// Copyright 2009 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 jpeg
import (
"io"
)
// maxCodeLength is the maximum (inclusive) number of bits in a Huffman code.
const maxCodeLength = 16
// maxNCodes is the maximum (inclusive) number of codes in a Huffman tree.
const maxNCodes = 256
// lutSize is the log-2 size of the Huffman decoder's look-up table.
const lutSize = 8
// huffman is a Huffman decoder, specified in section C.
type huffman struct {
// length is the number of codes in the tree.
nCodes int32
// lut is the look-up table for the next lutSize bits in the bit-stream.
// The high 8 bits of the uint16 are the encoded value. The low 8 bits
// are 1 plus the code length, or 0 if the value is too large to fit in
// lutSize bits.
lut [1 << lutSize]uint16
// vals are the decoded values, sorted by their encoding.
vals [maxNCodes]uint8
// minCodes[i] is the minimum code of length i, or -1 if there are no
// codes of that length.
minCodes [maxCodeLength]int32
// maxCodes[i] is the maximum code of length i, or -1 if there are no
// codes of that length.
maxCodes [maxCodeLength]int32
// valsIndices[i] is the index into vals of minCodes[i].
valsIndices [maxCodeLength]int32
}
// errShortHuffmanData means that an unexpected EOF occurred while decoding
// Huffman data.
var errShortHuffmanData = FormatError("short Huffman data")
// ensureNBits reads bytes from the byte buffer to ensure that d.bits.n is at
// least n. For best performance (avoiding function calls inside hot loops),
// the caller is the one responsible for first checking that d.bits.n < n.
func (d *decoder) ensureNBits(n int32) error {
for {
c, err := d.readByteStuffedByte()
if err != nil {
if err == io.EOF {
return errShortHuffmanData
}
return err
}
d.bits.a = d.bits.a<<8 | uint32(c)
d.bits.n += 8
if d.bits.m == 0 {
d.bits.m = 1 << 7
} else {
d.bits.m <<= 8
}
if d.bits.n >= n {
break
}
}
return nil
}
// receiveExtend is the composition of RECEIVE and EXTEND, specified in section
// F.2.2.1.
func (d *decoder) receiveExtend(t uint8) (int32, error) {
if d.bits.n < int32(t) {
if err := d.ensureNBits(int32(t)); err != nil {
return 0, err
}
}
d.bits.n -= int32(t)
d.bits.m >>= t
s := int32(1) << t
x := int32(d.bits.a>>uint8(d.bits.n)) & (s - 1)
if x < s>>1 {
x += ((-1) << t) + 1
}
return x, nil
}
// processDHT processes a Define Huffman Table marker, and initializes a huffman
// struct from its contents. Specified in section B.2.4.2.
func (d *decoder) processDHT(n int) error {
for n > 0 {
if n < 17 {
return FormatError("DHT has wrong length")
}
if err := d.readFull(d.tmp[:17]); err != nil {
return err
}
tc := d.tmp[0] >> 4
if tc > maxTc {
return FormatError("bad Tc value")
}
th := d.tmp[0] & 0x0f
// The baseline th <= 1 restriction is specified in table B.5.
if th > maxTh || (d.baseline && th > 1) {
return FormatError("bad Th value")
}
h := &d.huff[tc][th]
// Read nCodes and h.vals (and derive h.nCodes).
// nCodes[i] is the number of codes with code length i.
// h.nCodes is the total number of codes.
h.nCodes = 0
var nCodes [maxCodeLength]int32
for i := range nCodes {
nCodes[i] = int32(d.tmp[i+1])
h.nCodes += nCodes[i]
}
if h.nCodes == 0 {
return FormatError("Huffman table has zero length")
}
if h.nCodes > maxNCodes {
return FormatError("Huffman table has excessive length")
}
n -= int(h.nCodes) + 17
if n < 0 {
return FormatError("DHT has wrong length")
}
if err := d.readFull(h.vals[:h.nCodes]); err != nil {
return err
}
// Derive the look-up table.
for i := range h.lut {
h.lut[i] = 0
}
var x, code uint32
for i := uint32(0); i < lutSize; i++ {
code <<= 1
for j := int32(0); j < nCodes[i]; j++ {
// The codeLength is 1+i, so shift code by 8-(1+i) to
// calculate the high bits for every 8-bit sequence
// whose codeLength's high bits matches code.
// The high 8 bits of lutValue are the encoded value.
// The low 8 bits are 1 plus the codeLength.
base := uint8(code << (7 - i))
lutValue := uint16(h.vals[x])<<8 | uint16(2+i)
for k := uint8(0); k < 1<<(7-i); k++ {
h.lut[base|k] = lutValue
}
code++
x++
}
}
// Derive minCodes, maxCodes, and valsIndices.
var c, index int32
for i, n := range nCodes {
if n == 0 {
h.minCodes[i] = -1
h.maxCodes[i] = -1
h.valsIndices[i] = -1
} else {
h.minCodes[i] = c
h.maxCodes[i] = c + n - 1
h.valsIndices[i] = index
c += n
index += n
}
c <<= 1
}
}
return nil
}
// decodeHuffman returns the next Huffman-coded value from the bit-stream,
// decoded according to h.
func (d *decoder) decodeHuffman(h *huffman) (uint8, error) {
if h.nCodes == 0 {
return 0, FormatError("uninitialized Huffman table")
}
if d.bits.n < 8 {
if err := d.ensureNBits(8); err != nil {
if err != errMissingFF00 && err != errShortHuffmanData {
return 0, err
}
// There are no more bytes of data in this segment, but we may still
// be able to read the next symbol out of the previously read bits.
// First, undo the readByte that the ensureNBits call made.
if d.bytes.nUnreadable != 0 {
d.unreadByteStuffedByte()
}
goto slowPath
}
}
if v := h.lut[(d.bits.a>>uint32(d.bits.n-lutSize))&0xff]; v != 0 {
n := (v & 0xff) - 1
d.bits.n -= int32(n)
d.bits.m >>= n
return uint8(v >> 8), nil
}
slowPath:
for i, code := 0, int32(0); i < maxCodeLength; i++ {
if d.bits.n == 0 {
if err := d.ensureNBits(1); err != nil {
return 0, err
}
}
if d.bits.a&d.bits.m != 0 {
code |= 1
}
d.bits.n--
d.bits.m >>= 1
if code <= h.maxCodes[i] {
return h.vals[h.valsIndices[i]+code-h.minCodes[i]], nil
}
code <<= 1
}
return 0, FormatError("bad Huffman code")
}
func (d *decoder) decodeBit() (bool, error) {
if d.bits.n == 0 {
if err := d.ensureNBits(1); err != nil {
return false, err
}
}
ret := d.bits.a&d.bits.m != 0
d.bits.n--
d.bits.m >>= 1
return ret, nil
}
func (d *decoder) decodeBits(n int32) (uint32, error) {
if d.bits.n < n {
if err := d.ensureNBits(n); err != nil {
return 0, err
}
}
ret := d.bits.a >> uint32(d.bits.n-n)
ret &= (1 << uint32(n)) - 1
d.bits.n -= n
d.bits.m >>= uint32(n)
return ret, nil
}