Implement tree hash caching (#584)

* Implement basic tree hash caching

* Use spaces to indent top-level Cargo.toml

* Optimize BLS tree hash by hashing bytes directly

* Implement tree hash caching for validator registry

* Persist BeaconState tree hash cache to disk

* Address Paul's review comments

eth2/utils/eth2_hashing/src/lib.rs

@@ -10,6 +10,9 @@ use ring::digest::{digest, SHA256};
 #[cfg(target_arch = "wasm32")]
 use sha2::{Digest, Sha256};
 
+#[cfg(feature = "zero_hash_cache")]
+use lazy_static::lazy_static;
+
 /// Returns the digest of `input`.
 ///
 /// Uses `ring::digest::SHA256`.
@@ -23,6 +26,31 @@ pub fn hash(input: &[u8]) -> Vec<u8> {
     h
 }
 
+/// Compute the hash of two slices concatenated.
+pub fn hash_concat(h1: &[u8], h2: &[u8]) -> Vec<u8> {
+    let mut vec1 = h1.to_vec();
+    vec1.extend_from_slice(h2);
+    hash(&vec1)
+}
+
+/// The max index that can be used with `ZERO_HASHES`.
+#[cfg(feature = "zero_hash_cache")]
+pub const ZERO_HASHES_MAX_INDEX: usize = 48;
+
+#[cfg(feature = "zero_hash_cache")]
+lazy_static! {
+    /// Cached zero hashes where `ZERO_HASHES[i]` is the hash of a Merkle tree with 2^i zero leaves.
+    pub static ref ZERO_HASHES: Vec<Vec<u8>> = {
+        let mut hashes = vec![vec![0; 32]; ZERO_HASHES_MAX_INDEX + 1];
+
+        for i in 0..ZERO_HASHES_MAX_INDEX {
+            hashes[i + 1] = hash_concat(&hashes[i], &hashes[i]);
+        }
+
+        hashes
+    };
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;
@@ -41,4 +69,14 @@ mod tests {
         let expected: Vec<u8> = expected_hex.from_hex().unwrap();
         assert_eq!(expected, output);
     }
+
+    #[cfg(feature = "zero_hash_cache")]
+    mod zero_hash {
+        use super::*;
+
+        #[test]
+        fn zero_hash_zero() {
+            assert_eq!(ZERO_HASHES[0], vec![0; 32]);
+        }
+    }
 }