refactor: pre calculate the empty hash to save cost (#25359)

Co-authored-by: Alex | Interchain Labs <alex@cosmoslabs.io>

store/internal/tree/hash.go

@@ -4,6 +4,7 @@ import (
 	"crypto/sha256"
 	"hash"
 	"math/bits"
+	"slices"
 )
 
 var (
@@ -17,10 +18,13 @@ func HashFromByteSlices(items [][]byte) []byte {
 	return hashFromByteSlices(sha256.New(), items)
 }
 
+// emptyHash is the sha256 hash of the empty string.
+var emptyHash = [32]byte{0xe3, 0xb0, 0xc4, 0x42, 0x98, 0xfc, 0x1c, 0x14, 0x9a, 0xfb, 0xf4, 0xc8, 0x99, 0x6f, 0xb9, 0x24, 0x27, 0xae, 0x41, 0xe4, 0x64, 0x9b, 0x93, 0x4c, 0xa4, 0x95, 0x99, 0x1b, 0x78, 0x52, 0xb8, 0x55}
+
 func hashFromByteSlices(sha hash.Hash, items [][]byte) []byte {
 	switch len(items) {
 	case 0:
-		return emptyHash()
+		return slices.Clone(emptyHash[:])
 	case 1:
 		return leafHashOpt(sha, items[0])
 	default:
@@ -47,12 +51,6 @@ func innerHashOpt(s hash.Hash, left, right []byte) []byte {
 	return s.Sum(nil)
 }
 
-// emptyHash returns tmhash(<empty>)
-func emptyHash() []byte {
-	h := sha256.Sum256([]byte{})
-	return h[:]
-}
-
 // getSplitPoint returns the largest power of 2 less than length
 func getSplitPoint(length int64) int64 {
 	if length < 1 {

store/internal/tree/hash_test.go

@@ -0,0 +1,20 @@
+package tree
+
+import (
+	"crypto/sha256"
+	"testing"
+)
+
+func emptyHashF() []byte {
+	h := sha256.Sum256([]byte{})
+	return h[:]
+}
+
+func TestEmptyHashEqual(t *testing.T) {
+	var emptyHashBytes [32]byte
+	copy(emptyHashBytes[:], emptyHashF())
+	expected := emptyHash
+	if emptyHashBytes != expected {
+		t.Fatalf("empty hash mismatch: got=%#v expected=%#v", emptyHashBytes, expected)
+	}
+}