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