package org.jenetix;

import static java.lang.String.format;
import static java.util.Objects.requireNonNull;

import org.jenetics.Gene;
import org.jenetics.Optimize;
import org.jenetics.Phenotype;
import org.jenetics.Population;
import org.jenetics.Selector;
import org.jenetics.stat.MinMax;

/**
 * Selector implementation which is part of the
 * <a href="https://en.wikipedia.org/wiki/Weasel_program">Weasel program</a>
 * algorithm. The <i>Weasel program</i> is an thought experiment by Richard
 * Dawkins to illustrate the functioning of the evolution: random <i>mutation</i>
 * combined with non-random cumulative <i>selection</i>.
 * <p>
 * The selector always returns populations which only contains "{@code count}"
 * instances of the <i>best</i> {@link Phenotype}.
 * </p>
 * {@link org.jenetics.engine.Engine} setup for the <i>Weasel program:</i>
 * <pre>{@code
 * final Engine<CharacterGene, Integer> engine = Engine
 *     .builder(fitness, gtf)
 *      // Set the 'WeaselSelector'.
 *     .selector(new WeaselSelector<>())
 *      // Disable survivors selector.
 *     .offspringFraction(1)
 *      // Set the 'WeaselMutator'.
 *     .alterers(new WeaselMutator<>(0.05))
 *     .build();
 * }</pre>
 *
 * @see <a href="https://en.wikipedia.org/wiki/Weasel_program">Weasel program</a>
 * @see WeaselMutator
 *
 * @author <a href="mailto:franz.wilhelmstoetter@gmx.at">Franz Wilhelmstötter</a>
 * @since 3.5
 * @version 3.5
 */
public class WeaselSelector<
	G extends Gene<?, G>,
	C extends Comparable<? super C>
>
	implements Selector<G, C>
{
	@Override
	public Population<G, C> select(
		final Population<G, C> population,
		final int count,
		final Optimize opt
	) {
		requireNonNull(population, "Population");
		requireNonNull(opt, "Optimization");
		if (count < 0) {
			throw new IllegalArgumentException(format(
				"Selection count must be greater or equal then zero, but was %s",
				count
			));
		}

		final MinMax<Phenotype<G, C>> minMax = population.stream()
			.collect(MinMax.toMinMax(opt.ascending()));

		return new Population<G, C>(count).fill(minMax::getMax, count);
	}
}