1 /* 2 * Copyright (C) 2016 The Android Open Source Project 3 * 4 * Licensed under the Apache License, Version 2.0 (the "License"); 5 * you may not use this file except in compliance with the License. 6 * You may obtain a copy of the License at 7 * 8 * http://www.apache.org/licenses/LICENSE-2.0 9 * 10 * Unless required by applicable law or agreed to in writing, software 11 * distributed under the License is distributed on an "AS IS" BASIS, 12 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. 13 * See the License for the specific language governing permissions and 14 * limitations under the License 15 */ 16 17 package com.android.server.notification; 18 19 20 /** 21 * Exponentially weighted moving average estimator for event rate. 22 * 23 * {@hide} 24 */ 25 public class RateEstimator { 26 private static final double RATE_ALPHA = 0.8; 27 private static final double MINIMUM_DT = 0.0005; 28 private Long mLastEventTime; 29 private double mInterarrivalTime; 30 RateEstimator()31 public RateEstimator() { 32 // assume something generous if we have no information 33 mInterarrivalTime = 1000.0; 34 } 35 36 /** Update the estimate to account for an event that just happened. */ update(long now)37 public float update(long now) { 38 float rate; 39 if (mLastEventTime == null) { 40 // No last event time, rate is zero. 41 rate = 0f; 42 } else { 43 // Calculate the new inter-arrival time based on last event time. 44 mInterarrivalTime = getInterarrivalEstimate(now); 45 rate = (float) (1.0 / mInterarrivalTime); 46 } 47 mLastEventTime = now; 48 return rate; 49 } 50 51 /** @return the estimated rate if there were a new event right now. */ getRate(long now)52 public float getRate(long now) { 53 if (mLastEventTime == null) { 54 return 0f; 55 } 56 return (float) (1.0 / getInterarrivalEstimate(now)); 57 } 58 59 /** @return the average inter-arrival time if there were a new event right now. */ getInterarrivalEstimate(long now)60 private double getInterarrivalEstimate(long now) { 61 double dt = ((double) (now - mLastEventTime)) / 1000.0; 62 dt = Math.max(dt, MINIMUM_DT); 63 // a*iat_old + (1-a)*(t_now-t_last) 64 return (RATE_ALPHA * mInterarrivalTime + (1.0 - RATE_ALPHA) * dt); 65 } 66 } 67