001    /* ===========================================================
002     * JFreeChart : a free chart library for the Java(tm) platform
003     * ===========================================================
004     *
005     * (C) Copyright 2000-2011, 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     * [Oracle and Java are registered trademarks of Oracle and/or its affiliates. 
025     * Other names may be trademarks of their respective owners.]
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     * 28-May-2009 : Implemented equals() and hashCode(), and added serialization
042     *               support (DG);
043     *
044     */
045    
046    package org.jfree.data.function;
047    
048    import java.io.Serializable;
049    
050    import org.jfree.chart.HashUtilities;
051    
052    /**
053     * A normal distribution function.  See
054     * http://en.wikipedia.org/wiki/Normal_distribution.
055     */
056    public class NormalDistributionFunction2D implements Function2D, Serializable {
057    
058        /** The mean. */
059        private double mean;
060    
061        /** The standard deviation. */
062        private double std;
063    
064        /** Precomputed factor for the function value. */
065        private double factor;
066    
067        /** Precomputed denominator for the function value. */
068        private double denominator;
069    
070        /**
071         * Constructs a new normal distribution function.
072         *
073         * @param mean  the mean.
074         * @param std  the standard deviation (> 0).
075         */
076        public NormalDistributionFunction2D(double mean, double std) {
077            if (std <= 0) {
078                throw new IllegalArgumentException("Requires 'std' > 0.");
079            }
080            this.mean = mean;
081            this.std = std;
082            // calculate constant values
083            this.factor = 1 / (std * Math.sqrt(2.0 * Math.PI));
084            this.denominator = 2 * std * std;
085        }
086    
087        /**
088         * Returns the mean for the function.
089         *
090         * @return The mean.
091         */
092        public double getMean() {
093            return this.mean;
094        }
095        
096        /**
097         * Returns the standard deviation for the function.
098         *
099         * @return The standard deviation.
100         */
101        public double getStandardDeviation() {
102            return this.std;
103        }
104    
105        /**
106         * Returns the function value.
107         *
108         * @param x  the x-value.
109         *
110         * @return The value.
111         */
112        public double getValue(double x) {
113            double z = x - this.mean;
114            return this.factor * Math.exp(-z * z / this.denominator);
115        }
116    
117        /**
118         * Tests this function for equality with an arbitrary object.
119         *
120         * @param obj  the object (<code>null</code> permitted).
121         *
122         * @return A boolean.
123         */
124        public boolean equals(Object obj) {
125            if (!(obj instanceof NormalDistributionFunction2D)) {
126                return false;
127            }
128            NormalDistributionFunction2D that = (NormalDistributionFunction2D) obj;
129            if (this.mean != that.mean) {
130                return false;
131            }
132            if (this.std != that.std) {
133                return false;
134            }
135            return true;
136        }
137    
138        /**
139         * Returns a hash code for this instance.
140         *
141         * @return A hash code.
142         */
143        public int hashCode() {
144            int result = 29;
145            result = HashUtilities.hashCode(result, this.mean);
146            result = HashUtilities.hashCode(result, this.std);
147            return result;
148        }
149    
150    }