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    1   /*
    2    * Copyright 2003-2004 The Apache Software Foundation.
    3    *
    4    * Licensed under the Apache License, Version 2.0 (the "License");
    5    * you may not use this file except in compliance with the License.
    6    * You may obtain a copy of the License at
    7    *
    8    *      http://www.apache.org/licenses/LICENSE-2.0
    9    *
    10   * Unless required by applicable law or agreed to in writing, software
    11   * distributed under the License is distributed on an "AS IS" BASIS,
    12   * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
    13   * See the License for the specific language governing permissions and
    14   * limitations under the License.
    15   */
    16  package org.apache.commons.math.stat.descriptive.summary;
    17
    18  import java.io.Serializable;
    19
    20  import org.apache.commons.math.stat.descriptive.AbstractStorelessUnivariateStatistic;
    21
    22  /***
    23   * Returns the sum of the natural logs for this collection of values.
    24   * <p>
    25   * Uses {@link java.lang.Math#log(double)} to compute the logs.  Therefore,
    26   * <ul>
    27   * <li>If any of values are < 0, the result is <code>NaN.</code></li>
    28   * <li>If all values are non-negative and less than
    29   * <code>Double.POSITIVE_INFINITY</code>,  but at least one value is 0, the
    30   * result is <code>Double.NEGATIVE_INFINITY.</code></li>
    31   * <li>If both <code>Double.POSITIVE_INFINITY</code> and
    32   * <code>Double.NEGATIVE_INFINITY</code> are among the values, the result is
    33   * <code>NaN.</code></li>
    34   * </ul>
    35   * <p>
    36   * <strong>Note that this implementation is not synchronized.</strong> If
    37   * multiple threads access an instance of this class concurrently, and at least
    38   * one of the threads invokes the <code>increment()</code> or
    39   * <code>clear()</code> method, it must be synchronized externally.
    40   *
    41   * @version $Revision: 1.1 $ $Date: 2004/10/08 05:08:18 $
    42   */
    43  public class SumOfLogs extends AbstractStorelessUnivariateStatistic implements Serializable {
    44
    45      /*** Serializable version identifier */
    46      static final long serialVersionUID = -370076995648386763L;
    47
    48      /***Number of values that have been added */
    49      private int n;
    50
    51      /***
    52       * The currently running value
    53       */
    54      private double value;
    55
    56      /***
    57       * Create a SumOfLogs instance
    58       */
    59      public SumOfLogs() {
    60         value = 0d;
    61         n = 0;
    62      }
    63
    64      /***
    65       * @see org.apache.commons.math.stat.descriptive.StorelessUnivariateStatistic#increment(double)
    66       */
    67      public void increment(final double d) {
    68          value += Math.log(d);
    69          n++;
    70      }
    71
    72      /***
    73       * @see org.apache.commons.math.stat.descriptive.StorelessUnivariateStatistic#getResult()
    74       */
    75      public double getResult() {
    76          if (n > 0) {
    77              return value;
    78          } else {
    79              return Double.NaN;
    80          }
    81      }
    82
    83      /***
    84       * @see org.apache.commons.math.stat.descriptive.StorelessUnivariateStatistic#getN()
    85       */
    86      public long getN() {
    87          return n;
    88      }
    89
    90      /***
    91       * @see org.apache.commons.math.stat.descriptive.StorelessUnivariateStatistic#clear()
    92       */
    93      public void clear() {
    94          value = 0d;
    95          n = 0;
    96      }
    97
    98      /***
    99       * Returns the sum of the natural logs of the entries in the specified portion of
    100      * the input array, or <code>Double.NaN</code> if the designated subarray
    101      * is empty.
    102      * <p>
    103      * Throws <code>IllegalArgumentException</code> if the array is null.
    104      * <p>
    105      * See {@link SumOfLogs}.
    106      *
    107      * @param values the input array
    108      * @param begin index of the first array element to include
    109      * @param length the number of elements to include
    110      * @return the sum of the natural logs of the values or Double.NaN if
    111      * length = 0
    112      * @throws IllegalArgumentException if the array is null or the array index
    113      *  parameters are not valid
    114      */
    115     public double evaluate(final double[] values, final int begin, final int length) {
    116         double sumLog = Double.NaN;
    117         if (test(values, begin, length)) {
    118             sumLog = 0.0;
    119             for (int i = begin; i < begin + length; i++) {
    120                 sumLog += Math.log(values[i]);
    121             }
    122         }
    123         return sumLog;
    124     }
    125 }