/**
* @license
* Copyright 2016 Google Inc.
*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
goog.provide('shaka.abr.EwmaBandwidthEstimator');
goog.require('shaka.abr.Ewma');
/**
* Tracks bandwidth samples and estimates available bandwidth.
* Based on the minimum of two exponentially-weighted moving averages with
* different half-lives.
*
* @constructor
* @struct
*/
shaka.abr.EwmaBandwidthEstimator = function() {
/**
* 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.
*/
shaka.abr.EwmaBandwidthEstimator.prototype.sample = function(
durationMs, numBytes) {
if (numBytes < this.minBytes_) {
return;
}
let bandwidth = 8000 * numBytes / durationMs;
let 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.
*/
shaka.abr.EwmaBandwidthEstimator.prototype.getBandwidthEstimate =
function(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.
*/
shaka.abr.EwmaBandwidthEstimator.prototype.hasGoodEstimate = function() {
return this.bytesSampled_ >= this.minTotalBytes_;
};