1// Copyright 2014-2017 Ulrich Kunitz. All rights reserved. 2// Use of this source code is governed by a BSD-style 3// license that can be found in the LICENSE file. 4 5package lzma 6 7// movebits defines the number of bits used for the updates of probability 8// values. 9const movebits = 5 10 11// probbits defines the number of bits of a probability value. 12const probbits = 11 13 14// probInit defines 0.5 as initial value for prob values. 15const probInit prob = 1 << (probbits - 1) 16 17// Type prob represents probabilities. The type can also be used to encode and 18// decode single bits. 19type prob uint16 20 21// Dec decreases the probability. The decrease is proportional to the 22// probability value. 23func (p *prob) dec() { 24 *p -= *p >> movebits 25} 26 27// Inc increases the probability. The Increase is proportional to the 28// difference of 1 and the probability value. 29func (p *prob) inc() { 30 *p += ((1 << probbits) - *p) >> movebits 31} 32 33// Computes the new bound for a given range using the probability value. 34func (p prob) bound(r uint32) uint32 { 35 return (r >> probbits) * uint32(p) 36} 37 38// Bits returns 1. One is the number of bits that can be encoded or decoded 39// with a single prob value. 40func (p prob) Bits() int { 41 return 1 42} 43 44// Encode encodes the least-significant bit of v. Note that the p value will be 45// changed. 46func (p *prob) Encode(e *rangeEncoder, v uint32) error { 47 return e.EncodeBit(v, p) 48} 49 50// Decode decodes a single bit. Note that the p value will change. 51func (p *prob) Decode(d *rangeDecoder) (v uint32, err error) { 52 return d.DecodeBit(p) 53} 54