pdf.js/src/core/run_length_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.
 */

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

const RunLengthStream = (function RunLengthStreamClosure() {
  // eslint-disable-next-line no-shadow
  function RunLengthStream(str, maybeLength) {
    this.str = str;
    this.dict = str.dict;

    DecodeStream.call(this, maybeLength);
  }

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

  RunLengthStream.prototype.readBlock = function RunLengthStream_readBlock() {
    // The repeatHeader has following format. The first byte defines type of run
    // and amount of bytes to repeat/copy: n = 0 through 127 - copy next n bytes
    // (in addition to the second byte from the header), n = 129 through 255 -
    // duplicate the second byte from the header (257 - n) times, n = 128 - end.
    const repeatHeader = this.str.getBytes(2);
    if (!repeatHeader || repeatHeader.length < 2 || repeatHeader[0] === 128) {
      this.eof = true;
      return;
    }

    let buffer;
    let bufferLength = this.bufferLength;
    let n = repeatHeader[0];
    if (n < 128) {
      // copy n bytes
      buffer = this.ensureBuffer(bufferLength + n + 1);
      buffer[bufferLength++] = repeatHeader[1];
      if (n > 0) {
        const source = this.str.getBytes(n);
        buffer.set(source, bufferLength);
        bufferLength += n;
      }
    } else {
      n = 257 - n;
      const b = repeatHeader[1];
      buffer = this.ensureBuffer(bufferLength + n + 1);
      for (let i = 0; i < n; i++) {
        buffer[bufferLength++] = b;
      }
    }
    this.bufferLength = bufferLength;
  };

  return RunLengthStream;
})();

export { RunLengthStream };