001/* ===========================================================
002 * JFreeChart : a free chart library for the Java(tm) platform
003 * ===========================================================
004 *
005 * (C) Copyright 2000-2009, by Object Refinery Limited and Contributors.
006 *
007 * Project Info:  http://www.jfree.org/jfreechart/index.html
008 *
009 * This library is free software; you can redistribute it and/or modify it
010 * under the terms of the GNU Lesser General Public License as published by
011 * the Free Software Foundation; either version 2.1 of the License, or
012 * (at your option) any later version.
013 *
014 * This library is distributed in the hope that it will be useful, but
015 * WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY
016 * or FITNESS FOR A PARTICULAR PURPOSE. See the GNU Lesser General Public
017 * License for more details.
018 *
019 * You should have received a copy of the GNU Lesser General Public
020 * License along with this library; if not, write to the Free Software
021 * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301,
022 * USA.
023 *
024 * [Java is a trademark or registered trademark of Sun Microsystems, Inc.
025 * in the United States and other countries.]
026 *
027 * ---------------------------------
028 * NormalDistributionFunction2D.java
029 * ---------------------------------
030 * (C)opyright 2004-2009, by Object Refinery Limited.
031 *
032 * Original Author:  David Gilbert (for Object Refinery Limited);
033 * Contributor(s):   -;
034 *
035 * Changes
036 * -------
037 * 25-May-2004 : Version 1 (DG);
038 * 21-Nov-2005 : Added getters for the mean and standard deviation (DG);
039 * 12-Feb-2009 : Precompute some constants from the function - see bug
040 *               2572016 (DG);
041 *
042 */
043
044package org.jfree.data.function;
045
046/**
047 * A normal distribution function.  See
048 * http://en.wikipedia.org/wiki/Normal_distribution.
049 */
050public class NormalDistributionFunction2D implements Function2D {
051
052    /** The mean. */
053    private double mean;
054
055    /** The standard deviation. */
056    private double std;
057
058    /** Precomputed factor for the function value. */
059    private double factor;
060
061    /** Precomputed denominator for the function value. */
062    private double denominator;
063
064    /**
065     * Constructs a new normal distribution function.
066     *
067     * @param mean  the mean.
068     * @param std  the standard deviation (> 0).
069     */
070    public NormalDistributionFunction2D(double mean, double std) {
071        if (std <= 0) {
072            throw new IllegalArgumentException("Requires 'std' > 0.");
073        }
074        this.mean = mean;
075        this.std = std;
076        // calculate constant values
077        this.factor = 1 / (std * Math.sqrt(2.0 * Math.PI));
078        this.denominator = 2 * std * std;
079    }
080
081    /**
082     * Returns the mean for the function.
083     *
084     * @return The mean.
085     */
086    public double getMean() {
087        return this.mean;
088    }
089    
090    /**
091     * Returns the standard deviation for the function.
092     *
093     * @return The standard deviation.
094     */
095    public double getStandardDeviation() {
096        return this.std;
097    }
098
099    /**
100     * Returns the function value.
101     *
102     * @param x  the x-value.
103     *
104     * @return The value.
105     */
106    public double getValue(double x) {
107        double z = x - this.mean;
108        return this.factor * Math.exp(-z * z / this.denominator);
109    }
110
111}