lib/core/arithmetic_decoder.js

/**
 * @licstart The following is the entire license notice for the
 * Javascript code in this page
 *
 * Copyright 2020 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.
 *
 * @licend The above is the entire license notice for the
 * Javascript code in this page
 */
"use strict";

Object.defineProperty(exports, "__esModule", {
  value: true
});
exports.ArithmeticDecoder = void 0;
const 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
}];

class ArithmeticDecoder {
  constructor(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;
  }

  byteIn() {
    const data = this.data;
    let bp = this.bp;

    if (data[bp] === 0xff) {
      if (data[bp + 1] > 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;
    }
  }

  readBit(contexts, pos) {
    let cx_index = contexts[pos] >> 1,
        cx_mps = contexts[pos] & 1;
    const qeTableIcx = QeTable[cx_index];
    const qeIcx = qeTableIcx.qe;
    let d;
    let a = this.a - qeIcx;

    if (this.chigh < qeIcx) {
      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;
      }

      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;
      }
    }

    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;
  }

}

exports.ArithmeticDecoder = ArithmeticDecoder;