Fitness.java

package net.bmahe.genetics4j.core;

import java.util.Objects;

import java.util.function.Function;

import org.apache.commons.lang3.Validate;

/**

 * Functional interface for evaluating the fitness of a genotype in an evolutionary algorithm.

 * 

 * <p>The fitness function is a crucial component of evolutionary algorithms as it determines the quality or performance

 * of individual solutions. It maps a genotype to a fitness value that can be used for selection, ranking, and

 * determining evolutionary progress.

 * 

 * <p>Implementations should be:

 * <ul>

 * <li><strong>Deterministic</strong>: The same genotype should always produce the same fitness value</li>

 * <li><strong>Thread-safe</strong>: May be called concurrently from multiple threads</li>

 * <li><strong>Fast</strong>: Called frequently during evolution, performance matters</li>

 * </ul>

 * 

 * <p>Common fitness function patterns:

 * <ul>

 * <li><strong>Minimization</strong>: Lower values indicate better solutions (errors, costs)</li>

 * <li><strong>Maximization</strong>: Higher values indicate better solutions (profits, accuracy)</li>

 * <li><strong>Multi-objective</strong>: Use FitnessVector from the MOO module for multiple objectives</li>

 * </ul>

 * 

 * @param <T> the type of the fitness value, must be comparable for selection operations

 * @see Genotype

 * @see Individual

 * @see net.bmahe.genetics4j.core.evaluation.FitnessEvaluator

 */

@FunctionalInterface

public interface Fitness<T extends Comparable<T>> {

	/**

	 * Computes the fitness value for the specified genotype.

	 * 

	 * <p>This method should evaluate how well the genotype solves the problem and return a comparable fitness value. The

	 * interpretation of "better" depends on whether the optimization is for minimization or maximization.

	 * 

	 * @param genotype the genotype to evaluate

	 * @return the fitness value representing the quality of the genotype

	 * @throws RuntimeException if evaluation fails due to invalid genotype or computation error

	 */

	T compute(Genotype genotype);

	/**

	 * Creates a fitness function that evaluates a specific chromosome within a genotype.

	 *

	 * <p>This utility method simplifies fitness evaluation when the fitness depends on only one chromosome from the

	 * genotype. It extracts the chromosome at the specified index and applies the provided fitness function to it.

	 *

	 * <p>This is particularly useful for:

	 * <ul>

	 * <li>Single-chromosome problems where genotype contains only one chromosome</li>

	 * <li>Multi-chromosome problems where fitness depends on one specific chromosome</li>

	 * <li>Compositional fitness functions that evaluate chromosomes independently</li>

	 * </ul>

	 *

	 * @param <U>               the type of the chromosome to evaluate

	 * @param <V>               the type of the fitness value, must be comparable

	 * @param chromosomeIndex   the zero-based index of the chromosome to evaluate within the genotype

	 * @param chromosomeFitness the function that computes fitness for the extracted chromosome

	 * @return a fitness function that evaluates the specified chromosome

	 * @throws IllegalArgumentException if chromosomeIndex is negative

	 * @throws NullPointerException     if chromosomeFitness is null

	 * @throws ClassCastException       at runtime if the chromosome at the specified index cannot be cast to type U

	 * @throws IllegalArgumentException at runtime if chromosomeIndex exceeds the genotype size

	 */

	static <U, V extends Comparable<V>> Fitness<V> forChromosome(final int chromosomeIndex,

			final Function<U, V> chromosomeFitness) {

			@SuppressWarnings("unchecked")

		};

	}

}

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