001/* ===========================================================
002 * JFreeChart : a free chart library for the Java(tm) platform
003 * ===========================================================
004 *
005 * (C) Copyright 2000-2020, 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 2004-2016, 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
046package org.jfree.data.function;
047
048import java.io.Serializable;
049
050import org.jfree.chart.HashUtils;
051
052/**
053 * A normal distribution function.  See
054 * http://en.wikipedia.org/wiki/Normal_distribution.
055 */
056public 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    @Override
113    public double getValue(double x) {
114        double z = x - this.mean;
115        return this.factor * Math.exp(-z * z / this.denominator);
116    }
117
118    /**
119     * Tests this function for equality with an arbitrary object.
120     *
121     * @param obj  the object ({@code null} permitted).
122     *
123     * @return A boolean.
124     */
125    @Override
126    public boolean equals(Object obj) {
127        if (!(obj instanceof NormalDistributionFunction2D)) {
128            return false;
129        }
130        NormalDistributionFunction2D that = (NormalDistributionFunction2D) obj;
131        if (this.mean != that.mean) {
132            return false;
133        }
134        if (this.std != that.std) {
135            return false;
136        }
137        return true;
138    }
139
140    /**
141     * Returns a hash code for this instance.
142     *
143     * @return A hash code.
144     */
145    @Override
146    public int hashCode() {
147        int result = 29;
148        result = HashUtils.hashCode(result, this.mean);
149        result = HashUtils.hashCode(result, this.std);
150        return result;
151    }
152
153}