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      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 
     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. */
     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. */
     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. */
     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