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 class RateEstimator {
26     private static final double RATE_ALPHA = 0.7;
27     private static final double MINIMUM_DT = 0.0005;
28 
29     private Long mLastEventTime;
30     private double mInterarrivalTime;
31 
RateEstimator()32     public RateEstimator() {
33         // assume something generous if we have no information
34         mInterarrivalTime = 1000.0;
35     }
36 
37     /** Update the estimate to account for an event that just happened. */
update(long now)38     public void update(long now) {
39         if (mLastEventTime != null) {
40             // Calculate the new inter-arrival time based on last event time.
41             mInterarrivalTime = getInterarrivalEstimate(now);
42         }
43         mLastEventTime = now;
44     }
45 
46     /** @return the estimated rate if there were a new event right now. */
getRate(long now)47     public float getRate(long now) {
48         if (mLastEventTime == null) {
49             return 0f;
50         }
51         return (float) (1.0 / getInterarrivalEstimate(now));
52     }
53 
54     /** @return the average inter-arrival time if there were a new event right now. */
getInterarrivalEstimate(long now)55     private double getInterarrivalEstimate(long now) {
56         double dt = ((double) (now - mLastEventTime)) / 1000.0;
57         dt = Math.max(dt, MINIMUM_DT);
58         // a*iat_old + (1-a)*(t_now-t_last)
59         return (RATE_ALPHA * mInterarrivalTime + (1.0 - RATE_ALPHA) * dt);
60     }
61 }
62