/*
 * Java Genetic Algorithm Library (jenetics-8.0.0).
 * Copyright (c) 2007-2024 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 static java.util.Objects.requireNonNull;

import java.util.Optional;
import java.util.function.Function;

import io.jenetics.Gene;
import io.jenetics.Genotype;

/**
 * This interface describes a <i>problem</i> which can be solved by the GA
 * evolution {@code Engine}. It connects the actual {@link #fitness()} function
 * and the needed {@link #codec()}.
 *
 * {@snippet lang="java":
 * final Problem<ISeq<BitGene>, BitGene, Integer> counting = Problem.of(
 *     // Native fitness function
 *     genes -> (int)genes.stream()
 *         .filter(BitGene::bit)
 *         .count(),
 *     // Problem encoding
 *     Codec.of(
 *         Genotype.of(BitChromosome.of(100)),
 *         gt -> ISeq.of(gt.chromosome())
 *     )
 * );
 * }
 *
 * The example above shows the Ones-Counting problem definition.
 *
 * @see Codec
 * @see Engine
 *
 * @param <T> the (<i>native</i>) argument type of the problem fitness function
 * @param <G> the gene type the evolution engine is working with
 * @param <C> the result type of the fitness function
 *
 * @author <a href="mailto:franz.wilhelmstoetter@gmail.com">Franz Wilhelmstötter</a>
 * @version 6.1
 * @since 3.4
 */
public interface Problem<
        T,
        G extends Gene<?, G>,
        C extends Comparable<? super C>
> {

        /**
         * Return the fitness function of the <i>problem</i> in the <i>native</i>
         * problem domain.
         *
         * @return the fitness function
         */
        Function<T, C> fitness();

        /**
         * Return the codec, which translates the types of the problem domain into
         * types, which can be understood by the evolution {@code Engine}.
         *
         * @return the engine codec
         */
        Codec<T, G> codec();

        /**
         * Return the constraint, associated with {@code this} problem, if available.
         *
         * @since 6.1
         *
         * @return the constraint, associated with {@code this} problem
         */
        default Optional<Constraint<G, C>> constraint() {
                return Optional.empty();
        }

        /**
         * Converts the given {@link Genotype} to the target type {@link T}. This is
         * a shortcut for
         * {@snippet lang="java":
         * final Problem<SomeObject, DoubleGene, Double> problem = null; // @replace substring='null' replacement="..."
         * final Genotype<DoubleGene> gt = problem.codec().encoding().newInstance();
         *
         * final SomeObject arg = problem.decode(gt);
         * }
         *
         * @since 4.2
         *
         * @see Codec#decode(Genotype)
         *
         * @param genotype the genotype to be converted
         * @return the converted genotype
         * @throws NullPointerException if the given {@code genotype} is {@code null}
         */
        default T decode(final Genotype<G> genotype) {
                return codec().decode(genotype);
        }

        /**
         * Returns the fitness value for the given argument.
         *
         * @since 4.1
         *
         * @param arg the argument of the fitness function
         * @return the fitness value
         */
        default C fitness(final T arg) {
                return fitness().apply(arg);
        }

        /**
         * Returns the fitness value for the given argument.
         *
         * @since 4.1
         *
         * @param genotype the argument of the fitness function
         * @return the fitness value
         */
        default C fitness(final Genotype<G> genotype) {
                return fitness(codec().decode(genotype));
        }

        /**
         * Return a new optimization <i>problem</i> with the given parameters. The
         * given {@code constraint} is applied to the {@link Engine}, via
         * {@link Engine.Builder#constraint(Constraint)}, and the {@link Codec}, via
         * {@link Constraint#constrain(Codec)}.
         * <p>
         * <b>Note</b><br>
         *     When creating a new {@code Problem} instance with this factory method,
         *     there is no need for additionally <em>constraining</em> the given
         *     {@code codec} with {@link Constraint#constrain(Codec)}.
         *
         * @since 6.1
         *
         * @see Engine.Builder#constraint(Constraint)
         * @see Constraint#constrain(Codec)
         *
         * @param fitness the problem fitness function
         * @param codec the evolution engine codec
         * @param constraint the problem constraint, may be {@code null}
         * @param <T> the (<i>native</i>) argument type of the problem fitness function
         * @param <G> the gene type the evolution engine is working with
         * @param <C> the result type of the fitness function
         * @return a new problem object from the given parameters
         * @throws NullPointerException if the {@code fitness} or {@code codec} is
         *         {@code null}
         */
        static <T, G extends Gene<?, G>, C extends Comparable<? super C>>
        Problem<T, G, C> of(
                final Function<T, C> fitness,
                final Codec<T, G> codec,
                final Constraint<G, C> constraint
        ) {
                requireNonNull(fitness);
                requireNonNull(codec);

                final var constrainedCodec = wrap(constraint, codec);

                return new Problem<>() {
                        @Override
                        public Codec<T, G> codec() {
                                return constrainedCodec;
                        }
                        @Override
                        public Function<T, C> fitness() {
                                return fitness;
                        }
                        @Override
                        public Optional<Constraint<G, C>> constraint() {
                                return Optional.ofNullable(constraint);
                        }
                };
        }

        private static  <T, G extends Gene<?, G>, C extends Comparable<? super C>>
        Codec<T, G> wrap(final Constraint<G, C> constraint, final Codec<T, G> codec) {
                Codec<T, G> result = codec;
                if (constraint != null) {
                        result = codec instanceof InvertibleCodec<T, G> ic
                                ? constraint.constrain(ic)
                                : constraint.constrain(codec);
                }

                return result;
        }

        /**
         * Return a new optimization <i>problem</i> with the given parameters.
         *
         * @param fitness the problem fitness function
         * @param codec the evolution engine codec
         * @param <T> the (<i>native</i>) argument type of the problem fitness function
         * @param <G> the gene type the evolution engine is working with
         * @param <C> the result type of the fitness function
         * @return a new problem object from the given parameters
         * @throws NullPointerException if one of the arguments is {@code null}
         */
        static <T, G extends Gene<?, G>, C extends Comparable<? super C>>
        Problem<T, G, C> of(
                final Function<T, C> fitness,
                final Codec<T, G> codec
        ) {
                return of(fitness, codec, null);
        }

}