ShrinkMutation.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.node.Node;
import org.oakgp.node.NodeType;
import org.oakgp.node.walk.StrategyWalk;
import org.oakgp.primitive.PrimitiveSet;
import org.oakgp.select.NodeSelector;
import org.oakgp.util.Random;
import org.oakgp.util.Utils;

/**
 * Replaces a randomly selected function node of the parent with a terminal node.
 * <p>
 * The resulting offspring will be smaller than the parent.
 */
public final class ShrinkMutation implements GeneticOperator {
   private final Random random;
   private final PrimitiveSet primitiveSet;

   /**
    * Creates a {@code ShrinkMutation} that uses a {@code Random} to select function nodes to replace with terminals from a {@code PrimitiveSet}.
    *
    * @param random
    *           used to randomly select function nodes
    * @param primitiveSet
    *           used to select terminal nodes to replace function nodes with
    */
   public ShrinkMutation(Random random, PrimitiveSet primitiveSet) {
      this.random = random;
      this.primitiveSet = primitiveSet;
   }

   @Override
   public Node evolve(NodeSelector selector) {
      Node root = selector.next();
      int nodeCount = StrategyWalk.getNodeCount(root, NodeType::isFunction);
      if (nodeCount == 0) {
         // if nodeCount == 0 then that indicates that 'root' is a terminal node
         // (so can't be shrunk any further)
         return root;
      } else if (nodeCount == 1) {
         // if node count == 1 then that indicates that 'root' is a function node
         // that only has terminal nodes as arguments
         return primitiveSet.nextAlternativeTerminal(root);
      } else {
         int index = Utils.selectSubNodeIndex(random, nodeCount);
         return StrategyWalk.replaceAt(root, index, primitiveSet::nextAlternativeTerminal, NodeType::isFunction);
      }
   }
}