package org.apache.helix.healthcheck;
   
   /*
    * Licensed to the Apache Software Foundation (ASF) under one
    * or more contributor license agreements.  See the NOTICE file
    * distributed with this work for additional information
    * regarding copyright ownership.  The ASF licenses this file
    * to you 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.
   */
  
  import org.apache.log4j.Logger;
  
  public class DecayAggregationType implements AggregationType
  {
  
    private static final Logger logger = Logger
        .getLogger(DecayAggregationType.class);
  
    public final static String TYPE_NAME = "decay";
  
    double _decayFactor = 0.1;
  
    public DecayAggregationType(double df)
    {
      super();
      _decayFactor = df;
    }
  
    @Override
    public String getName()
    {
      StringBuilder sb = new StringBuilder();
      sb.append(TYPE_NAME);
      sb.append(DELIM);
      sb.append(_decayFactor);
      return sb.toString();
    }
  
    @Override
    public String merge(String iv, String ev, long prevTimestamp)
    {
      double incomingVal = Double.parseDouble(iv);
      double existingVal = Double.parseDouble(ev);
      long currTimestamp = System.currentTimeMillis();
      double minutesOld = (currTimestamp - prevTimestamp) / 60000.0;
      // come up with decay coeff for old value. More time passed, the more it
      // decays
      double oldDecayCoeff = Math.pow((1 - _decayFactor), minutesOld);
      return String
          .valueOf((double) (oldDecayCoeff * existingVal + (1 - oldDecayCoeff)
              * incomingVal));
    }
  }