shaka-player/lib/offline/stream_bandwidth_estimator.js

/*! @license
 * Shaka Player
 * Copyright 2016 Google LLC
 * SPDX-License-Identifier: Apache-2.0
 */

goog.provide('shaka.offline.StreamBandwidthEstimator');

goog.require('shaka.log');


/**
 * A utility class to help estimate the size of streams based on stream and
 * variant bandwidths. This class's main purpose is to isolate the logic in
 * creating non-zero bandwidth estimates for all streams so that each stream
 * will have some influence over the progress of the download.
 */
shaka.offline.StreamBandwidthEstimator = class {
  constructor() {
    /** @private {!Object.<number, number>} */
    this.estimateByStreamId_ = {};
  }

  /**
   * Add a new variant to the estimator. This will update the estimates for all
   * streams in the variant.
   *
   * @param {shaka.extern.Variant} variant
   */
  addVariant(variant) {
    // Three cases:
    //  1 - Only Audio
    //  2 - Only Video
    //  3 - Audio and Video

    const audio = variant.audio;
    const video = variant.video;

    // Case 1
    if (audio && !video) {
      const audioBitRate = audio.bandwidth || variant.bandwidth;
      this.setBitrate_(audio.id, audioBitRate);
    }

    // Case 2
    if (!audio && video) {
      const videoBitRate = video.bandwidth || variant.bandwidth;
      this.setBitrate_(video.id, videoBitRate);
    }

    // Case 3
    if (audio && video) {
      // Get the audio's bandwidth. If it is missing, default to our default
      // audio bandwidth.
      const audioBitRate =
          audio.bandwidth ||
          shaka.offline.StreamBandwidthEstimator.DEFAULT_AUDIO_BITRATE_;

      // Get the video's bandwidth. If it is missing, use the variant bandwidth
      // less the audio. If we get a negative bit rate, fall back to our
      // default video bandwidth.
      let videoBitRate = video.bandwidth || (variant.bandwidth - audioBitRate);
      if (videoBitRate <= 0) {
        shaka.log.warning(
            'Audio bit rate consumes variants bandwidth. Setting video ' +
            'bandwidth to match variant\'s bandwidth.');
        videoBitRate = variant.bandwidth;
      }

      this.setBitrate_(audio.id, audioBitRate);
      this.setBitrate_(video.id, videoBitRate);
    }
  }

  /**
   * @param {number} stream
   * @param {number} bitRate
   * @private
   */
  setBitrate_(stream, bitRate) {
    this.estimateByStreamId_[stream] = bitRate;
  }

  /**
   * Create an estimate for the text stream.
   *
   * @param {shaka.extern.Stream} text
   */
  addText(text) {
    this.estimateByStreamId_[text.id] =
        shaka.offline.StreamBandwidthEstimator.DEFAULT_TEXT_BITRATE_;
  }

  /**
   * Get the estimate for a segment that is part of a stream that has already
   * added to the estimator.
   *
   * @param {number} id
   * @param {!shaka.media.SegmentReference} segment
   * @return {number}
   */
  getSegmentEstimate(id, segment) {
    const duration = segment.endTime - segment.startTime;
    return this.getEstimate_(id) * duration;
  }

  /**
   * Get the estimate for an init segment for a stream that has already
   * added to the estimator.
   *
   * @param {number} id
   * @return {number}
   */
  getInitSegmentEstimate(id) {
    // Assume that the init segment is worth approximately half a second of
    // content.
    const duration = 0.5;
    return this.getEstimate_(id) * duration;
  }

  /**
   * @param {number} id
   * @return {number}
   * @private
   */
  getEstimate_(id) {
    let bitRate = this.estimateByStreamId_[id];

    if (bitRate == null) {
      bitRate = 0;
      shaka.log.error(
          'Asking for bitrate of stream not given to the estimator');
    }

    if (bitRate == 0) {
      shaka.log.warning(
          'Using bitrate of 0, this stream won\'t affect progress');
    }

    return bitRate;
  }
};


/**
 * Since audio bandwidth does not vary much, we are going to use a constant
 * approximation for audio bit rate allowing use to more accurately guess at
 * the video bitrate.
 *
 * YouTube's suggested bitrate for stereo audio is 384 kbps so we are going to
 * assume that: https://support.google.com/youtube/answer/1722171?hl=en
 *
 * @const {number}
 * @private
 */
shaka.offline.StreamBandwidthEstimator.DEFAULT_AUDIO_BITRATE_ = 393216;


/**
 * Since we don't normally get the bitrate for text, we still want to create
 * some approximation so that it can influence progress. This will use the
 * bitrate from "Tears of Steal" to give some kind of data-driven result.
 *
 * The file size for English subtitles is 4.7 KB. The video is 12:14 long,
 * which means that the text's bit rate is around 52 bps.
 *
 * @const {number}
 * @private
 */
shaka.offline.StreamBandwidthEstimator.DEFAULT_TEXT_BITRATE_ = 52;