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package jhplot.math.num.pdf;


/**
 * 

 * The Exponential distribution (1).
 * 
 * 

 * References:
 * 

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

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

    /** The mean. */
    private double mean;

    /**
     * Default constructor. Mean is set to 1.
     */
    public Exponential() {
        this(1.0);
    }

    /**
     * Create a distribution with the given mean.
     * 
     * @param m the mean.
     */
    public Exponential(double m) {
        super();
        setMean(m);
    }

    /**
     * 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 (x <= 0.0) {
            ret = 0.0;
        } else {
            ret = 1.0 - Math.exp(-x / getMean());
        }

        return ret;
    }

    /**
     * Access the mean.
     * 
     * @return the mean.
     */
    public double getMean() {
        return mean;
    }

    /**
     * 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 = 0.0;
        } else if (p == 1.0) {
            ret = Double.POSITIVE_INFINITY;
        } else {
            ret = -getMean() * Math.log(1.0 - p);
        }

        return ret;
    }

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