Source file src/pkg/compress/lzw/writer.go
1 // Copyright 2011 The Go Authors. All rights reserved. 2 // Use of this source code is governed by a BSD-style 3 // license that can be found in the LICENSE file. 4 5 package lzw 6 7 import ( 8 "bufio" 9 "errors" 10 "fmt" 11 "io" 12 ) 13 14 // A writer is a buffered, flushable writer. 15 type writer interface { 16 WriteByte(byte) error 17 Flush() error 18 } 19 20 // An errWriteCloser is an io.WriteCloser that always returns a given error. 21 type errWriteCloser struct { 22 err error 23 } 24 25 func (e *errWriteCloser) Write([]byte) (int, error) { 26 return 0, e.err 27 } 28 29 func (e *errWriteCloser) Close() error { 30 return e.err 31 } 32 33 const ( 34 // A code is a 12 bit value, stored as a uint32 when encoding to avoid 35 // type conversions when shifting bits. 36 maxCode = 1<<12 - 1 37 invalidCode = 1<<32 - 1 38 // There are 1<<12 possible codes, which is an upper bound on the number of 39 // valid hash table entries at any given point in time. tableSize is 4x that. 40 tableSize = 4 * 1 << 12 41 tableMask = tableSize - 1 42 // A hash table entry is a uint32. Zero is an invalid entry since the 43 // lower 12 bits of a valid entry must be a non-literal code. 44 invalidEntry = 0 45 ) 46 47 // encoder is LZW compressor. 48 type encoder struct { 49 // w is the writer that compressed bytes are written to. 50 w writer 51 // order, write, bits, nBits and width are the state for 52 // converting a code stream into a byte stream. 53 order Order 54 write func(*encoder, uint32) error 55 bits uint32 56 nBits uint 57 width uint 58 // litWidth is the width in bits of literal codes. 59 litWidth uint 60 // hi is the code implied by the next code emission. 61 // overflow is the code at which hi overflows the code width. 62 hi, overflow uint32 63 // savedCode is the accumulated code at the end of the most recent Write 64 // call. It is equal to invalidCode if there was no such call. 65 savedCode uint32 66 // err is the first error encountered during writing. Closing the encoder 67 // will make any future Write calls return errClosed 68 err error 69 // table is the hash table from 20-bit keys to 12-bit values. Each table 70 // entry contains key<<12|val and collisions resolve by linear probing. 71 // The keys consist of a 12-bit code prefix and an 8-bit byte suffix. 72 // The values are a 12-bit code. 73 table [tableSize]uint32 74 } 75 76 // writeLSB writes the code c for "Least Significant Bits first" data. 77 func (e *encoder) writeLSB(c uint32) error { 78 e.bits |= c << e.nBits 79 e.nBits += e.width 80 for e.nBits >= 8 { 81 if err := e.w.WriteByte(uint8(e.bits)); err != nil { 82 return err 83 } 84 e.bits >>= 8 85 e.nBits -= 8 86 } 87 return nil 88 } 89 90 // writeMSB writes the code c for "Most Significant Bits first" data. 91 func (e *encoder) writeMSB(c uint32) error { 92 e.bits |= c << (32 - e.width - e.nBits) 93 e.nBits += e.width 94 for e.nBits >= 8 { 95 if err := e.w.WriteByte(uint8(e.bits >> 24)); err != nil { 96 return err 97 } 98 e.bits <<= 8 99 e.nBits -= 8 100 } 101 return nil 102 } 103 104 // errOutOfCodes is an internal error that means that the encoder has run out 105 // of unused codes and a clear code needs to be sent next. 106 var errOutOfCodes = errors.New("lzw: out of codes") 107 108 // incHi increments e.hi and checks for both overflow and running out of 109 // unused codes. In the latter case, incHi sends a clear code, resets the 110 // encoder state and returns errOutOfCodes. 111 func (e *encoder) incHi() error { 112 e.hi++ 113 if e.hi == e.overflow { 114 e.width++ 115 e.overflow <<= 1 116 } 117 if e.hi == maxCode { 118 clear := uint32(1) << e.litWidth 119 if err := e.write(e, clear); err != nil { 120 return err 121 } 122 e.width = uint(e.litWidth) + 1 123 e.hi = clear + 1 124 e.overflow = clear << 1 125 for i := range e.table { 126 e.table[i] = invalidEntry 127 } 128 return errOutOfCodes 129 } 130 return nil 131 } 132 133 // Write writes a compressed representation of p to e's underlying writer. 134 func (e *encoder) Write(p []byte) (int, error) { 135 if e.err != nil { 136 return 0, e.err 137 } 138 if len(p) == 0 { 139 return 0, nil 140 } 141 litMask := uint32(1<<e.litWidth - 1) 142 code := e.savedCode 143 if code == invalidCode { 144 // The first code sent is always a literal code. 145 code, p = uint32(p[0])&litMask, p[1:] 146 } 147 loop: 148 for _, x := range p { 149 literal := uint32(x) & litMask 150 key := code<<8 | literal 151 // If there is a hash table hit for this key then we continue the loop 152 // and do not emit a code yet. 153 hash := (key>>12 ^ key) & tableMask 154 for h, t := hash, e.table[hash]; t != invalidEntry; { 155 if key == t>>12 { 156 code = t & maxCode 157 continue loop 158 } 159 h = (h + 1) & tableMask 160 t = e.table[h] 161 } 162 // Otherwise, write the current code, and literal becomes the start of 163 // the next emitted code. 164 if e.err = e.write(e, code); e.err != nil { 165 return 0, e.err 166 } 167 code = literal 168 // Increment e.hi, the next implied code. If we run out of codes, reset 169 // the encoder state (including clearing the hash table) and continue. 170 if err := e.incHi(); err != nil { 171 if err == errOutOfCodes { 172 continue 173 } 174 e.err = err 175 return 0, e.err 176 } 177 // Otherwise, insert key -> e.hi into the map that e.table represents. 178 for { 179 if e.table[hash] == invalidEntry { 180 e.table[hash] = (key << 12) | e.hi 181 break 182 } 183 hash = (hash + 1) & tableMask 184 } 185 } 186 e.savedCode = code 187 return len(p), nil 188 } 189 190 // Close closes the encoder, flushing any pending output. It does not close or 191 // flush e's underlying writer. 192 func (e *encoder) Close() error { 193 if e.err != nil { 194 if e.err == errClosed { 195 return nil 196 } 197 return e.err 198 } 199 // Make any future calls to Write return errClosed. 200 e.err = errClosed 201 // Write the savedCode if valid. 202 if e.savedCode != invalidCode { 203 if err := e.write(e, e.savedCode); err != nil { 204 return err 205 } 206 if err := e.incHi(); err != nil && err != errOutOfCodes { 207 return err 208 } 209 } 210 // Write the eof code. 211 eof := uint32(1)<<e.litWidth + 1 212 if err := e.write(e, eof); err != nil { 213 return err 214 } 215 // Write the final bits. 216 if e.nBits > 0 { 217 if e.order == MSB { 218 e.bits >>= 24 219 } 220 if err := e.w.WriteByte(uint8(e.bits)); err != nil { 221 return err 222 } 223 } 224 return e.w.Flush() 225 } 226 227 // NewWriter creates a new io.WriteCloser that satisfies writes by compressing 228 // the data and writing it to w. 229 // It is the caller's responsibility to call Close on the WriteCloser when 230 // finished writing. 231 // The number of bits to use for literal codes, litWidth, must be in the 232 // range [2,8] and is typically 8. 233 func NewWriter(w io.Writer, order Order, litWidth int) io.WriteCloser { 234 var write func(*encoder, uint32) error 235 switch order { 236 case LSB: 237 write = (*encoder).writeLSB 238 case MSB: 239 write = (*encoder).writeMSB 240 default: 241 return &errWriteCloser{errors.New("lzw: unknown order")} 242 } 243 if litWidth < 2 || 8 < litWidth { 244 return &errWriteCloser{fmt.Errorf("lzw: litWidth %d out of range", litWidth)} 245 } 246 bw, ok := w.(writer) 247 if !ok { 248 bw = bufio.NewWriter(w) 249 } 250 lw := uint(litWidth) 251 return &encoder{ 252 w: bw, 253 order: order, 254 write: write, 255 width: 1 + lw, 256 litWidth: lw, 257 hi: 1<<lw + 1, 258 overflow: 1 << (lw + 1), 259 savedCode: invalidCode, 260 } 261 }