src/demux/mp4demuxer.js

/**
 * MP4 demuxer
 */
import { logger } from '../utils/logger';
import Event from '../events';

const UINT32_MAX = Math.pow(2, 32) - 1;

class MP4Demuxer {
  constructor (observer, remuxer) {
    this.observer = observer;
    this.remuxer = remuxer;
  }

  resetTimeStamp (initPTS) {
    this.initPTS = initPTS;
  }

  resetInitSegment (initSegment, audioCodec, videoCodec, duration) {
    // jshint unused:false
    if (initSegment && initSegment.byteLength) {
      const initData = this.initData = MP4Demuxer.parseInitSegment(initSegment);

      // default audio codec if nothing specified
      // TODO : extract that from initsegment
      if (audioCodec == null) {
        audioCodec = 'mp4a.40.5';
      }

      if (videoCodec == null) {
        videoCodec = 'avc1.42e01e';
      }

      const tracks = {};
      if (initData.audio && initData.video) {
        tracks.audiovideo = { container: 'video/mp4', codec: audioCodec + ',' + videoCodec, initSegment: duration ? initSegment : null };
      } else {
        if (initData.audio) {
          tracks.audio = { container: 'audio/mp4', codec: audioCodec, initSegment: duration ? initSegment : null };
        }

        if (initData.video) {
          tracks.video = { container: 'video/mp4', codec: videoCodec, initSegment: duration ? initSegment : null };
        }
      }
      this.observer.trigger(Event.FRAG_PARSING_INIT_SEGMENT, { tracks });
    } else {
      if (audioCodec) {
        this.audioCodec = audioCodec;
      }

      if (videoCodec) {
        this.videoCodec = videoCodec;
      }
    }
  }

  static probe (data) {
    // ensure we find a moof box in the first 16 kB
    return MP4Demuxer.findBox({ data: data, start: 0, end: Math.min(data.length, 16384) }, ['moof']).length > 0;
  }

  static bin2str (buffer) {
    return String.fromCharCode.apply(null, buffer);
  }

  static readUint16 (buffer, offset) {
    if (buffer.data) {
      offset += buffer.start;
      buffer = buffer.data;
    }

    const val = buffer[offset] << 8 |
                buffer[offset + 1];

    return val < 0 ? 65536 + val : val;
  }

  static readUint32 (buffer, offset) {
    if (buffer.data) {
      offset += buffer.start;
      buffer = buffer.data;
    }

    const val = buffer[offset] << 24 |
                buffer[offset + 1] << 16 |
                buffer[offset + 2] << 8 |
                buffer[offset + 3];
    return val < 0 ? 4294967296 + val : val;
  }

  static writeUint32 (buffer, offset, value) {
    if (buffer.data) {
      offset += buffer.start;
      buffer = buffer.data;
    }
    buffer[offset] = value >> 24;
    buffer[offset + 1] = (value >> 16) & 0xff;
    buffer[offset + 2] = (value >> 8) & 0xff;
    buffer[offset + 3] = value & 0xff;
  }

  // Find the data for a box specified by its path
  static findBox (data, path) {
    let results = [],
      i, size, type, end, subresults, start, endbox;

    if (data.data) {
      start = data.start;
      end = data.end;
      data = data.data;
    } else {
      start = 0;
      end = data.byteLength;
    }

    if (!path.length) {
      // short-circuit the search for empty paths
      return null;
    }

    for (i = start; i < end;) {
      size = MP4Demuxer.readUint32(data, i);
      type = MP4Demuxer.bin2str(data.subarray(i + 4, i + 8));
      endbox = size > 1 ? i + size : end;

      if (type === path[0]) {
        if (path.length === 1) {
          // this is the end of the path and we've found the box we were
          // looking for
          results.push({ data: data, start: i + 8, end: endbox });
        } else {
          // recursively search for the next box along the path
          subresults = MP4Demuxer.findBox({ data: data, start: i + 8, end: endbox }, path.slice(1));
          if (subresults.length) {
            results = results.concat(subresults);
          }
        }
      }
      i = endbox;
    }

    // we've finished searching all of data
    return results;
  }

  static parseSegmentIndex (initSegment) {
    const moov = MP4Demuxer.findBox(initSegment, ['moov'])[0];
    const moovEndOffset = moov ? moov.end : null; // we need this in case we need to chop of garbage of the end of current data

    let index = 0;
    let sidx = MP4Demuxer.findBox(initSegment, ['sidx']);
    let references;

    if (!sidx || !sidx[0]) {
      return null;
    }

    references = [];
    sidx = sidx[0];

    const version = sidx.data[0];

    // set initial offset, we skip the reference ID (not needed)
    index = version === 0 ? 8 : 16;

    const timescale = MP4Demuxer.readUint32(sidx, index);
    index += 4;

    // TODO: parse earliestPresentationTime and firstOffset
    // usually zero in our case
    let earliestPresentationTime = 0;
    let firstOffset = 0;

    if (version === 0) {
      index += 8;
    } else {
      index += 16;
    }

    // skip reserved
    index += 2;

    let startByte = sidx.end + firstOffset;

    const referencesCount = MP4Demuxer.readUint16(sidx, index);
    index += 2;

    for (let i = 0; i < referencesCount; i++) {
      let referenceIndex = index;

      const referenceInfo = MP4Demuxer.readUint32(sidx, referenceIndex);
      referenceIndex += 4;

      const referenceSize = referenceInfo & 0x7FFFFFFF;
      const referenceType = (referenceInfo & 0x80000000) >>> 31;

      if (referenceType === 1) {
        console.warn('SIDX has hierarchical references (not supported)');
        return;
      }

      const subsegmentDuration = MP4Demuxer.readUint32(sidx, referenceIndex);
      referenceIndex += 4;

      references.push({
        referenceSize,
        subsegmentDuration, // unscaled
        info: {
          duration: subsegmentDuration / timescale,
          start: startByte,
          end: startByte + referenceSize - 1
        }
      });

      startByte += referenceSize;

      // Skipping 1 bit for |startsWithSap|, 3 bits for |sapType|, and 28 bits
      // for |sapDelta|.
      referenceIndex += 4;

      // skip to next ref
      index = referenceIndex;
    }

    return {
      earliestPresentationTime,
      timescale,
      version,
      referencesCount,
      references,
      moovEndOffset
    };
  }

  /**
   * Parses an MP4 initialization segment and extracts stream type and
   * timescale values for any declared tracks. Timescale values indicate the
   * number of clock ticks per second to assume for time-based values
   * elsewhere in the MP4.
   *
   * To determine the start time of an MP4, you need two pieces of
   * information: the timescale unit and the earliest base media decode
   * time. Multiple timescales can be specified within an MP4 but the
   * base media decode time is always expressed in the timescale from
   * the media header box for the track:
   * ```
   * moov > trak > mdia > mdhd.timescale
   * moov > trak > mdia > hdlr
   * ```
   * @param init {Uint8Array} the bytes of the init segment
   * @return {object} a hash of track type to timescale values or null if
   * the init segment is malformed.
   */
  static parseInitSegment (initSegment) {
    let result = [];
    let traks = MP4Demuxer.findBox(initSegment, ['moov', 'trak']);

    traks.forEach(trak => {
      const tkhd = MP4Demuxer.findBox(trak, ['tkhd'])[0];
      if (tkhd) {
        let version = tkhd.data[tkhd.start];
        let index = version === 0 ? 12 : 20;
        let trackId = MP4Demuxer.readUint32(tkhd, index);

        const mdhd = MP4Demuxer.findBox(trak, ['mdia', 'mdhd'])[0];
        if (mdhd) {
          version = mdhd.data[mdhd.start];
          index = version === 0 ? 12 : 20;
          const timescale = MP4Demuxer.readUint32(mdhd, index);

          const hdlr = MP4Demuxer.findBox(trak, ['mdia', 'hdlr'])[0];
          if (hdlr) {
            const hdlrType = MP4Demuxer.bin2str(hdlr.data.subarray(hdlr.start + 8, hdlr.start + 12));
            let type = { 'soun': 'audio', 'vide': 'video' }[hdlrType];
            if (type) {
              // extract codec info. TODO : parse codec details to be able to build MIME type
              let codecBox = MP4Demuxer.findBox(trak, ['mdia', 'minf', 'stbl', 'stsd']);
              if (codecBox.length) {
                codecBox = codecBox[0];
                let codecType = MP4Demuxer.bin2str(codecBox.data.subarray(codecBox.start + 12, codecBox.start + 16));
                logger.log(`MP4Demuxer:${type}:${codecType} found`);
              }
              result[trackId] = { timescale: timescale, type: type };
              result[type] = { timescale: timescale, id: trackId };
            }
          }
        }
      }
    });
    return result;
  }

  /**
 * Determine the base media decode start time, in seconds, for an MP4
 * fragment. If multiple fragments are specified, the earliest time is
 * returned.
 *
 * The base media decode time can be parsed from track fragment
 * metadata:
 * ```
 * moof > traf > tfdt.baseMediaDecodeTime
 * ```
 * It requires the timescale value from the mdhd to interpret.
 *
 * @param timescale {object} a hash of track ids to timescale values.
 * @return {number} the earliest base media decode start time for the
 * fragment, in seconds
 */
  static getStartDTS (initData, fragment) {
    let trafs, baseTimes, result;

    // we need info from two childrend of each track fragment box
    trafs = MP4Demuxer.findBox(fragment, ['moof', 'traf']);

    // determine the start times for each track
    baseTimes = [].concat.apply([], trafs.map(function (traf) {
      return MP4Demuxer.findBox(traf, ['tfhd']).map(function (tfhd) {
        let id, scale, baseTime;

        // get the track id from the tfhd
        id = MP4Demuxer.readUint32(tfhd, 4);
        // assume a 90kHz clock if no timescale was specified
        scale = initData[id].timescale || 90e3;

        // get the base media decode time from the tfdt
        baseTime = MP4Demuxer.findBox(traf, ['tfdt']).map(function (tfdt) {
          let version, result;

          version = tfdt.data[tfdt.start];
          result = MP4Demuxer.readUint32(tfdt, 4);
          if (version === 1) {
            result *= Math.pow(2, 32);

            result += MP4Demuxer.readUint32(tfdt, 8);
          }
          return result;
        })[0];
        // convert base time to seconds
        return baseTime / scale;
      });
    }));

    // return the minimum
    result = Math.min.apply(null, baseTimes);
    return isFinite(result) ? result : 0;
  }

  static offsetStartDTS (initData, fragment, timeOffset) {
    MP4Demuxer.findBox(fragment, ['moof', 'traf']).map(function (traf) {
      return MP4Demuxer.findBox(traf, ['tfhd']).map(function (tfhd) {
      // get the track id from the tfhd
        let id = MP4Demuxer.readUint32(tfhd, 4);
        // assume a 90kHz clock if no timescale was specified
        let timescale = initData[id].timescale || 90e3;

        // get the base media decode time from the tfdt
        MP4Demuxer.findBox(traf, ['tfdt']).map(function (tfdt) {
          let version = tfdt.data[tfdt.start];
          let baseMediaDecodeTime = MP4Demuxer.readUint32(tfdt, 4);
          if (version === 0) {
            MP4Demuxer.writeUint32(tfdt, 4, baseMediaDecodeTime - timeOffset * timescale);
          } else {
            baseMediaDecodeTime *= Math.pow(2, 32);
            baseMediaDecodeTime += MP4Demuxer.readUint32(tfdt, 8);
            baseMediaDecodeTime -= timeOffset * timescale;
            baseMediaDecodeTime = Math.max(baseMediaDecodeTime, 0);
            const upper = Math.floor(baseMediaDecodeTime / (UINT32_MAX + 1));
            const lower = Math.floor(baseMediaDecodeTime % (UINT32_MAX + 1));
            MP4Demuxer.writeUint32(tfdt, 4, upper);
            MP4Demuxer.writeUint32(tfdt, 8, lower);
          }
        });
      });
    });
  }

  // feed incoming data to the front of the parsing pipeline
  append (data, timeOffset, contiguous, accurateTimeOffset) {
    let initData = this.initData;
    if (!initData) {
      this.resetInitSegment(data, this.audioCodec, this.videoCodec, false);
      initData = this.initData;
    }
    let startDTS, initPTS = this.initPTS;
    if (initPTS === undefined) {
      let startDTS = MP4Demuxer.getStartDTS(initData, data);
      this.initPTS = initPTS = startDTS - timeOffset;
      this.observer.trigger(Event.INIT_PTS_FOUND, { initPTS: initPTS });
    }
    MP4Demuxer.offsetStartDTS(initData, data, initPTS);
    startDTS = MP4Demuxer.getStartDTS(initData, data);
    this.remuxer.remux(initData.audio, initData.video, null, null, startDTS, contiguous, accurateTimeOffset, data);
  }

  destroy () {}
}

export default MP4Demuxer;