crypto/merkle: optimize merkle tree hashing (#6513)

## Description 

Upstream https://github.com/lazyledger/lazyledger-core/pull/351 to optimize merkle tree hashing 

### Benchmarking:

```
benchmark                                 old ns/op     new ns/op     delta
BenchmarkHashAlternatives/recursive-8     22914         21949         -4.21%
BenchmarkHashAlternatives/iterative-8     21634         21939         +1.41%

benchmark                                 old allocs     new allocs     delta
BenchmarkHashAlternatives/recursive-8     398            200            -49.75%
BenchmarkHashAlternatives/iterative-8     399            301            -24.56%

benchmark                                 old bytes     new bytes     delta
BenchmarkHashAlternatives/recursive-8     19088         6496          -65.97%
BenchmarkHashAlternatives/iterative-8     21776         13984         -35.78%
```

cc @odeke-em @cuonglm

crypto/merkle/hash.go

@@ -1,6 +1,8 @@
 package merkle
 
 import (
+	"hash"
+
 	"github.com/tendermint/tendermint/crypto/tmhash"
 )
 
@@ -20,6 +22,14 @@ func leafHash(leaf []byte) []byte {
 	return tmhash.Sum(append(leafPrefix, leaf...))
 }
 
+// returns tmhash(0x00 || leaf)
+func leafHashOpt(s hash.Hash, leaf []byte) []byte {
+	s.Reset()
+	s.Write(leafPrefix)
+	s.Write(leaf)
+	return s.Sum(nil)
+}
+
 // returns tmhash(0x01 || left || right)
 func innerHash(left []byte, right []byte) []byte {
 	data := make([]byte, len(innerPrefix)+len(left)+len(right))
@@ -28,3 +38,11 @@ func innerHash(left []byte, right []byte) []byte {
 	copy(data[n:], right)
 	return tmhash.Sum(data)
 }
+
+func innerHashOpt(s hash.Hash, left []byte, right []byte) []byte {
+	s.Reset()
+	s.Write(innerPrefix)
+	s.Write(left)
+	s.Write(right)
+	return s.Sum(nil)
+}

crypto/merkle/proof.go

@@ -50,13 +50,13 @@ func ProofsFromByteSlices(items [][]byte) (rootHash []byte, proofs []*Proof) {
 // Verify that the Proof proves the root hash.
 // Check sp.Index/sp.Total manually if needed
 func (sp *Proof) Verify(rootHash []byte, leaf []byte) error {
-	leafHash := leafHash(leaf)
 	if sp.Total < 0 {
 		return errors.New("proof total must be positive")
 	}
 	if sp.Index < 0 {
 		return errors.New("proof index cannot be negative")
 	}
+	leafHash := leafHash(leaf)
 	if !bytes.Equal(sp.LeafHash, leafHash) {
 		return fmt.Errorf("invalid leaf hash: wanted %X got %X", leafHash, sp.LeafHash)
 	}

crypto/merkle/proof_key_path_test.go

@@ -35,6 +35,7 @@ func TestKeyPath(t *testing.T) {
 
 		res, err := KeyPathToKeys(path.String())
 		require.Nil(t, err)
+		require.Equal(t, len(keys), len(res))
 
 		for i, key := range keys {
 			require.Equal(t, key, res[i])

crypto/merkle/proof_test.go

@@ -171,12 +171,12 @@ func TestProofValidateBasic(t *testing.T) {
 	}
 }
 func TestVoteProtobuf(t *testing.T) {
-
 	_, proofs := ProofsFromByteSlices([][]byte{
 		[]byte("apple"),
 		[]byte("watermelon"),
 		[]byte("kiwi"),
 	})
+
 	testCases := []struct {
 		testName string
 		v1       *Proof

crypto/merkle/tree.go

@@ -1,22 +1,28 @@
 package merkle
 
 import (
+	"crypto/sha256"
+	"hash"
 	"math/bits"
 )
 
 // HashFromByteSlices computes a Merkle tree where the leaves are the byte slice,
 // in the provided order. It follows RFC-6962.
 func HashFromByteSlices(items [][]byte) []byte {
+	return hashFromByteSlices(sha256.New(), items)
+}
+
+func hashFromByteSlices(sha hash.Hash, items [][]byte) []byte {
 	switch len(items) {
 	case 0:
 		return emptyHash()
 	case 1:
-		return leafHash(items[0])
+		return leafHashOpt(sha, items[0])
 	default:
 		k := getSplitPoint(int64(len(items)))
-		left := HashFromByteSlices(items[:k])
-		right := HashFromByteSlices(items[k:])
-		return innerHash(left, right)
+		left := hashFromByteSlices(sha, items[:k])
+		right := hashFromByteSlices(sha, items[k:])
+		return innerHashOpt(sha, left, right)
 	}
 }
 
@@ -61,7 +67,7 @@ func HashFromByteSlices(items [][]byte) []byte {
 // implementation for so little benefit.
 func HashFromByteSlicesIterative(input [][]byte) []byte {
 	items := make([][]byte, len(input))
-
+	sha := sha256.New()
 	for i, leaf := range input {
 		items[i] = leafHash(leaf)
 	}
@@ -78,7 +84,7 @@ func HashFromByteSlicesIterative(input [][]byte) []byte {
 			wp := 0 // write position
 			for rp < size {
 				if rp+1 < size {
-					items[wp] = innerHash(items[rp], items[rp+1])
+					items[wp] = innerHashOpt(sha, items[rp], items[rp+1])
 					rp += 2
 				} else {
 					items[wp] = items[rp]