/*
* Copyright (C) 2016 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 com.android.server.notification;
/**
* Exponentially weighted moving average estimator for event rate.
*
* {@hide}
*/
public class RateEstimator {
private static final double RATE_ALPHA = 0.8;
private static final double MINIMUM_DT = 0.0005;
private Long mLastEventTime;
private double mInterarrivalTime;
public RateEstimator() {
// assume something generous if we have no information
mInterarrivalTime = 1000.0;
}
/** Update the estimate to account for an event that just happened. */
public float update(long now) {
float rate;
if (mLastEventTime == null) {
// No last event time, rate is zero.
rate = 0f;
} else {
// Calculate the new inter-arrival time based on last event time.
mInterarrivalTime = getInterarrivalEstimate(now);
rate = (float) (1.0 / mInterarrivalTime);
}
mLastEventTime = now;
return rate;
}
/** @return the estimated rate if there were a new event right now. */
public float getRate(long now) {
if (mLastEventTime == null) {
return 0f;
}
return (float) (1.0 / getInterarrivalEstimate(now));
}
/** @return the average inter-arrival time if there were a new event right now. */
private double getInterarrivalEstimate(long now) {
double dt = ((double) (now - mLastEventTime)) / 1000.0;
dt = Math.max(dt, MINIMUM_DT);
// a*iat_old + (1-a)*(t_now-t_last)
return (RATE_ALPHA * mInterarrivalTime + (1.0 - RATE_ALPHA) * dt);
}
}