/*
 * 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.moment;

import java.io.Serializable;

/**
 * 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>
 * 
 * @version $Revision$ $Date$
 */
public class FourthMoment extends ThirdMoment implements Serializable {

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

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

	/**
	 * Create a FourthMoment instance
	 */
	public FourthMoment() {
		super();
		m4 = Double.NaN;
	}

	/**
	 * Copy constructor, creates a new {@code FourthMoment} identical to the
	 * {@code original}
	 * 
	 * @param original
	 *            the {@code FourthMoment} instance to copy
	 */
	public FourthMoment(FourthMoment original) {
		super();
		copy(original, this);
	}

	/**
	 * {@inheritDoc}
	 */
	@Override
	public void increment(final double d) {
		if (n < 1) {
			m4 = 0.0;
			m3 = 0.0;
			m2 = 0.0;
			m1 = 0.0;
		}

		double prevM3 = m3;
		double prevM2 = m2;

		super.increment(d);

		double n0 = n;

		m4 = m4 - 4.0 * nDev * prevM3 + 6.0 * nDevSq * prevM2
				+ ((n0 * n0) - 3 * (n0 - 1)) * (nDevSq * nDevSq * (n0 - 1) * n0);
	}

	/**
	 * {@inheritDoc}
	 */
	@Override
	public double getResult() {
		return m4;
	}

	/**
	 * {@inheritDoc}
	 */
	@Override
	public void clear() {
		super.clear();
		m4 = Double.NaN;
	}

	/**
	 * {@inheritDoc}
	 */
	@Override
	public FourthMoment copy() {
		FourthMoment result = new FourthMoment();
		copy(this, result);
		return result;
	}

	/**
	 * Copies source to dest.
	 * <p>
	 * Neither source nor dest can be null.
	 * </p>
	 * 
	 * @param source
	 *            FourthMoment to copy
	 * @param dest
	 *            FourthMoment to copy to
	 * @throws NullPointerException
	 *             if either source or dest is null
	 */
	public static void copy(FourthMoment source, FourthMoment dest) {
		ThirdMoment.copy(source, dest);
		dest.m4 = source.m4;
	}
}
