Move tree_hash from ssz into own crate

eth2/utils/tree_hash/src/cached_tree_hash.rs

@@ -0,0 +1,193 @@
+use super::*;
+
+#[derive(Debug, PartialEq, Clone)]
+pub struct TreeHashCache {
+    cache: Vec<u8>,
+    chunk_modified: Vec<bool>,
+}
+
+impl Into<Vec<u8>> for TreeHashCache {
+    fn into(self) -> Vec<u8> {
+        self.cache
+    }
+}
+
+impl TreeHashCache {
+    pub fn new<T>(item: &T) -> Result<Self, Error>
+    where
+        T: CachedTreeHash<T>,
+    {
+        item.build_tree_hash_cache()
+    }
+
+    pub fn from_elems(cache: Vec<u8>, chunk_modified: Vec<bool>) -> Self {
+        Self {
+            cache,
+            chunk_modified,
+        }
+    }
+
+    pub fn from_leaves_and_subtrees<T>(
+        item: &T,
+        leaves_and_subtrees: Vec<Self>,
+    ) -> Result<Self, Error>
+    where
+        T: CachedTreeHash<T>,
+    {
+        let offset_handler = 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.
+        let num_provided_leaf_nodes = leaves_and_subtrees.len();
+
+        // 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 leaves_and_subtrees_bytes = leaves_and_subtrees
+            .iter()
+            .fold(0, |acc, t| acc + t.bytes_len());
+        let mut cache = Vec::with_capacity(leaves_and_subtrees_bytes + internal_node_bytes);
+        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);
+
+        // 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()?);
+            cache.append(&mut t.into_merkle_tree());
+        }
+
+        // Pad the leaves to an even power-of-two, using zeros.
+        pad_for_leaf_count(num_provided_leaf_nodes, &mut cache);
+
+        // Merkleize the leaves, then split the leaf nodes off them. Then, replace all-zeros
+        // internal nodes created earlier with the internal nodes generated by `merkleize`.
+        let mut merkleized = merkleize(leaves);
+        merkleized.split_off(internal_node_bytes);
+        cache.splice(0..internal_node_bytes, merkleized);
+
+        Ok(Self {
+            chunk_modified: vec![false; cache.len() / BYTES_PER_CHUNK],
+            cache,
+        })
+    }
+
+    pub fn from_bytes(bytes: Vec<u8>, initial_modified_state: bool) -> Result<Self, Error> {
+        if bytes.len() % BYTES_PER_CHUNK > 0 {
+            return Err(Error::BytesAreNotEvenChunks(bytes.len()));
+        }
+
+        Ok(Self {
+            chunk_modified: vec![initial_modified_state; bytes.len() / BYTES_PER_CHUNK],
+            cache: bytes,
+        })
+    }
+
+    pub fn bytes_len(&self) -> usize {
+        self.cache.len()
+    }
+
+    pub fn root(&self) -> Result<Vec<u8>, Error> {
+        self.cache
+            .get(0..HASHSIZE)
+            .ok_or_else(|| Error::NoBytesForRoot)
+            .and_then(|slice| Ok(slice.to_vec()))
+    }
+
+    pub fn splice(&mut self, chunk_range: Range<usize>, replace_with: Self) {
+        let (bytes, bools) = replace_with.into_components();
+
+        // Update the `chunk_modified` vec, marking all spliced-in nodes as changed.
+        self.chunk_modified.splice(chunk_range.clone(), bools);
+        self.cache
+            .splice(node_range_to_byte_range(&chunk_range), bytes);
+    }
+
+    pub fn maybe_update_chunk(&mut self, chunk: usize, to: &[u8]) -> Result<(), Error> {
+        let start = chunk * BYTES_PER_CHUNK;
+        let end = start + BYTES_PER_CHUNK;
+
+        if !self.chunk_equals(chunk, to)? {
+            self.cache
+                .get_mut(start..end)
+                .ok_or_else(|| Error::NoModifiedFieldForChunk(chunk))?
+                .copy_from_slice(to);
+            self.chunk_modified[chunk] = true;
+        }
+
+        Ok(())
+    }
+
+    pub fn slices(&self, chunk_range: Range<usize>) -> Option<(&[u8], &[bool])> {
+        Some((
+            self.cache.get(node_range_to_byte_range(&chunk_range))?,
+            self.chunk_modified.get(chunk_range)?,
+        ))
+    }
+
+    pub fn modify_chunk(&mut self, chunk: usize, to: &[u8]) -> Result<(), Error> {
+        let start = chunk * BYTES_PER_CHUNK;
+        let end = start + BYTES_PER_CHUNK;
+
+        self.cache
+            .get_mut(start..end)
+            .ok_or_else(|| Error::NoBytesForChunk(chunk))?
+            .copy_from_slice(to);
+
+        self.chunk_modified[chunk] = true;
+
+        Ok(())
+    }
+
+    pub fn get_chunk(&self, chunk: usize) -> Result<&[u8], Error> {
+        let start = chunk * BYTES_PER_CHUNK;
+        let end = start + BYTES_PER_CHUNK;
+
+        Ok(self
+            .cache
+            .get(start..end)
+            .ok_or_else(|| Error::NoModifiedFieldForChunk(chunk))?)
+    }
+
+    pub fn chunk_equals(&mut self, chunk: usize, other: &[u8]) -> Result<bool, Error> {
+        Ok(self.get_chunk(chunk)? == other)
+    }
+
+    pub fn changed(&self, chunk: usize) -> Result<bool, Error> {
+        self.chunk_modified
+            .get(chunk)
+            .cloned()
+            .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 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());
+
+        Ok(hash(&child_bytes))
+    }
+
+    pub fn mix_in_length(&self, chunk: usize, length: usize) -> Result<Vec<u8>, Error> {
+        let mut bytes = Vec::with_capacity(2 * BYTES_PER_CHUNK);
+
+        bytes.append(&mut self.get_chunk(chunk)?.to_vec());
+        bytes.append(&mut int_to_bytes32(length as u64));
+
+        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)
+    }
+}

eth2/utils/tree_hash/src/impls.rs

@@ -0,0 +1,239 @@
+use super::resize::{grow_merkle_cache, shrink_merkle_cache};
+use super::*;
+use ssz::ssz_encode;
+
+impl CachedTreeHash<u64> for u64 {
+    fn item_type() -> ItemType {
+        ItemType::Basic
+    }
+
+    fn build_tree_hash_cache(&self) -> Result<TreeHashCache, Error> {
+        Ok(TreeHashCache::from_bytes(
+            merkleize(ssz_encode(self)),
+            false,
+        )?)
+    }
+
+    fn num_bytes(&self) -> usize {
+        8
+    }
+
+    fn offsets(&self) -> Result<Vec<usize>, Error> {
+        Err(Error::ShouldNotProduceBTreeOverlay)
+    }
+
+    fn num_child_nodes(&self) -> usize {
+        0
+    }
+
+    fn packed_encoding(&self) -> Vec<u8> {
+        ssz_encode(self)
+    }
+
+    fn packing_factor() -> usize {
+        32 / 8
+    }
+
+    fn cached_hash_tree_root(
+        &self,
+        other: &Self,
+        cache: &mut TreeHashCache,
+        chunk: usize,
+    ) -> Result<usize, Error> {
+        if self != other {
+            let leaf = merkleize(ssz_encode(self));
+            cache.modify_chunk(chunk, &leaf)?;
+        }
+
+        Ok(chunk + 1)
+    }
+}
+
+impl<T> CachedTreeHash<Vec<T>> for Vec<T>
+where
+    T: CachedTreeHash<T>,
+{
+    fn item_type() -> ItemType {
+        ItemType::List
+    }
+
+    fn build_tree_hash_cache(&self) -> Result<TreeHashCache, Error> {
+        match T::item_type() {
+            ItemType::Basic => TreeHashCache::from_bytes(merkleize(get_packed_leaves(self)), false),
+            ItemType::Composite | ItemType::List => {
+                let subtrees = self
+                    .iter()
+                    .map(|item| TreeHashCache::new(item))
+                    .collect::<Result<Vec<TreeHashCache>, _>>()?;
+
+                TreeHashCache::from_leaves_and_subtrees(self, subtrees)
+            }
+        }
+    }
+
+    fn offsets(&self) -> Result<Vec<usize>, Error> {
+        let offsets = match T::item_type() {
+            ItemType::Basic => vec![1; self.len() / T::packing_factor()],
+            ItemType::Composite | ItemType::List => {
+                let mut offsets = vec![];
+
+                for item in self {
+                    offsets.push(BTreeOverlay::new(item, 0)?.total_nodes())
+                }
+
+                offsets
+            }
+        };
+
+        Ok(offsets)
+    }
+
+    fn num_child_nodes(&self) -> usize {
+        // TODO
+        42
+    }
+
+    fn num_bytes(&self) -> usize {
+        self.iter().fold(0, |acc, item| acc + item.num_bytes())
+    }
+
+    fn packed_encoding(&self) -> Vec<u8> {
+        panic!("List should never be packed")
+    }
+
+    fn packing_factor() -> usize {
+        1
+    }
+
+    fn cached_hash_tree_root(
+        &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)?;
+
+        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);
+        }
+
+        match T::item_type() {
+            ItemType::Basic => {
+                let leaves = get_packed_leaves(self);
+
+                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)?;
+                    }
+                }
+                let first_leaf_chunk = offset_handler.first_leaf_node()?;
+
+                cache.splice(
+                    first_leaf_chunk..offset_handler.next_node,
+                    TreeHashCache::from_bytes(leaves, true)?,
+                );
+            }
+            ItemType::Composite | ItemType::List => {
+                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.cached_hash_tree_root(old, cache, start_chunk)?;
+                        }
+                        // The item existed in the previous list but does not exist in this list.
+                        //
+                        // I.e., 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)?,
+                            );
+                        }
+                        // The item existed in the previous list but does exist in this list.
+                        //
+                        // I.e., the list has been lengthened.
+                        (None, Some(new)) => {
+                            let bytes: Vec<u8> = TreeHashCache::new(new)?.into();
+
+                            cache.splice(
+                                start_chunk..start_chunk + 1,
+                                TreeHashCache::from_bytes(bytes, true)?,
+                            );
+                        }
+                        // 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)?)?;
+            }
+        }
+
+        // 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())?)?;
+        }
+
+        Ok(offset_handler.next_node())
+    }
+}
+
+fn get_packed_leaves<T>(vec: &Vec<T>) -> Vec<u8>
+where
+    T: CachedTreeHash<T>,
+{
+    let num_packed_bytes = vec.num_bytes();
+    let num_leaves = num_sanitized_leaves(num_packed_bytes);
+
+    let mut packed = Vec::with_capacity(num_leaves * HASHSIZE);
+
+    for item in vec {
+        packed.append(&mut item.packed_encoding());
+    }
+
+    sanitise_bytes(packed)
+}

eth2/utils/tree_hash/src/lib.rs

@@ -0,0 +1,249 @@
+use hashing::hash;
+use int_to_bytes::int_to_bytes32;
+use std::fmt::Debug;
+use std::iter::Iterator;
+use std::ops::Range;
+
+mod cached_tree_hash;
+mod impls;
+mod resize;
+
+pub use cached_tree_hash::TreeHashCache;
+
+pub const BYTES_PER_CHUNK: usize = 32;
+pub const HASHSIZE: usize = 32;
+pub const MERKLE_HASH_CHUNCK: usize = 2 * BYTES_PER_CHUNK;
+
+#[derive(Debug, PartialEq, Clone)]
+pub enum Error {
+    ShouldNotProduceBTreeOverlay,
+    NoFirstNode,
+    NoBytesForRoot,
+    UnableToObtainSlices,
+    UnableToGrowMerkleTree,
+    UnableToShrinkMerkleTree,
+    BytesAreNotEvenChunks(usize),
+    NoModifiedFieldForChunk(usize),
+    NoBytesForChunk(usize),
+}
+
+#[derive(Debug, PartialEq, Clone)]
+pub enum ItemType {
+    Basic,
+    List,
+    Composite,
+}
+
+// TODO: remove debug requirement.
+pub trait CachedTreeHash<Item>: Debug {
+    fn item_type() -> ItemType;
+
+    fn build_tree_hash_cache(&self) -> Result<TreeHashCache, Error>;
+
+    /// Return the number of bytes when this element is encoded as raw SSZ _without_ length
+    /// prefixes.
+    fn num_bytes(&self) -> usize;
+
+    fn offsets(&self) -> Result<Vec<usize>, Error>;
+
+    fn num_child_nodes(&self) -> usize;
+
+    fn packed_encoding(&self) -> Vec<u8>;
+
+    fn packing_factor() -> usize;
+
+    fn cached_hash_tree_root(
+        &self,
+        other: &Item,
+        cache: &mut TreeHashCache,
+        chunk: usize,
+    ) -> Result<usize, Error>;
+}
+
+fn children(parent: usize) -> (usize, usize) {
+    ((2 * parent + 1), (2 * parent + 2))
+}
+
+fn num_nodes(num_leaves: usize) -> usize {
+    2 * num_leaves - 1
+}
+
+fn node_range_to_byte_range(node_range: &Range<usize>) -> Range<usize> {
+    node_range.start * HASHSIZE..node_range.end * HASHSIZE
+}
+
+#[derive(Debug)]
+pub struct BTreeOverlay {
+    num_internal_nodes: usize,
+    pub num_leaf_nodes: usize,
+    first_node: usize,
+    next_node: usize,
+    offsets: Vec<usize>,
+}
+
+impl BTreeOverlay {
+    pub fn new<T>(item: &T, initial_offset: usize) -> Result<Self, Error>
+    where
+        T: CachedTreeHash<T>,
+    {
+        Self::from_lengths(initial_offset, item.offsets()?)
+    }
+
+    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()
+        } 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 {
+            num_internal_nodes,
+            num_leaf_nodes,
+            offsets,
+            first_node: offset,
+            next_node,
+        })
+    }
+
+    pub fn root(&self) -> usize {
+        self.first_node
+    }
+
+    pub fn height(&self) -> usize {
+        self.num_leaf_nodes.trailing_zeros() as usize
+    }
+
+    pub fn chunk_range(&self) -> Range<usize> {
+        self.first_node..self.next_node
+    }
+
+    pub fn total_chunks(&self) -> usize {
+        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) -> Result<usize, Error> {
+        self.offsets
+            .get(self.num_internal_nodes)
+            .cloned()
+            .ok_or_else(|| Error::NoFirstNode)
+    }
+
+    pub fn next_node(&self) -> usize {
+        self.next_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]),
+            )
+        })
+    }
+
+    /// 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()
+    }
+}
+
+/// Split `values` into a power-of-two, identical-length chunks (padding with `0`) and merkleize
+/// them, returning the entire merkle tree.
+///
+/// The root hash is `merkleize(values)[0..BYTES_PER_CHUNK]`.
+pub fn merkleize(values: Vec<u8>) -> Vec<u8> {
+    let values = sanitise_bytes(values);
+
+    let leaves = values.len() / HASHSIZE;
+
+    if leaves == 0 {
+        panic!("No full leaves");
+    }
+
+    if !leaves.is_power_of_two() {
+        panic!("leaves is not power of two");
+    }
+
+    let mut o: Vec<u8> = vec![0; (num_nodes(leaves) - leaves) * HASHSIZE];
+    o.append(&mut values.to_vec());
+
+    let mut i = o.len();
+    let mut j = o.len() - values.len();
+
+    while i >= MERKLE_HASH_CHUNCK {
+        i -= MERKLE_HASH_CHUNCK;
+        let hash = hash(&o[i..i + MERKLE_HASH_CHUNCK]);
+
+        j -= HASHSIZE;
+        o[j..j + HASHSIZE].copy_from_slice(&hash);
+    }
+
+    o
+}
+
+pub fn sanitise_bytes(mut bytes: Vec<u8>) -> Vec<u8> {
+    let present_leaves = num_unsanitized_leaves(bytes.len());
+    let required_leaves = present_leaves.next_power_of_two();
+
+    if (present_leaves != required_leaves) | last_leaf_needs_padding(bytes.len()) {
+        bytes.resize(num_bytes(required_leaves), 0);
+    }
+
+    bytes
+}
+
+fn pad_for_leaf_count(num_leaves: usize, bytes: &mut Vec<u8>) {
+    let required_leaves = num_leaves.next_power_of_two();
+
+    bytes.resize(
+        bytes.len() + (required_leaves - num_leaves) * BYTES_PER_CHUNK,
+        0,
+    );
+}
+
+fn last_leaf_needs_padding(num_bytes: usize) -> bool {
+    num_bytes % HASHSIZE != 0
+}
+
+/// Rounds up
+fn num_unsanitized_leaves(num_bytes: usize) -> usize {
+    (num_bytes + HASHSIZE - 1) / HASHSIZE
+}
+
+/// Rounds up
+fn num_sanitized_leaves(num_bytes: usize) -> usize {
+    let leaves = (num_bytes + HASHSIZE - 1) / HASHSIZE;
+    leaves.next_power_of_two()
+}
+
+fn num_bytes(num_leaves: usize) -> usize {
+    num_leaves * HASHSIZE
+}

eth2/utils/tree_hash/src/resize.rs

@@ -0,0 +1,276 @@
+use super::*;
+
+/// New vec is bigger than old vec.
+pub fn grow_merkle_cache(
+    old_bytes: &[u8],
+    old_flags: &[bool],
+    from_height: usize,
+    to_height: usize,
+) -> Option<(Vec<u8>, Vec<bool>)> {
+    // Determine the size of our new tree. It is not just a simple `1 << to_height` as there can be
+    // an arbitrary number of nodes in `old_bytes` leaves if those leaves are subtrees.
+    let to_nodes = {
+        let old_nodes = old_bytes.len() / HASHSIZE;
+        let additional_nodes = old_nodes - nodes_in_tree_of_height(from_height);
+        nodes_in_tree_of_height(to_height) + additional_nodes
+    };
+
+    let mut bytes = vec![0; to_nodes * HASHSIZE];
+    let mut flags = vec![true; to_nodes];
+
+    let leaf_level = from_height;
+
+    for i in 0..=from_height as usize {
+        // If we're on the leaf slice, grab the first byte and all the of the bytes after that.
+        // This is required because we can have an arbitrary number of bytes at the leaf level
+        // (e.g., the case where there are subtrees as leaves).
+        //
+        // If we're not on a leaf level, the number of nodes is fixed and known.
+        let (old_byte_slice, old_flag_slice) = if i == leaf_level {
+            (
+                old_bytes.get(first_byte_at_height(i)..)?,
+                old_flags.get(first_node_at_height(i)..)?,
+            )
+        } else {
+            (
+                old_bytes.get(byte_range_at_height(i))?,
+                old_flags.get(node_range_at_height(i))?,
+            )
+        };
+
+        let new_i = i + to_height - from_height;
+        let (new_byte_slice, new_flag_slice) = if i == leaf_level {
+            (
+                bytes.get_mut(first_byte_at_height(new_i)..)?,
+                flags.get_mut(first_node_at_height(new_i)..)?,
+            )
+        } else {
+            (
+                bytes.get_mut(byte_range_at_height(new_i))?,
+                flags.get_mut(node_range_at_height(new_i))?,
+            )
+        };
+
+        new_byte_slice
+            .get_mut(0..old_byte_slice.len())?
+            .copy_from_slice(old_byte_slice);
+        new_flag_slice
+            .get_mut(0..old_flag_slice.len())?
+            .copy_from_slice(old_flag_slice);
+    }
+
+    Some((bytes, flags))
+}
+
+/// New vec is smaller than old vec.
+pub fn shrink_merkle_cache(
+    from_bytes: &[u8],
+    from_flags: &[bool],
+    from_height: usize,
+    to_height: usize,
+    to_nodes: usize,
+) -> Option<(Vec<u8>, Vec<bool>)> {
+    let mut bytes = vec![0; to_nodes * HASHSIZE];
+    let mut flags = vec![true; to_nodes];
+
+    for i in 0..=to_height as usize {
+        let from_i = i + from_height - to_height;
+
+        let (from_byte_slice, from_flag_slice) = if from_i == from_height {
+            (
+                from_bytes.get(first_byte_at_height(from_i)..)?,
+                from_flags.get(first_node_at_height(from_i)..)?,
+            )
+        } else {
+            (
+                from_bytes.get(byte_range_at_height(from_i))?,
+                from_flags.get(node_range_at_height(from_i))?,
+            )
+        };
+
+        let (to_byte_slice, to_flag_slice) = if i == to_height {
+            (
+                bytes.get_mut(first_byte_at_height(i)..)?,
+                flags.get_mut(first_node_at_height(i)..)?,
+            )
+        } else {
+            (
+                bytes.get_mut(byte_range_at_height(i))?,
+                flags.get_mut(node_range_at_height(i))?,
+            )
+        };
+
+        to_byte_slice.copy_from_slice(from_byte_slice.get(0..to_byte_slice.len())?);
+        to_flag_slice.copy_from_slice(from_flag_slice.get(0..to_flag_slice.len())?);
+    }
+
+    Some((bytes, flags))
+}
+
+fn nodes_in_tree_of_height(h: usize) -> usize {
+    2 * (1 << h) - 1
+}
+
+fn byte_range_at_height(h: usize) -> Range<usize> {
+    let node_range = node_range_at_height(h);
+    node_range.start * HASHSIZE..node_range.end * HASHSIZE
+}
+
+fn node_range_at_height(h: usize) -> Range<usize> {
+    first_node_at_height(h)..last_node_at_height(h) + 1
+}
+
+fn first_byte_at_height(h: usize) -> usize {
+    first_node_at_height(h) * HASHSIZE
+}
+
+fn first_node_at_height(h: usize) -> usize {
+    (1 << h) - 1
+}
+
+fn last_node_at_height(h: usize) -> usize {
+    (1 << (h + 1)) - 2
+}
+
+#[cfg(test)]
+mod test {
+    use super::*;
+
+    #[test]
+    fn can_grow_and_shrink_three_levels() {
+        let small: usize = 1;
+        let big: usize = 15;
+
+        let original_bytes = vec![42; small * HASHSIZE];
+        let original_flags = vec![false; small];
+
+        let (grown_bytes, grown_flags) = grow_merkle_cache(
+            &original_bytes,
+            &original_flags,
+            (small + 1).trailing_zeros() as usize - 1,
+            (big + 1).trailing_zeros() as usize - 1,
+        )
+        .unwrap();
+
+        let mut expected_bytes = vec![];
+        let mut expected_flags = vec![];
+        // First level
+        expected_bytes.append(&mut vec![0; 32]);
+        expected_flags.push(true);
+        // Second level
+        expected_bytes.append(&mut vec![0; 32]);
+        expected_bytes.append(&mut vec![0; 32]);
+        expected_flags.push(true);
+        expected_flags.push(true);
+        // Third level
+        expected_bytes.append(&mut vec![0; 32]);
+        expected_bytes.append(&mut vec![0; 32]);
+        expected_bytes.append(&mut vec![0; 32]);
+        expected_bytes.append(&mut vec![0; 32]);
+        expected_flags.push(true);
+        expected_flags.push(true);
+        expected_flags.push(true);
+        expected_flags.push(true);
+        // Fourth level
+        expected_bytes.append(&mut vec![42; 32]);
+        expected_bytes.append(&mut vec![0; 32]);
+        expected_bytes.append(&mut vec![0; 32]);
+        expected_bytes.append(&mut vec![0; 32]);
+        expected_bytes.append(&mut vec![0; 32]);
+        expected_bytes.append(&mut vec![0; 32]);
+        expected_bytes.append(&mut vec![0; 32]);
+        expected_bytes.append(&mut vec![0; 32]);
+        expected_flags.push(false);
+        expected_flags.push(true);
+        expected_flags.push(true);
+        expected_flags.push(true);
+        expected_flags.push(true);
+        expected_flags.push(true);
+        expected_flags.push(true);
+        expected_flags.push(true);
+
+        assert_eq!(expected_bytes, grown_bytes);
+        assert_eq!(expected_flags, grown_flags);
+
+        let (shrunk_bytes, shrunk_flags) = shrink_merkle_cache(
+            &grown_bytes,
+            &grown_flags,
+            (big + 1).trailing_zeros() as usize - 1,
+            (small + 1).trailing_zeros() as usize - 1,
+            small,
+        )
+        .unwrap();
+
+        assert_eq!(original_bytes, shrunk_bytes);
+        assert_eq!(original_flags, shrunk_flags);
+    }
+
+    #[test]
+    fn can_grow_and_shrink_one_level() {
+        let small: usize = 7;
+        let big: usize = 15;
+
+        let original_bytes = vec![42; small * HASHSIZE];
+        let original_flags = vec![false; small];
+
+        let (grown_bytes, grown_flags) = grow_merkle_cache(
+            &original_bytes,
+            &original_flags,
+            (small + 1).trailing_zeros() as usize - 1,
+            (big + 1).trailing_zeros() as usize - 1,
+        )
+        .unwrap();
+
+        let mut expected_bytes = vec![];
+        let mut expected_flags = vec![];
+        // First level
+        expected_bytes.append(&mut vec![0; 32]);
+        expected_flags.push(true);
+        // Second level
+        expected_bytes.append(&mut vec![42; 32]);
+        expected_bytes.append(&mut vec![0; 32]);
+        expected_flags.push(false);
+        expected_flags.push(true);
+        // Third level
+        expected_bytes.append(&mut vec![42; 32]);
+        expected_bytes.append(&mut vec![42; 32]);
+        expected_bytes.append(&mut vec![0; 32]);
+        expected_bytes.append(&mut vec![0; 32]);
+        expected_flags.push(false);
+        expected_flags.push(false);
+        expected_flags.push(true);
+        expected_flags.push(true);
+        // Fourth level
+        expected_bytes.append(&mut vec![42; 32]);
+        expected_bytes.append(&mut vec![42; 32]);
+        expected_bytes.append(&mut vec![42; 32]);
+        expected_bytes.append(&mut vec![42; 32]);
+        expected_bytes.append(&mut vec![0; 32]);
+        expected_bytes.append(&mut vec![0; 32]);
+        expected_bytes.append(&mut vec![0; 32]);
+        expected_bytes.append(&mut vec![0; 32]);
+        expected_flags.push(false);
+        expected_flags.push(false);
+        expected_flags.push(false);
+        expected_flags.push(false);
+        expected_flags.push(true);
+        expected_flags.push(true);
+        expected_flags.push(true);
+        expected_flags.push(true);
+
+        assert_eq!(expected_bytes, grown_bytes);
+        assert_eq!(expected_flags, grown_flags);
+
+        let (shrunk_bytes, shrunk_flags) = shrink_merkle_cache(
+            &grown_bytes,
+            &grown_flags,
+            (big + 1).trailing_zeros() as usize - 1,
+            (small + 1).trailing_zeros() as usize - 1,
+            small,
+        )
+        .unwrap();
+
+        assert_eq!(original_bytes, shrunk_bytes);
+        assert_eq!(original_flags, shrunk_flags);
+    }
+}