// 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 flate
import (
"io"
"math"
)
const (
// The largest offset code.
offsetCodeCount = 30
// The special code used to mark the end of a block.
endBlockMarker = 256
// The first length code.
lengthCodesStart = 257
// The number of codegen codes.
codegenCodeCount = 19
badCode = 255
)
// The number of extra bits needed by length code X - LENGTH_CODES_START.
var lengthExtraBits = []int8{
/* 257 */ 0, 0, 0,
/* 260 */ 0, 0, 0, 0, 0, 1, 1, 1, 1, 2,
/* 270 */ 2, 2, 2, 3, 3, 3, 3, 4, 4, 4,
/* 280 */ 4, 5, 5, 5, 5, 0,
}
// The length indicated by length code X - LENGTH_CODES_START.
var lengthBase = []uint32{
0, 1, 2, 3, 4, 5, 6, 7, 8, 10,
12, 14, 16, 20, 24, 28, 32, 40, 48, 56,
64, 80, 96, 112, 128, 160, 192, 224, 255,
}
// offset code word extra bits.
var offsetExtraBits = []int8{
0, 0, 0, 0, 1, 1, 2, 2, 3, 3,
4, 4, 5, 5, 6, 6, 7, 7, 8, 8,
9, 9, 10, 10, 11, 11, 12, 12, 13, 13,
/* extended window */
14, 14, 15, 15, 16, 16, 17, 17, 18, 18, 19, 19, 20, 20,
}
var offsetBase = []uint32{
/* normal deflate */
0x000000, 0x000001, 0x000002, 0x000003, 0x000004,
0x000006, 0x000008, 0x00000c, 0x000010, 0x000018,
0x000020, 0x000030, 0x000040, 0x000060, 0x000080,
0x0000c0, 0x000100, 0x000180, 0x000200, 0x000300,
0x000400, 0x000600, 0x000800, 0x000c00, 0x001000,
0x001800, 0x002000, 0x003000, 0x004000, 0x006000,
/* extended window */
0x008000, 0x00c000, 0x010000, 0x018000, 0x020000,
0x030000, 0x040000, 0x060000, 0x080000, 0x0c0000,
0x100000, 0x180000, 0x200000, 0x300000,
}
// The odd order in which the codegen code sizes are written.
var codegenOrder = []uint32{16, 17, 18, 0, 8, 7, 9, 6, 10, 5, 11, 4, 12, 3, 13, 2, 14, 1, 15}
type huffmanBitWriter struct {
w io.Writer
// Data waiting to be written is bytes[0:nbytes]
// and then the low nbits of bits.
bits uint32
nbits uint32
bytes [64]byte
nbytes int
literalFreq []int32
offsetFreq []int32
codegen []uint8
codegenFreq []int32
literalEncoding *huffmanEncoder
offsetEncoding *huffmanEncoder
codegenEncoding *huffmanEncoder
err error
}
func newHuffmanBitWriter(w io.Writer) *huffmanBitWriter {
return &huffmanBitWriter{
w: w,
literalFreq: make([]int32, maxNumLit),
offsetFreq: make([]int32, offsetCodeCount),
codegen: make([]uint8, maxNumLit+offsetCodeCount+1),
codegenFreq: make([]int32, codegenCodeCount),
literalEncoding: newHuffmanEncoder(maxNumLit),
offsetEncoding: newHuffmanEncoder(offsetCodeCount),
codegenEncoding: newHuffmanEncoder(codegenCodeCount),
}
}
func (w *huffmanBitWriter) reset(writer io.Writer) {
w.w = writer
w.bits, w.nbits, w.nbytes, w.err = 0, 0, 0, nil
w.bytes = [64]byte{}
for i := range w.codegen {
w.codegen[i] = 0
}
for _, s := range [...][]int32{w.literalFreq, w.offsetFreq, w.codegenFreq} {
for i := range s {
s[i] = 0
}
}
for _, enc := range [...]*huffmanEncoder{
w.literalEncoding,
w.offsetEncoding,
w.codegenEncoding} {
for i := range enc.code {
enc.code[i] = 0
}
for i := range enc.codeBits {
enc.codeBits[i] = 0
}
}
}
func (w *huffmanBitWriter) flushBits() {
if w.err != nil {
w.nbits = 0
return
}
bits := w.bits
w.bits >>= 16
w.nbits -= 16
n := w.nbytes
w.bytes[n] = byte(bits)
w.bytes[n+1] = byte(bits >> 8)
if n += 2; n >= len(w.bytes) {
_, w.err = w.w.Write(w.bytes[0:])
n = 0
}
w.nbytes = n
}
func (w *huffmanBitWriter) flush() {
if w.err != nil {
w.nbits = 0
return
}
n := w.nbytes
if w.nbits > 8 {
w.bytes[n] = byte(w.bits)
w.bits >>= 8
w.nbits -= 8
n++
}
if w.nbits > 0 {
w.bytes[n] = byte(w.bits)
w.nbits = 0
n++
}
w.bits = 0
_, w.err = w.w.Write(w.bytes[0:n])
w.nbytes = 0
}
func (w *huffmanBitWriter) writeBits(b, nb int32) {
w.bits |= uint32(b) << w.nbits
if w.nbits += uint32(nb); w.nbits >= 16 {
w.flushBits()
}
}
func (w *huffmanBitWriter) writeBytes(bytes []byte) {
if w.err != nil {
return
}
n := w.nbytes
if w.nbits == 8 {
w.bytes[n] = byte(w.bits)
w.nbits = 0
n++
}
if w.nbits != 0 {
w.err = InternalError("writeBytes with unfinished bits")
return
}
if n != 0 {
_, w.err = w.w.Write(w.bytes[0:n])
if w.err != nil {
return
}
}
w.nbytes = 0
_, w.err = w.w.Write(bytes)
}
// RFC 1951 3.2.7 specifies a special run-length encoding for specifying
// the literal and offset lengths arrays (which are concatenated into a single
// array). This method generates that run-length encoding.
//
// The result is written into the codegen array, and the frequencies
// of each code is written into the codegenFreq array.
// Codes 0-15 are single byte codes. Codes 16-18 are followed by additional
// information. Code badCode is an end marker
//
// numLiterals The number of literals in literalEncoding
// numOffsets The number of offsets in offsetEncoding
func (w *huffmanBitWriter) generateCodegen(numLiterals int, numOffsets int) {
for i := range w.codegenFreq {
w.codegenFreq[i] = 0
}
// Note that we are using codegen both as a temporary variable for holding
// a copy of the frequencies, and as the place where we put the result.
// This is fine because the output is always shorter than the input used
// so far.
codegen := w.codegen // cache
// Copy the concatenated code sizes to codegen. Put a marker at the end.
copy(codegen[0:numLiterals], w.literalEncoding.codeBits)
copy(codegen[numLiterals:numLiterals+numOffsets], w.offsetEncoding.codeBits)
codegen[numLiterals+numOffsets] = badCode
size := codegen[0]
count := 1
outIndex := 0
for inIndex := 1; size != badCode; inIndex++ {
// INVARIANT: We have seen "count" copies of size that have not yet
// had output generated for them.
nextSize := codegen[inIndex]
if nextSize == size {
count++
continue
}
// We need to generate codegen indicating "count" of size.
if size != 0 {
codegen[outIndex] = size
outIndex++
w.codegenFreq[size]++
count--
for count >= 3 {
n := 6
if n > count {
n = count
}
codegen[outIndex] = 16
outIndex++
codegen[outIndex] = uint8(n - 3)
outIndex++
w.codegenFreq[16]++
count -= n
}
} else {
for count >= 11 {
n := 138
if n > count {
n = count
}
codegen[outIndex] = 18
outIndex++
codegen[outIndex] = uint8(n - 11)
outIndex++
w.codegenFreq[18]++
count -= n
}
if count >= 3 {
// count >= 3 && count <= 10
codegen[outIndex] = 17
outIndex++
codegen[outIndex] = uint8(count - 3)
outIndex++
w.codegenFreq[17]++
count = 0
}
}
count--
for ; count >= 0; count-- {
codegen[outIndex] = size
outIndex++
w.codegenFreq[size]++
}
// Set up invariant for next time through the loop.
size = nextSize
count = 1
}
// Marker indicating the end of the codegen.
codegen[outIndex] = badCode
}
func (w *huffmanBitWriter) writeCode(code *huffmanEncoder, literal uint32) {
if w.err != nil {
return
}
w.writeBits(int32(code.code[literal]), int32(code.codeBits[literal]))
}
// Write the header of a dynamic Huffman block to the output stream.
//
// numLiterals The number of literals specified in codegen
// numOffsets The number of offsets specified in codegen
// numCodegens The number of codegens used in codegen
func (w *huffmanBitWriter) writeDynamicHeader(numLiterals int, numOffsets int, numCodegens int, isEof bool) {
if w.err != nil {
return
}
var firstBits int32 = 4
if isEof {
firstBits = 5
}
w.writeBits(firstBits, 3)
w.writeBits(int32(numLiterals-257), 5)
w.writeBits(int32(numOffsets-1), 5)
w.writeBits(int32(numCodegens-4), 4)
for i := 0; i < numCodegens; i++ {
value := w.codegenEncoding.codeBits[codegenOrder[i]]
w.writeBits(int32(value), 3)
}
i := 0
for {
var codeWord int = int(w.codegen[i])
i++
if codeWord == badCode {
break
}
// The low byte contains the actual code to generate.
w.writeCode(w.codegenEncoding, uint32(codeWord))
switch codeWord {
case 16:
w.writeBits(int32(w.codegen[i]), 2)
i++
break
case 17:
w.writeBits(int32(w.codegen[i]), 3)
i++
break
case 18:
w.writeBits(int32(w.codegen[i]), 7)
i++
break
}
}
}
func (w *huffmanBitWriter) writeStoredHeader(length int, isEof bool) {
if w.err != nil {
return
}
var flag int32
if isEof {
flag = 1
}
w.writeBits(flag, 3)
w.flush()
w.writeBits(int32(length), 16)
w.writeBits(int32(^uint16(length)), 16)
}
func (w *huffmanBitWriter) writeFixedHeader(isEof bool) {
if w.err != nil {
return
}
// Indicate that we are a fixed Huffman block
var value int32 = 2
if isEof {
value = 3
}
w.writeBits(value, 3)
}
func (w *huffmanBitWriter) writeBlock(tokens []token, eof bool, input []byte) {
if w.err != nil {
return
}
for i := range w.literalFreq {
w.literalFreq[i] = 0
}
for i := range w.offsetFreq {
w.offsetFreq[i] = 0
}
n := len(tokens)
tokens = tokens[0 : n+1]
tokens[n] = endBlockMarker
for _, t := range tokens {
switch t.typ() {
case literalType:
w.literalFreq[t.literal()]++
case matchType:
length := t.length()
offset := t.offset()
w.literalFreq[lengthCodesStart+lengthCode(length)]++
w.offsetFreq[offsetCode(offset)]++
}
}
// get the number of literals
numLiterals := len(w.literalFreq)
for w.literalFreq[numLiterals-1] == 0 {
numLiterals--
}
// get the number of offsets
numOffsets := len(w.offsetFreq)
for numOffsets > 0 && w.offsetFreq[numOffsets-1] == 0 {
numOffsets--
}
if numOffsets == 0 {
// We haven't found a single match. If we want to go with the dynamic encoding,
// we should count at least one offset to be sure that the offset huffman tree could be encoded.
w.offsetFreq[0] = 1
numOffsets = 1
}
w.literalEncoding.generate(w.literalFreq, 15)
w.offsetEncoding.generate(w.offsetFreq, 15)
storedBytes := 0
if input != nil {
storedBytes = len(input)
}
var extraBits int64
var storedSize int64 = math.MaxInt64
if storedBytes <= maxStoreBlockSize && input != nil {
storedSize = int64((storedBytes + 5) * 8)
// We only bother calculating the costs of the extra bits required by
// the length of offset fields (which will be the same for both fixed
// and dynamic encoding), if we need to compare those two encodings
// against stored encoding.
for lengthCode := lengthCodesStart + 8; lengthCode < numLiterals; lengthCode++ {
// First eight length codes have extra size = 0.
extraBits += int64(w.literalFreq[lengthCode]) * int64(lengthExtraBits[lengthCode-lengthCodesStart])
}
for offsetCode := 4; offsetCode < numOffsets; offsetCode++ {
// First four offset codes have extra size = 0.
extraBits += int64(w.offsetFreq[offsetCode]) * int64(offsetExtraBits[offsetCode])
}
}
// Figure out smallest code.
// Fixed Huffman baseline.
var size = int64(3) +
fixedLiteralEncoding.bitLength(w.literalFreq) +
fixedOffsetEncoding.bitLength(w.offsetFreq) +
extraBits
var literalEncoding = fixedLiteralEncoding
var offsetEncoding = fixedOffsetEncoding
// Dynamic Huffman?
var numCodegens int
// Generate codegen and codegenFrequencies, which indicates how to encode
// the literalEncoding and the offsetEncoding.
w.generateCodegen(numLiterals, numOffsets)
w.codegenEncoding.generate(w.codegenFreq, 7)
numCodegens = len(w.codegenFreq)
for numCodegens > 4 && w.codegenFreq[codegenOrder[numCodegens-1]] == 0 {
numCodegens--
}
dynamicHeader := int64(3+5+5+4+(3*numCodegens)) +
w.codegenEncoding.bitLength(w.codegenFreq) +
int64(extraBits) +
int64(w.codegenFreq[16]*2) +
int64(w.codegenFreq[17]*3) +
int64(w.codegenFreq[18]*7)
dynamicSize := dynamicHeader +
w.literalEncoding.bitLength(w.literalFreq) +
w.offsetEncoding.bitLength(w.offsetFreq)
if dynamicSize < size {
size = dynamicSize
literalEncoding = w.literalEncoding
offsetEncoding = w.offsetEncoding
}
// Stored bytes?
if storedSize < size {
w.writeStoredHeader(storedBytes, eof)
w.writeBytes(input[0:storedBytes])
return
}
// Huffman.
if literalEncoding == fixedLiteralEncoding {
w.writeFixedHeader(eof)
} else {
w.writeDynamicHeader(numLiterals, numOffsets, numCodegens, eof)
}
for _, t := range tokens {
switch t.typ() {
case literalType:
w.writeCode(literalEncoding, t.literal())
break
case matchType:
// Write the length
length := t.length()
lengthCode := lengthCode(length)
w.writeCode(literalEncoding, lengthCode+lengthCodesStart)
extraLengthBits := int32(lengthExtraBits[lengthCode])
if extraLengthBits > 0 {
extraLength := int32(length - lengthBase[lengthCode])
w.writeBits(extraLength, extraLengthBits)
}
// Write the offset
offset := t.offset()
offsetCode := offsetCode(offset)
w.writeCode(offsetEncoding, offsetCode)
extraOffsetBits := int32(offsetExtraBits[offsetCode])
if extraOffsetBits > 0 {
extraOffset := int32(offset - offsetBase[offsetCode])
w.writeBits(extraOffset, extraOffsetBits)
}
break
default:
panic("unknown token type: " + string(t))
}
}
}