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package jhplot.math.num.random;

/**
 * 

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

 * References:
 * 

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

 * 
 * @since 1.3
 * @version $Revision: 1.4 $ $Date: 2007/11/18 23:51:19 $
 */
public class ExponentialRandomVariable extends AbstractContinuousRandomVariable {

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

    /**
     * Default constructor. Mean is set to 1.
     */
    public ExponentialRandomVariable() {
        this(1.0);
    }

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

    /**
     * Create a random variable with the given parameters.
     * 
     * @param m the mean.
     * @param source the source generator.
     */
    public ExponentialRandomVariable(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 double nextRandomVariable(double m, RNG source) {
        double u;

        do {
            u = source.nextRandomNumber();
        } while (u <= 0.0);

        return -m * Math.log(u);
    }

    /**
     * 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 double nextRandomVariable() {
        return nextRandomVariable(getMean(), getSource());
    }

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