1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
|
// 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"
"os"
)
// 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 int // accumulator.
n int // the number of unread bits in a.
m int // mask. m==1<<(n-1) when n>0, with m==0 when n==0.
}
// 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) os.Error {
for d.b.n < n {
c, err := d.r.ReadByte()
if err != nil {
return err
}
d.b.a = d.b.a<<8 | int(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) (int, os.Error) {
err := d.ensureNBits(int(t))
if err != nil {
return 0, err
}
d.b.n -= int(t)
d.b.m >>= t
s := 1 << t
x := (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) os.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
const isBaseline = true // Progressive mode is not yet supported.
if th > maxTh || isBaseline && 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 at time, ought to be faster.
func (d *decoder) decodeHuffman(h *huffman) (uint8, os.Error) {
if h.length == 0 {
return 0, FormatError("uninitialized Huffman table")
}
for i, code := 0, 0; i < maxCodeLength; i++ {
err := d.ensureNBits(1)
if 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")
}
|
