// Copyright 2014 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 vp8l implements a decoder for the VP8L lossless image format.
 //
 // The VP8L specification is at:
 // https://developers.google.com/speed/webp/docs/riff_container
 package vp8l // import "golang.org/x/image/vp8l"
 
 import (
 	"bufio"
 	"errors"
 	"image"
 	"image/color"
 	"io"
 )
 
 var (
 	errInvalidCodeLengths = errors.New("vp8l: invalid code lengths")
 	errInvalidHuffmanTree = errors.New("vp8l: invalid Huffman tree")
 )
 
 // colorCacheMultiplier is the multiplier used for the color cache hash
 // function, specified in section 4.2.3.
 const colorCacheMultiplier = 0x1e35a7bd
 
 // distanceMapTable is the look-up table for distanceMap.
 var distanceMapTable = [120]uint8{
 	0x18, 0x07, 0x17, 0x19, 0x28, 0x06, 0x27, 0x29, 0x16, 0x1a,
 	0x26, 0x2a, 0x38, 0x05, 0x37, 0x39, 0x15, 0x1b, 0x36, 0x3a,
 	0x25, 0x2b, 0x48, 0x04, 0x47, 0x49, 0x14, 0x1c, 0x35, 0x3b,
 	0x46, 0x4a, 0x24, 0x2c, 0x58, 0x45, 0x4b, 0x34, 0x3c, 0x03,
 	0x57, 0x59, 0x13, 0x1d, 0x56, 0x5a, 0x23, 0x2d, 0x44, 0x4c,
 	0x55, 0x5b, 0x33, 0x3d, 0x68, 0x02, 0x67, 0x69, 0x12, 0x1e,
 	0x66, 0x6a, 0x22, 0x2e, 0x54, 0x5c, 0x43, 0x4d, 0x65, 0x6b,
 	0x32, 0x3e, 0x78, 0x01, 0x77, 0x79, 0x53, 0x5d, 0x11, 0x1f,
 	0x64, 0x6c, 0x42, 0x4e, 0x76, 0x7a, 0x21, 0x2f, 0x75, 0x7b,
 	0x31, 0x3f, 0x63, 0x6d, 0x52, 0x5e, 0x00, 0x74, 0x7c, 0x41,
 	0x4f, 0x10, 0x20, 0x62, 0x6e, 0x30, 0x73, 0x7d, 0x51, 0x5f,
 	0x40, 0x72, 0x7e, 0x61, 0x6f, 0x50, 0x71, 0x7f, 0x60, 0x70,
 }
 
 // distanceMap maps a LZ77 backwards reference distance to a two-dimensional
 // pixel offset, specified in section 4.2.2.
 func distanceMap(w int32, code uint32) int32 {
 	if int32(code) > int32(len(distanceMapTable)) {
 		return int32(code) - int32(len(distanceMapTable))
 	}
 	distCode := int32(distanceMapTable[code-1])
 	yOffset := distCode >> 4
 	xOffset := 8 - distCode&0xf
 	if d := yOffset*w + xOffset; d >= 1 {
 		return d
 	}
 	return 1
 }
 
 // decoder holds the bit-stream for a VP8L image.
 type decoder struct {
 	r     io.ByteReader
 	bits  uint32
 	nBits uint32
 }
 
 // read reads the next n bits from the decoder's bit-stream.
 func (d *decoder) read(n uint32) (uint32, error) {
 	for d.nBits < n {
 		c, err := d.r.ReadByte()
 		if err != nil {
 			if err == io.EOF {
 				err = io.ErrUnexpectedEOF
 			}
 			return 0, err
 		}
 		d.bits |= uint32(c) << d.nBits
 		d.nBits += 8
 	}
 	u := d.bits & (1<<n - 1)
 	d.bits >>= n
 	d.nBits -= n
 	return u, nil
 }
 
 // decodeTransform decodes the next transform and the width of the image after
 // transformation (or equivalently, before inverse transformation), specified
 // in section 3.
 func (d *decoder) decodeTransform(w int32, h int32) (t transform, newWidth int32, err error) {
 	t.oldWidth = w
 	t.transformType, err = d.read(2)
 	if err != nil {
 		return transform{}, 0, err
 	}
 	switch t.transformType {
 	case transformTypePredictor, transformTypeCrossColor:
 		t.bits, err = d.read(3)
 		if err != nil {
 			return transform{}, 0, err
 		}
 		t.bits += 2
 		t.pix, err = d.decodePix(nTiles(w, t.bits), nTiles(h, t.bits), 0, false)
 		if err != nil {
 			return transform{}, 0, err
 		}
 	case transformTypeSubtractGreen:
 		// No-op.
 	case transformTypeColorIndexing:
 		nColors, err := d.read(8)
 		if err != nil {
 			return transform{}, 0, err
 		}
 		nColors++
 		t.bits = 0
 		switch {
 		case nColors <= 2:
 			t.bits = 3
 		case nColors <= 4:
 			t.bits = 2
 		case nColors <= 16:
 			t.bits = 1
 		}
 		w = nTiles(w, t.bits)
 		pix, err := d.decodePix(int32(nColors), 1, 4*256, false)
 		if err != nil {
 			return transform{}, 0, err
 		}
 		for p := 4; p < len(pix); p += 4 {
 			pix[p+0] += pix[p-4]
 			pix[p+1] += pix[p-3]
 			pix[p+2] += pix[p-2]
 			pix[p+3] += pix[p-1]
 		}
 		// The spec says that "if the index is equal or larger than color_table_size,
 		// the argb color value should be set to 0x00000000 (transparent black)."
 		// We re-slice up to 256 4-byte pixels.
 		t.pix = pix[:4*256]
 	}
 	return t, w, nil
 }
 
 // repeatsCodeLength is the minimum code length for repeated codes.
 const repeatsCodeLength = 16
 
 // These magic numbers are specified at the end of section 5.2.2.
 // The 3-length arrays apply to code lengths >= repeatsCodeLength.
 var (
 	codeLengthCodeOrder = [19]uint8{
 		17, 18, 0, 1, 2, 3, 4, 5, 16, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15,
 	}
 	repeatBits    = [3]uint8{2, 3, 7}
 	repeatOffsets = [3]uint8{3, 3, 11}
 )
 
 // decodeCodeLengths decodes a Huffman tree's code lengths which are themselves
 // encoded via a Huffman tree, specified in section 5.2.2.
 func (d *decoder) decodeCodeLengths(dst []uint32, codeLengthCodeLengths []uint32) error {
 	h := hTree{}
 	if err := h.build(codeLengthCodeLengths); err != nil {
 		return err
 	}
 
 	maxSymbol := len(dst)
 	useLength, err := d.read(1)
 	if err != nil {
 		return err
 	}
 	if useLength != 0 {
 		n, err := d.read(3)
 		if err != nil {
 			return err
 		}
 		n = 2 + 2*n
 		ms, err := d.read(n)
 		if err != nil {
 			return err
 		}
 		maxSymbol = int(ms) + 2
 		if maxSymbol > len(dst) {
 			return errInvalidCodeLengths
 		}
 	}
 
 	// The spec says that "if code 16 [meaning repeat] is used before
 	// a non-zero value has been emitted, a value of 8 is repeated."
 	prevCodeLength := uint32(8)
 
 	for symbol := 0; symbol < len(dst); {
 		if maxSymbol == 0 {
 			break
 		}
 		maxSymbol--
 		codeLength, err := h.next(d)
 		if err != nil {
 			return err
 		}
 		if codeLength < repeatsCodeLength {
 			dst[symbol] = codeLength
 			symbol++
 			if codeLength != 0 {
 				prevCodeLength = codeLength
 			}
 			continue
 		}
 
 		repeat, err := d.read(uint32(repeatBits[codeLength-repeatsCodeLength]))
 		if err != nil {
 			return err
 		}
 		repeat += uint32(repeatOffsets[codeLength-repeatsCodeLength])
 		if symbol+int(repeat) > len(dst) {
 			return errInvalidCodeLengths
 		}
 		// A code length of 16 repeats the previous non-zero code.
 		// A code length of 17 or 18 repeats zeroes.
 		cl := uint32(0)
 		if codeLength == 16 {
 			cl = prevCodeLength
 		}
 		for ; repeat > 0; repeat-- {
 			dst[symbol] = cl
 			symbol++
 		}
 	}
 	return nil
 }
 
 // decodeHuffmanTree decodes a Huffman tree into h.
 func (d *decoder) decodeHuffmanTree(h *hTree, alphabetSize uint32) error {
 	useSimple, err := d.read(1)
 	if err != nil {
 		return err
 	}
 	if useSimple != 0 {
 		nSymbols, err := d.read(1)
 		if err != nil {
 			return err
 		}
 		nSymbols++
 		firstSymbolLengthCode, err := d.read(1)
 		if err != nil {
 			return err
 		}
 		firstSymbolLengthCode = 7*firstSymbolLengthCode + 1
 		var symbols [2]uint32
 		symbols[0], err = d.read(firstSymbolLengthCode)
 		if err != nil {
 			return err
 		}
 		if nSymbols == 2 {
 			symbols[1], err = d.read(8)
 			if err != nil {
 				return err
 			}
 		}
 		return h.buildSimple(nSymbols, symbols, alphabetSize)
 	}
 
 	nCodes, err := d.read(4)
 	if err != nil {
 		return err
 	}
 	nCodes += 4
 	if int(nCodes) > len(codeLengthCodeOrder) {
 		return errInvalidHuffmanTree
 	}
 	codeLengthCodeLengths := [len(codeLengthCodeOrder)]uint32{}
 	for i := uint32(0); i < nCodes; i++ {
 		codeLengthCodeLengths[codeLengthCodeOrder[i]], err = d.read(3)
 		if err != nil {
 			return err
 		}
 	}
 	codeLengths := make([]uint32, alphabetSize)
 	if err = d.decodeCodeLengths(codeLengths, codeLengthCodeLengths[:]); err != nil {
 		return err
 	}
 	return h.build(codeLengths)
 }
 
 const (
 	huffGreen    = 0
 	huffRed      = 1
 	huffBlue     = 2
 	huffAlpha    = 3
 	huffDistance = 4
 	nHuff        = 5
 )
 
 // hGroup is an array of 5 Huffman trees.
 type hGroup [nHuff]hTree
 
 // decodeHuffmanGroups decodes the one or more hGroups used to decode the pixel
 // data. If one hGroup is used for the entire image, then hPix and hBits will
 // be zero. If more than one hGroup is used, then hPix contains the meta-image
 // that maps tiles to hGroup index, and hBits contains the log-2 tile size.
 func (d *decoder) decodeHuffmanGroups(w int32, h int32, topLevel bool, ccBits uint32) (
 	hGroups []hGroup, hPix []byte, hBits uint32, err error) {
 
 	maxHGroupIndex := 0
 	if topLevel {
 		useMeta, err := d.read(1)
 		if err != nil {
 			return nil, nil, 0, err
 		}
 		if useMeta != 0 {
 			hBits, err = d.read(3)
 			if err != nil {
 				return nil, nil, 0, err
 			}
 			hBits += 2
 			hPix, err = d.decodePix(nTiles(w, hBits), nTiles(h, hBits), 0, false)
 			if err != nil {
 				return nil, nil, 0, err
 			}
 			for p := 0; p < len(hPix); p += 4 {
 				i := int(hPix[p])<<8 | int(hPix[p+1])
 				if maxHGroupIndex < i {
 					maxHGroupIndex = i
 				}
 			}
 		}
 	}
 	hGroups = make([]hGroup, maxHGroupIndex+1)
 	for i := range hGroups {
 		for j, alphabetSize := range alphabetSizes {
 			if j == 0 && ccBits > 0 {
 				alphabetSize += 1 << ccBits
 			}
 			if err := d.decodeHuffmanTree(&hGroups[i][j], alphabetSize); err != nil {
 				return nil, nil, 0, err
 			}
 		}
 	}
 	return hGroups, hPix, hBits, nil
 }
 
 const (
 	nLiteralCodes  = 256
 	nLengthCodes   = 24
 	nDistanceCodes = 40
 )
 
 var alphabetSizes = [nHuff]uint32{
 	nLiteralCodes + nLengthCodes,
 	nLiteralCodes,
 	nLiteralCodes,
 	nLiteralCodes,
 	nDistanceCodes,
 }
 
 // decodePix decodes pixel data, specified in section 5.2.2.
 func (d *decoder) decodePix(w int32, h int32, minCap int32, topLevel bool) ([]byte, error) {
 	// Decode the color cache parameters.
 	ccBits, ccShift, ccEntries := uint32(0), uint32(0), ([]uint32)(nil)
 	useColorCache, err := d.read(1)
 	if err != nil {
 		return nil, err
 	}
 	if useColorCache != 0 {
 		ccBits, err = d.read(4)
 		if err != nil {
 			return nil, err
 		}
 		if ccBits < 1 || 11 < ccBits {
 			return nil, errors.New("vp8l: invalid color cache parameters")
 		}
 		ccShift = 32 - ccBits
 		ccEntries = make([]uint32, 1<<ccBits)
 	}
 
 	// Decode the Huffman groups.
 	hGroups, hPix, hBits, err := d.decodeHuffmanGroups(w, h, topLevel, ccBits)
 	if err != nil {
 		return nil, err
 	}
 	hMask, tilesPerRow := int32(0), int32(0)
 	if hBits != 0 {
 		hMask, tilesPerRow = 1<<hBits-1, nTiles(w, hBits)
 	}
 
 	// Decode the pixels.
 	if minCap < 4*w*h {
 		minCap = 4 * w * h
 	}
 	pix := make([]byte, 4*w*h, minCap)
 	p, cachedP := 0, 0
 	x, y := int32(0), int32(0)
 	hg, lookupHG := &hGroups[0], hMask != 0
 	for p < len(pix) {
 		if lookupHG {
 			i := 4 * (tilesPerRow*(y>>hBits) + (x >> hBits))
 			hg = &hGroups[uint32(hPix[i])<<8|uint32(hPix[i+1])]
 		}
 
 		green, err := hg[huffGreen].next(d)
 		if err != nil {
 			return nil, err
 		}
 		switch {
 		case green < nLiteralCodes:
 			// We have a literal pixel.
 			red, err := hg[huffRed].next(d)
 			if err != nil {
 				return nil, err
 			}
 			blue, err := hg[huffBlue].next(d)
 			if err != nil {
 				return nil, err
 			}
 			alpha, err := hg[huffAlpha].next(d)
 			if err != nil {
 				return nil, err
 			}
 			pix[p+0] = uint8(red)
 			pix[p+1] = uint8(green)
 			pix[p+2] = uint8(blue)
 			pix[p+3] = uint8(alpha)
 			p += 4
 
 			x++
 			if x == w {
 				x, y = 0, y+1
 			}
 			lookupHG = hMask != 0 && x&hMask == 0
 
 		case green < nLiteralCodes+nLengthCodes:
 			// We have a LZ77 backwards reference.
 			length, err := d.lz77Param(green - nLiteralCodes)
 			if err != nil {
 				return nil, err
 			}
 			distSym, err := hg[huffDistance].next(d)
 			if err != nil {
 				return nil, err
 			}
 			distCode, err := d.lz77Param(distSym)
 			if err != nil {
 				return nil, err
 			}
 			dist := distanceMap(w, distCode)
 			pEnd := p + 4*int(length)
 			q := p - 4*int(dist)
 			qEnd := pEnd - 4*int(dist)
 			if p < 0 || len(pix) < pEnd || q < 0 || len(pix) < qEnd {
 				return nil, errors.New("vp8l: invalid LZ77 parameters")
 			}
 			for ; p < pEnd; p, q = p+1, q+1 {
 				pix[p] = pix[q]
 			}
 
 			x += int32(length)
 			for x >= w {
 				x, y = x-w, y+1
 			}
 			lookupHG = hMask != 0
 
 		default:
 			// We have a color cache lookup. First, insert previous pixels
 			// into the cache. Note that VP8L assumes ARGB order, but the
 			// Go image.RGBA type is in RGBA order.
 			for ; cachedP < p; cachedP += 4 {
 				argb := uint32(pix[cachedP+0])<<16 |
 					uint32(pix[cachedP+1])<<8 |
 					uint32(pix[cachedP+2])<<0 |
 					uint32(pix[cachedP+3])<<24
 				ccEntries[(argb*colorCacheMultiplier)>>ccShift] = argb
 			}
 			green -= nLiteralCodes + nLengthCodes
 			if int(green) >= len(ccEntries) {
 				return nil, errors.New("vp8l: invalid color cache index")
 			}
 			argb := ccEntries[green]
 			pix[p+0] = uint8(argb >> 16)
 			pix[p+1] = uint8(argb >> 8)
 			pix[p+2] = uint8(argb >> 0)
 			pix[p+3] = uint8(argb >> 24)
 			p += 4
 
 			x++
 			if x == w {
 				x, y = 0, y+1
 			}
 			lookupHG = hMask != 0 && x&hMask == 0
 		}
 	}
 	return pix, nil
 }
 
 // lz77Param returns the next LZ77 parameter: a length or a distance, specified
 // in section 4.2.2.
 func (d *decoder) lz77Param(symbol uint32) (uint32, error) {
 	if symbol < 4 {
 		return symbol + 1, nil
 	}
 	extraBits := (symbol - 2) >> 1
 	offset := (2 + symbol&1) << extraBits
 	n, err := d.read(extraBits)
 	if err != nil {
 		return 0, err
 	}
 	return offset + n + 1, nil
 }
 
 // decodeHeader decodes the VP8L header from r.
 func decodeHeader(r io.Reader) (d *decoder, w int32, h int32, err error) {
 	rr, ok := r.(io.ByteReader)
 	if !ok {
 		rr = bufio.NewReader(r)
 	}
 	d = &decoder{r: rr}
 	magic, err := d.read(8)
 	if err != nil {
 		return nil, 0, 0, err
 	}
 	if magic != 0x2f {
 		return nil, 0, 0, errors.New("vp8l: invalid header")
 	}
 	width, err := d.read(14)
 	if err != nil {
 		return nil, 0, 0, err
 	}
 	width++
 	height, err := d.read(14)
 	if err != nil {
 		return nil, 0, 0, err
 	}
 	height++
 	_, err = d.read(1) // Read and ignore the hasAlpha hint.
 	if err != nil {
 		return nil, 0, 0, err
 	}
 	version, err := d.read(3)
 	if err != nil {
 		return nil, 0, 0, err
 	}
 	if version != 0 {
 		return nil, 0, 0, errors.New("vp8l: invalid version")
 	}
 	return d, int32(width), int32(height), nil
 }
 
 // DecodeConfig decodes the color model and dimensions of a VP8L image from r.
 func DecodeConfig(r io.Reader) (image.Config, error) {
 	_, w, h, err := decodeHeader(r)
 	if err != nil {
 		return image.Config{}, err
 	}
 	return image.Config{
 		ColorModel: color.NRGBAModel,
 		Width:      int(w),
 		Height:     int(h),
 	}, nil
 }
 
 // Decode decodes a VP8L image from r.
 func Decode(r io.Reader) (image.Image, error) {
 	d, w, h, err := decodeHeader(r)
 	if err != nil {
 		return nil, err
 	}
 	// Decode the transforms.
 	var (
 		nTransforms    int
 		transforms     [nTransformTypes]transform
 		transformsSeen [nTransformTypes]bool
 		originalW      = w
 	)
 	for {
 		more, err := d.read(1)
 		if err != nil {
 			return nil, err
 		}
 		if more == 0 {
 			break
 		}
 		var t transform
 		t, w, err = d.decodeTransform(w, h)
 		if err != nil {
 			return nil, err
 		}
 		if transformsSeen[t.transformType] {
 			return nil, errors.New("vp8l: repeated transform")
 		}
 		transformsSeen[t.transformType] = true
 		transforms[nTransforms] = t
 		nTransforms++
 	}
 	// Decode the transformed pixels.
 	pix, err := d.decodePix(w, h, 0, true)
 	if err != nil {
 		return nil, err
 	}
 	// Apply the inverse transformations.
 	for i := nTransforms - 1; i >= 0; i-- {
 		t := &transforms[i]
 		pix = inverseTransforms[t.transformType](t, pix, h)
 	}
 	return &image.NRGBA{
 		Pix:    pix,
 		Stride: 4 * int(originalW),
 		Rect:   image.Rect(0, 0, int(originalW), int(h)),
 	}, nil
 }