}

// 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 {

// cdat is the current row's data, pdat is the previous row's data.
func filterPaeth(cdat, pdat []byte, bytesPerPixel int) {
	var a, b, c, pa, pb, pc int
	for i := 0; i < bytesPerPixel; i++ {
		a, c = 0, 0
		for j := i; j < len(cdat); j += bytesPerPixel {
			b = int(pdat[j])
			pa = b - c
			pb = a - c
			pc = abs(pa + pb)
			pa = abs(pa)
			pb = abs(pb)
			if pa <= pb && pa <= pc {

			// check above that width != 0, and so len(cdat) != 0.
			for i := 0; i < bytesPerPixel; i++ {
				cdat[i] += pdat[i] / 2
			}
			for i := bytesPerPixel; i < len(cdat); i++ {
				cdat[i] += uint8((int(cdat[i-bytesPerPixel]) + int(pdat[i])) / 2)
			}
		case ftPaeth:
			filterPaeth(cdat, pdat, bytesPerPixel)
		default:
			return nil, FormatError("bad filter type")
		}

	}
}

func benchmarkDecode(b *testing.B, filename string, bytesPerPixel int) {
	data, err := os.ReadFile(filename)
	if err != nil {
		b.Fatal(err)
	}
	cfg, err := DecodeConfig(bytes.NewReader(data))
	if err != nil {
		b.Fatal(err)
	}
	b.SetBytes(int64(cfg.Width * cfg.Height * bytesPerPixel))
	b.ReportAllocs()
	b.ResetTimer()
	for i := 0; i < b.N; i++ {

//
// This panics instead of returning an error because of backwards
// compatibility. The NewXxx functions do not return an error.
func pixelBufferLength(bytesPerPixel int, r Rectangle, imageTypeName string) int {
	totalLength := mul3NonNeg(bytesPerPixel, r.Dx(), r.Dy())
	if totalLength < 0 {
		panic("image: New" + imageTypeName + " Rectangle has huge or negative dimensions")
	}
	return totalLength
}