/**
* Copyright (c) 2011 Yahoo! Inc. All rights reserved.
*
* 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. See accompanying
* LICENSE file.
*/
package com.yahoo.ycsb.generator;
import java.util.Random;
import com.yahoo.ycsb.Utils;
/**
* A generator of an exponential distribution. It produces a sequence
* of time intervals (integers) according to an exponential
* distribution. Smaller intervals are more frequent than larger
* ones, and there is no bound on the length of an interval. When you
* construct an instance of this class, you specify a parameter gamma,
* which corresponds to the rate at which events occur.
* Alternatively, 1/gamma is the average length of an interval.
*/
public class ExponentialGenerator extends IntegerGenerator
{
// What percentage of the readings should be within the most recent exponential.frac portion of the dataset?
public static final String EXPONENTIAL_PERCENTILE_PROPERTY="exponential.percentile";
public static final String EXPONENTIAL_PERCENTILE_DEFAULT="95";
// What fraction of the dataset should be accessed exponential.percentile of the time?
public static final String EXPONENTIAL_FRAC_PROPERTY = "exponential.frac";
public static final String EXPONENTIAL_FRAC_DEFAULT = "0.8571428571"; // 1/7
/**
* The exponential constant to use.
*/
double _gamma;
/******************************* Constructors **************************************/
/**
* Create an exponential generator with a mean arrival rate of
* gamma. (And half life of 1/gamma).
*/
public ExponentialGenerator(double mean)
{
_gamma = 1.0/mean;
}
public ExponentialGenerator(double percentile, double range)
{
_gamma = -Math.log(1.0-percentile/100.0) / range; //1.0/mean;
}
/****************************************************************************************/
/**
* Generate the next item. this distribution will be skewed toward lower integers; e.g. 0 will
* be the most popular, 1 the next most popular, etc.
* @param itemcount The number of items in the distribution.
* @return The next item in the sequence.
*/
@Override
public int nextInt()
{
return (int)nextLong();
}
/**
* Generate the next item as a long.
*
* @param itemcount The number of items in the distribution.
* @return The next item in the sequence.
*/
public long nextLong()
{
return (long) (-Math.log(Utils.random().nextDouble()) / _gamma);
}
@Override
public double mean() {
return 1.0/_gamma;
}
public static void main(String args[]) {
ExponentialGenerator e = new ExponentialGenerator(90, 100);
int j = 0;
for(int i = 0; i < 1000; i++) {
if(e.nextInt() < 100) {
j++;
}
}
System.out.println("Got " + j + " hits. Expect 900");
}
}