pdf.js/src/core/arithmetic_decoder.js

/* -*- Mode: Java; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
/* vim: set shiftwidth=2 tabstop=2 autoindent cindent expandtab: */
/* Copyright 2012 Mozilla Foundation
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

'use strict';

/* This class implements the QM Coder decoding as defined in
 *   JPEG 2000 Part I Final Committee Draft Version 1.0
 *   Annex C.3 Arithmetic decoding procedure 
 * available at http://www.jpeg.org/public/fcd15444-1.pdf
 * 
 * The arithmetic decoder is used in conjunction with context models to decode
 * JPEG2000 and JBIG2 streams.
 */
var ArithmeticDecoder = (function ArithmeticDecoderClosure() {
  // Table C-2
  var QeTable = [
    {qe: 0x5601, nmps: 1, nlps: 1, switchFlag: 1},
    {qe: 0x3401, nmps: 2, nlps: 6, switchFlag: 0},
    {qe: 0x1801, nmps: 3, nlps: 9, switchFlag: 0},
    {qe: 0x0AC1, nmps: 4, nlps: 12, switchFlag: 0},
    {qe: 0x0521, nmps: 5, nlps: 29, switchFlag: 0},
    {qe: 0x0221, nmps: 38, nlps: 33, switchFlag: 0},
    {qe: 0x5601, nmps: 7, nlps: 6, switchFlag: 1},
    {qe: 0x5401, nmps: 8, nlps: 14, switchFlag: 0},
    {qe: 0x4801, nmps: 9, nlps: 14, switchFlag: 0},
    {qe: 0x3801, nmps: 10, nlps: 14, switchFlag: 0},
    {qe: 0x3001, nmps: 11, nlps: 17, switchFlag: 0},
    {qe: 0x2401, nmps: 12, nlps: 18, switchFlag: 0},
    {qe: 0x1C01, nmps: 13, nlps: 20, switchFlag: 0},
    {qe: 0x1601, nmps: 29, nlps: 21, switchFlag: 0},
    {qe: 0x5601, nmps: 15, nlps: 14, switchFlag: 1},
    {qe: 0x5401, nmps: 16, nlps: 14, switchFlag: 0},
    {qe: 0x5101, nmps: 17, nlps: 15, switchFlag: 0},
    {qe: 0x4801, nmps: 18, nlps: 16, switchFlag: 0},
    {qe: 0x3801, nmps: 19, nlps: 17, switchFlag: 0},
    {qe: 0x3401, nmps: 20, nlps: 18, switchFlag: 0},
    {qe: 0x3001, nmps: 21, nlps: 19, switchFlag: 0},
    {qe: 0x2801, nmps: 22, nlps: 19, switchFlag: 0},
    {qe: 0x2401, nmps: 23, nlps: 20, switchFlag: 0},
    {qe: 0x2201, nmps: 24, nlps: 21, switchFlag: 0},
    {qe: 0x1C01, nmps: 25, nlps: 22, switchFlag: 0},
    {qe: 0x1801, nmps: 26, nlps: 23, switchFlag: 0},
    {qe: 0x1601, nmps: 27, nlps: 24, switchFlag: 0},
    {qe: 0x1401, nmps: 28, nlps: 25, switchFlag: 0},
    {qe: 0x1201, nmps: 29, nlps: 26, switchFlag: 0},
    {qe: 0x1101, nmps: 30, nlps: 27, switchFlag: 0},
    {qe: 0x0AC1, nmps: 31, nlps: 28, switchFlag: 0},
    {qe: 0x09C1, nmps: 32, nlps: 29, switchFlag: 0},
    {qe: 0x08A1, nmps: 33, nlps: 30, switchFlag: 0},
    {qe: 0x0521, nmps: 34, nlps: 31, switchFlag: 0},
    {qe: 0x0441, nmps: 35, nlps: 32, switchFlag: 0},
    {qe: 0x02A1, nmps: 36, nlps: 33, switchFlag: 0},
    {qe: 0x0221, nmps: 37, nlps: 34, switchFlag: 0},
    {qe: 0x0141, nmps: 38, nlps: 35, switchFlag: 0},
    {qe: 0x0111, nmps: 39, nlps: 36, switchFlag: 0},
    {qe: 0x0085, nmps: 40, nlps: 37, switchFlag: 0},
    {qe: 0x0049, nmps: 41, nlps: 38, switchFlag: 0},
    {qe: 0x0025, nmps: 42, nlps: 39, switchFlag: 0},
    {qe: 0x0015, nmps: 43, nlps: 40, switchFlag: 0},
    {qe: 0x0009, nmps: 44, nlps: 41, switchFlag: 0},
    {qe: 0x0005, nmps: 45, nlps: 42, switchFlag: 0},
    {qe: 0x0001, nmps: 45, nlps: 43, switchFlag: 0},
    {qe: 0x5601, nmps: 46, nlps: 46, switchFlag: 0}
  ];

  // C.3.5 Initialisation of the decoder (INITDEC)
  function ArithmeticDecoder(data, start, end) {
    this.data = data;
    this.bp = start;
    this.dataEnd = end;

    this.chigh = data[start];
    this.clow = 0;

    this.byteIn();

    this.chigh = ((this.chigh << 7) & 0xFFFF) | ((this.clow >> 9) & 0x7F);
    this.clow = (this.clow << 7) & 0xFFFF;
    this.ct -= 7;
    this.a = 0x8000;
  }

  ArithmeticDecoder.prototype = {
    // C.3.4 Compressed data input (BYTEIN)
    byteIn: function ArithmeticDecoder_byteIn() {
      var data = this.data;
      var bp = this.bp;
      if (data[bp] === 0xFF) {
        var b1 = data[bp + 1];
        if (b1 > 0x8F) {
          this.clow += 0xFF00;
          this.ct = 8;
        } else {
          bp++;
          this.clow += (data[bp] << 9);
          this.ct = 7;
          this.bp = bp;
        }
      } else {
        bp++;
        this.clow += bp < this.dataEnd ? (data[bp] << 8) : 0xFF00;
        this.ct = 8;
        this.bp = bp;
      }
      if (this.clow > 0xFFFF) {
        this.chigh += (this.clow >> 16);
        this.clow &= 0xFFFF;
      }
    },
    // C.3.2 Decoding a decision (DECODE)
    readBit: function ArithmeticDecoder_readBit(contexts, pos) {
      // contexts are packed into 1 byte:
      // highest 7 bits carry cx.index, lowest bit carries cx.mps
      var cx_index = contexts[pos] >> 1, cx_mps = contexts[pos] & 1;
      var qeTableIcx = QeTable[cx_index];
      var qeIcx = qeTableIcx.qe;
      var d;
      var a = this.a - qeIcx;

      if (this.chigh < qeIcx) {
        // exchangeLps
        if (a < qeIcx) {
          a = qeIcx;
          d = cx_mps;
          cx_index = qeTableIcx.nmps;
        } else {
          a = qeIcx;
          d = 1 ^ cx_mps;
          if (qeTableIcx.switchFlag === 1) {
            cx_mps = d;
          }
          cx_index = qeTableIcx.nlps;
        }
      } else {
        this.chigh -= qeIcx;
        if ((a & 0x8000) !== 0) {
          this.a = a;
          return cx_mps;
        }
        // exchangeMps
        if (a < qeIcx) {
          d = 1 ^ cx_mps;
          if (qeTableIcx.switchFlag === 1) {
            cx_mps = d;
          }
          cx_index = qeTableIcx.nlps;
        } else {
          d = cx_mps;
          cx_index = qeTableIcx.nmps;
        }
      }
      // C.3.3 renormD;
      do {
        if (this.ct === 0) {
          this.byteIn();
        }

        a <<= 1;
        this.chigh = ((this.chigh << 1) & 0xFFFF) | ((this.clow >> 15) & 1);
        this.clow = (this.clow << 1) & 0xFFFF;
        this.ct--;
      } while ((a & 0x8000) === 0);
      this.a = a;

      contexts[pos] = cx_index << 1 | cx_mps;
      return d;
    }
  };

  return ArithmeticDecoder;
})();