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package jhplot.math.num.random;

/**
 * 

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

 * References:
 * 

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

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

    /** The median. */
    private double median;

    /** The standard deviation. */
    private double scale;

    /**
     * Default constructor. The location parameter is set to zero and the scale
     * parameter is set to one.
     */
    public CauchyRandomVariable() {
        this(0.0, 1.0);
    }

    /**
     * Create a random variable with the given location and scale parameters.
     * 
     * @param m the location parameter.
     * @param s the scale parameter.
     */
    public CauchyRandomVariable(double m, double s) {
        this(m, s, new RandomRNG());
    }

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

    /**
     * Access the next random variable using the given generator.
     * 
     * @param m the location parameter.
     * @param s the scale parameter.
     * @param source the source generator.
     * @return the next random variable.
     */
    public static double nextRandomVariable(double m, double s, RNG source) {
        double v;
        double u1;

        do {
            u1 = source.nextRandomNumber();
            double u2 = source.nextRandomNumber();
            v = 2.0 * u2 - 1.0;
        } while (u1 * u1 + v * v >= 1);

        return (v / u1) * s + m;
    }

    /**
     * Access the location parameter.
     * 
     * @return the location parameter.
     */
    private double getMedian() {
        return median;
    }

    /**
     * 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(getMedian(), getScale(), getSource());
    }

    /**
     * Modify the location parameter.
     * 
     * @param m The new location parameter value.
     */
    private void setMedian(double m) {
        if (Double.isNaN(m)) {
            throw new IllegalArgumentException(
                "location parameter must be a valid number.");
        }
        this.median = m;
    }

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