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 */
package jhplot.math.num.random;


/**
 * 

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

 * References:
 * 

 * 
Wikipedia contributors, "F-distribution," Wikipedia, The Free
 * Encyclopedia, 
 * http://en.wikipedia.org/wiki/F_distribution
 * 
 * 

 * 
 * @since 1.3
 * @version $Revision: 1.2 $ $Date: 2007/11/08 17:37:57 $
 */
public class FRandomVariable extends AbstractContinuousRandomVariable {

    /** 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 FRandomVariable() {
        this(1.0, 1.0);
    }

    /**
     * Create a random variable 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 FRandomVariable(double dfn, double dfd) {
        this(dfn, dfd, new RandomRNG());
    }

    /**
     * Create a random variable with the given parameters.
     * 
     * @param dfn the numerator degrees of freedom.
     * @param dfd the denominator degrees of freedom.
     * @param source the source generator.
     */
    public FRandomVariable(double dfn, double dfd, RNG source) {
        super(source);
        setNumeratorDegreesOfFreedom(dfn);
        setDenominatorDegreesOfFreedom(dfd);
    }

    /**
     * Access the next random variable using the given generator.
     * 
     * @param dfn the numerator degrees of freedom.
     * @param dfd the denominator degrees of freedom.
     * @param source the source generator.
     * @return the next random variable.
     */
    public static double nextRandomVariable(double dfn, double dfd, RNG source) {
        double x1 = ChiSquaredRandomVariable.nextRandomVariable(dfn, source)
            / dfn;
        double x2 = ChiSquaredRandomVariable.nextRandomVariable(dfd, source);
        return x1 / x2;
    }

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

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

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

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