Padding efficent merkle root algo (#436)

* Add initial work on padding efficent merkle roots

* Improve merklize_padded

* Improve tree_hash crate -- fix bugs, docs

* Update codebase for tree_hash API change

* Remove dbg statements, fix import error

* Fix clippy lints, doc error

* Tidy tree hash comments

* Increase tree_hash max tree height

* Fix PR review comments

* Fix typos

* Fix cache access off-by-one in tree hash

* Set max tree depth to 48 (from 64)

eth2/utils/tree_hash/src/merkleize_standard.rs

@@ -0,0 +1,80 @@
+use super::*;
+use hashing::hash;
+
+/// Merkleizes bytes and returns the root, using a simple algorithm that does not optimize to avoid
+/// processing or storing padding bytes.
+///
+/// The input `bytes` will be padded to ensure that the number of leaves is a power-of-two.
+///
+/// It is likely a better choice to use [merkleize_padded](fn.merkleize_padded.html) instead.
+///
+/// ## CPU Performance
+///
+/// Will hash all nodes in the tree, even if they are padding and pre-determined.
+///
+/// ## Memory Performance
+///
+///  - Duplicates the input `bytes`.
+///  - Stores all internal nodes, even if they are padding.
+///  - Does not free up unused memory during operation.
+pub fn merkleize_standard(bytes: &[u8]) -> Vec<u8> {
+    // If the bytes are just one chunk (or less than one chunk) just return them.
+    if bytes.len() <= HASHSIZE {
+        let mut o = bytes.to_vec();
+        o.resize(HASHSIZE, 0);
+        return o;
+    }
+
+    let leaves = num_sanitized_leaves(bytes.len());
+    let nodes = num_nodes(leaves);
+    let internal_nodes = nodes - leaves;
+
+    let num_bytes = std::cmp::max(internal_nodes, 1) * HASHSIZE + bytes.len();
+
+    let mut o: Vec<u8> = vec![0; internal_nodes * HASHSIZE];
+
+    o.append(&mut bytes.to_vec());
+
+    assert_eq!(o.len(), num_bytes);
+
+    let empty_chunk_hash = hash(&[0; MERKLE_HASH_CHUNK]);
+
+    let mut i = nodes * HASHSIZE;
+    let mut j = internal_nodes * HASHSIZE;
+
+    while i >= MERKLE_HASH_CHUNK {
+        i -= MERKLE_HASH_CHUNK;
+
+        j -= HASHSIZE;
+        let hash = match o.get(i..i + MERKLE_HASH_CHUNK) {
+            // All bytes are available, hash as ususal.
+            Some(slice) => hash(slice),
+            // Unable to get all the bytes.
+            None => {
+                match o.get(i..) {
+                    // Able to get some of the bytes, pad them out.
+                    Some(slice) => {
+                        let mut bytes = slice.to_vec();
+                        bytes.resize(MERKLE_HASH_CHUNK, 0);
+                        hash(&bytes)
+                    }
+                    // Unable to get any bytes, use the empty-chunk hash.
+                    None => empty_chunk_hash.clone(),
+                }
+            }
+        };
+
+        o[j..j + HASHSIZE].copy_from_slice(&hash);
+    }
+
+    o
+}
+
+fn num_sanitized_leaves(num_bytes: usize) -> usize {
+    let leaves = (num_bytes + HASHSIZE - 1) / HASHSIZE;
+    leaves.next_power_of_two()
+}
+
+fn num_nodes(num_leaves: usize) -> usize {
+    2 * num_leaves - 1
+}