/*
 * 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 Third Central Moment. Specifically, what
 * is computed is the sum of cubed deviations from the sample mean.
 * <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>n = number of observations (including current obs)</li>
 * </ul>
 * Then
 * </p>
 * <p>
 * new value = old value - 3 * (dev/n) * m2 + (n-1) * (n -2) * (dev^3/n^2)
 * </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 ThirdMoment extends SecondMoment implements Serializable {

	/** Serializable version identifier */
	private static final long serialVersionUID = -7818711964045118679L;

	/** third moment of values that have been added */
	protected double m3;

	/**
	 * Square of deviation of most recently added value from previous first
	 * moment, normalized by previous sample size. Retained to prevent repeated
	 * computation in higher order moments. nDevSq = nDev * nDev.
	 */
	protected double nDevSq;

	/**
	 * Create a FourthMoment instance
	 */
	public ThirdMoment() {
		super();
		m3 = Double.NaN;
		nDevSq = Double.NaN;
	}

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

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

		double prevM2 = m2;
		super.increment(d);
		nDevSq = nDev * nDev;
		double n0 = n;
		m3 = m3 - 3.0 * nDev * prevM2 + (n0 - 1) * (n0 - 2) * nDevSq * dev;
	}

	/**
	 * {@inheritDoc}
	 */
	@Override
	public double getResult() {
		return m3;
	}

	/**
	 * {@inheritDoc}
	 */
	@Override
	public void clear() {
		super.clear();
		m3 = Double.NaN;
		nDevSq = Double.NaN;
	}

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

	/**
	 * Copies source to dest.
	 * <p>
	 * Neither source nor dest can be null.
	 * </p>
	 * 
	 * @param source
	 *            ThirdMoment to copy
	 * @param dest
	 *            ThirdMoment to copy to
	 * @throws NullPointerException
	 *             if either source or dest is null
	 */
	public static void copy(ThirdMoment source, ThirdMoment dest) {
		SecondMoment.copy(source, dest);
		dest.m3 = source.m3;
		dest.nDevSq = source.nDevSq;
	}

}
