1 files changed, 91 insertions, 0 deletions
diff --git a/src/image/png/paeth_test.go b/src/image/png/paeth_test.go
new file mode 100644
index 0000000..cfc1896
--- /dev/null
+++ b/src/image/png/paeth_test.go
@@ -0,0 +1,91 @@
+// Copyright 2012 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package png
+
+import (
+	"bytes"
+	"math/rand"
+	"testing"
+)
+
+func slowAbs(x int) int {
+	if x < 0 {
+		return -x
+	}
+	return x
+}
+
+// slowPaeth is a slow but simple implementation of the Paeth function.
+// It is a straight port of the sample code in the PNG spec, section 9.4.
+func slowPaeth(a, b, c uint8) uint8 {
+	p := int(a) + int(b) - int(c)
+	pa := slowAbs(p - int(a))
+	pb := slowAbs(p - int(b))
+	pc := slowAbs(p - int(c))
+	if pa <= pb && pa <= pc {
+		return a
+	} else if pb <= pc {
+		return b
+	}
+	return c
+}
+
+// slowFilterPaeth is a slow but simple implementation of func filterPaeth.
+func slowFilterPaeth(cdat, pdat []byte, bytesPerPixel int) {
+	for i := 0; i < bytesPerPixel; i++ {
+		cdat[i] += paeth(0, pdat[i], 0)
+	}
+	for i := bytesPerPixel; i < len(cdat); i++ {
+		cdat[i] += paeth(cdat[i-bytesPerPixel], pdat[i], pdat[i-bytesPerPixel])
+	}
+}
+
+func TestPaeth(t *testing.T) {
+	for a := 0; a < 256; a += 15 {
+		for b := 0; b < 256; b += 15 {
+			for c := 0; c < 256; c += 15 {
+				got := paeth(uint8(a), uint8(b), uint8(c))
+				want := slowPaeth(uint8(a), uint8(b), uint8(c))
+				if got != want {
+					t.Errorf("a, b, c = %d, %d, %d: got %d, want %d", a, b, c, got, want)
+				}
+			}
+		}
+	}
+}
+
+func BenchmarkPaeth(b *testing.B) {
+	for i := 0; i < b.N; i++ {
+		paeth(uint8(i>>16), uint8(i>>8), uint8(i))
+	}
+}
+
+func TestPaethDecode(t *testing.T) {
+	pdat0 := make([]byte, 32)
+	pdat1 := make([]byte, 32)
+	pdat2 := make([]byte, 32)
+	cdat0 := make([]byte, 32)
+	cdat1 := make([]byte, 32)
+	cdat2 := make([]byte, 32)
+	r := rand.New(rand.NewSource(1))
+	for bytesPerPixel := 1; bytesPerPixel <= 8; bytesPerPixel++ {
+		for i := 0; i < 100; i++ {
+			for j := range pdat0 {
+				pdat0[j] = uint8(r.Uint32())
+				cdat0[j] = uint8(r.Uint32())
+			}
+			copy(pdat1, pdat0)
+			copy(pdat2, pdat0)
+			copy(cdat1, cdat0)
+			copy(cdat2, cdat0)
+			filterPaeth(cdat1, pdat1, bytesPerPixel)
+			slowFilterPaeth(cdat2, pdat2, bytesPerPixel)
+			if !bytes.Equal(cdat1, cdat2) {
+				t.Errorf("bytesPerPixel: %d\npdat0: % x\ncdat0: % x\ngot:   % x\nwant:  % x", bytesPerPixel, pdat0, cdat0, cdat1, cdat2)
+				break
+			}
+		}
+	}
+}