/*
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* Copyright (C) 2016 The Android Open Source Project
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*
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* Licensed under the Apache License, Version 2.0 (the "License");
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* you may not use this file except in compliance with the License.
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* You may obtain a copy of the License at
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*
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* http://www.apache.org/licenses/LICENSE-2.0
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*
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* Unless required by applicable law or agreed to in writing, software
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* distributed under the License is distributed on an "AS IS" BASIS,
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* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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* See the License for the specific language governing permissions and
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* limitations under the License
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*/
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package com.android.server.notification;
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/**
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* Exponentially weighted moving average estimator for event rate.
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*
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* {@hide}
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*/
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public class RateEstimator {
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private static final double RATE_ALPHA = 0.8;
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private static final double MINIMUM_DT = 0.0005;
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private Long mLastEventTime;
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private double mInterarrivalTime;
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public RateEstimator() {
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// assume something generous if we have no information
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mInterarrivalTime = 1000.0;
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}
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/** Update the estimate to account for an event that just happened. */
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public float update(long now) {
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float rate;
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if (mLastEventTime == null) {
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// No last event time, rate is zero.
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rate = 0f;
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} else {
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// Calculate the new inter-arrival time based on last event time.
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mInterarrivalTime = getInterarrivalEstimate(now);
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rate = (float) (1.0 / mInterarrivalTime);
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}
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mLastEventTime = now;
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return rate;
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}
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/** @return the estimated rate if there were a new event right now. */
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public float getRate(long now) {
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if (mLastEventTime == null) {
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return 0f;
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}
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return (float) (1.0 / getInterarrivalEstimate(now));
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}
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/** @return the average inter-arrival time if there were a new event right now. */
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private double getInterarrivalEstimate(long now) {
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double dt = ((double) (now - mLastEventTime)) / 1000.0;
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dt = Math.max(dt, MINIMUM_DT);
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// a*iat_old + (1-a)*(t_now-t_last)
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return (RATE_ALPHA * mInterarrivalTime + (1.0 - RATE_ALPHA) * dt);
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}
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}
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