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1   /*
2    * Licensed to the Apache Software Foundation (ASF) under one or more
3    * contributor license agreements.  See the NOTICE file distributed with
4    * this work for additional information regarding copyright ownership.
5    * The ASF licenses this file to You under the Apache License, Version 2.0
6    * (the "License"); you may not use this file except in compliance with
7    * the License.  You may obtain a copy of the License at
8    *
9    *      http://www.apache.org/licenses/LICENSE-2.0
10   *
11   * Unless required by applicable law or agreed to in writing, software
12   * distributed under the License is distributed on an "AS IS" BASIS,
13   * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
14   * See the License for the specific language governing permissions and
15   * limitations under the License.
16   */
17  package org.apache.commons.math.stat.descriptive.moment;
18  
19  import java.io.Serializable;
20  
21  import org.apache.commons.math.MathRuntimeException;
22  import org.apache.commons.math.stat.descriptive.AbstractStorelessUnivariateStatistic;
23  import org.apache.commons.math.stat.descriptive.StorelessUnivariateStatistic;
24  import org.apache.commons.math.stat.descriptive.summary.SumOfLogs;
25  
26  /**
27   * Returns the <a href="http://www.xycoon.com/geometric_mean.htm">
28   * geometric mean </a> of the available values.
29   * <p>
30   * Uses a {@link SumOfLogs} instance to compute sum of logs and returns
31   * <code> exp( 1/n  (sum of logs) ).</code>  Therefore, </p>
32   * <ul>
33   * <li>If any of values are < 0, the result is <code>NaN.</code></li>
34   * <li>If all values are non-negative and less than 
35   * <code>Double.POSITIVE_INFINITY</code>,  but at least one value is 0, the 
36   * result is <code>0.</code></li>
37   * <li>If both <code>Double.POSITIVE_INFINITY</code> and 
38   * <code>Double.NEGATIVE_INFINITY</code> are among the values, the result is
39   * <code>NaN.</code></li>
40   * </ul> </p>
41   * <p>
42   * <strong>Note that this implementation is not synchronized.</strong> If 
43   * multiple threads access an instance of this class concurrently, and at least
44   * one of the threads invokes the <code>increment()</code> or 
45   * <code>clear()</code> method, it must be synchronized externally.</p>
46   * 
47   *
48   * @version $Revision: 780541 $ $Date: 2009-05-31 20:47:02 -0400 (Sun, 31 May 2009) $
49   */
50  public class GeometricMean extends AbstractStorelessUnivariateStatistic implements Serializable {
51  
52      /** Serializable version identifier */
53      private static final long serialVersionUID = -8178734905303459453L;  
54      
55      /** Wrapped SumOfLogs instance */
56      private StorelessUnivariateStatistic sumOfLogs;
57  
58      /**
59       * Create a GeometricMean instance
60       */
61      public GeometricMean() {
62          sumOfLogs = new SumOfLogs();
63      }
64      
65      /**
66       * Copy constructor, creates a new {@code GeometricMean} identical
67       * to the {@code original}
68       * 
69       * @param original the {@code GeometricMean} instance to copy
70       */
71      public GeometricMean(GeometricMean original) {
72          super();
73          copy(original, this);
74      }
75      
76      /**
77       * {@inheritDoc}
78       */
79      @Override
80      public GeometricMean copy() {
81          GeometricMean result = new GeometricMean();
82          copy(this, result);
83          return result;
84      }
85      
86      /**
87       * Create a GeometricMean instance using the given SumOfLogs instance
88       * @param sumOfLogs sum of logs instance to use for computation
89       */
90      public GeometricMean(SumOfLogs sumOfLogs) {
91          this.sumOfLogs = sumOfLogs;
92      }
93      
94      /**
95       * {@inheritDoc}
96       */
97      @Override
98      public void increment(final double d) {
99          sumOfLogs.increment(d);
100     }
101 
102     /**
103      * {@inheritDoc}
104      */
105     @Override
106     public double getResult() {
107         if (sumOfLogs.getN() > 0) {
108             return Math.exp(sumOfLogs.getResult() / sumOfLogs.getN());
109         } else {
110             return Double.NaN;
111         }
112     }
113 
114     /**
115      * {@inheritDoc}
116      */
117     @Override
118     public void clear() {
119         sumOfLogs.clear();
120     }
121 
122     /**
123      * Returns the geometric mean of the entries in the specified portion
124      * of the input array.
125      * <p>
126      * See {@link GeometricMean} for details on the computing algorithm.</p>
127      * <p>
128      * Throws <code>IllegalArgumentException</code> if the array is null.</p>
129      * 
130      * @param values input array containing the values
131      * @param begin first array element to include
132      * @param length the number of elements to include
133      * @return the geometric mean or Double.NaN if length = 0 or
134      * any of the values are &lt;= 0.
135      * @throws IllegalArgumentException if the input array is null or the array
136      * index parameters are not valid
137      */
138     @Override
139     public double evaluate(
140         final double[] values, final int begin, final int length) {
141         return Math.exp(
142             sumOfLogs.evaluate(values, begin, length) / length);
143     }
144     
145     /**
146      * {@inheritDoc}
147      */
148     public long getN() {
149         return sumOfLogs.getN();
150     }
151     
152     /**
153      * <p>Sets the implementation for the sum of logs.</p>
154      * <p>This method must be activated before any data has been added - i.e.,
155      * before {@link #increment(double) increment} has been used to add data; 
156      * otherwise an IllegalStateException will be thrown.</p>
157      * 
158      * @param sumLogImpl the StorelessUnivariateStatistic instance to use
159      * for computing the log sum
160      * @throws IllegalStateException if data has already been added 
161      *  (i.e if n > 0)
162      */
163     public void setSumLogImpl(
164             StorelessUnivariateStatistic sumLogImpl) {
165         checkEmpty();
166         this.sumOfLogs = sumLogImpl;
167     }
168     
169     /**
170      * Returns the currently configured sum of logs implementation
171      * 
172      * @return the StorelessUnivariateStatistic implementing the log sum
173      */
174     public StorelessUnivariateStatistic getSumLogImpl() {
175         return sumOfLogs;
176     }
177     
178     /**
179      * Copies source to dest.
180      * <p>Neither source nor dest can be null.</p>
181      * 
182      * @param source GeometricMean to copy
183      * @param dest GeometricMean to copy to
184      * @throws NullPointerException if either source or dest is null
185      */
186     public static void copy(GeometricMean source, GeometricMean dest) {
187         dest.sumOfLogs = source.sumOfLogs.copy();
188     }
189     
190     
191     /**
192      * Throws IllegalStateException if n > 0.
193      */
194     private void checkEmpty() {
195         if (getN() > 0) {
196             throw MathRuntimeException.createIllegalStateException(
197                     "{0} values have been added before statistic is configured",
198                     getN());
199         }
200     }
201 
202 }