feat: reverse mutations generation, mutations serialization (#355)

* feat: revert mutations generation, mutations serialization
* tests: check both `apply_mutations` and `apply_mutations_with_reversion`
* feat: add `num_leaves` method for `Smt`
* refactor: improve ad-hoc benchmarks
* chore: update crate version to v0.13.1

src/merkle/index.rs

@@ -128,7 +128,7 @@ impl NodeIndex {
         self.value
     }
 
-    /// Returns true if the current instance points to a right sibling node.
+    /// Returns `true` if the current instance points to a right sibling node.
     pub const fn is_value_odd(&self) -> bool {
         (self.value & 1) == 1
     }

src/merkle/mmr/partial.rs

@@ -303,7 +303,7 @@ impl PartialMmr {
 
         if leaf_pos + 1 == self.forest
             && path.depth() == 0
-            && self.peaks.last().map_or(false, |v| *v == leaf)
+            && self.peaks.last().is_some_and(|v| *v == leaf)
         {
             self.track_latest = true;
             return Ok(());

src/merkle/smt/full/leaf.rs

@@ -70,7 +70,7 @@ impl SmtLeaf {
         Self::Single((key, value))
     }
 
-    /// Returns a new single leaf with the specified entry. The leaf index is derived from the
+    /// Returns a new multiple leaf with the specified entries. The leaf index is derived from the
     /// entries' keys.
     ///
     /// # Errors

src/merkle/smt/full/mod.rs

@@ -114,6 +114,11 @@ impl Smt {
         <Self as SparseMerkleTree<SMT_DEPTH>>::root(self)
     }
 
+    /// Returns the number of non-empty leaves in this tree.
+    pub fn num_leaves(&self) -> usize {
+        self.leaves.len()
+    }
+
     /// Returns the leaf to which `key` maps
     pub fn get_leaf(&self, key: &RpoDigest) -> SmtLeaf {
         <Self as SparseMerkleTree<SMT_DEPTH>>::get_leaf(self, key)
@@ -200,7 +205,7 @@ impl Smt {
         <Self as SparseMerkleTree<SMT_DEPTH>>::compute_mutations(self, kv_pairs)
     }
 
-    /// Apply the prospective mutations computed with [`Smt::compute_mutations()`] to this tree.
+    /// Applies the prospective mutations computed with [`Smt::compute_mutations()`] to this tree.
     ///
     /// # Errors
     /// If `mutations` was computed on a tree with a different root than this one, returns
@@ -214,6 +219,23 @@ impl Smt {
         <Self as SparseMerkleTree<SMT_DEPTH>>::apply_mutations(self, mutations)
     }
 
+    /// Applies the prospective mutations computed with [`Smt::compute_mutations()`] to this tree
+    /// and returns the reverse mutation set.
+    ///
+    /// Applying the reverse mutation sets to the updated tree will revert the changes.
+    ///
+    /// # Errors
+    /// If `mutations` was computed on a tree with a different root than this one, returns
+    /// [`MerkleError::ConflictingRoots`] with a two-item [`Vec`]. The first item is the root hash
+    /// the `mutations` were computed against, and the second item is the actual current root of
+    /// this tree.
+    pub fn apply_mutations_with_reversion(
+        &mut self,
+        mutations: MutationSet<SMT_DEPTH, RpoDigest, Word>,
+    ) -> Result<MutationSet<SMT_DEPTH, RpoDigest, Word>, MerkleError> {
+        <Self as SparseMerkleTree<SMT_DEPTH>>::apply_mutations_with_reversion(self, mutations)
+    }
+
     // HELPERS
     // --------------------------------------------------------------------------------------------
 
@@ -275,12 +297,12 @@ impl SparseMerkleTree<SMT_DEPTH> for Smt {
             .unwrap_or_else(|| EmptySubtreeRoots::get_inner_node(SMT_DEPTH, index.depth()))
     }
 
-    fn insert_inner_node(&mut self, index: NodeIndex, inner_node: InnerNode) {
-        self.inner_nodes.insert(index, inner_node);
+    fn insert_inner_node(&mut self, index: NodeIndex, inner_node: InnerNode) -> Option<InnerNode> {
+        self.inner_nodes.insert(index, inner_node)
     }
 
-    fn remove_inner_node(&mut self, index: NodeIndex) {
-        let _ = self.inner_nodes.remove(&index);
+    fn remove_inner_node(&mut self, index: NodeIndex) -> Option<InnerNode> {
+        self.inner_nodes.remove(&index)
     }
 
     fn insert_value(&mut self, key: Self::Key, value: Self::Value) -> Option<Self::Value> {

src/merkle/smt/full/tests.rs

@@ -1,12 +1,14 @@
-use alloc::vec::Vec;
+use alloc::{collections::BTreeMap, vec::Vec};
 
 use super::{Felt, LeafIndex, NodeIndex, Rpo256, RpoDigest, Smt, SmtLeaf, EMPTY_WORD, SMT_DEPTH};
 use crate::{
-    merkle::{smt::SparseMerkleTree, EmptySubtreeRoots, MerkleStore},
+    merkle::{
+        smt::{NodeMutation, SparseMerkleTree},
+        EmptySubtreeRoots, MerkleStore, MutationSet,
+    },
     utils::{Deserializable, Serializable},
     Word, ONE, WORD_SIZE,
 };
-
 // SMT
 // --------------------------------------------------------------------------------------------
 
@@ -412,21 +414,49 @@ fn test_prospective_insertion() {
 
     let mutations = smt.compute_mutations(vec![(key_1, value_1)]);
     assert_eq!(mutations.root(), root_1, "prospective root 1 did not match actual root 1");
-    smt.apply_mutations(mutations).unwrap();
+    let revert = apply_mutations(&mut smt, mutations);
     assert_eq!(smt.root(), root_1, "mutations before and after apply did not match");
+    assert_eq!(revert.old_root, smt.root(), "reverse mutations old root did not match");
+    assert_eq!(revert.root(), root_empty, "reverse mutations new root did not match");
+    assert_eq!(
+        revert.new_pairs,
+        BTreeMap::from_iter([(key_1, EMPTY_WORD)]),
+        "reverse mutations pairs did not match"
+    );
+    assert_eq!(
+        revert.node_mutations,
+        smt.inner_nodes.keys().map(|key| (*key, NodeMutation::Removal)).collect(),
+        "reverse mutations inner nodes did not match"
+    );
 
     let mutations = smt.compute_mutations(vec![(key_2, value_2)]);
     assert_eq!(mutations.root(), root_2, "prospective root 2 did not match actual root 2");
     let mutations =
         smt.compute_mutations(vec![(key_3, EMPTY_WORD), (key_2, value_2), (key_3, value_3)]);
     assert_eq!(mutations.root(), root_3, "mutations before and after apply did not match");
-    smt.apply_mutations(mutations).unwrap();
+    let old_root = smt.root();
+    let revert = apply_mutations(&mut smt, mutations);
+    assert_eq!(revert.old_root, smt.root(), "reverse mutations old root did not match");
+    assert_eq!(revert.root(), old_root, "reverse mutations new root did not match");
+    assert_eq!(
+        revert.new_pairs,
+        BTreeMap::from_iter([(key_2, EMPTY_WORD), (key_3, EMPTY_WORD)]),
+        "reverse mutations pairs did not match"
+    );
 
     // Edge case: multiple values at the same key, where a later pair restores the original value.
     let mutations = smt.compute_mutations(vec![(key_3, EMPTY_WORD), (key_3, value_3)]);
     assert_eq!(mutations.root(), root_3);
-    smt.apply_mutations(mutations).unwrap();
+    let old_root = smt.root();
+    let revert = apply_mutations(&mut smt, mutations);
     assert_eq!(smt.root(), root_3);
+    assert_eq!(revert.old_root, smt.root(), "reverse mutations old root did not match");
+    assert_eq!(revert.root(), old_root, "reverse mutations new root did not match");
+    assert_eq!(
+        revert.new_pairs,
+        BTreeMap::from_iter([(key_3, value_3)]),
+        "reverse mutations pairs did not match"
+    );
 
     // Test batch updates, and that the order doesn't matter.
     let pairs =
@@ -437,8 +467,16 @@ fn test_prospective_insertion() {
         root_empty,
         "prospective root for batch removal did not match actual root",
     );
-    smt.apply_mutations(mutations).unwrap();
+    let old_root = smt.root();
+    let revert = apply_mutations(&mut smt, mutations);
     assert_eq!(smt.root(), root_empty, "mutations before and after apply did not match");
+    assert_eq!(revert.old_root, smt.root(), "reverse mutations old root did not match");
+    assert_eq!(revert.root(), old_root, "reverse mutations new root did not match");
+    assert_eq!(
+        revert.new_pairs,
+        BTreeMap::from_iter([(key_1, value_1), (key_2, value_2), (key_3, value_3)]),
+        "reverse mutations pairs did not match"
+    );
 
     let pairs = vec![(key_3, value_3), (key_1, value_1), (key_2, value_2)];
     let mutations = smt.compute_mutations(pairs);
@@ -447,6 +485,72 @@ fn test_prospective_insertion() {
     assert_eq!(smt.root(), root_3);
 }
 
+#[test]
+fn test_mutations_revert() {
+    let mut smt = Smt::default();
+
+    let key_1: RpoDigest = RpoDigest::from([ONE, ONE, ONE, Felt::new(1)]);
+    let key_2: RpoDigest =
+        RpoDigest::from([2_u32.into(), 2_u32.into(), 2_u32.into(), Felt::new(2)]);
+    let key_3: RpoDigest =
+        RpoDigest::from([0_u32.into(), 0_u32.into(), 0_u32.into(), Felt::new(3)]);
+
+    let value_1 = [ONE; WORD_SIZE];
+    let value_2 = [2_u32.into(); WORD_SIZE];
+    let value_3 = [3_u32.into(); WORD_SIZE];
+
+    smt.insert(key_1, value_1);
+    smt.insert(key_2, value_2);
+
+    let mutations =
+        smt.compute_mutations(vec![(key_1, EMPTY_WORD), (key_2, value_1), (key_3, value_3)]);
+
+    let original = smt.clone();
+
+    let revert = smt.apply_mutations_with_reversion(mutations).unwrap();
+    assert_eq!(revert.old_root, smt.root(), "reverse mutations old root did not match");
+    assert_eq!(revert.root(), original.root(), "reverse mutations new root did not match");
+
+    smt.apply_mutations(revert).unwrap();
+
+    assert_eq!(smt, original, "SMT with applied revert mutations did not match original SMT");
+}
+
+#[test]
+fn test_mutation_set_serialization() {
+    let mut smt = Smt::default();
+
+    let key_1: RpoDigest = RpoDigest::from([ONE, ONE, ONE, Felt::new(1)]);
+    let key_2: RpoDigest =
+        RpoDigest::from([2_u32.into(), 2_u32.into(), 2_u32.into(), Felt::new(2)]);
+    let key_3: RpoDigest =
+        RpoDigest::from([0_u32.into(), 0_u32.into(), 0_u32.into(), Felt::new(3)]);
+
+    let value_1 = [ONE; WORD_SIZE];
+    let value_2 = [2_u32.into(); WORD_SIZE];
+    let value_3 = [3_u32.into(); WORD_SIZE];
+
+    smt.insert(key_1, value_1);
+    smt.insert(key_2, value_2);
+
+    let mutations =
+        smt.compute_mutations(vec![(key_1, EMPTY_WORD), (key_2, value_1), (key_3, value_3)]);
+
+    let serialized = mutations.to_bytes();
+    let deserialized =
+        MutationSet::<SMT_DEPTH, RpoDigest, Word>::read_from_bytes(&serialized).unwrap();
+
+    assert_eq!(deserialized, mutations, "deserialized mutations did not match original");
+
+    let revert = smt.apply_mutations_with_reversion(mutations).unwrap();
+
+    let serialized = revert.to_bytes();
+    let deserialized =
+        MutationSet::<SMT_DEPTH, RpoDigest, Word>::read_from_bytes(&serialized).unwrap();
+
+    assert_eq!(deserialized, revert, "deserialized mutations did not match original");
+}
+
 /// Tests that 2 key-value pairs stored in the same leaf have the same path
 #[test]
 fn test_smt_path_to_keys_in_same_leaf_are_equal() {
@@ -602,3 +706,19 @@ fn build_multiple_leaf_node(kv_pairs: &[(RpoDigest, Word)]) -> RpoDigest {
 
     Rpo256::hash_elements(&elements)
 }
+
+/// Applies mutations with and without reversion to the given SMT, comparing resulting SMTs,
+/// returning mutation set for reversion.
+fn apply_mutations(
+    smt: &mut Smt,
+    mutation_set: MutationSet<SMT_DEPTH, RpoDigest, Word>,
+) -> MutationSet<SMT_DEPTH, RpoDigest, Word> {
+    let mut smt2 = smt.clone();
+
+    let reversion = smt.apply_mutations_with_reversion(mutation_set.clone()).unwrap();
+    smt2.apply_mutations(mutation_set).unwrap();
+
+    assert_eq!(&smt2, smt);
+
+    reversion
+}

src/merkle/smt/mod.rs

@@ -1,5 +1,7 @@
 use alloc::{collections::BTreeMap, vec::Vec};
 
+use winter_utils::{ByteReader, ByteWriter, Deserializable, DeserializationError, Serializable};
+
 use super::{EmptySubtreeRoots, InnerNodeInfo, MerkleError, MerklePath, NodeIndex};
 use crate::{
     hash::rpo::{Rpo256, RpoDigest},
@@ -135,7 +137,7 @@ pub(crate) trait SparseMerkleTree<const DEPTH: u8> {
             if node_hash == *EmptySubtreeRoots::entry(DEPTH, node_depth) {
                 // If a subtree is empty, then can remove the inner node, since it's equal to the
                 // default value
-                self.remove_inner_node(index)
+                self.remove_inner_node(index);
             } else {
                 self.insert_inner_node(index, InnerNode { left, right });
             }
@@ -241,7 +243,7 @@ pub(crate) trait SparseMerkleTree<const DEPTH: u8> {
         }
     }
 
-    /// Apply the prospective mutations computed with [`SparseMerkleTree::compute_mutations()`] to
+    /// Applies the prospective mutations computed with [`SparseMerkleTree::compute_mutations()`] to
     /// this tree.
     ///
     /// # Errors
@@ -275,8 +277,12 @@ pub(crate) trait SparseMerkleTree<const DEPTH: u8> {
 
         for (index, mutation) in node_mutations {
             match mutation {
-                Removal => self.remove_inner_node(index),
-                Addition(node) => self.insert_inner_node(index, node),
+                Removal => {
+                    self.remove_inner_node(index);
+                },
+                Addition(node) => {
+                    self.insert_inner_node(index, node);
+                },
             }
         }
 
@@ -289,6 +295,76 @@ pub(crate) trait SparseMerkleTree<const DEPTH: u8> {
         Ok(())
     }
 
+    /// Applies the prospective mutations computed with [`SparseMerkleTree::compute_mutations()`] to
+    /// this tree and returns the reverse mutation set. Applying the reverse mutation sets to the
+    /// updated tree will revert the changes.
+    ///
+    /// # Errors
+    /// If `mutations` was computed on a tree with a different root than this one, returns
+    /// [`MerkleError::ConflictingRoots`] with a two-item [`Vec`]. The first item is the root hash
+    /// the `mutations` were computed against, and the second item is the actual current root of
+    /// this tree.
+    fn apply_mutations_with_reversion(
+        &mut self,
+        mutations: MutationSet<DEPTH, Self::Key, Self::Value>,
+    ) -> Result<MutationSet<DEPTH, Self::Key, Self::Value>, MerkleError>
+    where
+        Self: Sized,
+    {
+        use NodeMutation::*;
+        let MutationSet {
+            old_root,
+            node_mutations,
+            new_pairs,
+            new_root,
+        } = mutations;
+
+        // Guard against accidentally trying to apply mutations that were computed against a
+        // different tree, including a stale version of this tree.
+        if old_root != self.root() {
+            return Err(MerkleError::ConflictingRoots {
+                expected_root: self.root(),
+                actual_root: old_root,
+            });
+        }
+
+        let mut reverse_mutations = BTreeMap::new();
+        for (index, mutation) in node_mutations {
+            match mutation {
+                Removal => {
+                    if let Some(node) = self.remove_inner_node(index) {
+                        reverse_mutations.insert(index, Addition(node));
+                    }
+                },
+                Addition(node) => {
+                    if let Some(old_node) = self.insert_inner_node(index, node) {
+                        reverse_mutations.insert(index, Addition(old_node));
+                    } else {
+                        reverse_mutations.insert(index, Removal);
+                    }
+                },
+            }
+        }
+
+        let mut reverse_pairs = BTreeMap::new();
+        for (key, value) in new_pairs {
+            if let Some(old_value) = self.insert_value(key.clone(), value) {
+                reverse_pairs.insert(key, old_value);
+            } else {
+                reverse_pairs.insert(key, Self::EMPTY_VALUE);
+            }
+        }
+
+        self.set_root(new_root);
+
+        Ok(MutationSet {
+            old_root: new_root,
+            node_mutations: reverse_mutations,
+            new_pairs: reverse_pairs,
+            new_root: old_root,
+        })
+    }
+
     // REQUIRED METHODS
     // ---------------------------------------------------------------------------------------------
 
@@ -302,10 +378,10 @@ pub(crate) trait SparseMerkleTree<const DEPTH: u8> {
     fn get_inner_node(&self, index: NodeIndex) -> InnerNode;
 
     /// Inserts an inner node at the given index
-    fn insert_inner_node(&mut self, index: NodeIndex, inner_node: InnerNode);
+    fn insert_inner_node(&mut self, index: NodeIndex, inner_node: InnerNode) -> Option<InnerNode>;
 
     /// Removes an inner node at the given index
-    fn remove_inner_node(&mut self, index: NodeIndex);
+    fn remove_inner_node(&mut self, index: NodeIndex) -> Option<InnerNode>;
 
     /// Inserts a leaf node, and returns the value at the key if already exists
     fn insert_value(&mut self, key: Self::Key, value: Self::Value) -> Option<Self::Value>;
@@ -352,7 +428,7 @@ pub(crate) trait SparseMerkleTree<const DEPTH: u8> {
 
 #[derive(Debug, Default, Clone, PartialEq, Eq)]
 #[cfg_attr(feature = "serde", derive(serde::Deserialize, serde::Serialize))]
-pub(crate) struct InnerNode {
+pub struct InnerNode {
     pub left: RpoDigest,
     pub right: RpoDigest,
 }
@@ -423,7 +499,7 @@ impl<const DEPTH: u8> TryFrom<NodeIndex> for LeafIndex<DEPTH> {
 /// [`MutationSet`] stores this type in relation to a [`NodeIndex`] to keep track of what changes
 /// need to occur at which node indices.
 #[derive(Debug, Clone, PartialEq, Eq)]
-pub(crate) enum NodeMutation {
+pub enum NodeMutation {
     /// Corresponds to [`SparseMerkleTree::remove_inner_node()`].
     Removal,
     /// Corresponds to [`SparseMerkleTree::insert_inner_node()`].
@@ -456,9 +532,94 @@ pub struct MutationSet<const DEPTH: u8, K, V> {
 }
 
 impl<const DEPTH: u8, K, V> MutationSet<DEPTH, K, V> {
-    /// Queries the root that was calculated during `SparseMerkleTree::compute_mutations()`. See
+    /// Returns the SMT root that was calculated during `SparseMerkleTree::compute_mutations()`. See
     /// that method for more information.
     pub fn root(&self) -> RpoDigest {
         self.new_root
     }
+
+    /// Returns the SMT root before the mutations were applied.
+    pub fn old_root(&self) -> RpoDigest {
+        self.old_root
+    }
+
+    /// Returns the set of inner nodes that need to be removed or added.
+    pub fn node_mutations(&self) -> &BTreeMap<NodeIndex, NodeMutation> {
+        &self.node_mutations
+    }
+
+    /// Returns the set of top-level key-value pairs that need to be added, updated or deleted
+    /// (i.e. set to `EMPTY_WORD`).
+    pub fn new_pairs(&self) -> &BTreeMap<K, V> {
+        &self.new_pairs
+    }
+}
+
+// SERIALIZATION
+// ================================================================================================
+
+impl Serializable for InnerNode {
+    fn write_into<W: ByteWriter>(&self, target: &mut W) {
+        self.left.write_into(target);
+        self.right.write_into(target);
+    }
+}
+
+impl Deserializable for InnerNode {
+    fn read_from<R: ByteReader>(source: &mut R) -> Result<Self, DeserializationError> {
+        let left = source.read()?;
+        let right = source.read()?;
+
+        Ok(Self { left, right })
+    }
+}
+
+impl Serializable for NodeMutation {
+    fn write_into<W: ByteWriter>(&self, target: &mut W) {
+        match self {
+            NodeMutation::Removal => target.write_bool(false),
+            NodeMutation::Addition(inner_node) => {
+                target.write_bool(true);
+                inner_node.write_into(target);
+            },
+        }
+    }
+}
+
+impl Deserializable for NodeMutation {
+    fn read_from<R: ByteReader>(source: &mut R) -> Result<Self, DeserializationError> {
+        if source.read_bool()? {
+            let inner_node = source.read()?;
+            return Ok(NodeMutation::Addition(inner_node));
+        }
+
+        Ok(NodeMutation::Removal)
+    }
+}
+
+impl<const DEPTH: u8, K: Serializable, V: Serializable> Serializable for MutationSet<DEPTH, K, V> {
+    fn write_into<W: ByteWriter>(&self, target: &mut W) {
+        target.write(self.old_root);
+        target.write(self.new_root);
+        self.node_mutations.write_into(target);
+        self.new_pairs.write_into(target);
+    }
+}
+
+impl<const DEPTH: u8, K: Deserializable + Ord, V: Deserializable> Deserializable
+    for MutationSet<DEPTH, K, V>
+{
+    fn read_from<R: ByteReader>(source: &mut R) -> Result<Self, DeserializationError> {
+        let old_root = source.read()?;
+        let new_root = source.read()?;
+        let node_mutations = source.read()?;
+        let new_pairs = source.read()?;
+
+        Ok(Self {
+            old_root,
+            node_mutations,
+            new_pairs,
+            new_root,
+        })
+    }
 }

src/merkle/smt/simple/mod.rs

@@ -221,7 +221,7 @@ impl<const DEPTH: u8> SimpleSmt<DEPTH> {
         <Self as SparseMerkleTree<DEPTH>>::compute_mutations(self, kv_pairs)
     }
 
-    /// Apply the prospective mutations computed with [`SimpleSmt::compute_mutations()`] to this
+    /// Applies the prospective mutations computed with [`SimpleSmt::compute_mutations()`] to this
     /// tree.
     ///
     /// # Errors
@@ -236,6 +236,23 @@ impl<const DEPTH: u8> SimpleSmt<DEPTH> {
         <Self as SparseMerkleTree<DEPTH>>::apply_mutations(self, mutations)
     }
 
+    /// Applies the prospective mutations computed with [`SimpleSmt::compute_mutations()`] to
+    /// this tree and returns the reverse mutation set.
+    ///
+    /// Applying the reverse mutation sets to the updated tree will revert the changes.
+    ///
+    /// # Errors
+    /// If `mutations` was computed on a tree with a different root than this one, returns
+    /// [`MerkleError::ConflictingRoots`] with a two-item [`alloc::vec::Vec`]. The first item is the
+    /// root hash the `mutations` were computed against, and the second item is the actual
+    /// current root of this tree.
+    pub fn apply_mutations_with_reversion(
+        &mut self,
+        mutations: MutationSet<DEPTH, LeafIndex<DEPTH>, Word>,
+    ) -> Result<MutationSet<DEPTH, LeafIndex<DEPTH>, Word>, MerkleError> {
+        <Self as SparseMerkleTree<DEPTH>>::apply_mutations_with_reversion(self, mutations)
+    }
+
     /// Inserts a subtree at the specified index. The depth at which the subtree is inserted is
     /// computed as `DEPTH - SUBTREE_DEPTH`.
     ///
@@ -321,12 +338,12 @@ impl<const DEPTH: u8> SparseMerkleTree<DEPTH> for SimpleSmt<DEPTH> {
             .unwrap_or_else(|| EmptySubtreeRoots::get_inner_node(DEPTH, index.depth()))
     }
 
-    fn insert_inner_node(&mut self, index: NodeIndex, inner_node: InnerNode) {
-        self.inner_nodes.insert(index, inner_node);
+    fn insert_inner_node(&mut self, index: NodeIndex, inner_node: InnerNode) -> Option<InnerNode> {
+        self.inner_nodes.insert(index, inner_node)
     }
 
-    fn remove_inner_node(&mut self, index: NodeIndex) {
-        let _ = self.inner_nodes.remove(&index);
+    fn remove_inner_node(&mut self, index: NodeIndex) -> Option<InnerNode> {
+        self.inner_nodes.remove(&index)
     }
 
     fn insert_value(&mut self, key: LeafIndex<DEPTH>, value: Word) -> Option<Word> {