<commit_before>/*
 * 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.math3.stat.descriptive.moment;

import java.io.Serializable;

import org.apache.commons.math3.exception.NullArgumentException;
import org.apache.commons.math3.util.MathUtils;

/**
 * Computes a statistic related to the Fourth Central Moment.  Specifically,
 * what is computed is the sum of
 * <p>
 * (x_i - xbar) ^ 4, </p>
 * <p>
 * where the x_i are the
 * sample observations and xbar is the sample mean. </p>
 * <p>
 * The following recursive updating formula is used: </p>
 * <p>
 * Let <ul>
 * <li> dev = (current obs - previous mean) </li>
 * <li> m2 = previous value of {@link SecondMoment} </li>
 * <li> m2 = previous value of {@link ThirdMoment} </li>
 * <li> n = number of observations (including current obs) </li>
 * </ul>
 * Then </p>
 * <p>
 * new value = old value - 4 * (dev/n) * m3 + 6 * (dev/n)^2 * m2 + <br>
 * [n^2 - 3 * (n-1)] * dev^4 * (n-1) / n^3 </p>
 * <p>
 * Returns <code>Double.NaN</code> if no data values have been added and
 * returns <code>0</code> if there is just one value in the data set. </p>
 * <p>
 * <strong>Note that this implementation is not synchronized.</strong> If
 * multiple threads access an instance of this class concurrently, and at least
 * one of the threads invokes the <code>increment()</code> or
 * <code>clear()</code> method, it must be synchronized externally. </p>
 *
 */
class FourthMoment extends ThirdMoment implements Serializable{

    /** Serializable version identifier */
    private static final long serialVersionUID = 4763990447117157611L;

    /** fourth moment of values that have been added */
    private double m4;

    /**
     * Create a FourthMoment instance
     */<commit_msg>Made it clear that NaNs or infinities may yield NaN in moment computations.  JIRA: MATH-1146.<commit_after>/*
 * 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.math3.stat.descriptive.moment;

import java.io.Serializable;

import org.apache.commons.math3.exception.NullArgumentException;
import org.apache.commons.math3.util.MathUtils;

/**
 * Computes a statistic related to the Fourth Central Moment.  Specifically,
 * what is computed is the sum of
 * <p>
 * (x_i - xbar) ^ 4, </p>
 * <p>
 * where the x_i are the
 * sample observations and xbar is the sample mean. </p>
 * <p>
 * The following recursive updating formula is used: </p>
 * <p>
 * Let <ul>
 * <li> dev = (current obs - previous mean) </li>
 * <li> m2 = previous value of {@link SecondMoment} </li>
 * <li> m2 = previous value of {@link ThirdMoment} </li>
 * <li> n = number of observations (including current obs) </li>
 * </ul>
 * Then </p>
 * <p>
 * new value = old value - 4 * (dev/n) * m3 + 6 * (dev/n)^2 * m2 + <br>
 * [n^2 - 3 * (n-1)] * dev^4 * (n-1) / n^3 </p>
 * <p>
 * Returns <code>Double.NaN</code> if no data values have been added and
 * returns <code>0</code> if there is just one value in the data set. Note that
 * Double.NaN may also be returned if the input includes NaN and / or infinite
 * values. </p>
 * <p>
 * <strong>Note that this implementation is not synchronized.</strong> If
 * multiple threads access an instance of this class concurrently, and at least
 * one of the threads invokes the <code>increment()</code> or
 * <code>clear()</code> method, it must be synchronized externally. </p>
 *
 */
class FourthMoment extends ThirdMoment implements Serializable{

    /** Serializable version identifier */
    private static final long serialVersionUID = 4763990447117157611L;

    /** fourth moment of values that have been added */
    private double m4;

    /**
     * Create a FourthMoment instance
     */
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