Remove "old" item requirement from treehash

eth2/utils/tree_hash/src/cached_tree_hash.rs

@@ -17,10 +17,13 @@ pub enum Error {
     UnableToObtainSlices,
     UnableToGrowMerkleTree,
     UnableToShrinkMerkleTree,
+    TreeCannotHaveZeroNodes,
     ShouldNeverBePacked(TreeHashType),
     BytesAreNotEvenChunks(usize),
     NoModifiedFieldForChunk(usize),
     NoBytesForChunk(usize),
+    NoOverlayForIndex(usize),
+    NotLeafNode(usize),
 }
 
 pub trait CachedTreeHash<T>: CachedTreeHashSubTree<T> + Sized {
@@ -36,12 +39,7 @@ pub trait CachedTreeHashSubTree<Item>: TreeHash {
 
     fn new_tree_hash_cache(&self) -> Result<TreeHashCache, Error>;
 
-    fn update_tree_hash_cache(
-        &self,
-        other: &Item,
-        cache: &mut TreeHashCache,
-        chunk: usize,
-    ) -> Result<usize, Error>;
+    fn update_tree_hash_cache(&self, cache: &mut TreeHashCache) -> Result<(), Error>;
 }
 
 fn children(parent: usize) -> (usize, usize) {
@@ -123,6 +121,10 @@ fn num_bytes(num_leaves: usize) -> usize {
 pub struct TreeHashCache {
     cache: Vec<u8>,
     chunk_modified: Vec<bool>,
+    overlays: Vec<BTreeOverlay>,
+
+    pub chunk_index: usize,
+    pub overlay_index: usize,
 }
 
 impl Into<Vec<u8>> for TreeHashCache {
@@ -139,10 +141,17 @@ impl TreeHashCache {
         item.new_tree_hash_cache()
     }
 
-    pub fn from_elems(cache: Vec<u8>, chunk_modified: Vec<bool>) -> Self {
+    pub fn from_elems(
+        cache: Vec<u8>,
+        chunk_modified: Vec<bool>,
+        overlays: Vec<BTreeOverlay>,
+    ) -> Self {
         Self {
             cache,
             chunk_modified,
+            overlays,
+            chunk_index: 0,
+            overlay_index: 0,
         }
     }
 
@@ -153,7 +162,7 @@ impl TreeHashCache {
     where
         T: CachedTreeHashSubTree<T>,
     {
-        let offset_handler = BTreeOverlay::new(item, 0)?;
+        let overlay = BTreeOverlay::new(item, 0)?;
 
         // Note how many leaves were provided. If is not a power-of-two, we'll need to pad it out
         // later.
@@ -161,7 +170,7 @@ impl TreeHashCache {
 
         // Allocate enough bytes to store the internal nodes and the leaves and subtrees, then fill
         // all the to-be-built internal nodes with zeros and append the leaves and subtrees.
-        let internal_node_bytes = offset_handler.num_internal_nodes * BYTES_PER_CHUNK;
+        let internal_node_bytes = overlay.num_internal_nodes() * BYTES_PER_CHUNK;
         let leaves_and_subtrees_bytes = leaves_and_subtrees
             .iter()
             .fold(0, |acc, t| acc + t.bytes_len());
@@ -169,13 +178,19 @@ impl TreeHashCache {
         cache.resize(internal_node_bytes, 0);
 
         // Allocate enough bytes to store all the leaves.
-        let mut leaves = Vec::with_capacity(offset_handler.num_leaf_nodes * HASHSIZE);
+        let mut leaves = Vec::with_capacity(overlay.num_leaf_nodes() * HASHSIZE);
+        let mut overlays = Vec::with_capacity(leaves_and_subtrees.len());
+        overlays.push(overlay);
 
         // Iterate through all of the leaves/subtrees, adding their root as a leaf node and then
         // concatenating their merkle trees.
         for t in leaves_and_subtrees {
             leaves.append(&mut t.root().ok_or_else(|| Error::NoBytesForRoot)?.to_vec());
-            cache.append(&mut t.into_merkle_tree());
+
+            let (mut bytes, _bools, mut t_overlays) = t.into_components();
+
+            cache.append(&mut bytes);
+            overlays.append(&mut t_overlays);
         }
 
         // Pad the leaves to an even power-of-two, using zeros.
@@ -190,10 +205,17 @@ impl TreeHashCache {
         Ok(Self {
             chunk_modified: vec![false; cache.len() / BYTES_PER_CHUNK],
             cache,
+            overlays,
+            chunk_index: 0,
+            overlay_index: 0,
         })
     }
 
-    pub fn from_bytes(bytes: Vec<u8>, initial_modified_state: bool) -> Result<Self, Error> {
+    pub fn from_bytes(
+        bytes: Vec<u8>,
+        initial_modified_state: bool,
+        overlay: BTreeOverlay,
+    ) -> Result<Self, Error> {
         if bytes.len() % BYTES_PER_CHUNK > 0 {
             return Err(Error::BytesAreNotEvenChunks(bytes.len()));
         }
@@ -201,9 +223,84 @@ impl TreeHashCache {
         Ok(Self {
             chunk_modified: vec![initial_modified_state; bytes.len() / BYTES_PER_CHUNK],
             cache: bytes,
+            overlays: vec![overlay],
+            chunk_index: 0,
+            overlay_index: 0,
         })
     }
 
+    pub fn get_overlay(
+        &self,
+        overlay_index: usize,
+        chunk_index: usize,
+    ) -> Result<BTreeOverlay, Error> {
+        let mut overlay = self
+            .overlays
+            .get(overlay_index)
+            .ok_or_else(|| Error::NoOverlayForIndex(overlay_index))?
+            .clone();
+
+        overlay.offset = chunk_index;
+
+        Ok(overlay)
+    }
+
+    pub fn replace_overlay(
+        &mut self,
+        overlay_index: usize,
+        new_overlay: BTreeOverlay,
+    ) -> Result<BTreeOverlay, Error> {
+        let old_overlay = self
+            .overlays
+            .get(overlay_index)
+            .ok_or_else(|| Error::NoOverlayForIndex(overlay_index))?;
+
+        // Get slices of the exsiting tree from the cache.
+        let (old_bytes, old_flags) = self
+            .slices(old_overlay.chunk_range())
+            .ok_or_else(|| Error::UnableToObtainSlices)?;
+
+        let (new_bytes, new_bools) = if new_overlay.num_leaf_nodes() > old_overlay.num_leaf_nodes()
+        {
+            resize::grow_merkle_cache(
+                old_bytes,
+                old_flags,
+                old_overlay.height(),
+                new_overlay.height(),
+            )
+            .ok_or_else(|| Error::UnableToGrowMerkleTree)?
+        } else {
+            resize::shrink_merkle_cache(
+                old_bytes,
+                old_flags,
+                old_overlay.height(),
+                new_overlay.height(),
+                new_overlay.total_chunks(),
+            )
+            .ok_or_else(|| Error::UnableToShrinkMerkleTree)?
+        };
+
+        // Splice the newly created `TreeHashCache` over the existing elements.
+        self.splice(old_overlay.chunk_range(), new_bytes, new_bools);
+
+        Ok(std::mem::replace(
+            &mut self.overlays[overlay_index],
+            new_overlay,
+        ))
+    }
+
+    pub fn update_internal_nodes(&mut self, overlay: &BTreeOverlay) -> Result<(), Error> {
+        for (parent, children) in overlay.internal_parents_and_children().into_iter().rev() {
+            dbg!(parent);
+            dbg!(&children);
+            if self.either_modified(children)? {
+                self.modify_chunk(parent, &self.hash_children(children)?)?;
+            }
+        }
+
+        Ok(())
+    }
+
     pub fn bytes_len(&self) -> usize {
         self.cache.len()
     }
@@ -212,9 +309,7 @@ impl TreeHashCache {
         self.cache.get(0..HASHSIZE)
     }
 
-    pub fn splice(&mut self, chunk_range: Range<usize>, replace_with: Self) {
-        let (bytes, bools) = replace_with.into_components();
-
+    pub fn splice(&mut self, chunk_range: Range<usize>, bytes: Vec<u8>, bools: Vec<bool>) {
         // Update the `chunk_modified` vec, marking all spliced-in nodes as changed.
         self.chunk_modified.splice(chunk_range.clone(), bools);
         self.cache
@@ -278,14 +373,14 @@ impl TreeHashCache {
             .ok_or_else(|| Error::NoModifiedFieldForChunk(chunk))
     }
 
-    pub fn either_modified(&self, children: (&usize, &usize)) -> Result<bool, Error> {
-        Ok(self.changed(*children.0)? | self.changed(*children.1)?)
+    pub fn either_modified(&self, children: (usize, usize)) -> Result<bool, Error> {
+        Ok(self.changed(children.0)? | self.changed(children.1)?)
     }
 
-    pub fn hash_children(&self, children: (&usize, &usize)) -> Result<Vec<u8>, Error> {
+    pub fn hash_children(&self, children: (usize, usize)) -> Result<Vec<u8>, Error> {
         let mut child_bytes = Vec::with_capacity(BYTES_PER_CHUNK * 2);
-        child_bytes.append(&mut self.get_chunk(*children.0)?.to_vec());
-        child_bytes.append(&mut self.get_chunk(*children.1)?.to_vec());
+        child_bytes.append(&mut self.get_chunk(children.0)?.to_vec());
+        child_bytes.append(&mut self.get_chunk(children.1)?.to_vec());
 
         Ok(hash(&child_bytes))
     }
@@ -299,11 +394,7 @@ impl TreeHashCache {
         Ok(hash(&bytes))
     }
 
-    pub fn into_merkle_tree(self) -> Vec<u8> {
-        self.cache
-    }
-
-    pub fn into_components(self) -> (Vec<u8>, Vec<bool>) {
-        (self.cache, self.chunk_modified)
+    pub fn into_components(self) -> (Vec<u8>, Vec<bool>, Vec<BTreeOverlay>) {
+        (self.cache, self.chunk_modified, self.overlays)
     }
 }

eth2/utils/tree_hash/src/cached_tree_hash/btree_overlay.rs

@@ -1,12 +1,9 @@
 use super::*;
 
-#[derive(Debug)]
+#[derive(Debug, PartialEq, Clone)]
 pub struct BTreeOverlay {
-    pub num_internal_nodes: usize,
-    pub num_leaf_nodes: usize,
-    pub first_node: usize,
-    pub next_node: usize,
-    offsets: Vec<usize>,
+    pub offset: usize,
+    lengths: Vec<usize>,
 }
 
 impl BTreeOverlay {
@@ -17,84 +14,87 @@ impl BTreeOverlay {
         item.tree_hash_cache_overlay(initial_offset)
     }
 
-    pub fn from_lengths(offset: usize, mut lengths: Vec<usize>) -> Result<Self, Error> {
-        // Extend it to the next power-of-two, if it is not already.
-        let num_leaf_nodes = if lengths.len().is_power_of_two() {
-            lengths.len()
+    pub fn from_lengths(offset: usize, lengths: Vec<usize>) -> Result<Self, Error> {
+        if lengths.is_empty() {
+            Err(Error::TreeCannotHaveZeroNodes)
         } else {
-            let num_leaf_nodes = lengths.len().next_power_of_two();
-            lengths.resize(num_leaf_nodes, 1);
-            num_leaf_nodes
-        };
-
-        let num_nodes = num_nodes(num_leaf_nodes);
-        let num_internal_nodes = num_nodes - num_leaf_nodes;
-
-        let mut offsets = Vec::with_capacity(num_nodes);
-        offsets.append(&mut (offset..offset + num_internal_nodes).collect());
-
-        let mut next_node = num_internal_nodes + offset;
-        for i in 0..num_leaf_nodes {
-            offsets.push(next_node);
-            next_node += lengths[i];
+            Ok(Self { offset, lengths })
         }
+    }
 
-        Ok(Self {
-            num_internal_nodes,
-            num_leaf_nodes,
-            offsets,
-            first_node: offset,
-            next_node,
-        })
+    pub fn num_leaf_nodes(&self) -> usize {
+        self.lengths.len().next_power_of_two()
+    }
+
+    fn num_padding_leaves(&self) -> usize {
+        self.num_leaf_nodes() - self.lengths.len()
+    }
+
+    pub fn num_nodes(&self) -> usize {
+        2 * self.num_leaf_nodes() - 1
+    }
+
+    pub fn num_internal_nodes(&self) -> usize {
+        self.num_leaf_nodes() - 1
+    }
+
+    fn first_node(&self) -> usize {
+        self.offset
     }
 
     pub fn root(&self) -> usize {
-        self.first_node
+        self.first_node()
+    }
+
+    pub fn next_node(&self) -> usize {
+        self.first_node() + self.lengths.iter().sum::<usize>()
     }
 
     pub fn height(&self) -> usize {
-        self.num_leaf_nodes.trailing_zeros() as usize
+        self.num_leaf_nodes().trailing_zeros() as usize
     }
 
     pub fn chunk_range(&self) -> Range<usize> {
-        self.first_node..self.next_node
+        self.first_node()..self.next_node()
     }
 
     pub fn total_chunks(&self) -> usize {
-        self.next_node - self.first_node
+        self.next_node() - self.first_node()
     }
 
-    pub fn total_nodes(&self) -> usize {
-        self.num_internal_nodes + self.num_leaf_nodes
+    pub fn first_leaf_node(&self) -> usize {
+        self.offset + self.num_internal_nodes()
     }
 
-    pub fn first_leaf_node(&self) -> Result<usize, Error> {
-        self.offsets
-            .get(self.num_internal_nodes)
-            .cloned()
-            .ok_or_else(|| Error::NoFirstNode)
+    pub fn get_leaf_node(&self, i: usize) -> Result<Option<Range<usize>>, Error> {
+        if i >= self.num_leaf_nodes() {
+            return Err(Error::NotLeafNode(i));
+        } else if i >= self.num_leaf_nodes() - self.num_padding_leaves() {
+            Ok(None)
+        } else {
+            let first_node = self.offset + self.lengths.iter().take(i).sum::<usize>();
+            let last_node = first_node + self.lengths[i];
+            Ok(Some(first_node..last_node))
+        }
     }
 
     /// Returns an iterator visiting each internal node, providing the left and right child chunks
     /// for the node.
-    pub fn iter_internal_nodes<'a>(
-        &'a self,
-    ) -> impl DoubleEndedIterator<Item = (&'a usize, (&'a usize, &'a usize))> {
-        let internal_nodes = &self.offsets[0..self.num_internal_nodes];
-
-        internal_nodes.iter().enumerate().map(move |(i, parent)| {
-            let children = children(i);
-            (
-                parent,
-                (&self.offsets[children.0], &self.offsets[children.1]),
-            )
-        })
+    pub fn internal_parents_and_children(&self) -> Vec<(usize, (usize, usize))> {
+        (0..self.num_internal_nodes())
+            .into_iter()
+            .map(|parent| {
+                let children = children(parent);
+                (
+                    parent + self.offset,
+                    (children.0 + self.offset, children.1 + self.offset),
+                )
+            })
+            .collect()
     }
 
-    /// Returns an iterator visiting each leaf node, providing the chunk for that node.
-    pub fn iter_leaf_nodes<'a>(&'a self) -> impl DoubleEndedIterator<Item = &'a usize> {
-        let leaf_nodes = &self.offsets[self.num_internal_nodes..];
-
-        leaf_nodes.iter()
+    // Returns a `Vec` of chunk indices for each internal node of the tree.
+    pub fn internal_node_chunks(&self) -> Vec<usize> {
+        (self.offset..self.offset + self.num_internal_nodes()).collect()
     }
 }

eth2/utils/tree_hash/src/cached_tree_hash/impls.rs

@@ -8,6 +8,7 @@ impl CachedTreeHashSubTree<u64> for u64 {
         Ok(TreeHashCache::from_bytes(
             merkleize(self.to_le_bytes().to_vec()),
             false,
+            self.tree_hash_cache_overlay(0)?,
         )?)
     }
 
@@ -15,17 +16,13 @@ impl CachedTreeHashSubTree<u64> for u64 {
         BTreeOverlay::from_lengths(chunk_offset, vec![1])
     }
 
-    fn update_tree_hash_cache(
-        &self,
-        other: &Self,
-        cache: &mut TreeHashCache,
-        chunk: usize,
-    ) -> Result<usize, Error> {
-        if self != other {
-            let leaf = merkleize(self.to_le_bytes().to_vec());
-            cache.modify_chunk(chunk, &leaf)?;
-        }
+    fn update_tree_hash_cache(&self, cache: &mut TreeHashCache) -> Result<(), Error> {
+        let leaf = merkleize(self.to_le_bytes().to_vec());
+        cache.maybe_update_chunk(cache.chunk_index, &leaf)?;
 
-        Ok(chunk + 1)
+        cache.chunk_index += 1;
+        cache.overlay_index += 1;
+
+        Ok(())
     }
 }

eth2/utils/tree_hash/src/cached_tree_hash/impls/vec.rs

@@ -5,10 +5,14 @@ where
     T: CachedTreeHashSubTree<T> + TreeHash,
 {
     fn new_tree_hash_cache(&self) -> Result<TreeHashCache, Error> {
-        match T::tree_hash_type() {
-            TreeHashType::Basic => {
-                TreeHashCache::from_bytes(merkleize(get_packed_leaves(self)?), false)
-            }
+        let overlay = self.tree_hash_cache_overlay(0)?;
+
+        let mut cache = match T::tree_hash_type() {
+            TreeHashType::Basic => TreeHashCache::from_bytes(
+                merkleize(get_packed_leaves(self)?),
+                false,
+                overlay.clone(),
+            ),
             TreeHashType::Container | TreeHashType::List | TreeHashType::Vector => {
                 let subtrees = self
                     .iter()
@@ -17,17 +21,29 @@ where
 
                 TreeHashCache::from_leaves_and_subtrees(self, subtrees)
             }
-        }
+        }?;
+
+        // Mix in the length of the list.
+        let root_node = overlay.root();
+        cache.modify_chunk(root_node, &cache.mix_in_length(root_node, self.len())?)?;
+
+        Ok(cache)
     }
 
     fn tree_hash_cache_overlay(&self, chunk_offset: usize) -> Result<BTreeOverlay, Error> {
         let lengths = match T::tree_hash_type() {
-            TreeHashType::Basic => vec![1; self.len() / T::tree_hash_packing_factor()],
+            TreeHashType::Basic => {
+                // Ceil division.
+                let num_leaves = (self.len() + T::tree_hash_packing_factor() - 1)
+                    / T::tree_hash_packing_factor();
+
+                vec![1; num_leaves]
+            }
             TreeHashType::Container | TreeHashType::List | TreeHashType::Vector => {
                 let mut lengths = vec![];
 
                 for item in self {
-                    lengths.push(BTreeOverlay::new(item, 0)?.total_nodes())
+                    lengths.push(BTreeOverlay::new(item, 0)?.num_nodes())
                 }
 
                 lengths
@@ -37,120 +53,93 @@ where
         BTreeOverlay::from_lengths(chunk_offset, lengths)
     }
 
-    fn update_tree_hash_cache(
-        &self,
-        other: &Vec<T>,
-        cache: &mut TreeHashCache,
-        chunk: usize,
-    ) -> Result<usize, Error> {
-        let offset_handler = BTreeOverlay::new(self, chunk)?;
-        let old_offset_handler = BTreeOverlay::new(other, chunk)?;
+    fn update_tree_hash_cache(&self, cache: &mut TreeHashCache) -> Result<(), Error> {
+        let new_overlay = BTreeOverlay::new(self, cache.chunk_index)?;
+        let old_overlay = cache
+            .get_overlay(cache.overlay_index, cache.chunk_index)?
+            .clone();
 
-        if offset_handler.num_leaf_nodes != old_offset_handler.num_leaf_nodes {
-            let old_offset_handler = BTreeOverlay::new(other, chunk)?;
-
-            // Get slices of the exsiting tree from the cache.
-            let (old_bytes, old_flags) = cache
-                .slices(old_offset_handler.chunk_range())
-                .ok_or_else(|| Error::UnableToObtainSlices)?;
-
-            let (new_bytes, new_flags) =
-                if offset_handler.num_leaf_nodes > old_offset_handler.num_leaf_nodes {
-                    grow_merkle_cache(
-                        old_bytes,
-                        old_flags,
-                        old_offset_handler.height(),
-                        offset_handler.height(),
-                    )
-                    .ok_or_else(|| Error::UnableToGrowMerkleTree)?
-                } else {
-                    shrink_merkle_cache(
-                        old_bytes,
-                        old_flags,
-                        old_offset_handler.height(),
-                        offset_handler.height(),
-                        offset_handler.total_chunks(),
-                    )
-                    .ok_or_else(|| Error::UnableToShrinkMerkleTree)?
-                };
-
-            // Create a `TreeHashCache` from the raw elements.
-            let modified_cache = TreeHashCache::from_elems(new_bytes, new_flags);
-
-            // Splice the newly created `TreeHashCache` over the existing elements.
-            cache.splice(old_offset_handler.chunk_range(), modified_cache);
+        // If the merkle tree required to represent the new list is of a different size to the one
+        // required for the previous list, then update our cache.
+        //
+        // This grows/shrinks the bytes to accomodate the new tree, preserving as much of the tree
+        // as possible.
+        if new_overlay.num_leaf_nodes() != old_overlay.num_leaf_nodes() {
+            cache.replace_overlay(cache.overlay_index, new_overlay.clone())?;
         }
 
         match T::tree_hash_type() {
             TreeHashType::Basic => {
-                let leaves = get_packed_leaves(self)?;
+                let mut buf = vec![0; HASHSIZE];
+                let item_bytes = HASHSIZE / T::tree_hash_packing_factor();
 
-                for (i, chunk) in offset_handler.iter_leaf_nodes().enumerate() {
-                    if let Some(latest) = leaves.get(i * HASHSIZE..(i + 1) * HASHSIZE) {
-                        cache.maybe_update_chunk(*chunk, latest)?;
+                // Iterate through each of the leaf nodes.
+                for i in 0..new_overlay.num_leaf_nodes() {
+                    // Iterate through the number of items that may be packing into the leaf node.
+                    for j in 0..T::tree_hash_packing_factor() {
+                        // Create a mut slice that can either be filled with a serialized item or
+                        // padding.
+                        let buf_slice = &mut buf[j * item_bytes..(j + 1) * item_bytes];
+
+                        // Attempt to get the item for this portion of the chunk. If it exists,
+                        // update `buf` with it's serialized bytes. If it doesn't exist, update
+                        // `buf` with padding.
+                        match self.get(i * T::tree_hash_packing_factor() + j) {
+                            Some(item) => {
+                                buf_slice.copy_from_slice(&item.tree_hash_packed_encoding());
+                            }
+                            None => buf_slice.copy_from_slice(&vec![0; item_bytes]),
+                        }
                     }
-                }
-                let first_leaf_chunk = offset_handler.first_leaf_node()?;
 
-                cache.splice(
-                    first_leaf_chunk..offset_handler.next_node,
-                    TreeHashCache::from_bytes(leaves, true)?,
-                );
+                    // Update the chunk if the generated `buf` is not the same as the cache.
+                    let chunk = new_overlay.first_leaf_node() + i;
+                    cache.maybe_update_chunk(chunk, &buf)?;
+                }
             }
             TreeHashType::Container | TreeHashType::List | TreeHashType::Vector => {
-                let mut i = offset_handler.num_leaf_nodes;
-                for &start_chunk in offset_handler.iter_leaf_nodes().rev() {
-                    i -= 1;
-                    match (other.get(i), self.get(i)) {
-                        // The item existed in the previous list and exsits in the current list.
-                        (Some(old), Some(new)) => {
-                            new.update_tree_hash_cache(old, cache, start_chunk)?;
+                for i in (0..new_overlay.num_leaf_nodes()).rev() {
+                    match (old_overlay.get_leaf_node(i)?, new_overlay.get_leaf_node(i)?) {
+                        // The item existed in the previous list and exists in the current list.
+                        (Some(_old), Some(new)) => {
+                            cache.chunk_index = new.start;
+                            self[i].update_tree_hash_cache(cache)?;
                         }
                         // The item existed in the previous list but does not exist in this list.
                         //
-                        // I.e., the list has been shortened.
+                        // Viz., the list has been shortened.
                         (Some(old), None) => {
                             // Splice out the entire tree of the removed node, replacing it with a
                             // single padding node.
-                            let end_chunk = BTreeOverlay::new(old, start_chunk)?.next_node;
-
-                            cache.splice(
-                                start_chunk..end_chunk,
-                                TreeHashCache::from_bytes(vec![0; HASHSIZE], true)?,
-                            );
+                            cache.splice(old, vec![0; HASHSIZE], vec![true]);
                         }
-                        // The item existed in the previous list but does exist in this list.
+                        // The item did not exist in the previous list but does exist in this list.
                         //
-                        // I.e., the list has been lengthened.
+                        // Viz., the list has been lengthened.
                         (None, Some(new)) => {
-                            let bytes: Vec<u8> = TreeHashCache::new(new)?.into();
+                            let bytes: Vec<u8> = TreeHashCache::new(&self[i])?.into();
+                            let bools = vec![true; bytes.len() / HASHSIZE];
 
-                            cache.splice(
-                                start_chunk..start_chunk + 1,
-                                TreeHashCache::from_bytes(bytes, true)?,
-                            );
+                            cache.splice(new.start..new.start + 1, bytes, bools);
                         }
                         // The item didn't exist in the old list and doesn't exist in the new list,
                         // nothing to do.
                         (None, None) => {}
-                    };
+                    }
                 }
             }
         }
 
-        for (&parent, children) in offset_handler.iter_internal_nodes().rev() {
-            if cache.either_modified(children)? {
-                cache.modify_chunk(parent, &cache.hash_children(children)?)?;
-            }
-        }
+        cache.update_internal_nodes(&new_overlay)?;
 
-        // If the root node or the length has changed, mix in the length of the list.
-        let root_node = offset_handler.root();
-        if cache.changed(root_node)? | (self.len() != other.len()) {
-            cache.modify_chunk(root_node, &cache.mix_in_length(root_node, self.len())?)?;
-        }
+        // Always update the root node as we don't have a reliable check to know if the list len
+        // has changed.
+        let root_node = new_overlay.root();
+        cache.modify_chunk(root_node, &cache.mix_in_length(root_node, self.len())?)?;
 
-        Ok(offset_handler.next_node)
+        cache.chunk_index = new_overlay.next_node();
+
+        Ok(())
     }
 }