forgejo/vendor/github.com/ulikunitz/xz/lzma/lengthcodec.go

// 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
}