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/*
* 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;
private defaultTTFB_: number;
private ttfb_: EWMA;
constructor(
slow: number,
fast: number,
defaultEstimate: number,
defaultTTFB: number = 100
) {
this.defaultEstimate_ = defaultEstimate;
this.minWeight_ = 0.001;
this.minDelayMs_ = 50;
this.slow_ = new EWMA(slow);
this.fast_ = new EWMA(fast);
this.defaultTTFB_ = defaultTTFB;
this.ttfb_ = new EWMA(slow);
}
update(slow: number, fast: number) {
const { slow_, fast_, ttfb_ } = this;
if (slow_.halfLife !== slow) {
this.slow_ = new EWMA(slow, slow_.getEstimate(), slow_.getTotalWeight());
}
if (fast_.halfLife !== fast) {
this.fast_ = new EWMA(fast, fast_.getEstimate(), fast_.getTotalWeight());
}
if (ttfb_.halfLife !== slow) {
this.ttfb_ = new EWMA(slow, ttfb_.getEstimate(), ttfb_.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);
}
sampleTTFB(ttfb: number) {
// weight is frequency curve applied to TTFB in seconds
// (longer times have less weight with expected input under 1 second)
const seconds = ttfb / 1000;
const weight = Math.sqrt(2) * Math.exp(-Math.pow(seconds, 2) / 2);
this.ttfb_.sample(weight, Math.max(ttfb, 5));
}
canEstimate(): boolean {
return this.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_;
}
}
getEstimateTTFB(): number {
if (this.ttfb_.getTotalWeight() >= this.minWeight_) {
return this.ttfb_.getEstimate();
} else {
return this.defaultTTFB_;
}
}
destroy() {}
}
export default EwmaBandWidthEstimator;