vtt-segment-loader.js

/**
 * @file vtt-segment-loader.js
 */
import SegmentLoader from './segment-loader';
import videojs from 'video.js';
import window from 'global/window';
import removeCuesFromTrack from
  'videojs-contrib-media-sources/es5/remove-cues-from-track.js';
import { initSegmentId } from './bin-utils';

const VTT_LINE_TERMINATORS =
  new Uint8Array('\n\n'.split('').map(char => char.charCodeAt(0)));

const uintToString = function(uintArray) {
  return String.fromCharCode.apply(null, uintArray);
};

/**
 * An object that manages segment loading and appending.
 *
 * @class VTTSegmentLoader
 * @param {Object} options required and optional options
 * @extends videojs.EventTarget
 */
export default class VTTSegmentLoader extends SegmentLoader {
  constructor(settings, options = {}) {
    super(settings, options);

    // SegmentLoader requires a MediaSource be specified or it will throw an error;
    // however, VTTSegmentLoader has no need of a media source, so delete the reference
    this.mediaSource_ = null;

    this.subtitlesTrack_ = null;
  }

  /**
   * Indicates which time ranges are buffered
   *
   * @return {TimeRange}
   *         TimeRange object representing the current buffered ranges
   */
  buffered_() {
    if (!this.subtitlesTrack_ || !this.subtitlesTrack_.cues.length) {
      return videojs.createTimeRanges();
    }

    const cues = this.subtitlesTrack_.cues;
    let start = cues[0].startTime;
    let end = cues[cues.length - 1].startTime;

    return videojs.createTimeRanges([[start, end]]);
  }

  /**
   * Gets and sets init segment for the provided map
   *
   * @param {Object} map
   *        The map object representing the init segment to get or set
   * @param {Boolean=} set
   *        If true, the init segment for the provided map should be saved
   * @return {Object}
   *         map object for desired init segment
   */
  initSegment(map, set = false) {
    if (!map) {
      return null;
    }

    const id = initSegmentId(map);
    let storedMap = this.initSegments_[id];

    if (set && !storedMap && map.bytes) {
      // append WebVTT line terminators to the media initialization segment if it exists
      // to follow the WebVTT spec (https://w3c.github.io/webvtt/#file-structure) that
      // requires two or more WebVTT line terminators between the WebVTT header and the
      // rest of the file
      const combinedByteLength = VTT_LINE_TERMINATORS.byteLength + map.bytes.byteLength;
      const combinedSegment = new Uint8Array(combinedByteLength);

      combinedSegment.set(map.bytes);
      combinedSegment.set(VTT_LINE_TERMINATORS, map.bytes.byteLength);

      this.initSegments_[id] = storedMap = {
        resolvedUri: map.resolvedUri,
        byterange: map.byterange,
        bytes: combinedSegment
      };
    }

    return storedMap || map;
  }

  /**
   * Returns true if all configuration required for loading is present, otherwise false.
   *
   * @return {Boolean} True if the all configuration is ready for loading
   * @private
   */
  couldBeginLoading_() {
    return this.playlist_ &&
           this.subtitlesTrack_ &&
           !this.paused();
  }

  /**
   * Once all the starting parameters have been specified, begin
   * operation. This method should only be invoked from the INIT
   * state.
   *
   * @private
   */
  init_() {
    this.state = 'READY';
    this.resetEverything();
    return this.monitorBuffer_();
  }

  /**
   * Set a subtitle track on the segment loader to add subtitles to
   *
   * @param {TextTrack=} track
   *        The text track to add loaded subtitles to
   * @return {TextTrack}
   *        Returns the subtitles track
   */
  track(track) {
    if (typeof track === 'undefined') {
      return this.subtitlesTrack_;
    }

    this.subtitlesTrack_ = track;

    // if we were unpaused but waiting for a sourceUpdater, start
    // buffering now
    if (this.state === 'INIT' && this.couldBeginLoading_()) {
      this.init_();
    }

    return this.subtitlesTrack_;
  }

  /**
   * Remove any data in the source buffer between start and end times
   * @param {Number} start - the start time of the region to remove from the buffer
   * @param {Number} end - the end time of the region to remove from the buffer
   */
  remove(start, end) {
    removeCuesFromTrack(start, end, this.subtitlesTrack_);
  }

  /**
   * fill the buffer with segements unless the sourceBuffers are
   * currently updating
   *
   * Note: this function should only ever be called by monitorBuffer_
   * and never directly
   *
   * @private
   */
  fillBuffer_() {
    if (!this.syncPoint_) {
      this.syncPoint_ = this.syncController_.getSyncPoint(this.playlist_,
                                                          this.duration_(),
                                                          this.currentTimeline_,
                                                          this.currentTime_());
    }

    // see if we need to begin loading immediately
    let segmentInfo = this.checkBuffer_(this.buffered_(),
                                        this.playlist_,
                                        this.mediaIndex,
                                        this.hasPlayed_(),
                                        this.currentTime_(),
                                        this.syncPoint_);

    segmentInfo = this.skipEmptySegments_(segmentInfo);

    if (!segmentInfo) {
      return;
    }

    if (this.syncController_.timestampOffsetForTimeline(segmentInfo.timeline) === null) {
      // We don't have the timestamp offset that we need to sync subtitles.
      // Rerun on a timestamp offset or user interaction.
      let checkTimestampOffset = () => {
        this.state = 'READY';
        if (!this.paused()) {
          // if not paused, queue a buffer check as soon as possible
          this.monitorBuffer_();
        }
      };

      this.syncController_.one('timestampoffset', checkTimestampOffset);
      this.state = 'WAITING_ON_TIMELINE';
      return;
    }

    this.loadSegment_(segmentInfo);
  }

  /**
   * Prevents the segment loader from requesting segments we know contain no subtitles
   * by walking forward until we find the next segment that we don't know whether it is
   * empty or not.
   *
   * @param {Object} segmentInfo
   *        a segment info object that describes the current segment
   * @return {Object}
   *         a segment info object that describes the current segment
   */
  skipEmptySegments_(segmentInfo) {
    while (segmentInfo && segmentInfo.segment.empty) {
      segmentInfo = this.generateSegmentInfo_(
        segmentInfo.playlist,
        segmentInfo.mediaIndex + 1,
        segmentInfo.startOfSegment + segmentInfo.duration,
        segmentInfo.isSyncRequest);
    }
    return segmentInfo;
  }

  /**
   * append a decrypted segement to the SourceBuffer through a SourceUpdater
   *
   * @private
   */
  handleSegment_() {
    if (!this.pendingSegment_ || !this.subtitlesTrack_) {
      this.state = 'READY';
      return;
    }

    this.state = 'APPENDING';

    let segmentInfo = this.pendingSegment_;
    let segment = segmentInfo.segment;

    // Make sure that vttjs has loaded, otherwise, wait till it finished loading
    if (typeof window.WebVTT !== 'function' &&
        this.subtitlesTrack_ &&
        this.subtitlesTrack_.tech_) {

      const loadHandler = () => {
        this.handleSegment_();
      };

      this.state = 'WAITING_ON_VTTJS';
      this.subtitlesTrack_.tech_.one('vttjsloaded', loadHandler);
      this.subtitlesTrack_.tech_.one('vttjserror', () => {
        this.subtitlesTrack_.tech_.off('vttjsloaded', loadHandler);
        this.error({
          message: 'Error loading vtt.js'
        });
        this.state = 'READY';
        this.pause();
        this.trigger('error');
      });

      return;
    }

    segment.requested = true;

    try {
      this.parseVTTCues_(segmentInfo);
    } catch (e) {
      this.error({
        message: e.message
      });
      this.state = 'READY';
      this.pause();
      return this.trigger('error');
    }

    this.updateTimeMapping_(segmentInfo,
                            this.syncController_.timelines[segmentInfo.timeline],
                            this.playlist_);

    if (segmentInfo.isSyncRequest) {
      this.trigger('syncinfoupdate');
      this.pendingSegment_ = null;
      this.state = 'READY';
      return;
    }

    segmentInfo.byteLength = segmentInfo.bytes.byteLength;

    this.mediaSecondsLoaded += segment.duration;

    if (segmentInfo.cues.length) {
     // remove any overlapping cues to prevent doubling
      this.remove(segmentInfo.cues[0].endTime,
                  segmentInfo.cues[segmentInfo.cues.length - 1].endTime);
    }

    segmentInfo.cues.forEach((cue) => {
      this.subtitlesTrack_.addCue(cue);
    });

    this.handleUpdateEnd_();
  }

  /**
   * Uses the WebVTT parser to parse the segment response
   *
   * @param {Object} segmentInfo
   *        a segment info object that describes the current segment
   * @private
   */
  parseVTTCues_(segmentInfo) {
    let decoder;
    let decodeBytesToString = false;

    if (typeof window.TextDecoder === 'function') {
      decoder = new window.TextDecoder('utf8');
    } else {
      decoder = window.WebVTT.StringDecoder();
      decodeBytesToString = true;
    }

    const parser = new window.WebVTT.Parser(window,
                                            window.vttjs,
                                            decoder);

    segmentInfo.cues = [];
    segmentInfo.timestampmap = { MPEGTS: 0, LOCAL: 0 };

    parser.oncue = segmentInfo.cues.push.bind(segmentInfo.cues);
    parser.ontimestampmap = (map) => segmentInfo.timestampmap = map;
    parser.onparsingerror = (error) => {
      videojs.log.warn('Error encountered when parsing cues: ' + error.message);
    };

    if (segmentInfo.segment.map) {
      let mapData = segmentInfo.segment.map.bytes;

      if (decodeBytesToString) {
        mapData = uintToString(mapData);
      }

      parser.parse(mapData);
    }

    let segmentData = segmentInfo.bytes;

    if (decodeBytesToString) {
      segmentData = uintToString(segmentData);
    }

    parser.parse(segmentData);
    parser.flush();
  }

  /**
   * Updates the start and end times of any cues parsed by the WebVTT parser using
   * the information parsed from the X-TIMESTAMP-MAP header and a TS to media time mapping
   * from the SyncController
   *
   * @param {Object} segmentInfo
   *        a segment info object that describes the current segment
   * @param {Object} mappingObj
   *        object containing a mapping from TS to media time
   * @param {Object} playlist
   *        the playlist object containing the segment
   * @private
   */
  updateTimeMapping_(segmentInfo, mappingObj, playlist) {
    const segment = segmentInfo.segment;

    if (!mappingObj) {
      // If the sync controller does not have a mapping of TS to Media Time for the
      // timeline, then we don't have enough information to update the cue
      // start/end times
      return;
    }

    if (!segmentInfo.cues.length) {
      // If there are no cues, we also do not have enough information to figure out
      // segment timing. Mark that the segment contains no cues so we don't re-request
      // an empty segment.
      segment.empty = true;
      return;
    }

    const timestampmap = segmentInfo.timestampmap;
    const diff = (timestampmap.MPEGTS / 90000) - timestampmap.LOCAL + mappingObj.mapping;

    segmentInfo.cues.forEach((cue) => {
      // First convert cue time to TS time using the timestamp-map provided within the vtt
      cue.startTime += diff;
      cue.endTime += diff;
    });

    if (!playlist.syncInfo) {
      const firstStart = segmentInfo.cues[0].startTime;
      const lastStart = segmentInfo.cues[segmentInfo.cues.length - 1].startTime;

      playlist.syncInfo = {
        mediaSequence: playlist.mediaSequence + segmentInfo.mediaIndex,
        time: Math.min(firstStart, lastStart - segment.duration)
      };
    }
  }
}