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package jhplot.math.num.random;

/**
 * 

 * A random variable generator for the Chi-Squared distribution.
 * 
 * 

 * References:
 * 

 * 
Wikipedia contributors, "Chi-square Distribution," Wikipedia, The Free
 * Encyclopedia, 
 * http://en.wikipedia.org/wiki/Chi-square_distribution
 * 
 * 

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

    /** The degrees of freedom. */
    private double degreesOfFreedom;

    /**
     * Default constructor. Degrees of freedom is set to 1.
     */
    public ChiSquaredRandomVariable() {
        this(1.0);
    }

    /**
     * Create a random variable with the given degrees of freedom.
     * 
     * @param df degrees of freedom.
     */
    public ChiSquaredRandomVariable(double df) {
        this(df, new RandomRNG());
    }

    /**
     * Create a random variable with the given parameters.
     * 
     * @param df degrees of freedom.
     * @param source the source generator.
     */
    public ChiSquaredRandomVariable(double df, RNG source) {
        super(source);
        setDegreesOfFreedom(df);
    }

    /**
     * Access the next random variable using the given generator.
     * 
     * @param df degrees of freedom.
     * @param source the source generator.
     * @return the next random variable.
     */
    public static double nextRandomVariable(double df, RNG source) {
        return GammaRandomVariable.nextRandomVariable(df / 2.0, 2.0, source);
    }

    /**
     * Access the degrees of freedom.
     * 
     * @return the degrees of freedom.
     */
    private double getDegreesOfFreedom() {
        return degreesOfFreedom;
    }

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

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