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package jhplot.math.num.pdf;

import jhplot.math.num.NumericException;
import jhplot.math.num.special.Beta;

/**
 * 

 * The F distribution (1).
 * 
 * 

 * References:
 * 

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

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

    /** The denominator degrees of freedom. */
    private double denominatorDegreesOfFreedom;

    /** The numerator degrees of freedom. */
    private double numeratorDegreesOfFreedom;

    /**
     * Default constructor. Numerator degrees of freedom and denominator degrees
     * of freedom are both set to 1.
     */
    public FDistribution() {
        this(1.0, 1.0);
    }

    /**
     * Create a distribution with the given numerator degrees of freedom and
     * denominator degrees of freedom.
     * 
     * @param dfn the numerator degrees of freedom.
     * @param dfd the denominator degrees of freedom.
     */
    public FDistribution(double dfn, double dfd) {
        super();
        setNumeratorDegreesOfFreedom(dfn);
        setDenominatorDegreesOfFreedom(dfd);
    }

    /**
     * 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.
     * @throws NumericException if the cumulative probability can not be
     *         computed.
     */
    public double cumulativeProbability(double x) throws NumericException {
        double ret;
        if (Double.isNaN(x)) {
            ret = Double.NaN;
        } else if (x <= 0.0) {
            ret = 0.0;
        } else if (Double.isInfinite(x)) {
            ret = 1.0;
        } else {
            double n = getNumeratorDegreesOfFreedom();
            double m = getDenominatorDegreesOfFreedom();

            ret = Beta.regularizedBeta((n * x) / (m + n * x), 0.5 * n, 0.5 * m);
        }
        return ret;
    }

    /**
     * Access the denominator degrees of freedom.
     * 
     * @return the denominator degrees of freedom.
     */
    public double getDenominatorDegreesOfFreedom() {
        return denominatorDegreesOfFreedom;
    }

    /**
     * Access the numerator degrees of freedom.
     * 
     * @return the numerator degrees of freedom.
     */
    public double getNumeratorDegreesOfFreedom() {
        return numeratorDegreesOfFreedom;
    }

    /**
     * The inverse CDF for this distribution. This method returns x such that,
     * P(X < x) = p.
     * 
     * @param p the cumulative probability.
     * @return x
     * @throws NumericException if the inverse cumulative probability can not be
     *         computed.
     */
    public double inverseCumulativeProbability(double p)
        throws NumericException {
        double ret;

        if (p < 0.0 || p > 1.0 || Double.isNaN(p)) {
            ret = Double.NaN;
        } else if (p == 0.0) {
            ret = 0.0;
        } else if (p == 1.0) {
            ret = Double.POSITIVE_INFINITY;
        } else {
            ret = findInverseCumulativeProbability(p, 0.0, 10,
                Double.POSITIVE_INFINITY);
        }

        return ret;
    }

    /**
     * Modify the denominator degrees of freedom.
     * 
     * @param degreesOfFreedom the new denominator degrees of freedom.
     */
    public void setDenominatorDegreesOfFreedom(double degreesOfFreedom) {
        if (degreesOfFreedom <= 0.0 || Double.isNaN(degreesOfFreedom)) {
            throw new IllegalArgumentException(
                "degrees of freedom must be positive.");
        }
        this.denominatorDegreesOfFreedom = degreesOfFreedom;
    }

    /**
     * Modify the numerator degrees of freedom.
     * 
     * @param degreesOfFreedom the new numerator degrees of freedom.
     */
    public void setNumeratorDegreesOfFreedom(double degreesOfFreedom) {
        if (degreesOfFreedom <= 0.0 || Double.isNaN(degreesOfFreedom)) {
            throw new IllegalArgumentException(
                "degrees of freedom must be positive.");
        }
        this.numeratorDegreesOfFreedom = degreesOfFreedom;
    }
}