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185 lines
6 KiB
Go
185 lines
6 KiB
Go
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// Copyright 2016 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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// dictDecoder implements the LZ77 sliding dictionary as used in decompression.
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// LZ77 decompresses data through sequences of two forms of commands:
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//
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// * Literal insertions: Runs of one or more symbols are inserted into the data
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// stream as is. This is accomplished through the writeByte method for a
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// single symbol, or combinations of writeSlice/writeMark for multiple symbols.
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// Any valid stream must start with a literal insertion if no preset dictionary
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// is used.
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//
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// * Backward copies: Runs of one or more symbols are copied from previously
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// emitted data. Backward copies come as the tuple (dist, length) where dist
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// determines how far back in the stream to copy from and length determines how
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// many bytes to copy. Note that it is valid for the length to be greater than
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// the distance. Since LZ77 uses forward copies, that situation is used to
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// perform a form of run-length encoding on repeated runs of symbols.
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// The writeCopy and tryWriteCopy are used to implement this command.
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//
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// For performance reasons, this implementation performs little to no sanity
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// checks about the arguments. As such, the invariants documented for each
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// method call must be respected.
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type dictDecoder struct {
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hist []byte // Sliding window history
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// Invariant: 0 <= rdPos <= wrPos <= len(hist)
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wrPos int // Current output position in buffer
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rdPos int // Have emitted hist[:rdPos] already
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full bool // Has a full window length been written yet?
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}
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// init initializes dictDecoder to have a sliding window dictionary of the given
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// size. If a preset dict is provided, it will initialize the dictionary with
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// the contents of dict.
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func (dd *dictDecoder) init(size int, dict []byte) {
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*dd = dictDecoder{hist: dd.hist}
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if cap(dd.hist) < size {
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dd.hist = make([]byte, size)
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}
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dd.hist = dd.hist[:size]
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if len(dict) > len(dd.hist) {
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dict = dict[len(dict)-len(dd.hist):]
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}
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dd.wrPos = copy(dd.hist, dict)
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if dd.wrPos == len(dd.hist) {
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dd.wrPos = 0
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dd.full = true
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}
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dd.rdPos = dd.wrPos
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}
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// histSize reports the total amount of historical data in the dictionary.
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func (dd *dictDecoder) histSize() int {
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if dd.full {
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return len(dd.hist)
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}
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return dd.wrPos
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}
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// availRead reports the number of bytes that can be flushed by readFlush.
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func (dd *dictDecoder) availRead() int {
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return dd.wrPos - dd.rdPos
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}
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// availWrite reports the available amount of output buffer space.
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func (dd *dictDecoder) availWrite() int {
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return len(dd.hist) - dd.wrPos
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}
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// writeSlice returns a slice of the available buffer to write data to.
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//
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// This invariant will be kept: len(s) <= availWrite()
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func (dd *dictDecoder) writeSlice() []byte {
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return dd.hist[dd.wrPos:]
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}
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// writeMark advances the writer pointer by cnt.
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//
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// This invariant must be kept: 0 <= cnt <= availWrite()
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func (dd *dictDecoder) writeMark(cnt int) {
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dd.wrPos += cnt
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}
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// writeByte writes a single byte to the dictionary.
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//
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// This invariant must be kept: 0 < availWrite()
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func (dd *dictDecoder) writeByte(c byte) {
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dd.hist[dd.wrPos] = c
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dd.wrPos++
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}
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// writeCopy copies a string at a given (dist, length) to the output.
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// This returns the number of bytes copied and may be less than the requested
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// length if the available space in the output buffer is too small.
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//
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// This invariant must be kept: 0 < dist <= histSize()
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func (dd *dictDecoder) writeCopy(dist, length int) int {
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dstBase := dd.wrPos
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dstPos := dstBase
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srcPos := dstPos - dist
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endPos := dstPos + length
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if endPos > len(dd.hist) {
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endPos = len(dd.hist)
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}
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// Copy non-overlapping section after destination position.
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//
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// This section is non-overlapping in that the copy length for this section
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// is always less than or equal to the backwards distance. This can occur
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// if a distance refers to data that wraps-around in the buffer.
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// Thus, a backwards copy is performed here; that is, the exact bytes in
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// the source prior to the copy is placed in the destination.
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if srcPos < 0 {
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srcPos += len(dd.hist)
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dstPos += copy(dd.hist[dstPos:endPos], dd.hist[srcPos:])
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srcPos = 0
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}
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// Copy possibly overlapping section before destination position.
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//
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// This section can overlap if the copy length for this section is larger
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// than the backwards distance. This is allowed by LZ77 so that repeated
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// strings can be succinctly represented using (dist, length) pairs.
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// Thus, a forwards copy is performed here; that is, the bytes copied is
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// possibly dependent on the resulting bytes in the destination as the copy
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// progresses along. This is functionally equivalent to the following:
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//
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// for i := 0; i < endPos-dstPos; i++ {
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// dd.hist[dstPos+i] = dd.hist[srcPos+i]
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// }
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// dstPos = endPos
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//
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for dstPos < endPos {
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dstPos += copy(dd.hist[dstPos:endPos], dd.hist[srcPos:dstPos])
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}
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dd.wrPos = dstPos
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return dstPos - dstBase
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}
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// tryWriteCopy tries to copy a string at a given (distance, length) to the
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// output. This specialized version is optimized for short distances.
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//
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// This method is designed to be inlined for performance reasons.
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//
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// This invariant must be kept: 0 < dist <= histSize()
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func (dd *dictDecoder) tryWriteCopy(dist, length int) int {
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dstPos := dd.wrPos
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endPos := dstPos + length
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if dstPos < dist || endPos > len(dd.hist) {
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return 0
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}
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dstBase := dstPos
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srcPos := dstPos - dist
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// Copy possibly overlapping section before destination position.
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loop:
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dstPos += copy(dd.hist[dstPos:endPos], dd.hist[srcPos:dstPos])
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if dstPos < endPos {
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goto loop // Avoid for-loop so that this function can be inlined
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}
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dd.wrPos = dstPos
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return dstPos - dstBase
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}
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// readFlush returns a slice of the historical buffer that is ready to be
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// emitted to the user. The data returned by readFlush must be fully consumed
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// before calling any other dictDecoder methods.
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func (dd *dictDecoder) readFlush() []byte {
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toRead := dd.hist[dd.rdPos:dd.wrPos]
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dd.rdPos = dd.wrPos
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if dd.wrPos == len(dd.hist) {
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dd.wrPos, dd.rdPos = 0, 0
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dd.full = true
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}
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return toRead
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}
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