ArtificialAntFitnessFunction.java

/*
 * Copyright 2015 S. Webber
 *
 * 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.
 */
package org.oakgp.examples.ant;

import org.oakgp.Assignments;
import org.oakgp.node.Node;
import org.oakgp.rank.fitness.FitnessFunction;

/** Calculates the fitness of candidate solutions to navigate an artificial ant around a two-dimensional grid. */
class ArtificialAntFitnessFunction implements FitnessFunction {
   private static final int MAX_MOVES = 600;

   @Override
   public double evaluate(Node candidate) {
      MutableState state = new MutableState(GridReader.copyGrid());
      evaluate(candidate, state);
      return calculateFitness(state);
   }

   /** Keeps evaluating the given candidate using the given state until either the maximum number of moves have been taken or all the food has been eaten. */
   private void evaluate(Node candidate, MutableState state) {
      Assignments assignments = Assignments.createAssignments(state);
      int movesTaken = 0;
      while (movesTaken < MAX_MOVES && isRemainingFood(state)) {
         candidate.evaluate(assignments);

         // if last call to evaluate did not result in the state being updated
         // then exit now to avoid getting stuck in loop when evaluating candidates that do not do anything
         if (movesTaken == state.getMovesTaken()) {
            return;
         }

         movesTaken = state.getMovesTaken();
      }
   }

   /**
    * Calculate fitness of the given state.
    * <p>
    * Any state where all the food has been eaten is better than any state where there is food remaining. The best solution is the one where all the food has
    * been eaten in the quickest time.
    */
   private double calculateFitness(MutableState state) {
      if (isRemainingFood(state)) {
         return Integer.MAX_VALUE - state.getFoodEaten();
      } else {
         return state.getMovesTaken();
      }
   }

   /** Returns {@code true} if the given state contains cells with food that have not yet been visited. */
   private boolean isRemainingFood(MutableState state) {
      return state.getFoodEaten() < GridReader.getNumberOfFoodCells();
   }
}