Source file src/compress/flate/huffman_bit_writer.go

Documentation: compress/flate

		 1  // Copyright 2009 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 flate
		 6  
		 7  import (
		 8  	"io"
		 9  )
		10  
		11  const (
		12  	// The largest offset code.
		13  	offsetCodeCount = 30
		14  
		15  	// The special code used to mark the end of a block.
		16  	endBlockMarker = 256
		17  
		18  	// The first length code.
		19  	lengthCodesStart = 257
		20  
		21  	// The number of codegen codes.
		22  	codegenCodeCount = 19
		23  	badCode					= 255
		24  
		25  	// bufferFlushSize indicates the buffer size
		26  	// after which bytes are flushed to the writer.
		27  	// Should preferably be a multiple of 6, since
		28  	// we accumulate 6 bytes between writes to the buffer.
		29  	bufferFlushSize = 240
		30  
		31  	// bufferSize is the actual output byte buffer size.
		32  	// It must have additional headroom for a flush
		33  	// which can contain up to 8 bytes.
		34  	bufferSize = bufferFlushSize + 8
		35  )
		36  
		37  // The number of extra bits needed by length code X - LENGTH_CODES_START.
		38  var lengthExtraBits = []int8{
		39  	/* 257 */ 0, 0, 0,
		40  	/* 260 */ 0, 0, 0, 0, 0, 1, 1, 1, 1, 2,
		41  	/* 270 */ 2, 2, 2, 3, 3, 3, 3, 4, 4, 4,
		42  	/* 280 */ 4, 5, 5, 5, 5, 0,
		43  }
		44  
		45  // The length indicated by length code X - LENGTH_CODES_START.
		46  var lengthBase = []uint32{
		47  	0, 1, 2, 3, 4, 5, 6, 7, 8, 10,
		48  	12, 14, 16, 20, 24, 28, 32, 40, 48, 56,
		49  	64, 80, 96, 112, 128, 160, 192, 224, 255,
		50  }
		51  
		52  // offset code word extra bits.
		53  var offsetExtraBits = []int8{
		54  	0, 0, 0, 0, 1, 1, 2, 2, 3, 3,
		55  	4, 4, 5, 5, 6, 6, 7, 7, 8, 8,
		56  	9, 9, 10, 10, 11, 11, 12, 12, 13, 13,
		57  }
		58  
		59  var offsetBase = []uint32{
		60  	0x000000, 0x000001, 0x000002, 0x000003, 0x000004,
		61  	0x000006, 0x000008, 0x00000c, 0x000010, 0x000018,
		62  	0x000020, 0x000030, 0x000040, 0x000060, 0x000080,
		63  	0x0000c0, 0x000100, 0x000180, 0x000200, 0x000300,
		64  	0x000400, 0x000600, 0x000800, 0x000c00, 0x001000,
		65  	0x001800, 0x002000, 0x003000, 0x004000, 0x006000,
		66  }
		67  
		68  // The odd order in which the codegen code sizes are written.
		69  var codegenOrder = []uint32{16, 17, 18, 0, 8, 7, 9, 6, 10, 5, 11, 4, 12, 3, 13, 2, 14, 1, 15}
		70  
		71  type huffmanBitWriter struct {
		72  	// writer is the underlying writer.
		73  	// Do not use it directly; use the write method, which ensures
		74  	// that Write errors are sticky.
		75  	writer io.Writer
		76  
		77  	// Data waiting to be written is bytes[0:nbytes]
		78  	// and then the low nbits of bits.	Data is always written
		79  	// sequentially into the bytes array.
		80  	bits						uint64
		81  	nbits					 uint
		82  	bytes					 [bufferSize]byte
		83  	codegenFreq		 [codegenCodeCount]int32
		84  	nbytes					int
		85  	literalFreq		 []int32
		86  	offsetFreq			[]int32
		87  	codegen				 []uint8
		88  	literalEncoding *huffmanEncoder
		89  	offsetEncoding	*huffmanEncoder
		90  	codegenEncoding *huffmanEncoder
		91  	err						 error
		92  }
		93  
		94  func newHuffmanBitWriter(w io.Writer) *huffmanBitWriter {
		95  	return &huffmanBitWriter{
		96  		writer:					w,
		97  		literalFreq:		 make([]int32, maxNumLit),
		98  		offsetFreq:			make([]int32, offsetCodeCount),
		99  		codegen:				 make([]uint8, maxNumLit+offsetCodeCount+1),
	 100  		literalEncoding: newHuffmanEncoder(maxNumLit),
	 101  		codegenEncoding: newHuffmanEncoder(codegenCodeCount),
	 102  		offsetEncoding:	newHuffmanEncoder(offsetCodeCount),
	 103  	}
	 104  }
	 105  
	 106  func (w *huffmanBitWriter) reset(writer io.Writer) {
	 107  	w.writer = writer
	 108  	w.bits, w.nbits, w.nbytes, w.err = 0, 0, 0, nil
	 109  }
	 110  
	 111  func (w *huffmanBitWriter) flush() {
	 112  	if w.err != nil {
	 113  		w.nbits = 0
	 114  		return
	 115  	}
	 116  	n := w.nbytes
	 117  	for w.nbits != 0 {
	 118  		w.bytes[n] = byte(w.bits)
	 119  		w.bits >>= 8
	 120  		if w.nbits > 8 { // Avoid underflow
	 121  			w.nbits -= 8
	 122  		} else {
	 123  			w.nbits = 0
	 124  		}
	 125  		n++
	 126  	}
	 127  	w.bits = 0
	 128  	w.write(w.bytes[:n])
	 129  	w.nbytes = 0
	 130  }
	 131  
	 132  func (w *huffmanBitWriter) write(b []byte) {
	 133  	if w.err != nil {
	 134  		return
	 135  	}
	 136  	_, w.err = w.writer.Write(b)
	 137  }
	 138  
	 139  func (w *huffmanBitWriter) writeBits(b int32, nb uint) {
	 140  	if w.err != nil {
	 141  		return
	 142  	}
	 143  	w.bits |= uint64(b) << w.nbits
	 144  	w.nbits += nb
	 145  	if w.nbits >= 48 {
	 146  		bits := w.bits
	 147  		w.bits >>= 48
	 148  		w.nbits -= 48
	 149  		n := w.nbytes
	 150  		bytes := w.bytes[n : n+6]
	 151  		bytes[0] = byte(bits)
	 152  		bytes[1] = byte(bits >> 8)
	 153  		bytes[2] = byte(bits >> 16)
	 154  		bytes[3] = byte(bits >> 24)
	 155  		bytes[4] = byte(bits >> 32)
	 156  		bytes[5] = byte(bits >> 40)
	 157  		n += 6
	 158  		if n >= bufferFlushSize {
	 159  			w.write(w.bytes[:n])
	 160  			n = 0
	 161  		}
	 162  		w.nbytes = n
	 163  	}
	 164  }
	 165  
	 166  func (w *huffmanBitWriter) writeBytes(bytes []byte) {
	 167  	if w.err != nil {
	 168  		return
	 169  	}
	 170  	n := w.nbytes
	 171  	if w.nbits&7 != 0 {
	 172  		w.err = InternalError("writeBytes with unfinished bits")
	 173  		return
	 174  	}
	 175  	for w.nbits != 0 {
	 176  		w.bytes[n] = byte(w.bits)
	 177  		w.bits >>= 8
	 178  		w.nbits -= 8
	 179  		n++
	 180  	}
	 181  	if n != 0 {
	 182  		w.write(w.bytes[:n])
	 183  	}
	 184  	w.nbytes = 0
	 185  	w.write(bytes)
	 186  }
	 187  
	 188  // RFC 1951 3.2.7 specifies a special run-length encoding for specifying
	 189  // the literal and offset lengths arrays (which are concatenated into a single
	 190  // array).	This method generates that run-length encoding.
	 191  //
	 192  // The result is written into the codegen array, and the frequencies
	 193  // of each code is written into the codegenFreq array.
	 194  // Codes 0-15 are single byte codes. Codes 16-18 are followed by additional
	 195  // information. Code badCode is an end marker
	 196  //
	 197  //	numLiterals			The number of literals in literalEncoding
	 198  //	numOffsets			 The number of offsets in offsetEncoding
	 199  //	litenc, offenc	 The literal and offset encoder to use
	 200  func (w *huffmanBitWriter) generateCodegen(numLiterals int, numOffsets int, litEnc, offEnc *huffmanEncoder) {
	 201  	for i := range w.codegenFreq {
	 202  		w.codegenFreq[i] = 0
	 203  	}
	 204  	// Note that we are using codegen both as a temporary variable for holding
	 205  	// a copy of the frequencies, and as the place where we put the result.
	 206  	// This is fine because the output is always shorter than the input used
	 207  	// so far.
	 208  	codegen := w.codegen // cache
	 209  	// Copy the concatenated code sizes to codegen. Put a marker at the end.
	 210  	cgnl := codegen[:numLiterals]
	 211  	for i := range cgnl {
	 212  		cgnl[i] = uint8(litEnc.codes[i].len)
	 213  	}
	 214  
	 215  	cgnl = codegen[numLiterals : numLiterals+numOffsets]
	 216  	for i := range cgnl {
	 217  		cgnl[i] = uint8(offEnc.codes[i].len)
	 218  	}
	 219  	codegen[numLiterals+numOffsets] = badCode
	 220  
	 221  	size := codegen[0]
	 222  	count := 1
	 223  	outIndex := 0
	 224  	for inIndex := 1; size != badCode; inIndex++ {
	 225  		// INVARIANT: We have seen "count" copies of size that have not yet
	 226  		// had output generated for them.
	 227  		nextSize := codegen[inIndex]
	 228  		if nextSize == size {
	 229  			count++
	 230  			continue
	 231  		}
	 232  		// We need to generate codegen indicating "count" of size.
	 233  		if size != 0 {
	 234  			codegen[outIndex] = size
	 235  			outIndex++
	 236  			w.codegenFreq[size]++
	 237  			count--
	 238  			for count >= 3 {
	 239  				n := 6
	 240  				if n > count {
	 241  					n = count
	 242  				}
	 243  				codegen[outIndex] = 16
	 244  				outIndex++
	 245  				codegen[outIndex] = uint8(n - 3)
	 246  				outIndex++
	 247  				w.codegenFreq[16]++
	 248  				count -= n
	 249  			}
	 250  		} else {
	 251  			for count >= 11 {
	 252  				n := 138
	 253  				if n > count {
	 254  					n = count
	 255  				}
	 256  				codegen[outIndex] = 18
	 257  				outIndex++
	 258  				codegen[outIndex] = uint8(n - 11)
	 259  				outIndex++
	 260  				w.codegenFreq[18]++
	 261  				count -= n
	 262  			}
	 263  			if count >= 3 {
	 264  				// count >= 3 && count <= 10
	 265  				codegen[outIndex] = 17
	 266  				outIndex++
	 267  				codegen[outIndex] = uint8(count - 3)
	 268  				outIndex++
	 269  				w.codegenFreq[17]++
	 270  				count = 0
	 271  			}
	 272  		}
	 273  		count--
	 274  		for ; count >= 0; count-- {
	 275  			codegen[outIndex] = size
	 276  			outIndex++
	 277  			w.codegenFreq[size]++
	 278  		}
	 279  		// Set up invariant for next time through the loop.
	 280  		size = nextSize
	 281  		count = 1
	 282  	}
	 283  	// Marker indicating the end of the codegen.
	 284  	codegen[outIndex] = badCode
	 285  }
	 286  
	 287  // dynamicSize returns the size of dynamically encoded data in bits.
	 288  func (w *huffmanBitWriter) dynamicSize(litEnc, offEnc *huffmanEncoder, extraBits int) (size, numCodegens int) {
	 289  	numCodegens = len(w.codegenFreq)
	 290  	for numCodegens > 4 && w.codegenFreq[codegenOrder[numCodegens-1]] == 0 {
	 291  		numCodegens--
	 292  	}
	 293  	header := 3 + 5 + 5 + 4 + (3 * numCodegens) +
	 294  		w.codegenEncoding.bitLength(w.codegenFreq[:]) +
	 295  		int(w.codegenFreq[16])*2 +
	 296  		int(w.codegenFreq[17])*3 +
	 297  		int(w.codegenFreq[18])*7
	 298  	size = header +
	 299  		litEnc.bitLength(w.literalFreq) +
	 300  		offEnc.bitLength(w.offsetFreq) +
	 301  		extraBits
	 302  
	 303  	return size, numCodegens
	 304  }
	 305  
	 306  // fixedSize returns the size of dynamically encoded data in bits.
	 307  func (w *huffmanBitWriter) fixedSize(extraBits int) int {
	 308  	return 3 +
	 309  		fixedLiteralEncoding.bitLength(w.literalFreq) +
	 310  		fixedOffsetEncoding.bitLength(w.offsetFreq) +
	 311  		extraBits
	 312  }
	 313  
	 314  // storedSize calculates the stored size, including header.
	 315  // The function returns the size in bits and whether the block
	 316  // fits inside a single block.
	 317  func (w *huffmanBitWriter) storedSize(in []byte) (int, bool) {
	 318  	if in == nil {
	 319  		return 0, false
	 320  	}
	 321  	if len(in) <= maxStoreBlockSize {
	 322  		return (len(in) + 5) * 8, true
	 323  	}
	 324  	return 0, false
	 325  }
	 326  
	 327  func (w *huffmanBitWriter) writeCode(c hcode) {
	 328  	if w.err != nil {
	 329  		return
	 330  	}
	 331  	w.bits |= uint64(c.code) << w.nbits
	 332  	w.nbits += uint(c.len)
	 333  	if w.nbits >= 48 {
	 334  		bits := w.bits
	 335  		w.bits >>= 48
	 336  		w.nbits -= 48
	 337  		n := w.nbytes
	 338  		bytes := w.bytes[n : n+6]
	 339  		bytes[0] = byte(bits)
	 340  		bytes[1] = byte(bits >> 8)
	 341  		bytes[2] = byte(bits >> 16)
	 342  		bytes[3] = byte(bits >> 24)
	 343  		bytes[4] = byte(bits >> 32)
	 344  		bytes[5] = byte(bits >> 40)
	 345  		n += 6
	 346  		if n >= bufferFlushSize {
	 347  			w.write(w.bytes[:n])
	 348  			n = 0
	 349  		}
	 350  		w.nbytes = n
	 351  	}
	 352  }
	 353  
	 354  // Write the header of a dynamic Huffman block to the output stream.
	 355  //
	 356  //	numLiterals	The number of literals specified in codegen
	 357  //	numOffsets	 The number of offsets specified in codegen
	 358  //	numCodegens	The number of codegens used in codegen
	 359  func (w *huffmanBitWriter) writeDynamicHeader(numLiterals int, numOffsets int, numCodegens int, isEof bool) {
	 360  	if w.err != nil {
	 361  		return
	 362  	}
	 363  	var firstBits int32 = 4
	 364  	if isEof {
	 365  		firstBits = 5
	 366  	}
	 367  	w.writeBits(firstBits, 3)
	 368  	w.writeBits(int32(numLiterals-257), 5)
	 369  	w.writeBits(int32(numOffsets-1), 5)
	 370  	w.writeBits(int32(numCodegens-4), 4)
	 371  
	 372  	for i := 0; i < numCodegens; i++ {
	 373  		value := uint(w.codegenEncoding.codes[codegenOrder[i]].len)
	 374  		w.writeBits(int32(value), 3)
	 375  	}
	 376  
	 377  	i := 0
	 378  	for {
	 379  		var codeWord int = int(w.codegen[i])
	 380  		i++
	 381  		if codeWord == badCode {
	 382  			break
	 383  		}
	 384  		w.writeCode(w.codegenEncoding.codes[uint32(codeWord)])
	 385  
	 386  		switch codeWord {
	 387  		case 16:
	 388  			w.writeBits(int32(w.codegen[i]), 2)
	 389  			i++
	 390  			break
	 391  		case 17:
	 392  			w.writeBits(int32(w.codegen[i]), 3)
	 393  			i++
	 394  			break
	 395  		case 18:
	 396  			w.writeBits(int32(w.codegen[i]), 7)
	 397  			i++
	 398  			break
	 399  		}
	 400  	}
	 401  }
	 402  
	 403  func (w *huffmanBitWriter) writeStoredHeader(length int, isEof bool) {
	 404  	if w.err != nil {
	 405  		return
	 406  	}
	 407  	var flag int32
	 408  	if isEof {
	 409  		flag = 1
	 410  	}
	 411  	w.writeBits(flag, 3)
	 412  	w.flush()
	 413  	w.writeBits(int32(length), 16)
	 414  	w.writeBits(int32(^uint16(length)), 16)
	 415  }
	 416  
	 417  func (w *huffmanBitWriter) writeFixedHeader(isEof bool) {
	 418  	if w.err != nil {
	 419  		return
	 420  	}
	 421  	// Indicate that we are a fixed Huffman block
	 422  	var value int32 = 2
	 423  	if isEof {
	 424  		value = 3
	 425  	}
	 426  	w.writeBits(value, 3)
	 427  }
	 428  
	 429  // writeBlock will write a block of tokens with the smallest encoding.
	 430  // The original input can be supplied, and if the huffman encoded data
	 431  // is larger than the original bytes, the data will be written as a
	 432  // stored block.
	 433  // If the input is nil, the tokens will always be Huffman encoded.
	 434  func (w *huffmanBitWriter) writeBlock(tokens []token, eof bool, input []byte) {
	 435  	if w.err != nil {
	 436  		return
	 437  	}
	 438  
	 439  	tokens = append(tokens, endBlockMarker)
	 440  	numLiterals, numOffsets := w.indexTokens(tokens)
	 441  
	 442  	var extraBits int
	 443  	storedSize, storable := w.storedSize(input)
	 444  	if storable {
	 445  		// We only bother calculating the costs of the extra bits required by
	 446  		// the length of offset fields (which will be the same for both fixed
	 447  		// and dynamic encoding), if we need to compare those two encodings
	 448  		// against stored encoding.
	 449  		for lengthCode := lengthCodesStart + 8; lengthCode < numLiterals; lengthCode++ {
	 450  			// First eight length codes have extra size = 0.
	 451  			extraBits += int(w.literalFreq[lengthCode]) * int(lengthExtraBits[lengthCode-lengthCodesStart])
	 452  		}
	 453  		for offsetCode := 4; offsetCode < numOffsets; offsetCode++ {
	 454  			// First four offset codes have extra size = 0.
	 455  			extraBits += int(w.offsetFreq[offsetCode]) * int(offsetExtraBits[offsetCode])
	 456  		}
	 457  	}
	 458  
	 459  	// Figure out smallest code.
	 460  	// Fixed Huffman baseline.
	 461  	var literalEncoding = fixedLiteralEncoding
	 462  	var offsetEncoding = fixedOffsetEncoding
	 463  	var size = w.fixedSize(extraBits)
	 464  
	 465  	// Dynamic Huffman?
	 466  	var numCodegens int
	 467  
	 468  	// Generate codegen and codegenFrequencies, which indicates how to encode
	 469  	// the literalEncoding and the offsetEncoding.
	 470  	w.generateCodegen(numLiterals, numOffsets, w.literalEncoding, w.offsetEncoding)
	 471  	w.codegenEncoding.generate(w.codegenFreq[:], 7)
	 472  	dynamicSize, numCodegens := w.dynamicSize(w.literalEncoding, w.offsetEncoding, extraBits)
	 473  
	 474  	if dynamicSize < size {
	 475  		size = dynamicSize
	 476  		literalEncoding = w.literalEncoding
	 477  		offsetEncoding = w.offsetEncoding
	 478  	}
	 479  
	 480  	// Stored bytes?
	 481  	if storable && storedSize < size {
	 482  		w.writeStoredHeader(len(input), eof)
	 483  		w.writeBytes(input)
	 484  		return
	 485  	}
	 486  
	 487  	// Huffman.
	 488  	if literalEncoding == fixedLiteralEncoding {
	 489  		w.writeFixedHeader(eof)
	 490  	} else {
	 491  		w.writeDynamicHeader(numLiterals, numOffsets, numCodegens, eof)
	 492  	}
	 493  
	 494  	// Write the tokens.
	 495  	w.writeTokens(tokens, literalEncoding.codes, offsetEncoding.codes)
	 496  }
	 497  
	 498  // writeBlockDynamic encodes a block using a dynamic Huffman table.
	 499  // This should be used if the symbols used have a disproportionate
	 500  // histogram distribution.
	 501  // If input is supplied and the compression savings are below 1/16th of the
	 502  // input size the block is stored.
	 503  func (w *huffmanBitWriter) writeBlockDynamic(tokens []token, eof bool, input []byte) {
	 504  	if w.err != nil {
	 505  		return
	 506  	}
	 507  
	 508  	tokens = append(tokens, endBlockMarker)
	 509  	numLiterals, numOffsets := w.indexTokens(tokens)
	 510  
	 511  	// Generate codegen and codegenFrequencies, which indicates how to encode
	 512  	// the literalEncoding and the offsetEncoding.
	 513  	w.generateCodegen(numLiterals, numOffsets, w.literalEncoding, w.offsetEncoding)
	 514  	w.codegenEncoding.generate(w.codegenFreq[:], 7)
	 515  	size, numCodegens := w.dynamicSize(w.literalEncoding, w.offsetEncoding, 0)
	 516  
	 517  	// Store bytes, if we don't get a reasonable improvement.
	 518  	if ssize, storable := w.storedSize(input); storable && ssize < (size+size>>4) {
	 519  		w.writeStoredHeader(len(input), eof)
	 520  		w.writeBytes(input)
	 521  		return
	 522  	}
	 523  
	 524  	// Write Huffman table.
	 525  	w.writeDynamicHeader(numLiterals, numOffsets, numCodegens, eof)
	 526  
	 527  	// Write the tokens.
	 528  	w.writeTokens(tokens, w.literalEncoding.codes, w.offsetEncoding.codes)
	 529  }
	 530  
	 531  // indexTokens indexes a slice of tokens, and updates
	 532  // literalFreq and offsetFreq, and generates literalEncoding
	 533  // and offsetEncoding.
	 534  // The number of literal and offset tokens is returned.
	 535  func (w *huffmanBitWriter) indexTokens(tokens []token) (numLiterals, numOffsets int) {
	 536  	for i := range w.literalFreq {
	 537  		w.literalFreq[i] = 0
	 538  	}
	 539  	for i := range w.offsetFreq {
	 540  		w.offsetFreq[i] = 0
	 541  	}
	 542  
	 543  	for _, t := range tokens {
	 544  		if t < matchType {
	 545  			w.literalFreq[t.literal()]++
	 546  			continue
	 547  		}
	 548  		length := t.length()
	 549  		offset := t.offset()
	 550  		w.literalFreq[lengthCodesStart+lengthCode(length)]++
	 551  		w.offsetFreq[offsetCode(offset)]++
	 552  	}
	 553  
	 554  	// get the number of literals
	 555  	numLiterals = len(w.literalFreq)
	 556  	for w.literalFreq[numLiterals-1] == 0 {
	 557  		numLiterals--
	 558  	}
	 559  	// get the number of offsets
	 560  	numOffsets = len(w.offsetFreq)
	 561  	for numOffsets > 0 && w.offsetFreq[numOffsets-1] == 0 {
	 562  		numOffsets--
	 563  	}
	 564  	if numOffsets == 0 {
	 565  		// We haven't found a single match. If we want to go with the dynamic encoding,
	 566  		// we should count at least one offset to be sure that the offset huffman tree could be encoded.
	 567  		w.offsetFreq[0] = 1
	 568  		numOffsets = 1
	 569  	}
	 570  	w.literalEncoding.generate(w.literalFreq, 15)
	 571  	w.offsetEncoding.generate(w.offsetFreq, 15)
	 572  	return
	 573  }
	 574  
	 575  // writeTokens writes a slice of tokens to the output.
	 576  // codes for literal and offset encoding must be supplied.
	 577  func (w *huffmanBitWriter) writeTokens(tokens []token, leCodes, oeCodes []hcode) {
	 578  	if w.err != nil {
	 579  		return
	 580  	}
	 581  	for _, t := range tokens {
	 582  		if t < matchType {
	 583  			w.writeCode(leCodes[t.literal()])
	 584  			continue
	 585  		}
	 586  		// Write the length
	 587  		length := t.length()
	 588  		lengthCode := lengthCode(length)
	 589  		w.writeCode(leCodes[lengthCode+lengthCodesStart])
	 590  		extraLengthBits := uint(lengthExtraBits[lengthCode])
	 591  		if extraLengthBits > 0 {
	 592  			extraLength := int32(length - lengthBase[lengthCode])
	 593  			w.writeBits(extraLength, extraLengthBits)
	 594  		}
	 595  		// Write the offset
	 596  		offset := t.offset()
	 597  		offsetCode := offsetCode(offset)
	 598  		w.writeCode(oeCodes[offsetCode])
	 599  		extraOffsetBits := uint(offsetExtraBits[offsetCode])
	 600  		if extraOffsetBits > 0 {
	 601  			extraOffset := int32(offset - offsetBase[offsetCode])
	 602  			w.writeBits(extraOffset, extraOffsetBits)
	 603  		}
	 604  	}
	 605  }
	 606  
	 607  // huffOffset is a static offset encoder used for huffman only encoding.
	 608  // It can be reused since we will not be encoding offset values.
	 609  var huffOffset *huffmanEncoder
	 610  
	 611  func init() {
	 612  	offsetFreq := make([]int32, offsetCodeCount)
	 613  	offsetFreq[0] = 1
	 614  	huffOffset = newHuffmanEncoder(offsetCodeCount)
	 615  	huffOffset.generate(offsetFreq, 15)
	 616  }
	 617  
	 618  // writeBlockHuff encodes a block of bytes as either
	 619  // Huffman encoded literals or uncompressed bytes if the
	 620  // results only gains very little from compression.
	 621  func (w *huffmanBitWriter) writeBlockHuff(eof bool, input []byte) {
	 622  	if w.err != nil {
	 623  		return
	 624  	}
	 625  
	 626  	// Clear histogram
	 627  	for i := range w.literalFreq {
	 628  		w.literalFreq[i] = 0
	 629  	}
	 630  
	 631  	// Add everything as literals
	 632  	histogram(input, w.literalFreq)
	 633  
	 634  	w.literalFreq[endBlockMarker] = 1
	 635  
	 636  	const numLiterals = endBlockMarker + 1
	 637  	w.offsetFreq[0] = 1
	 638  	const numOffsets = 1
	 639  
	 640  	w.literalEncoding.generate(w.literalFreq, 15)
	 641  
	 642  	// Figure out smallest code.
	 643  	// Always use dynamic Huffman or Store
	 644  	var numCodegens int
	 645  
	 646  	// Generate codegen and codegenFrequencies, which indicates how to encode
	 647  	// the literalEncoding and the offsetEncoding.
	 648  	w.generateCodegen(numLiterals, numOffsets, w.literalEncoding, huffOffset)
	 649  	w.codegenEncoding.generate(w.codegenFreq[:], 7)
	 650  	size, numCodegens := w.dynamicSize(w.literalEncoding, huffOffset, 0)
	 651  
	 652  	// Store bytes, if we don't get a reasonable improvement.
	 653  	if ssize, storable := w.storedSize(input); storable && ssize < (size+size>>4) {
	 654  		w.writeStoredHeader(len(input), eof)
	 655  		w.writeBytes(input)
	 656  		return
	 657  	}
	 658  
	 659  	// Huffman.
	 660  	w.writeDynamicHeader(numLiterals, numOffsets, numCodegens, eof)
	 661  	encoding := w.literalEncoding.codes[:257]
	 662  	n := w.nbytes
	 663  	for _, t := range input {
	 664  		// Bitwriting inlined, ~30% speedup
	 665  		c := encoding[t]
	 666  		w.bits |= uint64(c.code) << w.nbits
	 667  		w.nbits += uint(c.len)
	 668  		if w.nbits < 48 {
	 669  			continue
	 670  		}
	 671  		// Store 6 bytes
	 672  		bits := w.bits
	 673  		w.bits >>= 48
	 674  		w.nbits -= 48
	 675  		bytes := w.bytes[n : n+6]
	 676  		bytes[0] = byte(bits)
	 677  		bytes[1] = byte(bits >> 8)
	 678  		bytes[2] = byte(bits >> 16)
	 679  		bytes[3] = byte(bits >> 24)
	 680  		bytes[4] = byte(bits >> 32)
	 681  		bytes[5] = byte(bits >> 40)
	 682  		n += 6
	 683  		if n < bufferFlushSize {
	 684  			continue
	 685  		}
	 686  		w.write(w.bytes[:n])
	 687  		if w.err != nil {
	 688  			return // Return early in the event of write failures
	 689  		}
	 690  		n = 0
	 691  	}
	 692  	w.nbytes = n
	 693  	w.writeCode(encoding[endBlockMarker])
	 694  }
	 695  
	 696  // histogram accumulates a histogram of b in h.
	 697  //
	 698  // len(h) must be >= 256, and h's elements must be all zeroes.
	 699  func histogram(b []byte, h []int32) {
	 700  	h = h[:256]
	 701  	for _, t := range b {
	 702  		h[t]++
	 703  	}
	 704  }
	 705  

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