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// 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"
// Each code is at most 16 bits long.
const maxCodeLength = 16
// Each decoded value is a uint8, so there are at most 256 such values.
const maxNumValues = 256
// Bit stream for the Huffman decoder.
// The n least significant bits of a form the unread bits, to be read in MSB to LSB order.
type bits struct {
a uint32 // accumulator.
m uint32 // mask. m==1<<(n-1) when n>0, with m==0 when n==0.
n int // the number of unread bits in a.
}
// Huffman table decoder, specified in section C.
type huffman struct {
l [maxCodeLength]int
length int // sum of l[i].
val [maxNumValues]uint8 // the decoded values, as sorted by their encoding.
size [maxNumValues]int // size[i] is the number of bits to encode val[i].
code [maxNumValues]int // code[i] is the encoding of val[i].
minCode [maxCodeLength]int // min codes of length i, or -1 if no codes of that length.
maxCode [maxCodeLength]int // max codes of length i, or -1 if no codes of that length.
valIndex [maxCodeLength]int // index into val of minCode[i].
}
// Reads bytes from the io.Reader to ensure that bits.n is at least n.
func (d *decoder) ensureNBits(n int) error {
for d.b.n < n {
c, err := d.r.ReadByte()
if err != nil {
return err
}
d.b.a = d.b.a<<8 | uint32(c)
d.b.n += 8
if d.b.m == 0 {
d.b.m = 1 << 7
} else {
d.b.m <<= 8
}
// Byte stuffing, specified in section F.1.2.3.
if c == 0xff {
c, err = d.r.ReadByte()
if err != nil {
return err
}
if c != 0x00 {
return FormatError("missing 0xff00 sequence")
}
}
}
return nil
}
// The composition of RECEIVE and EXTEND, specified in section F.2.2.1.
func (d *decoder) receiveExtend(t uint8) (int32, error) {
if d.b.n < int(t) {
if err := d.ensureNBits(int(t)); err != nil {
return 0, err
}
}
d.b.n -= int(t)
d.b.m >>= t
s := int32(1) << t
x := int32(d.b.a>>uint8(d.b.n)) & (s - 1)
if x < s>>1 {
x += ((-1) << t) + 1
}
return x, nil
}
// 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")
}
_, err := io.ReadFull(d.r, d.tmp[0:17])
if err != nil {
return err
}
tc := d.tmp[0] >> 4
if tc > maxTc {
return FormatError("bad Tc value")
}
th := d.tmp[0] & 0x0f
if th > maxTh || !d.progressive && th > 1 {
return FormatError("bad Th value")
}
h := &d.huff[tc][th]
// Read l and val (and derive length).
h.length = 0
for i := 0; i < maxCodeLength; i++ {
h.l[i] = int(d.tmp[i+1])
h.length += h.l[i]
}
if h.length == 0 {
return FormatError("Huffman table has zero length")
}
if h.length > maxNumValues {
return FormatError("Huffman table has excessive length")
}
n -= h.length + 17
if n < 0 {
return FormatError("DHT has wrong length")
}
_, err = io.ReadFull(d.r, h.val[0:h.length])
if err != nil {
return err
}
// Derive size.
k := 0
for i := 0; i < maxCodeLength; i++ {
for j := 0; j < h.l[i]; j++ {
h.size[k] = i + 1
k++
}
}
// Derive code.
code := 0
size := h.size[0]
for i := 0; i < h.length; i++ {
if size != h.size[i] {
code <<= uint8(h.size[i] - size)
size = h.size[i]
}
h.code[i] = code
code++
}
// Derive minCode, maxCode, and valIndex.
k = 0
index := 0
for i := 0; i < maxCodeLength; i++ {
if h.l[i] == 0 {
h.minCode[i] = -1
h.maxCode[i] = -1
h.valIndex[i] = -1
} else {
h.minCode[i] = k
h.maxCode[i] = k + h.l[i] - 1
h.valIndex[i] = index
k += h.l[i]
index += h.l[i]
}
k <<= 1
}
}
return nil
}
// Returns the next Huffman-coded value from the bit stream, decoded according to h.
// TODO(nigeltao): This decoding algorithm is simple, but slow. A lookahead table, instead of always
// peeling off only 1 bit at time, ought to be faster.
func (d *decoder) decodeHuffman(h *huffman) (uint8, error) {
if h.length == 0 {
return 0, FormatError("uninitialized Huffman table")
}
for i, code := 0, 0; i < maxCodeLength; i++ {
if d.b.n == 0 {
if err := d.ensureNBits(1); err != nil {
return 0, err
}
}
if d.b.a&d.b.m != 0 {
code |= 1
}
d.b.n--
d.b.m >>= 1
if code <= h.maxCode[i] {
return h.val[h.valIndex[i]+code-h.minCode[i]], nil
}
code <<= 1
}
return 0, FormatError("bad Huffman code")
}
func (d *decoder) decodeBit() (bool, error) {
if d.b.n == 0 {
if err := d.ensureNBits(1); err != nil {
return false, err
}
}
ret := d.b.a&d.b.m != 0
d.b.n--
d.b.m >>= 1
return ret, nil
}
func (d *decoder) decodeBits(n int) (uint32, error) {
if d.b.n < n {
if err := d.ensureNBits(n); err != nil {
return 0, err
}
}
ret := d.b.a >> uint(d.b.n-n)
ret &= (1 << uint(n)) - 1
d.b.n -= n
d.b.m >>= uint(n)
return ret, nil
}
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