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/*
 * Licensed to the Apache Software Foundation (ASF) under one or more
 * contributor license agreements.  See the NOTICE file distributed with
 * this work for additional information regarding copyright ownership.
 * The ASF licenses this file to You under the Apache License, Version 2.0
 * (the "License"); you may not use this file except in compliance with
 * the License.  You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */
package org.apache.commons.math.stat.descriptive.summary;

import java.io.Serializable;

import org.apache.commons.math.stat.descriptive.AbstractStorelessUnivariateStatistic;

/**
 * Returns the sum of the natural logs for this collection of values.  
 * 
 * Uses {@link java.lang.Math#log(double)} to compute the logs.  Therefore,
 * 

 * If any of values are < 0, the result is NaN.
 * If all values are non-negative and less than 
 * Double.POSITIVE_INFINITY,  but at least one value is 0, the
 * result is Double.NEGATIVE_INFINITY.
 * If both Double.POSITIVE_INFINITY and 
 * Double.NEGATIVE_INFINITY are among the values, the result is
 * NaN.
 * 

 * 
 * Note that this implementation is not synchronized. If 
 * multiple threads access an instance of this class concurrently, and at least
 * one of the threads invokes the increment() or 
 * clear() method, it must be synchronized externally.
 * 
 * @version $Revision: 617953 $ $Date: 2008-02-02 22:54:00 -0700 (Sat, 02 Feb 2008) $
 */
public class SumOfLogs extends AbstractStorelessUnivariateStatistic implements Serializable {

    /** Serializable version identifier */
    private static final long serialVersionUID = -370076995648386763L;    

    /**Number of values that have been added */
    private int n;
    
    /**
     * The currently running value
     */
    private double value;
    
    /**
     * Create a SumOfLogs instance
     */
    public SumOfLogs() {
       value = 0d;
       n = 0;
    }

    /**
     * @see org.apache.commons.math.stat.descriptive.StorelessUnivariateStatistic#increment(double)
     */
    public void increment(final double d) {
        value += Math.log(d);
        n++;
    }

    /**
     * @see org.apache.commons.math.stat.descriptive.StorelessUnivariateStatistic#getResult()
     */
    public double getResult() {
        if (n > 0) {
            return value;
        } else {
            return Double.NaN;
        }
    }

    /**
     * @see org.apache.commons.math.stat.descriptive.StorelessUnivariateStatistic#getN()
     */
    public long getN() {
        return n;
    }
    
    /**
     * @see org.apache.commons.math.stat.descriptive.StorelessUnivariateStatistic#clear()
     */
    public void clear() {
        value = 0d;
        n = 0;
    }

    /**
     * Returns the sum of the natural logs of the entries in the specified portion of
     * the input array, or Double.NaN if the designated subarray
     * is empty.
     * 
     * Throws IllegalArgumentException if the array is null.
     * 
     * See {@link SumOfLogs}.
     * 
     * @param values the input array
     * @param begin index of the first array element to include
     * @param length the number of elements to include
     * @return the sum of the natural logs of the values or Double.NaN if 
     * length = 0
     * @throws IllegalArgumentException if the array is null or the array index
     *  parameters are not valid
     */
    public double evaluate(final double[] values, final int begin, final int length) {
        double sumLog = Double.NaN;
        if (test(values, begin, length)) {
            sumLog = 0.0;
            for (int i = begin; i < begin + length; i++) {
                sumLog += Math.log(values[i]);
            }
        }
        return sumLog;
    }
}