SubTreeMutation.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.mutate;

import org.oakgp.evolve.GeneticOperator;
import org.oakgp.generate.TreeGenerator;
import org.oakgp.node.Node;
import org.oakgp.node.walk.NodeWalk;
import org.oakgp.select.NodeSelector;
import org.oakgp.util.Random;
import org.oakgp.util.Utils;

/** Randomly replaces a subtree of the parent with a newly generated subtree. */
public final class SubTreeMutation implements GeneticOperator {
   private final Random random;
   private final TreeGenerator treeGenerator;

   /**
    * Creates a {@code SubTreeMutation} that uses the given {@code TreeGenerator} to generate new subtrees to replace existing subtrees.
    *
    * @param random
    *           used to randomly select nodes to mutate
    * @param treeGenerator
    *           used to generate new subtrees to replace nodes selected for mutation
    */
   public SubTreeMutation(Random random, TreeGenerator treeGenerator) {
      this.random = random;
      this.treeGenerator = treeGenerator;
   }

   @Override
   public Node evolve(NodeSelector selector) {
      Node root = selector.next();
      int mutationPoint = Utils.selectSubNodeIndex(random, root);
      return NodeWalk.replaceAt(root, mutationPoint, node -> {
         return treeGenerator.generate(node.getType(), node.getHeight());
      });
   }
}