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// Copyright 2011 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.
package vp8
// Each VP8 frame consists of between 2 and 9 bitstream partitions.
// Each partition is byte-aligned and is independently arithmetic-encoded.
//
// This file implements decoding a partition's bitstream, as specified in
// chapter 7. The implementation follows libwebp's approach instead of the
// specification's reference C implementation. For example, we use a look-up
// table instead of a for loop to recalibrate the encoded range.
var (
lutShift = [127]uint8{
7, 6, 6, 5, 5, 5, 5, 4, 4, 4, 4, 4, 4, 4, 4,
3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3,
2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2,
2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2,
1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1,
1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1,
1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1,
1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1,
}
lutRangeM1 = [127]uint8{
127,
127, 191,
127, 159, 191, 223,
127, 143, 159, 175, 191, 207, 223, 239,
127, 135, 143, 151, 159, 167, 175, 183, 191, 199, 207, 215, 223, 231, 239, 247,
127, 131, 135, 139, 143, 147, 151, 155, 159, 163, 167, 171, 175, 179, 183, 187,
191, 195, 199, 203, 207, 211, 215, 219, 223, 227, 231, 235, 239, 243, 247, 251,
127, 129, 131, 133, 135, 137, 139, 141, 143, 145, 147, 149, 151, 153, 155, 157,
159, 161, 163, 165, 167, 169, 171, 173, 175, 177, 179, 181, 183, 185, 187, 189,
191, 193, 195, 197, 199, 201, 203, 205, 207, 209, 211, 213, 215, 217, 219, 221,
223, 225, 227, 229, 231, 233, 235, 237, 239, 241, 243, 245, 247, 249, 251, 253,
}
)
// uniformProb represents a 50% probability that the next bit is 0.
const uniformProb = 128
// partition holds arithmetic-coded bits.
type partition struct {
// buf is the input bytes.
buf []byte
// r is how many of buf's bytes have been consumed.
r int
// rangeM1 is range minus 1, where range is in the arithmetic coding sense,
// not the Go language sense.
rangeM1 uint32
// bits and nBits hold those bits shifted out of buf but not yet consumed.
bits uint32
nBits uint8
// unexpectedEOF tells whether we tried to read past buf.
unexpectedEOF bool
}
// init initializes the partition.
func (p *partition) init(buf []byte) {
p.buf = buf
p.r = 0
p.rangeM1 = 254
p.bits = 0
p.nBits = 0
p.unexpectedEOF = false
}
// readBit returns the next bit.
func (p *partition) readBit(prob uint8) bool {
if p.nBits < 8 {
if p.r >= len(p.buf) {
p.unexpectedEOF = true
return false
}
// Expression split for 386 compiler.
x := uint32(p.buf[p.r])
p.bits |= x << (8 - p.nBits)
p.r++
p.nBits += 8
}
split := (p.rangeM1*uint32(prob))>>8 + 1
bit := p.bits >= split<<8
if bit {
p.rangeM1 -= split
p.bits -= split << 8
} else {
p.rangeM1 = split - 1
}
if p.rangeM1 < 127 {
shift := lutShift[p.rangeM1]
p.rangeM1 = uint32(lutRangeM1[p.rangeM1])
p.bits <<= shift
p.nBits -= shift
}
return bit
}
// readUint returns the next n-bit unsigned integer.
func (p *partition) readUint(prob, n uint8) uint32 {
var u uint32
for n > 0 {
n--
if p.readBit(prob) {
u |= 1 << n
}
}
return u
}
// readInt returns the next n-bit signed integer.
func (p *partition) readInt(prob, n uint8) int32 {
u := p.readUint(prob, n)
b := p.readBit(prob)
if b {
return -int32(u)
}
return int32(u)
}
// readOptionalInt returns the next n-bit signed integer in an encoding
// where the likely result is zero.
func (p *partition) readOptionalInt(prob, n uint8) int32 {
if !p.readBit(prob) {
return 0
}
return p.readInt(prob, n)
}
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