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range.go
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range.go
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package lzma
import (
"bytes"
)
const (
kTopValue = uint32(1) << 24
)
type rangedecoder struct {
nrange uint32
code uint32
reader bytes.Reader
}
func (rd *rangedecoder) init() error {
rd.nrange = 0xffffffff
rd.code = 0
b, err := rd.reader.ReadByte()
if err != nil {
return err
}
if b != 0 {
return lzmaInvalidFormat
}
for i := 0; i < 4; i++ {
if err := rd.updateCode(); err != nil {
return err
}
}
if rd.code >= rd.nrange {
return lzmaInvalidFormat
}
return nil
}
func (rd *rangedecoder) isFinishedOK() bool {
return rd.code == 0
}
func (rd *rangedecoder) updateCode() error {
b, err := rd.reader.ReadByte()
if err != nil {
return err
}
rd.code = (rd.code << 8) | uint32(b)
return nil
}
func (rd *rangedecoder) normalize() error {
if rd.nrange < kTopValue {
rd.nrange <<= 8
if err := rd.updateCode(); err != nil {
return err
}
}
return nil
}
func (rd *rangedecoder) decodeDirectBits(numBits uint32) (uint32, error) {
res := uint32(0)
for {
rd.nrange >>= 1
rd.code -= rd.nrange
t := 0 - (rd.code >> 31)
rd.code += rd.nrange & t
if err := rd.normalize(); err != nil {
return 0, err
}
res <<= 1
res += t + 1
numBits--
if numBits <= 0 {
break
}
}
return res, nil
}
func (rd *rangedecoder) decodeBit(prob *prob) (uint, error) {
symbol := uint(0)
bound := prob.bound(rd.nrange)
if rd.code < bound {
prob.inc()
rd.nrange = bound
} else {
prob.dec()
rd.code -= bound
rd.nrange -= bound
symbol = 1
}
if err := rd.normalize() ; err != nil {
return 0, err
}
return symbol, nil
}