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 */
package jhplot.math.num.random;


/**
 * 

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

 * References:
 * 

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

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

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

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

    /** The shape parameter. */
    private double shape;

    /**
     * Default constructor. Shape and scale are both set to 1 and location is set to 0.
     */
    public WeibullRandomVariable() {
        this(1.0, 1.0, 0.0);
    }

    /**
     * Create a rnadom variable with the given shape and scale values.  Location is set to 0.
     * 
     * @param a the shape parameter.
     * @param b the scale parameter.
     */
    public WeibullRandomVariable(double a, double b) {
        this(a, b, 0.0, new RandomRNG());
    }

    /**
     * Create a random variable with the given parameters.
     * 
     * @param a the shape parameter.
     * @param b the scale parameter.
     * @param c the location parameter.
     */
    public WeibullRandomVariable(double a, double b, double c) {
        this(a, b, c, new RandomRNG());
    }

    /**
     * Create a random variable with the given parameters.
     * 
     * @param a the shape parameter.
     * @param b the scale parameter.
     * @param c the location parameter.
     * @param source the source generator.
     */
    public WeibullRandomVariable(double a, double b, double c, RNG source) {
        super(source);
        setShape(a);
        setScale(b);
        setLocation(c);
    }

    /**
     * Create a random variable with the given shape and scale values.  Location is set to 0.
     * 
     * @param a the shape parameter.
     * @param b the scale parameter.
     * @param source the source generator.
     */
    public WeibullRandomVariable(double a, double b, RNG source) {
        this(a, b, 0.0, source);
    }

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

        do {
            u = source.nextRandomNumber();
        } while (u <= 0.0);

        return Math.pow(-Math.log(u), 1.0 / shape) * scale + location;
    }

    /**
     * Access the location value.
     * 
     * @return the location.
     */
    private double getLocation() {
        return location;
    }

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

    /**
     * Access the shape parameter.
     * 
     * @return the shape parameter.
     */
    private double getShape() {
        return shape;
    }

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

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

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

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