// 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 (
"fmt"
"io"
"math"
)
const (
NoCompression = 0
BestSpeed = 1
fastCompression = 3
BestCompression = 9
DefaultCompression = -1
logWindowSize = 15
windowSize = 1 << logWindowSize
windowMask = windowSize - 1
logMaxOffsetSize = 15 // Standard DEFLATE
minMatchLength = 3 // The smallest match that the compressor looks for
maxMatchLength = 258 // The longest match for the compressor
minOffsetSize = 1 // The shortest offset that makes any sense
// The maximum number of tokens we put into a single flat block, just to
// stop things from getting too large.
maxFlateBlockTokens = 1 << 14
maxStoreBlockSize = 65535
hashBits = 17
hashSize = 1 << hashBits
hashMask = (1 << hashBits) - 1
hashShift = (hashBits + minMatchLength - 1) / minMatchLength
maxHashOffset = 1 << 24
skipNever = math.MaxInt32
)
type compressionLevel struct {
good, lazy, nice, chain, fastSkipHashing int
}
var levels = []compressionLevel{
{}, // 0
// For levels 1-3 we don't bother trying with lazy matches
{3, 0, 8, 4, 4},
{3, 0, 16, 8, 5},
{3, 0, 32, 32, 6},
// Levels 4-9 use increasingly more lazy matching
// and increasingly stringent conditions for "good enough".
{4, 4, 16, 16, skipNever},
{8, 16, 32, 32, skipNever},
{8, 16, 128, 128, skipNever},
{8, 32, 128, 256, skipNever},
{32, 128, 258, 1024, skipNever},
{32, 258, 258, 4096, skipNever},
}
type compressor struct {
compressionLevel
w *huffmanBitWriter
// compression algorithm
fill func(*compressor, []byte) int // copy data to window
step func(*compressor) // process window
sync bool // requesting flush
// Input hash chains
// hashHead[hashValue] contains the largest inputIndex with the specified hash value
// If hashHead[hashValue] is within the current window, then
// hashPrev[hashHead[hashValue] & windowMask] contains the previous index
// with the same hash value.
chainHead int
hashHead []int
hashPrev []int
hashOffset int
// input window: unprocessed data is window[index:windowEnd]
index int
window []byte
windowEnd int
blockStart int // window index where current tokens start
byteAvailable bool // if true, still need to process window[index-1].
// queued output tokens
tokens []token
// deflate state
length int
offset int
hash int
maxInsertIndex int
err error
}
func (d *compressor) fillDeflate(b []byte) int {
if d.index >= 2*windowSize-(minMatchLength+maxMatchLength) {
// shift the window by windowSize
copy(d.window, d.window[windowSize:2*windowSize])
d.index -= windowSize
d.windowEnd -= windowSize
if d.blockStart >= windowSize {
d.blockStart -= windowSize
} else {
d.blockStart = math.MaxInt32
}
d.hashOffset += windowSize
if d.hashOffset > maxHashOffset {
delta := d.hashOffset - 1
d.hashOffset -= delta
d.chainHead -= delta
for i, v := range d.hashPrev {
if v > delta {
d.hashPrev[i] -= delta
} else {
d.hashPrev[i] = 0
}
}
for i, v := range d.hashHead {
if v > delta {
d.hashHead[i] -= delta
} else {
d.hashHead[i] = 0
}
}
}
}
n := copy(d.window[d.windowEnd:], b)
d.windowEnd += n
return n
}
func (d *compressor) writeBlock(tokens []token, index int, eof bool) error {
if index > 0 || eof {
var window []byte
if d.blockStart <= index {
window = d.window[d.blockStart:index]
}
d.blockStart = index
d.w.writeBlock(tokens, eof, window)
return d.w.err
}
return nil
}
// Try to find a match starting at index whose length is greater than prevSize.
// We only look at chainCount possibilities before giving up.
func (d *compressor) findMatch(pos int, prevHead int, prevLength int, lookahead int) (length, offset int, ok bool) {
minMatchLook := maxMatchLength
if lookahead < minMatchLook {
minMatchLook = lookahead
}
win := d.window[0 : pos+minMatchLook]
// We quit when we get a match that's at least nice long
nice := len(win) - pos
if d.nice < nice {
nice = d.nice
}
// If we've got a match that's good enough, only look in 1/4 the chain.
tries := d.chain
length = prevLength
if length >= d.good {
tries >>= 2
}
w0 := win[pos]
w1 := win[pos+1]
wEnd := win[pos+length]
minIndex := pos - windowSize
for i := prevHead; tries > 0; tries-- {
if w0 == win[i] && w1 == win[i+1] && wEnd == win[i+length] {
// The hash function ensures that if win[i] and win[i+1] match, win[i+2] matches
n := 3
for pos+n < len(win) && win[i+n] == win[pos+n] {
n++
}
if n > length && (n > 3 || pos-i <= 4096) {
length = n
offset = pos - i
ok = true
if n >= nice {
// The match is good enough that we don't try to find a better one.
break
}
wEnd = win[pos+n]
}
}
if i == minIndex {
// hashPrev[i & windowMask] has already been overwritten, so stop now.
break
}
if i = d.hashPrev[i&windowMask] - d.hashOffset; i < minIndex || i < 0 {
break
}
}
return
}
func (d *compressor) writeStoredBlock(buf []byte) error {
if d.w.writeStoredHeader(len(buf), false); d.w.err != nil {
return d.w.err
}
d.w.writeBytes(buf)
return d.w.err
}
func (d *compressor) initDeflate() {
d.hashHead = make([]int, hashSize)
d.hashPrev = make([]int, windowSize)
d.window = make([]byte, 2*windowSize)
d.hashOffset = 1
d.tokens = make([]token, 0, maxFlateBlockTokens+1)
d.length = minMatchLength - 1
d.offset = 0
d.byteAvailable = false
d.index = 0
d.hash = 0
d.chainHead = -1
}
func (d *compressor) deflate() {
if d.windowEnd-d.index < minMatchLength+maxMatchLength && !d.sync {
return
}
d.maxInsertIndex = d.windowEnd - (minMatchLength - 1)
if d.index < d.maxInsertIndex {
d.hash = int(d.window[d.index])<<hashShift + int(d.window[d.index+1])
}
Loop:
for {
if d.index > d.windowEnd {
panic("index > windowEnd")
}
lookahead := d.windowEnd - d.index
if lookahead < minMatchLength+maxMatchLength {
if !d.sync {
break Loop
}
if d.index > d.windowEnd {
panic("index > windowEnd")
}
if lookahead == 0 {
// Flush current output block if any.
if d.byteAvailable {
// There is still one pending token that needs to be flushed
d.tokens = append(d.tokens, literalToken(uint32(d.window[d.index-1])))
d.byteAvailable = false
}
if len(d.tokens) > 0 {
if d.err = d.writeBlock(d.tokens, d.index, false); d.err != nil {
return
}
d.tokens = d.tokens[:0]
}
break Loop
}
}
if d.index < d.maxInsertIndex {
// Update the hash
d.hash = (d.hash<<hashShift + int(d.window[d.index+2])) & hashMask
d.chainHead = d.hashHead[d.hash]
d.hashPrev[d.index&windowMask] = d.chainHead
d.hashHead[d.hash] = d.index + d.hashOffset
}
prevLength := d.length
prevOffset := d.offset
d.length = minMatchLength - 1
d.offset = 0
minIndex := d.index - windowSize
if minIndex < 0 {
minIndex = 0
}
if d.chainHead-d.hashOffset >= minIndex &&
(d.fastSkipHashing != skipNever && lookahead > minMatchLength-1 ||
d.fastSkipHashing == skipNever && lookahead > prevLength && prevLength < d.lazy) {
if newLength, newOffset, ok := d.findMatch(d.index, d.chainHead-d.hashOffset, minMatchLength-1, lookahead); ok {
d.length = newLength
d.offset = newOffset
}
}
if d.fastSkipHashing != skipNever && d.length >= minMatchLength ||
d.fastSkipHashing == skipNever && prevLength >= minMatchLength && d.length <= prevLength {
// There was a match at the previous step, and the current match is
// not better. Output the previous match.
if d.fastSkipHashing != skipNever {
d.tokens = append(d.tokens, matchToken(uint32(d.length-minMatchLength), uint32(d.offset-minOffsetSize)))
} else {
d.tokens = append(d.tokens, matchToken(uint32(prevLength-minMatchLength), uint32(prevOffset-minOffsetSize)))
}
// Insert in the hash table all strings up to the end of the match.
// index and index-1 are already inserted. If there is not enough
// lookahead, the last two strings are not inserted into the hash
// table.
if d.length <= d.fastSkipHashing {
var newIndex int
if d.fastSkipHashing != skipNever {
newIndex = d.index + d.length
} else {
newIndex = d.index + prevLength - 1
}
for d.index++; d.index < newIndex; d.index++ {
if d.index < d.maxInsertIndex {
d.hash = (d.hash<<hashShift + int(d.window[d.index+2])) & hashMask
// Get previous value with the same hash.
// Our chain should point to the previous value.
d.hashPrev[d.index&windowMask] = d.hashHead[d.hash]
// Set the head of the hash chain to us.
d.hashHead[d.hash] = d.index + d.hashOffset
}
}
if d.fastSkipHashing == skipNever {
d.byteAvailable = false
d.length = minMatchLength - 1
}
} else {
// For matches this long, we don't bother inserting each individual
// item into the table.
d.index += d.length
if d.index < d.maxInsertIndex {
d.hash = (int(d.window[d.index])<<hashShift + int(d.window[d.index+1]))
}
}
if len(d.tokens) == maxFlateBlockTokens {
// The block includes the current character
if d.err = d.writeBlock(d.tokens, d.index, false); d.err != nil {
return
}
d.tokens = d.tokens[:0]
}
} else {
if d.fastSkipHashing != skipNever || d.byteAvailable {
i := d.index - 1
if d.fastSkipHashing != skipNever {
i = d.index
}
d.tokens = append(d.tokens, literalToken(uint32(d.window[i])))
if len(d.tokens) == maxFlateBlockTokens {
if d.err = d.writeBlock(d.tokens, i+1, false); d.err != nil {
return
}
d.tokens = d.tokens[:0]
}
}
d.index++
if d.fastSkipHashing == skipNever {
d.byteAvailable = true
}
}
}
}
func (d *compressor) fillStore(b []byte) int {
n := copy(d.window[d.windowEnd:], b)
d.windowEnd += n
return n
}
func (d *compressor) store() {
if d.windowEnd > 0 {
d.err = d.writeStoredBlock(d.window[:d.windowEnd])
}
d.windowEnd = 0
}
func (d *compressor) write(b []byte) (n int, err error) {
n = len(b)
b = b[d.fill(d, b):]
for len(b) > 0 {
d.step(d)
b = b[d.fill(d, b):]
}
return n, d.err
}
func (d *compressor) syncFlush() error {
d.sync = true
d.step(d)
if d.err == nil {
d.w.writeStoredHeader(0, false)
d.w.flush()
d.err = d.w.err
}
d.sync = false
return d.err
}
func (d *compressor) init(w io.Writer, level int) (err error) {
d.w = newHuffmanBitWriter(w)
switch {
case level == NoCompression:
d.window = make([]byte, maxStoreBlockSize)
d.fill = (*compressor).fillStore
d.step = (*compressor).store
case level == DefaultCompression:
level = 6
fallthrough
case 1 <= level && level <= 9:
d.compressionLevel = levels[level]
d.initDeflate()
d.fill = (*compressor).fillDeflate
d.step = (*compressor).deflate
default:
return fmt.Errorf("flate: invalid compression level %d: want value in range [-1, 9]", level)
}
return nil
}
var zeroes [32]int
var bzeroes [256]byte
func (d *compressor) reset(w io.Writer) {
d.w.reset(w)
d.sync = false
d.err = nil
switch d.compressionLevel.chain {
case 0:
// level was NoCompression.
for i := range d.window {
d.window[i] = 0
}
d.windowEnd = 0
default:
d.chainHead = -1
for s := d.hashHead; len(s) > 0; {
n := copy(s, zeroes[:])
s = s[n:]
}
for s := d.hashPrev; len(s) > 0; s = s[len(zeroes):] {
copy(s, zeroes[:])
}
d.hashOffset = 1
d.index, d.windowEnd = 0, 0
for s := d.window; len(s) > 0; {
n := copy(s, bzeroes[:])
s = s[n:]
}
d.blockStart, d.byteAvailable = 0, false
d.tokens = d.tokens[:maxFlateBlockTokens+1]
for i := 0; i <= maxFlateBlockTokens; i++ {
d.tokens[i] = 0
}
d.tokens = d.tokens[:0]
d.length = minMatchLength - 1
d.offset = 0
d.hash = 0
d.maxInsertIndex = 0
}
}
func (d *compressor) close() error {
d.sync = true
d.step(d)
if d.err != nil {
return d.err
}
if d.w.writeStoredHeader(0, true); d.w.err != nil {
return d.w.err
}
d.w.flush()
return d.w.err
}
// NewWriter returns a new Writer compressing data at the given level.
// Following zlib, levels range from 1 (BestSpeed) to 9 (BestCompression);
// higher levels typically run slower but compress more. Level 0
// (NoCompression) does not attempt any compression; it only adds the
// necessary DEFLATE framing. Level -1 (DefaultCompression) uses the default
// compression level.
//
// If level is in the range [-1, 9] then the error returned will be nil.
// Otherwise the error returned will be non-nil.
func NewWriter(w io.Writer, level int) (*Writer, error) {
var dw Writer
if err := dw.d.init(w, level); err != nil {
return nil, err
}
return &dw, nil
}
// NewWriterDict is like NewWriter but initializes the new
// Writer with a preset dictionary. The returned Writer behaves
// as if the dictionary had been written to it without producing
// any compressed output. The compressed data written to w
// can only be decompressed by a Reader initialized with the
// same dictionary.
func NewWriterDict(w io.Writer, level int, dict []byte) (*Writer, error) {
dw := &dictWriter{w, false}
zw, err := NewWriter(dw, level)
if err != nil {
return nil, err
}
zw.Write(dict)
zw.Flush()
dw.enabled = true
zw.dict = append(zw.dict, dict...) // duplicate dictionary for Reset method.
return zw, err
}
type dictWriter struct {
w io.Writer
enabled bool
}
func (w *dictWriter) Write(b []byte) (n int, err error) {
if w.enabled {
return w.w.Write(b)
}
return len(b), nil
}
// A Writer takes data written to it and writes the compressed
// form of that data to an underlying writer (see NewWriter).
type Writer struct {
d compressor
dict []byte
}
// Write writes data to w, which will eventually write the
// compressed form of data to its underlying writer.
func (w *Writer) Write(data []byte) (n int, err error) {
return w.d.write(data)
}
// Flush flushes any pending compressed data to the underlying writer.
// It is useful mainly in compressed network protocols, to ensure that
// a remote reader has enough data to reconstruct a packet.
// Flush does not return until the data has been written.
// If the underlying writer returns an error, Flush returns that error.
//
// In the terminology of the zlib library, Flush is equivalent to Z_SYNC_FLUSH.
func (w *Writer) Flush() error {
// For more about flushing:
// http://www.bolet.org/~pornin/deflate-flush.html
return w.d.syncFlush()
}
// Close flushes and closes the writer.
func (w *Writer) Close() error {
return w.d.close()
}
// Reset discards the writer's state and makes it equivalent to
// the result of NewWriter or NewWriterDict called with dst
// and w's level and dictionary.
func (w *Writer) Reset(dst io.Writer) {
if dw, ok := w.d.w.w.(*dictWriter); ok {
// w was created with NewWriterDict
dw.w = dst
w.d.reset(dw)
dw.enabled = false
w.Write(w.dict)
w.Flush()
dw.enabled = true
} else {
// w was created with NewWriter
w.d.reset(dst)
}
}