/*
 * Copyright (c) 2007, DoodleProject
 * All rights reserved.
 * 
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
 * are met:
 * 
 * Redistributions of source code must retain the above copyright
 * notice, this list of conditions and the following disclaimer.
 * 
 * Redistributions in binary form must reproduce the above copyright
 * notice, this list of conditions and the following disclaimer in
 * the documentation and/or other materials provided with the
 * distribution.
 * 
 * Neither the name of DoodleProject nor the names of its
 * contributors may be used to endorse or promote products derived
 * from this software without specific prior written permission.
 * 
 * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND
 * CONTRIBUTORS "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES,
 * INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF
 * MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
 * DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR CONTRIBUTORS
 * BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL,
 * EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED
 * TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
 * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON
 * ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR
 * TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF
 * THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
 * SUCH DAMAGE.
 */

package jhplot.math.num.random;


/**
 * 

 * A random variable generator for the Poisson distribution.
 * 
 * 

 * References:
 * 

 * 
Wikipedia contributors, "Poisson Distribution," Wikipedia, The Free
 * Encyclopedia, 
 * http://en.wikipedia.org/wiki/Poisson_distribution
 * 
 * 

 * 
 * @since 1.3
 * @version $Revision: 1.3 $ $Date: 2007/11/18 23:51:19 $
 */
public class PoissonRandomVariable extends AbstractDiscreteRandomVariable {

    /** The mean. */
    private double mean;

    /**
     * Default constructor. Mean is set to zero and standard deviation is set to
     * one.
     */
    public PoissonRandomVariable() {
        this(1.0);
    }

    /**
     * Create a random variable with the given mean.
     * 
     * @param m the mean.
     */
    public PoissonRandomVariable(double m) {
        this(m, new RandomRNG());
    }

    /**
     * Create a random variable with the given parameters.
     * 
     * @param m the mean.
     * @param source the source generator.
     */
    public PoissonRandomVariable(double m, RNG source) {
        super(source);
        setMean(m);
    }

    /**
     * Access the next random variable using the given generator.
     * 
     * @param m the mean.
     * @param source the source generator.
     * @return the next random variable.
     */
    public static int nextRandomVariable(double m, RNG source) {
        double u = Math.exp(-m);
        double p = 1.0;
        int k = 0;

        do {
            ++k;
            p *= source.nextRandomNumber();
        } while (p >= u);

        return k - 1;
    }

    /**
     * Access the mean.
     * 
     * @return the mean.
     */
    private double getMean() {
        return mean;
    }

    /**
     * Access the next random variable from this generator.
     * 
     * @return the next random variable.
     */
    public int nextRandomVariable() {
        return nextRandomVariable(getMean(), getSource());
    }

    /**
     * Modify the mean.
     * 
     * @param m the new mean value.
     */
    private void setMean(double m) {
        if (Double.isNaN(m) || m <= 0.0) {
            throw new IllegalArgumentException("mean must be positive.");
        }
        this.mean = m;
    }
}