/*
 * Java Genetic Algorithm Library (jenetics-7.1.0).
 * Copyright (c) 2007-2022 Franz Wilhelmstötter
 *
 * Licensed 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.
 *
 * Author:
 *    Franz Wilhelmstötter (franz.wilhelmstoetter@gmail.com)
 */
package io.jenetics.engine;

import java.util.function.Function;
import java.util.function.Predicate;
import java.util.function.Supplier;
import java.util.stream.Stream;

import io.jenetics.Gene;
import io.jenetics.internal.engine.EvolutionStreamImpl;

/**
 * The {@code EvolutionStream} class extends the Java {@link Stream} and adds a
 * method for limiting the evolution by a given predicate.
 *
 * @implNote Collecting an <em>empty</em> {@code EvolutionStream} will return
 *           {@code null}.
 * <pre>{@code
 * final EvolutionResult<DoubleGene, Double> result = engine.stream()
 *     .limit(0)
 *     .collect(toBestEvolutionResult());
 *
 * assert result == null;
 * }</pre>
 *
 * @see java.util.stream.Stream
 * @see Engine
 * @see EvolutionStreamable
 *
 * @author <a href="mailto:franz.wilhelmstoetter@gmail.com">Franz Wilhelmstötter</a>
 * @since 3.0
 * @version 6.0
 */
public interface EvolutionStream<
        G extends Gene<?, G>,
        C extends Comparable<? super C>
>
        extends Stream<EvolutionResult<G, C>>
{

        /**
         * Returns a stream consisting of the elements of this stream, truncated
         * when the given {@code proceed} predicate returns {@code false}.
         * <p>
         * <i>General usage example:</i>
         * <pre>{@code
         * final Phenotype<DoubleGene, Double> result = engine.stream()
         *      // Truncate the evolution stream after 5 "steady" generations.
         *     .limit(bySteadyFitness(5))
         *      // The evolution will stop after maximal 100 generations.
         *     .limit(100)
         *     .collect(toBestPhenotype());
         * }</pre>
         *
         * <b>Note:</b>
         * The evolution result may be {@code null}, if your <em>truncation</em>
         * predicate returns {@code false} for the initial population.
         * <pre>{@code
         * final EvolutionResult<DoubleGene, Double> result = engine.stream()
         *     .limit(er -> false)
         *     .collect(toBestEvolutionResult());
         *
         * assert result == null;
         * }</pre>
         *
         * @see Limits
         *
         * @param proceed the predicate which determines whether the stream is
         *        truncated or not. <i>If the predicate returns {@code false}, the
         *        evolution stream is truncated.</i>
         * @return the new stream
         * @throws NullPointerException if the given predicate is {@code null}.
         */
        EvolutionStream<G, C>
        limit(final Predicate<? super EvolutionResult<G, C>> proceed);

        /**
         * Create a new {@code EvolutionStream} from the given {@code start}
         * population and {@code evolution} function. The main purpose of this
         * factory method is to simplify the creation of an {@code EvolutionStream}
         * from an own evolution (GA) engine.
         *
         * <pre>{@code
         * final Supplier<EvolutionStart<DoubleGene, Double>> start = ...
         * final EvolutionStream<DoubleGene, Double> stream =
         *     EvolutionStream.of(start, new MySpecialEngine());
         * }</pre>
         *
         * A more complete example for would look like as:
         *
         * <pre>{@code
         * public final class SpecialEngine {
         *
         *     // The fitness function.
         *     private static Double fitness(final Genotype<DoubleGene> gt) {
         *         return gt.gene().allele();
         *     }
         *
         *     // Create new evolution start object.
         *     private static EvolutionStart<DoubleGene, Double>
         *     start(final int populationSize, final long generation) {
         *         final Population<DoubleGene, Double> population =
         *             Genotype.of(DoubleChromosome.of(0, 1)).instances()
         *                 .map(gt -> Phenotype.of(gt, generation, SpecialEngine::fitness))
         *                 .limit(populationSize)
         *                 .collect(Population.toPopulation());
         *
         *         return EvolutionStart.of(population, generation);
         *     }
         *
         *     // The special evolution function.
         *     private static EvolutionResult<DoubleGene, Double>
         *     evolve(final EvolutionStart<DoubleGene, Double> start) {
         *         // Your special evolution implementation comes here!
         *         return null;
         *     }
         *
         *     public static void main(final String[] args) {
         *         final Genotype<DoubleGene> best = EvolutionStream
         *             .ofEvolution(() -> start(50, 0), SpecialEngine::evolve)
         *             .limit(Limits.bySteadyFitness(10))
         *             .limit(1000)
         *             .collect(EvolutionResult.toBestGenotype());
         *
         *         System.out.println(String.format("Best Genotype: %s", best));
         *     }
         * }
         * }</pre>
         *
         *
         * @since 5.1
         *
         * @see #ofAdjustableEvolution(Supplier, Function)
         *
         * @param <G> the gene type
         * @param <C> the fitness type
         * @param start the evolution start
         * @param evolution the evolution function
         * @return a new {@code EvolutionStream} with the given {@code start} and
         *         {@code evolution} function
         * @throws java.lang.NullPointerException if one of the arguments is
         *         {@code null}
         */
        static <G extends Gene<?, G>, C extends Comparable<? super C>>
        EvolutionStream<G, C> ofEvolution(
                final Supplier<EvolutionStart<G, C>> start,
                final Evolution<G, C> evolution
        ) {
                return new EvolutionStreamImpl<>(start, evolution);
        }

        /**
         * Create a new evolution stream with an <em>adjustable</em> evolution
         * function.
         *
         * <pre>{@code
         * public static void main(final String[] args) {
         *     final Problem<double[], DoubleGene, Double> problem = Problem.of(
         *         v -> Math.sin(v[0])*Math.cos(v[1]),
         *         Codecs.ofVector(DoubleRange.of(0, 2*Math.PI), 2)
         *     );
         *
         *     // Engine builder template.
         *     final Engine.Builder<DoubleGene, Double> builder = Engine
         *         .builder(problem)
         *         .minimizing();
         *
         *     // Evolution used for low fitness variance.
         *     final Evolution<DoubleGene, Double> lowVar = builder.copy()
         *         .alterers(new Mutator<>(0.5))
         *         .selector(new MonteCarloSelector<>())
         *         .build();
         *
         *     // Evolution used for high fitness variance.
         *     final Evolution<DoubleGene, Double> highVar = builder.copy()
         *         .alterers(
         *             new Mutator<>(0.05),
         *             new MeanAlterer<>())
         *         .selector(new RouletteWheelSelector<>())
         *         .build();
         *
         *     final EvolutionStream<DoubleGene, Double> stream =
         *         EvolutionStream.ofAdjustableEvolution(
         *             EvolutionStart::empty,
         *             er -> var(er) < 0.2 ? lowVar : highVar
         *         );
         *
         *     final Genotype<DoubleGene> result = stream
         *         .limit(Limits.bySteadyFitness(50))
         *         .collect(EvolutionResult.toBestGenotype());
         *
         *     System.out.println(result + ": " +
         *         problem.fitness().apply(problem.codec().decode(result)));
         * }
         *
         * private static double var(final EvolutionStart<DoubleGene, Double> result) {
         *     return result != null
         *         ? result.getPopulation().stream()
         *             .map(Phenotype::fitness)
         *             .collect(DoubleMoments.toDoubleMoments())
         *             .variance()
         *         : 0.0;
         * }
         * }</pre>
         *
         * @see #ofEvolution(Supplier, Evolution)
         *
         * @param start the evolution start object
         * @param evolution the adaptable evolution function
         * @param <G> the gene type
         * @param <C> the fitness type
         * @return a new {@code EvolutionStream} with the given {@code start} and
         *         {@code evolution} function
         * @throws java.lang.NullPointerException if one of the arguments is
         *         {@code null}
         */
        static <G extends Gene<?, G>, C extends Comparable<? super C>>
        EvolutionStream<G, C> ofAdjustableEvolution(
                final Supplier<EvolutionStart<G, C>> start,
                final Function<
                        ? super EvolutionStart<G, C>,
                        ? extends Evolution<G, C>> evolution
        ) {
                return EvolutionStreamImpl.of(start, evolution);
        }

}