githaven-fork/vendor/github.com/ulikunitz/xz/lzma/prob.go

// Copyright 2014-2017 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

// movebits defines the number of bits used for the updates of probability
// values.
const movebits = 5

// probbits defines the number of bits of a probability value.
const probbits = 11

// probInit defines 0.5 as initial value for prob values.
const probInit prob = 1 << (probbits - 1)

// Type prob represents probabilities. The type can also be used to encode and
// decode single bits.
type prob uint16

// Dec decreases the probability. The decrease is proportional to the
// probability value.
func (p *prob) dec() {
	*p -= *p >> movebits
}

// Inc increases the probability. The Increase is proportional to the
// difference of 1 and the probability value.
func (p *prob) inc() {
	*p += ((1 << probbits) - *p) >> movebits
}

// Computes the new bound for a given range using the probability value.
func (p prob) bound(r uint32) uint32 {
	return (r >> probbits) * uint32(p)
}

// Bits returns 1. One is the number of bits that can be encoded or decoded
// with a single prob value.
func (p prob) Bits() int {
	return 1
}

// Encode encodes the least-significant bit of v. Note that the p value will be
// changed.
func (p *prob) Encode(e *rangeEncoder, v uint32) error {
	return e.EncodeBit(v, p)
}

// Decode decodes a single bit. Note that the p value will change.
func (p *prob) Decode(d *rangeDecoder) (v uint32, err error) {
	return d.DecodeBit(p)
}