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