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
|
// Copyright 2012 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.
// +build ignore
// This program generates fixedhuff.go
// Invoke as
//
// go run gen.go |gofmt >fixedhuff.go
package main
import (
"fmt"
)
const maxCodeLen = 16
// Note: the definition of the huffmanDecoder struct is copied from
// inflate.go, as it is private to the implementation.
// chunk & 15 is number of bits
// chunk >> 4 is value, including table link
const (
huffmanChunkBits = 9
huffmanNumChunks = 1 << huffmanChunkBits
huffmanCountMask = 15
huffmanValueShift = 4
)
type huffmanDecoder struct {
min int // the minimum code length
chunks [huffmanNumChunks]uint32 // chunks as described above
links [][]uint32 // overflow links
linkMask uint32 // mask the width of the link table
}
// Initialize Huffman decoding tables from array of code lengths.
func (h *huffmanDecoder) init(bits []int) bool {
// Count number of codes of each length,
// compute min and max length.
var count [maxCodeLen]int
var min, max int
for _, n := range bits {
if n == 0 {
continue
}
if min == 0 || n < min {
min = n
}
if n > max {
max = n
}
count[n]++
}
if max == 0 {
return false
}
h.min = min
var linkBits uint
var numLinks int
if max > huffmanChunkBits {
linkBits = uint(max) - huffmanChunkBits
numLinks = 1 << linkBits
h.linkMask = uint32(numLinks - 1)
}
code := 0
var nextcode [maxCodeLen]int
for i := min; i <= max; i++ {
if i == huffmanChunkBits+1 {
// create link tables
link := code >> 1
h.links = make([][]uint32, huffmanNumChunks-link)
for j := uint(link); j < huffmanNumChunks; j++ {
reverse := int(reverseByte[j>>8]) | int(reverseByte[j&0xff])<<8
reverse >>= uint(16 - huffmanChunkBits)
off := j - uint(link)
h.chunks[reverse] = uint32(off<<huffmanValueShift + uint(i))
h.links[off] = make([]uint32, 1<<linkBits)
}
}
n := count[i]
nextcode[i] = code
code += n
code <<= 1
}
for i, n := range bits {
if n == 0 {
continue
}
code := nextcode[n]
nextcode[n]++
chunk := uint32(i<<huffmanValueShift | n)
reverse := int(reverseByte[code>>8]) | int(reverseByte[code&0xff])<<8
reverse >>= uint(16 - n)
if n <= huffmanChunkBits {
for off := reverse; off < huffmanNumChunks; off += 1 << uint(n) {
h.chunks[off] = chunk
}
} else {
linktab := h.links[h.chunks[reverse&(huffmanNumChunks-1)]>>huffmanValueShift]
reverse >>= huffmanChunkBits
for off := reverse; off < numLinks; off += 1 << uint(n-huffmanChunkBits) {
linktab[off] = chunk
}
}
}
return true
}
func main() {
var h huffmanDecoder
var bits [288]int
initReverseByte()
for i := 0; i < 144; i++ {
bits[i] = 8
}
for i := 144; i < 256; i++ {
bits[i] = 9
}
for i := 256; i < 280; i++ {
bits[i] = 7
}
for i := 280; i < 288; i++ {
bits[i] = 8
}
h.init(bits[:])
fmt.Println("package flate")
fmt.Println()
fmt.Println("// autogenerated by gen.go, DO NOT EDIT")
fmt.Println()
fmt.Println("var fixedHuffmanDecoder = huffmanDecoder{")
fmt.Printf("\t%d,\n", h.min)
fmt.Println("\t[huffmanNumChunks]uint32{")
for i := 0; i < huffmanNumChunks; i++ {
if i&7 == 0 {
fmt.Printf("\t\t")
} else {
fmt.Printf(" ")
}
fmt.Printf("0x%04x,", h.chunks[i])
if i&7 == 7 {
fmt.Println()
}
}
fmt.Println("\t},")
fmt.Println("\tnil, 0,")
fmt.Println("}")
}
var reverseByte [256]byte
func initReverseByte() {
for x := 0; x < 256; x++ {
var result byte
for i := uint(0); i < 8; i++ {
result |= byte(((x >> i) & 1) << (7 - i))
}
reverseByte[x] = result
}
}
|
