lzma-purejs-requirejs/lib/RangeCoder/Decoder.js

pure JavaScript LZMA de/compression, for node.js, volo, and the browser.

'use strict';

const freeze = require('../freeze');

var Decoder = function(stream){
  if (stream) {
    this.setStream(stream);
    this.init();
  }
};

Decoder.prototype.setStream = function(stream){
  this._stream = stream;
};

Decoder.prototype.releaseStream = function(){
  this._stream = null;
};

Decoder.prototype.init = function(){
  var i = 5;

  this._code = 0;
  this._range = -1;

  while(i --){
    this._code = (this._code << 8) | this._stream.readByte();
  }
};

Decoder.prototype.decodeDirectBits = function(numTotalBits){
  var result = 0, i = numTotalBits, t;

  while(i --){
    this._range >>>= 1;
    t = (this._code - this._range) >>> 31;
    this._code -= this._range & (t - 1);
    result = (result << 1) | (1 - t);

    if ( (this._range & 0xff000000) === 0){
      this._code = (this._code << 8) | this._stream.readByte();
      this._range <<= 8;
    }
  }

  return result;
};

Decoder.prototype.decodeBit = function(probs, index){
  var prob = probs[index],
      newBound = (this._range >>> 11) * prob;

  if ( (this._code ^ 0x80000000) < (newBound ^ 0x80000000) ){
    this._range = newBound;
    probs[index] += (2048 - prob) >>> 5;
    if ( (this._range & 0xff000000) === 0){
      this._code = (this._code << 8) | this._stream.readByte();
      this._range <<= 8;
    }
    return 0;
  }

  this._range -= newBound;
  this._code -= newBound;
  probs[index] -= prob >>> 5;
  if ( (this._range & 0xff000000) === 0){
    this._code = (this._code << 8) | this._stream.readByte();
    this._range <<= 8;
  }
  return 1;
};

freeze(Decoder.prototype);
module.exports = freeze(Decoder);