/*
 * 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 Second Central Moment.  Specifically,
what is computed is the sum of squared deviations from the sample mean.

The following recursive updating formula is used:

Let 

 dev = (current obs - previous mean) 
 n = number of observations (including current obs) 

Then


new value = old value + dev^2 * (n -1) / n.

Returns Double.NaN if no data values have been added and
returns 0 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.

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.
/**
 * Computes a statistic related to the Second Central Moment.  Specifically,
 * what is computed is the sum of squared 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> n = number of observations (including current obs) </li>
 * </ul>
 * Then</p>
 * <p>
 * new value = old value + dev^2 * (n -1) / n.</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>
 *
 */
public class SecondMoment extends FirstMoment implements Serializable {

    
Serializable version identifier /** Serializable version identifier */
    private static final long serialVersionUID = 3942403127395076445L;

    
second moment of values that have been added /** second moment of values that have been added */
    protected double m2;

    
Create a SecondMoment instance
/**
     * Create a SecondMoment instance
     */
    public SecondMoment() {
        super();
        m2 = Double.NaN;
    }

    
Copy constructor, creates a new SecondMoment identical to the original 
Params:
original – the SecondMoment instance to copy
Throws:
NullArgumentException – if original is null/**
     * Copy constructor, creates a new {@code SecondMoment} identical
     * to the {@code original}
     *
     * @param original the {@code SecondMoment} instance to copy
     * @throws NullArgumentException if original is null
     */
    public SecondMoment(SecondMoment original)
    throws NullArgumentException {
        super(original);
        this.m2 = original.m2;
    }

    
{@inheritDoc}
/**
     * {@inheritDoc}
     */
    @Override
    public void increment(final double d) {
        if (n < 1) {
            m1 = m2 = 0.0;
        }
        super.increment(d);
        m2 += ((double) n - 1) * dev * nDev;
    }

    
{@inheritDoc}
/**
     * {@inheritDoc}
     */
    @Override
    public void clear() {
        super.clear();
        m2 = Double.NaN;
    }

    
{@inheritDoc}
/**
     * {@inheritDoc}
     */
    @Override
    public double getResult() {
        return m2;
    }

    
{@inheritDoc}
/**
     * {@inheritDoc}
     */
    @Override
    public SecondMoment copy() {
        SecondMoment result = new SecondMoment();
        // no try-catch or advertised NAE because args are guaranteed non-null
        copy(this, result);
        return result;
    }

    
Copies source to dest.
Neither source nor dest can be null.
Params:
source – SecondMoment to copy
dest – SecondMoment to copy to
Throws:
NullArgumentException – if either source or dest is null/**
     * Copies source to dest.
     * <p>Neither source nor dest can be null.</p>
     *
     * @param source SecondMoment to copy
     * @param dest SecondMoment to copy to
     * @throws NullArgumentException if either source or dest is null
     */
    public static void copy(SecondMoment source, SecondMoment dest)
        throws NullArgumentException {
        MathUtils.checkNotNull(source);
        MathUtils.checkNotNull(dest);
        FirstMoment.copy(source, dest);
        dest.m2 = source.m2;
    }

}