chore: add TieredSmt to readme

Merge pull request #140 from 0xPolygonMiden/next
Tracking PR for v0.5 release
2026-01-12 00:51:29 +01:00 · 2023-05-26 14:41:22 -07:00 · 2023-05-26 14:36:20 -07:00 · 2023-05-26 14:32:17 -07:00 · 2023-05-20 22:52:59 -07:00 · 2023-05-20 22:47:07 -07:00
25 changed files with 1783 additions and 905 deletions
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,3 +1,17 @@
+## 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).
+- Allowed merging of leaves in `MerkleStore` (#138).
+- [BREAKING] Refactored how existing data structures are added to `MerkleStore` (#139).
+
 ## 0.3.0 (2023-04-08)

 - Added `depth` parameter to SMT constructors in `MerkleStore` (#115).
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -1,12 +1,12 @@
 [package]
 name = "miden-crypto"
-version = "0.3.0"
+version = "0.5.0"
 description = "Miden Cryptographic primitives"
 authors = ["miden contributors"]
 readme = "README.md"
 license = "MIT"
 repository = "https://github.com/0xPolygonMiden/crypto"
-documentation = "https://docs.rs/miden-crypto/0.3.0"
+documentation = "https://docs.rs/miden-crypto/0.5.0"
 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" }
--- a/README.md
+++ b/README.md
@@ -13,16 +13,14 @@ For performance benchmarks of these hash functions and their comparison to other
 [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.
+* `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:

--- a/benches/hash.rs
+++ b/benches/hash.rs
@@ -106,11 +106,5 @@ fn blake3_sequential(c: &mut Criterion) {
    });
 }

-criterion_group!(
-    hash_group,
-    rpo256_2to1,
-    rpo256_sequential,
-    blake3_2to1,
-    blake3_sequential
-);
+criterion_group!(hash_group, rpo256_2to1, rpo256_sequential, blake3_2to1, blake3_sequential);
 criterion_main!(hash_group);
--- a/benches/smt.rs
+++ b/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,
@@ -75,10 +73,5 @@ criterion_main!(smt_group);
 fn generate_word(seed: &mut [u8; 32]) -> Word {
    swap(seed, &mut prng_array(*seed));
    let nums: [u64; 4] = prng_array(*seed);
-    [
-        Felt::new(nums[0]),
-        Felt::new(nums[1]),
-        Felt::new(nums[2]),
-        Felt::new(nums[3]),
-    ]
+    [Felt::new(nums[0]), Felt::new(nums[1]), Felt::new(nums[2]), Felt::new(nums[3])]
 }
--- a/benches/store.rs
+++ b/benches/store.rs
@@ -34,7 +34,7 @@ fn get_empty_leaf_simplesmt(c: &mut Criterion) {
    // both SMT and the store are pre-populated with empty hashes, accessing these values is what is
    // being benchmarked here, so no values are inserted into the backends
    let smt = SimpleSmt::new(depth).unwrap();
-    let store = MerkleStore::new();
+    let store = MerkleStore::from(&smt);
    let root = smt.root();

    group.bench_function(BenchmarkId::new("SimpleSmt", depth), |b| {
@@ -66,7 +66,7 @@ fn get_leaf_merkletree(c: &mut Criterion) {

        let mtree_leaves: Vec<Word> = leaves.iter().map(|v| v.into()).collect();
        let mtree = MerkleTree::new(mtree_leaves.clone()).unwrap();
-        let store = MerkleStore::new().with_merkle_tree(mtree_leaves).unwrap();
+        let store = MerkleStore::from(&mtree);
        let depth = mtree.depth();
        let root = mtree.root();
        let size_u64 = size as u64;
@@ -104,13 +104,8 @@ 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 store = MerkleStore::new()
-            .with_sparse_merkle_tree(SimpleSmt::MAX_DEPTH, smt_leaves)
-            .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();
        let size_u64 = size as u64;
@@ -143,7 +138,7 @@ fn get_node_of_empty_simplesmt(c: &mut Criterion) {
    // of these values is what is being benchmarked here, so no values are inserted into the
    // backends.
    let smt = SimpleSmt::new(depth).unwrap();
-    let store = MerkleStore::new();
+    let store = MerkleStore::from(&smt);
    let root = smt.root();
    let half_depth = depth / 2;
    let half_size = 2_u64.pow(half_depth as u32);
@@ -178,7 +173,7 @@ fn get_node_merkletree(c: &mut Criterion) {

        let mtree_leaves: Vec<Word> = leaves.iter().map(|v| v.into()).collect();
        let mtree = MerkleTree::new(mtree_leaves.clone()).unwrap();
-        let store = MerkleStore::new().with_merkle_tree(mtree_leaves).unwrap();
+        let store = MerkleStore::from(&mtree);
        let root = mtree.root();
        let half_depth = mtree.depth() / 2;
        let half_size = 2_u64.pow(half_depth as u32);
@@ -217,13 +212,8 @@ 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 store = MerkleStore::new()
-            .with_sparse_merkle_tree(SimpleSmt::MAX_DEPTH, smt_leaves)
-            .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;
        let half_size = 2_u64.pow(half_depth as u32);
@@ -258,7 +248,7 @@ fn get_leaf_path_merkletree(c: &mut Criterion) {

        let mtree_leaves: Vec<Word> = leaves.iter().map(|v| v.into()).collect();
        let mtree = MerkleTree::new(mtree_leaves.clone()).unwrap();
-        let store = MerkleStore::new().with_merkle_tree(mtree_leaves).unwrap();
+        let store = MerkleStore::from(&mtree);
        let depth = mtree.depth();
        let root = mtree.root();
        let size_u64 = size as u64;
@@ -296,13 +286,8 @@ 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 store = MerkleStore::new()
-            .with_sparse_merkle_tree(SimpleSmt::MAX_DEPTH, smt_leaves)
-            .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();
        let size_u64 = size as u64;
@@ -347,16 +332,16 @@ fn new(c: &mut Criterion) {

        // This could be done with `bench_with_input`, however to remove variables while comparing
        // with MerkleTree it is using `iter_batched`
-        group.bench_function(
-            BenchmarkId::new("MerkleStore::with_merkle_tree", size),
-            |b| {
+        group.bench_function(BenchmarkId::new("MerkleStore::extend::MerkleTree", size), |b| {
            b.iter_batched(
                || leaves.iter().map(|v| v.into()).collect::<Vec<Word>>(),
-                    |l| black_box(MerkleStore::new().with_merkle_tree(l)),
+                |l| {
+                    let mtree = MerkleTree::new(l).unwrap();
+                    black_box(MerkleStore::from(&mtree));
+                },
                BatchSize::SmallInput,
            )
-            },
-        );
+        });

        group.bench_function(BenchmarkId::new("SimpleSmt::new", size), |b| {
            b.iter_batched(
@@ -367,14 +352,12 @@ 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,
            )
        });

-        group.bench_function(
-            BenchmarkId::new("MerkleStore::with_sparse_merkle_tree", size),
-            |b| {
+        group.bench_function(BenchmarkId::new("MerkleStore::extend::SimpleSmt", size), |b| {
            b.iter_batched(
                || {
                    leaves
@@ -384,14 +367,12 @@ fn new(c: &mut Criterion) {
                        .collect::<Vec<(u64, Word)>>()
                },
                |l| {
-                        black_box(
-                            MerkleStore::new().with_sparse_merkle_tree(SimpleSmt::MAX_DEPTH, l),
-                        )
+                    let smt = SimpleSmt::with_leaves(SimpleSmt::MAX_DEPTH, l).unwrap();
+                    black_box(MerkleStore::from(&smt));
                },
                BatchSize::SmallInput,
            )
-            },
-        );
+        });
    }
 }

@@ -407,7 +388,7 @@ fn update_leaf_merkletree(c: &mut Criterion) {

        let mtree_leaves: Vec<Word> = leaves.iter().map(|v| v.into()).collect();
        let mut mtree = MerkleTree::new(mtree_leaves.clone()).unwrap();
-        let mut store = MerkleStore::new().with_merkle_tree(mtree_leaves).unwrap();
+        let mut store = MerkleStore::from(&mtree);
        let depth = mtree.depth();
        let root = mtree.root();
        let size_u64 = size as u64;
@@ -452,13 +433,8 @@ 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 store = MerkleStore::new()
-            .with_sparse_merkle_tree(SimpleSmt::MAX_DEPTH, smt_leaves)
-            .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();
        let size_u64 = size as u64;
--- a/rustfmt.toml
+++ b/rustfmt.toml
@@ -0,0 +1,20 @@
+edition = "2021"
+array_width = 80
+attr_fn_like_width = 80
+chain_width = 80
+#condense_wildcard_suffixes = true
+#enum_discrim_align_threshold = 40
+fn_call_width = 80
+#fn_single_line = true
+#format_code_in_doc_comments = true
+#format_macro_matchers = true
+#format_strings = true
+#group_imports = "StdExternalCrate"
+#hex_literal_case = "Lower"
+#imports_granularity = "Crate"
+newline_style = "Unix"
+#normalize_doc_attributes = true
+#reorder_impl_items = true
+single_line_if_else_max_width = 60
+use_field_init_shorthand = true
+use_try_shorthand = true
--- a/src/hash/blake/mod.rs
+++ b/src/hash/blake/mod.rs
@@ -270,10 +270,7 @@ impl Blake3_160 {
 /// Zero-copy ref shrink to array.
 fn shrink_bytes<const M: usize, const N: usize>(bytes: &[u8; M]) -> &[u8; N] {
    // compile-time assertion
-    assert!(
-        M >= N,
-        "N should fit in M so it can be safely transmuted into a smaller slice!"
-    );
+    assert!(M >= N, "N should fit in M so it can be safely transmuted into a smaller slice!");
    // safety: bytes len is asserted
    unsafe { transmute(bytes) }
 }
--- a/src/hash/rpo/digest.rs
+++ b/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()
@@ -118,14 +140,13 @@ impl Ord for RpoDigest {
        // finally, we use `Felt::inner` instead of `Felt::as_int` so we avoid performing a
        // montgomery reduction for every limb. that is safe because every inner element of the
        // digest is guaranteed to be in its canonical form (that is, `x in [0,p)`).
-        self.0
-            .iter()
-            .map(Felt::inner)
-            .zip(other.0.iter().map(Felt::inner))
-            .fold(Ordering::Equal, |ord, (a, b)| match ord {
+        self.0.iter().map(Felt::inner).zip(other.0.iter().map(Felt::inner)).fold(
+            Ordering::Equal,
+            |ord, (a, b)| match ord {
                Ordering::Equal => a.cmp(&b),
                _ => ord,
-            })
+            },
+        )
    }
 }

@@ -135,6 +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
 // ================================================================================================

--- a/src/merkle/empty_roots.rs
+++ b/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
--- a/src/merkle/index.rs
+++ b/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)
    }
 }

@@ -132,10 +168,7 @@ mod tests {

    #[test]
    fn test_node_index_value_too_high() {
-        assert_eq!(
-            NodeIndex::new(0, 0).unwrap(),
-            NodeIndex { depth: 0, value: 0 }
-        );
+        assert_eq!(NodeIndex::new(0, 0).unwrap(), NodeIndex { depth: 0, value: 0 });
        match NodeIndex::new(0, 1) {
            Err(MerkleError::InvalidIndex { depth, value }) => {
                assert_eq!(depth, 0);
--- a/src/merkle/merkle_tree.rs
+++ b/src/merkle/merkle_tree.rs
@@ -109,14 +109,33 @@ impl MerkleTree {
            index.move_up();
        }

-        debug_assert!(
-            index.is_root(),
-            "the path walk must go all the way to the root"
-        );
+        debug_assert!(index.is_root(), "the path walk must go all the way to the root");

        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))
+    }
+
+    /// 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
@@ -152,14 +171,6 @@ impl MerkleTree {

        Ok(())
    }
-
-    /// An iterator over every inner node in the tree. The iterator order is unspecified.
-    pub fn inner_nodes(&self) -> MerkleTreeNodes<'_> {
-        MerkleTreeNodes {
-            nodes: &self.nodes,
-            index: 1, // index 0 is just padding, start at 1
-        }
-    }
 }

 // ITERATORS
@@ -168,12 +179,12 @@ impl MerkleTree {
 /// An iterator over every inner node of the [MerkleTree].
 ///
 /// Use this to extract the data of the tree, there is no guarantee on the order of the elements.
-pub struct MerkleTreeNodes<'a> {
+pub struct InnerNodeIterator<'a> {
    nodes: &'a Vec<Word>,
    index: usize,
 }

-impl<'a> Iterator for MerkleTreeNodes<'a> {
+impl<'a> Iterator for InnerNodeIterator<'a> {
    type Item = InnerNodeInfo;

    fn next(&mut self) -> Option<Self::Item> {
@@ -195,6 +206,9 @@ impl<'a> Iterator for MerkleTreeNodes<'a> {
    }
 }

+// UTILITY FUNCTIONS
+// ================================================================================================
+
 /// Utility to visualize a [MerkleTree] in text.
 pub fn tree_to_text(tree: &MerkleTree) -> Result<String, fmt::Error> {
    let indent = "  ";
@@ -248,12 +262,7 @@ mod tests {
    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: [Word; 4] = [int_to_node(1), int_to_node(2), int_to_node(3), int_to_node(4)];

    const LEAVES8: [Word; 8] = [
        int_to_node(1),
@@ -309,22 +318,10 @@ mod tests {
        let (_, node2, node3) = compute_internal_nodes();

        // check depth 2
-        assert_eq!(
-            vec![LEAVES4[1], node3],
-            *tree.get_path(NodeIndex::make(2, 0)).unwrap()
-        );
-        assert_eq!(
-            vec![LEAVES4[0], node3],
-            *tree.get_path(NodeIndex::make(2, 1)).unwrap()
-        );
-        assert_eq!(
-            vec![LEAVES4[3], node2],
-            *tree.get_path(NodeIndex::make(2, 2)).unwrap()
-        );
-        assert_eq!(
-            vec![LEAVES4[2], node2],
-            *tree.get_path(NodeIndex::make(2, 3)).unwrap()
-        );
+        assert_eq!(vec![LEAVES4[1], node3], *tree.get_path(NodeIndex::make(2, 0)).unwrap());
+        assert_eq!(vec![LEAVES4[0], node3], *tree.get_path(NodeIndex::make(2, 1)).unwrap());
+        assert_eq!(vec![LEAVES4[3], node2], *tree.get_path(NodeIndex::make(2, 2)).unwrap());
+        assert_eq!(vec![LEAVES4[2], node2], *tree.get_path(NodeIndex::make(2, 3)).unwrap());

        // check depth 1
        assert_eq!(vec![node3], *tree.get_path(NodeIndex::make(1, 0)).unwrap());
--- a/src/merkle/mmr/accumulator.rs
+++ b/src/merkle/mmr/accumulator.rs
@@ -1,8 +1,4 @@
-use super::{
-    super::Vec,
-    super::{WORD_SIZE, ZERO},
-    MmrProof, Rpo256, Word,
-};
+use super::{super::Vec, super::ZERO, Felt, MmrProof, Rpo256, Word};

 #[derive(Debug, Clone, PartialEq)]
 pub struct MmrPeaks {
@@ -35,27 +31,49 @@ pub struct MmrPeaks {
 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 {
        let root = &self.peaks[opening.peak_index()];
-        opening
-            .merkle_path
-            .verify(opening.relative_pos() as u64, value, root)
+        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().concat());
+        elements.resize(len, ZERO);
+        elements
    }
 }
--- a/src/merkle/mmr/full.rs
+++ b/src/merkle/mmr/full.rs
@@ -280,10 +280,7 @@ impl<'a> Iterator for MmrNodes<'a> {
    type Item = InnerNodeInfo;

    fn next(&mut self) -> Option<Self::Item> {
-        debug_assert!(
-            self.last_right.count_ones() <= 1,
-            "last_right tracks zero or one element"
-        );
+        debug_assert!(self.last_right.count_ones() <= 1, "last_right tracks zero or one element");

        // only parent nodes are emitted, remove the single node tree from the forest
        let target = self.mmr.forest & (usize::MAX << 1);
--- a/src/merkle/mmr/mod.rs
+++ b/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
 // ================================================================================================
--- a/src/merkle/mmr/tests.rs
+++ b/src/merkle/mmr/tests.rs
@@ -118,9 +118,7 @@ fn test_mmr_simple() {
    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::hash_elements(&[postorder[2], postorder[5]].concat()));
    postorder.push(LEAVES[4]);
    postorder.push(LEAVES[5]);
    postorder.push(*Rpo256::hash_elements(&[LEAVES[4], LEAVES[5]].concat()));
@@ -201,10 +199,7 @@ fn test_mmr_open() {
    let h23: Word = Rpo256::hash_elements(&LEAVES[2..4].concat()).into();

    // node at pos 7 is the root
-    assert!(
-        mmr.open(7).is_err(),
-        "Element 7 is not in the tree, result should be None"
-    );
+    assert!(mmr.open(7).is_err(), "Element 7 is not in the tree, result should be None");

    // node at pos 6 is the root
    let empty: MerklePath = MerklePath::new(vec![]);
@@ -297,41 +292,13 @@ fn test_mmr_open() {
 #[test]
 fn test_mmr_get() {
    let mmr: Mmr = LEAVES.into();
-    assert_eq!(
-        mmr.get(0).unwrap(),
-        LEAVES[0],
-        "value at pos 0 must correspond"
-    );
-    assert_eq!(
-        mmr.get(1).unwrap(),
-        LEAVES[1],
-        "value at pos 1 must correspond"
-    );
-    assert_eq!(
-        mmr.get(2).unwrap(),
-        LEAVES[2],
-        "value at pos 2 must correspond"
-    );
-    assert_eq!(
-        mmr.get(3).unwrap(),
-        LEAVES[3],
-        "value at pos 3 must correspond"
-    );
-    assert_eq!(
-        mmr.get(4).unwrap(),
-        LEAVES[4],
-        "value at pos 4 must correspond"
-    );
-    assert_eq!(
-        mmr.get(5).unwrap(),
-        LEAVES[5],
-        "value at pos 5 must correspond"
-    );
-    assert_eq!(
-        mmr.get(6).unwrap(),
-        LEAVES[6],
-        "value at pos 6 must correspond"
-    );
+    assert_eq!(mmr.get(0).unwrap(), LEAVES[0], "value at pos 0 must correspond");
+    assert_eq!(mmr.get(1).unwrap(), LEAVES[1], "value at pos 1 must correspond");
+    assert_eq!(mmr.get(2).unwrap(), LEAVES[2], "value at pos 2 must correspond");
+    assert_eq!(mmr.get(3).unwrap(), LEAVES[3], "value at pos 3 must correspond");
+    assert_eq!(mmr.get(4).unwrap(), LEAVES[4], "value at pos 4 must correspond");
+    assert_eq!(mmr.get(5).unwrap(), LEAVES[5], "value at pos 5 must correspond");
+    assert_eq!(mmr.get(6).unwrap(), LEAVES[6], "value at pos 6 must correspond");
    assert!(mmr.get(7).is_err());
 }

@@ -341,11 +308,7 @@ fn test_mmr_invariants() {
    for v in 1..=1028 {
        mmr.add(int_to_node(v));
        let accumulator = mmr.accumulator();
-        assert_eq!(
-            v as usize,
-            mmr.forest(),
-            "MMR leaf count must increase by one on every add"
-        );
+        assert_eq!(v as usize, mmr.forest(), "MMR leaf count must increase by one on every add");
        assert_eq!(
            v as usize, accumulator.num_leaves,
            "MMR and its accumulator must match leaves count"
@@ -374,41 +337,21 @@ fn test_bit_position_iterator() {
    assert_eq!(TrueBitPositionIterator::new(0).count(), 0);
    assert_eq!(TrueBitPositionIterator::new(0).rev().count(), 0);

-    assert_eq!(
-        TrueBitPositionIterator::new(1).collect::<Vec<u32>>(),
-        vec![0]
-    );
-    assert_eq!(
-        TrueBitPositionIterator::new(1).rev().collect::<Vec<u32>>(),
-        vec![0],
-    );
+    assert_eq!(TrueBitPositionIterator::new(1).collect::<Vec<u32>>(), vec![0]);
+    assert_eq!(TrueBitPositionIterator::new(1).rev().collect::<Vec<u32>>(), vec![0],);

-    assert_eq!(
-        TrueBitPositionIterator::new(2).collect::<Vec<u32>>(),
-        vec![1]
-    );
-    assert_eq!(
-        TrueBitPositionIterator::new(2).rev().collect::<Vec<u32>>(),
-        vec![1],
-    );
+    assert_eq!(TrueBitPositionIterator::new(2).collect::<Vec<u32>>(), vec![1]);
+    assert_eq!(TrueBitPositionIterator::new(2).rev().collect::<Vec<u32>>(), vec![1],);

-    assert_eq!(
-        TrueBitPositionIterator::new(3).collect::<Vec<u32>>(),
-        vec![0, 1],
-    );
-    assert_eq!(
-        TrueBitPositionIterator::new(3).rev().collect::<Vec<u32>>(),
-        vec![1, 0],
-    );
+    assert_eq!(TrueBitPositionIterator::new(3).collect::<Vec<u32>>(), vec![0, 1],);
+    assert_eq!(TrueBitPositionIterator::new(3).rev().collect::<Vec<u32>>(), vec![1, 0],);

    assert_eq!(
        TrueBitPositionIterator::new(0b11010101).collect::<Vec<u32>>(),
        vec![0, 2, 4, 6, 7],
    );
    assert_eq!(
-        TrueBitPositionIterator::new(0b11010101)
-            .rev()
-            .collect::<Vec<u32>>(),
+        TrueBitPositionIterator::new(0b11010101).rev().collect::<Vec<u32>>(),
        vec![7, 6, 4, 2, 0],
    );
 }
@@ -463,10 +406,7 @@ fn test_mmr_hash_peaks() {
    // 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())
-    );
+    assert_eq!(peaks.hash_peaks(), *Rpo256::hash_elements(&expected_peaks.as_slice().concat()));
 }

 #[test]
--- a/src/merkle/mod.rs
+++ b/src/merkle/mod.rs
@@ -1,6 +1,6 @@
 use super::{
    hash::rpo::{Rpo256, RpoDigest},
-    utils::collections::{vec, BTreeMap, Vec},
+    utils::collections::{vec, BTreeMap, BTreeSet, Vec},
    Felt, StarkField, Word, WORD_SIZE, ZERO,
 };
 use core::fmt;
@@ -10,6 +10,7 @@ use core::fmt;

 mod empty_roots;
 pub use empty_roots::EmptySubtreeRoots;
+use empty_roots::EMPTY_WORD;

 mod index;
 pub use index::NodeIndex;
@@ -26,8 +27,11 @@ 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};
+pub use mmr::{Mmr, MmrPeaks, MmrProof};

 mod store;
 pub use store::MerkleStore;
@@ -43,13 +47,15 @@ pub enum MerkleError {
    ConflictingRoots(Vec<Word>),
    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),
+    InvalidNumEntries(usize, usize),
+    NodeNotInSet(NodeIndex),
+    NodeNotInStore(Word, NodeIndex),
+    NumLeavesNotPowerOfTwo(usize),
    RootNotInStore(Word),
 }

@@ -60,9 +66,8 @@ 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 +77,12 @@ 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),
        }
    }
--- a/src/merkle/path.rs
+++ b/src/merkle/path.rs
@@ -1,4 +1,4 @@
-use super::{vec, MerkleError, NodeIndex, Rpo256, Vec, Word};
+use super::{vec, InnerNodeInfo, MerkleError, NodeIndex, Rpo256, Vec, Word};
 use core::ops::{Deref, DerefMut};

 // MERKLE PATH
@@ -22,6 +22,11 @@ impl MerklePath {
    // PROVIDERS
    // --------------------------------------------------------------------------------------------

+    /// Returns the depth in which this Merkle path proof is valid.
+    pub fn depth(&self) -> u8 {
+        self.nodes.len() as u8
+    }
+
    /// Computes the merkle root for this opening.
    pub fn compute_root(&self, index: u64, node: Word) -> Result<Word, MerkleError> {
        let mut index = NodeIndex::new(self.depth(), index)?;
@@ -34,11 +39,6 @@ impl MerklePath {
        Ok(root)
    }

-    /// Returns the depth in which this Merkle path proof is valid.
-    pub fn depth(&self) -> u8 {
-        self.nodes.len() as u8
-    }
-
    /// Verifies the Merkle opening proof towards the provided root.
    ///
    /// Returns `true` if `node` exists at `index` in a Merkle tree with `root`.
@@ -48,6 +48,20 @@ impl MerklePath {
            Err(_) => false,
        }
    }
+
+    /// Returns an iterator over every inner node of this [MerklePath].
+    ///
+    /// The iteration order is unspecified.
+    ///
+    /// # 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> {
+        Ok(InnerNodeIterator {
+            nodes: &self.nodes,
+            index: NodeIndex::new(self.depth(), index)?,
+            value: node,
+        })
+    }
 }

 impl From<Vec<Word>> for MerklePath {
@@ -72,6 +86,9 @@ impl DerefMut for MerklePath {
    }
 }

+// ITERATORS
+// ================================================================================================
+
 impl FromIterator<Word> for MerklePath {
    fn from_iter<T: IntoIterator<Item = Word>>(iter: T) -> Self {
        Self::new(iter.into_iter().collect())
@@ -87,6 +104,39 @@ impl IntoIterator for MerklePath {
    }
 }

+/// An iterator over internal nodes of a [MerklePath].
+pub struct InnerNodeIterator<'a> {
+    nodes: &'a Vec<Word>,
+    index: NodeIndex,
+    value: Word,
+}
+
+impl<'a> Iterator for InnerNodeIterator<'a> {
+    type Item = InnerNodeInfo;
+
+    fn next(&mut self) -> Option<Self::Item> {
+        if !self.index.is_root() {
+            let sibling_pos = self.nodes.len() - self.index.depth() as usize;
+            let (left, right) = if self.index.is_value_odd() {
+                (self.nodes[sibling_pos], self.value)
+            } else {
+                (self.value, self.nodes[sibling_pos])
+            };
+
+            self.value = Rpo256::merge(&[left.into(), right.into()]).into();
+            self.index.move_up();
+
+            Some(InnerNodeInfo {
+                value: self.value,
+                left,
+                right,
+            })
+        } else {
+            None
+        }
+    }
+}
+
 // MERKLE PATH CONTAINERS
 // ================================================================================================

@@ -110,3 +160,25 @@ pub struct RootPath {
    /// The path from `value` to `root` (exclusive).
    pub path: MerklePath,
 }
+
+// TESTS
+// ================================================================================================
+
+#[cfg(test)]
+mod tests {
+    use crate::merkle::{int_to_node, MerklePath};
+
+    #[test]
+    fn test_inner_nodes() {
+        let nodes = vec![int_to_node(1), int_to_node(2), int_to_node(3), int_to_node(4)];
+        let merkle_path = MerklePath::new(nodes);
+
+        let index = 6;
+        let node = int_to_node(5);
+        let root = merkle_path.compute_root(index, node).unwrap();
+
+        let inner_root = merkle_path.inner_nodes(index, node).unwrap().last().unwrap().value;
+
+        assert_eq!(root, inner_root);
+    }
+}
--- a/src/merkle/path_set.rs
+++ b/src/merkle/path_set.rs
@@ -34,9 +34,7 @@ impl MerklePathSet {
    where
        I: IntoIterator<Item = (u64, Word, MerklePath)>,
    {
-        paths
-            .into_iter()
-            .try_fold(self, |mut set, (index, value, path)| {
+        paths.into_iter().try_fold(self, |mut set, (index, value, path)| {
            set.add_path(index, value, path)?;
            Ok(set)
        })
@@ -75,7 +73,7 @@ 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])
    }

@@ -106,11 +104,8 @@ 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)
    }
@@ -202,7 +197,7 @@ 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 -----------------------------------------------------------------
@@ -291,14 +286,9 @@ mod tests {
        let hash_6 = int_to_node(6);
        let index = 6_u64;
        let depth = 3_u8;
-        let set = MerklePathSet::new(depth)
-            .with_paths([(index, hash_6, path_6.into())])
-            .unwrap();
+        let set = MerklePathSet::new(depth).with_paths([(index, hash_6, path_6.into())]).unwrap();

-        assert_eq!(
-            int_to_node(6u64),
-            set.get_node(NodeIndex::make(depth, index)).unwrap()
-        );
+        assert_eq!(int_to_node(6u64), set.get_node(NodeIndex::make(depth, index)).unwrap());
    }

    #[test]
--- a/src/merkle/simple_smt/mod.rs
+++ b/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, EMPTY_WORD,
 };

 #[cfg(test)]
@@ -10,6 +10,7 @@ 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 {
@@ -20,18 +21,6 @@ pub struct SimpleSmt {
    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
    // --------------------------------------------------------------------------------------------
@@ -45,7 +34,12 @@ impl SimpleSmt {
    // 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 {
@@ -66,36 +60,47 @@ 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)
+        // 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 != EMPTY_WORD || empty_entries.contains(&key) {
+                return Err(MerkleError::DuplicateValuesForIndex(key));
            }
-
-    /// 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
+            // 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 == EMPTY_WORD && !empty_entries.insert(key) {
+                return Err(MerkleError::DuplicateValuesForIndex(key));
+            }
+        }
+        Ok(tree)
    }

    // PUBLIC ACCESSORS
@@ -111,45 +116,43 @@ 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.
+    /// 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<Word, 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(self
+                .get_leaf_node(index.value())
+                .unwrap_or_else(|| self.empty_hashes[index.depth() as usize].into()))
        } 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().into())
        }
    }

-    /// 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)?;
+        self.get_node(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 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()));
@@ -168,18 +171,26 @@ impl SimpleSmt {
        Ok(path.into())
    }

-    /// 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(),
@@ -191,61 +202,43 @@ impl SimpleSmt {
    // 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()));
-        }
-        self.insert_leaf(key, 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(EMPTY_WORD);

-        Ok(())
+        // if the old value and new value are the same, there is nothing to update
+        if value == old_value {
+            return Ok(value);
        }

-    /// 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();
            index.move_up();
            let BranchNode { left, right } = self.get_branch_node(&index);
-            let (left, right) = if is_right {
-                (left, value)
-            } else {
-                (value, right)
-            };
+            let (left, right) = if is_right { (left, value) } else { (value, right) };
            self.insert_branch_node(index, left, right);
            value = Rpo256::merge(&[left, right]);
        }
        self.root = value.into();
-        Ok(())
+        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 {
@@ -263,3 +256,18 @@ 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])
+    }
+}
--- a/src/merkle/simple_smt/tests.rs
+++ b/src/merkle/simple_smt/tests.rs
@@ -1,19 +1,15 @@
 use super::{
    super::{int_to_node, InnerNodeInfo, MerkleError, MerkleTree, RpoDigest, SimpleSmt},
-    NodeIndex, Rpo256, Vec, Word,
+    NodeIndex, Rpo256, Vec, Word, EMPTY_WORD,
 };
-use proptest::prelude::*;
-use rand_utils::prng_array;
+
+// 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: [Word; 4] = [int_to_node(1), int_to_node(2), int_to_node(3), int_to_node(4)];

 const VALUES8: [Word; 8] = [
    int_to_node(1),
@@ -28,25 +24,17 @@ const VALUES8: [Word; 8] = [

 const ZERO_VALUES8: [Word; 8] = [int_to_node(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();
@@ -56,94 +44,59 @@ fn build_sparse_tree() {
    let key = 6;
    let new_node = int_to_node(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);
    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(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();
+    // 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());

-    let (_, node2, node3) = compute_internal_nodes();
-
-    // check 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(VALUES4.into_iter())).unwrap();

    // check depth 2
    assert_eq!(VALUES4[0], tree.get_node(NodeIndex::make(2, 0)).unwrap());
@@ -185,35 +138,28 @@ 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(VALUES8.into_iter())).unwrap();

    // update one value
    let key = 3;
    let new_node = int_to_node(9);
    let mut expected_values = VALUES8.to_vec();
    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);
    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]
@@ -245,7 +191,7 @@ fn small_tree_opening_is_consistent() {

    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));

@@ -269,56 +215,30 @@ 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_node(1)), (5, int_to_node(2)), (1_u64, int_to_node(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_node(0)), (5, int_to_node(2)), (1_u64, int_to_node(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_node(0)), (5, int_to_node(2)), (1_u64, int_to_node(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_node(1)), (5, int_to_node(2)), (1_u64, int_to_node(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_node(0)), (5, int_to_node(2))];
+    let smt = SimpleSmt::with_leaves(64, entries);
+    assert!(smt.is_ok());
 }

 // HELPER FUNCTIONS
--- a/src/merkle/store/mod.rs
+++ b/src/merkle/store/mod.rs
@@ -1,9 +1,9 @@
-use super::mmr::{Mmr, MmrPeaks};
 use super::{
-    BTreeMap, EmptySubtreeRoots, MerkleError, MerklePath, MerklePathSet, MerkleTree, NodeIndex,
-    RootPath, Rpo256, RpoDigest, SimpleSmt, ValuePath, Vec, Word,
+    mmr::Mmr, BTreeMap, EmptySubtreeRoots, InnerNodeInfo, MerkleError, MerklePath, MerklePathSet,
+    MerkleTree, NodeIndex, RootPath, Rpo256, RpoDigest, SimpleSmt, TieredSmt, ValuePath, Vec, Word,
 };
 use crate::utils::{ByteReader, ByteWriter, Deserializable, DeserializationError, Serializable};
+use core::borrow::Borrow;

 #[cfg(test)]
 mod tests;
@@ -14,7 +14,7 @@ pub struct Node {
    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
@@ -47,9 +47,13 @@ pub struct Node {
 /// // the store is initialized with the SMT empty nodes
 /// assert_eq!(store.num_internal_nodes(), 255);
 ///
+/// let tree1 = MerkleTree::new(vec![A, B, C, D, E, F, G, H0]).unwrap();
+/// 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.add_merkle_tree([A, B, C, D, E, F, G, H0]);
-/// store.add_merkle_tree([A, B, C, D, E, F, G, H1]);
+/// store
+///     .extend(tree1.inner_nodes())
+///     .extend(tree2.inner_nodes());
 ///
 /// // every leaf except the last are the same
 /// for i in 0..7 {
@@ -111,60 +115,6 @@ impl MerkleStore {
        MerkleStore { nodes }
    }

-    /// Appends the provided merkle tree represented by its `leaves` to the set.
-    pub fn with_merkle_tree<I>(mut self, leaves: I) -> Result<Self, MerkleError>
-    where
-        I: IntoIterator<Item = Word>,
-    {
-        self.add_merkle_tree(leaves)?;
-        Ok(self)
-    }
-
-    /// Appends the provided Sparse Merkle tree represented by its `entries` to the set.
-    ///
-    /// For more information, check [MerkleStore::add_sparse_merkle_tree].
-    pub fn with_sparse_merkle_tree<R, I>(
-        mut self,
-        depth: u8,
-        entries: R,
-    ) -> Result<Self, MerkleError>
-    where
-        R: IntoIterator<IntoIter = I>,
-        I: Iterator<Item = (u64, Word)> + ExactSizeIterator,
-    {
-        self.add_sparse_merkle_tree(depth, entries)?;
-        Ok(self)
-    }
-
-    /// Appends the provided merkle path set.
-    pub fn with_merkle_path(
-        mut self,
-        index_value: u64,
-        node: Word,
-        path: MerklePath,
-    ) -> Result<Self, MerkleError> {
-        self.add_merkle_path(index_value, node, path)?;
-        Ok(self)
-    }
-
-    /// Appends the provided merkle path set.
-    pub fn with_merkle_paths<I>(mut self, paths: I) -> Result<Self, MerkleError>
-    where
-        I: IntoIterator<Item = (u64, Word, MerklePath)>,
-    {
-        self.add_merkle_paths(paths)?;
-        Ok(self)
-    }
-
-    /// Appends the provided [Mmr] represented by its `leaves` to the set.
-    pub fn with_mmr<I>(mut self, leaves: I) -> Result<Self, MerkleError>
-    where
-        I: IntoIterator<Item = Word>,
-    {
-        self.add_mmr(leaves)?;
-        Ok(self)
-    }
-
    // PUBLIC ACCESSORS
    // --------------------------------------------------------------------------------------------

@@ -184,15 +134,11 @@ impl MerkleStore {
        let mut hash: RpoDigest = root.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.into()))?;

        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.into(), index))?;

            let bit = (index.value() >> i) & 1;
            hash = if bit == 0 { node.left } else { node.right }
@@ -206,7 +152,6 @@ 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.
@@ -215,15 +160,11 @@ impl MerkleStore {
        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.into()))?;

        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.into(), index))?;

            let bit = (index.value() >> i) & 1;
            hash = if bit == 0 {
@@ -302,11 +243,7 @@ impl MerkleStore {
            };

            // traverse down
-            hash = if path & 1 == 0 {
-                children.left
-            } else {
-                children.right
-            };
+            hash = if path & 1 == 0 { children.left } else { children.right };
            path >>= 1;
        }

@@ -319,71 +256,53 @@ impl MerkleStore {
        Ok(tree_depth)
    }

+    // DATA EXTRACTORS
+    // --------------------------------------------------------------------------------------------
+
+    /// 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
+    where
+        I: Iterator<Item = R>,
+        R: Borrow<Word>,
+    {
+        let mut store = MerkleStore::new();
+        for root in roots {
+            let root = RpoDigest::from(*root.borrow());
+            store.clone_tree_from(root, 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.into(),
+            left: n.left.into(),
+            right: n.right.into(),
+        })
+    }
+
    // STATE MUTATORS
    // --------------------------------------------------------------------------------------------

-    /// Adds all the nodes of a Merkle tree represented by `leaves`.
-    ///
-    /// This will instantiate a Merkle tree using `leaves` and include all the nodes into the
-    /// store.
-    ///
-    /// # Errors
-    ///
-    /// This method may return the following errors:
-    /// - `DepthTooSmall` if leaves is empty or contains only 1 element
-    /// - `NumLeavesNotPowerOfTwo` if the number of leaves is not a power-of-two
-    pub fn add_merkle_tree<I>(&mut self, leaves: I) -> Result<Word, MerkleError>
+    /// Adds a sequence of nodes yielded by the provided iterator into the store.
+    pub fn extend<I>(&mut self, iter: I) -> &mut MerkleStore
    where
-        I: IntoIterator<Item = Word>,
+        I: Iterator<Item = InnerNodeInfo>,
    {
-        let leaves: Vec<_> = leaves.into_iter().collect();
-        if leaves.len() < 2 {
-            return Err(MerkleError::DepthTooSmall(leaves.len() as u8));
+        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 tree = MerkleTree::new(leaves)?;
-        for node in tree.inner_nodes() {
-            self.nodes.insert(
-                node.value.into(),
-                Node {
-                    left: node.left.into(),
-                    right: node.right.into(),
-                },
-            );
-        }
-
-        Ok(tree.root())
-    }
-
-    /// Adds a Sparse Merkle tree defined by the specified `entries` to the store, and returns the
-    /// root of the added tree.
-    ///
-    /// The entries are expected to contain tuples of `(index, node)` describing nodes in the tree
-    /// at `depth`.
-    ///
-    /// # Errors
-    /// Returns an error if the provided `depth` is greater than [SimpleSmt::MAX_DEPTH].
-    pub fn add_sparse_merkle_tree<R, I>(
-        &mut self,
-        depth: u8,
-        entries: R,
-    ) -> Result<Word, MerkleError>
-    where
-        R: IntoIterator<IntoIter = I>,
-        I: Iterator<Item = (u64, Word)> + ExactSizeIterator,
-    {
-        let smt = SimpleSmt::new(depth)?.with_leaves(entries)?;
-        for node in smt.inner_nodes() {
-            self.nodes.insert(
-                node.value.into(),
-                Node {
-                    left: node.left.into(),
-                    right: node.right.into(),
-                },
-            );
-        }
-
-        Ok(smt.root())
+        self
    }

    /// Adds all the nodes of a Merkle path represented by `path`, opening to `node`. Returns the
@@ -393,31 +312,21 @@ impl MerkleStore {
    /// include all the nodes into the store.
    pub fn add_merkle_path(
        &mut self,
-        index_value: u64,
-        mut node: Word,
+        index: u64,
+        node: Word,
        path: MerklePath,
    ) -> Result<Word, MerkleError> {
-        let mut index = NodeIndex::new(path.len() as u8, index_value)?;
+        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();

-        for sibling in path {
-            let (left, right) = match index.is_value_odd() {
-                true => (sibling, node),
-                false => (node, sibling),
-            };
-            let parent = Rpo256::merge(&[left.into(), right.into()]);
-            self.nodes.insert(
-                parent,
-                Node {
-                    left: left.into(),
-                    right: right.into(),
-                },
-            );
+            debug_assert_eq!(Rpo256::merge(&[left, right]), value);
+            self.nodes.insert(value, Node { left, right });

-            index.move_up();
-            node = parent.into();
-        }
-
-        Ok(node)
+            node.value
+        });
+        Ok(root)
    }

    /// Adds all the nodes of multiple Merkle paths into the store.
@@ -447,25 +356,6 @@ impl MerkleStore {
        Ok(root)
    }

-    /// Appends the provided [Mmr] into the store.
-    pub fn add_mmr<I>(&mut self, leaves: I) -> Result<MmrPeaks, MerkleError>
-    where
-        I: IntoIterator<Item = Word>,
-    {
-        let mmr = Mmr::from(leaves);
-        for node in mmr.inner_nodes() {
-            self.nodes.insert(
-                node.value.into(),
-                Node {
-                    left: node.left.into(),
-                    right: node.right.into(),
-                },
-            );
-        }
-
-        Ok(mmr.accumulator())
-    }
-
    /// Sets a node to `value`.
    ///
    /// # Errors
@@ -490,26 +380,87 @@ impl MerkleStore {
        Ok(RootPath { root, path })
    }

+    /// 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 root1: RpoDigest = root1.into();
-        let root2: RpoDigest = root2.into();
+        let left: RpoDigest = root1.into();
+        let right: RpoDigest = root2.into();

-        if !self.nodes.contains_key(&root1) {
-            Err(MerkleError::NodeNotInStore(root1.into(), NodeIndex::root()))
-        } else if !self.nodes.contains_key(&root1) {
-            Err(MerkleError::NodeNotInStore(root2.into(), NodeIndex::root()))
-        } else {
-            let parent: Word = Rpo256::merge(&[root1, root2]).into();
-            self.nodes.insert(
-                parent.into(),
-                Node {
-                    left: root1,
-                    right: root2,
-                },
-            );
+        let parent = Rpo256::merge(&[left, right]);
+        self.nodes.insert(parent, Node { left, right });

-            Ok(parent)
+        Ok(parent.into())
    }
+
+    // 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 matches!(self.nodes.insert(root, *node), None) {
+                self.clone_tree_from(node.left, source);
+                self.clone_tree_from(node.right, source);
+            }
+        }
+    }
+}
+
+// CONVERSIONS
+// ================================================================================================
+
+impl From<&MerkleTree> for MerkleStore {
+    fn from(value: &MerkleTree) -> Self {
+        let mut store = MerkleStore::new();
+        store.extend(value.inner_nodes());
+        store
+    }
+}
+
+impl From<&SimpleSmt> for MerkleStore {
+    fn from(value: &SimpleSmt) -> Self {
+        let mut store = MerkleStore::new();
+        store.extend(value.inner_nodes());
+        store
+    }
+}
+
+impl From<&Mmr> for MerkleStore {
+    fn from(value: &Mmr) -> Self {
+        let mut store = MerkleStore::new();
+        store.extend(value.inner_nodes());
+        store
+    }
+}
+
+impl From<&TieredSmt> for MerkleStore {
+    fn from(value: &TieredSmt) -> Self {
+        let mut store = MerkleStore::new();
+        store.extend(value.inner_nodes());
+        store
+    }
+}
+
+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
+    }
+}
+
+// ITERATORS
+// ================================================================================================
+
+impl Extend<InnerNodeInfo> for MerkleStore {
+    fn extend<T: IntoIterator<Item = InnerNodeInfo>>(&mut self, iter: T) {
+        self.extend(iter.into_iter());
    }
 }

--- a/src/merkle/store/tests.rs
+++ b/src/merkle/store/tests.rs
@@ -1,34 +1,48 @@
-use super::*;
+use super::{
+    super::EMPTY_WORD, Deserializable, EmptySubtreeRoots, MerkleError, MerklePath, MerkleStore,
+    NodeIndex, RpoDigest, Serializable,
+};
 use crate::{
    hash::rpo::Rpo256,
-    merkle::{int_to_node, MerklePathSet},
-    Felt, Word, WORD_SIZE, ZERO,
+    merkle::{int_to_node, MerklePathSet, MerkleTree, SimpleSmt},
+    Felt, Word, WORD_SIZE,
 };

-#[cfg(std)]
+#[cfg(feature = "std")]
 use std::error::Error;

+// TEST DATA
+// ================================================================================================
+
 const KEYS4: [u64; 4] = [0, 1, 2, 3];
-const LEAVES4: [Word; 4] = [
+const VALUES4: [Word; 4] = [int_to_node(1), int_to_node(2), int_to_node(3), int_to_node(4)];
+
+const VALUES8: [Word; 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),
 ];
-const EMPTY: Word = [ZERO; WORD_SIZE];
+
+// TESTS
+// ================================================================================================

 #[test]
 fn test_root_not_in_store() -> Result<(), MerkleError> {
-    let mtree = MerkleTree::new(LEAVES4.to_vec())?;
-    let store = MerkleStore::default().with_merkle_tree(LEAVES4)?;
+    let mtree = MerkleTree::new(VALUES4.to_vec())?;
+    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"
    );

@@ -37,35 +51,33 @@ fn test_root_not_in_store() -> Result<(), MerkleError> {

 #[test]
 fn test_merkle_tree() -> Result<(), MerkleError> {
-    let mut store = MerkleStore::default();
+    let mtree = MerkleTree::new(VALUES4.to_vec())?;
+    let store = MerkleStore::from(&mtree);

-    let mtree = MerkleTree::new(LEAVES4.to_vec())?;
-    store.add_merkle_tree(LEAVES4.to_vec())?;
-
-    // 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)),
@@ -90,11 +102,9 @@ fn test_merkle_tree() -> Result<(), MerkleError> {

    // STORE MERKLE PATH MATCHS ==============================================================
    // 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();
+    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!(
@@ -103,11 +113,9 @@ fn test_merkle_tree() -> Result<(), MerkleError> {
        "merkle path for index 0 must be the same for the MerkleTree and MerkleStore"
    );

-    let result = store
-        .get_path(mtree.root(), NodeIndex::make(mtree.depth(), 1))
-        .unwrap();
+    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!(
@@ -116,11 +124,9 @@ fn test_merkle_tree() -> Result<(), MerkleError> {
        "merkle path for index 1 must be the same for the MerkleTree and MerkleStore"
    );

-    let result = store
-        .get_path(mtree.root(), NodeIndex::make(mtree.depth(), 2))
-        .unwrap();
+    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!(
@@ -129,11 +135,9 @@ fn test_merkle_tree() -> Result<(), MerkleError> {
        "merkle path for index 0 must be the same for the MerkleTree and MerkleStore"
    );

-    let result = store
-        .get_path(mtree.root(), NodeIndex::make(mtree.depth(), 3))
-        .unwrap();
+    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!(
@@ -148,7 +152,7 @@ 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::new(EMPTY_WORD);

    for depth in 0..255 {
        root = Rpo256::merge(&[root; 2]);
@@ -172,16 +176,13 @@ 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, EMPTY_WORD, "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(), EMPTY_WORD).unwrap(),
            smt.root(),
            "computed root from the path must match the empty tree root"
        );
@@ -192,80 +193,62 @@ fn test_leaf_paths_for_empty_trees() -> Result<(), MerkleError> {

 #[test]
 fn test_get_invalid_node() {
-    let mut store = MerkleStore::default();
-    let mtree = MerkleTree::new(LEAVES4.to_vec()).expect("creating a merkle tree must work");
-    store
-        .add_merkle_tree(LEAVES4.to_vec())
-        .expect("adding a merkle tree to the store must work");
+    let mtree = MerkleTree::new(VALUES4.to_vec()).expect("creating a merkle tree must work");
+    let store = MerkleStore::from(&mtree);
    let _ = store.get_node(mtree.root(), NodeIndex::make(mtree.depth(), 3));
 }

 #[test]
 fn test_add_sparse_merkle_tree_one_level() -> Result<(), MerkleError> {
-    let mut store = MerkleStore::default();
    let keys2: [u64; 2] = [0, 1];
    let leaves2: [Word; 2] = [int_to_node(1), int_to_node(2)];
-    store.add_sparse_merkle_tree(48, keys2.into_iter().zip(leaves2.into_iter()))?;
-    let smt = SimpleSmt::new(1)
-        .unwrap()
-        .with_leaves(keys2.into_iter().zip(leaves2.into_iter()))
-        .unwrap();
+    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!(
-        store.get_node(smt.root(), idx).unwrap(),
-        smt.get_node(idx).unwrap()
-    );
+    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!(
-        store.get_node(smt.root(), idx).unwrap(),
-        smt.get_node(idx).unwrap()
-    );
+    assert_eq!(store.get_node(smt.root(), idx).unwrap(), smt.get_node(idx).unwrap());

    Ok(())
 }

 #[test]
 fn test_sparse_merkle_tree() -> Result<(), MerkleError> {
-    let mut store = MerkleStore::default();
-    store.add_sparse_merkle_tree(
-        SimpleSmt::MAX_DEPTH,
-        KEYS4.into_iter().zip(LEAVES4.into_iter()),
-    )?;
-
-    let smt = SimpleSmt::new(SimpleSmt::MAX_DEPTH)
-        .unwrap()
-        .with_leaves(KEYS4.into_iter().zip(LEAVES4.into_iter()))
+    let smt =
+        SimpleSmt::with_leaves(SimpleSmt::MAX_DEPTH, KEYS4.into_iter().zip(VALUES4.into_iter()))
            .unwrap();

+    let store = MerkleStore::from(&smt);
+
    // STORE LEAVES ARE CORRECT ==============================================================
    // 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(EMPTY_WORD),
        "unmodified node 4 must be ZERO"
    );

@@ -299,11 +282,9 @@ fn test_sparse_merkle_tree() -> Result<(), MerkleError> {

    // STORE MERKLE PATH MATCHS ==============================================================
    // 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();
+    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!(
@@ -312,11 +293,9 @@ fn test_sparse_merkle_tree() -> Result<(), MerkleError> {
        "merkle path for index 0 must be the same for the MerkleTree and MerkleStore"
    );

-    let result = store
-        .get_path(smt.root(), NodeIndex::make(smt.depth(), 1))
-        .unwrap();
+    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!(
@@ -325,11 +304,9 @@ fn test_sparse_merkle_tree() -> Result<(), MerkleError> {
        "merkle path for index 1 must be the same for the MerkleTree and MerkleStore"
    );

-    let result = store
-        .get_path(smt.root(), NodeIndex::make(smt.depth(), 2))
-        .unwrap();
+    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!(
@@ -338,11 +315,9 @@ fn test_sparse_merkle_tree() -> Result<(), MerkleError> {
        "merkle path for index 2 must be the same for the MerkleTree and MerkleStore"
    );

-    let result = store
-        .get_path(smt.root(), NodeIndex::make(smt.depth(), 3))
-        .unwrap();
+    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!(
@@ -351,11 +326,9 @@ fn test_sparse_merkle_tree() -> Result<(), MerkleError> {
        "merkle path for index 3 must be the same for the MerkleTree and MerkleStore"
    );

-    let result = store
-        .get_path(smt.root(), NodeIndex::make(smt.depth(), 4))
-        .unwrap();
+    let result = store.get_path(smt.root(), NodeIndex::make(smt.depth(), 4)).unwrap();
    assert_eq!(
-        EMPTY, result.value,
+        EMPTY_WORD, result.value,
        "Value for merkle path at index 4 must match leaf value"
    );
    assert_eq!(
@@ -369,7 +342,7 @@ 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(VALUES4.to_vec())?;

    let i0 = 0;
    let p0 = mtree.get_path(NodeIndex::make(2, i0)).unwrap();
@@ -384,16 +357,14 @@ 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();
-    store
-        .add_merkle_paths(paths.clone())
-        .expect("the valid paths must work");
+    store.add_merkle_paths(paths.clone()).expect("the valid paths must work");

    let depth = 2;
    let set = MerklePathSet::new(depth).with_paths(paths).unwrap();
@@ -402,22 +373,22 @@ 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"
    );

@@ -446,11 +417,9 @@ fn test_add_merkle_paths() -> Result<(), MerkleError> {

    // STORE MERKLE PATH MATCHS ==============================================================
    // 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();
+    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!(
@@ -459,11 +428,9 @@ fn test_add_merkle_paths() -> Result<(), MerkleError> {
        "merkle path for index 0 must be the same for the MerkleTree and MerkleStore"
    );

-    let result = store
-        .get_path(set.root(), NodeIndex::make(set.depth(), 1))
-        .unwrap();
+    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!(
@@ -472,11 +439,9 @@ fn test_add_merkle_paths() -> Result<(), MerkleError> {
        "merkle path for index 1 must be the same for the MerkleTree and MerkleStore"
    );

-    let result = store
-        .get_path(set.root(), NodeIndex::make(set.depth(), 2))
-        .unwrap();
+    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!(
@@ -485,11 +450,9 @@ fn test_add_merkle_paths() -> Result<(), MerkleError> {
        "merkle path for index 0 must be the same for the MerkleTree and MerkleStore"
    );

-    let result = store
-        .get_path(set.root(), NodeIndex::make(set.depth(), 3))
-        .unwrap();
+    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!(
@@ -519,7 +482,8 @@ fn wont_open_to_different_depth_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
    // exist for the set.
-    let store = MerkleStore::default().with_merkle_tree([a, b]).unwrap();
+    let mtree = MerkleTree::new(vec![a, b]).unwrap();
+    let store = MerkleStore::from(&mtree);
    let index = NodeIndex::root();
    let err = store.get_node(root, index).err().unwrap();
    assert_eq!(err, MerkleError::RootNotInStore(root));
@@ -546,13 +510,9 @@ fn store_path_opens_from_leaf() {

    let root = Rpo256::merge(&[m.into(), n.into()]);

-    let store = MerkleStore::default()
-        .with_merkle_tree([a, b, c, d, e, f, g, h])
-        .unwrap();
-    let path = store
-        .get_path(root.into(), NodeIndex::make(3, 1))
-        .unwrap()
-        .path;
+    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 expected = MerklePath::new([a.into(), j.into(), n.into()].to_vec());
    assert_eq!(path, expected);
@@ -560,24 +520,20 @@ fn store_path_opens_from_leaf() {

 #[test]
 fn test_set_node() -> Result<(), MerkleError> {
-    let mtree = MerkleTree::new(LEAVES4.to_vec())?;
-    let mut store = MerkleStore::default().with_merkle_tree(LEAVES4)?;
+    let mtree = MerkleTree::new(VALUES4.to_vec())?;
+    let mut store = MerkleStore::from(&mtree);
    let value = int_to_node(42);
    let index = NodeIndex::make(mtree.depth(), 0);
    let new_root = store.set_node(mtree.root(), index, value)?.root;
-    assert_eq!(
-        store.get_node(new_root, index),
-        Ok(value),
-        "Value must have changed"
-    );
+    assert_eq!(store.get_node(new_root, index), Ok(value), "Value must have changed");

    Ok(())
 }

 #[test]
 fn test_constructors() -> Result<(), MerkleError> {
-    let store = MerkleStore::new().with_merkle_tree(LEAVES4)?;
-    let mtree = MerkleTree::new(LEAVES4.to_vec())?;
+    let mtree = MerkleTree::new(VALUES4.to_vec())?;
+    let store = MerkleStore::from(&mtree);

    let depth = mtree.depth();
    let leaves = 2u64.pow(depth.into());
@@ -588,12 +544,8 @@ fn test_constructors() -> Result<(), MerkleError> {
    }

    let depth = 32;
-    let store = MerkleStore::default()
-        .with_sparse_merkle_tree(depth, KEYS4.into_iter().zip(LEAVES4.into_iter()))?;
-    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(VALUES4.into_iter())).unwrap();
+    let store = MerkleStore::from(&smt);
    let depth = smt.depth();

    for key in KEYS4 {
@@ -604,34 +556,20 @@ 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 store1 = MerkleStore::default().with_merkle_paths(paths.clone())?;
-    let store2 = MerkleStore::default()
-        .with_merkle_path(0, LEAVES4[0], mtree.get_path(NodeIndex::make(d, 0))?)?
-        .with_merkle_path(1, LEAVES4[1], mtree.get_path(NodeIndex::make(d, 1))?)?
-        .with_merkle_path(2, LEAVES4[2], mtree.get_path(NodeIndex::make(d, 2))?)?
-        .with_merkle_path(3, LEAVES4[3], mtree.get_path(NodeIndex::make(d, 3))?)?;
+    let mut store1 = MerkleStore::default();
+    store1.add_merkle_paths(paths.clone())?;
+
+    let mut store2 = MerkleStore::default();
+    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] {
@@ -792,17 +730,72 @@ fn get_leaf_depth_works_with_depth_8() {
    // duplicate the tree on `a` and assert the depth is short-circuited by such sub-tree
    let index = NodeIndex::new(8, a).unwrap();
    root = store.set_node(root, index, root).unwrap().root;
-    assert_eq!(
-        Err(MerkleError::DepthTooBig(9)),
-        store.get_leaf_depth(root, 8, a)
-    );
+    assert_eq!(Err(MerkleError::DepthTooBig(9)), store.get_leaf_depth(root, 8, a));
 }

-#[cfg(std)]
+// SUBSET EXTRACTION
+// ================================================================================================
+
+#[test]
+fn mstore_subset() {
+    // add a Merkle tree of depth 3 to the store
+    let mtree = MerkleTree::new(VALUES8.to_vec()).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(VALUES8[..4].to_vec()).unwrap();
+    let subtree2 = MerkleTree::new(VALUES8[2..4].to_vec()).unwrap();
+    let subtree3 = MerkleTree::new(VALUES8[6..].to_vec()).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 original = MerkleStore::new().with_merkle_tree(LEAVES4)?;
-    let decoded = MerkleStore::read_from_bytes(&original.to_bytes())?;
-    assert_eq!(original, decoded);
+    let mtree = MerkleTree::new(VALUES4.to_vec())?;
+    let store = MerkleStore::from(&mtree);
+    let decoded = MerkleStore::read_from_bytes(&store.to_bytes()).expect("deserialization failed");
+    assert_eq!(store, decoded);
    Ok(())
 }
--- a/src/merkle/tiered_smt/mod.rs
+++ b/src/merkle/tiered_smt/mod.rs
@@ -0,0 +1,482 @@
+use super::{
+    BTreeMap, BTreeSet, EmptySubtreeRoots, Felt, InnerNodeInfo, MerkleError, MerklePath, NodeIndex,
+    Rpo256, RpoDigest, StarkField, Vec, Word, EMPTY_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;
+
+    // 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 != EMPTY_WORD || 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 == EMPTY_WORD && !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.into());
+            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 => EMPTY_WORD,
+        }
+    }
+
+    // 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(EMPTY_WORD);
+        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.into(),
+                    left: self.get_node_unchecked(&index.left_child()).into(),
+                    right: self.get_node_unchecked(&index.right_child()).into(),
+                })
+            } 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);
+        }
+        // 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()
+    }
+}
--- a/src/merkle/tiered_smt/tests.rs
+++ b/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().into(), index, leaf_node.into()).unwrap().root;
+
+    smt.insert(key, value);
+
+    assert_eq!(smt.root(), tree_root.into());
+
+    // 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.into()).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.into()).unwrap().root;
+
+    // --- verify that data is consistent between store and tree --------------
+
+    assert_eq!(smt.root(), tree_root.into());
+
+    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.into()).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.into()).unwrap().root;
+
+    // --- verify that data is consistent between store and tree --------------
+
+    assert_eq!(smt.root(), tree_root.into());
+
+    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.into()).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.into()).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.into()).unwrap().root;
+
+    // --- verify that data is consistent between store and tree --------------
+
+    assert_eq!(smt.root(), tree_root.into());
+
+    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.into()).unwrap().root;
+
+    assert_eq!(smt.root(), tree_root.into());
+
+    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.into()).unwrap().root;
+
+    // --- verify that data is consistent between store and tree --------------
+
+    assert_eq!(smt.root(), tree_root.into());
+
+    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.into()).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.into()).unwrap().root;
+
+    // --- verify that data is consistent between store and tree --------------
+
+    assert_eq!(smt.root(), tree_root.into());
+
+    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() -> Word {
+    EmptySubtreeRoots::empty_hashes(64)[0].into()
+}
+
+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);
+    }
+
+    Rpo256::hash_elements(&elements)
+}
+
+fn get_non_empty_nodes(store: &MerkleStore) -> Vec<InnerNodeInfo> {
+    store
+        .inner_nodes()
+        .filter(|node| !is_empty_subtree(&RpoDigest::from(node.value)))
+        .collect::<Vec<_>>()
+}
+
+fn is_empty_subtree(node: &RpoDigest) -> bool {
+    EmptySubtreeRoots::empty_hashes(255).contains(&node)
+}
Author	SHA1	Message	Date
Bobbin Threadbare	2ed880d976	chore: add TieredSmt to readme	2023-05-26 14:41:22 -07:00
Bobbin Threadbare	daa27f49f2	Merge pull request #140 from 0xPolygonMiden/next Tracking PR for v0.5 release	2023-05-26 14:36:20 -07:00
Bobbin Threadbare	dcda57f71a	chore: update changelog	2023-05-26 14:32:17 -07:00
Bobbin Threadbare	d9e3211418	Merge pull request #153 from 0xPolygonMiden/bobbin-tsmt-iter Tiered SMT iterators	2023-05-20 22:52:59 -07:00
Bobbin Threadbare	21e7a5c07d	feat: implement iterators over contents of TieredSmt	2023-05-20 22:47:07 -07:00
Bobbin Threadbare	02673ff87e	Merge pull request #152 from 0xPolygonMiden/bobbin-tsmt Basic Tiered MST	2023-05-16 15:42:34 -07:00
Bobbin Threadbare	b768eade4d	feat: added handling of bottom tier to TieredSmt	2023-05-16 15:38:05 -07:00
Bobbin Threadbare	51ce07cc34	feat: implement basic TieredSmt	2023-05-12 11:33:34 -07:00
Bobbin Threadbare	550738bd94	Merge pull request #151 from 0xPolygonMiden/bobbin-mstore-subset MerkleStore subset and more	2023-05-11 00:41:01 -07:00
Bobbin Threadbare	629494b601	feat: add leaves() iterator to SimpleSmt	2023-05-11 00:37:16 -07:00
Bobbin Threadbare	13aeda5a27	feat: add subset() to MerkleStore	2023-05-09 18:38:21 -07:00
Bobbin Threadbare	e5aba870a2	Merge pull request #149 from 0xPolygonMiden/bobbin-simple-smt SimpleSmt updates	2023-05-08 07:35:00 -07:00
Bobbin Threadbare	fcf03478ba	refactor: update SimpleSmt interfaces	2023-05-08 00:12:24 -07:00
frisitano	0ddd0db89b	Merge pull request #148 from 0xPolygonMiden/frisitano-mmr-accumulator refactor: Mmr accumulator	2023-05-05 17:56:46 +08:00
frisitano	2100d6c861	refactor(mmr): expose method to join mmr peaks in a vector and pad	2023-05-05 12:03:32 +08:00
Bobbin Threadbare	52409ac039	Merge pull request #146 from 0xPolygonMiden/frisitano-merkle-store-inner-nodes feat: add .inner_nodes() to [MerkleStore]	2023-05-04 13:49:36 -07:00
frisitano	4555fc918f	feat: add .inner_nodes() to [MerkleStore]	2023-05-04 19:15:52 +07:00
Bobbin Threadbare	52db23cd42	chore: update crate version to v0.5.0	2023-04-21 15:48:18 -07:00
Bobbin Threadbare	09025b4014	Merge pull request #129 from 0xPolygonMiden/next Tracking PR for v0.4 release	2023-04-21 15:38:33 -07:00
Bobbin Threadbare	e983e940b2	chore: update changelog	2023-04-21 14:42:08 -07:00
Bobbin Threadbare	ae4e27b6c7	Merge pull request #139 from 0xPolygonMiden/hacka-support-adding-existing-structures-to-store store: support adding existing structures	2023-04-21 14:32:52 -07:00
Bobbin Threadbare	130ae3d12a	feat: add inner node iterator to MerklePath	2023-04-21 14:27:58 -07:00
Bobbin Threadbare	22c9f382c4	fix: serialization test	2023-04-21 11:39:49 -07:00
Bobbin Threadbare	9be4253f19	feat: remove clone requirement for MerkleStore From constructors	2023-04-21 11:22:36 -07:00
Augusto F. Hack	59595a2e04	feat: added From convertions for the MerkleStore	2023-04-21 14:47:58 +02:00
Augusto F. Hack	eb316f51bc	store: remove SimpleSMT/MerkleTree/Mmr add/with methods	2023-04-21 14:47:48 +02:00
Augusto F. Hack	8161477d6a	store: support adding existing structures	2023-04-20 13:45:31 +02:00
Augusto Hack	158167356d	Merge pull request #138 from 0xPolygonMiden/hacka-merge-support-for-leaves feat: allow merging of leaves	2023-04-17 12:29:13 +02:00
Augusto F. Hack	3996374a8b	feat: allow merging of leaves Consider the case of a MMR with one entry, and a new entry is being added. Both of these values are quite unique, they are at the same time the root and only leaf of their corresponding tree. Currently this representation is not supported by the [MerkleStore], so the leaves are not in it. Once the two values are merged, they both become leaves of a new tree under the new parent, and the existing validation didn't permit that promotion from happening. This lifts the validation, and changes the method to clarify that not only `root` are being merged, by arbitrary nodes of a tree (leafs, internal, or roots), with arbitrary mixing of each.	2023-04-17 12:21:51 +02:00
Augusto Hack	7fa03c7967	Merge pull request #137 from 0xPolygonMiden/frisitano-reexport-mmr-proof re-export mmr proof	2023-04-14 14:34:58 +02:00
frisitano	79915cc346	feat: re-export MmrProof	2023-04-14 13:25:19 +01:00
Augusto Hack	45412b5cec	Merge pull request #134 from 0xPolygonMiden/add-rustfmt-config config: add rustfmt config	2023-04-11 17:58:06 +02:00
Augusto F. Hack	bbb1e641a3	config: add rustfmt config	2023-04-11 17:38:39 +02:00
Bobbin Threadbare	e02507d11e	chore: update version to v0.4.0	2023-04-08 12:46:53 -07:00