130 lines
2.9 KiB
Go
130 lines
2.9 KiB
Go
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// Copyright 2014-2017 Ulrich Kunitz. 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 lzma
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import "errors"
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// maxPosBits defines the number of bits of the position value that are used to
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// to compute the posState value. The value is used to select the tree codec
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// for length encoding and decoding.
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const maxPosBits = 4
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// minMatchLen and maxMatchLen give the minimum and maximum values for
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// encoding and decoding length values. minMatchLen is also used as base
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// for the encoded length values.
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const (
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minMatchLen = 2
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maxMatchLen = minMatchLen + 16 + 256 - 1
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)
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// lengthCodec support the encoding of the length value.
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type lengthCodec struct {
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choice [2]prob
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low [1 << maxPosBits]treeCodec
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mid [1 << maxPosBits]treeCodec
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high treeCodec
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}
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// deepcopy initializes the lc value as deep copy of the source value.
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func (lc *lengthCodec) deepcopy(src *lengthCodec) {
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if lc == src {
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return
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}
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lc.choice = src.choice
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for i := range lc.low {
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lc.low[i].deepcopy(&src.low[i])
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}
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for i := range lc.mid {
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lc.mid[i].deepcopy(&src.mid[i])
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}
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lc.high.deepcopy(&src.high)
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}
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// init initializes a new length codec.
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func (lc *lengthCodec) init() {
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for i := range lc.choice {
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lc.choice[i] = probInit
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}
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for i := range lc.low {
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lc.low[i] = makeTreeCodec(3)
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}
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for i := range lc.mid {
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lc.mid[i] = makeTreeCodec(3)
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}
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lc.high = makeTreeCodec(8)
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}
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// lBits gives the number of bits used for the encoding of the l value
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// provided to the range encoder.
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func lBits(l uint32) int {
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switch {
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case l < 8:
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return 4
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case l < 16:
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return 5
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default:
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return 10
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}
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}
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// Encode encodes the length offset. The length offset l can be compute by
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// subtracting minMatchLen (2) from the actual length.
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//
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// l = length - minMatchLen
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//
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func (lc *lengthCodec) Encode(e *rangeEncoder, l uint32, posState uint32,
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) (err error) {
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if l > maxMatchLen-minMatchLen {
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return errors.New("lengthCodec.Encode: l out of range")
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}
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if l < 8 {
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if err = lc.choice[0].Encode(e, 0); err != nil {
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return
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}
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return lc.low[posState].Encode(e, l)
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}
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if err = lc.choice[0].Encode(e, 1); err != nil {
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return
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}
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if l < 16 {
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if err = lc.choice[1].Encode(e, 0); err != nil {
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return
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}
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return lc.mid[posState].Encode(e, l-8)
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}
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if err = lc.choice[1].Encode(e, 1); err != nil {
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return
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}
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if err = lc.high.Encode(e, l-16); err != nil {
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return
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}
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return nil
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}
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// Decode reads the length offset. Add minMatchLen to compute the actual length
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// to the length offset l.
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func (lc *lengthCodec) Decode(d *rangeDecoder, posState uint32,
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) (l uint32, err error) {
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var b uint32
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if b, err = lc.choice[0].Decode(d); err != nil {
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return
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}
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if b == 0 {
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l, err = lc.low[posState].Decode(d)
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return
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}
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if b, err = lc.choice[1].Decode(d); err != nil {
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return
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}
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if b == 0 {
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l, err = lc.mid[posState].Decode(d)
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l += 8
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return
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}
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l, err = lc.high.Decode(d)
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l += 16
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return
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}
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