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CloudSim Plus: A modern, highly extensible and easier-to-use Java 8 Framework for Modeling and Simulation of Cloud Computing Infrastructures and Services

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/*
 * Title:        CloudSim Toolkit
 * Descripimport java.util.Random;
mulation) Toolkit for Modeling and Simulation of Clouds
 * Licence:      GPL - http://www.gnu.org/copyleft/gpl.html
 *
 * Copyright (c) 2009-2012, The University of Melbourne, Australia
 */
package org.cloudbus.cloudsim.distributions;

import org.apache.commons.math3.distribution.UniformRealDistribution;

/**
 * A pseudo random number generator following the
 * Zipf distribution.
 *
 * @author Marcos Dias de Assuncao
 * @since CloudSim Toolkit 1.0
 */
public class ZipfDistr extends ContinuousDistributionAbstract {

    /**
     * The shape.
     */
    private final double shape;

    /**
     * The den.
     */
    private double den;

    /**
     * Instantiates a new Zipf pseudo random number generator.
     *
     * @param seed the seed
     * @param shape the shape
     * @param population the population
     */
    public ZipfDistr(long seed, double shape, int population) {
        super(new UniformRealDistribution(0, 1), seed);
        if (shape <= 0.0 || population < 1) {
            throw new IllegalArgumentException("Mean must be greater than 0.0 and population greater than 0");
        }

        this.shape = shape;
        computeDen(shape, population);
    }

    /**
     * Instantiates a new Zipf pseudo random number generator.
     *
     * @param shape the shape
     * @param population the population
     */
    public ZipfDistr(double shape, int population) {
        this(-1, shape, population);
    }

    @Override
    public double sample() {
        final double variate = super.sample();
        double num = 1;
        double nextNum = 1 + 1 / Math.pow(2, shape);
        double j = 3;

        while (variate > nextNum / den) {
            num = nextNum;
            nextNum += 1 / Math.pow(j, shape);
            j++;
        }

        return num / den;
    }

    /**
     * Compute the den.
     *
     * @param shape the shape
     * @param population the population
     */
    private void computeDen(double shape, int population) {
        den = 0.0;
        for (int j = 1; j <= population; j++) {
            den += 1 / Math.pow(j, shape);
        }
    }

}