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/*
 * #%L
 * Image processing operations for SciJava Ops.
 * %%
 * Copyright (C) 2014 - 2024 SciJava developers.
 * %%
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions are met:
 * 
 * 1. Redistributions of source code must retain the above copyright notice,
 *    this list of conditions and the following disclaimer.
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 *    this list of conditions and the following disclaimer in the documentation
 *    and/or other materials provided with the distribution.
 * 
 * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
 * AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
 * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
 * ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDERS OR CONTRIBUTORS BE
 * LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
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 * INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
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package org.scijava.ops.image.stats;

import net.imglib2.type.numeric.RealType;

import org.scijava.function.Computers;
import org.scijava.ops.spi.Op;

/**
 * Op to calculate the {@code stats.variance} using the online algorithm from
 * Knuth and Welford.
 *
 * @author Daniel Seebacher (University of Konstanz)
 * @author Christian Dietz (University of Konstanz)
 * @param  input type
 * @param  output type
 * @see 
 *      Wikipedia
 * @implNote op names='stats.variance'
 */
public class IterableVariance, O extends RealType>
	implements Computers.Arity1, O>
{

	/**
	 * TODO
	 *
	 * @param input
	 * @param variance
	 */
	@Override
	public void compute(final Iterable input, final O variance) {
		int n = 0;
		double mean = 0.0;
		double M2 = 0.0;

		for (final I in : input) {
			double x = in.getRealDouble();

			n = n + 1;
			double delta = x - mean;
			mean = mean + delta / n;
			M2 = M2 + delta * (x - mean);
		}

		if (n < 2) {
			variance.setReal(Double.NaN);
		}
		else {
			variance.setReal(M2 / (n - 1));
		}
	}

}