pdf.js/src/core/lzw_stream.js

/* 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.
 */
/* eslint-disable no-var */

import { DecodeStream } from "./stream.js";

var LZWStream = (function LZWStreamClosure() {
  // eslint-disable-next-line no-shadow
  function LZWStream(str, maybeLength, earlyChange) {
    this.str = str;
    this.dict = str.dict;
    this.cachedData = 0;
    this.bitsCached = 0;

    var maxLzwDictionarySize = 4096;
    var lzwState = {
      earlyChange,
      codeLength: 9,
      nextCode: 258,
      dictionaryValues: new Uint8Array(maxLzwDictionarySize),
      dictionaryLengths: new Uint16Array(maxLzwDictionarySize),
      dictionaryPrevCodes: new Uint16Array(maxLzwDictionarySize),
      currentSequence: new Uint8Array(maxLzwDictionarySize),
      currentSequenceLength: 0,
    };
    for (var i = 0; i < 256; ++i) {
      lzwState.dictionaryValues[i] = i;
      lzwState.dictionaryLengths[i] = 1;
    }
    this.lzwState = lzwState;

    DecodeStream.call(this, maybeLength);
  }

  LZWStream.prototype = Object.create(DecodeStream.prototype);

  LZWStream.prototype.readBits = function LZWStream_readBits(n) {
    var bitsCached = this.bitsCached;
    var cachedData = this.cachedData;
    while (bitsCached < n) {
      var c = this.str.getByte();
      if (c === -1) {
        this.eof = true;
        return null;
      }
      cachedData = (cachedData << 8) | c;
      bitsCached += 8;
    }
    this.bitsCached = bitsCached -= n;
    this.cachedData = cachedData;
    this.lastCode = null;
    return (cachedData >>> bitsCached) & ((1 << n) - 1);
  };

  LZWStream.prototype.readBlock = function LZWStream_readBlock() {
    var blockSize = 512;
    var estimatedDecodedSize = blockSize * 2,
      decodedSizeDelta = blockSize;
    var i, j, q;

    var lzwState = this.lzwState;
    if (!lzwState) {
      return; // eof was found
    }

    var earlyChange = lzwState.earlyChange;
    var nextCode = lzwState.nextCode;
    var dictionaryValues = lzwState.dictionaryValues;
    var dictionaryLengths = lzwState.dictionaryLengths;
    var dictionaryPrevCodes = lzwState.dictionaryPrevCodes;
    var codeLength = lzwState.codeLength;
    var prevCode = lzwState.prevCode;
    var currentSequence = lzwState.currentSequence;
    var currentSequenceLength = lzwState.currentSequenceLength;

    var decodedLength = 0;
    var currentBufferLength = this.bufferLength;
    var buffer = this.ensureBuffer(this.bufferLength + estimatedDecodedSize);

    for (i = 0; i < blockSize; i++) {
      var code = this.readBits(codeLength);
      var hasPrev = currentSequenceLength > 0;
      if (code < 256) {
        currentSequence[0] = code;
        currentSequenceLength = 1;
      } else if (code >= 258) {
        if (code < nextCode) {
          currentSequenceLength = dictionaryLengths[code];
          for (j = currentSequenceLength - 1, q = code; j >= 0; j--) {
            currentSequence[j] = dictionaryValues[q];
            q = dictionaryPrevCodes[q];
          }
        } else {
          currentSequence[currentSequenceLength++] = currentSequence[0];
        }
      } else if (code === 256) {
        codeLength = 9;
        nextCode = 258;
        currentSequenceLength = 0;
        continue;
      } else {
        this.eof = true;
        delete this.lzwState;
        break;
      }

      if (hasPrev) {
        dictionaryPrevCodes[nextCode] = prevCode;
        dictionaryLengths[nextCode] = dictionaryLengths[prevCode] + 1;
        dictionaryValues[nextCode] = currentSequence[0];
        nextCode++;
        codeLength =
          (nextCode + earlyChange) & (nextCode + earlyChange - 1)
            ? codeLength
            : Math.min(
                Math.log(nextCode + earlyChange) / 0.6931471805599453 + 1,
                12
              ) | 0;
      }
      prevCode = code;

      decodedLength += currentSequenceLength;
      if (estimatedDecodedSize < decodedLength) {
        do {
          estimatedDecodedSize += decodedSizeDelta;
        } while (estimatedDecodedSize < decodedLength);
        buffer = this.ensureBuffer(this.bufferLength + estimatedDecodedSize);
      }
      for (j = 0; j < currentSequenceLength; j++) {
        buffer[currentBufferLength++] = currentSequence[j];
      }
    }
    lzwState.nextCode = nextCode;
    lzwState.codeLength = codeLength;
    lzwState.prevCode = prevCode;
    lzwState.currentSequenceLength = currentSequenceLength;

    this.bufferLength = currentBufferLength;
  };

  return LZWStream;
})();

export { LZWStream };