SlidingPercentileBandwidthStatistic.java
/*
* Copyright 2021 The Android Open Source Project
*
* 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.
*/
package androidx.media3.exoplayer.upstream.experimental;
import static androidx.media3.common.util.Assertions.checkArgument;
import static androidx.media3.exoplayer.upstream.experimental.BandwidthEstimator.ESTIMATE_NOT_AVAILABLE;
import androidx.media3.common.util.UnstableApi;
import androidx.media3.common.util.Util;
import java.util.ArrayDeque;
import java.util.TreeSet;
/**
* A {@link BandwidthStatistic} that calculates estimates based on a sliding window weighted
* percentile.
*/
@UnstableApi
public class SlidingPercentileBandwidthStatistic implements BandwidthStatistic {
/** The default maximum number of samples. */
public static final int DEFAULT_MAX_SAMPLES_COUNT = 10;
/** The default percentile to return. */
public static final double DEFAULT_PERCENTILE = 0.5;
private final int maxSampleCount;
private final double percentile;
private final ArrayDeque<Sample> samples;
private final TreeSet<Sample> sortedSamples;
private double weightSum;
private long bitrateEstimate;
/**
* Creates an instance with a maximum of {@link #DEFAULT_MAX_SAMPLES_COUNT} samples, returning the
* {@link #DEFAULT_PERCENTILE}.
*/
public SlidingPercentileBandwidthStatistic() {
this(DEFAULT_MAX_SAMPLES_COUNT, DEFAULT_PERCENTILE);
}
/**
* Creates an instance.
*
* @param maxSampleCount The maximum number of samples.
* @param percentile The percentile to return. Must be in the range of [0-1].
*/
public SlidingPercentileBandwidthStatistic(int maxSampleCount, double percentile) {
checkArgument(percentile >= 0 && percentile <= 1);
this.maxSampleCount = maxSampleCount;
this.percentile = percentile;
this.samples = new ArrayDeque<>();
this.sortedSamples = new TreeSet<>();
this.bitrateEstimate = ESTIMATE_NOT_AVAILABLE;
}
@Override
public void addSample(long bytes, long durationUs) {
while (samples.size() >= maxSampleCount) {
Sample removedSample = samples.remove();
sortedSamples.remove(removedSample);
weightSum -= removedSample.weight;
}
double weight = Math.sqrt((double) bytes);
long bitrate = bytes * 8_000_000 / durationUs;
Sample sample = new Sample(bitrate, weight);
samples.add(sample);
sortedSamples.add(sample);
weightSum += weight;
bitrateEstimate = calculateBitrateEstimate();
}
@Override
public long getBandwidthEstimate() {
return bitrateEstimate;
}
@Override
public void reset() {
samples.clear();
sortedSamples.clear();
weightSum = 0;
bitrateEstimate = ESTIMATE_NOT_AVAILABLE;
}
private long calculateBitrateEstimate() {
if (samples.isEmpty()) {
return ESTIMATE_NOT_AVAILABLE;
}
double targetWeightSum = weightSum * percentile;
double previousPartialWeightSum = 0;
long previousSampleBitrate = 0;
double nextPartialWeightSum = 0;
for (Sample sample : sortedSamples) {
// The percentile position of each sample is the middle of its weight. Hence, we need to add
// half the weight to check whether the target percentile is before or after this sample.
nextPartialWeightSum += sample.weight / 2;
if (nextPartialWeightSum >= targetWeightSum) {
if (previousSampleBitrate == 0) {
return sample.bitrate;
}
// Interpolate between samples to get an estimate for the target percentile.
double partialBitrateBetweenSamples =
(sample.bitrate - previousSampleBitrate)
* (targetWeightSum - previousPartialWeightSum)
/ (nextPartialWeightSum - previousPartialWeightSum);
return previousSampleBitrate + (long) partialBitrateBetweenSamples;
}
previousSampleBitrate = sample.bitrate;
previousPartialWeightSum = nextPartialWeightSum;
nextPartialWeightSum += sample.weight / 2;
}
return previousSampleBitrate;
}
private static class Sample implements Comparable<Sample> {
private final long bitrate;
private final double weight;
public Sample(long bitrate, double weight) {
this.bitrate = bitrate;
this.weight = weight;
}
@Override
public int compareTo(Sample other) {
return Util.compareLong(this.bitrate, other.bitrate);
}
}
}