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// Copyright 2014-2021 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"
"io"
)
// rangeEncoder implements range encoding of single bits. The low value can
// overflow therefore we need uint64. The cache value is used to handle
// overflows.
type rangeEncoder struct {
lbw *LimitedByteWriter
nrange uint32
low uint64
cacheLen int64
cache byte
}
// maxInt64 provides the maximal value of the int64 type
const maxInt64 = 1<<63 - 1
// newRangeEncoder creates a new range encoder.
func newRangeEncoder(bw io.ByteWriter) (re *rangeEncoder, err error) {
lbw, ok := bw.(*LimitedByteWriter)
if !ok {
lbw = &LimitedByteWriter{BW: bw, N: maxInt64}
}
return &rangeEncoder{
lbw: lbw,
nrange: 0xffffffff,
cacheLen: 1}, nil
}
// Available returns the number of bytes that still can be written. The
// method takes the bytes that will be currently written by Close into
// account.
func (e *rangeEncoder) Available() int64 {
return e.lbw.N - (e.cacheLen + 4)
}
// writeByte writes a single byte to the underlying writer. An error is
// returned if the limit is reached. The written byte will be counted if
// the underlying writer doesn't return an error.
func (e *rangeEncoder) writeByte(c byte) error {
if e.Available() < 1 {
return ErrLimit
}
return e.lbw.WriteByte(c)
}
// DirectEncodeBit encodes the least-significant bit of b with probability 1/2.
func (e *rangeEncoder) DirectEncodeBit(b uint32) error {
e.nrange >>= 1
e.low += uint64(e.nrange) & (0 - (uint64(b) & 1))
// normalize
const top = 1 << 24
if e.nrange >= top {
return nil
}
e.nrange <<= 8
return e.shiftLow()
}
// EncodeBit encodes the least significant bit of b. The p value will be
// updated by the function depending on the bit encoded.
func (e *rangeEncoder) EncodeBit(b uint32, p *prob) error {
bound := p.bound(e.nrange)
if b&1 == 0 {
e.nrange = bound
p.inc()
} else {
e.low += uint64(bound)
e.nrange -= bound
p.dec()
}
// normalize
const top = 1 << 24
if e.nrange >= top {
return nil
}
e.nrange <<= 8
return e.shiftLow()
}
// Close writes a complete copy of the low value.
func (e *rangeEncoder) Close() error {
for i := 0; i < 5; i++ {
if err := e.shiftLow(); err != nil {
return err
}
}
return nil
}
// shiftLow shifts the low value for 8 bit. The shifted byte is written into
// the byte writer. The cache value is used to handle overflows.
func (e *rangeEncoder) shiftLow() error {
if uint32(e.low) < 0xff000000 || (e.low>>32) != 0 {
tmp := e.cache
for {
err := e.writeByte(tmp + byte(e.low>>32))
if err != nil {
return err
}
tmp = 0xff
e.cacheLen--
if e.cacheLen <= 0 {
if e.cacheLen < 0 {
panic("negative cacheLen")
}
break
}
}
e.cache = byte(uint32(e.low) >> 24)
}
e.cacheLen++
e.low = uint64(uint32(e.low) << 8)
return nil
}
// rangeDecoder decodes single bits of the range encoding stream.
type rangeDecoder struct {
br io.ByteReader
nrange uint32
code uint32
}
// newRangeDecoder initializes a range decoder. It reads five bytes from the
// reader and therefore may return an error.
func newRangeDecoder(br io.ByteReader) (d *rangeDecoder, err error) {
d = &rangeDecoder{br: br, nrange: 0xffffffff}
b, err := d.br.ReadByte()
if err != nil {
return nil, err
}
if b != 0 {
return nil, errors.New("newRangeDecoder: first byte not zero")
}
for i := 0; i < 4; i++ {
if err = d.updateCode(); err != nil {
return nil, err
}
}
if d.code >= d.nrange {
return nil, errors.New("newRangeDecoder: d.code >= d.nrange")
}
return d, nil
}
// possiblyAtEnd checks whether the decoder may be at the end of the stream.
func (d *rangeDecoder) possiblyAtEnd() bool {
return d.code == 0
}
// DirectDecodeBit decodes a bit with probability 1/2. The return value b will
// contain the bit at the least-significant position. All other bits will be
// zero.
func (d *rangeDecoder) DirectDecodeBit() (b uint32, err error) {
d.nrange >>= 1
d.code -= d.nrange
t := 0 - (d.code >> 31)
d.code += d.nrange & t
b = (t + 1) & 1
// d.code will stay less then d.nrange
// normalize
// assume d.code < d.nrange
const top = 1 << 24
if d.nrange >= top {
return b, nil
}
d.nrange <<= 8
// d.code < d.nrange will be maintained
return b, d.updateCode()
}
// decodeBit decodes a single bit. The bit will be returned at the
// least-significant position. All other bits will be zero. The probability
// value will be updated.
func (d *rangeDecoder) DecodeBit(p *prob) (b uint32, err error) {
bound := p.bound(d.nrange)
if d.code < bound {
d.nrange = bound
p.inc()
b = 0
} else {
d.code -= bound
d.nrange -= bound
p.dec()
b = 1
}
// normalize
// assume d.code < d.nrange
const top = 1 << 24
if d.nrange >= top {
return b, nil
}
d.nrange <<= 8
// d.code < d.nrange will be maintained
return b, d.updateCode()
}
// updateCode reads a new byte into the code.
func (d *rangeDecoder) updateCode() error {
b, err := d.br.ReadByte()
if err != nil {
return err
}
d.code = (d.code << 8) | uint32(b)
return nil
}
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