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

/**
 * Computes the first moment (arithmetic mean). Uses the definitional formula:
 * <p>
 * mean = sum(x_i) / n
 * </p>
 * <p>
 * where <code>n</code> is the number of observations.
 * </p>
 * <p>
 * To limit numeric errors, the value of the statistic is computed using the
 * following recursive updating algorithm:
 * </p>
 * <p>
 * <ol>
 * <li>Initialize <code>m = </code> the first value</li>
 * <li>For each additional value, update using <br>
 * <code>m = m + (new value - m) / (number of observations)</code></li>
 * </ol>
 * </p>
 * <p>
 * Returns <code>Double.NaN</code> if the dataset is empty.
 * </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 FirstMoment extends AbstractStorelessUnivariateStatistic implements Serializable {

	/** Serializable version identifier */
	private static final long serialVersionUID = 6112755307178490473L;

	/** Count of values that have been added */
	protected long n;

	/** First moment of values that have been added */
	protected double m1;

	/**
	 * Deviation of most recently added value from previous first moment.
	 * Retained to prevent repeated computation in higher order moments.
	 */
	protected double dev;

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

	/**
	 * Create a FirstMoment instance
	 */
	public FirstMoment() {
		n = 0;
		m1 = Double.NaN;
		dev = Double.NaN;
		nDev = Double.NaN;
	}

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

	/**
	 * {@inheritDoc}
	 */
	@Override
	public void increment(final double d) {
		if (n == 0) {
			m1 = 0.0;
		}
		n++;
		double n0 = n;
		dev = d - m1;
		nDev = dev / n0;
		m1 += nDev;
	}

	/**
	 * {@inheritDoc}
	 */
	@Override
	public void clear() {
		m1 = Double.NaN;
		n = 0;
		dev = Double.NaN;
		nDev = Double.NaN;
	}

	/**
	 * {@inheritDoc}
	 */
	@Override
	public double getResult() {
		return m1;
	}

	/**
	 * {@inheritDoc}
	 */
	public long getN() {
		return n;
	}

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

	/**
	 * Copies source to dest.
	 * <p>
	 * Neither source nor dest can be null.
	 * </p>
	 * 
	 * @param source
	 *            FirstMoment to copy
	 * @param dest
	 *            FirstMoment to copy to
	 * @throws NullPointerException
	 *             if either source or dest is null
	 */
	public static void copy(FirstMoment source, FirstMoment dest) {
		dest.n = source.n;
		dest.m1 = source.m1;
		dest.dev = source.dev;
		dest.nDev = dest.nDev;
	}
}
