videojs-http-streaming/src/segment-loader.js

/**
 * @file segment-loader.js
 */
import {getMediaInfoForTime_ as getMediaInfoForTime} from './playlist';
import videojs from 'video.js';
import SourceUpdater from './source-updater';
import Config from './config';
import window from 'global/window';
import { createTransferableMessage } from './bin-utils';

// in ms
const CHECK_BUFFER_DELAY = 500;

/**
 * Determines if we should call endOfStream on the media source based
 * on the state of the buffer or if appened segment was the final
 * segment in the playlist.
 *
 * @param {Object} playlist a media playlist object
 * @param {Object} mediaSource the MediaSource object
 * @param {Number} segmentIndex the index of segment we last appended
 * @returns {Boolean} do we need to call endOfStream on the MediaSource
 */
const detectEndOfStream = function(playlist, mediaSource, segmentIndex) {
  if (!playlist) {
    return false;
  }

  let segments = playlist.segments;

  // determine a few boolean values to help make the branch below easier
  // to read
  let appendedLastSegment = segmentIndex === segments.length;

  // if we've buffered to the end of the video, we need to call endOfStream
  // so that MediaSources can trigger the `ended` event when it runs out of
  // buffered data instead of waiting for me
  return playlist.endList &&
    mediaSource.readyState === 'open' &&
    appendedLastSegment;
};

/**
 * Turns segment byterange into a string suitable for use in
 * HTTP Range requests
 */
const byterangeStr = function(byterange) {
  let byterangeStart;
  let byterangeEnd;

  // `byterangeEnd` is one less than `offset + length` because the HTTP range
  // header uses inclusive ranges
  byterangeEnd = byterange.offset + byterange.length - 1;
  byterangeStart = byterange.offset;
  return 'bytes=' + byterangeStart + '-' + byterangeEnd;
};

/**
 * Defines headers for use in the xhr request for a particular segment.
 */
const segmentXhrHeaders = function(segment) {
  let headers = {};

  if ('byterange' in segment) {
    headers.Range = byterangeStr(segment.byterange);
  }
  return headers;
};

/**
 * Returns a unique string identifier for a media initialization
 * segment.
 */
const initSegmentId = function(initSegment) {
  let byterange = initSegment.byterange || {
    length: Infinity,
    offset: 0
  };

  return [
    byterange.length, byterange.offset, initSegment.resolvedUri
  ].join(',');
};

/**
 * An object that manages segment loading and appending.
 *
 * @class SegmentLoader
 * @param {Object} options required and optional options
 * @extends videojs.EventTarget
 */
export default class SegmentLoader extends videojs.EventTarget {
  constructor(options) {
    super();
    // check pre-conditions
    if (!options) {
      throw new TypeError('Initialization options are required');
    }
    if (typeof options.currentTime !== 'function') {
      throw new TypeError('No currentTime getter specified');
    }
    if (!options.mediaSource) {
      throw new TypeError('No MediaSource specified');
    }
    let settings = videojs.mergeOptions(videojs.options.hls, options);

    // public properties
    this.state = 'INIT';
    this.bandwidth = settings.bandwidth;
    this.throughput = {rate: 0, count: 0};
    this.roundTrip = NaN;
    this.resetStats_();
    this.mediaIndex = null;

    // private settings
    this.hasPlayed_ = settings.hasPlayed;
    this.currentTime_ = settings.currentTime;
    this.seekable_ = settings.seekable;
    this.seeking_ = settings.seeking;
    this.setCurrentTime_ = settings.setCurrentTime;
    this.mediaSource_ = settings.mediaSource;
    this.hls_ = settings.hls;
    this.loaderType_ = settings.loaderType;

    // private instance variables
    this.checkBufferTimeout_ = null;
    this.error_ = void 0;
    this.currentTimeline_ = -1;
    this.xhr_ = null;
    this.pendingSegment_ = null;
    this.mimeType_ = null;
    this.sourceUpdater_ = null;
    this.xhrOptions_ = null;

    // Fragmented mp4 playback
    this.activeInitSegmentId_ = null;
    this.initSegments_ = {};

    this.decrypter_ = settings.decrypter;

    // Manages the tracking and generation of sync-points, mappings
    // between a time in the display time and a segment index within
    // a playlist
    this.syncController_ = settings.syncController;
    this.syncPoint_ = {
      segmentIndex: 0,
      time: 0
    };

    this.syncController_.on('syncinfoupdate', () => this.trigger('syncinfoupdate'));

    // ...for determining the fetch location
    this.fetchAtBuffer_ = false;

    if (settings.debug) {
      this.logger_ = videojs.log.bind(videojs, 'segment-loader', this.loaderType_, '->');
    }
  }

  /**
   * reset all of our media stats
   *
   * @private
   */
  resetStats_() {
    this.mediaBytesTransferred = 0;
    this.mediaRequests = 0;
    this.mediaTransferDuration = 0;
    this.mediaSecondsLoaded = 0;
  }

  /**
   * dispose of the SegmentLoader and reset to the default state
   */
  dispose() {
    this.state = 'DISPOSED';
    this.abort_();
    if (this.sourceUpdater_) {
      this.sourceUpdater_.dispose();
    }
    this.resetStats_();
  }

  /**
   * abort anything that is currently doing on with the SegmentLoader
   * and reset to a default state
   */
  abort() {
    if (this.state !== 'WAITING') {
      if (this.pendingSegment_) {
        this.pendingSegment_ = null;
      }
      return;
    }

    this.abort_();

    // don't wait for buffer check timeouts to begin fetching the
    // next segment
    if (!this.paused()) {
      this.state = 'READY';
      this.monitorBuffer_();
    }
  }

  /**
   * abort all pending xhr requests and null any pending segements
   *
   * @private
   */
  abort_() {
    if (this.xhr_) {
      this.xhr_.abort();
    }

    // clear out the segment being processed
    this.pendingSegment_ = null;
  }

  /**
   * set an error on the segment loader and null out any pending segements
   *
   * @param {Error} error the error to set on the SegmentLoader
   * @return {Error} the error that was set or that is currently set
   */
  error(error) {
    if (typeof error !== 'undefined') {
      this.error_ = error;
    }

    this.pendingSegment_ = null;
    return this.error_;
  }

  /**
   * load a playlist and start to fill the buffer
   */
  load() {
    // un-pause
    this.monitorBuffer_();

    // if we don't have a playlist yet, keep waiting for one to be
    // specified
    if (!this.playlist_) {
      return;
    }

    // not sure if this is the best place for this
    this.syncController_.setDateTimeMapping(this.playlist_);

    // if all the configuration is ready, initialize and begin loading
    if (this.state === 'INIT' && this.mimeType_) {
      return this.init_();
    }

    // if we're in the middle of processing a segment already, don't
    // kick off an additional segment request
    if (!this.sourceUpdater_ ||
        (this.state !== 'READY' &&
        this.state !== 'INIT')) {
      return;
    }

    this.state = 'READY';
  }

  /**
   * 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.sourceUpdater_ = new SourceUpdater(this.mediaSource_, this.mimeType_);
    this.resetEverything();
    return this.monitorBuffer_();
  }

  /**
   * set a playlist on the segment loader
   *
   * @param {PlaylistLoader} media the playlist to set on the segment loader
   */
  playlist(newPlaylist, options = {}) {
    if (!newPlaylist) {
      return;
    }

    let oldPlaylist = this.playlist_;
    let segmentInfo = this.pendingSegment_;

    this.playlist_ = newPlaylist;
    this.xhrOptions_ = options;

    // when we haven't started playing yet, the start of a live playlist
    // is always our zero-time so force a sync update each time the playlist
    // is refreshed from the server
    if (!this.hasPlayed_()) {
      newPlaylist.syncInfo = {
        mediaSequence: newPlaylist.mediaSequence,
        time: 0
      };
    }

    // in VOD, this is always a rendition switch (or we updated our syncInfo above)
    // in LIVE, we always want to update with new playlists (including refreshes)
    this.trigger('syncinfoupdate');

    // if we were unpaused but waiting for a playlist, start
    // buffering now
    if (this.mimeType_ && this.state === 'INIT' && !this.paused()) {
      return this.init_();
    }

    if (!oldPlaylist || oldPlaylist.uri !== newPlaylist.uri) {
      if (this.mediaIndex !== null) {
        // we must "resync" the segment loader when we switch renditions and
        // the segment loader is already synced to the previous rendition
        this.resyncLoader();
      }

      // the rest of this function depends on `oldPlaylist` being defined
      return;
    }

    // we reloaded the same playlist so we are in a live scenario
    // and we will likely need to adjust the mediaIndex
    let mediaSequenceDiff = newPlaylist.mediaSequence - oldPlaylist.mediaSequence;

    this.logger_('mediaSequenceDiff', mediaSequenceDiff);

    // update the mediaIndex on the SegmentLoader
    // this is important because we can abort a request and this value must be
    // equal to the last appended mediaIndex
    if (this.mediaIndex !== null) {
      this.mediaIndex -= mediaSequenceDiff;
    }

    // update the mediaIndex on the SegmentInfo object
    // this is important because we will update this.mediaIndex with this value
    // in `handleUpdateEnd_` after the segment has been successfully appended
    if (segmentInfo) {
      segmentInfo.mediaIndex -= mediaSequenceDiff;

      // we need to update the referenced segment so that timing information is
      // saved for the new playlist's segment, however, if the segment fell off the
      // playlist, we can leave the old reference and just lose the timing info
      if (segmentInfo.mediaIndex >= 0) {
        segmentInfo.segment = newPlaylist.segments[segmentInfo.mediaIndex];
      }
    }

    this.syncController_.saveExpiredSegmentInfo(oldPlaylist, newPlaylist);
  }

  /**
   * Prevent the loader from fetching additional segments. If there
   * is a segment request outstanding, it will finish processing
   * before the loader halts. A segment loader can be unpaused by
   * calling load().
   */
  pause() {
    if (this.checkBufferTimeout_) {
      window.clearTimeout(this.checkBufferTimeout_);

      this.checkBufferTimeout_ = null;
    }
  }

  /**
   * Returns whether the segment loader is fetching additional
   * segments when given the opportunity. This property can be
   * modified through calls to pause() and load().
   */
  paused() {
    return this.checkBufferTimeout_ === null;
  }

  /**
   * create/set the following mimetype on the SourceBuffer through a
   * SourceUpdater
   *
   * @param {String} mimeType the mime type string to use
   */
  mimeType(mimeType) {
    if (this.mimeType_) {
      return;
    }

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

  /**
   * Delete all the buffered data and reset the SegmentLoader
   */
  resetEverything() {
    this.resetLoader();
    this.remove(0, Infinity);
  }

  /**
   * Force the SegmentLoader to resync and start loading around the currentTime instead
   * of starting at the end of the buffer
   *
   * Useful for fast quality changes
   */
  resetLoader() {
    this.fetchAtBuffer_ = false;
    this.resyncLoader();
  }

  /**
   * Force the SegmentLoader to restart synchronization and make a conservative guess
   * before returning to the simple walk-forward method
   */
  resyncLoader() {
    this.mediaIndex = null;
    this.syncPoint_ = null;
  }

  /**
   * 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) {
    if (this.sourceUpdater_) {
      this.sourceUpdater_.remove(start, end);
    }
  }

  /**
   * (re-)schedule monitorBufferTick_ to run as soon as possible
   *
   * @private
   */
  monitorBuffer_() {
    if (this.checkBufferTimeout_) {
      window.clearTimeout(this.checkBufferTimeout_);
    }

    this.checkBufferTimeout_ = window.setTimeout(this.monitorBufferTick_.bind(this), 1);
  }

  /**
   * As long as the SegmentLoader is in the READY state, periodically
   * invoke fillBuffer_().
   *
   * @private
   */
  monitorBufferTick_() {
    if (this.state === 'READY') {
      this.fillBuffer_();
    }

    if (this.checkBufferTimeout_) {
      window.clearTimeout(this.checkBufferTimeout_);
    }

    this.checkBufferTimeout_ = window.setTimeout(this.monitorBufferTick_.bind(this),
                                                 CHECK_BUFFER_DELAY);
  }

  /**
   * 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.sourceUpdater_.updating()) {
      return;
    }

    if (!this.syncPoint_) {
      this.syncPoint_ = this.syncController_.getSyncPoint(this.playlist_,
                                                          this.mediaSource_.duration,
                                                          this.currentTimeline_,
                                                          this.currentTime_());
    }

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

    if (!segmentInfo) {
      return;
    }

    let isEndOfStream = detectEndOfStream(this.playlist_,
                                          this.mediaSource_,
                                          segmentInfo.mediaIndex);

    if (isEndOfStream) {
      this.mediaSource_.endOfStream();
      return;
    }

    if (segmentInfo.mediaIndex === this.playlist_.segments.length - 1 &&
        this.mediaSource_.readyState === 'ended' &&
        !this.seeking_()) {
      return;
    }

    // We will need to change timestampOffset of the sourceBuffer if either of
    // the following conditions are true:
    // - The segment.timeline !== this.currentTimeline
    //   (we are crossing a discontinuity somehow)
    // - The "timestampOffset" for the start of this segment is less than
    //   the currently set timestampOffset
    if (segmentInfo.timeline !== this.currentTimeline_ ||
        ((segmentInfo.startOfSegment !== null) &&
        segmentInfo.startOfSegment < this.sourceUpdater_.timestampOffset())) {
      this.syncController_.reset();
      segmentInfo.timestampOffset = segmentInfo.startOfSegment;
    }

    this.loadSegment_(segmentInfo);
  }

  /**
   * Determines what segment request should be made, given current playback
   * state.
   *
   * @param {TimeRanges} buffered - the state of the buffer
   * @param {Object} playlist - the playlist object to fetch segments from
   * @param {Number} mediaIndex - the previous mediaIndex fetched or null
   * @param {Boolean} hasPlayed - a flag indicating whether we have played or not
   * @param {Number} currentTime - the playback position in seconds
   * @param {Object} syncPoint - a segment info object that describes the
   * @returns {Object} a segment request object that describes the segment to load
   */
  checkBuffer_(buffered, playlist, mediaIndex, hasPlayed, currentTime, syncPoint) {
    let lastBufferedEnd = 0;
    let startOfSegment;

    if (buffered.length) {
      lastBufferedEnd = buffered.end(buffered.length - 1);
    }

    let bufferedTime = Math.max(0, lastBufferedEnd - currentTime);

    if (!playlist.segments.length) {
      return null;
    }

    // if there is plenty of content buffered, and the video has
    // been played before relax for awhile
    if (bufferedTime >= Config.GOAL_BUFFER_LENGTH) {
      return null;
    }

    // if the video has not yet played once, and we already have
    // one segment downloaded do nothing
    if (!hasPlayed && bufferedTime >= 1) {
      return null;
    }

    this.logger_('checkBuffer_',
      'mediaIndex:', mediaIndex,
      'hasPlayed:', hasPlayed,
      'currentTime:', currentTime,
      'syncPoint:', syncPoint,
      'fetchAtBuffer:', this.fetchAtBuffer_,
      'bufferedTime:', bufferedTime);

    // When the syncPoint is null, there is no way of determining a good
    // conservative segment index to fetch from
    // The best thing to do here is to get the kind of sync-point data by
    // making a request
    if (syncPoint === null) {
      mediaIndex = this.getSyncSegmentCandidate_(playlist);
      this.logger_('getSync', 'mediaIndex:', mediaIndex);
      return this.generateSegmentInfo_(playlist, mediaIndex, null, true);
    }

    // Under normal playback conditions fetching is a simple walk forward
    if (mediaIndex !== null) {
      this.logger_('walkForward', 'mediaIndex:', mediaIndex + 1);
      let segment = playlist.segments[mediaIndex];

      if (segment && segment.end) {
        startOfSegment = segment.end;
      } else {
        startOfSegment = lastBufferedEnd;
      }
      return this.generateSegmentInfo_(playlist, mediaIndex + 1, startOfSegment, false);
    }

    // There is a sync-point but the lack of a mediaIndex indicates that
    // we need to make a good conservative guess about which segment to
    // fetch
    if (this.fetchAtBuffer_) {
      // Find the segment containing the end of the buffer
      let mediaSourceInfo = getMediaInfoForTime(playlist,
                                                lastBufferedEnd,
                                                syncPoint.segmentIndex,
                                                syncPoint.time);

      mediaIndex = mediaSourceInfo.mediaIndex;
      startOfSegment = mediaSourceInfo.startTime;
    } else {
      // Find the segment containing currentTime
      let mediaSourceInfo = getMediaInfoForTime(playlist,
                                                currentTime,
                                                syncPoint.segmentIndex,
                                                syncPoint.time);

      mediaIndex = mediaSourceInfo.mediaIndex;
      startOfSegment = mediaSourceInfo.startTime;
    }
    this.logger_('getMediaIndexForTime',
      'mediaIndex:', mediaIndex,
      'startOfSegment:', startOfSegment);

    return this.generateSegmentInfo_(playlist, mediaIndex, startOfSegment, false);
  }

  /**
   * The segment loader has no recourse except to fetch a segment in the
   * current playlist and use the internal timestamps in that segment to
   * generate a syncPoint. This function returns a good candidate index
   * for that process.
   *
   * @param {Object} playlist - the playlist object to look for a
   * @returns {Number} An index of a segment from the playlist to load
   */
  getSyncSegmentCandidate_(playlist) {
    if (this.currentTimeline_ === -1) {
      return 0;
    }

    let segmentIndexArray = playlist.segments
      .map((s, i) => {
        return {
          timeline: s.timeline,
          segmentIndex: i
        };
      }).filter(s => s.timeline === this.currentTimeline_);

    if (segmentIndexArray.length) {
      return segmentIndexArray[Math.min(segmentIndexArray.length - 1, 1)].segmentIndex;
    }

    return Math.max(playlist.segments.length - 1, 0);
  }

  generateSegmentInfo_(playlist, mediaIndex, startOfSegment, isSyncRequest) {
    if (mediaIndex < 0 || mediaIndex >= playlist.segments.length) {
      return null;
    }

    let segment = playlist.segments[mediaIndex];

    return {
      // resolve the segment URL relative to the playlist
      uri: segment.resolvedUri,
      // the segment's mediaIndex at the time it was requested
      mediaIndex,
      // whether or not to update the SegmentLoader's state with this
      // segment's mediaIndex
      isSyncRequest,
      startOfSegment,
      // the segment's playlist
      playlist,
      // unencrypted bytes of the segment
      bytes: null,
      // when a key is defined for this segment, the encrypted bytes
      encryptedBytes: null,
      // The target timestampOffset for this segment when we append it
      // to the source buffer
      timestampOffset: null,
      // The timeline that the segment is in
      timeline: segment.timeline,
      // The expected duration of the segment in seconds
      duration: segment.duration,
      // retain the segment in case the playlist updates while doing an async process
      segment
    };
  }

  /**
   * load a specific segment from a request into the buffer
   *
   * @private
   */
  loadSegment_(segmentInfo) {
    let segment;
    let keyXhr;
    let initSegmentXhr;
    let segmentXhr;
    let removeToTime = 0;

    removeToTime = this.trimBuffer_(segmentInfo);

    if (removeToTime > 0) {
      this.sourceUpdater_.remove(0, removeToTime);
    }

    segment = segmentInfo.segment;

    // optionally, request the decryption key
    if (segment.key) {
      let keyRequestOptions = videojs.mergeOptions(this.xhrOptions_, {
        uri: segment.key.resolvedUri,
        responseType: 'arraybuffer'
      });

      keyXhr = this.hls_.xhr(keyRequestOptions, this.handleResponse_.bind(this));
    }

    // optionally, request the associated media init segment
    if (segment.map &&
        !this.initSegments_[initSegmentId(segment.map)]) {
      let initSegmentOptions = videojs.mergeOptions(this.xhrOptions_, {
        uri: segment.map.resolvedUri,
        responseType: 'arraybuffer',
        headers: segmentXhrHeaders(segment.map)
      });

      initSegmentXhr = this.hls_.xhr(initSegmentOptions,
                                     this.handleResponse_.bind(this));
    }
    this.pendingSegment_ = segmentInfo;

    let segmentRequestOptions = videojs.mergeOptions(this.xhrOptions_, {
      uri: segmentInfo.uri,
      responseType: 'arraybuffer',
      headers: segmentXhrHeaders(segment)
    });

    segmentXhr = this.hls_.xhr(segmentRequestOptions, this.handleResponse_.bind(this));
    segmentXhr.addEventListener('progress', (event) => {
      this.trigger(event);
    });

    this.xhr_ = {
      keyXhr,
      initSegmentXhr,
      segmentXhr,
      abort() {
        if (this.segmentXhr) {
          // Prevent error handler from running.
          this.segmentXhr.onreadystatechange = null;
          this.segmentXhr.abort();
          this.segmentXhr = null;
        }
        if (this.initSegmentXhr) {
          // Prevent error handler from running.
          this.initSegmentXhr.onreadystatechange = null;
          this.initSegmentXhr.abort();
          this.initSegmentXhr = null;
        }
        if (this.keyXhr) {
          // Prevent error handler from running.
          this.keyXhr.onreadystatechange = null;
          this.keyXhr.abort();
          this.keyXhr = null;
        }
      }
    };

    this.state = 'WAITING';
  }

  /**
   * trim the back buffer so we only remove content
   * on segment boundaries
   *
   * @private
   *
   * @param {Object} segmentInfo - the current segment
   * @returns {Number} removeToTime - the end point in time, in seconds
   * that the the buffer should be trimmed.
   */
  trimBuffer_(segmentInfo) {
    let seekable = this.seekable_();
    let currentTime = this.currentTime_();
    let removeToTime;

    // Chrome has a hard limit of 150mb of
    // buffer and a very conservative "garbage collector"
    // We manually clear out the old buffer to ensure
    // we don't trigger the QuotaExceeded error
    // on the source buffer during subsequent appends

    // If we have a seekable range use that as the limit for what can be removed safely
    // otherwise remove anything older than 1 minute before the current play head
    if (seekable.length &&
        seekable.start(0) > 0 &&
        seekable.start(0) < currentTime) {
      return seekable.start(0);
    }

    removeToTime = currentTime - 60;

    return removeToTime;
  }

  /**
   * triggered when a segment response is received
   *
   * @private
   */
  handleResponse_(error, request) {
    let segmentInfo;
    let segment;
    let view;

    // timeout of previously aborted request
    if (!this.xhr_ ||
        (request !== this.xhr_.segmentXhr &&
         request !== this.xhr_.keyXhr &&
         request !== this.xhr_.initSegmentXhr)) {
      return;
    }

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

    // if a request times out, reset bandwidth tracking
    if (request.timedout) {
      this.abort_();
      this.bandwidth = 1;
      this.roundTrip = NaN;
      this.state = 'READY';
      return this.trigger('progress');
    }

    // trigger an event for other errors
    if (!request.aborted && error) {
      // abort will clear xhr_
      let keyXhrRequest = this.xhr_.keyXhr;

      this.abort_();
      this.error({
        status: request.status,
        message: request === keyXhrRequest ?
          'HLS key request error at URL: ' + segment.key.uri :
          'HLS segment request error at URL: ' + segmentInfo.uri,
        code: 2,
        xhr: request
      });
      this.state = 'READY';
      this.pause();
      return this.trigger('error');
    }

    // stop processing if the request was aborted
    if (!request.response) {
      this.abort_();
      return;
    }

    if (request === this.xhr_.segmentXhr) {
      // the segment request is no longer outstanding
      this.xhr_.segmentXhr = null;
      segmentInfo.startOfAppend = Date.now();

      // calculate the download bandwidth based on segment request
      this.roundTrip = request.roundTripTime;
      this.bandwidth = request.bandwidth;

      // update analytics stats
      this.mediaBytesTransferred += request.bytesReceived || 0;
      this.mediaRequests += 1;
      this.mediaTransferDuration += request.roundTripTime || 0;

      if (segment.key) {
        segmentInfo.encryptedBytes = new Uint8Array(request.response);
      } else {
        segmentInfo.bytes = new Uint8Array(request.response);
      }
    }

    if (request === this.xhr_.keyXhr) {
      // the key request is no longer outstanding
      this.xhr_.keyXhr = null;

      if (request.response.byteLength !== 16) {
        this.abort_();
        this.error({
          status: request.status,
          message: 'Invalid HLS key at URL: ' + segment.key.uri,
          code: 2,
          xhr: request
        });
        this.state = 'READY';
        this.pause();
        return this.trigger('error');
      }

      view = new DataView(request.response);
      segment.key.bytes = new Uint32Array([
        view.getUint32(0),
        view.getUint32(4),
        view.getUint32(8),
        view.getUint32(12)
      ]);

      // if the media sequence is greater than 2^32, the IV will be incorrect
      // assuming 10s segments, that would be about 1300 years
      segment.key.iv = segment.key.iv || new Uint32Array([
        0, 0, 0, segmentInfo.mediaIndex + segmentInfo.playlist.mediaSequence
      ]);
    }

    if (request === this.xhr_.initSegmentXhr) {
      // the init segment request is no longer outstanding
      this.xhr_.initSegmentXhr = null;
      segment.map.bytes = new Uint8Array(request.response);
      this.initSegments_[initSegmentId(segment.map)] = segment.map;
    }

    if (!this.xhr_.segmentXhr && !this.xhr_.keyXhr && !this.xhr_.initSegmentXhr) {
      this.xhr_ = null;
      this.processResponse_();
    }
  }

  /**
   * Decrypt the segment that is being loaded if necessary
   *
   * @private
   */
  processResponse_() {
    if (!this.pendingSegment_) {
      this.state = 'READY';
      return;
    }

    this.state = 'DECRYPTING';

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

    if (segment.key) {
      // this is an encrypted segment
      // incrementally decrypt the segment
      this.decrypter_.postMessage(createTransferableMessage({
        source: this.loaderType_,
        encrypted: segmentInfo.encryptedBytes,
        key: segment.key.bytes,
        iv: segment.key.iv
      }), [
        segmentInfo.encryptedBytes.buffer,
        segment.key.bytes.buffer
      ]);
    } else {
      this.handleSegment_();
    }
  }

  /**
   * Handles response from the decrypter and attaches the decrypted bytes to the pending
   * segment
   *
   * @param {Object} data
   *        Response from decrypter
   * @method handleDecrypted_
   */
  handleDecrypted_(data) {
    const segmentInfo = this.pendingSegment_;
    const decrypted = data.decrypted;

    if (segmentInfo) {
      segmentInfo.bytes = new Uint8Array(decrypted.bytes,
                                         decrypted.byteOffset,
                                         decrypted.byteLength);
    }
    this.handleSegment_();
  }

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

    this.state = 'APPENDING';

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

    this.syncController_.probeSegmentInfo(segmentInfo);

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

    if (segmentInfo.timestampOffset !== null &&
        segmentInfo.timestampOffset !== this.sourceUpdater_.timestampOffset()) {
      this.sourceUpdater_.timestampOffset(segmentInfo.timestampOffset);
    }

    // if the media initialization segment is changing, append it
    // before the content segment
    if (segment.map) {
      let initId = initSegmentId(segment.map);

      if (!this.activeInitSegmentId_ ||
          this.activeInitSegmentId_ !== initId) {
        let initSegment = this.initSegments_[initId];

        this.sourceUpdater_.appendBuffer(initSegment.bytes, () => {
          this.activeInitSegmentId_ = initId;
        });
      }
    }

    segmentInfo.byteLength = segmentInfo.bytes.byteLength;
    if (typeof segment.start === 'number' && typeof segment.end === 'number') {
      this.mediaSecondsLoaded += segment.end - segment.start;
    } else {
      this.mediaSecondsLoaded += segment.duration;
    }

    this.sourceUpdater_.appendBuffer(segmentInfo.bytes,
                                     this.handleUpdateEnd_.bind(this));
  }

  /**
   * callback to run when appendBuffer is finished. detects if we are
   * in a good state to do things with the data we got, or if we need
   * to wait for more
   *
   * @private
   */
  handleUpdateEnd_() {
    this.logger_('handleUpdateEnd_', 'segmentInfo:', this.pendingSegment_);

    if (!this.pendingSegment_) {
      this.state = 'READY';
      if (!this.paused()) {
        this.monitorBuffer_();
      }
      return;
    }

    let segmentInfo = this.pendingSegment_;
    let segment = segmentInfo.segment;
    let isWalkingForward = this.mediaIndex !== null;

    this.pendingSegment_ = null;
    this.recordThroughput_(segmentInfo);

    this.state = 'READY';

    this.mediaIndex = segmentInfo.mediaIndex;
    this.fetchAtBuffer_ = true;
    this.currentTimeline_ = segmentInfo.timeline;

    // We must update the syncinfo to recalculate the seekable range before
    // the following conditional otherwise it may consider this a bad "guess"
    // and attempt to resync when the post-update seekable window and live
    // point would mean that this was the perfect segment to fetch
    this.trigger('syncinfoupdate');

    // If we previously appended a segment that ends more than 3 targetDurations before
    // the currentTime_ that means that our conservative guess was too conservative.
    // In that case, reset the loader state so that we try to use any information gained
    // from the previous request to create a new, more accurate, sync-point.
    if (segment.end &&
        this.currentTime_() - segment.end > segmentInfo.playlist.targetDuration * 3) {
      this.resetEverything();
      return;
    }

    // Don't do a rendition switch unless the SegmentLoader is already walking forward
    if (isWalkingForward) {
      this.trigger('progress');
    }

    // any time an update finishes and the last segment is in the
    // buffer, end the stream. this ensures the "ended" event will
    // fire if playback reaches that point.
    let isEndOfStream = detectEndOfStream(segmentInfo.playlist,
                                          this.mediaSource_,
                                          this.mediaIndex + 1);

    if (isEndOfStream) {
      this.mediaSource_.endOfStream();
    }

    if (!this.paused()) {
      this.monitorBuffer_();
    }
  }

  /**
   * Records the current throughput of the decrypt, transmux, and append
   * portion of the semgment pipeline. `throughput.rate` is a the cumulative
   * moving average of the throughput. `throughput.count` is the number of
   * data points in the average.
   *
   * @private
   * @param {Object} segmentInfo the object returned by loadSegment
   */
  recordThroughput_(segmentInfo) {
    let rate = this.throughput.rate;
    // Add one to the time to ensure that we don't accidentally attempt to divide
    // by zero in the case where the throughput is ridiculously high
    let segmentProcessingTime =
      Date.now() - segmentInfo.startOfAppend + 1;
    // Multiply by 8000 to convert from bytes/millisecond to bits/second
    let segmentProcessingThroughput =
      Math.floor((segmentInfo.byteLength / segmentProcessingTime) * 8 * 1000);

    // This is just a cumulative moving average calculation:
    //   newAvg = oldAvg + (sample - oldAvg) / (sampleCount + 1)
    this.throughput.rate +=
      (segmentProcessingThroughput - rate) / (++this.throughput.count);
  }

  /**
   * A debugging logger noop that is set to console.log only if debugging
   * is enabled globally
   *
   * @private
   */
  logger_() {}
}