/* 
 * Copyright 2007-2013
 * Licensed under GNU Lesser General Public License
 * 
 * This file is part of EpochX: genetic programming software for research
 * 
 * EpochX is free software: you can redistribute it and/or modify
 * it under the terms of the GNU Lesser General Public License as published by
 * the Free Software Foundation, either version 3 of the License, or
 * (at your option) any later version.
 * 
 * EpochX is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
 * GNU Lesser General Public License for more details.
 * 
 * You should have received a copy of the GNU Lesser General Public License
 * along with EpochX. If not, see <http://www.gnu.org/licenses/>.
 * 
 * The latest version is available from: http://www.epochx.org
 */
package org.epochx.fitness;

import java.util.HashMap;
import java.util.Map;

import org.epochx.AbstractFitnessFunction;
import org.epochx.Fitness;
import org.epochx.Individual;

/**
 * A fitness function which caches fitness scores and delegates fitness calculations to a
 * delegate fitness function. For the caching to work correctly the fitness cases must not 
 * be changed, or the cache must be cleared when they are changed. Caching is based on 
 * hash code so it is important that Individuals have a suitable hashCode implementation.
 * 
 * @since 2.0
 */
public class CachedFitnessFunction extends AbstractFitnessFunction {

	// The cache of fitness scores
	private Map<Object, Fitness> cache;
	
	// The fitness function to delegate to when not in cache
	private AbstractFitnessFunction delegate;
	
	/**
	 * Constructs a <code>CachedFitnessFunction</code> fitness function. Fitness calculations
	 * are performed by the given delegate, unless the individual's fitness has been 
	 * cached.
	 * 
	 * @param delegate the fitness function the fitness calculations should be delegated to
	 */
	public CachedFitnessFunction(AbstractFitnessFunction delegate) {
		this.delegate = delegate;
		cache = new HashMap<Object, Fitness>();
	}
	
	/**
	 * Returns the fitness of the given individual. If the cache contains a fitness score
	 * for the given individual then the fitness value from the cache is returned. 
	 * Otherwise the fitness is calculated using the delegate fitness function and then 
	 * cached for future evaluations.
	 * 
	 * The caching of individuals is based on hash codes. If two individuals return the 
	 * same hash code then its assumed they have equal fitness scores.
	 *  
	 * @param individual the program to evaluate
	 * @return the fitness of the given individual
	 */
	@Override
	public Fitness evaluate(Individual individual) {
		Object key = individual.hashCode();
		
		//TODO Use source generator if one is set
		
		Fitness fitness = cache.get(key);
		if (fitness == null) {
			fitness = delegate.evaluate(individual);
			cache.put(key, fitness);
		}
		
		return fitness;
	}
	
	/**
	 * Clears the cache.
	 */
	public void clear() {
		cache.clear();
	}

}