compress/flate/dict_decoder.go

// Copyright 2016 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

// dictDecoder implements the LZ77 sliding dictionary as used in decompression.
// LZ77 decompresses data through sequences of two forms of commands:
//
//	* Literal insertions: Runs of one or more symbols are inserted into the data
//	stream as is. This is accomplished through the writeByte method for a
//	single symbol, or combinations of writeSlice/writeMark for multiple symbols.
//	Any valid stream must start with a literal insertion if no preset dictionary
//	is used.
//
//	* Backward copies: Runs of one or more symbols are copied from previously
//	emitted data. Backward copies come as the tuple (dist, length) where dist
//	determines how far back in the stream to copy from and length determines how
//	many bytes to copy. Note that it is valid for the length to be greater than
//	the distance. Since LZ77 uses forward copies, that situation is used to
//	perform a form of run-length encoding on repeated runs of symbols.
//	The writeCopy and tryWriteCopy are used to implement this command.
//
// For performance reasons, this implementation performs little to no sanity
// checks about the arguments. As such, the invariants documented for each
// method call must be respected.
type dictDecoder struct {
	hist []byte // Sliding window history

	// Invariant: 0 <= rdPos <= wrPos <= len(hist)
	wrPos int  // Current output position in buffer
	rdPos int  // Have emitted hist[:rdPos] already
	full  bool // Has a full window length been written yet?
}

// init initializes dictDecoder to have a sliding window dictionary of the given
// size. If a preset dict is provided, it will initialize the dictionary with
// the contents of dict.
func (dd *dictDecoder) init(size int, dict []byte) {
	*dd = dictDecoder{hist: dd.hist}

	if cap(dd.hist) < size {
		dd.hist = make([]byte, size)
	}
	dd.hist = dd.hist[:size]

	if len(dict) > len(dd.hist) {
		dict = dict[len(dict)-len(dd.hist):]
	}
	dd.wrPos = copy(dd.hist, dict)
	if dd.wrPos == len(dd.hist) {
		dd.wrPos = 0
		dd.full = true
	}
	dd.rdPos = dd.wrPos
}

// histSize reports the total amount of historical data in the dictionary.
func (dd *dictDecoder) histSize() int {
	if dd.full {
		return len(dd.hist)
	}
	return dd.wrPos
}

// availRead reports the number of bytes that can be flushed by readFlush.
func (dd *dictDecoder) availRead() int {
	return dd.wrPos - dd.rdPos
}

// availWrite reports the available amount of output buffer space.
func (dd *dictDecoder) availWrite() int {
	return len(dd.hist) - dd.wrPos
}

// writeSlice returns a slice of the available buffer to write data to.
//
// This invariant will be kept: len(s) <= availWrite()
func (dd *dictDecoder) writeSlice() []byte {
	return dd.hist[dd.wrPos:]
}

// writeMark advances the writer pointer by cnt.
//
// This invariant must be kept: 0 <= cnt <= availWrite()
func (dd *dictDecoder) writeMark(cnt int) {
	dd.wrPos += cnt
}

// writeByte writes a single byte to the dictionary.
//
// This invariant must be kept: 0 < availWrite()
func (dd *dictDecoder) writeByte(c byte) {
	dd.hist[dd.wrPos] = c
	dd.wrPos++
}

// writeCopy copies a string at a given (dist, length) to the output.
// This returns the number of bytes copied and may be less than the requested
// length if the available space in the output buffer is too small.
//
// This invariant must be kept: 0 < dist <= histSize()
func (dd *dictDecoder) writeCopy(dist, length int) int {
	dstBase := dd.wrPos
	dstPos := dstBase
	srcPos := dstPos - dist
	endPos := dstPos + length
	if endPos > len(dd.hist) {
		endPos = len(dd.hist)
	}

	// Copy non-overlapping section after destination position.
	//
	// This section is non-overlapping in that the copy length for this section
	// is always less than or equal to the backwards distance. This can occur
	// if a distance refers to data that wraps-around in the buffer.
	// Thus, a backwards copy is performed here; that is, the exact bytes in
	// the source prior to the copy is placed in the destination.
	if srcPos < 0 {
		srcPos += len(dd.hist)
		dstPos += copy(dd.hist[dstPos:endPos], dd.hist[srcPos:])
		srcPos = 0
	}

	// Copy possibly overlapping section before destination position.
	//
	// This section can overlap if the copy length for this section is larger
	// than the backwards distance. This is allowed by LZ77 so that repeated
	// strings can be succinctly represented using (dist, length) pairs.
	// Thus, a forwards copy is performed here; that is, the bytes copied is
	// possibly dependent on the resulting bytes in the destination as the copy
	// progresses along. This is functionally equivalent to the following:
	//
	//	for i := 0; i < endPos-dstPos; i++ {
	//		dd.hist[dstPos+i] = dd.hist[srcPos+i]
	//	}
	//	dstPos = endPos
	//
	for dstPos < endPos {
		dstPos += copy(dd.hist[dstPos:endPos], dd.hist[srcPos:dstPos])
	}

	dd.wrPos = dstPos
	return dstPos - dstBase
}

// tryWriteCopy tries to copy a string at a given (distance, length) to the
// output. This specialized version is optimized for short distances.
//
// This method is designed to be inlined for performance reasons.
//
// This invariant must be kept: 0 < dist <= histSize()
func (dd *dictDecoder) tryWriteCopy(dist, length int) int {
	dstPos := dd.wrPos
	endPos := dstPos + length
	if dstPos < dist || endPos > len(dd.hist) {
		return 0
	}
	dstBase := dstPos
	srcPos := dstPos - dist

	// Copy possibly overlapping section before destination position.
	for dstPos < endPos {
		dstPos += copy(dd.hist[dstPos:endPos], dd.hist[srcPos:dstPos])
	}

	dd.wrPos = dstPos
	return dstPos - dstBase
}

// readFlush returns a slice of the historical buffer that is ready to be
// emitted to the user. The data returned by readFlush must be fully consumed
// before calling any other dictDecoder methods.
func (dd *dictDecoder) readFlush() []byte {
	toRead := dd.hist[dd.rdPos:dd.wrPos]
	dd.rdPos = dd.wrPos
	if dd.wrPos == len(dd.hist) {
		dd.wrPos, dd.rdPos = 0, 0
		dd.full = true
	}
	return toRead
}