zstd: Rewrite matchLen to make it inlineable

zstd/enc_base.go

@@ -126,24 +126,7 @@ func (e *fastBase) matchlen(s, t int32, src []byte) int32 {
 			panic(fmt.Sprintf("len(src)-s (%d) > maxCompressedBlockSize (%d)", len(src)-int(s), maxCompressedBlockSize))
 		}
 	}
-	a := src[s:]
-	b := src[t:]
-	b = b[:len(a)]
-	end := int32((len(a) >> 3) << 3)
-	for i := int32(0); i < end; i += 8 {
-		if diff := load6432(a, i) ^ load6432(b, i); diff != 0 {
-			return i + int32(bits.TrailingZeros64(diff)>>3)
-		}
-	}
-
-	a = a[end:]
-	b = b[end:]
-	for i := range a {
-		if a[i] != b[i] {
-			return int32(i) + end
-		}
-	}
-	return int32(len(a)) + end
+	return int32(matchLen(src[s:], src[t:]))
 }
 
 // Reset the encoding table.

zstd/zstd.go

@@ -110,26 +110,25 @@ func printf(format string, a ...interface{}) {
 	}
 }
 
-// matchLen returns the maximum length.
+// matchLen returns the maximum common prefix length of a and b.
 // a must be the shortest of the two.
-// The function also returns whether all bytes matched.
-func matchLen(a, b []byte) int {
-	b = b[:len(a)]
-	for i := 0; i < len(a)-7; i += 8 {
-		if diff := load64(a, i) ^ load64(b, i); diff != 0 {
-			return i + (bits.TrailingZeros64(diff) >> 3)
+func matchLen(a, b []byte) (n int) {
+	for ; len(a) >= 8 && len(b) >= 8; a, b = a[8:], b[8:] {
+		diff := binary.LittleEndian.Uint64(a) ^ binary.LittleEndian.Uint64(b)
+		if diff != 0 {
+			return n + bits.TrailingZeros64(diff)>>3
 		}
+		n += 8
 	}
 
-	checked := (len(a) >> 3) << 3
-	a = a[checked:]
-	b = b[checked:]
 	for i := range a {
 		if a[i] != b[i] {
-			return i + checked
+			break
 		}
+		n++
 	}
-	return len(a) + checked
+	return n
+
 }
 
 func load3232(b []byte, i int32) uint32 {
@@ -140,10 +139,6 @@ func load6432(b []byte, i int32) uint64 {
 	return binary.LittleEndian.Uint64(b[i:])
 }
 
-func load64(b []byte, i int) uint64 {
-	return binary.LittleEndian.Uint64(b[i:])
-}
-
 type byter interface {
 	Bytes() []byte
 	Len() int

zstd/zstd_test.go

@@ -31,3 +31,24 @@ func TestMain(m *testing.M) {
 	}
 	os.Exit(ec)
 }
+
+func TestMatchLen(t *testing.T) {
+	a := make([]byte, 130)
+	for i := range a {
+		a[i] = byte(i)
+	}
+	b := append([]byte{}, a...)
+
+	check := func(x, y []byte, l int) {
+		if m := matchLen(x, y); m != l {
+			t.Error("expected", l, "got", m)
+		}
+	}
+
+	for l := range a {
+		a[l] = ^a[l]
+		check(a, b, l)
+		check(a[:l], b, l)
+		a[l] = ^a[l]
+	}
+}