src/utils/ewma-bandwidth-estimator.ts

/*
 * EWMA Bandwidth Estimator
 *  - heavily inspired from shaka-player
 * Tracks bandwidth samples and estimates available bandwidth.
 * Based on the minimum of two exponentially-weighted moving averages with
 * different half-lives.
 */

import EWMA from '../utils/ewma';

class EwmaBandWidthEstimator {
  private defaultEstimate_: number;
  private minWeight_: number;
  private minDelayMs_: number;
  private slow_: EWMA;
  private fast_: EWMA;

  constructor (slow: number, fast: number, defaultEstimate: number) {
    this.defaultEstimate_ = defaultEstimate;
    this.minWeight_ = 0.001;
    this.minDelayMs_ = 50;
    this.slow_ = new EWMA(slow);
    this.fast_ = new EWMA(fast);
  }

  update (slow: number, fast: number) {
    const { slow_, fast_ } = this;
    if (this.slow_.halfLife !== slow) {
      this.slow_ = new EWMA(slow, slow_.getEstimate(), slow_.getTotalWeight());
    }
    if (this.fast_.halfLife !== fast) {
      this.fast_ = new EWMA(fast, fast_.getEstimate(), fast_.getTotalWeight());
    }
  }

  sample (durationMs: number, numBytes: number) {
    durationMs = Math.max(durationMs, this.minDelayMs_);
    const numBits = 8 * numBytes;
    // weight is duration in seconds
    const durationS = durationMs / 1000;
    // value is bandwidth in bits/s
    const bandwidthInBps = numBits / durationS;
    this.fast_.sample(durationS, bandwidthInBps);
    this.slow_.sample(durationS, bandwidthInBps);
  }

  canEstimate (): boolean {
    const fast = this.fast_;
    return (fast && fast.getTotalWeight() >= this.minWeight_);
  }

  getEstimate (): number {
    if (this.canEstimate()) {
      // console.log('slow estimate:'+ Math.round(this.slow_.getEstimate()));
      // console.log('fast estimate:'+ Math.round(this.fast_.getEstimate()));
      // Take the minimum of these two estimates.  This should have the effect of
      // adapting down quickly, but up more slowly.
      return Math.min(this.fast_.getEstimate(), this.slow_.getEstimate());
    } else {
      return this.defaultEstimate_;
    }
  }

  destroy () {
  }
}
export default EwmaBandWidthEstimator;