/*
 * #%L
 * Image processing operations for SciJava Ops.
 * %%
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package org.scijava.ops.image.stats;

import net.imglib2.type.numeric.RealType;
import org.scijava.function.Computers;

import java.util.ArrayList;

import static java.util.Collections.swap;

/**
 * Op to calculate the n-th {@code stats.percentile}.
 *
 * @author Daniel Seebacher (University of Konstanz)
 * @author Christian Dietz (University of Konstanz)
 * @author Jan Eglinger
 * @param <I> input type
 * @param <O> output type
 * @implNote op names='stats.quantile'
 */
public class DefaultQuantile<I extends RealType<I>, N extends Number, O extends RealType<O>>
	implements Computers.Arity2<Iterable<I>, N, O>
{

	/**
	 * TODO
	 *
	 * @param input
	 * @param quantile
	 * @param output
	 */
	@Override
	public void compute(final Iterable<I> input, final N quantile,
		final O output)
	{
		if (quantile.doubleValue() < 0 || quantile.doubleValue() > 1) {
			throw new IllegalArgumentException(
				"Quantile must be between 0 and 1 (inclusive) but is " + quantile);
		}

		final var statistics = new ArrayList<Double>();

		for (var i : input) {
			statistics.add(i.getRealDouble());
		}

		output.setReal(select(statistics, (int) (statistics.size() * quantile
			.doubleValue())));
	}

	/**
	 * Returns the value of the kth lowest element. Do note that for nth lowest
	 * element, k = n - 1.
	 * <p>
	 * This an all-in-one method version of your basic quick select algorithm.
	 * </p>
	 */
	static double select(final ArrayList<Double> array, final int k) {

        var left = 0;
        var right = array.size() - 1;

		while (true) {

			if (right <= left + 1) {

				if (right == left + 1 && array.get(right) < array.get(left)) {
					swap(array, left, right);
				}

				return array.get(k);

			}
			final var middle = (left + right) >>> 1;
			swap(array, middle, left + 1);

			if (array.get(left) > array.get(right)) {
				swap(array, left, right);
			}

			if (array.get(left + 1) > array.get(right)) {
				swap(array, left + 1, right);
			}

			if (array.get(left) > array.get(left + 1)) {
				swap(array, left, left + 1);
			}

            var i = left + 1;
            var j = right;
			final double pivot = array.get(left + 1);

			while (true) {
				do
					++i;
				while (array.get(i) < pivot);
				do
					--j;
				while (array.get(j) > pivot);

				if (j < i) {
					break;
				}

				swap(array, i, j);
			}

			array.set(left + 1, array.get(j));
			array.set(j, pivot);

			if (j >= k) {
				right = j - 1;
			}

			if (j <= k) {
				left = i;
			}
		}
	}
}
