# All files / src/demux mp4demuxer.js

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    1x

     /**
     * 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;