert(node_index, value)
+            .ok_or(MerkleError::NodeNotInSet(node_index))?;
+
+        // if the old value and new value are the same, there is nothing to update
+        if value == old_value {
+            return Ok(old_value);
+        }
+
+        let mut node_index = node_index;
+        let mut value = value;
+        for _ in 0..node_index.depth() {
+            let sibling = self.nodes.get(&node_index.sibling()).expect("sibling should exist");
+            value = Rpo256::merge(&node_index.build_node(value, *sibling));
+            node_index.move_up();
+            self.nodes.insert(node_index, value);
+        }
+
+        Ok(old_value)
+    }
+
+    // UTILITY FUNCTIONS
+    // --------------------------------------------------------------------------------------------
+
+    /// Utility to visualize a [PartialMerkleTree] in text.
+    pub fn print(&self) -> Result<String, fmt::Error> {
+        let indent = "  ";
+        let mut s = String::new();
+        s.push_str("root: ");
+        s.push_str(&word_to_hex(&self.root())?);
+        s.push('\n');
+        for d in 1..=self.max_depth() {
+            let entries = 2u64.pow(d.into());
+            for i in 0..entries {
+                let index = NodeIndex::new(d, i).expect("The index must always be valid");
+                let node = self.get_node(index);
+                let node = match node {
+                    Err(_) => continue,
+                    Ok(node) => node,
+                };
+
+                for _ in 0..d {
+                    s.push_str(indent);
+                }
+                s.push_str(&format!("({}, {}): ", index.depth(), index.value()));
+                s.push_str(&word_to_hex(&node)?);
+                s.push('\n');
+            }
+        }
+
+        Ok(s)
+    }
+
+    // HELPER METHODS
+    // --------------------------------------------------------------------------------------------
+
+    /// Updates depth value with the maximum of current and provided depth.
+    fn update_depth(&mut self, new_depth: u8) {
+        self.max_depth = new_depth.max(self.max_depth);
+    }
+
+    /// Returns an error if the depth is 0 or is greater than 64.
+    fn check_depth(depth: u8) -> Result<(), MerkleError> {
+        // validate the range of the depth.
+        if depth < Self::MIN_DEPTH {
+            return Err(MerkleError::DepthTooSmall(depth));
+        } else if Self::MAX_DEPTH < depth {
+            return Err(MerkleError::DepthTooBig(depth as u64));
+        }
+        Ok(())
+    }
+}

src/merkle/partial_mt/tests.rs

@@ -0,0 +1,313 @@
+use super::{
+    super::{
+        digests_to_words, int_to_node, DefaultMerkleStore as MerkleStore, MerkleTree, NodeIndex,
+        PartialMerkleTree,
+    },
+    RpoDigest, ValuePath, Vec,
+};
+
+// TEST DATA
+// ================================================================================================
+
+const NODE10: NodeIndex = NodeIndex::new_unchecked(1, 0);
+const NODE11: NodeIndex = NodeIndex::new_unchecked(1, 1);
+
+const NODE20: NodeIndex = NodeIndex::new_unchecked(2, 0);
+const NODE22: NodeIndex = NodeIndex::new_unchecked(2, 2);
+const NODE23: NodeIndex = NodeIndex::new_unchecked(2, 3);
+
+const NODE30: NodeIndex = NodeIndex::new_unchecked(3, 0);
+const NODE31: NodeIndex = NodeIndex::new_unchecked(3, 1);
+const NODE32: NodeIndex = NodeIndex::new_unchecked(3, 2);
+const NODE33: NodeIndex = NodeIndex::new_unchecked(3, 3);
+
+const VALUES8: [RpoDigest; 8] = [
+    int_to_node(30),
+    int_to_node(31),
+    int_to_node(32),
+    int_to_node(33),
+    int_to_node(34),
+    int_to_node(35),
+    int_to_node(36),
+    int_to_node(37),
+];
+
+// TESTS
+// ================================================================================================
+
+// For the Partial Merkle Tree tests we will use parts of the Merkle Tree which full form is
+// illustrated below:
+//
+//              __________ root __________
+//             /                          \
+//       ____ 10 ____                ____ 11 ____
+//      /            \              /            \
+//     20            21            22            23
+//   /    \        /    \        /    \        /    \
+// (30)  (31)    (32)  (33)    (34)  (35)    (36)  (37)
+//
+// Where node number is a concatenation of its depth and index. For example, node with
+// NodeIndex(3, 5) will be labeled as `35`. Leaves of the tree are shown as nodes with parenthesis
+// (33).
+
+/// Checks that root returned by `root()` function is equal to the expected one.
+#[test]
+fn get_root() {
+    let mt = MerkleTree::new(digests_to_words(&VALUES8)).unwrap();
+    let expected_root = mt.root();
+
+    let ms = MerkleStore::from(&mt);
+    let path33 = ms.get_path(expected_root, NODE33).unwrap();
+
+    let pmt = PartialMerkleTree::with_paths([(3, path33.value, path33.path)]).unwrap();
+
+    assert_eq!(pmt.root(), expected_root);
+}
+
+/// This test checks correctness of the `add_path()` and `get_path()` functions. First it creates a
+/// PMT using `add_path()` by adding Merkle Paths from node 33 and node 22 to the empty PMT. Then
+/// it checks that paths returned by `get_path()` function are equal to the expected ones.
+#[test]
+fn add_and_get_paths() {
+    let mt = MerkleTree::new(digests_to_words(&VALUES8)).unwrap();
+    let expected_root = mt.root();
+
+    let ms = MerkleStore::from(&mt);
+
+    let expected_path33 = ms.get_path(expected_root, NODE33).unwrap();
+    let expected_path22 = ms.get_path(expected_root, NODE22).unwrap();
+
+    let mut pmt = PartialMerkleTree::new();
+    pmt.add_path(3, expected_path33.value, expected_path33.path.clone()).unwrap();
+    pmt.add_path(2, expected_path22.value, expected_path22.path.clone()).unwrap();
+
+    let path33 = pmt.get_path(NODE33).unwrap();
+    let path22 = pmt.get_path(NODE22).unwrap();
+    let actual_root = pmt.root();
+
+    assert_eq!(expected_path33.path, path33);
+    assert_eq!(expected_path22.path, path22);
+    assert_eq!(expected_root, actual_root);
+}
+
+/// Checks that function `get_node` used on nodes 10 and 32 returns expected values.
+#[test]
+fn get_node() {
+    let mt = MerkleTree::new(digests_to_words(&VALUES8)).unwrap();
+    let expected_root = mt.root();
+
+    let ms = MerkleStore::from(&mt);
+
+    let path33 = ms.get_path(expected_root, NODE33).unwrap();
+
+    let pmt = PartialMerkleTree::with_paths([(3, path33.value, path33.path)]).unwrap();
+
+    assert_eq!(ms.get_node(expected_root, NODE32).unwrap(), pmt.get_node(NODE32).unwrap());
+    assert_eq!(ms.get_node(expected_root, NODE10).unwrap(), pmt.get_node(NODE10).unwrap());
+}
+
+/// Updates leaves of the PMT using `update_leaf()` function and checks that new root of the tree
+/// is equal to the expected one.
+#[test]
+fn update_leaf() {
+    let mt = MerkleTree::new(digests_to_words(&VALUES8)).unwrap();
+    let root = mt.root();
+
+    let mut ms = MerkleStore::from(&mt);
+    let path33 = ms.get_path(root, NODE33).unwrap();
+
+    let mut pmt = PartialMerkleTree::with_paths([(3, path33.value, path33.path)]).unwrap();
+
+    let new_value32 = int_to_node(132);
+    let expected_root = ms.set_node(root, NODE32, new_value32).unwrap().root;
+
+    pmt.update_leaf(NODE32, new_value32).unwrap();
+    let actual_root = pmt.root();
+
+    assert_eq!(expected_root, actual_root);
+
+    let new_value20 = int_to_node(120);
+    let expected_root = ms.set_node(expected_root, NODE20, new_value20).unwrap().root;
+
+    pmt.update_leaf(NODE20, new_value20).unwrap();
+    let actual_root = pmt.root();
+
+    assert_eq!(expected_root, actual_root);
+}
+
+/// Checks that paths of the PMT returned by `paths()` function are equal to the expected ones.
+#[test]
+fn get_paths() {
+    let mt = MerkleTree::new(digests_to_words(&VALUES8)).unwrap();
+    let expected_root = mt.root();
+
+    let ms = MerkleStore::from(&mt);
+
+    let path33 = ms.get_path(expected_root, NODE33).unwrap();
+    let path22 = ms.get_path(expected_root, NODE22).unwrap();
+
+    let mut pmt = PartialMerkleTree::new();
+    pmt.add_path(3, path33.value, path33.path).unwrap();
+    pmt.add_path(2, path22.value, path22.path).unwrap();
+    // After PMT creation with path33 (33; 32, 20, 11) and path22 (22; 23, 10) we will have this
+    // tree:
+    //
+    //           ______root______
+    //          /                \
+    //      ___10___           ___11___
+    //     /        \         /        \
+    //   (20)       21      (22)      (23)
+    //            /    \
+    //          (32)  (33)
+    //
+    // Which have leaf nodes 20, 22, 23, 32 and 33. Hence overall we will have 5 paths -- one path
+    // for each leaf.
+
+    let leaves = vec![NODE20, NODE22, NODE23, NODE32, NODE33];
+    let expected_paths: Vec<(NodeIndex, ValuePath)> = leaves
+        .iter()
+        .map(|&leaf| {
+            (
+                leaf,
+                ValuePath {
+                    value: mt.get_node(leaf).unwrap(),
+                    path: mt.get_path(leaf).unwrap(),
+                },
+            )
+        })
+        .collect();
+
+    let actual_paths = pmt.paths();
+
+    assert_eq!(expected_paths, actual_paths);
+}
+
+// Checks correctness of leaves determination when using the `leaves()` function.
+#[test]
+fn leaves() {
+    let mt = MerkleTree::new(digests_to_words(&VALUES8)).unwrap();
+    let expected_root = mt.root();
+
+    let ms = MerkleStore::from(&mt);
+
+    let path33 = ms.get_path(expected_root, NODE33).unwrap();
+    let path22 = ms.get_path(expected_root, NODE22).unwrap();
+
+    let mut pmt = PartialMerkleTree::with_paths([(3, path33.value, path33.path)]).unwrap();
+    // After PMT creation with path33 (33; 32, 20, 11) we will have this tree:
+    //
+    //           ______root______
+    //          /                \
+    //      ___10___            (11)
+    //     /         \
+    //   (20)        21
+    //             /    \
+    //           (32)  (33)
+    //
+    // Which have leaf nodes 11, 20, 32 and 33.
+
+    let value11 = mt.get_node(NODE11).unwrap();
+    let value20 = mt.get_node(NODE20).unwrap();
+    let value32 = mt.get_node(NODE32).unwrap();
+    let value33 = mt.get_node(NODE33).unwrap();
+
+    let leaves = vec![(NODE11, value11), (NODE20, value20), (NODE32, value32), (NODE33, value33)];
+
+    let expected_leaves = leaves.iter().copied();
+    assert!(expected_leaves.eq(pmt.leaves()));
+
+    pmt.add_path(2, path22.value, path22.path).unwrap();
+    // After adding the path22 (22; 23, 10) to the existing PMT we will have this tree:
+    //
+    //           ______root______
+    //          /                \
+    //      ___10___           ___11___
+    //     /        \         /        \
+    //   (20)       21      (22)      (23)
+    //            /    \
+    //          (32)  (33)
+    //
+    // Which have leaf nodes 20, 22, 23, 32 and 33.
+
+    let value20 = mt.get_node(NODE20).unwrap();
+    let value22 = mt.get_node(NODE22).unwrap();
+    let value23 = mt.get_node(NODE23).unwrap();
+    let value32 = mt.get_node(NODE32).unwrap();
+    let value33 = mt.get_node(NODE33).unwrap();
+
+    let leaves = vec![
+        (NODE20, value20),
+        (NODE22, value22),
+        (NODE23, value23),
+        (NODE32, value32),
+        (NODE33, value33),
+    ];
+
+    let expected_leaves = leaves.iter().copied();
+    assert!(expected_leaves.eq(pmt.leaves()));
+}
+
+/// Checks that addition of the path with different root will cause an error.
+#[test]
+fn err_add_path() {
+    let path33 = vec![int_to_node(1), int_to_node(2), int_to_node(3)].into();
+    let path22 = vec![int_to_node(4), int_to_node(5)].into();
+
+    let mut pmt = PartialMerkleTree::new();
+    pmt.add_path(3, int_to_node(6), path33).unwrap();
+
+    assert!(pmt.add_path(2, int_to_node(7), path22).is_err());
+}
+
+/// Checks that the request of the node which is not in the PMT will cause an error.
+#[test]
+fn err_get_node() {
+    let mt = MerkleTree::new(digests_to_words(&VALUES8)).unwrap();
+    let expected_root = mt.root();
+
+    let ms = MerkleStore::from(&mt);
+
+    let path33 = ms.get_path(expected_root, NODE33).unwrap();
+
+    let pmt = PartialMerkleTree::with_paths([(3, path33.value, path33.path)]).unwrap();
+
+    assert!(pmt.get_node(NODE22).is_err());
+    assert!(pmt.get_node(NODE23).is_err());
+    assert!(pmt.get_node(NODE30).is_err());
+    assert!(pmt.get_node(NODE31).is_err());
+}
+
+/// Checks that the request of the path from the leaf which is not in the PMT will cause an error.
+#[test]
+fn err_get_path() {
+    let mt = MerkleTree::new(digests_to_words(&VALUES8)).unwrap();
+    let expected_root = mt.root();
+
+    let ms = MerkleStore::from(&mt);
+
+    let path33 = ms.get_path(expected_root, NODE33).unwrap();
+
+    let pmt = PartialMerkleTree::with_paths([(3, path33.value, path33.path)]).unwrap();
+
+    assert!(pmt.get_path(NODE22).is_err());
+    assert!(pmt.get_path(NODE23).is_err());
+    assert!(pmt.get_path(NODE30).is_err());
+    assert!(pmt.get_path(NODE31).is_err());
+}
+
+#[test]
+fn err_update_leaf() {
+    let mt = MerkleTree::new(digests_to_words(&VALUES8)).unwrap();
+    let expected_root = mt.root();
+
+    let ms = MerkleStore::from(&mt);
+
+    let path33 = ms.get_path(expected_root, NODE33).unwrap();
+
+    let mut pmt = PartialMerkleTree::with_paths([(3, path33.value, path33.path)]).unwrap();
+
+    assert!(pmt.update_leaf(NODE22, int_to_node(22)).is_err());
+    assert!(pmt.update_leaf(NODE23, int_to_node(23)).is_err());
+    assert!(pmt.update_leaf(NODE30, int_to_node(30)).is_err());
+    assert!(pmt.update_leaf(NODE31, int_to_node(31)).is_err());
+}

src/merkle/path.rs

@@ -1,4 +1,4 @@
-use super::{vec, InnerNodeInfo, MerkleError, NodeIndex, Rpo256, Vec, Word};
+use super::{vec, InnerNodeInfo, MerkleError, NodeIndex, Rpo256, RpoDigest, Vec};
 use core::ops::{Deref, DerefMut};
 
 // MERKLE PATH
@@ -7,7 +7,7 @@ use core::ops::{Deref, DerefMut};
 /// A merkle path container, composed of a sequence of nodes of a Merkle tree.
 #[derive(Clone, Debug, Default, PartialEq, Eq)]
 pub struct MerklePath {
-    nodes: Vec<Word>,
+    nodes: Vec<RpoDigest>,
 }
 
 impl MerklePath {
@@ -15,7 +15,7 @@ impl MerklePath {
     // --------------------------------------------------------------------------------------------
 
     /// Creates a new Merkle path from a list of nodes.
-    pub fn new(nodes: Vec<Word>) -> Self {
+    pub fn new(nodes: Vec<RpoDigest>) -> Self {
         Self { nodes }
     }
 
@@ -28,13 +28,13 @@ impl MerklePath {
     }
 
     /// Computes the merkle root for this opening.
-    pub fn compute_root(&self, index: u64, node: Word) -> Result<Word, MerkleError> {
+    pub fn compute_root(&self, index: u64, node: RpoDigest) -> Result<RpoDigest, MerkleError> {
         let mut index = NodeIndex::new(self.depth(), index)?;
         let root = self.nodes.iter().copied().fold(node, |node, sibling| {
             // compute the node and move to the next iteration.
-            let input = index.build_node(node.into(), sibling.into());
+            let input = index.build_node(node, sibling);
             index.move_up();
-            Rpo256::merge(&input).into()
+            Rpo256::merge(&input)
         });
         Ok(root)
     }
@@ -42,7 +42,7 @@ impl MerklePath {
     /// Verifies the Merkle opening proof towards the provided root.
     ///
     /// Returns `true` if `node` exists at `index` in a Merkle tree with `root`.
-    pub fn verify(&self, index: u64, node: Word, root: &Word) -> bool {
+    pub fn verify(&self, index: u64, node: RpoDigest, root: &RpoDigest) -> bool {
         match self.compute_root(index, node) {
             Ok(computed_root) => root == &computed_root,
             Err(_) => false,
@@ -55,7 +55,11 @@ impl MerklePath {
     ///
     /// # Errors
     /// Returns an error if the specified index is not valid for this path.
-    pub fn inner_nodes(&self, index: u64, node: Word) -> Result<InnerNodeIterator, MerkleError> {
+    pub fn inner_nodes(
+        &self,
+        index: u64,
+        node: RpoDigest,
+    ) -> Result<InnerNodeIterator, MerkleError> {
         Ok(InnerNodeIterator {
             nodes: &self.nodes,
             index: NodeIndex::new(self.depth(), index)?,
@@ -64,8 +68,14 @@ impl MerklePath {
     }
 }
 
-impl From<Vec<Word>> for MerklePath {
-    fn from(path: Vec<Word>) -> Self {
+impl From<MerklePath> for Vec<RpoDigest> {
+    fn from(path: MerklePath) -> Self {
+        path.nodes
+    }
+}
+
+impl From<Vec<RpoDigest>> for MerklePath {
+    fn from(path: Vec<RpoDigest>) -> Self {
         Self::new(path)
     }
 }
@@ -73,7 +83,7 @@ impl From<Vec<Word>> for MerklePath {
 impl Deref for MerklePath {
     // we use `Vec` here instead of slice so we can call vector mutation methods directly from the
     // merkle path (example: `Vec::remove`).
-    type Target = Vec<Word>;
+    type Target = Vec<RpoDigest>;
 
     fn deref(&self) -> &Self::Target {
         &self.nodes
@@ -89,15 +99,15 @@ impl DerefMut for MerklePath {
 // ITERATORS
 // ================================================================================================
 
-impl FromIterator<Word> for MerklePath {
-    fn from_iter<T: IntoIterator<Item = Word>>(iter: T) -> Self {
+impl FromIterator<RpoDigest> for MerklePath {
+    fn from_iter<T: IntoIterator<Item = RpoDigest>>(iter: T) -> Self {
         Self::new(iter.into_iter().collect())
     }
 }
 
 impl IntoIterator for MerklePath {
-    type Item = Word;
-    type IntoIter = vec::IntoIter<Word>;
+    type Item = RpoDigest;
+    type IntoIter = vec::IntoIter<RpoDigest>;
 
     fn into_iter(self) -> Self::IntoIter {
         self.nodes.into_iter()
@@ -106,9 +116,9 @@ impl IntoIterator for MerklePath {
 
 /// An iterator over internal nodes of a [MerklePath].
 pub struct InnerNodeIterator<'a> {
-    nodes: &'a Vec<Word>,
+    nodes: &'a Vec<RpoDigest>,
     index: NodeIndex,
-    value: Word,
+    value: RpoDigest,
 }
 
 impl<'a> Iterator for InnerNodeIterator<'a> {
@@ -123,7 +133,7 @@ impl<'a> Iterator for InnerNodeIterator<'a> {
                 (self.value, self.nodes[sibling_pos])
             };
 
-            self.value = Rpo256::merge(&[left.into(), right.into()]).into();
+            self.value = Rpo256::merge(&[left, right]);
             self.index.move_up();
 
             Some(InnerNodeInfo {
@@ -144,7 +154,7 @@ impl<'a> Iterator for InnerNodeIterator<'a> {
 #[derive(Clone, Debug, Default, PartialEq, Eq)]
 pub struct ValuePath {
     /// The node value opening for `path`.
-    pub value: Word,
+    pub value: RpoDigest,
     /// The path from `value` to `root` (exclusive).
     pub path: MerklePath,
 }
@@ -156,7 +166,7 @@ pub struct ValuePath {
 #[derive(Clone, Debug, Default, PartialEq, Eq)]
 pub struct RootPath {
     /// The node value opening for `path`.
-    pub root: Word,
+    pub root: RpoDigest,
     /// The path from `value` to `root` (exclusive).
     pub path: MerklePath,
 }

src/merkle/path_set.rs

@@ -1,4 +1,5 @@
-use super::{BTreeMap, MerkleError, MerklePath, NodeIndex, Rpo256, ValuePath, Vec, Word, ZERO};
+use super::{BTreeMap, MerkleError, MerklePath, NodeIndex, Rpo256, ValuePath, Vec};
+use crate::{hash::rpo::RpoDigest, Word};
 
 // MERKLE PATH SET
 // ================================================================================================
@@ -6,7 +7,7 @@ use super::{BTreeMap, MerkleError, MerklePath, NodeIndex, Rpo256, ValuePath, Vec
 /// A set of Merkle paths.
 #[derive(Debug, Clone, PartialEq, Eq)]
 pub struct MerklePathSet {
-    root: Word,
+    root: RpoDigest,
     total_depth: u8,
     paths: BTreeMap<u64, MerklePath>,
 }
@@ -17,7 +18,7 @@ impl MerklePathSet {
 
     /// Returns an empty MerklePathSet.
     pub fn new(depth: u8) -> Self {
-        let root = [ZERO; 4];
+        let root = RpoDigest::default();
         let paths = BTreeMap::new();
 
         Self {
@@ -32,10 +33,10 @@ impl MerklePathSet {
     /// Analogous to `[Self::add_path]`.
     pub fn with_paths<I>(self, paths: I) -> Result<Self, MerkleError>
     where
-        I: IntoIterator<Item = (u64, Word, MerklePath)>,
+        I: IntoIterator<Item = (u64, RpoDigest, MerklePath)>,
     {
         paths.into_iter().try_fold(self, |mut set, (index, value, path)| {
-            set.add_path(index, value, path)?;
+            set.add_path(index, value.into(), path)?;
             Ok(set)
         })
     }
@@ -44,7 +45,7 @@ impl MerklePathSet {
     // --------------------------------------------------------------------------------------------
 
     /// Returns the root to which all paths in this set resolve.
-    pub const fn root(&self) -> Word {
+    pub const fn root(&self) -> RpoDigest {
         self.root
     }
 
@@ -61,7 +62,7 @@ impl MerklePathSet {
     /// Returns an error if:
     /// * The specified index is not valid for the depth of structure.
     /// * Requested node does not exist in the set.
-    pub fn get_node(&self, index: NodeIndex) -> Result<Word, MerkleError> {
+    pub fn get_node(&self, index: NodeIndex) -> Result<RpoDigest, MerkleError> {
         if index.depth() != self.total_depth {
             return Err(MerkleError::InvalidDepth {
                 expected: self.total_depth,
@@ -73,7 +74,7 @@ impl MerklePathSet {
         let path_key = index.value() - parity;
         self.paths
             .get(&path_key)
-            .ok_or(MerkleError::NodeNotInSet(path_key))
+            .ok_or(MerkleError::NodeNotInSet(index))
             .map(|path| path[parity as usize])
     }
 
@@ -84,7 +85,7 @@ impl MerklePathSet {
     /// * Leaf with the requested path does not exist in the set.
     pub fn get_leaf(&self, index: u64) -> Result<Word, MerkleError> {
         let index = NodeIndex::new(self.depth(), index)?;
-        self.get_node(index)
+        Ok(self.get_node(index)?.into())
     }
 
     /// Returns a Merkle path to the node at the specified index. The node itself is
@@ -104,11 +105,8 @@ impl MerklePathSet {
 
         let parity = index.value() & 1;
         let path_key = index.value() - parity;
-        let mut path = self
-            .paths
-            .get(&path_key)
-            .cloned()
-            .ok_or(MerkleError::NodeNotInSet(index.value()))?;
+        let mut path =
+            self.paths.get(&path_key).cloned().ok_or(MerkleError::NodeNotInSet(index))?;
         path.remove(parity as usize);
         Ok(path)
     }
@@ -166,18 +164,18 @@ impl MerklePathSet {
 
         // update the current path
         let parity = index_value & 1;
-        path.insert(parity as usize, value);
+        path.insert(parity as usize, value.into());
 
         // traverse to the root, updating the nodes
-        let root: Word = Rpo256::merge(&[path[0].into(), path[1].into()]).into();
+        let root = Rpo256::merge(&[path[0], path[1]]);
         let root = path.iter().skip(2).copied().fold(root, |root, hash| {
             index.move_up();
-            Rpo256::merge(&index.build_node(root.into(), hash.into())).into()
+            Rpo256::merge(&index.build_node(root, hash))
         });
 
         // if the path set is empty (the root is all ZEROs), set the root to the root of the added
         // path; otherwise, the root of the added path must be identical to the current root
-        if self.root == [ZERO; 4] {
+        if self.root == RpoDigest::default() {
             self.root = root;
         } else if self.root != root {
             return Err(MerkleError::ConflictingRoots([self.root, root].to_vec()));
@@ -200,30 +198,30 @@ impl MerklePathSet {
         let path_key = index.value() - parity;
         let path = match self.paths.get_mut(&path_key) {
             Some(path) => path,
-            None => return Err(MerkleError::NodeNotInSet(base_index_value)),
+            None => return Err(MerkleError::NodeNotInSet(index)),
         };
 
         // Fill old_hashes vector -----------------------------------------------------------------
         let mut current_index = index;
         let mut old_hashes = Vec::with_capacity(path.len().saturating_sub(2));
-        let mut root: Word = Rpo256::merge(&[path[0].into(), path[1].into()]).into();
+        let mut root = Rpo256::merge(&[path[0], path[1]]);
         for hash in path.iter().skip(2).copied() {
             old_hashes.push(root);
             current_index.move_up();
-            let input = current_index.build_node(hash.into(), root.into());
-            root = Rpo256::merge(&input).into();
+            let input = current_index.build_node(hash, root);
+            root = Rpo256::merge(&input);
         }
 
         // Fill new_hashes vector -----------------------------------------------------------------
-        path[index.is_value_odd() as usize] = value;
+        path[index.is_value_odd() as usize] = value.into();
 
         let mut new_hashes = Vec::with_capacity(path.len().saturating_sub(2));
-        let mut new_root: Word = Rpo256::merge(&[path[0].into(), path[1].into()]).into();
+        let mut new_root = Rpo256::merge(&[path[0], path[1]]);
         for path_hash in path.iter().skip(2).copied() {
             new_hashes.push(new_root);
             index.move_up();
-            let input = current_index.build_node(path_hash.into(), new_root.into());
-            new_root = Rpo256::merge(&input).into();
+            let input = current_index.build_node(path_hash, new_root);
+            new_root = Rpo256::merge(&input);
         }
 
         self.root = new_root;
@@ -248,7 +246,7 @@ impl MerklePathSet {
 #[cfg(test)]
 mod tests {
     use super::*;
-    use crate::merkle::int_to_node;
+    use crate::merkle::{int_to_leaf, int_to_node};
 
     #[test]
     fn get_root() {
@@ -321,20 +319,20 @@ mod tests {
             ])
             .unwrap();
 
-        let new_hash_6 = int_to_node(100);
-        let new_hash_5 = int_to_node(55);
+        let new_hash_6 = int_to_leaf(100);
+        let new_hash_5 = int_to_leaf(55);
 
         set.update_leaf(index_6, new_hash_6).unwrap();
         let new_path_4 = set.get_path(NodeIndex::make(depth, index_4)).unwrap();
-        let new_hash_67 = calculate_parent_hash(new_hash_6, 14_u64, hash_7);
+        let new_hash_67 = calculate_parent_hash(new_hash_6.into(), 14_u64, hash_7);
         assert_eq!(new_hash_67, new_path_4[1]);
 
         set.update_leaf(index_5, new_hash_5).unwrap();
         let new_path_4 = set.get_path(NodeIndex::make(depth, index_4)).unwrap();
         let new_path_6 = set.get_path(NodeIndex::make(depth, index_6)).unwrap();
-        let new_hash_45 = calculate_parent_hash(new_hash_5, 13_u64, hash_4);
+        let new_hash_45 = calculate_parent_hash(new_hash_5.into(), 13_u64, hash_4);
         assert_eq!(new_hash_45, new_path_6[1]);
-        assert_eq!(new_hash_5, new_path_4[0]);
+        assert_eq!(RpoDigest::from(new_hash_5), new_path_4[0]);
     }
 
     #[test]
@@ -348,45 +346,45 @@ mod tests {
         let g = int_to_node(7);
         let h = int_to_node(8);
 
-        let i = Rpo256::merge(&[a.into(), b.into()]);
-        let j = Rpo256::merge(&[c.into(), d.into()]);
-        let k = Rpo256::merge(&[e.into(), f.into()]);
-        let l = Rpo256::merge(&[g.into(), h.into()]);
+        let i = Rpo256::merge(&[a, b]);
+        let j = Rpo256::merge(&[c, d]);
+        let k = Rpo256::merge(&[e, f]);
+        let l = Rpo256::merge(&[g, h]);
 
-        let m = Rpo256::merge(&[i.into(), j.into()]);
-        let n = Rpo256::merge(&[k.into(), l.into()]);
+        let m = Rpo256::merge(&[i, j]);
+        let n = Rpo256::merge(&[k, l]);
 
-        let root = Rpo256::merge(&[m.into(), n.into()]);
+        let root = Rpo256::merge(&[m, n]);
 
         let mut set = MerklePathSet::new(3);
 
         let value = b;
         let index = 1;
-        let path = MerklePath::new([a.into(), j.into(), n.into()].to_vec());
-        set.add_path(index, value, path.clone()).unwrap();
-        assert_eq!(value, set.get_leaf(index).unwrap());
-        assert_eq!(Word::from(root), set.root());
+        let path = MerklePath::new([a, j, n].to_vec());
+        set.add_path(index, value.into(), path).unwrap();
+        assert_eq!(*value, set.get_leaf(index).unwrap());
+        assert_eq!(root, set.root());
 
         let value = e;
         let index = 4;
-        let path = MerklePath::new([f.into(), l.into(), m.into()].to_vec());
-        set.add_path(index, value, path.clone()).unwrap();
-        assert_eq!(value, set.get_leaf(index).unwrap());
-        assert_eq!(Word::from(root), set.root());
+        let path = MerklePath::new([f, l, m].to_vec());
+        set.add_path(index, value.into(), path).unwrap();
+        assert_eq!(*value, set.get_leaf(index).unwrap());
+        assert_eq!(root, set.root());
 
         let value = a;
         let index = 0;
-        let path = MerklePath::new([b.into(), j.into(), n.into()].to_vec());
-        set.add_path(index, value, path.clone()).unwrap();
-        assert_eq!(value, set.get_leaf(index).unwrap());
-        assert_eq!(Word::from(root), set.root());
+        let path = MerklePath::new([b, j, n].to_vec());
+        set.add_path(index, value.into(), path).unwrap();
+        assert_eq!(*value, set.get_leaf(index).unwrap());
+        assert_eq!(root, set.root());
 
         let value = h;
         let index = 7;
-        let path = MerklePath::new([g.into(), k.into(), m.into()].to_vec());
-        set.add_path(index, value, path.clone()).unwrap();
-        assert_eq!(value, set.get_leaf(index).unwrap());
-        assert_eq!(Word::from(root), set.root());
+        let path = MerklePath::new([g, k, m].to_vec());
+        set.add_path(index, value.into(), path).unwrap();
+        assert_eq!(*value, set.get_leaf(index).unwrap());
+        assert_eq!(root, set.root());
     }
 
     // HELPER FUNCTIONS
@@ -400,11 +398,11 @@ mod tests {
     /// - node — current node
     /// - node_pos — position of the current node
     /// - sibling — neighboring vertex in the tree
-    fn calculate_parent_hash(node: Word, node_pos: u64, sibling: Word) -> Word {
+    fn calculate_parent_hash(node: RpoDigest, node_pos: u64, sibling: RpoDigest) -> RpoDigest {
         if is_even(node_pos) {
-            Rpo256::merge(&[node.into(), sibling.into()]).into()
+            Rpo256::merge(&[node, sibling])
         } else {
-            Rpo256::merge(&[sibling.into(), node.into()]).into()
+            Rpo256::merge(&[sibling, node])
         }
     }
 }

src/merkle/simple_smt/mod.rs

@@ -1,6 +1,6 @@
 use super::{
-    BTreeMap, EmptySubtreeRoots, InnerNodeInfo, MerkleError, MerklePath, NodeIndex, Rpo256,
-    RpoDigest, Vec, Word,
+    BTreeMap, BTreeSet, EmptySubtreeRoots, InnerNodeInfo, MerkleError, MerklePath, NodeIndex,
+    Rpo256, RpoDigest, Vec, Word,
 };
 
 #[cfg(test)]
@@ -10,28 +10,17 @@ mod tests;
 // ================================================================================================
 
 /// A sparse Merkle tree with 64-bit keys and 4-element leaf values, without compaction.
+///
 /// The root of the tree is recomputed on each new leaf update.
 #[derive(Debug, Clone, PartialEq, Eq)]
 pub struct SimpleSmt {
     depth: u8,
-    root: Word,
+    root: RpoDigest,
     leaves: BTreeMap<u64, Word>,
     branches: BTreeMap<NodeIndex, BranchNode>,
     empty_hashes: Vec<RpoDigest>,
 }
 
-#[derive(Debug, Default, Clone, PartialEq, Eq)]
-struct BranchNode {
-    left: RpoDigest,
-    right: RpoDigest,
-}
-
-impl BranchNode {
-    fn parent(&self) -> RpoDigest {
-        Rpo256::merge(&[self.left, self.right])
-    }
-}
-
 impl SimpleSmt {
     // CONSTANTS
     // --------------------------------------------------------------------------------------------
@@ -42,10 +31,18 @@ impl SimpleSmt {
     /// Maximum supported depth.
     pub const MAX_DEPTH: u8 = 64;
 
+    /// Value of an empty leaf.
+    pub const EMPTY_VALUE: Word = super::empty_roots::EMPTY_WORD;
+
     // CONSTRUCTORS
     // --------------------------------------------------------------------------------------------
 
-    /// Creates a new simple SMT with the provided depth.
+    /// Returns a new [SimpleSmt] instantiated with the specified depth.
+    ///
+    /// All leaves in the returned tree are set to [ZERO; 4].
+    ///
+    /// # Errors
+    /// Returns an error if the depth is 0 or is greater than 64.
     pub fn new(depth: u8) -> Result<Self, MerkleError> {
         // validate the range of the depth.
         if depth < Self::MIN_DEPTH {
@@ -55,7 +52,7 @@ impl SimpleSmt {
         }
 
         let empty_hashes = EmptySubtreeRoots::empty_hashes(depth).to_vec();
-        let root = empty_hashes[0].into();
+        let root = empty_hashes[0];
 
         Ok(Self {
             root,
@@ -66,43 +63,54 @@ impl SimpleSmt {
         })
     }
 
-    /// Appends the provided entries as leaves of the tree.
+    /// Returns a new [SimpleSmt] instantiated with the specified depth and with leaves
+    /// set as specified by the provided entries.
+    ///
+    /// All leaves omitted from the entries list are set to [ZERO; 4].
     ///
     /// # Errors
-    ///
-    /// The function will fail if the provided entries count exceed the maximum tree capacity, that
-    /// is `2^{depth}`.
-    pub fn with_leaves<R, I>(mut self, entries: R) -> Result<Self, MerkleError>
+    /// Returns an error if:
+    /// - If the depth is 0 or is greater than 64.
+    /// - The number of entries exceeds the maximum tree capacity, that is 2^{depth}.
+    /// - The provided entries contain multiple values for the same key.
+    pub fn with_leaves<R, I>(depth: u8, entries: R) -> Result<Self, MerkleError>
     where
         R: IntoIterator<IntoIter = I>,
         I: Iterator<Item = (u64, Word)> + ExactSizeIterator,
     {
-        // check if the leaves count will fit the depth setup
-        let mut entries = entries.into_iter();
-        let max = 1 << self.depth.min(63);
+        // create an empty tree
+        let mut tree = Self::new(depth)?;
+
+        // check if the number of leaves can be accommodated by the tree's depth; we use a min
+        // depth of 63 because we consider passing in a vector of size 2^64 infeasible.
+        let entries = entries.into_iter();
+        let max = 1 << tree.depth.min(63);
         if entries.len() > max {
-            return Err(MerkleError::InvalidEntriesCount(max, entries.len()));
+            return Err(MerkleError::InvalidNumEntries(max, entries.len()));
         }
 
-        // append leaves and return
-        entries.try_for_each(|(key, leaf)| self.insert_leaf(key, leaf))?;
-        Ok(self)
-    }
-
-    /// Replaces the internal empty digests used when a given depth doesn't contain a node.
-    pub fn with_empty_subtrees<I>(mut self, hashes: I) -> Self
-    where
-        I: IntoIterator<Item = RpoDigest>,
-    {
-        self.replace_empty_subtrees(hashes.into_iter().collect());
-        self
+        // append leaves to the tree returning an error if a duplicate entry for the same key
+        // is found
+        let mut empty_entries = BTreeSet::new();
+        for (key, value) in entries {
+            let old_value = tree.update_leaf(key, value)?;
+            if old_value != Self::EMPTY_VALUE || empty_entries.contains(&key) {
+                return Err(MerkleError::DuplicateValuesForIndex(key));
+            }
+            // if we've processed an empty entry, add the key to the set of empty entry keys, and
+            // if this key was already in the set, return an error
+            if value == Self::EMPTY_VALUE && !empty_entries.insert(key) {
+                return Err(MerkleError::DuplicateValuesForIndex(key));
+            }
+        }
+        Ok(tree)
     }
 
     // PUBLIC ACCESSORS
     // --------------------------------------------------------------------------------------------
 
     /// Returns the root of this Merkle tree.
-    pub const fn root(&self) -> Word {
+    pub const fn root(&self) -> RpoDigest {
         self.root
     }
 
@@ -111,40 +119,44 @@ impl SimpleSmt {
         self.depth
     }
 
-    // PROVIDERS
-    // --------------------------------------------------------------------------------------------
-
-    /// Returns the set count of the keys of the leaves.
-    pub fn leaves_count(&self) -> usize {
-        self.leaves.len()
-    }
-
     /// Returns a node at the specified index.
     ///
     /// # Errors
-    /// Returns an error if:
-    /// * The specified depth is greater than the depth of the tree.
-    pub fn get_node(&self, index: NodeIndex) -> Result<Word, MerkleError> {
+    /// Returns an error if the specified index has depth set to 0 or the depth is greater than
+    /// the depth of this Merkle tree.
+    pub fn get_node(&self, index: NodeIndex) -> Result<RpoDigest, MerkleError> {
         if index.is_root() {
             Err(MerkleError::DepthTooSmall(index.depth()))
         } else if index.depth() > self.depth() {
             Err(MerkleError::DepthTooBig(index.depth() as u64))
         } else if index.depth() == self.depth() {
-            self.get_leaf_node(index.value())
-                .or_else(|| self.empty_hashes.get(index.depth() as usize).copied().map(Word::from))
-                .ok_or(MerkleError::NodeNotInSet(index.value()))
+            // the lookup in empty_hashes could fail only if empty_hashes were not built correctly
+            // by the constructor as we check the depth of the lookup above.
+            Ok(RpoDigest::from(
+                self.get_leaf_node(index.value())
+                    .unwrap_or_else(|| *self.empty_hashes[index.depth() as usize]),
+            ))
         } else {
-            let branch_node = self.get_branch_node(&index);
-            Ok(Rpo256::merge(&[branch_node.left, branch_node.right]).into())
+            Ok(self.get_branch_node(&index).parent())
         }
     }
 
-    /// Returns a Merkle path from the node at the specified key to the root. The node itself is
-    /// not included in the path.
+    /// Returns a value of the leaf at the specified index.
     ///
     /// # Errors
-    /// Returns an error if:
-    /// * The specified depth is greater than the depth of the tree.
+    /// Returns an error if the index is greater than the maximum tree capacity, that is 2^{depth}.
+    pub fn get_leaf(&self, index: u64) -> Result<Word, MerkleError> {
+        let index = NodeIndex::new(self.depth, index)?;
+        Ok(self.get_node(index)?.into())
+    }
+
+    /// Returns a Merkle path from the node at the specified index to the root.
+    ///
+    /// The node itself is not included in the path.
+    ///
+    /// # Errors
+    /// Returns an error if the specified index has depth set to 0 or the depth is greater than
+    /// the depth of this Merkle tree.
     pub fn get_path(&self, mut index: NodeIndex) -> Result<MerklePath, MerkleError> {
         if index.is_root() {
             return Err(MerkleError::DepthTooSmall(index.depth()));
@@ -158,55 +170,57 @@ impl SimpleSmt {
             index.move_up();
             let BranchNode { left, right } = self.get_branch_node(&index);
             let value = if is_right { left } else { right };
-            path.push(*value);
+            path.push(value);
         }
-        Ok(path.into())
+        Ok(MerklePath::new(path))
     }
 
-    /// Return a Merkle path from the leaf at the specified key to the root. The leaf itself is not
-    /// included in the path.
+    /// Return a Merkle path from the leaf at the specified index to the root.
+    ///
+    /// The leaf itself is not included in the path.
     ///
     /// # Errors
-    /// Returns an error if:
-    /// * The specified key does not exist as a leaf node.
-    pub fn get_leaf_path(&self, key: u64) -> Result<MerklePath, MerkleError> {
-        let index = NodeIndex::new(self.depth(), key)?;
+    /// Returns an error if the index is greater than the maximum tree capacity, that is 2^{depth}.
+    pub fn get_leaf_path(&self, index: u64) -> Result<MerklePath, MerkleError> {
+        let index = NodeIndex::new(self.depth(), index)?;
         self.get_path(index)
     }
 
-    /// Iterator over the inner nodes of the [SimpleSmt].
+    // ITERATORS
+    // --------------------------------------------------------------------------------------------
+
+    /// Returns an iterator over the leaves of this [SimpleSmt].
+    pub fn leaves(&self) -> impl Iterator<Item = (u64, &Word)> {
+        self.leaves.iter().map(|(i, w)| (*i, w))
+    }
+
+    /// Returns an iterator over the inner nodes of this Merkle tree.
     pub fn inner_nodes(&self) -> impl Iterator<Item = InnerNodeInfo> + '_ {
         self.branches.values().map(|e| InnerNodeInfo {
-            value: e.parent().into(),
-            left: e.left.into(),
-            right: e.right.into(),
+            value: e.parent(),
+            left: e.left,
+            right: e.right,
         })
     }
 
     // STATE MUTATORS
     // --------------------------------------------------------------------------------------------
 
-    /// Replaces the leaf located at the specified key, and recomputes hashes by walking up the
-    /// tree.
+    /// Updates value of the leaf at the specified index returning the old leaf value.
+    ///
+    /// This also recomputes all hashes between the leaf and the root, updating the root itself.
     ///
     /// # Errors
-    /// Returns an error if the specified key is not a valid leaf index for this tree.
-    pub fn update_leaf(&mut self, key: u64, value: Word) -> Result<(), MerkleError> {
-        let index = NodeIndex::new(self.depth(), key)?;
-        if !self.check_leaf_node_exists(key) {
-            return Err(MerkleError::NodeNotInSet(index.value()));
+    /// Returns an error if the index is greater than the maximum tree capacity, that is 2^{depth}.
+    pub fn update_leaf(&mut self, index: u64, value: Word) -> Result<Word, MerkleError> {
+        let old_value = self.insert_leaf_node(index, value).unwrap_or(Self::EMPTY_VALUE);
+
+        // if the old value and new value are the same, there is nothing to update
+        if value == old_value {
+            return Ok(value);
         }
-        self.insert_leaf(key, value)?;
 
-        Ok(())
-    }
-
-    /// Inserts a leaf located at the specified key, and recomputes hashes by walking up the tree
-    pub fn insert_leaf(&mut self, key: u64, value: Word) -> Result<(), MerkleError> {
-        self.insert_leaf_node(key, value);
-
-        // TODO consider using a map `index |-> word` instead of `index |-> (word, word)`
-        let mut index = NodeIndex::new(self.depth(), key)?;
+        let mut index = NodeIndex::new(self.depth(), index)?;
         let mut value = RpoDigest::from(value);
         for _ in 0..index.depth() {
             let is_right = index.is_value_odd();
@@ -216,27 +230,19 @@ impl SimpleSmt {
             self.insert_branch_node(index, left, right);
             value = Rpo256::merge(&[left, right]);
         }
-        self.root = value.into();
-        Ok(())
+        self.root = value;
+        Ok(old_value)
     }
 
     // HELPER METHODS
     // --------------------------------------------------------------------------------------------
 
-    fn replace_empty_subtrees(&mut self, hashes: Vec<RpoDigest>) {
-        self.empty_hashes = hashes;
-    }
-
-    fn check_leaf_node_exists(&self, key: u64) -> bool {
-        self.leaves.contains_key(&key)
-    }
-
     fn get_leaf_node(&self, key: u64) -> Option<Word> {
         self.leaves.get(&key).copied()
     }
 
-    fn insert_leaf_node(&mut self, key: u64, node: Word) {
-        self.leaves.insert(key, node);
+    fn insert_leaf_node(&mut self, key: u64, node: Word) -> Option<Word> {
+        self.leaves.insert(key, node)
     }
 
     fn get_branch_node(&self, index: &NodeIndex) -> BranchNode {
@@ -254,3 +260,18 @@ impl SimpleSmt {
         self.branches.insert(index, branch);
     }
 }
+
+// BRANCH NODE
+// ================================================================================================
+
+#[derive(Debug, Default, Clone, PartialEq, Eq)]
+struct BranchNode {
+    left: RpoDigest,
+    right: RpoDigest,
+}
+
+impl BranchNode {
+    fn parent(&self) -> RpoDigest {
+        Rpo256::merge(&[self.left, self.right])
+    }
+}

src/merkle/simple_smt/tests.rs

@@ -1,16 +1,21 @@
 use super::{
-    super::{int_to_node, InnerNodeInfo, MerkleError, MerkleTree, RpoDigest, SimpleSmt},
-    NodeIndex, Rpo256, Vec, Word,
+    super::{InnerNodeInfo, MerkleError, MerkleTree, RpoDigest, SimpleSmt},
+    NodeIndex, Rpo256, Vec,
 };
-use proptest::prelude::*;
-use rand_utils::prng_array;
+use crate::{
+    merkle::{digests_to_words, empty_roots::EMPTY_WORD, int_to_leaf, int_to_node},
+    Word,
+};
+
+// TEST DATA
+// ================================================================================================
 
 const KEYS4: [u64; 4] = [0, 1, 2, 3];
 const KEYS8: [u64; 8] = [0, 1, 2, 3, 4, 5, 6, 7];
 
-const VALUES4: [Word; 4] = [int_to_node(1), int_to_node(2), int_to_node(3), int_to_node(4)];
+const VALUES4: [RpoDigest; 4] = [int_to_node(1), int_to_node(2), int_to_node(3), int_to_node(4)];
 
-const VALUES8: [Word; 8] = [
+const VALUES8: [RpoDigest; 8] = [
     int_to_node(1),
     int_to_node(2),
     int_to_node(3),
@@ -21,27 +26,19 @@ const VALUES8: [Word; 8] = [
     int_to_node(8),
 ];
 
-const ZERO_VALUES8: [Word; 8] = [int_to_node(0); 8];
+const ZERO_VALUES8: [Word; 8] = [int_to_leaf(0); 8];
+
+// TESTS
+// ================================================================================================
 
 #[test]
 fn build_empty_tree() {
+    // tree of depth 3
     let smt = SimpleSmt::new(3).unwrap();
     let mt = MerkleTree::new(ZERO_VALUES8.to_vec()).unwrap();
     assert_eq!(mt.root(), smt.root());
 }
 
-#[test]
-fn empty_digests_are_consistent() {
-    let depth = 5;
-    let root = SimpleSmt::new(depth).unwrap().root();
-    let computed: [RpoDigest; 2] = (0..depth).fold([Default::default(); 2], |state, _| {
-        let digest = Rpo256::merge(&state);
-        [digest; 2]
-    });
-
-    assert_eq!(Word::from(computed[0]), root);
-}
-
 #[test]
 fn build_sparse_tree() {
     let mut smt = SimpleSmt::new(3).unwrap();
@@ -49,82 +46,65 @@ fn build_sparse_tree() {
 
     // insert single value
     let key = 6;
-    let new_node = int_to_node(7);
+    let new_node = int_to_leaf(7);
     values[key as usize] = new_node;
-    smt.insert_leaf(key, new_node).expect("Failed to insert leaf");
+    let old_value = smt.update_leaf(key, new_node).expect("Failed to update leaf");
     let mt2 = MerkleTree::new(values.clone()).unwrap();
     assert_eq!(mt2.root(), smt.root());
     assert_eq!(
         mt2.get_path(NodeIndex::make(3, 6)).unwrap(),
         smt.get_path(NodeIndex::make(3, 6)).unwrap()
     );
+    assert_eq!(old_value, EMPTY_WORD);
 
     // insert second value at distinct leaf branch
     let key = 2;
-    let new_node = int_to_node(3);
+    let new_node = int_to_leaf(3);
     values[key as usize] = new_node;
-    smt.insert_leaf(key, new_node).expect("Failed to insert leaf");
+    let old_value = smt.update_leaf(key, new_node).expect("Failed to update leaf");
     let mt3 = MerkleTree::new(values).unwrap();
     assert_eq!(mt3.root(), smt.root());
     assert_eq!(
         mt3.get_path(NodeIndex::make(3, 2)).unwrap(),
         smt.get_path(NodeIndex::make(3, 2)).unwrap()
     );
+    assert_eq!(old_value, EMPTY_WORD);
 }
 
 #[test]
-fn build_full_tree() {
-    let tree = SimpleSmt::new(2)
-        .unwrap()
-        .with_leaves(KEYS4.into_iter().zip(VALUES4.into_iter()))
-        .unwrap();
+fn test_depth2_tree() {
+    let tree =
+        SimpleSmt::with_leaves(2, KEYS4.into_iter().zip(digests_to_words(&VALUES4).into_iter()))
+            .unwrap();
 
+    // check internal structure
     let (root, node2, node3) = compute_internal_nodes();
     assert_eq!(root, tree.root());
     assert_eq!(node2, tree.get_node(NodeIndex::make(1, 0)).unwrap());
     assert_eq!(node3, tree.get_node(NodeIndex::make(1, 1)).unwrap());
-}
 
-#[test]
-fn get_values() {
-    let tree = SimpleSmt::new(2)
-        .unwrap()
-        .with_leaves(KEYS4.into_iter().zip(VALUES4.into_iter()))
-        .unwrap();
-
-    // check depth 2
+    // check get_node()
     assert_eq!(VALUES4[0], tree.get_node(NodeIndex::make(2, 0)).unwrap());
     assert_eq!(VALUES4[1], tree.get_node(NodeIndex::make(2, 1)).unwrap());
     assert_eq!(VALUES4[2], tree.get_node(NodeIndex::make(2, 2)).unwrap());
     assert_eq!(VALUES4[3], tree.get_node(NodeIndex::make(2, 3)).unwrap());
-}
 
-#[test]
-fn get_path() {
-    let tree = SimpleSmt::new(2)
-        .unwrap()
-        .with_leaves(KEYS4.into_iter().zip(VALUES4.into_iter()))
-        .unwrap();
-
-    let (_, node2, node3) = compute_internal_nodes();
-
-    // check depth 2
+    // check get_path(): depth 2
     assert_eq!(vec![VALUES4[1], node3], *tree.get_path(NodeIndex::make(2, 0)).unwrap());
     assert_eq!(vec![VALUES4[0], node3], *tree.get_path(NodeIndex::make(2, 1)).unwrap());
     assert_eq!(vec![VALUES4[3], node2], *tree.get_path(NodeIndex::make(2, 2)).unwrap());
     assert_eq!(vec![VALUES4[2], node2], *tree.get_path(NodeIndex::make(2, 3)).unwrap());
 
-    // check depth 1
+    // check get_path(): depth 1
     assert_eq!(vec![node3], *tree.get_path(NodeIndex::make(1, 0)).unwrap());
     assert_eq!(vec![node2], *tree.get_path(NodeIndex::make(1, 1)).unwrap());
 }
 
 #[test]
-fn test_parent_node_iterator() -> Result<(), MerkleError> {
-    let tree = SimpleSmt::new(2)
-        .unwrap()
-        .with_leaves(KEYS4.into_iter().zip(VALUES4.into_iter()))
-        .unwrap();
+fn test_inner_node_iterator() -> Result<(), MerkleError> {
+    let tree =
+        SimpleSmt::with_leaves(2, KEYS4.into_iter().zip(digests_to_words(&VALUES4).into_iter()))
+            .unwrap();
 
     // check depth 2
     assert_eq!(VALUES4[0], tree.get_node(NodeIndex::make(2, 0)).unwrap());
@@ -144,19 +124,19 @@ fn test_parent_node_iterator() -> Result<(), MerkleError> {
     let nodes: Vec<InnerNodeInfo> = tree.inner_nodes().collect();
     let expected = vec![
         InnerNodeInfo {
-            value: root.into(),
-            left: l1n0.into(),
-            right: l1n1.into(),
+            value: root,
+            left: l1n0,
+            right: l1n1,
         },
         InnerNodeInfo {
-            value: l1n0.into(),
-            left: l2n0.into(),
-            right: l2n1.into(),
+            value: l1n0,
+            left: l2n0,
+            right: l2n1,
         },
         InnerNodeInfo {
-            value: l1n1.into(),
-            left: l2n2.into(),
-            right: l2n3.into(),
+            value: l1n1,
+            left: l2n2,
+            right: l2n3,
         },
     ];
     assert_eq!(nodes, expected);
@@ -166,35 +146,30 @@ fn test_parent_node_iterator() -> Result<(), MerkleError> {
 
 #[test]
 fn update_leaf() {
-    let mut tree = SimpleSmt::new(3)
-        .unwrap()
-        .with_leaves(KEYS8.into_iter().zip(VALUES8.into_iter()))
-        .unwrap();
+    let mut tree =
+        SimpleSmt::with_leaves(3, KEYS8.into_iter().zip(digests_to_words(&VALUES8).into_iter()))
+            .unwrap();
 
     // update one value
     let key = 3;
-    let new_node = int_to_node(9);
-    let mut expected_values = VALUES8.to_vec();
+    let new_node = int_to_leaf(9);
+    let mut expected_values = digests_to_words(&VALUES8);
     expected_values[key] = new_node;
-    let expected_tree = SimpleSmt::new(3)
-        .unwrap()
-        .with_leaves(KEYS8.into_iter().zip(expected_values.clone().into_iter()))
-        .unwrap();
+    let expected_tree = MerkleTree::new(expected_values.clone()).unwrap();
 
-    tree.update_leaf(key as u64, new_node).unwrap();
-    assert_eq!(expected_tree.root, tree.root);
+    let old_leaf = tree.update_leaf(key as u64, new_node).unwrap();
+    assert_eq!(expected_tree.root(), tree.root);
+    assert_eq!(old_leaf, *VALUES8[key]);
 
     // update another value
     let key = 6;
-    let new_node = int_to_node(10);
+    let new_node = int_to_leaf(10);
     expected_values[key] = new_node;
-    let expected_tree = SimpleSmt::new(3)
-        .unwrap()
-        .with_leaves(KEYS8.into_iter().zip(expected_values.into_iter()))
-        .unwrap();
+    let expected_tree = MerkleTree::new(expected_values.clone()).unwrap();
 
-    tree.update_leaf(key as u64, new_node).unwrap();
-    assert_eq!(expected_tree.root, tree.root);
+    let old_leaf = tree.update_leaf(key as u64, new_node).unwrap();
+    assert_eq!(expected_tree.root(), tree.root);
+    assert_eq!(old_leaf, *VALUES8[key]);
 }
 
 #[test]
@@ -207,34 +182,34 @@ fn small_tree_opening_is_consistent() {
     //  / \   / \   / \   / \
     // a   b 0   0 c   0 0   d
 
-    let z = Word::from(RpoDigest::default());
+    let z = EMPTY_WORD;
 
     let a = Word::from(Rpo256::merge(&[z.into(); 2]));
     let b = Word::from(Rpo256::merge(&[a.into(); 2]));
     let c = Word::from(Rpo256::merge(&[b.into(); 2]));
     let d = Word::from(Rpo256::merge(&[c.into(); 2]));
 
-    let e = Word::from(Rpo256::merge(&[a.into(), b.into()]));
-    let f = Word::from(Rpo256::merge(&[z.into(), z.into()]));
-    let g = Word::from(Rpo256::merge(&[c.into(), z.into()]));
-    let h = Word::from(Rpo256::merge(&[z.into(), d.into()]));
+    let e = Rpo256::merge(&[a.into(), b.into()]);
+    let f = Rpo256::merge(&[z.into(), z.into()]);
+    let g = Rpo256::merge(&[c.into(), z.into()]);
+    let h = Rpo256::merge(&[z.into(), d.into()]);
 
-    let i = Word::from(Rpo256::merge(&[e.into(), f.into()]));
-    let j = Word::from(Rpo256::merge(&[g.into(), h.into()]));
+    let i = Rpo256::merge(&[e, f]);
+    let j = Rpo256::merge(&[g, h]);
 
-    let k = Word::from(Rpo256::merge(&[i.into(), j.into()]));
+    let k = Rpo256::merge(&[i, j]);
 
     let depth = 3;
     let entries = vec![(0, a), (1, b), (4, c), (7, d)];
-    let tree = SimpleSmt::new(depth).unwrap().with_leaves(entries).unwrap();
+    let tree = SimpleSmt::with_leaves(depth, entries).unwrap();
 
-    assert_eq!(tree.root(), Word::from(k));
+    assert_eq!(tree.root(), k);
 
-    let cases: Vec<(u8, u64, Vec<Word>)> = vec![
-        (3, 0, vec![b, f, j]),
-        (3, 1, vec![a, f, j]),
-        (3, 4, vec![z, h, i]),
-        (3, 7, vec![z, g, i]),
+    let cases: Vec<(u8, u64, Vec<RpoDigest>)> = vec![
+        (3, 0, vec![b.into(), f, j]),
+        (3, 1, vec![a.into(), f, j]),
+        (3, 4, vec![z.into(), h, i]),
+        (3, 7, vec![z.into(), g, i]),
         (2, 0, vec![f, j]),
         (2, 1, vec![e, j]),
         (2, 2, vec![h, i]),
@@ -250,65 +225,39 @@ fn small_tree_opening_is_consistent() {
     }
 }
 
-proptest! {
-    #[test]
-    fn arbitrary_openings_single_leaf(
-        depth in SimpleSmt::MIN_DEPTH..SimpleSmt::MAX_DEPTH,
-        key in prop::num::u64::ANY,
-        leaf in prop::num::u64::ANY,
-    ) {
-        let mut tree = SimpleSmt::new(depth).unwrap();
+#[test]
+fn fail_on_duplicates() {
+    let entries = [(1_u64, int_to_leaf(1)), (5, int_to_leaf(2)), (1_u64, int_to_leaf(3))];
+    let smt = SimpleSmt::with_leaves(64, entries);
+    assert!(smt.is_err());
 
-        let key = key % (1 << depth as u64);
-        let leaf = int_to_node(leaf);
+    let entries = [(1_u64, int_to_leaf(0)), (5, int_to_leaf(2)), (1_u64, int_to_leaf(0))];
+    let smt = SimpleSmt::with_leaves(64, entries);
+    assert!(smt.is_err());
 
-        tree.insert_leaf(key, leaf.into()).unwrap();
-        tree.get_leaf_path(key).unwrap();
+    let entries = [(1_u64, int_to_leaf(0)), (5, int_to_leaf(2)), (1_u64, int_to_leaf(1))];
+    let smt = SimpleSmt::with_leaves(64, entries);
+    assert!(smt.is_err());
 
-        // traverse to root, fetching all paths
-        for d in 1..depth {
-            let k = key >> (depth - d);
-            tree.get_path(NodeIndex::make(d, k)).unwrap();
-        }
-    }
+    let entries = [(1_u64, int_to_leaf(1)), (5, int_to_leaf(2)), (1_u64, int_to_leaf(0))];
+    let smt = SimpleSmt::with_leaves(64, entries);
+    assert!(smt.is_err());
+}
 
-    #[test]
-    fn arbitrary_openings_multiple_leaves(
-        depth in SimpleSmt::MIN_DEPTH..SimpleSmt::MAX_DEPTH,
-        count in 2u8..10u8,
-        ref seed in any::<[u8; 32]>()
-    ) {
-        let mut tree = SimpleSmt::new(depth).unwrap();
-        let mut seed = *seed;
-        let leaves = (1 << depth) - 1;
-
-        for _ in 0..count {
-            seed = prng_array(seed);
-
-            let mut key = [0u8; 8];
-            let mut leaf = [0u8; 8];
-
-            key.copy_from_slice(&seed[..8]);
-            leaf.copy_from_slice(&seed[8..16]);
-
-            let key = u64::from_le_bytes(key);
-            let key = key % leaves;
-            let leaf = u64::from_le_bytes(leaf);
-            let leaf = int_to_node(leaf);
-
-            tree.insert_leaf(key, leaf).unwrap();
-            tree.get_leaf_path(key).unwrap();
-        }
-    }
+#[test]
+fn with_no_duplicates_empty_node() {
+    let entries = [(1_u64, int_to_leaf(0)), (5, int_to_leaf(2))];
+    let smt = SimpleSmt::with_leaves(64, entries);
+    assert!(smt.is_ok());
 }
 
 // HELPER FUNCTIONS
 // --------------------------------------------------------------------------------------------
 
-fn compute_internal_nodes() -> (Word, Word, Word) {
-    let node2 = Rpo256::hash_elements(&[VALUES4[0], VALUES4[1]].concat());
-    let node3 = Rpo256::hash_elements(&[VALUES4[2], VALUES4[3]].concat());
+fn compute_internal_nodes() -> (RpoDigest, RpoDigest, RpoDigest) {
+    let node2 = Rpo256::merge(&[VALUES4[0], VALUES4[1]]);
+    let node3 = Rpo256::merge(&[VALUES4[2], VALUES4[3]]);
     let root = Rpo256::merge(&[node2, node3]);
 
-    (root.into(), node2.into(), node3.into())
+    (root, node2, node3)
 }

src/merkle/store/mod.rs

@@ -1,20 +1,30 @@
-use super::mmr::Mmr;
 use super::{
-    BTreeMap, EmptySubtreeRoots, InnerNodeInfo, MerkleError, MerklePath, MerklePathSet, MerkleTree,
-    NodeIndex, RootPath, Rpo256, RpoDigest, SimpleSmt, ValuePath, Vec, Word,
+    mmr::Mmr, BTreeMap, EmptySubtreeRoots, InnerNodeInfo, KvMap, MerkleError, MerklePath,
+    MerklePathSet, MerkleTree, NodeIndex, RecordingMap, RootPath, Rpo256, RpoDigest, SimpleSmt,
+    TieredSmt, ValuePath, Vec,
 };
 use crate::utils::{ByteReader, ByteWriter, Deserializable, DeserializationError, Serializable};
+use core::borrow::Borrow;
 
 #[cfg(test)]
 mod tests;
 
+// MERKLE STORE
+// ================================================================================================
+
+/// A default [MerkleStore] which uses a simple [BTreeMap] as the backing storage.
+pub type DefaultMerkleStore = MerkleStore<BTreeMap<RpoDigest, StoreNode>>;
+
+/// A [MerkleStore] with recording capabilities which uses [RecordingMap] as the backing storage.
+pub type RecordingMerkleStore = MerkleStore<RecordingMap<RpoDigest, StoreNode>>;
+
 #[derive(Debug, Default, Copy, Clone, Eq, PartialEq)]
-pub struct Node {
+pub struct StoreNode {
     left: RpoDigest,
     right: RpoDigest,
 }
 
-/// An in-memory data store for Merkle-lized data.
+/// An in-memory data store for Merkelized data.
 ///
 /// This is a in memory data store for Merkle trees, this store allows all the nodes of multiple
 /// trees to live as long as necessary and without duplication, this allows the implementation of
@@ -42,7 +52,7 @@ pub struct Node {
 /// # let T1 = MerkleTree::new([A, B, C, D, E, F, G, H1].to_vec()).expect("even number of leaves provided");
 /// # let ROOT0 = T0.root();
 /// # let ROOT1 = T1.root();
-/// let mut store = MerkleStore::new();
+/// let mut store: MerkleStore = MerkleStore::new();
 ///
 /// // the store is initialized with the SMT empty nodes
 /// assert_eq!(store.num_internal_nodes(), 255);
@@ -51,9 +61,8 @@ pub struct Node {
 /// let tree2 = MerkleTree::new(vec![A, B, C, D, E, F, G, H1]).unwrap();
 ///
 /// // populates the store with two merkle trees, common nodes are shared
-/// store
-///     .extend(tree1.inner_nodes())
-///     .extend(tree2.inner_nodes());
+/// store.extend(tree1.inner_nodes());
+/// store.extend(tree2.inner_nodes());
 ///
 /// // every leaf except the last are the same
 /// for i in 0..7 {
@@ -78,40 +87,24 @@ pub struct Node {
 /// assert_eq!(store.num_internal_nodes() - 255, 10);
 /// ```
 #[derive(Debug, Clone, Eq, PartialEq)]
-pub struct MerkleStore {
-    nodes: BTreeMap<RpoDigest, Node>,
+pub struct MerkleStore<T: KvMap<RpoDigest, StoreNode> = BTreeMap<RpoDigest, StoreNode>> {
+    nodes: T,
 }
 
-impl Default for MerkleStore {
+impl<T: KvMap<RpoDigest, StoreNode>> Default for MerkleStore<T> {
     fn default() -> Self {
         Self::new()
     }
 }
 
-impl MerkleStore {
+impl<T: KvMap<RpoDigest, StoreNode>> MerkleStore<T> {
     // CONSTRUCTORS
     // --------------------------------------------------------------------------------------------
 
     /// Creates an empty `MerkleStore` instance.
-    pub fn new() -> MerkleStore {
+    pub fn new() -> MerkleStore<T> {
         // pre-populate the store with the empty hashes
-        let subtrees = EmptySubtreeRoots::empty_hashes(255);
-        let nodes = subtrees
-            .iter()
-            .rev()
-            .copied()
-            .zip(subtrees.iter().rev().skip(1).copied())
-            .map(|(child, parent)| {
-                (
-                    parent,
-                    Node {
-                        left: child,
-                        right: child,
-                    },
-                )
-            })
-            .collect();
-
+        let nodes = empty_hashes().into_iter().collect();
         MerkleStore { nodes }
     }
 
@@ -126,25 +119,24 @@ impl MerkleStore {
     /// Returns the node at `index` rooted on the tree `root`.
     ///
     /// # Errors
-    ///
     /// This method can return the following errors:
     /// - `RootNotInStore` if the `root` is not present in the store.
-    /// - `NodeNotInStore` if a node needed to traverse from `root` to `index` is not present in the store.
-    pub fn get_node(&self, root: Word, index: NodeIndex) -> Result<Word, MerkleError> {
-        let mut hash: RpoDigest = root.into();
+    /// - `NodeNotInStore` if a node needed to traverse from `root` to `index` is not present in
+    ///   the store.
+    pub fn get_node(&self, root: RpoDigest, index: NodeIndex) -> Result<RpoDigest, MerkleError> {
+        let mut hash = root;
 
         // corner case: check the root is in the store when called with index `NodeIndex::root()`
-        self.nodes.get(&hash).ok_or(MerkleError::RootNotInStore(hash.into()))?;
+        self.nodes.get(&hash).ok_or(MerkleError::RootNotInStore(hash))?;
 
         for i in (0..index.depth()).rev() {
-            let node =
-                self.nodes.get(&hash).ok_or(MerkleError::NodeNotInStore(hash.into(), index))?;
+            let node = self.nodes.get(&hash).ok_or(MerkleError::NodeNotInStore(hash, index))?;
 
             let bit = (index.value() >> i) & 1;
             hash = if bit == 0 { node.left } else { node.right }
         }
 
-        Ok(hash.into())
+        Ok(hash)
     }
 
     /// Returns the node at the specified `index` and its opening to the `root`.
@@ -152,27 +144,26 @@ impl MerkleStore {
     /// The path starts at the sibling of the target leaf.
     ///
     /// # Errors
-    ///
     /// This method can return the following errors:
     /// - `RootNotInStore` if the `root` is not present in the store.
-    /// - `NodeNotInStore` if a node needed to traverse from `root` to `index` is not present in the store.
-    pub fn get_path(&self, root: Word, index: NodeIndex) -> Result<ValuePath, MerkleError> {
-        let mut hash: RpoDigest = root.into();
+    /// - `NodeNotInStore` if a node needed to traverse from `root` to `index` is not present in
+    ///   the store.
+    pub fn get_path(&self, root: RpoDigest, index: NodeIndex) -> Result<ValuePath, MerkleError> {
+        let mut hash = root;
         let mut path = Vec::with_capacity(index.depth().into());
 
         // corner case: check the root is in the store when called with index `NodeIndex::root()`
-        self.nodes.get(&hash).ok_or(MerkleError::RootNotInStore(hash.into()))?;
+        self.nodes.get(&hash).ok_or(MerkleError::RootNotInStore(hash))?;
 
         for i in (0..index.depth()).rev() {
-            let node =
-                self.nodes.get(&hash).ok_or(MerkleError::NodeNotInStore(hash.into(), index))?;
+            let node = self.nodes.get(&hash).ok_or(MerkleError::NodeNotInStore(hash, index))?;
 
             let bit = (index.value() >> i) & 1;
             hash = if bit == 0 {
-                path.push(node.right.into());
+                path.push(node.right);
                 node.left
             } else {
-                path.push(node.left.into());
+                path.push(node.left);
                 node.right
             }
         }
@@ -181,7 +172,7 @@ impl MerkleStore {
         path.reverse();
 
         Ok(ValuePath {
-            value: hash.into(),
+            value: hash,
             path: MerklePath::new(path),
         })
     }
@@ -200,10 +191,10 @@ impl MerkleStore {
     /// - The path from the root continues to a depth greater than `tree_depth`.
     /// - The provided `tree_depth` is greater than `64.
     /// - The provided `index` is not valid for a depth equivalent to `tree_depth`. For more
-    /// information, check [NodeIndex::new].
+    ///   information, check [NodeIndex::new].
     pub fn get_leaf_depth(
         &self,
-        root: Word,
+        root: RpoDigest,
         tree_depth: u8,
         index: u64,
     ) -> Result<u8, MerkleError> {
@@ -222,9 +213,9 @@ impl MerkleStore {
 
         // check if the root exists, providing the proper error report if it doesn't
         let empty = EmptySubtreeRoots::empty_hashes(tree_depth);
-        let mut hash: RpoDigest = root.into();
+        let mut hash = root;
         if !self.nodes.contains_key(&hash) {
-            return Err(MerkleError::RootNotInStore(hash.into()));
+            return Err(MerkleError::RootNotInStore(hash));
         }
 
         // we traverse from root to leaf, so the path is reversed
@@ -257,26 +248,38 @@ impl MerkleStore {
         Ok(tree_depth)
     }
 
-    // STATE MUTATORS
+    // DATA EXTRACTORS
     // --------------------------------------------------------------------------------------------
 
-    /// Adds a sequence of nodes yielded by the provided iterator into the store.
-    pub fn extend<I>(&mut self, iter: I) -> &mut MerkleStore
+    /// Returns a subset of this Merkle store such that the returned Merkle store contains all
+    /// nodes which are descendants of the specified roots.
+    ///
+    /// The roots for which no descendants exist in this Merkle store are ignored.
+    pub fn subset<I, R>(&self, roots: I) -> MerkleStore<T>
     where
-        I: Iterator<Item = InnerNodeInfo>,
+        I: Iterator<Item = R>,
+        R: Borrow<RpoDigest>,
     {
-        for node in iter {
-            let value: RpoDigest = node.value.into();
-            let left: RpoDigest = node.left.into();
-            let right: RpoDigest = node.right.into();
-
-            debug_assert_eq!(Rpo256::merge(&[left, right]), value);
-            self.nodes.insert(value, Node { left, right });
+        let mut store = MerkleStore::new();
+        for root in roots {
+            let root = *root.borrow();
+            store.clone_tree_from(root, self);
         }
-
-        self
+        store
     }
 
+    /// Iterator over the inner nodes of the [MerkleStore].
+    pub fn inner_nodes(&self) -> impl Iterator<Item = InnerNodeInfo> + '_ {
+        self.nodes.iter().map(|(r, n)| InnerNodeInfo {
+            value: *r,
+            left: n.left,
+            right: n.right,
+        })
+    }
+
+    // STATE MUTATORS
+    // --------------------------------------------------------------------------------------------
+
     /// Adds all the nodes of a Merkle path represented by `path`, opening to `node`. Returns the
     /// new root.
     ///
@@ -285,16 +288,16 @@ impl MerkleStore {
     pub fn add_merkle_path(
         &mut self,
         index: u64,
-        node: Word,
+        node: RpoDigest,
         path: MerklePath,
-    ) -> Result<Word, MerkleError> {
-        let root = path.inner_nodes(index, node)?.fold(Word::default(), |_, node| {
-            let value: RpoDigest = node.value.into();
-            let left: RpoDigest = node.left.into();
-            let right: RpoDigest = node.right.into();
+    ) -> Result<RpoDigest, MerkleError> {
+        let root = path.inner_nodes(index, node)?.fold(RpoDigest::default(), |_, node| {
+            let value: RpoDigest = node.value;
+            let left: RpoDigest = node.left;
+            let right: RpoDigest = node.right;
 
             debug_assert_eq!(Rpo256::merge(&[left, right]), value);
-            self.nodes.insert(value, Node { left, right });
+            self.nodes.insert(value, StoreNode { left, right });
 
             node.value
         });
@@ -309,7 +312,7 @@ impl MerkleStore {
     /// For further reference, check [MerkleStore::add_merkle_path].
     pub fn add_merkle_paths<I>(&mut self, paths: I) -> Result<(), MerkleError>
     where
-        I: IntoIterator<Item = (u64, Word, MerklePath)>,
+        I: IntoIterator<Item = (u64, RpoDigest, MerklePath)>,
     {
         for (index_value, node, path) in paths.into_iter() {
             self.add_merkle_path(index_value, node, path)?;
@@ -320,7 +323,10 @@ impl MerkleStore {
     /// Appends the provided [MerklePathSet] into the store.
     ///
     /// For further reference, check [MerkleStore::add_merkle_path].
-    pub fn add_merkle_path_set(&mut self, path_set: &MerklePathSet) -> Result<Word, MerkleError> {
+    pub fn add_merkle_path_set(
+        &mut self,
+        path_set: &MerklePathSet,
+    ) -> Result<RpoDigest, MerkleError> {
         let root = path_set.root();
         for (index, path) in path_set.to_paths() {
             self.add_merkle_path(index, path.value, path.path)?;
@@ -331,15 +337,15 @@ impl MerkleStore {
     /// Sets a node to `value`.
     ///
     /// # Errors
-    ///
     /// This method can return the following errors:
     /// - `RootNotInStore` if the `root` is not present in the store.
-    /// - `NodeNotInStore` if a node needed to traverse from `root` to `index` is not present in the store.
+    /// - `NodeNotInStore` if a node needed to traverse from `root` to `index` is not present in
+    ///   the store.
     pub fn set_node(
         &mut self,
-        mut root: Word,
+        mut root: RpoDigest,
         index: NodeIndex,
-        value: Word,
+        value: RpoDigest,
     ) -> Result<RootPath, MerkleError> {
         let node = value;
         let ValuePath { value, path } = self.get_path(root, index)?;
@@ -355,80 +361,138 @@ impl MerkleStore {
     /// Merges two elements and adds the resulting node into the store.
     ///
     /// Merges arbitrary values. They may be leafs, nodes, or a mixture of both.
-    pub fn merge_roots(&mut self, root1: Word, root2: Word) -> Result<Word, MerkleError> {
-        let left: RpoDigest = root1.into();
-        let right: RpoDigest = root2.into();
+    pub fn merge_roots(
+        &mut self,
+        left_root: RpoDigest,
+        right_root: RpoDigest,
+    ) -> Result<RpoDigest, MerkleError> {
+        let parent = Rpo256::merge(&[left_root, right_root]);
+        self.nodes.insert(
+            parent,
+            StoreNode {
+                left: left_root,
+                right: right_root,
+            },
+        );
 
-        let parent = Rpo256::merge(&[left, right]);
-        self.nodes.insert(parent, Node { left, right });
+        Ok(parent)
+    }
 
-        Ok(parent.into())
+    // DESTRUCTURING
+    // --------------------------------------------------------------------------------------------
+
+    /// Returns the inner storage of this MerkleStore while consuming `self`.
+    pub fn into_inner(self) -> T {
+        self.nodes
+    }
+
+    // HELPER METHODS
+    // --------------------------------------------------------------------------------------------
+
+    /// Recursively clones a tree with the specified root from the specified source into self.
+    ///
+    /// If the source store does not contain a tree with the specified root, this is a noop.
+    fn clone_tree_from(&mut self, root: RpoDigest, source: &Self) {
+        // process the node only if it is in the source
+        if let Some(node) = source.nodes.get(&root) {
+            // if the node has already been inserted, no need to process it further as all of its
+            // descendants should be already cloned from the source store
+            if self.nodes.insert(root, *node).is_none() {
+                self.clone_tree_from(node.left, source);
+                self.clone_tree_from(node.right, source);
+            }
+        }
     }
 }
 
 // CONVERSIONS
 // ================================================================================================
 
-impl From<&MerkleTree> for MerkleStore {
+impl<T: KvMap<RpoDigest, StoreNode>> From<&MerkleTree> for MerkleStore<T> {
     fn from(value: &MerkleTree) -> Self {
-        let mut store = MerkleStore::new();
-        store.extend(value.inner_nodes());
-        store
+        let nodes = combine_nodes_with_empty_hashes(value.inner_nodes()).collect();
+        Self { nodes }
     }
 }
 
-impl From<&SimpleSmt> for MerkleStore {
+impl<T: KvMap<RpoDigest, StoreNode>> From<&SimpleSmt> for MerkleStore<T> {
     fn from(value: &SimpleSmt) -> Self {
-        let mut store = MerkleStore::new();
-        store.extend(value.inner_nodes());
-        store
+        let nodes = combine_nodes_with_empty_hashes(value.inner_nodes()).collect();
+        Self { nodes }
     }
 }
 
-impl From<&Mmr> for MerkleStore {
+impl<T: KvMap<RpoDigest, StoreNode>> From<&Mmr> for MerkleStore<T> {
     fn from(value: &Mmr) -> Self {
-        let mut store = MerkleStore::new();
-        store.extend(value.inner_nodes());
-        store
+        let nodes = combine_nodes_with_empty_hashes(value.inner_nodes()).collect();
+        Self { nodes }
     }
 }
 
-impl FromIterator<InnerNodeInfo> for MerkleStore {
-    fn from_iter<T: IntoIterator<Item = InnerNodeInfo>>(iter: T) -> Self {
-        let mut store = MerkleStore::new();
-        store.extend(iter.into_iter());
-        store
+impl<T: KvMap<RpoDigest, StoreNode>> From<&TieredSmt> for MerkleStore<T> {
+    fn from(value: &TieredSmt) -> Self {
+        let nodes = combine_nodes_with_empty_hashes(value.inner_nodes()).collect();
+        Self { nodes }
+    }
+}
+
+impl<T: KvMap<RpoDigest, StoreNode>> From<T> for MerkleStore<T> {
+    fn from(values: T) -> Self {
+        let nodes = values.into_iter().chain(empty_hashes().into_iter()).collect();
+        Self { nodes }
+    }
+}
+
+impl<T: KvMap<RpoDigest, StoreNode>> FromIterator<InnerNodeInfo> for MerkleStore<T> {
+    fn from_iter<I: IntoIterator<Item = InnerNodeInfo>>(iter: I) -> Self {
+        let nodes = combine_nodes_with_empty_hashes(iter.into_iter()).collect();
+        Self { nodes }
+    }
+}
+
+impl<T: KvMap<RpoDigest, StoreNode>> FromIterator<(RpoDigest, StoreNode)> for MerkleStore<T> {
+    fn from_iter<I: IntoIterator<Item = (RpoDigest, StoreNode)>>(iter: I) -> Self {
+        let nodes = iter.into_iter().chain(empty_hashes().into_iter()).collect();
+        Self { nodes }
     }
 }
 
 // ITERATORS
 // ================================================================================================
 
-impl Extend<InnerNodeInfo> for MerkleStore {
-    fn extend<T: IntoIterator<Item = InnerNodeInfo>>(&mut self, iter: T) {
-        self.extend(iter.into_iter());
+impl<T: KvMap<RpoDigest, StoreNode>> Extend<InnerNodeInfo> for MerkleStore<T> {
+    fn extend<I: IntoIterator<Item = InnerNodeInfo>>(&mut self, iter: I) {
+        self.nodes.extend(iter.into_iter().map(|info| {
+            (
+                info.value,
+                StoreNode {
+                    left: info.left,
+                    right: info.right,
+                },
+            )
+        }));
     }
 }
 
 // SERIALIZATION
 // ================================================================================================
 
-impl Serializable for Node {
+impl Serializable for StoreNode {
     fn write_into<W: ByteWriter>(&self, target: &mut W) {
         self.left.write_into(target);
         self.right.write_into(target);
     }
 }
 
-impl Deserializable for Node {
+impl Deserializable for StoreNode {
     fn read_from<R: ByteReader>(source: &mut R) -> Result<Self, DeserializationError> {
         let left = RpoDigest::read_from(source)?;
         let right = RpoDigest::read_from(source)?;
-        Ok(Node { left, right })
+        Ok(StoreNode { left, right })
     }
 }
 
-impl Serializable for MerkleStore {
+impl<T: KvMap<RpoDigest, StoreNode>> Serializable for MerkleStore<T> {
     fn write_into<W: ByteWriter>(&self, target: &mut W) {
         target.write_u64(self.nodes.len() as u64);
 
@@ -439,17 +503,55 @@ impl Serializable for MerkleStore {
     }
 }
 
-impl Deserializable for MerkleStore {
+impl<T: KvMap<RpoDigest, StoreNode>> Deserializable for MerkleStore<T> {
     fn read_from<R: ByteReader>(source: &mut R) -> Result<Self, DeserializationError> {
         let len = source.read_u64()?;
-        let mut nodes: BTreeMap<RpoDigest, Node> = BTreeMap::new();
+        let mut nodes: Vec<(RpoDigest, StoreNode)> = Vec::with_capacity(len as usize);
 
         for _ in 0..len {
             let key = RpoDigest::read_from(source)?;
-            let value = Node::read_from(source)?;
-            nodes.insert(key, value);
+            let value = StoreNode::read_from(source)?;
+            nodes.push((key, value));
         }
 
-        Ok(MerkleStore { nodes })
+        Ok(nodes.into_iter().collect())
     }
 }
+
+// HELPER FUNCTIONS
+// ================================================================================================
+
+/// Creates empty hashes for all the subtrees of a tree with a max depth of 255.
+fn empty_hashes() -> impl IntoIterator<Item = (RpoDigest, StoreNode)> {
+    let subtrees = EmptySubtreeRoots::empty_hashes(255);
+    subtrees.iter().rev().copied().zip(subtrees.iter().rev().skip(1).copied()).map(
+        |(child, parent)| {
+            (
+                parent,
+                StoreNode {
+                    left: child,
+                    right: child,
+                },
+            )
+        },
+    )
+}
+
+/// Consumes an iterator of [InnerNodeInfo] and returns an iterator of `(value, node)` tuples
+/// which includes the nodes associate with roots of empty subtrees up to a depth of 255.
+fn combine_nodes_with_empty_hashes(
+    nodes: impl IntoIterator<Item = InnerNodeInfo>,
+) -> impl Iterator<Item = (RpoDigest, StoreNode)> {
+    nodes
+        .into_iter()
+        .map(|info| {
+            (
+                info.value,
+                StoreNode {
+                    left: info.left,
+                    right: info.right,
+                },
+            )
+        })
+        .chain(empty_hashes().into_iter())
+}

src/merkle/store/tests.rs

@@ -1,29 +1,52 @@
-use super::*;
+use super::{
+    DefaultMerkleStore as MerkleStore, EmptySubtreeRoots, MerkleError, MerklePath, NodeIndex,
+    RecordingMerkleStore, RpoDigest,
+};
 use crate::{
     hash::rpo::Rpo256,
-    merkle::{int_to_node, MerklePathSet, MerkleTree, SimpleSmt},
-    Felt, Word, WORD_SIZE, ZERO,
+    merkle::{digests_to_words, int_to_leaf, int_to_node, MerklePathSet, MerkleTree, SimpleSmt},
+    Felt, Word, ONE, WORD_SIZE, ZERO,
 };
 
+#[cfg(feature = "std")]
+use super::{Deserializable, Serializable};
+
 #[cfg(feature = "std")]
 use std::error::Error;
 
+// TEST DATA
+// ================================================================================================
+
 const KEYS4: [u64; 4] = [0, 1, 2, 3];
-const LEAVES4: [Word; 4] = [int_to_node(1), int_to_node(2), int_to_node(3), int_to_node(4)];
-const EMPTY: Word = [ZERO; WORD_SIZE];
+const VALUES4: [RpoDigest; 4] = [int_to_node(1), int_to_node(2), int_to_node(3), int_to_node(4)];
+
+const KEYS8: [u64; 8] = [0, 1, 2, 3, 4, 5, 6, 7];
+const VALUES8: [RpoDigest; 8] = [
+    int_to_node(1),
+    int_to_node(2),
+    int_to_node(3),
+    int_to_node(4),
+    int_to_node(5),
+    int_to_node(6),
+    int_to_node(7),
+    int_to_node(8),
+];
+
+// TESTS
+// ================================================================================================
 
 #[test]
 fn test_root_not_in_store() -> Result<(), MerkleError> {
-    let mtree = MerkleTree::new(LEAVES4.to_vec())?;
+    let mtree = MerkleTree::new(digests_to_words(&VALUES4))?;
     let store = MerkleStore::from(&mtree);
     assert_eq!(
-        store.get_node(LEAVES4[0], NodeIndex::make(mtree.depth(), 0)),
-        Err(MerkleError::RootNotInStore(LEAVES4[0])),
+        store.get_node(VALUES4[0], NodeIndex::make(mtree.depth(), 0)),
+        Err(MerkleError::RootNotInStore(VALUES4[0])),
         "Leaf 0 is not a root"
     );
     assert_eq!(
-        store.get_path(LEAVES4[0], NodeIndex::make(mtree.depth(), 0)),
-        Err(MerkleError::RootNotInStore(LEAVES4[0])),
+        store.get_path(VALUES4[0], NodeIndex::make(mtree.depth(), 0)),
+        Err(MerkleError::RootNotInStore(VALUES4[0])),
         "Leaf 0 is not a root"
     );
 
@@ -32,33 +55,33 @@ fn test_root_not_in_store() -> Result<(), MerkleError> {
 
 #[test]
 fn test_merkle_tree() -> Result<(), MerkleError> {
-    let mtree = MerkleTree::new(LEAVES4.to_vec())?;
+    let mtree = MerkleTree::new(digests_to_words(&VALUES4))?;
     let store = MerkleStore::from(&mtree);
 
-    // STORE LEAVES ARE CORRECT ==============================================================
+    // STORE LEAVES ARE CORRECT -------------------------------------------------------------------
     // checks the leaves in the store corresponds to the expected values
     assert_eq!(
         store.get_node(mtree.root(), NodeIndex::make(mtree.depth(), 0)),
-        Ok(LEAVES4[0]),
+        Ok(VALUES4[0]),
         "node 0 must be in the tree"
     );
     assert_eq!(
         store.get_node(mtree.root(), NodeIndex::make(mtree.depth(), 1)),
-        Ok(LEAVES4[1]),
+        Ok(VALUES4[1]),
         "node 1 must be in the tree"
     );
     assert_eq!(
         store.get_node(mtree.root(), NodeIndex::make(mtree.depth(), 2)),
-        Ok(LEAVES4[2]),
+        Ok(VALUES4[2]),
         "node 2 must be in the tree"
     );
     assert_eq!(
         store.get_node(mtree.root(), NodeIndex::make(mtree.depth(), 3)),
-        Ok(LEAVES4[3]),
+        Ok(VALUES4[3]),
         "node 3 must be in the tree"
     );
 
-    // STORE LEAVES MATCH TREE ===============================================================
+    // STORE LEAVES MATCH TREE --------------------------------------------------------------------
     // sanity check the values returned by the store and the tree
     assert_eq!(
         mtree.get_node(NodeIndex::make(mtree.depth(), 0)),
@@ -85,7 +108,7 @@ fn test_merkle_tree() -> Result<(), MerkleError> {
     // assert the merkle path returned by the store is the same as the one in the tree
     let result = store.get_path(mtree.root(), NodeIndex::make(mtree.depth(), 0)).unwrap();
     assert_eq!(
-        LEAVES4[0], result.value,
+        VALUES4[0], result.value,
         "Value for merkle path at index 0 must match leaf value"
     );
     assert_eq!(
@@ -96,7 +119,7 @@ fn test_merkle_tree() -> Result<(), MerkleError> {
 
     let result = store.get_path(mtree.root(), NodeIndex::make(mtree.depth(), 1)).unwrap();
     assert_eq!(
-        LEAVES4[1], result.value,
+        VALUES4[1], result.value,
         "Value for merkle path at index 0 must match leaf value"
     );
     assert_eq!(
@@ -107,7 +130,7 @@ fn test_merkle_tree() -> Result<(), MerkleError> {
 
     let result = store.get_path(mtree.root(), NodeIndex::make(mtree.depth(), 2)).unwrap();
     assert_eq!(
-        LEAVES4[2], result.value,
+        VALUES4[2], result.value,
         "Value for merkle path at index 0 must match leaf value"
     );
     assert_eq!(
@@ -118,7 +141,7 @@ fn test_merkle_tree() -> Result<(), MerkleError> {
 
     let result = store.get_path(mtree.root(), NodeIndex::make(mtree.depth(), 3)).unwrap();
     assert_eq!(
-        LEAVES4[3], result.value,
+        VALUES4[3], result.value,
         "Value for merkle path at index 0 must match leaf value"
     );
     assert_eq!(
@@ -133,12 +156,12 @@ fn test_merkle_tree() -> Result<(), MerkleError> {
 #[test]
 fn test_empty_roots() {
     let store = MerkleStore::default();
-    let mut root = RpoDigest::new(EMPTY);
+    let mut root = RpoDigest::default();
 
     for depth in 0..255 {
         root = Rpo256::merge(&[root; 2]);
         assert!(
-            store.get_node(root.into(), NodeIndex::make(0, 0)).is_ok(),
+            store.get_node(root, NodeIndex::make(0, 0)).is_ok(),
             "The root of the empty tree of depth {depth} must be registered"
         );
     }
@@ -157,13 +180,17 @@ fn test_leaf_paths_for_empty_trees() -> Result<(), MerkleError> {
         let index = NodeIndex::make(depth, 0);
         let store_path = store.get_path(smt.root(), index)?;
         let smt_path = smt.get_path(index)?;
-        assert_eq!(store_path.value, EMPTY, "the leaf of an empty tree is always ZERO");
+        assert_eq!(
+            store_path.value,
+            RpoDigest::default(),
+            "the leaf of an empty tree is always ZERO"
+        );
         assert_eq!(
             store_path.path, smt_path,
             "the returned merkle path does not match the computed values"
         );
         assert_eq!(
-            store_path.path.compute_root(depth.into(), EMPTY).unwrap(),
+            store_path.path.compute_root(depth.into(), RpoDigest::default()).unwrap(),
             smt.root(),
             "computed root from the path must match the empty tree root"
         );
@@ -174,7 +201,8 @@ fn test_leaf_paths_for_empty_trees() -> Result<(), MerkleError> {
 
 #[test]
 fn test_get_invalid_node() {
-    let mtree = MerkleTree::new(LEAVES4.to_vec()).expect("creating a merkle tree must work");
+    let mtree =
+        MerkleTree::new(digests_to_words(&VALUES4)).expect("creating a merkle tree must work");
     let store = MerkleStore::from(&mtree);
     let _ = store.get_node(mtree.root(), NodeIndex::make(mtree.depth(), 3));
 }
@@ -182,19 +210,16 @@ fn test_get_invalid_node() {
 #[test]
 fn test_add_sparse_merkle_tree_one_level() -> Result<(), MerkleError> {
     let keys2: [u64; 2] = [0, 1];
-    let leaves2: [Word; 2] = [int_to_node(1), int_to_node(2)];
-    let smt = SimpleSmt::new(1)
-        .unwrap()
-        .with_leaves(keys2.into_iter().zip(leaves2.into_iter()))
-        .unwrap();
+    let leaves2: [Word; 2] = [int_to_leaf(1), int_to_leaf(2)];
+    let smt = SimpleSmt::with_leaves(1, keys2.into_iter().zip(leaves2.into_iter())).unwrap();
     let store = MerkleStore::from(&smt);
 
     let idx = NodeIndex::make(1, 0);
-    assert_eq!(smt.get_node(idx).unwrap(), leaves2[0]);
+    assert_eq!(smt.get_node(idx).unwrap(), leaves2[0].into());
     assert_eq!(store.get_node(smt.root(), idx).unwrap(), smt.get_node(idx).unwrap());
 
     let idx = NodeIndex::make(1, 1);
-    assert_eq!(smt.get_node(idx).unwrap(), leaves2[1]);
+    assert_eq!(smt.get_node(idx).unwrap(), leaves2[1].into());
     assert_eq!(store.get_node(smt.root(), idx).unwrap(), smt.get_node(idx).unwrap());
 
     Ok(())
@@ -202,10 +227,11 @@ fn test_add_sparse_merkle_tree_one_level() -> Result<(), MerkleError> {
 
 #[test]
 fn test_sparse_merkle_tree() -> Result<(), MerkleError> {
-    let smt = SimpleSmt::new(SimpleSmt::MAX_DEPTH)
-        .unwrap()
-        .with_leaves(KEYS4.into_iter().zip(LEAVES4.into_iter()))
-        .unwrap();
+    let smt = SimpleSmt::with_leaves(
+        SimpleSmt::MAX_DEPTH,
+        KEYS4.into_iter().zip(digests_to_words(&VALUES4).into_iter()),
+    )
+    .unwrap();
 
     let store = MerkleStore::from(&smt);
 
@@ -213,27 +239,27 @@ fn test_sparse_merkle_tree() -> Result<(), MerkleError> {
     // checks the leaves in the store corresponds to the expected values
     assert_eq!(
         store.get_node(smt.root(), NodeIndex::make(smt.depth(), 0)),
-        Ok(LEAVES4[0]),
+        Ok(VALUES4[0]),
         "node 0 must be in the tree"
     );
     assert_eq!(
         store.get_node(smt.root(), NodeIndex::make(smt.depth(), 1)),
-        Ok(LEAVES4[1]),
+        Ok(VALUES4[1]),
         "node 1 must be in the tree"
     );
     assert_eq!(
         store.get_node(smt.root(), NodeIndex::make(smt.depth(), 2)),
-        Ok(LEAVES4[2]),
+        Ok(VALUES4[2]),
         "node 2 must be in the tree"
     );
     assert_eq!(
         store.get_node(smt.root(), NodeIndex::make(smt.depth(), 3)),
-        Ok(LEAVES4[3]),
+        Ok(VALUES4[3]),
         "node 3 must be in the tree"
     );
     assert_eq!(
         store.get_node(smt.root(), NodeIndex::make(smt.depth(), 4)),
-        Ok(EMPTY),
+        Ok(RpoDigest::default()),
         "unmodified node 4 must be ZERO"
     );
 
@@ -269,7 +295,7 @@ fn test_sparse_merkle_tree() -> Result<(), MerkleError> {
     // assert the merkle path returned by the store is the same as the one in the tree
     let result = store.get_path(smt.root(), NodeIndex::make(smt.depth(), 0)).unwrap();
     assert_eq!(
-        LEAVES4[0], result.value,
+        VALUES4[0], result.value,
         "Value for merkle path at index 0 must match leaf value"
     );
     assert_eq!(
@@ -280,7 +306,7 @@ fn test_sparse_merkle_tree() -> Result<(), MerkleError> {
 
     let result = store.get_path(smt.root(), NodeIndex::make(smt.depth(), 1)).unwrap();
     assert_eq!(
-        LEAVES4[1], result.value,
+        VALUES4[1], result.value,
         "Value for merkle path at index 1 must match leaf value"
     );
     assert_eq!(
@@ -291,7 +317,7 @@ fn test_sparse_merkle_tree() -> Result<(), MerkleError> {
 
     let result = store.get_path(smt.root(), NodeIndex::make(smt.depth(), 2)).unwrap();
     assert_eq!(
-        LEAVES4[2], result.value,
+        VALUES4[2], result.value,
         "Value for merkle path at index 2 must match leaf value"
     );
     assert_eq!(
@@ -302,7 +328,7 @@ fn test_sparse_merkle_tree() -> Result<(), MerkleError> {
 
     let result = store.get_path(smt.root(), NodeIndex::make(smt.depth(), 3)).unwrap();
     assert_eq!(
-        LEAVES4[3], result.value,
+        VALUES4[3], result.value,
         "Value for merkle path at index 3 must match leaf value"
     );
     assert_eq!(
@@ -312,7 +338,11 @@ fn test_sparse_merkle_tree() -> Result<(), MerkleError> {
     );
 
     let result = store.get_path(smt.root(), NodeIndex::make(smt.depth(), 4)).unwrap();
-    assert_eq!(EMPTY, result.value, "Value for merkle path at index 4 must match leaf value");
+    assert_eq!(
+        RpoDigest::default(),
+        result.value,
+        "Value for merkle path at index 4 must match leaf value"
+    );
     assert_eq!(
         smt.get_path(NodeIndex::make(smt.depth(), 4)),
         Ok(result.path),
@@ -324,7 +354,7 @@ fn test_sparse_merkle_tree() -> Result<(), MerkleError> {
 
 #[test]
 fn test_add_merkle_paths() -> Result<(), MerkleError> {
-    let mtree = MerkleTree::new(LEAVES4.to_vec())?;
+    let mtree = MerkleTree::new(digests_to_words(&VALUES4))?;
 
     let i0 = 0;
     let p0 = mtree.get_path(NodeIndex::make(2, i0)).unwrap();
@@ -339,10 +369,10 @@ fn test_add_merkle_paths() -> Result<(), MerkleError> {
     let p3 = mtree.get_path(NodeIndex::make(2, i3)).unwrap();
 
     let paths = [
-        (i0, LEAVES4[i0 as usize], p0),
-        (i1, LEAVES4[i1 as usize], p1),
-        (i2, LEAVES4[i2 as usize], p2),
-        (i3, LEAVES4[i3 as usize], p3),
+        (i0, VALUES4[i0 as usize], p0),
+        (i1, VALUES4[i1 as usize], p1),
+        (i2, VALUES4[i2 as usize], p2),
+        (i3, VALUES4[i3 as usize], p3),
     ];
 
     let mut store = MerkleStore::default();
@@ -355,22 +385,22 @@ fn test_add_merkle_paths() -> Result<(), MerkleError> {
     // checks the leaves in the store corresponds to the expected values
     assert_eq!(
         store.get_node(set.root(), NodeIndex::make(set.depth(), 0)),
-        Ok(LEAVES4[0]),
+        Ok(VALUES4[0]),
         "node 0 must be in the set"
     );
     assert_eq!(
         store.get_node(set.root(), NodeIndex::make(set.depth(), 1)),
-        Ok(LEAVES4[1]),
+        Ok(VALUES4[1]),
         "node 1 must be in the set"
     );
     assert_eq!(
         store.get_node(set.root(), NodeIndex::make(set.depth(), 2)),
-        Ok(LEAVES4[2]),
+        Ok(VALUES4[2]),
         "node 2 must be in the set"
     );
     assert_eq!(
         store.get_node(set.root(), NodeIndex::make(set.depth(), 3)),
-        Ok(LEAVES4[3]),
+        Ok(VALUES4[3]),
         "node 3 must be in the set"
     );
 
@@ -401,7 +431,7 @@ fn test_add_merkle_paths() -> Result<(), MerkleError> {
     // assert the merkle path returned by the store is the same as the one in the set
     let result = store.get_path(set.root(), NodeIndex::make(set.depth(), 0)).unwrap();
     assert_eq!(
-        LEAVES4[0], result.value,
+        VALUES4[0], result.value,
         "Value for merkle path at index 0 must match leaf value"
     );
     assert_eq!(
@@ -412,7 +442,7 @@ fn test_add_merkle_paths() -> Result<(), MerkleError> {
 
     let result = store.get_path(set.root(), NodeIndex::make(set.depth(), 1)).unwrap();
     assert_eq!(
-        LEAVES4[1], result.value,
+        VALUES4[1], result.value,
         "Value for merkle path at index 0 must match leaf value"
     );
     assert_eq!(
@@ -423,7 +453,7 @@ fn test_add_merkle_paths() -> Result<(), MerkleError> {
 
     let result = store.get_path(set.root(), NodeIndex::make(set.depth(), 2)).unwrap();
     assert_eq!(
-        LEAVES4[2], result.value,
+        VALUES4[2], result.value,
         "Value for merkle path at index 0 must match leaf value"
     );
     assert_eq!(
@@ -434,7 +464,7 @@ fn test_add_merkle_paths() -> Result<(), MerkleError> {
 
     let result = store.get_path(set.root(), NodeIndex::make(set.depth(), 3)).unwrap();
     assert_eq!(
-        LEAVES4[3], result.value,
+        VALUES4[3], result.value,
         "Value for merkle path at index 0 must match leaf value"
     );
     assert_eq!(
@@ -459,7 +489,6 @@ fn wont_open_to_different_depth_root() {
     for depth in (1..=63).rev() {
         root = Rpo256::merge(&[root, empty[depth]]);
     }
-    let root = Word::from(root);
 
     // For this example, the depth of the Merkle tree is 1, as we have only two leaves. Here we
     // attempt to fetch a node on the maximum depth, and it should fail because the root shouldn't
@@ -487,22 +516,22 @@ fn store_path_opens_from_leaf() {
     let k = Rpo256::merge(&[e.into(), f.into()]);
     let l = Rpo256::merge(&[g.into(), h.into()]);
 
-    let m = Rpo256::merge(&[i.into(), j.into()]);
-    let n = Rpo256::merge(&[k.into(), l.into()]);
+    let m = Rpo256::merge(&[i, j]);
+    let n = Rpo256::merge(&[k, l]);
 
-    let root = Rpo256::merge(&[m.into(), n.into()]);
+    let root = Rpo256::merge(&[m, n]);
 
     let mtree = MerkleTree::new(vec![a, b, c, d, e, f, g, h]).unwrap();
     let store = MerkleStore::from(&mtree);
-    let path = store.get_path(root.into(), NodeIndex::make(3, 1)).unwrap().path;
+    let path = store.get_path(root, NodeIndex::make(3, 1)).unwrap().path;
 
-    let expected = MerklePath::new([a.into(), j.into(), n.into()].to_vec());
+    let expected = MerklePath::new([a.into(), j, n].to_vec());
     assert_eq!(path, expected);
 }
 
 #[test]
 fn test_set_node() -> Result<(), MerkleError> {
-    let mtree = MerkleTree::new(LEAVES4.to_vec())?;
+    let mtree = MerkleTree::new(digests_to_words(&VALUES4))?;
     let mut store = MerkleStore::from(&mtree);
     let value = int_to_node(42);
     let index = NodeIndex::make(mtree.depth(), 0);
@@ -514,7 +543,7 @@ fn test_set_node() -> Result<(), MerkleError> {
 
 #[test]
 fn test_constructors() -> Result<(), MerkleError> {
-    let mtree = MerkleTree::new(LEAVES4.to_vec())?;
+    let mtree = MerkleTree::new(digests_to_words(&VALUES4))?;
     let store = MerkleStore::from(&mtree);
 
     let depth = mtree.depth();
@@ -526,10 +555,11 @@ fn test_constructors() -> Result<(), MerkleError> {
     }
 
     let depth = 32;
-    let smt = SimpleSmt::new(depth)
-        .unwrap()
-        .with_leaves(KEYS4.into_iter().zip(LEAVES4.into_iter()))
-        .unwrap();
+    let smt = SimpleSmt::with_leaves(
+        depth,
+        KEYS4.into_iter().zip(digests_to_words(&VALUES4).into_iter()),
+    )
+    .unwrap();
     let store = MerkleStore::from(&smt);
     let depth = smt.depth();
 
@@ -541,20 +571,20 @@ fn test_constructors() -> Result<(), MerkleError> {
 
     let d = 2;
     let paths = [
-        (0, LEAVES4[0], mtree.get_path(NodeIndex::make(d, 0)).unwrap()),
-        (1, LEAVES4[1], mtree.get_path(NodeIndex::make(d, 1)).unwrap()),
-        (2, LEAVES4[2], mtree.get_path(NodeIndex::make(d, 2)).unwrap()),
-        (3, LEAVES4[3], mtree.get_path(NodeIndex::make(d, 3)).unwrap()),
+        (0, VALUES4[0], mtree.get_path(NodeIndex::make(d, 0)).unwrap()),
+        (1, VALUES4[1], mtree.get_path(NodeIndex::make(d, 1)).unwrap()),
+        (2, VALUES4[2], mtree.get_path(NodeIndex::make(d, 2)).unwrap()),
+        (3, VALUES4[3], mtree.get_path(NodeIndex::make(d, 3)).unwrap()),
     ];
 
     let mut store1 = MerkleStore::default();
     store1.add_merkle_paths(paths.clone())?;
 
     let mut store2 = MerkleStore::default();
-    store2.add_merkle_path(0, LEAVES4[0], mtree.get_path(NodeIndex::make(d, 0))?)?;
-    store2.add_merkle_path(1, LEAVES4[1], mtree.get_path(NodeIndex::make(d, 1))?)?;
-    store2.add_merkle_path(2, LEAVES4[2], mtree.get_path(NodeIndex::make(d, 2))?)?;
-    store2.add_merkle_path(3, LEAVES4[3], mtree.get_path(NodeIndex::make(d, 3))?)?;
+    store2.add_merkle_path(0, VALUES4[0], mtree.get_path(NodeIndex::make(d, 0))?)?;
+    store2.add_merkle_path(1, VALUES4[1], mtree.get_path(NodeIndex::make(d, 1))?)?;
+    store2.add_merkle_path(2, VALUES4[2], mtree.get_path(NodeIndex::make(d, 2))?)?;
+    store2.add_merkle_path(3, VALUES4[3], mtree.get_path(NodeIndex::make(d, 3))?)?;
     let set = MerklePathSet::new(d).with_paths(paths).unwrap();
 
     for key in [0, 1, 2, 3] {
@@ -575,11 +605,11 @@ fn node_path_should_be_truncated_by_midtier_insert() {
     let key = 0b11010010_11001100_11001100_11001100_11001100_11001100_11001100_11001100_u64;
 
     let mut store = MerkleStore::new();
-    let root: Word = EmptySubtreeRoots::empty_hashes(64)[0].into();
+    let root: RpoDigest = EmptySubtreeRoots::empty_hashes(64)[0];
 
     // insert first node - works as expected
     let depth = 64;
-    let node = [Felt::new(key); WORD_SIZE];
+    let node = RpoDigest::from([Felt::new(key); WORD_SIZE]);
     let index = NodeIndex::new(depth, key).unwrap();
     let root = store.set_node(root, index, node).unwrap().root;
     let result = store.get_node(root, index).unwrap();
@@ -592,7 +622,7 @@ fn node_path_should_be_truncated_by_midtier_insert() {
     let key = key ^ (1 << 63);
     let key = key >> 8;
     let depth = 56;
-    let node = [Felt::new(key); WORD_SIZE];
+    let node = RpoDigest::from([Felt::new(key); WORD_SIZE]);
     let index = NodeIndex::new(depth, key).unwrap();
     let root = store.set_node(root, index, node).unwrap().root;
     let result = store.get_node(root, index).unwrap();
@@ -611,13 +641,13 @@ fn node_path_should_be_truncated_by_midtier_insert() {
 #[test]
 fn get_leaf_depth_works_depth_64() {
     let mut store = MerkleStore::new();
-    let mut root: Word = EmptySubtreeRoots::empty_hashes(64)[0].into();
+    let mut root: RpoDigest = EmptySubtreeRoots::empty_hashes(64)[0];
     let key = u64::MAX;
 
     // this will create a rainbow tree and test all opening to depth 64
     for d in 0..64 {
         let k = key & (u64::MAX >> d);
-        let node = [Felt::new(k); WORD_SIZE];
+        let node = RpoDigest::from([Felt::new(k); WORD_SIZE]);
         let index = NodeIndex::new(64, k).unwrap();
 
         // assert the leaf doesn't exist before the insert. the returned depth should always
@@ -634,14 +664,14 @@ fn get_leaf_depth_works_depth_64() {
 #[test]
 fn get_leaf_depth_works_with_incremental_depth() {
     let mut store = MerkleStore::new();
-    let mut root: Word = EmptySubtreeRoots::empty_hashes(64)[0].into();
+    let mut root: RpoDigest = EmptySubtreeRoots::empty_hashes(64)[0];
 
     // insert some path to the left of the root and assert it
     let key = 0b01001011_10110110_00001101_01110100_00111011_10101101_00000100_01000001_u64;
     assert_eq!(0, store.get_leaf_depth(root, 64, key).unwrap());
     let depth = 64;
     let index = NodeIndex::new(depth, key).unwrap();
-    let node = [Felt::new(key); WORD_SIZE];
+    let node = RpoDigest::from([Felt::new(key); WORD_SIZE]);
     root = store.set_node(root, index, node).unwrap().root;
     assert_eq!(depth, store.get_leaf_depth(root, 64, key).unwrap());
 
@@ -650,7 +680,7 @@ fn get_leaf_depth_works_with_incremental_depth() {
     assert_eq!(1, store.get_leaf_depth(root, 64, key).unwrap());
     let depth = 16;
     let index = NodeIndex::new(depth, key >> (64 - depth)).unwrap();
-    let node = [Felt::new(key); WORD_SIZE];
+    let node = RpoDigest::from([Felt::new(key); WORD_SIZE]);
     root = store.set_node(root, index, node).unwrap().root;
     assert_eq!(depth, store.get_leaf_depth(root, 64, key).unwrap());
 
@@ -658,7 +688,7 @@ fn get_leaf_depth_works_with_incremental_depth() {
     let key = 0b11001011_10110111_00000000_00000000_00000000_00000000_00000000_00000000_u64;
     assert_eq!(16, store.get_leaf_depth(root, 64, key).unwrap());
     let index = NodeIndex::new(depth, key >> (64 - depth)).unwrap();
-    let node = [Felt::new(key); WORD_SIZE];
+    let node = RpoDigest::from([Felt::new(key); WORD_SIZE]);
     root = store.set_node(root, index, node).unwrap().root;
     assert_eq!(depth, store.get_leaf_depth(root, 64, key).unwrap());
 
@@ -667,7 +697,7 @@ fn get_leaf_depth_works_with_incremental_depth() {
     assert_eq!(15, store.get_leaf_depth(root, 64, key).unwrap());
     let depth = 17;
     let index = NodeIndex::new(depth, key >> (64 - depth)).unwrap();
-    let node = [Felt::new(key); WORD_SIZE];
+    let node = RpoDigest::from([Felt::new(key); WORD_SIZE]);
     root = store.set_node(root, index, node).unwrap().root;
     assert_eq!(depth, store.get_leaf_depth(root, 64, key).unwrap());
 }
@@ -675,7 +705,7 @@ fn get_leaf_depth_works_with_incremental_depth() {
 #[test]
 fn get_leaf_depth_works_with_depth_8() {
     let mut store = MerkleStore::new();
-    let mut root: Word = EmptySubtreeRoots::empty_hashes(8)[0].into();
+    let mut root: RpoDigest = EmptySubtreeRoots::empty_hashes(8)[0];
 
     // insert some random, 8 depth keys. `a` diverges from the first bit
     let a = 0b01101001_u64;
@@ -685,7 +715,7 @@ fn get_leaf_depth_works_with_depth_8() {
 
     for k in [a, b, c, d] {
         let index = NodeIndex::new(8, k).unwrap();
-        let node = [Felt::new(k); WORD_SIZE];
+        let node = RpoDigest::from([Felt::new(k); WORD_SIZE]);
         root = store.set_node(root, index, node).unwrap().root;
     }
 
@@ -718,12 +748,120 @@ fn get_leaf_depth_works_with_depth_8() {
     assert_eq!(Err(MerkleError::DepthTooBig(9)), store.get_leaf_depth(root, 8, a));
 }
 
+// SUBSET EXTRACTION
+// ================================================================================================
+
+#[test]
+fn mstore_subset() {
+    // add a Merkle tree of depth 3 to the store
+    let mtree = MerkleTree::new(digests_to_words(&VALUES8)).unwrap();
+    let mut store = MerkleStore::default();
+    let empty_store_num_nodes = store.nodes.len();
+    store.extend(mtree.inner_nodes());
+
+    // build 3 subtrees contained within the above Merkle tree; note that subtree2 is a subset
+    // of subtree1
+    let subtree1 = MerkleTree::new(digests_to_words(&VALUES8[..4])).unwrap();
+    let subtree2 = MerkleTree::new(digests_to_words(&VALUES8[2..4])).unwrap();
+    let subtree3 = MerkleTree::new(digests_to_words(&VALUES8[6..])).unwrap();
+
+    // --- extract all 3 subtrees ---------------------------------------------
+
+    let substore = store.subset([subtree1.root(), subtree2.root(), subtree3.root()].iter());
+
+    // number of nodes should increase by 4: 3 nodes form subtree1 and 1 node from subtree3
+    assert_eq!(substore.nodes.len(), empty_store_num_nodes + 4);
+
+    // make sure paths that all subtrees are in the store
+    check_mstore_subtree(&substore, &subtree1);
+    check_mstore_subtree(&substore, &subtree2);
+    check_mstore_subtree(&substore, &subtree3);
+
+    // --- extract subtrees 1 and 3 -------------------------------------------
+    // this should give the same result as above as subtree2 is nested withing subtree1
+
+    let substore = store.subset([subtree1.root(), subtree3.root()].iter());
+
+    // number of nodes should increase by 4: 3 nodes form subtree1 and 1 node from subtree3
+    assert_eq!(substore.nodes.len(), empty_store_num_nodes + 4);
+
+    // make sure paths that all subtrees are in the store
+    check_mstore_subtree(&substore, &subtree1);
+    check_mstore_subtree(&substore, &subtree2);
+    check_mstore_subtree(&substore, &subtree3);
+}
+
+fn check_mstore_subtree(store: &MerkleStore, subtree: &MerkleTree) {
+    for (i, value) in subtree.leaves() {
+        let index = NodeIndex::new(subtree.depth(), i).unwrap();
+        let path1 = store.get_path(subtree.root(), index).unwrap();
+        assert_eq!(*path1.value, *value);
+
+        let path2 = subtree.get_path(index).unwrap();
+        assert_eq!(path1.path, path2);
+    }
+}
+
+// SERIALIZATION
+// ================================================================================================
+
 #[cfg(feature = "std")]
 #[test]
 fn test_serialization() -> Result<(), Box<dyn Error>> {
-    let mtree = MerkleTree::new(LEAVES4.to_vec())?;
+    let mtree = MerkleTree::new(digests_to_words(&VALUES4))?;
     let store = MerkleStore::from(&mtree);
     let decoded = MerkleStore::read_from_bytes(&store.to_bytes()).expect("deserialization failed");
     assert_eq!(store, decoded);
     Ok(())
 }
+
+// MERKLE RECORDER
+// ================================================================================================
+#[test]
+fn test_recorder() {
+    // instantiate recorder from MerkleTree and SimpleSmt
+    let mtree = MerkleTree::new(digests_to_words(&VALUES4)).unwrap();
+    let smtree = SimpleSmt::with_leaves(
+        64,
+        KEYS8.into_iter().zip(VALUES8.into_iter().map(|x| x.into()).rev()),
+    )
+    .unwrap();
+
+    let mut recorder: RecordingMerkleStore =
+        mtree.inner_nodes().chain(smtree.inner_nodes()).collect();
+
+    // get nodes from both trees and make sure they are correct
+    let index_0 = NodeIndex::new(mtree.depth(), 0).unwrap();
+    let node = recorder.get_node(mtree.root(), index_0).unwrap();
+    assert_eq!(node, mtree.get_node(index_0).unwrap());
+
+    let index_1 = NodeIndex::new(smtree.depth(), 1).unwrap();
+    let node = recorder.get_node(smtree.root(), index_1).unwrap();
+    assert_eq!(node, smtree.get_node(index_1).unwrap());
+
+    // insert a value and assert that when we request it next time it is accurate
+    let new_value = [ZERO, ZERO, ONE, ONE].into();
+    let index_2 = NodeIndex::new(smtree.depth(), 2).unwrap();
+    let root = recorder.set_node(smtree.root(), index_2, new_value).unwrap().root;
+    assert_eq!(recorder.get_node(root, index_2).unwrap(), new_value);
+
+    // construct the proof
+    let rec_map = recorder.into_inner();
+    let proof = rec_map.into_proof();
+    let merkle_store: MerkleStore = proof.into();
+
+    // make sure the proof contains all nodes from both trees
+    let node = merkle_store.get_node(mtree.root(), index_0).unwrap();
+    assert_eq!(node, mtree.get_node(index_0).unwrap());
+
+    let node = merkle_store.get_node(smtree.root(), index_1).unwrap();
+    assert_eq!(node, smtree.get_node(index_1).unwrap());
+
+    let node = merkle_store.get_node(smtree.root(), index_2).unwrap();
+    assert_eq!(node, smtree.get_leaf(index_2.value()).unwrap().into());
+
+    // assert that is doesnt contain nodes that were not recorded
+    let not_recorded_index = NodeIndex::new(smtree.depth(), 4).unwrap();
+    assert!(merkle_store.get_node(smtree.root(), not_recorded_index).is_err());
+    assert!(smtree.get_node(not_recorded_index).is_ok());
+}

src/merkle/tiered_smt/mod.rs

@@ -0,0 +1,485 @@
+use super::{
+    BTreeMap, BTreeSet, EmptySubtreeRoots, Felt, InnerNodeInfo, MerkleError, MerklePath, NodeIndex,
+    Rpo256, RpoDigest, StarkField, Vec, Word, ZERO,
+};
+use core::cmp;
+
+#[cfg(test)]
+mod tests;
+
+// TIERED SPARSE MERKLE TREE
+// ================================================================================================
+
+/// Tiered (compacted) Sparse Merkle tree mapping 256-bit keys to 256-bit values. Both keys and
+/// values are represented by 4 field elements.
+///
+/// Leaves in the tree can exist only on specific depths called "tiers". These depths are: 16, 32,
+/// 48, and 64. Initially, when a tree is empty, it is equivalent to an empty Sparse Merkle tree
+/// of depth 64 (i.e., leaves at depth 64 are set to [ZERO; 4]). As non-empty values are inserted
+/// into the tree they are added to the first available tier.
+///
+/// For example, when the first key-value is inserted, it will be stored in a node at depth 16
+/// such that the first 16 bits of the key determine the position of the node at depth 16. If
+/// another value with a key sharing the same 16-bit prefix is inserted, both values move into
+/// the next tier (depth 32). This process is repeated until values end up at tier 64. If multiple
+/// values have keys with a common 64-bit prefix, such key-value pairs are stored in a sorted list
+/// at the last tier (depth = 64).
+///
+/// To differentiate between internal and leaf nodes, node values are computed as follows:
+/// - Internal nodes: hash(left_child, right_child).
+/// - Leaf node at depths 16, 32, or 64: hash(rem_key, value, domain=depth).
+/// - Leaf node at depth 64: hash([rem_key_0, value_0, ..., rem_key_n, value_n, domain=64]).
+///
+/// Where rem_key is computed by replacing d most significant bits of the key with zeros where d
+/// is depth (i.e., for a leaf at depth 16, we replace 16 most significant bits of the key with 0).
+#[derive(Debug, Clone, PartialEq, Eq)]
+pub struct TieredSmt {
+    root: RpoDigest,
+    nodes: BTreeMap<NodeIndex, RpoDigest>,
+    upper_leaves: BTreeMap<NodeIndex, RpoDigest>, // node_index |-> key map
+    bottom_leaves: BTreeMap<u64, BottomLeaf>,     // leaves of depth 64
+    values: BTreeMap<RpoDigest, Word>,
+}
+
+impl TieredSmt {
+    // CONSTANTS
+    // --------------------------------------------------------------------------------------------
+
+    /// The number of levels between tiers.
+    const TIER_SIZE: u8 = 16;
+
+    /// Depths at which leaves can exist in a tiered SMT.
+    const TIER_DEPTHS: [u8; 4] = [16, 32, 48, 64];
+
+    /// Maximum node depth. This is also the bottom tier of the tree.
+    const MAX_DEPTH: u8 = 64;
+
+    /// Value of an empty leaf.
+    pub const EMPTY_VALUE: Word = super::empty_roots::EMPTY_WORD;
+
+    // CONSTRUCTORS
+    // --------------------------------------------------------------------------------------------
+
+    /// Returns a new [TieredSmt] instantiated with the specified key-value pairs.
+    ///
+    /// # Errors
+    /// Returns an error if the provided entries contain multiple values for the same key.
+    pub fn with_leaves<R, I>(entries: R) -> Result<Self, MerkleError>
+    where
+        R: IntoIterator<IntoIter = I>,
+        I: Iterator<Item = (RpoDigest, Word)> + ExactSizeIterator,
+    {
+        // create an empty tree
+        let mut tree = Self::default();
+
+        // append leaves to the tree returning an error if a duplicate entry for the same key
+        // is found
+        let mut empty_entries = BTreeSet::new();
+        for (key, value) in entries {
+            let old_value = tree.insert(key, value);
+            if old_value != Self::EMPTY_VALUE || empty_entries.contains(&key) {
+                return Err(MerkleError::DuplicateValuesForKey(key));
+            }
+            // if we've processed an empty entry, add the key to the set of empty entry keys, and
+            // if this key was already in the set, return an error
+            if value == Self::EMPTY_VALUE && !empty_entries.insert(key) {
+                return Err(MerkleError::DuplicateValuesForKey(key));
+            }
+        }
+        Ok(tree)
+    }
+
+    // PUBLIC ACCESSORS
+    // --------------------------------------------------------------------------------------------
+
+    /// Returns the root of this Merkle tree.
+    pub const fn root(&self) -> RpoDigest {
+        self.root
+    }
+
+    /// Returns a node at the specified index.
+    ///
+    /// # Errors
+    /// Returns an error if:
+    /// - The specified index depth is 0 or greater than 64.
+    /// - The node with the specified index does not exists in the Merkle tree. This is possible
+    ///   when a leaf node with the same index prefix exists at a tier higher than the requested
+    ///   node.
+    pub fn get_node(&self, index: NodeIndex) -> Result<RpoDigest, MerkleError> {
+        self.validate_node_access(index)?;
+        Ok(self.get_node_unchecked(&index))
+    }
+
+    /// Returns a Merkle path from the node at the specified index to the root.
+    ///
+    /// The node itself is not included in the path.
+    ///
+    /// # Errors
+    /// Returns an error if:
+    /// - The specified index depth is 0 or greater than 64.
+    /// - The node with the specified index does not exists in the Merkle tree. This is possible
+    ///   when a leaf node with the same index prefix exists at a tier higher than the node to
+    ///   which the path is requested.
+    pub fn get_path(&self, mut index: NodeIndex) -> Result<MerklePath, MerkleError> {
+        self.validate_node_access(index)?;
+
+        let mut path = Vec::with_capacity(index.depth() as usize);
+        for _ in 0..index.depth() {
+            let node = self.get_node_unchecked(&index.sibling());
+            path.push(node);
+            index.move_up();
+        }
+
+        Ok(path.into())
+    }
+
+    /// Returns the value associated with the specified key.
+    ///
+    /// If nothing was inserted into this tree for the specified key, [ZERO; 4] is returned.
+    pub fn get_value(&self, key: RpoDigest) -> Word {
+        match self.values.get(&key) {
+            Some(value) => *value,
+            None => Self::EMPTY_VALUE,
+        }
+    }
+
+    // STATE MUTATORS
+    // --------------------------------------------------------------------------------------------
+
+    /// Inserts the provided value into the tree under the specified key and returns the value
+    /// previously stored under this key.
+    ///
+    /// If the value for the specified key was not previously set, [ZERO; 4] is returned.
+    pub fn insert(&mut self, key: RpoDigest, value: Word) -> Word {
+        // insert the value into the key-value map, and if nothing has changed, return
+        let old_value = self.values.insert(key, value).unwrap_or(Self::EMPTY_VALUE);
+        if old_value == value {
+            return old_value;
+        }
+
+        // determine the index for the value node; this index could have 3 different meanings:
+        // - it points to a root of an empty subtree (excluding depth = 64); in this case, we can
+        //   replace the node with the value node immediately.
+        // - it points to a node at the bottom tier (i.e., depth = 64); in this case, we need to
+        //   process bottom-tier insertion which will be handled by insert_node().
+        // - it points to a leaf node; this node could be a node with the same key or a different
+        //   key with a common prefix; in the latter case, we'll need to move the leaf to a lower
+        //   tier; for this scenario the `leaf_key` will contain the key of the leaf node
+        let (mut index, leaf_key) = self.get_insert_location(&key);
+
+        // if the returned index points to a leaf, and this leaf is for a different key, we need
+        // to move the leaf to a lower tier
+        if let Some(other_key) = leaf_key {
+            if other_key != key {
+                // determine how far down the tree should we move the existing leaf
+                let common_prefix_len = get_common_prefix_tier(&key, &other_key);
+                let depth = cmp::min(common_prefix_len + Self::TIER_SIZE, Self::MAX_DEPTH);
+
+                // move the leaf to the new location; this requires first removing the existing
+                // index, re-computing node value, and inserting the node at a new location
+                let other_index = key_to_index(&other_key, depth);
+                let other_value = *self.values.get(&other_key).expect("no value for other key");
+                self.upper_leaves.remove(&index).expect("other node key not in map");
+                self.insert_node(other_index, other_key, other_value);
+
+                // the new leaf also needs to move down to the same tier
+                index = key_to_index(&key, depth);
+            }
+        }
+
+        // insert the node and return the old value
+        self.insert_node(index, key, value);
+        old_value
+    }
+
+    // ITERATORS
+    // --------------------------------------------------------------------------------------------
+
+    /// Returns an iterator over all inner nodes of this [TieredSmt] (i.e., nodes not at depths 16
+    /// 32, 48, or 64).
+    ///
+    /// The iterator order is unspecified.
+    pub fn inner_nodes(&self) -> impl Iterator<Item = InnerNodeInfo> + '_ {
+        self.nodes.iter().filter_map(|(index, node)| {
+            if is_inner_node(index) {
+                Some(InnerNodeInfo {
+                    value: *node,
+                    left: self.get_node_unchecked(&index.left_child()),
+                    right: self.get_node_unchecked(&index.right_child()),
+                })
+            } else {
+                None
+            }
+        })
+    }
+
+    /// Returns an iterator over upper leaves (i.e., depth = 16, 32, or 48) for this [TieredSmt].
+    ///
+    /// Each yielded item is a (node, key, value) tuple where key is a full un-truncated key (i.e.,
+    /// with key[3] element unmodified).
+    ///
+    /// The iterator order is unspecified.
+    pub fn upper_leaves(&self) -> impl Iterator<Item = (RpoDigest, RpoDigest, Word)> + '_ {
+        self.upper_leaves.iter().map(|(index, key)| {
+            let node = self.get_node_unchecked(index);
+            let value = self.get_value(*key);
+            (node, *key, value)
+        })
+    }
+
+    /// Returns an iterator over bottom leaves (i.e., depth = 64) of this [TieredSmt].
+    ///
+    /// Each yielded item consists of the hash of the leaf and its contents, where contents is
+    /// a vector containing key-value pairs of entries storied in this leaf. Note that keys are
+    /// un-truncated keys (i.e., with key[3] element unmodified).
+    ///
+    /// The iterator order is unspecified.
+    pub fn bottom_leaves(&self) -> impl Iterator<Item = (RpoDigest, Vec<(RpoDigest, Word)>)> + '_ {
+        self.bottom_leaves.values().map(|leaf| (leaf.hash(), leaf.contents()))
+    }
+
+    // HELPER METHODS
+    // --------------------------------------------------------------------------------------------
+
+    /// Checks if the specified index is valid in the context of this Merkle tree.
+    ///
+    /// # Errors
+    /// Returns an error if:
+    /// - The specified index depth is 0 or greater than 64.
+    /// - The node for the specified index does not exists in the Merkle tree. This is possible
+    ///   when an ancestors of the specified index is a leaf node.
+    fn validate_node_access(&self, index: NodeIndex) -> Result<(), MerkleError> {
+        if index.is_root() {
+            return Err(MerkleError::DepthTooSmall(index.depth()));
+        } else if index.depth() > Self::MAX_DEPTH {
+            return Err(MerkleError::DepthTooBig(index.depth() as u64));
+        } else {
+            // make sure that there are no leaf nodes in the ancestors of the index; since leaf
+            // nodes can live at specific depth, we just need to check these depths.
+            let tier = get_index_tier(&index);
+            let mut tier_index = index;
+            for &depth in Self::TIER_DEPTHS[..tier].iter().rev() {
+                tier_index.move_up_to(depth);
+                if self.upper_leaves.contains_key(&tier_index) {
+                    return Err(MerkleError::NodeNotInSet(index));
+                }
+            }
+        }
+
+        Ok(())
+    }
+
+    /// Returns a node at the specified index. If the node does not exist at this index, a root
+    /// for an empty subtree at the index's depth is returned.
+    ///
+    /// Unlike [TieredSmt::get_node()] this does not perform any checks to verify that the returned
+    /// node is valid in the context of this tree.
+    fn get_node_unchecked(&self, index: &NodeIndex) -> RpoDigest {
+        match self.nodes.get(index) {
+            Some(node) => *node,
+            None => EmptySubtreeRoots::empty_hashes(Self::MAX_DEPTH)[index.depth() as usize],
+        }
+    }
+
+    /// Returns an index at which a node for the specified key should be inserted. If a leaf node
+    /// already exists at that index, returns the key associated with that leaf node.
+    ///
+    /// In case the index falls into the bottom tier (depth = 64), leaf node key is not returned
+    /// as the bottom tier may contain multiple key-value pairs in the same leaf.
+    fn get_insert_location(&self, key: &RpoDigest) -> (NodeIndex, Option<RpoDigest>) {
+        // traverse the tree from the root down checking nodes at tiers 16, 32, and 48. Return if
+        // a node at any of the tiers is either a leaf or a root of an empty subtree.
+        let mse = Word::from(key)[3].as_int();
+        for depth in (Self::TIER_DEPTHS[0]..Self::MAX_DEPTH).step_by(Self::TIER_SIZE as usize) {
+            let index = NodeIndex::new_unchecked(depth, mse >> (Self::MAX_DEPTH - depth));
+            if let Some(leaf_key) = self.upper_leaves.get(&index) {
+                return (index, Some(*leaf_key));
+            } else if !self.nodes.contains_key(&index) {
+                return (index, None);
+            }
+        }
+
+        // if we got here, that means all of the nodes checked so far are internal nodes, and
+        // the new node would need to be inserted in the bottom tier.
+        let index = NodeIndex::new_unchecked(Self::MAX_DEPTH, mse);
+        (index, None)
+    }
+
+    /// Inserts the provided key-value pair at the specified index and updates the root of this
+    /// Merkle tree by recomputing the path to the root.
+    fn insert_node(&mut self, mut index: NodeIndex, key: RpoDigest, value: Word) {
+        let depth = index.depth();
+
+        // insert the key into index-key map and compute the new value of the node
+        let mut node = if index.depth() == Self::MAX_DEPTH {
+            // for the bottom tier, we add the key-value pair to the existing leaf, or create a
+            // new leaf with this key-value pair
+            self.bottom_leaves
+                .entry(index.value())
+                .and_modify(|leaves| leaves.add_value(key, value))
+                .or_insert(BottomLeaf::new(key, value))
+                .hash()
+        } else {
+            // for the upper tiers, we just update the index-key map and compute the value of the
+            // node
+            self.upper_leaves.insert(index, key);
+            // the node value is computed as: hash(remaining_key || value, domain = depth)
+            let remaining_path = get_remaining_path(key, depth.into());
+            Rpo256::merge_in_domain(&[remaining_path, value.into()], depth.into())
+        };
+
+        // insert the node and update the path from the node to the root
+        for _ in 0..index.depth() {
+            self.nodes.insert(index, node);
+            let sibling = self.get_node_unchecked(&index.sibling());
+            node = Rpo256::merge(&index.build_node(node, sibling));
+            index.move_up();
+        }
+
+        // update the root
+        self.nodes.insert(NodeIndex::root(), node);
+        self.root = node;
+    }
+}
+
+impl Default for TieredSmt {
+    fn default() -> Self {
+        Self {
+            root: EmptySubtreeRoots::empty_hashes(Self::MAX_DEPTH)[0],
+            nodes: BTreeMap::new(),
+            upper_leaves: BTreeMap::new(),
+            bottom_leaves: BTreeMap::new(),
+            values: BTreeMap::new(),
+        }
+    }
+}
+
+// HELPER FUNCTIONS
+// ================================================================================================
+
+/// Returns the remaining path for the specified key at the specified depth.
+///
+/// Remaining path is computed by setting n most significant bits of the key to zeros, where n is
+/// the specified depth.
+fn get_remaining_path(key: RpoDigest, depth: u32) -> RpoDigest {
+    let mut key = Word::from(key);
+    key[3] = if depth == 64 {
+        ZERO
+    } else {
+        // remove `depth` bits from the most significant key element
+        ((key[3].as_int() << depth) >> depth).into()
+    };
+    key.into()
+}
+
+/// Returns index for the specified key inserted at the specified depth.
+///
+/// The value for the key is computed by taking n most significant bits from the most significant
+/// element of the key, where n is the specified depth.
+fn key_to_index(key: &RpoDigest, depth: u8) -> NodeIndex {
+    let mse = Word::from(key)[3].as_int();
+    let value = match depth {
+        16 | 32 | 48 | 64 => mse >> ((TieredSmt::MAX_DEPTH - depth) as u32),
+        _ => unreachable!("invalid depth: {depth}"),
+    };
+    NodeIndex::new_unchecked(depth, value)
+}
+
+/// Returns tiered common prefix length between the most significant elements of the provided keys.
+///
+/// Specifically:
+/// - returns 64 if the most significant elements are equal.
+/// - returns 48 if the common prefix is between 48 and 63 bits.
+/// - returns 32 if the common prefix is between 32 and 47 bits.
+/// - returns 16 if the common prefix is between 16 and 31 bits.
+/// - returns 0 if the common prefix is fewer than 16 bits.
+fn get_common_prefix_tier(key1: &RpoDigest, key2: &RpoDigest) -> u8 {
+    let e1 = Word::from(key1)[3].as_int();
+    let e2 = Word::from(key2)[3].as_int();
+    let ex = (e1 ^ e2).leading_zeros() as u8;
+    (ex / 16) * 16
+}
+
+/// Returns a tier for the specified index.
+///
+/// The tiers are defined as follows:
+/// - Tier 0: depth 0 through 16 (inclusive).
+/// - Tier 1: depth 17 through 32 (inclusive).
+/// - Tier 2: depth 33 through 48 (inclusive).
+/// - Tier 3: depth 49 through 64 (inclusive).
+const fn get_index_tier(index: &NodeIndex) -> usize {
+    debug_assert!(index.depth() <= TieredSmt::MAX_DEPTH, "invalid depth");
+    match index.depth() {
+        0..=16 => 0,
+        17..=32 => 1,
+        33..=48 => 2,
+        _ => 3,
+    }
+}
+
+/// Returns true if the specified index is an index for an inner node (i.e., the depth is not 16,
+/// 32, 48, or 64).
+const fn is_inner_node(index: &NodeIndex) -> bool {
+    !matches!(index.depth(), 16 | 32 | 48 | 64)
+}
+
+// BOTTOM LEAF
+// ================================================================================================
+
+/// Stores contents of the bottom leaf (i.e., leaf at depth = 64) in a [TieredSmt].
+///
+/// Bottom leaf can contain one or more key-value pairs all sharing the same 64-bit key prefix.
+/// The values are sorted by key to make sure the structure of the leaf is independent of the
+/// insertion order. This guarantees that a leaf with the same set of key-value pairs always has
+/// the same hash value.
+#[derive(Debug, Clone, PartialEq, Eq)]
+struct BottomLeaf {
+    prefix: u64,
+    values: BTreeMap<[u64; 4], Word>,
+}
+
+impl BottomLeaf {
+    /// Returns a new [BottomLeaf] with a single key-value pair added.
+    pub fn new(key: RpoDigest, value: Word) -> Self {
+        let prefix = Word::from(key)[3].as_int();
+        let mut values = BTreeMap::new();
+        let key = get_remaining_path(key, TieredSmt::MAX_DEPTH as u32);
+        values.insert(key.into(), value);
+        Self { prefix, values }
+    }
+
+    /// Adds a new key-value pair to this leaf.
+    pub fn add_value(&mut self, key: RpoDigest, value: Word) {
+        let key = get_remaining_path(key, TieredSmt::MAX_DEPTH as u32);
+        self.values.insert(key.into(), value);
+    }
+
+    /// Computes a hash of this leaf.
+    pub fn hash(&self) -> RpoDigest {
+        let mut elements = Vec::with_capacity(self.values.len() * 2);
+        for (key, val) in self.values.iter() {
+            key.iter().for_each(|&v| elements.push(Felt::new(v)));
+            elements.extend_from_slice(val.as_slice());
+        }
+        // TODO: hash in domain
+        Rpo256::hash_elements(&elements)
+    }
+
+    /// Returns contents of this leaf as a vector of (key, value) pairs.
+    ///
+    /// The keys are returned in their un-truncated form.
+    pub fn contents(&self) -> Vec<(RpoDigest, Word)> {
+        self.values
+            .iter()
+            .map(|(key, val)| {
+                let key = RpoDigest::from([
+                    Felt::new(key[0]),
+                    Felt::new(key[1]),
+                    Felt::new(key[2]),
+                    Felt::new(self.prefix),
+                ]);
+                (key, *val)
+            })
+            .collect()
+    }
+}

src/merkle/tiered_smt/tests.rs

@@ -0,0 +1,441 @@
+use super::{
+    super::{super::ONE, Felt, MerkleStore, WORD_SIZE, ZERO},
+    get_remaining_path, EmptySubtreeRoots, InnerNodeInfo, NodeIndex, Rpo256, RpoDigest, TieredSmt,
+    Vec, Word,
+};
+
+#[test]
+fn tsmt_insert_one() {
+    let mut smt = TieredSmt::default();
+    let mut store = MerkleStore::default();
+
+    let raw = 0b_01101001_01101100_00011111_11111111_10010110_10010011_11100000_00000000_u64;
+    let key = RpoDigest::from([ONE, ONE, ONE, Felt::new(raw)]);
+    let value = [ONE; WORD_SIZE];
+
+    // since the tree is empty, the first node will be inserted at depth 16 and the index will be
+    // 16 most significant bits of the key
+    let index = NodeIndex::make(16, raw >> 48);
+    let leaf_node = build_leaf_node(key, value, 16);
+    let tree_root = store.set_node(smt.root(), index, leaf_node).unwrap().root;
+
+    smt.insert(key, value);
+
+    assert_eq!(smt.root(), tree_root);
+
+    // make sure the value was inserted, and the node is at the expected index
+    assert_eq!(smt.get_value(key), value);
+    assert_eq!(smt.get_node(index).unwrap(), leaf_node);
+
+    // make sure the paths we get from the store and the tree match
+    let expected_path = store.get_path(tree_root, index).unwrap();
+    assert_eq!(smt.get_path(index).unwrap(), expected_path.path);
+
+    // make sure inner nodes match
+    let expected_nodes = get_non_empty_nodes(&store);
+    let actual_nodes = smt.inner_nodes().collect::<Vec<_>>();
+    assert_eq!(actual_nodes.len(), expected_nodes.len());
+    actual_nodes.iter().for_each(|node| assert!(expected_nodes.contains(node)));
+
+    // make sure leaves are returned correctly
+    let mut leaves = smt.upper_leaves();
+    assert_eq!(leaves.next(), Some((leaf_node, key, value)));
+    assert_eq!(leaves.next(), None);
+}
+
+#[test]
+fn tsmt_insert_two_16() {
+    let mut smt = TieredSmt::default();
+    let mut store = MerkleStore::default();
+
+    // --- insert the first value ---------------------------------------------
+    let raw_a = 0b_10101010_10101010_00011111_11111111_10010110_10010011_11100000_00000000_u64;
+    let key_a = RpoDigest::from([ONE, ONE, ONE, Felt::new(raw_a)]);
+    let val_a = [ONE; WORD_SIZE];
+    smt.insert(key_a, val_a);
+
+    // --- insert the second value --------------------------------------------
+    // the key for this value has the same 16-bit prefix as the key for the first value,
+    // thus, on insertions, both values should be pushed to depth 32 tier
+    let raw_b = 0b_10101010_10101010_10011111_11111111_10010110_10010011_11100000_00000000_u64;
+    let key_b = RpoDigest::from([ONE, ONE, ONE, Felt::new(raw_b)]);
+    let val_b = [Felt::new(2); WORD_SIZE];
+    smt.insert(key_b, val_b);
+
+    // --- build Merkle store with equivalent data ----------------------------
+    let mut tree_root = get_init_root();
+    let index_a = NodeIndex::make(32, raw_a >> 32);
+    let leaf_node_a = build_leaf_node(key_a, val_a, 32);
+    tree_root = store.set_node(tree_root, index_a, leaf_node_a).unwrap().root;
+
+    let index_b = NodeIndex::make(32, raw_b >> 32);
+    let leaf_node_b = build_leaf_node(key_b, val_b, 32);
+    tree_root = store.set_node(tree_root, index_b, leaf_node_b).unwrap().root;
+
+    // --- verify that data is consistent between store and tree --------------
+
+    assert_eq!(smt.root(), tree_root);
+
+    assert_eq!(smt.get_value(key_a), val_a);
+    assert_eq!(smt.get_node(index_a).unwrap(), leaf_node_a);
+    let expected_path = store.get_path(tree_root, index_a).unwrap().path;
+    assert_eq!(smt.get_path(index_a).unwrap(), expected_path);
+
+    assert_eq!(smt.get_value(key_b), val_b);
+    assert_eq!(smt.get_node(index_b).unwrap(), leaf_node_b);
+    let expected_path = store.get_path(tree_root, index_b).unwrap().path;
+    assert_eq!(smt.get_path(index_b).unwrap(), expected_path);
+
+    // make sure inner nodes match - the store contains more entries because it keeps track of
+    // all prior state - so, we don't check that the number of inner nodes is the same in both
+    let expected_nodes = get_non_empty_nodes(&store);
+    let actual_nodes = smt.inner_nodes().collect::<Vec<_>>();
+    actual_nodes.iter().for_each(|node| assert!(expected_nodes.contains(node)));
+
+    // make sure leaves are returned correctly
+    let mut leaves = smt.upper_leaves();
+    assert_eq!(leaves.next(), Some((leaf_node_a, key_a, val_a)));
+    assert_eq!(leaves.next(), Some((leaf_node_b, key_b, val_b)));
+    assert_eq!(leaves.next(), None);
+}
+
+#[test]
+fn tsmt_insert_two_32() {
+    let mut smt = TieredSmt::default();
+    let mut store = MerkleStore::default();
+
+    // --- insert the first value ---------------------------------------------
+    let raw_a = 0b_10101010_10101010_00011111_11111111_10010110_10010011_11100000_00000000_u64;
+    let key_a = RpoDigest::from([ONE, ONE, ONE, Felt::new(raw_a)]);
+    let val_a = [ONE; WORD_SIZE];
+    smt.insert(key_a, val_a);
+
+    // --- insert the second value --------------------------------------------
+    // the key for this value has the same 32-bit prefix as the key for the first value,
+    // thus, on insertions, both values should be pushed to depth 48 tier
+    let raw_b = 0b_10101010_10101010_00011111_11111111_00010110_10010011_11100000_00000000_u64;
+    let key_b = RpoDigest::from([ONE, ONE, ONE, Felt::new(raw_b)]);
+    let val_b = [Felt::new(2); WORD_SIZE];
+    smt.insert(key_b, val_b);
+
+    // --- build Merkle store with equivalent data ----------------------------
+    let mut tree_root = get_init_root();
+    let index_a = NodeIndex::make(48, raw_a >> 16);
+    let leaf_node_a = build_leaf_node(key_a, val_a, 48);
+    tree_root = store.set_node(tree_root, index_a, leaf_node_a).unwrap().root;
+
+    let index_b = NodeIndex::make(48, raw_b >> 16);
+    let leaf_node_b = build_leaf_node(key_b, val_b, 48);
+    tree_root = store.set_node(tree_root, index_b, leaf_node_b).unwrap().root;
+
+    // --- verify that data is consistent between store and tree --------------
+
+    assert_eq!(smt.root(), tree_root);
+
+    assert_eq!(smt.get_value(key_a), val_a);
+    assert_eq!(smt.get_node(index_a).unwrap(), leaf_node_a);
+    let expected_path = store.get_path(tree_root, index_a).unwrap().path;
+    assert_eq!(smt.get_path(index_a).unwrap(), expected_path);
+
+    assert_eq!(smt.get_value(key_b), val_b);
+    assert_eq!(smt.get_node(index_b).unwrap(), leaf_node_b);
+    let expected_path = store.get_path(tree_root, index_b).unwrap().path;
+    assert_eq!(smt.get_path(index_b).unwrap(), expected_path);
+
+    // make sure inner nodes match - the store contains more entries because it keeps track of
+    // all prior state - so, we don't check that the number of inner nodes is the same in both
+    let expected_nodes = get_non_empty_nodes(&store);
+    let actual_nodes = smt.inner_nodes().collect::<Vec<_>>();
+    actual_nodes.iter().for_each(|node| assert!(expected_nodes.contains(node)));
+}
+
+#[test]
+fn tsmt_insert_three() {
+    let mut smt = TieredSmt::default();
+    let mut store = MerkleStore::default();
+
+    // --- insert the first value ---------------------------------------------
+    let raw_a = 0b_10101010_10101010_00011111_11111111_10010110_10010011_11100000_00000000_u64;
+    let key_a = RpoDigest::from([ONE, ONE, ONE, Felt::new(raw_a)]);
+    let val_a = [ONE; WORD_SIZE];
+    smt.insert(key_a, val_a);
+
+    // --- insert the second value --------------------------------------------
+    // the key for this value has the same 16-bit prefix as the key for the first value,
+    // thus, on insertions, both values should be pushed to depth 32 tier
+    let raw_b = 0b_10101010_10101010_10011111_11111111_10010110_10010011_11100000_00000000_u64;
+    let key_b = RpoDigest::from([ONE, ONE, ONE, Felt::new(raw_b)]);
+    let val_b = [Felt::new(2); WORD_SIZE];
+    smt.insert(key_b, val_b);
+
+    // --- insert the third value ---------------------------------------------
+    // the key for this value has the same 16-bit prefix as the keys for the first two,
+    // values; thus, on insertions, it will be inserted into depth 32 tier, but will not
+    // affect locations of the other two values
+    let raw_c = 0b_10101010_10101010_11011111_11111111_10010110_10010011_11100000_00000000_u64;
+    let key_c = RpoDigest::from([ONE, ONE, ONE, Felt::new(raw_c)]);
+    let val_c = [Felt::new(3); WORD_SIZE];
+    smt.insert(key_c, val_c);
+
+    // --- build Merkle store with equivalent data ----------------------------
+    let mut tree_root = get_init_root();
+    let index_a = NodeIndex::make(32, raw_a >> 32);
+    let leaf_node_a = build_leaf_node(key_a, val_a, 32);
+    tree_root = store.set_node(tree_root, index_a, leaf_node_a).unwrap().root;
+
+    let index_b = NodeIndex::make(32, raw_b >> 32);
+    let leaf_node_b = build_leaf_node(key_b, val_b, 32);
+    tree_root = store.set_node(tree_root, index_b, leaf_node_b).unwrap().root;
+
+    let index_c = NodeIndex::make(32, raw_c >> 32);
+    let leaf_node_c = build_leaf_node(key_c, val_c, 32);
+    tree_root = store.set_node(tree_root, index_c, leaf_node_c).unwrap().root;
+
+    // --- verify that data is consistent between store and tree --------------
+
+    assert_eq!(smt.root(), tree_root);
+
+    assert_eq!(smt.get_value(key_a), val_a);
+    assert_eq!(smt.get_node(index_a).unwrap(), leaf_node_a);
+    let expected_path = store.get_path(tree_root, index_a).unwrap().path;
+    assert_eq!(smt.get_path(index_a).unwrap(), expected_path);
+
+    assert_eq!(smt.get_value(key_b), val_b);
+    assert_eq!(smt.get_node(index_b).unwrap(), leaf_node_b);
+    let expected_path = store.get_path(tree_root, index_b).unwrap().path;
+    assert_eq!(smt.get_path(index_b).unwrap(), expected_path);
+
+    assert_eq!(smt.get_value(key_c), val_c);
+    assert_eq!(smt.get_node(index_c).unwrap(), leaf_node_c);
+    let expected_path = store.get_path(tree_root, index_c).unwrap().path;
+    assert_eq!(smt.get_path(index_c).unwrap(), expected_path);
+
+    // make sure inner nodes match - the store contains more entries because it keeps track of
+    // all prior state - so, we don't check that the number of inner nodes is the same in both
+    let expected_nodes = get_non_empty_nodes(&store);
+    let actual_nodes = smt.inner_nodes().collect::<Vec<_>>();
+    actual_nodes.iter().for_each(|node| assert!(expected_nodes.contains(node)));
+}
+
+#[test]
+fn tsmt_update() {
+    let mut smt = TieredSmt::default();
+    let mut store = MerkleStore::default();
+
+    // --- insert a value into the tree ---------------------------------------
+    let raw = 0b_01101001_01101100_00011111_11111111_10010110_10010011_11100000_00000000_u64;
+    let key = RpoDigest::from([ONE, ONE, ONE, Felt::new(raw)]);
+    let value_a = [ONE; WORD_SIZE];
+    smt.insert(key, value_a);
+
+    // --- update the value ---------------------------------------------------
+    let value_b = [Felt::new(2); WORD_SIZE];
+    smt.insert(key, value_b);
+
+    // --- verify consistency -------------------------------------------------
+    let mut tree_root = get_init_root();
+    let index = NodeIndex::make(16, raw >> 48);
+    let leaf_node = build_leaf_node(key, value_b, 16);
+    tree_root = store.set_node(tree_root, index, leaf_node).unwrap().root;
+
+    assert_eq!(smt.root(), tree_root);
+
+    assert_eq!(smt.get_value(key), value_b);
+    assert_eq!(smt.get_node(index).unwrap(), leaf_node);
+    let expected_path = store.get_path(tree_root, index).unwrap().path;
+    assert_eq!(smt.get_path(index).unwrap(), expected_path);
+
+    // make sure inner nodes match - the store contains more entries because it keeps track of
+    // all prior state - so, we don't check that the number of inner nodes is the same in both
+    let expected_nodes = get_non_empty_nodes(&store);
+    let actual_nodes = smt.inner_nodes().collect::<Vec<_>>();
+    actual_nodes.iter().for_each(|node| assert!(expected_nodes.contains(node)));
+}
+
+// BOTTOM TIER TESTS
+// ================================================================================================
+
+#[test]
+fn tsmt_bottom_tier() {
+    let mut smt = TieredSmt::default();
+    let mut store = MerkleStore::default();
+
+    // common prefix for the keys
+    let prefix = 0b_10101010_10101010_00011111_11111111_10010110_10010011_11100000_00000000_u64;
+
+    // --- insert the first value ---------------------------------------------
+    let key_a = RpoDigest::from([ONE, ONE, ONE, Felt::new(prefix)]);
+    let val_a = [ONE; WORD_SIZE];
+    smt.insert(key_a, val_a);
+
+    // --- insert the second value --------------------------------------------
+    // this key has the same 64-bit prefix and thus both values should end up in the same
+    // node at depth 64
+    let key_b = RpoDigest::from([ZERO, ONE, ONE, Felt::new(prefix)]);
+    let val_b = [Felt::new(2); WORD_SIZE];
+    smt.insert(key_b, val_b);
+
+    // --- build Merkle store with equivalent data ----------------------------
+    let index = NodeIndex::make(64, prefix);
+    // to build bottom leaf we sort by key starting with the least significant element, thus
+    // key_b is smaller than key_a.
+    let leaf_node = build_bottom_leaf_node(&[key_b, key_a], &[val_b, val_a]);
+    let mut tree_root = get_init_root();
+    tree_root = store.set_node(tree_root, index, leaf_node).unwrap().root;
+
+    // --- verify that data is consistent between store and tree --------------
+
+    assert_eq!(smt.root(), tree_root);
+
+    assert_eq!(smt.get_value(key_a), val_a);
+    assert_eq!(smt.get_value(key_b), val_b);
+
+    assert_eq!(smt.get_node(index).unwrap(), leaf_node);
+    let expected_path = store.get_path(tree_root, index).unwrap().path;
+    assert_eq!(smt.get_path(index).unwrap(), expected_path);
+
+    // make sure inner nodes match - the store contains more entries because it keeps track of
+    // all prior state - so, we don't check that the number of inner nodes is the same in both
+    let expected_nodes = get_non_empty_nodes(&store);
+    let actual_nodes = smt.inner_nodes().collect::<Vec<_>>();
+    actual_nodes.iter().for_each(|node| assert!(expected_nodes.contains(node)));
+
+    // make sure leaves are returned correctly
+    let mut leaves = smt.bottom_leaves();
+    assert_eq!(leaves.next(), Some((leaf_node, vec![(key_b, val_b), (key_a, val_a)])));
+    assert_eq!(leaves.next(), None);
+}
+
+#[test]
+fn tsmt_bottom_tier_two() {
+    let mut smt = TieredSmt::default();
+    let mut store = MerkleStore::default();
+
+    // --- insert the first value ---------------------------------------------
+    let raw_a = 0b_10101010_10101010_00011111_11111111_10010110_10010011_11100000_00000000_u64;
+    let key_a = RpoDigest::from([ONE, ONE, ONE, Felt::new(raw_a)]);
+    let val_a = [ONE; WORD_SIZE];
+    smt.insert(key_a, val_a);
+
+    // --- insert the second value --------------------------------------------
+    // the key for this value has the same 48-bit prefix as the key for the first value,
+    // thus, on insertions, both should end up in different nodes at depth 64
+    let raw_b = 0b_10101010_10101010_00011111_11111111_10010110_10010011_01100000_00000000_u64;
+    let key_b = RpoDigest::from([ONE, ONE, ONE, Felt::new(raw_b)]);
+    let val_b = [Felt::new(2); WORD_SIZE];
+    smt.insert(key_b, val_b);
+
+    // --- build Merkle store with equivalent data ----------------------------
+    let mut tree_root = get_init_root();
+    let index_a = NodeIndex::make(64, raw_a);
+    let leaf_node_a = build_bottom_leaf_node(&[key_a], &[val_a]);
+    tree_root = store.set_node(tree_root, index_a, leaf_node_a).unwrap().root;
+
+    let index_b = NodeIndex::make(64, raw_b);
+    let leaf_node_b = build_bottom_leaf_node(&[key_b], &[val_b]);
+    tree_root = store.set_node(tree_root, index_b, leaf_node_b).unwrap().root;
+
+    // --- verify that data is consistent between store and tree --------------
+
+    assert_eq!(smt.root(), tree_root);
+
+    assert_eq!(smt.get_value(key_a), val_a);
+    assert_eq!(smt.get_node(index_a).unwrap(), leaf_node_a);
+    let expected_path = store.get_path(tree_root, index_a).unwrap().path;
+    assert_eq!(smt.get_path(index_a).unwrap(), expected_path);
+
+    assert_eq!(smt.get_value(key_b), val_b);
+    assert_eq!(smt.get_node(index_b).unwrap(), leaf_node_b);
+    let expected_path = store.get_path(tree_root, index_b).unwrap().path;
+    assert_eq!(smt.get_path(index_b).unwrap(), expected_path);
+
+    // make sure inner nodes match - the store contains more entries because it keeps track of
+    // all prior state - so, we don't check that the number of inner nodes is the same in both
+    let expected_nodes = get_non_empty_nodes(&store);
+    let actual_nodes = smt.inner_nodes().collect::<Vec<_>>();
+    actual_nodes.iter().for_each(|node| assert!(expected_nodes.contains(node)));
+
+    // make sure leaves are returned correctly
+    let mut leaves = smt.bottom_leaves();
+    assert_eq!(leaves.next(), Some((leaf_node_b, vec![(key_b, val_b)])));
+    assert_eq!(leaves.next(), Some((leaf_node_a, vec![(key_a, val_a)])));
+    assert_eq!(leaves.next(), None);
+}
+
+// ERROR TESTS
+// ================================================================================================
+
+#[test]
+fn tsmt_node_not_available() {
+    let mut smt = TieredSmt::default();
+
+    let raw = 0b_10101010_10101010_00011111_11111111_10010110_10010011_11100000_00000000_u64;
+    let key = RpoDigest::from([ONE, ONE, ONE, Felt::new(raw)]);
+    let value = [ONE; WORD_SIZE];
+
+    // build an index which is just below the inserted leaf node
+    let index = NodeIndex::make(17, raw >> 47);
+
+    // since we haven't inserted the node yet, we should be able to get node and path to this index
+    assert!(smt.get_node(index).is_ok());
+    assert!(smt.get_path(index).is_ok());
+
+    smt.insert(key, value);
+
+    // but once the node is inserted, everything under it should be unavailable
+    assert!(smt.get_node(index).is_err());
+    assert!(smt.get_path(index).is_err());
+
+    let index = NodeIndex::make(32, raw >> 32);
+    assert!(smt.get_node(index).is_err());
+    assert!(smt.get_path(index).is_err());
+
+    let index = NodeIndex::make(34, raw >> 30);
+    assert!(smt.get_node(index).is_err());
+    assert!(smt.get_path(index).is_err());
+
+    let index = NodeIndex::make(50, raw >> 14);
+    assert!(smt.get_node(index).is_err());
+    assert!(smt.get_path(index).is_err());
+
+    let index = NodeIndex::make(64, raw);
+    assert!(smt.get_node(index).is_err());
+    assert!(smt.get_path(index).is_err());
+}
+
+// HELPER FUNCTIONS
+// ================================================================================================
+
+fn get_init_root() -> RpoDigest {
+    EmptySubtreeRoots::empty_hashes(64)[0]
+}
+
+fn build_leaf_node(key: RpoDigest, value: Word, depth: u8) -> RpoDigest {
+    let remaining_path = get_remaining_path(key, depth as u32);
+    Rpo256::merge_in_domain(&[remaining_path, value.into()], depth.into())
+}
+
+fn build_bottom_leaf_node(keys: &[RpoDigest], values: &[Word]) -> RpoDigest {
+    assert_eq!(keys.len(), values.len());
+
+    let mut elements = Vec::with_capacity(keys.len());
+    for (key, val) in keys.iter().zip(values.iter()) {
+        let mut key = Word::from(key);
+        key[3] = ZERO;
+        elements.extend_from_slice(&key);
+        elements.extend_from_slice(val.as_slice());
+    }
+
+    Rpo256::hash_elements(&elements)
+}
+
+fn get_non_empty_nodes(store: &MerkleStore) -> Vec<InnerNodeInfo> {
+    store
+        .inner_nodes()
+        .filter(|node| !is_empty_subtree(&node.value))
+        .collect::<Vec<_>>()
+}
+
+fn is_empty_subtree(node: &RpoDigest) -> bool {
+    EmptySubtreeRoots::empty_hashes(255).contains(node)
+}

src/utils.rs

@@ -1,21 +0,0 @@
-use super::Word;
-use crate::utils::string::String;
-use core::fmt::{self, Write};
-
-// RE-EXPORTS
-// ================================================================================================
-pub use winter_utils::{
-    collections, string, uninit_vector, ByteReader, ByteWriter, Deserializable,
-    DeserializationError, Serializable, SliceReader,
-};
-
-/// Converts a [Word] into hex.
-pub fn word_to_hex(w: &Word) -> Result<String, fmt::Error> {
-    let mut s = String::new();
-
-    for byte in w.iter().flat_map(|e| e.to_bytes()) {
-        write!(s, "{byte:02x}")?;
-    }
-
-    Ok(s)
-}

src/utils/kv_map.rs

@@ -0,0 +1,324 @@
+use core::cell::RefCell;
+use winter_utils::{
+    collections::{btree_map::IntoIter, BTreeMap, BTreeSet},
+    Box,
+};
+
+// KEY-VALUE MAP TRAIT
+// ================================================================================================
+
+/// A trait that defines the interface for a key-value map.
+pub trait KvMap<K: Ord + Clone, V: Clone>:
+    Extend<(K, V)> + FromIterator<(K, V)> + IntoIterator<Item = (K, V)>
+{
+    fn get(&self, key: &K) -> Option<&V>;
+    fn contains_key(&self, key: &K) -> bool;
+    fn len(&self) -> usize;
+    fn is_empty(&self) -> bool {
+        self.len() == 0
+    }
+    fn insert(&mut self, key: K, value: V) -> Option<V>;
+
+    fn iter(&self) -> Box<dyn Iterator<Item = (&K, &V)> + '_>;
+}
+
+// BTREE MAP `KvMap` IMPLEMENTATION
+// ================================================================================================
+
+impl<K: Ord + Clone, V: Clone> KvMap<K, V> for BTreeMap<K, V> {
+    fn get(&self, key: &K) -> Option<&V> {
+        self.get(key)
+    }
+
+    fn contains_key(&self, key: &K) -> bool {
+        self.contains_key(key)
+    }
+
+    fn len(&self) -> usize {
+        self.len()
+    }
+
+    fn insert(&mut self, key: K, value: V) -> Option<V> {
+        self.insert(key, value)
+    }
+
+    fn iter(&self) -> Box<dyn Iterator<Item = (&K, &V)> + '_> {
+        Box::new(self.iter())
+    }
+}
+
+// RECORDING MAP
+// ================================================================================================
+
+/// A [RecordingMap] that records read requests to the underlying key-value map.
+///
+/// The data recorder is used to generate a proof for read requests.
+///
+/// The [RecordingMap] is composed of three parts:
+/// - `data`: which contains the current set of key-value pairs in the map.
+/// - `updates`: which tracks keys for which values have been since the map was instantiated.
+///   updates include both insertions and updates of values under existing keys.
+/// - `trace`: which contains the key-value pairs from the original data which have been accesses
+///   since the map was instantiated.
+#[derive(Debug, Default, Clone, Eq, PartialEq)]
+pub struct RecordingMap<K, V> {
+    data: BTreeMap<K, V>,
+    updates: BTreeSet<K>,
+    trace: RefCell<BTreeMap<K, V>>,
+}
+
+impl<K: Ord + Clone, V: Clone> RecordingMap<K, V> {
+    // CONSTRUCTOR
+    // --------------------------------------------------------------------------------------------
+    /// Returns a new [RecordingMap] instance initialized with the provided key-value pairs.
+    /// ([BTreeMap]).
+    pub fn new(init: impl IntoIterator<Item = (K, V)>) -> Self {
+        RecordingMap {
+            data: init.into_iter().collect(),
+            updates: BTreeSet::new(),
+            trace: RefCell::new(BTreeMap::new()),
+        }
+    }
+
+    // FINALIZER
+    // --------------------------------------------------------------------------------------------
+
+    /// Consumes the [RecordingMap] and returns a [BTreeMap] containing the key-value pairs from
+    /// the initial data set that were read during recording.
+    pub fn into_proof(self) -> BTreeMap<K, V> {
+        self.trace.take()
+    }
+
+    // TEST HELPERS
+    // --------------------------------------------------------------------------------------------
+
+    #[cfg(test)]
+    pub fn trace_len(&self) -> usize {
+        self.trace.borrow().len()
+    }
+
+    #[cfg(test)]
+    pub fn updates_len(&self) -> usize {
+        self.updates.len()
+    }
+}
+
+impl<K: Ord + Clone, V: Clone> KvMap<K, V> for RecordingMap<K, V> {
+    // PUBLIC ACCESSORS
+    // --------------------------------------------------------------------------------------------
+
+    /// Returns a reference to the value associated with the given key if the value exists.
+    ///
+    /// If the key is part of the initial data set, the key access is recorded.
+    fn get(&self, key: &K) -> Option<&V> {
+        self.data.get(key).map(|value| {
+            if !self.updates.contains(key) {
+                self.trace.borrow_mut().insert(key.clone(), value.clone());
+            }
+            value
+        })
+    }
+
+    /// Returns a boolean to indicate whether the given key exists in the data set.
+    ///
+    /// If the key is part of the initial data set, the key access is recorded.
+    fn contains_key(&self, key: &K) -> bool {
+        self.get(key).is_some()
+    }
+
+    /// Returns the number of key-value pairs in the data set.
+    fn len(&self) -> usize {
+        self.data.len()
+    }
+
+    // MUTATORS
+    // --------------------------------------------------------------------------------------------
+
+    /// Inserts a key-value pair into the data set.
+    ///
+    /// If the key already exists in the data set, the value is updated and the old value is
+    /// returned.
+    fn insert(&mut self, key: K, value: V) -> Option<V> {
+        let new_update = self.updates.insert(key.clone());
+        self.data.insert(key.clone(), value).map(|old_value| {
+            if new_update {
+                self.trace.borrow_mut().insert(key, old_value.clone());
+            }
+            old_value
+        })
+    }
+
+    // ITERATION
+    // --------------------------------------------------------------------------------------------
+
+    /// Returns an iterator over the key-value pairs in the data set.
+    fn iter(&self) -> Box<dyn Iterator<Item = (&K, &V)> + '_> {
+        Box::new(self.data.iter())
+    }
+}
+
+impl<K: Clone + Ord, V: Clone> Extend<(K, V)> for RecordingMap<K, V> {
+    fn extend<T: IntoIterator<Item = (K, V)>>(&mut self, iter: T) {
+        iter.into_iter().for_each(move |(k, v)| {
+            self.insert(k, v);
+        });
+    }
+}
+
+impl<K: Clone + Ord, V: Clone> FromIterator<(K, V)> for RecordingMap<K, V> {
+    fn from_iter<T: IntoIterator<Item = (K, V)>>(iter: T) -> Self {
+        Self::new(iter)
+    }
+}
+
+impl<K: Clone + Ord, V: Clone> IntoIterator for RecordingMap<K, V> {
+    type Item = (K, V);
+    type IntoIter = IntoIter<K, V>;
+
+    fn into_iter(self) -> Self::IntoIter {
+        self.data.into_iter()
+    }
+}
+
+// TESTS
+// ================================================================================================
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    const ITEMS: [(u64, u64); 5] = [(0, 0), (1, 1), (2, 2), (3, 3), (4, 4)];
+
+    #[test]
+    fn test_get_item() {
+        // instantiate a recording map
+        let map = RecordingMap::new(ITEMS.to_vec());
+
+        // get a few items
+        let get_items = [0, 1, 2];
+        for key in get_items.iter() {
+            map.get(key);
+        }
+
+        // convert the map into a proof
+        let proof = map.into_proof();
+
+        // check that the proof contains the expected values
+        for (key, value) in ITEMS.iter() {
+            match get_items.contains(key) {
+                true => assert_eq!(proof.get(key), Some(value)),
+                false => assert_eq!(proof.get(key), None),
+            }
+        }
+    }
+
+    #[test]
+    fn test_contains_key() {
+        // instantiate a recording map
+        let map = RecordingMap::new(ITEMS.to_vec());
+
+        // check if the map contains a few items
+        let get_items = [0, 1, 2];
+        for key in get_items.iter() {
+            map.contains_key(key);
+        }
+
+        // convert the map into a proof
+        let proof = map.into_proof();
+
+        // check that the proof contains the expected values
+        for (key, _) in ITEMS.iter() {
+            match get_items.contains(key) {
+                true => assert_eq!(proof.contains_key(key), true),
+                false => assert_eq!(proof.contains_key(key), false),
+            }
+        }
+    }
+
+    #[test]
+    fn test_len() {
+        // instantiate a recording map
+        let mut map = RecordingMap::new(ITEMS.to_vec());
+        // length of the map should be equal to the number of items
+        assert_eq!(map.len(), ITEMS.len());
+
+        // inserting entry with key that already exists should not change the length, but it does
+        // add entries to the trace and update sets
+        map.insert(4, 5);
+        assert_eq!(map.len(), ITEMS.len());
+        assert_eq!(map.trace_len(), 1);
+        assert_eq!(map.updates_len(), 1);
+
+        // inserting entry with new key should increase the length; it should also record the key
+        // as an updated key, but the trace length does not change since old values were not touched
+        map.insert(5, 5);
+        assert_eq!(map.len(), ITEMS.len() + 1);
+        assert_eq!(map.trace_len(), 1);
+        assert_eq!(map.updates_len(), 2);
+
+        // get some items so that they are saved in the trace; this should record original items
+        // in the trace, but should not affect the set of updates
+        let get_items = [0, 1, 2];
+        for key in get_items.iter() {
+            map.contains_key(key);
+        }
+        assert_eq!(map.trace_len(), 4);
+        assert_eq!(map.updates_len(), 2);
+
+        // read the same items again, this should not have any effect on either length, trace, or
+        // the set of updates
+        let get_items = [0, 1, 2];
+        for key in get_items.iter() {
+            map.contains_key(key);
+        }
+        assert_eq!(map.trace_len(), 4);
+        assert_eq!(map.updates_len(), 2);
+
+        // read a newly inserted item; this should not affect either length, trace, or the set of
+        // updates
+        let _val = map.get(&5).unwrap();
+        assert_eq!(map.trace_len(), 4);
+        assert_eq!(map.updates_len(), 2);
+
+        // update a newly inserted item; this should not affect either length, trace, or the set
+        // of updates
+        map.insert(5, 11);
+        assert_eq!(map.trace_len(), 4);
+        assert_eq!(map.updates_len(), 2);
+
+        // Note: The length reported by the proof will be different to the length originally
+        // reported by the map.
+        let proof = map.into_proof();
+
+        // length of the proof should be equal to get_items + 1. The extra item is the original
+        // value at key = 4u64
+        assert_eq!(proof.len(), get_items.len() + 1);
+    }
+
+    #[test]
+    fn test_iter() {
+        let mut map = RecordingMap::new(ITEMS.to_vec());
+        assert!(map.iter().all(|(x, y)| ITEMS.contains(&(*x, *y))));
+
+        // when inserting entry with key that already exists the iterator should return the new value
+        let new_value = 5;
+        map.insert(4, new_value);
+        assert_eq!(map.iter().count(), ITEMS.len());
+        assert!(map.iter().all(|(x, y)| if x == &4 {
+            y == &new_value
+        } else {
+            ITEMS.contains(&(*x, *y))
+        }));
+    }
+
+    #[test]
+    fn test_is_empty() {
+        // instantiate an empty recording map
+        let empty_map: RecordingMap<u64, u64> = RecordingMap::default();
+        assert!(empty_map.is_empty());
+
+        // instantiate a non-empty recording map
+        let map = RecordingMap::new(ITEMS.to_vec());
+        assert!(!map.is_empty());
+    }
+}

src/utils/mod.rs

@@ -0,0 +1,36 @@
+use super::{utils::string::String, Word};
+use core::fmt::{self, Write};
+
+#[cfg(not(feature = "std"))]
+pub use alloc::format;
+
+#[cfg(feature = "std")]
+pub use std::format;
+
+mod kv_map;
+
+// RE-EXPORTS
+// ================================================================================================
+pub use winter_utils::{
+    string, uninit_vector, Box, ByteReader, ByteWriter, Deserializable, DeserializationError,
+    Serializable, SliceReader,
+};
+
+pub mod collections {
+    pub use super::kv_map::*;
+    pub use winter_utils::collections::*;
+}
+
+// UTILITY FUNCTIONS
+// ================================================================================================
+
+/// Converts a [Word] into hex.
+pub fn word_to_hex(w: &Word) -> Result<String, fmt::Error> {
+    let mut s = String::new();
+
+    for byte in w.iter().flat_map(|e| e.to_bytes()) {
+        write!(s, "{byte:02x}")?;
+    }
+
+    Ok(s)
+}