701 lines
18 KiB
Go
701 lines
18 KiB
Go
// Copyright 2009 The Go Authors. All rights reserved.
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// Use of this source code is governed by a BSD-style
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// license that can be found in the LICENSE file.
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package flate
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import (
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"io"
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)
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const (
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// The largest offset code.
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offsetCodeCount = 30
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// The special code used to mark the end of a block.
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endBlockMarker = 256
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// The first length code.
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lengthCodesStart = 257
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// The number of codegen codes.
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codegenCodeCount = 19
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badCode = 255
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// bufferFlushSize indicates the buffer size
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// after which bytes are flushed to the writer.
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// Should preferably be a multiple of 6, since
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// we accumulate 6 bytes between writes to the buffer.
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bufferFlushSize = 240
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// bufferSize is the actual output byte buffer size.
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// It must have additional headroom for a flush
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// which can contain up to 8 bytes.
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bufferSize = bufferFlushSize + 8
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)
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// The number of extra bits needed by length code X - LENGTH_CODES_START.
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var lengthExtraBits = []int8{
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/* 257 */ 0, 0, 0,
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/* 260 */ 0, 0, 0, 0, 0, 1, 1, 1, 1, 2,
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/* 270 */ 2, 2, 2, 3, 3, 3, 3, 4, 4, 4,
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/* 280 */ 4, 5, 5, 5, 5, 0,
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}
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// The length indicated by length code X - LENGTH_CODES_START.
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var lengthBase = []uint32{
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0, 1, 2, 3, 4, 5, 6, 7, 8, 10,
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12, 14, 16, 20, 24, 28, 32, 40, 48, 56,
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64, 80, 96, 112, 128, 160, 192, 224, 255,
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}
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// offset code word extra bits.
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var offsetExtraBits = []int8{
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0, 0, 0, 0, 1, 1, 2, 2, 3, 3,
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4, 4, 5, 5, 6, 6, 7, 7, 8, 8,
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9, 9, 10, 10, 11, 11, 12, 12, 13, 13,
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/* extended window */
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14, 14, 15, 15, 16, 16, 17, 17, 18, 18, 19, 19, 20, 20,
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}
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var offsetBase = []uint32{
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/* normal deflate */
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0x000000, 0x000001, 0x000002, 0x000003, 0x000004,
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0x000006, 0x000008, 0x00000c, 0x000010, 0x000018,
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0x000020, 0x000030, 0x000040, 0x000060, 0x000080,
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0x0000c0, 0x000100, 0x000180, 0x000200, 0x000300,
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0x000400, 0x000600, 0x000800, 0x000c00, 0x001000,
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0x001800, 0x002000, 0x003000, 0x004000, 0x006000,
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/* extended window */
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0x008000, 0x00c000, 0x010000, 0x018000, 0x020000,
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0x030000, 0x040000, 0x060000, 0x080000, 0x0c0000,
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0x100000, 0x180000, 0x200000, 0x300000,
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}
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// The odd order in which the codegen code sizes are written.
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var codegenOrder = []uint32{16, 17, 18, 0, 8, 7, 9, 6, 10, 5, 11, 4, 12, 3, 13, 2, 14, 1, 15}
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type huffmanBitWriter struct {
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// writer is the underlying writer.
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// Do not use it directly; use the write method, which ensures
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// that Write errors are sticky.
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writer io.Writer
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// Data waiting to be written is bytes[0:nbytes]
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// and then the low nbits of bits.
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bits uint64
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nbits uint
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bytes [bufferSize]byte
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codegenFreq [codegenCodeCount]int32
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nbytes int
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literalFreq []int32
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offsetFreq []int32
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codegen []uint8
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literalEncoding *huffmanEncoder
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offsetEncoding *huffmanEncoder
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codegenEncoding *huffmanEncoder
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err error
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}
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func newHuffmanBitWriter(w io.Writer) *huffmanBitWriter {
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return &huffmanBitWriter{
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writer: w,
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literalFreq: make([]int32, maxNumLit),
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offsetFreq: make([]int32, offsetCodeCount),
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codegen: make([]uint8, maxNumLit+offsetCodeCount+1),
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literalEncoding: newHuffmanEncoder(maxNumLit),
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codegenEncoding: newHuffmanEncoder(codegenCodeCount),
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offsetEncoding: newHuffmanEncoder(offsetCodeCount),
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}
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}
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func (w *huffmanBitWriter) reset(writer io.Writer) {
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w.writer = writer
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w.bits, w.nbits, w.nbytes, w.err = 0, 0, 0, nil
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w.bytes = [bufferSize]byte{}
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}
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func (w *huffmanBitWriter) flush() {
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if w.err != nil {
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w.nbits = 0
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return
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}
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n := w.nbytes
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for w.nbits != 0 {
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w.bytes[n] = byte(w.bits)
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w.bits >>= 8
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if w.nbits > 8 { // Avoid underflow
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w.nbits -= 8
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} else {
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w.nbits = 0
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}
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n++
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}
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w.bits = 0
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w.write(w.bytes[:n])
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w.nbytes = 0
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}
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func (w *huffmanBitWriter) write(b []byte) {
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if w.err != nil {
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return
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}
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_, w.err = w.writer.Write(b)
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}
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func (w *huffmanBitWriter) writeBits(b int32, nb uint) {
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if w.err != nil {
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return
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}
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w.bits |= uint64(b) << w.nbits
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w.nbits += nb
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if w.nbits >= 48 {
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bits := w.bits
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w.bits >>= 48
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w.nbits -= 48
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n := w.nbytes
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bytes := w.bytes[n : n+6]
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bytes[0] = byte(bits)
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bytes[1] = byte(bits >> 8)
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bytes[2] = byte(bits >> 16)
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bytes[3] = byte(bits >> 24)
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bytes[4] = byte(bits >> 32)
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bytes[5] = byte(bits >> 40)
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n += 6
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if n >= bufferFlushSize {
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w.write(w.bytes[:n])
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n = 0
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}
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w.nbytes = n
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}
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}
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func (w *huffmanBitWriter) writeBytes(bytes []byte) {
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if w.err != nil {
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return
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}
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n := w.nbytes
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if w.nbits&7 != 0 {
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w.err = InternalError("writeBytes with unfinished bits")
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return
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}
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for w.nbits != 0 {
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w.bytes[n] = byte(w.bits)
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w.bits >>= 8
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w.nbits -= 8
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n++
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}
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if n != 0 {
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w.write(w.bytes[:n])
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}
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w.nbytes = 0
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w.write(bytes)
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}
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// RFC 1951 3.2.7 specifies a special run-length encoding for specifying
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// the literal and offset lengths arrays (which are concatenated into a single
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// array). This method generates that run-length encoding.
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//
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// The result is written into the codegen array, and the frequencies
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// of each code is written into the codegenFreq array.
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// Codes 0-15 are single byte codes. Codes 16-18 are followed by additional
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// information. Code badCode is an end marker
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//
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// numLiterals The number of literals in literalEncoding
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// numOffsets The number of offsets in offsetEncoding
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// litenc, offenc The literal and offset encoder to use
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func (w *huffmanBitWriter) generateCodegen(numLiterals int, numOffsets int, litEnc, offEnc *huffmanEncoder) {
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for i := range w.codegenFreq {
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w.codegenFreq[i] = 0
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}
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// Note that we are using codegen both as a temporary variable for holding
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// a copy of the frequencies, and as the place where we put the result.
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// This is fine because the output is always shorter than the input used
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// so far.
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codegen := w.codegen // cache
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// Copy the concatenated code sizes to codegen. Put a marker at the end.
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cgnl := codegen[:numLiterals]
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for i := range cgnl {
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cgnl[i] = uint8(litEnc.codes[i].len)
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}
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cgnl = codegen[numLiterals : numLiterals+numOffsets]
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for i := range cgnl {
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cgnl[i] = uint8(offEnc.codes[i].len)
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}
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codegen[numLiterals+numOffsets] = badCode
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size := codegen[0]
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count := 1
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outIndex := 0
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for inIndex := 1; size != badCode; inIndex++ {
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// INVARIANT: We have seen "count" copies of size that have not yet
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// had output generated for them.
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nextSize := codegen[inIndex]
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if nextSize == size {
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count++
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continue
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}
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// We need to generate codegen indicating "count" of size.
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if size != 0 {
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codegen[outIndex] = size
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outIndex++
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w.codegenFreq[size]++
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count--
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for count >= 3 {
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n := 6
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if n > count {
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n = count
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}
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codegen[outIndex] = 16
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outIndex++
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codegen[outIndex] = uint8(n - 3)
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outIndex++
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w.codegenFreq[16]++
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count -= n
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}
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} else {
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for count >= 11 {
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n := 138
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if n > count {
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n = count
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}
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codegen[outIndex] = 18
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outIndex++
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codegen[outIndex] = uint8(n - 11)
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outIndex++
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w.codegenFreq[18]++
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count -= n
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}
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if count >= 3 {
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// count >= 3 && count <= 10
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codegen[outIndex] = 17
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outIndex++
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codegen[outIndex] = uint8(count - 3)
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outIndex++
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w.codegenFreq[17]++
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count = 0
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}
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}
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count--
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for ; count >= 0; count-- {
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codegen[outIndex] = size
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outIndex++
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w.codegenFreq[size]++
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}
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// Set up invariant for next time through the loop.
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size = nextSize
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count = 1
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}
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// Marker indicating the end of the codegen.
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codegen[outIndex] = badCode
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}
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// dynamicSize returns the size of dynamically encoded data in bits.
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func (w *huffmanBitWriter) dynamicSize(litEnc, offEnc *huffmanEncoder, extraBits int) (size, numCodegens int) {
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numCodegens = len(w.codegenFreq)
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for numCodegens > 4 && w.codegenFreq[codegenOrder[numCodegens-1]] == 0 {
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numCodegens--
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}
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header := 3 + 5 + 5 + 4 + (3 * numCodegens) +
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w.codegenEncoding.bitLength(w.codegenFreq[:]) +
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int(w.codegenFreq[16])*2 +
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int(w.codegenFreq[17])*3 +
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int(w.codegenFreq[18])*7
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size = header +
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litEnc.bitLength(w.literalFreq) +
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offEnc.bitLength(w.offsetFreq) +
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extraBits
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return size, numCodegens
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}
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// fixedSize returns the size of dynamically encoded data in bits.
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func (w *huffmanBitWriter) fixedSize(extraBits int) int {
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return 3 +
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fixedLiteralEncoding.bitLength(w.literalFreq) +
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fixedOffsetEncoding.bitLength(w.offsetFreq) +
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extraBits
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}
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// storedSize calculates the stored size, including header.
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// The function returns the size in bits and whether the block
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// fits inside a single block.
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func (w *huffmanBitWriter) storedSize(in []byte) (int, bool) {
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if in == nil {
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return 0, false
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}
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if len(in) <= maxStoreBlockSize {
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return (len(in) + 5) * 8, true
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}
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return 0, false
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}
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func (w *huffmanBitWriter) writeCode(c hcode) {
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if w.err != nil {
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return
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}
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w.bits |= uint64(c.code) << w.nbits
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w.nbits += uint(c.len)
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if w.nbits >= 48 {
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bits := w.bits
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w.bits >>= 48
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w.nbits -= 48
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n := w.nbytes
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bytes := w.bytes[n : n+6]
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bytes[0] = byte(bits)
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bytes[1] = byte(bits >> 8)
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bytes[2] = byte(bits >> 16)
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bytes[3] = byte(bits >> 24)
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bytes[4] = byte(bits >> 32)
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bytes[5] = byte(bits >> 40)
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n += 6
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if n >= bufferFlushSize {
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w.write(w.bytes[:n])
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n = 0
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}
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w.nbytes = n
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}
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}
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// Write the header of a dynamic Huffman block to the output stream.
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//
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// numLiterals The number of literals specified in codegen
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// numOffsets The number of offsets specified in codegen
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// numCodegens The number of codegens used in codegen
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func (w *huffmanBitWriter) writeDynamicHeader(numLiterals int, numOffsets int, numCodegens int, isEof bool) {
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if w.err != nil {
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return
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}
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var firstBits int32 = 4
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if isEof {
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firstBits = 5
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}
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w.writeBits(firstBits, 3)
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w.writeBits(int32(numLiterals-257), 5)
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w.writeBits(int32(numOffsets-1), 5)
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w.writeBits(int32(numCodegens-4), 4)
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for i := 0; i < numCodegens; i++ {
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value := uint(w.codegenEncoding.codes[codegenOrder[i]].len)
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w.writeBits(int32(value), 3)
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}
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i := 0
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for {
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var codeWord int = int(w.codegen[i])
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i++
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if codeWord == badCode {
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break
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}
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w.writeCode(w.codegenEncoding.codes[uint32(codeWord)])
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switch codeWord {
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case 16:
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w.writeBits(int32(w.codegen[i]), 2)
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i++
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break
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case 17:
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w.writeBits(int32(w.codegen[i]), 3)
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i++
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break
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case 18:
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w.writeBits(int32(w.codegen[i]), 7)
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i++
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break
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}
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}
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}
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func (w *huffmanBitWriter) writeStoredHeader(length int, isEof bool) {
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if w.err != nil {
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return
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}
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var flag int32
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if isEof {
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flag = 1
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}
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w.writeBits(flag, 3)
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w.flush()
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w.writeBits(int32(length), 16)
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w.writeBits(int32(^uint16(length)), 16)
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}
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func (w *huffmanBitWriter) writeFixedHeader(isEof bool) {
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if w.err != nil {
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return
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}
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// Indicate that we are a fixed Huffman block
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var value int32 = 2
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if isEof {
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value = 3
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}
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w.writeBits(value, 3)
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}
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// writeBlock will write a block of tokens with the smallest encoding.
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// The original input can be supplied, and if the huffman encoded data
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// is larger than the original bytes, the data will be written as a
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// stored block.
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// If the input is nil, the tokens will always be Huffman encoded.
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func (w *huffmanBitWriter) writeBlock(tokens []token, eof bool, input []byte) {
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if w.err != nil {
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return
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}
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tokens = append(tokens, endBlockMarker)
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numLiterals, numOffsets := w.indexTokens(tokens)
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var extraBits int
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storedSize, storable := w.storedSize(input)
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if storable {
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// We only bother calculating the costs of the extra bits required by
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// the length of offset fields (which will be the same for both fixed
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// and dynamic encoding), if we need to compare those two encodings
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// against stored encoding.
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for lengthCode := lengthCodesStart + 8; lengthCode < numLiterals; lengthCode++ {
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// First eight length codes have extra size = 0.
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extraBits += int(w.literalFreq[lengthCode]) * int(lengthExtraBits[lengthCode-lengthCodesStart])
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}
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for offsetCode := 4; offsetCode < numOffsets; offsetCode++ {
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// First four offset codes have extra size = 0.
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extraBits += int(w.offsetFreq[offsetCode]) * int(offsetExtraBits[offsetCode])
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}
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}
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// Figure out smallest code.
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// Fixed Huffman baseline.
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var literalEncoding = fixedLiteralEncoding
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var offsetEncoding = fixedOffsetEncoding
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var size = w.fixedSize(extraBits)
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// Dynamic Huffman?
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var numCodegens int
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// Generate codegen and codegenFrequencies, which indicates how to encode
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// the literalEncoding and the offsetEncoding.
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w.generateCodegen(numLiterals, numOffsets, w.literalEncoding, w.offsetEncoding)
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w.codegenEncoding.generate(w.codegenFreq[:], 7)
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dynamicSize, numCodegens := w.dynamicSize(w.literalEncoding, w.offsetEncoding, extraBits)
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if dynamicSize < size {
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size = dynamicSize
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literalEncoding = w.literalEncoding
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offsetEncoding = w.offsetEncoding
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}
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// Stored bytes?
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if storable && storedSize < size {
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w.writeStoredHeader(len(input), eof)
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w.writeBytes(input)
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return
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}
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// Huffman.
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if literalEncoding == fixedLiteralEncoding {
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w.writeFixedHeader(eof)
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} else {
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w.writeDynamicHeader(numLiterals, numOffsets, numCodegens, eof)
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}
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// Write the tokens.
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w.writeTokens(tokens, literalEncoding.codes, offsetEncoding.codes)
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}
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// writeBlockDynamic encodes a block using a dynamic Huffman table.
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// This should be used if the symbols used have a disproportionate
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// histogram distribution.
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// If input is supplied and the compression savings are below 1/16th of the
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// input size the block is stored.
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func (w *huffmanBitWriter) writeBlockDynamic(tokens []token, eof bool, input []byte) {
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if w.err != nil {
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return
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}
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tokens = append(tokens, endBlockMarker)
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numLiterals, numOffsets := w.indexTokens(tokens)
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// Generate codegen and codegenFrequencies, which indicates how to encode
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// the literalEncoding and the offsetEncoding.
|
|
w.generateCodegen(numLiterals, numOffsets, w.literalEncoding, w.offsetEncoding)
|
|
w.codegenEncoding.generate(w.codegenFreq[:], 7)
|
|
size, numCodegens := w.dynamicSize(w.literalEncoding, w.offsetEncoding, 0)
|
|
|
|
// Store bytes, if we don't get a reasonable improvement.
|
|
if ssize, storable := w.storedSize(input); storable && ssize < (size+size>>4) {
|
|
w.writeStoredHeader(len(input), eof)
|
|
w.writeBytes(input)
|
|
return
|
|
}
|
|
|
|
// Write Huffman table.
|
|
w.writeDynamicHeader(numLiterals, numOffsets, numCodegens, eof)
|
|
|
|
// Write the tokens.
|
|
w.writeTokens(tokens, w.literalEncoding.codes, w.offsetEncoding.codes)
|
|
}
|
|
|
|
// indexTokens indexes a slice of tokens, and updates
|
|
// literalFreq and offsetFreq, and generates literalEncoding
|
|
// and offsetEncoding.
|
|
// The number of literal and offset tokens is returned.
|
|
func (w *huffmanBitWriter) indexTokens(tokens []token) (numLiterals, numOffsets int) {
|
|
for i := range w.literalFreq {
|
|
w.literalFreq[i] = 0
|
|
}
|
|
for i := range w.offsetFreq {
|
|
w.offsetFreq[i] = 0
|
|
}
|
|
|
|
for _, t := range tokens {
|
|
if t < matchType {
|
|
w.literalFreq[t.literal()]++
|
|
continue
|
|
}
|
|
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)
|
|
return
|
|
}
|
|
|
|
// writeTokens writes a slice of tokens to the output.
|
|
// codes for literal and offset encoding must be supplied.
|
|
func (w *huffmanBitWriter) writeTokens(tokens []token, leCodes, oeCodes []hcode) {
|
|
if w.err != nil {
|
|
return
|
|
}
|
|
for _, t := range tokens {
|
|
if t < matchType {
|
|
w.writeCode(leCodes[t.literal()])
|
|
continue
|
|
}
|
|
// Write the length
|
|
length := t.length()
|
|
lengthCode := lengthCode(length)
|
|
w.writeCode(leCodes[lengthCode+lengthCodesStart])
|
|
extraLengthBits := uint(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(oeCodes[offsetCode])
|
|
extraOffsetBits := uint(offsetExtraBits[offsetCode])
|
|
if extraOffsetBits > 0 {
|
|
extraOffset := int32(offset - offsetBase[offsetCode])
|
|
w.writeBits(extraOffset, extraOffsetBits)
|
|
}
|
|
}
|
|
}
|
|
|
|
// huffOffset is a static offset encoder used for huffman only encoding.
|
|
// It can be reused since we will not be encoding offset values.
|
|
var huffOffset *huffmanEncoder
|
|
|
|
func init() {
|
|
w := newHuffmanBitWriter(nil)
|
|
w.offsetFreq[0] = 1
|
|
huffOffset = newHuffmanEncoder(offsetCodeCount)
|
|
huffOffset.generate(w.offsetFreq, 15)
|
|
}
|
|
|
|
// writeBlockHuff encodes a block of bytes as either
|
|
// Huffman encoded literals or uncompressed bytes if the
|
|
// results only gains very little from compression.
|
|
func (w *huffmanBitWriter) writeBlockHuff(eof bool, input []byte) {
|
|
if w.err != nil {
|
|
return
|
|
}
|
|
|
|
// Clear histogram
|
|
for i := range w.literalFreq {
|
|
w.literalFreq[i] = 0
|
|
}
|
|
|
|
// Add everything as literals
|
|
histogram(input, w.literalFreq)
|
|
|
|
w.literalFreq[endBlockMarker] = 1
|
|
|
|
const numLiterals = endBlockMarker + 1
|
|
const numOffsets = 1
|
|
|
|
w.literalEncoding.generate(w.literalFreq, 15)
|
|
|
|
// Figure out smallest code.
|
|
// Always use dynamic Huffman or Store
|
|
var numCodegens int
|
|
|
|
// Generate codegen and codegenFrequencies, which indicates how to encode
|
|
// the literalEncoding and the offsetEncoding.
|
|
w.generateCodegen(numLiterals, numOffsets, w.literalEncoding, huffOffset)
|
|
w.codegenEncoding.generate(w.codegenFreq[:], 7)
|
|
size, numCodegens := w.dynamicSize(w.literalEncoding, huffOffset, 0)
|
|
|
|
// Store bytes, if we don't get a reasonable improvement.
|
|
if ssize, storable := w.storedSize(input); storable && ssize < (size+size>>4) {
|
|
w.writeStoredHeader(len(input), eof)
|
|
w.writeBytes(input)
|
|
return
|
|
}
|
|
|
|
// Huffman.
|
|
w.writeDynamicHeader(numLiterals, numOffsets, numCodegens, eof)
|
|
encoding := w.literalEncoding.codes[:257]
|
|
n := w.nbytes
|
|
for _, t := range input {
|
|
// Bitwriting inlined, ~30% speedup
|
|
c := encoding[t]
|
|
w.bits |= uint64(c.code) << w.nbits
|
|
w.nbits += uint(c.len)
|
|
if w.nbits < 48 {
|
|
continue
|
|
}
|
|
// Store 6 bytes
|
|
bits := w.bits
|
|
w.bits >>= 48
|
|
w.nbits -= 48
|
|
bytes := w.bytes[n : n+6]
|
|
bytes[0] = byte(bits)
|
|
bytes[1] = byte(bits >> 8)
|
|
bytes[2] = byte(bits >> 16)
|
|
bytes[3] = byte(bits >> 24)
|
|
bytes[4] = byte(bits >> 32)
|
|
bytes[5] = byte(bits >> 40)
|
|
n += 6
|
|
if n < bufferFlushSize {
|
|
continue
|
|
}
|
|
w.write(w.bytes[:n])
|
|
if w.err != nil {
|
|
return // Return early in the event of write failures
|
|
}
|
|
n = 0
|
|
}
|
|
w.nbytes = n
|
|
w.writeCode(encoding[endBlockMarker])
|
|
}
|