text: // 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)
  }
  