SamePopulationTermination.java

import static java.lang.Math.PI;
import static org.jenetics.engine.EvolutionResult.toBestPhenotype;

import java.util.Set;
import java.util.function.Predicate;
import java.util.stream.Collectors;

import org.jenetics.DoubleGene;
import org.jenetics.Genotype;
import org.jenetics.Phenotype;
import org.jenetics.engine.Engine;
import org.jenetics.engine.EvolutionResult;
import org.jenetics.engine.codecs;
import org.jenetics.util.DoubleRange;

final class SamePopulationTermination
	implements Predicate<EvolutionResult<?, ?>>
{
	private final double _threshold;

	private SamePopulationTermination(final double threshold) {
		_threshold = threshold;
	}

	@Override
	public boolean test(final EvolutionResult<?, ?> result) {
		final Set<Genotype<?>> genotypes = result.getPopulation().stream()
			.map(Phenotype::getGenotype)
			.collect(Collectors.toSet());

		final double sameElementRate =
			(double)(result.getPopulation().size() - genotypes.size())/
				(double)result.getPopulation().size();

		return sameElementRate > _threshold;
	}

	public static void main(final String[] args) {
		final Engine<DoubleGene, Double> engine = Engine
			.builder(
				x -> x[0] + x[1], // Fitness function
				codecs.ofVector(DoubleRange.of(0.0, 2.0*PI), 2))
			.build();

		final Phenotype<DoubleGene, Double> best = engine.stream()
			.limit(new SamePopulationTermination(0.6))
			.collect(toBestPhenotype());

		System.out.println(best);
	}
}