RateEstimator.java revision fd303327083fdf309b9d8a15343f9eb70c014b11 
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
17package com.android.server.notification;
18
19
20/**
21 * Exponentially weighted moving average estimator for event rate.
22 *
23 * {@hide}
24 */
25public 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 Float mInterarrivalTime;
30
31    public RateEstimator() {}
32
33    /** Update the estimate to account for an event that just happened. */
34    public float update(long now) {
35        float rate;
36        if (mLastEventTime == null) {
37            // No last event time, rate is zero.
38            rate = 0f;
39        } else {
40            // Calculate the new inter-arrival time based on last event time.
41            mInterarrivalTime = (float) getInterarrivalEstimate(now);
42            rate = (float) (1.0 / mInterarrivalTime);
43        }
44        mLastEventTime = now;
45        return rate;
46    }
47
48    /** @return the estimated rate if there were a new event right now. */
49    public float getRate(long now) {
50        if (mLastEventTime == null) {
51            return 0f;
52        }
53        return (float) (1.0 / getInterarrivalEstimate(now));
54    }
55
56    /** @return the average inter-arrival time if there were a new event right now. */
57    private double getInterarrivalEstimate(long now) {
58        double dt = ((double) (now - mLastEventTime)) / 1000.0;
59        dt = Math.max(dt, MINIMUM_DT);
60        if (mInterarrivalTime == null) {
61            // No last inter-arrival time, return the new value directly.
62            return dt;
63        }
64        // a*iat_old + (1-a)*(t_now-t_last)
65        return (RATE_ALPHA * mInterarrivalTime + (1.0 - RATE_ALPHA) * dt);
66    }
67}
68