SubtreeCrossover.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.evolve.crossover;

import java.util.function.Predicate;

import org.oakgp.Type;
import org.oakgp.evolve.GeneticOperator;
import org.oakgp.node.Node;
import org.oakgp.node.walk.DepthWalk;
import org.oakgp.node.walk.StrategyWalk;
import org.oakgp.select.NodeSelector;
import org.oakgp.util.Random;
import org.oakgp.util.Utils;

/**
 * Replaces a randomly selected subtree of one parent with a randomly selected subtree of another parent.
 * <p>
 * A subtree is selected at random from each of the parents. The subtree of the first is replaced by the subtree of the second. The structure of the resulting
 * tree can differ significantly from either of the trees that were combined to produce it.
 * </p>
 */
public final class SubtreeCrossover implements GeneticOperator {
   private final Random random;
   private final int maxDepth;

   /**
    * Creates a {@code SubtreeCrossover} that uses the given {@code Random} to select subtrees from parents.
    *
    * @param random
    *           used to randomly select subtrees to replace and the subtrees to replace them with
    * @param maxDepth
    *           used to enforce a maximum depth of any offspring
    */
   public SubtreeCrossover(Random random, int maxDepth) {
      this.random = random;
      this.maxDepth = maxDepth;
   }

   @Override
   public Node evolve(NodeSelector selector) {
      Node parent1 = selector.next();
      Node parent2 = selector.next();
      int to = Utils.selectSubNodeIndex(random, parent1);
      return DepthWalk.replaceAt(parent1, to, (t, d) -> {
         int maxHeightParent2 = maxDepth - d;
         Type toType = t.getType();
         Predicate<Node> treeWalkerStrategy = n -> n.getType() == toType && n.getHeight() <= maxHeightParent2 + 1;
         int nodeCount = StrategyWalk.getNodeCount(parent2, treeWalkerStrategy);
         if (nodeCount == 0) {
            return t;
         } else {
            int from = random.nextInt(nodeCount);
            return StrategyWalk.getAt(parent2, from, treeWalkerStrategy);
         }
      });
   }
}