/*! @license
* Shaka Player
* Copyright 2016 Google LLC
* SPDX-License-Identifier: Apache-2.0
*/
goog.provide('shaka.abr.EwmaBandwidthEstimator');
goog.require('shaka.abr.Ewma');
/**
* @summary
* This class tracks bandwidth samples and estimates available bandwidth.
* Based on the minimum of two exponentially-weighted moving averages with
* different half-lives.
*
*/
shaka.abr.EwmaBandwidthEstimator = class {
/** */
constructor() {
/**
* A fast-moving average.
* Half of the estimate is based on the last 2 seconds of sample history.
* @private {!shaka.abr.Ewma}
*/
this.fast_ = new shaka.abr.Ewma(2);
/**
* A slow-moving average.
* Half of the estimate is based on the last 5 seconds of sample history.
* @private {!shaka.abr.Ewma}
*/
this.slow_ = new shaka.abr.Ewma(5);
/**
* Number of bytes sampled.
* @private {number}
*/
this.bytesSampled_ = 0;
/**
* Minimum number of bytes sampled before we trust the estimate. If we have
* not sampled much data, our estimate may not be accurate enough to trust.
* If bytesSampled_ is less than minTotalBytes_, we use defaultEstimate_.
* This specific value is based on experimentation.
*
* @private {number}
* @const
*/
this.minTotalBytes_ = 128e3; // 128kB
/**
* Minimum number of bytes, under which samples are discarded. Our models
* do not include latency information, so connection startup time (time to
* first byte) is considered part of the download time. Because of this, we
* should ignore very small downloads which would cause our estimate to be
* too low.
* This specific value is based on experimentation.
*
* @private {number}
* @const
*/
this.minBytes_ = 16e3; // 16kB
}
/**
* Takes a bandwidth sample.
*
* @param {number} durationMs The amount of time, in milliseconds, for a
* particular request.
* @param {number} numBytes The total number of bytes transferred in that
* request.
*/
sample(
durationMs, numBytes) {
if (numBytes < this.minBytes_) {
return;
}
const bandwidth = 8000 * numBytes / durationMs;
const weight = durationMs / 1000;
this.bytesSampled_ += numBytes;
this.fast_.sample(weight, bandwidth);
this.slow_.sample(weight, bandwidth);
}
/**
* Gets the current bandwidth estimate.
*
* @param {number} defaultEstimate
* @return {number} The bandwidth estimate in bits per second.
*/
getBandwidthEstimate(defaultEstimate) {
if (this.bytesSampled_ < this.minTotalBytes_) {
return defaultEstimate;
}
// 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());
}
/**
* @return {boolean} True if there is enough data to produce a meaningful
* estimate.
*/
hasGoodEstimate() {
return this.bytesSampled_ >= this.minTotalBytes_;
}
};