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package jhplot.math.num.random;



/**
 * 

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

 * References:
 * 

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

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

    /** The scale parameter. */
    private double scale;

    /**
     * Default constructor. Alpha and scale are both set to 1.
     */
    public RayleighRandomVariable() {
        this(1.0);
    }

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

    /**
     * Create a random variable with the given parameters.
     * 
     * @param s the scale parameter.
     * @param source the source generator.
     */
    public RayleighRandomVariable(double s, RNG source) {
        super(source);
        setScale(s);
    }

    /**
     * Access the next random variable using the given generator.
     * 
     * @param s the scale parameter.
     * @param source the source generator.
     * @return the next random variable.
     */
    public static double nextRandomVariable(double s, RNG source) {
        double u;

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

        return Math.sqrt(-2.0 * Math.log(u)) * s;
    }

    /**
     * Access the scale parameter.
     * 
     * @return the scale parameter.
     */
    private double getScale() {
        return scale;
    }

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

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