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54 lines
1.5 KiB
54 lines
1.5 KiB
// Copyright 2014-2021 Ulrich Kunitz. All rights reserved.
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// Use of this source code is governed by a BSD-style
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// license that can be found in the LICENSE file.
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package lzma
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// movebits defines the number of bits used for the updates of probability
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// values.
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const movebits = 5
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// probbits defines the number of bits of a probability value.
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const probbits = 11
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// probInit defines 0.5 as initial value for prob values.
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const probInit prob = 1 << (probbits - 1)
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// Type prob represents probabilities. The type can also be used to encode and
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// decode single bits.
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type prob uint16
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// Dec decreases the probability. The decrease is proportional to the
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// probability value.
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func (p *prob) dec() {
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*p -= *p >> movebits
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}
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// Inc increases the probability. The Increase is proportional to the
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// difference of 1 and the probability value.
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func (p *prob) inc() {
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*p += ((1 << probbits) - *p) >> movebits
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}
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// Computes the new bound for a given range using the probability value.
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func (p prob) bound(r uint32) uint32 {
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return (r >> probbits) * uint32(p)
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}
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// Bits returns 1. One is the number of bits that can be encoded or decoded
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// with a single prob value.
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func (p prob) Bits() int {
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return 1
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}
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// Encode encodes the least-significant bit of v. Note that the p value will be
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// changed.
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func (p *prob) Encode(e *rangeEncoder, v uint32) error {
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return e.EncodeBit(v, p)
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}
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// Decode decodes a single bit. Note that the p value will change.
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func (p *prob) Decode(d *rangeDecoder) (v uint32, err error) {
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return d.DecodeBit(p)
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}
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