Source file src/pkg/compress/flate/huffman_bit_writer.go

// 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"
    "os"
    "strconv"
)

const (
    // The largest offset code.
    offsetCodeCount = 30

    // The largest offset code in the extensions.
    extendedOffsetCodeCount = 42

    // 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             os.Error
}

type WrongValueError struct {
    name  string
    from  int32
    to    int32
    value int32
}

func newHuffmanBitWriter(w io.Writer) *huffmanBitWriter {
    return &huffmanBitWriter{
        w:               w,
        literalFreq:     make([]int32, maxLit),
        offsetFreq:      make([]int32, extendedOffsetCodeCount),
        codegen:         make([]uint8, maxLit+extendedOffsetCodeCount+1),
        codegenFreq:     make([]int32, codegenCodeCount),
        literalEncoding: newHuffmanEncoder(maxLit),
        offsetEncoding:  newHuffmanEncoder(extendedOffsetCodeCount),
        codegenEncoding: newHuffmanEncoder(codegenCodeCount),
    }
}

func (err WrongValueError) String() string {
    return "huffmanBitWriter: " + err.name + " should belong to [" + strconv.Itoa64(int64(err.from)) + ";" +
        strconv.Itoa64(int64(err.to)) + "] but actual value is " + strconv.Itoa64(int64(err.value))
}

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 specifiying
// 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) {
    fillInt32s(w.codegenFreq, 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.
    copyUint8s(codegen[0:numLiterals], w.literalEncoding.codeBits)
    copyUint8s(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 := min(count, 6)
                codegen[outIndex] = 16
                outIndex++
                codegen[outIndex] = uint8(n - 3)
                outIndex++
                w.codegenFreq[16]++
                count -= n
            }
        } else {
            for count >= 11 {
                n := min(count, 138)
                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  Tne 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)
    if numOffsets > offsetCodeCount {
        // Extended version of decompressor
        w.writeBits(int32(offsetCodeCount+((numOffsets-(1+offsetCodeCount))>>3)), 5)
        w.writeBits(int32((numOffsets-(1+offsetCodeCount))&0x7), 3)
    } else {
        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
    }
    fillInt32s(w.literalFreq, 0)
    fillInt32s(w.offsetFreq, 0)

    n := len(tokens)
    tokens = tokens[0 : n+1]
    tokens[n] = endBlockMarker

    totalLength := -1 // Subtract 1 for endBlock.
    for _, t := range tokens {
        switch t.typ() {
        case literalType:
            w.literalFreq[t.literal()]++
            totalLength++
            break
        case matchType:
            length := t.length()
            offset := t.offset()
            totalLength += int(length + 3)
            w.literalFreq[lengthCodesStart+lengthCode(length)]++
            w.offsetFreq[offsetCode(offset)]++
            break
        }
    }
    w.literalEncoding.generate(w.literalFreq, 15)
    w.offsetEncoding.generate(w.offsetFreq, 15)

    // 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 > 1 && w.offsetFreq[numOffsets-1] == 0 {
        numOffsets--
    }
    storedBytes := 0
    if input != nil {
        storedBytes = len(input)
    }
    var extraBits int64
    var storedSize int64
    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])
        }
    } else {
        storedSize = math.MaxInt32
    }

    // Figure out which generates smaller code, fixed Huffman, dynamic
    // Huffman, or just storing the data.
    var fixedSize int64 = math.MaxInt64
    if numOffsets <= offsetCodeCount {
        fixedSize = int64(3) +
            fixedLiteralEncoding.bitLength(w.literalFreq) +
            fixedOffsetEncoding.bitLength(w.offsetFreq) +
            extraBits
    }
    // 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--
    }
    extensionSummand := 0
    if numOffsets > offsetCodeCount {
        extensionSummand = 3
    }
    dynamicHeader := int64(3+5+5+4+(3*numCodegens)) +
        // Following line is an extension.
        int64(extensionSummand) +
        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 storedSize < fixedSize && storedSize < dynamicSize {
        w.writeStoredHeader(storedBytes, eof)
        w.writeBytes(input[0:storedBytes])
        return
    }
    var literalEncoding *huffmanEncoder
    var offsetEncoding *huffmanEncoder

    if fixedSize <= dynamicSize {
        w.writeFixedHeader(eof)
        literalEncoding = fixedLiteralEncoding
        offsetEncoding = fixedOffsetEncoding
    } else {
        // Write the header.
        w.writeDynamicHeader(numLiterals, numOffsets, numCodegens, eof)
        literalEncoding = w.literalEncoding
        offsetEncoding = w.offsetEncoding
    }

    // Write the tokens.
    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))
        }
    }
}