Merge pull request cosmos#26 from odeke-em/improve-reverse-bytes+tests

hd: optimize ReverseBytes + add tests

hd/address.go

@@ -282,9 +282,20 @@ func CalcSha512(buf []byte) []byte {
 }
 
 func ReverseBytes(buf []byte) []byte {
-	res := []byte{}
-	for i := len(buf) - 1; i >= 0; i-- {
-		res = append(res, buf[i])
+	var res []byte
+	if len(buf) == 0 {
+		return res
+	}
+
+	// Walk till mid-way, swapping bytes from each end:
+	// b[i] and b[len-i-1]
+	blen := len(buf)
+	res = make([]byte, blen)
+	mid := blen / 2
+	for left := 0; left <= mid; left++ {
+		right := blen - left - 1
+		res[left] = buf[right]
+		res[right] = buf[left]
 	}
 	return res
 }

hd/hd_test.go

@@ -89,6 +89,26 @@ func TestHDToAddr(t *testing.T) {
 	}
 }
 
+func TestReverseBytes(t *testing.T) {
+	tests := [...]struct {
+		v    []byte
+		want []byte
+	}{
+		{[]byte(""), []byte("")},
+		{nil, nil},
+		{[]byte("Tendermint"), []byte("tnimredneT")},
+		{[]byte("T"), []byte("T")},
+		{[]byte("Te"), []byte("eT")},
+	}
+
+	for i, tt := range tests {
+		got := ReverseBytes(tt.v)
+		if !bytes.Equal(got, tt.want) {
+			t.Errorf("#%d:\ngot= (%x)\nwant=(%x)", i, got, tt.want)
+		}
+	}
+}
+
 func ifExit(err error, n int) {
 	if err != nil {
 		fmt.Println(n, err)
@@ -187,3 +207,36 @@ func tylerSmith(seed []byte) ([]byte, []byte, []byte) {
 	pub := k.PublicKey().Key
 	return masterKey.Key, priv, pub
 }
+
+// Benchmarks
+
+var revBytesCases = [][]byte{
+	nil,
+	[]byte(""),
+
+	[]byte("12"),
+
+	// 16byte case
+	[]byte("abcdefghijklmnop"),
+
+	// 32byte case
+	[]byte("abcdefghijklmnopqrstuvwxyz123456"),
+
+	// 64byte case
+	[]byte("abcdefghijklmnopqrstuvwxyz123456abcdefghijklmnopqrstuvwxyz123456"),
+}
+
+func BenchmarkReverseBytes(b *testing.B) {
+	var sink []byte
+	for i := 0; i < b.N; i++ {
+		for _, tt := range revBytesCases {
+			sink = ReverseBytes(tt)
+		}
+	}
+	b.ReportAllocs()
+
+	// sink is necessary to ensure if the compiler tries
+	// to smart, that it won't optimize away the benchmarks.
+	if sink != nil {
+	}
+}