Remove leaf hashing in MerkleRoot function (#1653)

* Remove leaf hashing in MerkleRoot function

See https://github.com/ethereum/eth2.0-specs/pull/646 for clarification.

* Add comment clarifying leaf hashing removal

shared/hashutil/merkleRoot.go

@@ -3,6 +3,8 @@ package hashutil
 // MerkleRoot derives the merkle root from a 2d byte array with each element
 // in the outer array signifying the data that is to be represented in the
 // merkle tree.
+// Note: This function is only used to merklize a list of block root hashes.
+// As such, we assume the input comes pre-hashed and do NOT hash the leaves.
 // Spec:
 //	def merkle_root(values):
 //		o = [0] * len(values) + values
@@ -12,12 +14,6 @@ package hashutil
 func MerkleRoot(values [][]byte) []byte {
 	length := len(values)
 
-	// Data is hashed so as to be stored as leaves in the tree.
-	for i, v := range values {
-		hashedValue := Hash(v)
-		values[i] = hashedValue[:]
-	}
-
 	newSet := make([][]byte, length, length*2)
 	newSet = append(newSet, values...)
 

shared/hashutil/merkleRoot_test.go

@@ -13,10 +13,10 @@ func TestMerkleRoot(t *testing.T) {
 		{'d'},
 	}
 
-	hashedV1 := Hash([]byte{'a'})
-	hashedV2 := Hash([]byte{'b'})
-	hashedV3 := Hash([]byte{'c'})
-	hashedV4 := Hash([]byte{'d'})
+	hashedV1 := []byte{'a'}
+	hashedV2 := []byte{'b'}
+	hashedV3 := []byte{'c'}
+	hashedV4 := []byte{'d'}
 
 	leftNode := Hash(append(hashedV1[:], hashedV2[:]...))
 	rightNode := Hash(append(hashedV3[:], hashedV4[:]...))