source: code/trunk/vendor/github.com/klauspost/compress/flate/dict_decoder.go@ 822

// 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.
loop:
        dstPos += copy(dd.hist[dstPos:endPos], dd.hist[srcPos:dstPos])
        if dstPos < endPos {
                goto loop // Avoid for-loop so that this function can be inlined
        }

        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
}