Tidy tree hash cache, add new trait

eth2/utils/tree_hash/src/btree_overlay.rs

@@ -12,7 +12,7 @@ pub struct BTreeOverlay {
 impl BTreeOverlay {
     pub fn new<T>(item: &T, initial_offset: usize) -> Result<Self, Error>
     where
-        T: CachedTreeHash<T>,
+        T: CachedTreeHashSubtree<T>,
     {
         item.btree_overlay(initial_offset)
     }

eth2/utils/tree_hash/src/cached_tree_hash.rs

@@ -15,7 +15,7 @@ impl Into<Vec<u8>> for TreeHashCache {
 impl TreeHashCache {
     pub fn new<T>(item: &T) -> Result<Self, Error>
     where
-        T: CachedTreeHash<T>,
+        T: CachedTreeHashSubtree<T>,
     {
         item.new_cache()
     }
@@ -32,7 +32,7 @@ impl TreeHashCache {
         leaves_and_subtrees: Vec<Self>,
     ) -> Result<Self, Error>
     where
-        T: CachedTreeHash<T>,
+        T: CachedTreeHashSubtree<T>,
     {
         let offset_handler = BTreeOverlay::new(item, 0)?;
 
@@ -55,7 +55,7 @@ impl TreeHashCache {
         // 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()?);
+            leaves.append(&mut t.root().ok_or_else(|| Error::NoBytesForRoot)?.to_vec());
             cache.append(&mut t.into_merkle_tree());
         }
 
@@ -89,11 +89,8 @@ impl TreeHashCache {
         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 root(&self) -> Option<&[u8]> {
+        self.cache.get(0..HASHSIZE)
     }
 
     pub fn splice(&mut self, chunk_range: Range<usize>, replace_with: Self) {

eth2/utils/tree_hash/src/impls.rs

@@ -4,7 +4,7 @@ use ssz::ssz_encode;
 
 mod vec;
 
-impl CachedTreeHash<u64> for u64 {
+impl CachedTreeHashSubtree<u64> for u64 {
     fn item_type() -> ItemType {
         ItemType::Basic
     }

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

@@ -1,8 +1,8 @@
 use super::*;
 
-impl<T> CachedTreeHash<Vec<T>> for Vec<T>
+impl<T> CachedTreeHashSubtree<Vec<T>> for Vec<T>
 where
-    T: CachedTreeHash<T>,
+    T: CachedTreeHashSubtree<T>,
 {
     fn item_type() -> ItemType {
         ItemType::List
@@ -168,7 +168,7 @@ where
 
 fn get_packed_leaves<T>(vec: &Vec<T>) -> Result<Vec<u8>, Error>
 where
-    T: CachedTreeHash<T>,
+    T: CachedTreeHashSubtree<T>,
 {
     let num_packed_bytes = (BYTES_PER_CHUNK / T::packing_factor()) * vec.len();
     let num_leaves = num_sanitized_leaves(num_packed_bytes);

eth2/utils/tree_hash/src/lib.rs

@@ -1,6 +1,5 @@
 use hashing::hash;
 use int_to_bytes::int_to_bytes32;
-use std::fmt::Debug;
 use std::ops::Range;
 
 mod btree_overlay;
@@ -36,8 +35,15 @@ pub enum ItemType {
     Composite,
 }
 
-// TODO: remove debug requirement.
-pub trait CachedTreeHash<Item>: Debug {
+pub trait CachedTreeHash<T>: CachedTreeHashSubtree<T> + Sized {
+    fn update_internal_tree_hash_cache(self, old: T) -> Result<(Self, Self), Error>;
+
+    fn cached_tree_hash_root(&self) -> Option<Vec<u8>>;
+
+    fn clone_without_tree_hash_cache(&self) -> Self;
+}
+
+pub trait CachedTreeHashSubtree<Item> {
     fn item_type() -> ItemType;
 
     fn btree_overlay(&self, chunk_offset: usize) -> Result<BTreeOverlay, Error>;