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package jhplot.math.num.pdf;


/**
 * 

 * The Cauchy distribution (1).
 * 
 * 

 * References:
 * 

 * 
 Eric W. Weisstein. "Cauchy Distribution." From MathWorld--A Wolfram Web
 * Resource. 
 * http://mathworld.wolfram.com/Cauchy.html 
 * 
 * 

 * 
 * @since 1.1
 * @version $Revision: 1.2 $ $Date: 2007/10/25 04:44:10 $
 */
public class Cauchy extends ContinuousDistribution {

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

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

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

    /**
     * Create a distribution with the given location and scale parameters.
     * 
     * @param m the location parameter.
     * @param s the scale parameter.
     */
    public Cauchy(double m, double s) {
        super();
        setMedian(m);
        setScale(s);
    }

    /**
     * The CDF for this distribution. This method returns P(X < x).
     * 
     * @param x the value at which the CDF is evaluated.
     * @return CDF for this distribution.
     */
    public double cumulativeProbability(double x) {
        double ret;

        if (Double.isNaN(x)) {
            ret = Double.NaN;
        } else if (Double.isInfinite(x)) {
            if (x < 0.0) {
                ret = 0.0;
            } else {
                ret = 1.0;
            }
        } else if (x == median) {
            ret = 0.5;
        } else {
            ret = 0.5 + (Math.atan((x - median) / scale) / Math.PI);
        }

        return ret;
    }

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

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

    /**
     * The inverse CDF for this distribution. This method returns x such that,
     * P(X < x) = p.
     * 
     * @param p the cumulative probability.
     * @return x
     */
    public double inverseCumulativeProbability(double p) {
        double ret;

        if (p < 0.0 || p > 1.0 || Double.isNaN(p)) {
            ret = Double.NaN;
        } else if (p == 0.0) {
            ret = Double.NEGATIVE_INFINITY;
        } else if (p == 1.0) {
            ret = Double.POSITIVE_INFINITY;
        } else if (p == 0.5) {
            ret = median;
        } else {
            ret = median + scale * Math.tan(Math.PI * (p - .5));
        }

        return ret;
    }

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