source: trunk/sources/HeuristicLab.GP.Operators/3.3/Manipulation/ChangeNodeTypeManipulation.cs @ 2222

#region License Information
/* HeuristicLab
 * Copyright (C) 2002-2008 Heuristic and Evolutionary Algorithms Laboratory (HEAL)
 *
 * This file is part of HeuristicLab.
 *
 * HeuristicLab is free software: you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation, either version 3 of the License, or
 * (at your option) any later version.
 *
 * HeuristicLab 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 General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with HeuristicLab. If not, see <http://www.gnu.org/licenses/>.
 */
#endregion

using System.Collections.Generic;
using System.Linq;
using HeuristicLab.Core;
using HeuristicLab.Random;
using System.Diagnostics;
using HeuristicLab.GP.Interfaces;

namespace HeuristicLab.GP.Operators {
  public class ChangeNodeTypeManipulation : GPManipulatorBase {

    public override string Description {
      get {
        return @"This manipulation operator selects a random tree-node and changes the function type.
If this leads to a constraint-violation (wrong number or type of sub-trees) the sub-trees are repaired
resulting in a valid tree again.";
      }
    }

    public ChangeNodeTypeManipulation()
      : base() {
    }

    internal override IOperation Manipulate(MersenneTwister random, IGeneticProgrammingModel gpModel, FunctionLibrary library, int maxTreeSize, int maxTreeHeight, IScope scope) {
      TreeGardener gardener = new TreeGardener(random, library);
      IFunctionTree parent = gardener.GetRandomParentNode(gpModel.FunctionTree);
      IFunctionTree selectedChild;
      int selectedChildIndex;
      if (parent == null) {
        selectedChildIndex = 0;
        selectedChild = gpModel.FunctionTree;
      } else {
        selectedChildIndex = random.Next(parent.SubTrees.Count);
        selectedChild = parent.SubTrees[selectedChildIndex];
      }

      if (selectedChild.SubTrees.Count == 0) {
        IFunctionTree newTerminal = ChangeTerminalType(parent, selectedChild, selectedChildIndex, gardener, random);
        if (parent == null) {
          // no parent means the new child is the initial operator
          gpModel.FunctionTree = newTerminal;
        } else {
          parent.RemoveSubTree(selectedChildIndex);
          parent.InsertSubTree(selectedChildIndex, newTerminal);
          // updating the variable is not necessary because it stays the same
        }
        Debug.Assert(gardener.IsValidTree(gpModel.FunctionTree));
        // size and height stays the same when changing a terminal so no need to update the variables
        // schedule an operation to initialize the new terminal
        return Util.CreateInitializationOperation(TreeGardener.GetAllSubTrees(newTerminal), scope);
      } else {
        List<IFunctionTree> uninitializedBranches;
        IFunctionTree newFunctionTree = ChangeFunctionType(parent, selectedChild, selectedChildIndex, gardener, random, out uninitializedBranches);
        // in rare cases the function creates a tree that breaks the size limits
        // calculate the height and size difference and 
        // check if the size of the new tree is still in the allowed bounds
        int oldChildSize = selectedChild.GetSize();
        int oldChildHeight = selectedChild.GetHeight();
        int newChildSize = newFunctionTree.GetSize();
        int newChildHeight = newFunctionTree.GetHeight();
        if ((gpModel.Height - oldChildHeight) + newChildHeight > maxTreeHeight ||
          (gpModel.Size - oldChildSize) + newChildSize > maxTreeSize) {
          // if size-constraints are violated don't change anything
          return null;
        }
        if (parent == null) {
          // no parent means the new function is the initial operator
          gpModel.FunctionTree = newFunctionTree;
        } else {
          // remove the old child
          parent.RemoveSubTree(selectedChildIndex);
          // add the new child as sub-tree of parent
          parent.InsertSubTree(selectedChildIndex, newFunctionTree);
        }
        // update size and height
        gpModel.Size = gpModel.Size - oldChildSize + newChildSize;
        gpModel.Height = gpModel.FunctionTree.GetHeight(); // must recalculate height because we can't know wether the manipulated branch was the deepest branch
        // check if whole tree is ok
        Debug.Assert(gardener.IsValidTree(gpModel.FunctionTree));
        // return a composite operation that initializes all created sub-trees
        return Util.CreateInitializationOperation(uninitializedBranches, scope);
      }
    }

    private IFunctionTree ChangeTerminalType(IFunctionTree parent, IFunctionTree child, int childIndex, TreeGardener gardener, MersenneTwister random) {
      IList<IFunction> allowedTerminals;
      if (parent == null) {
        allowedTerminals = gardener.Terminals;
      } else {
        allowedTerminals = new List<IFunction>();
        var allAllowedChildren = gardener.GetAllowedSubFunctions(parent.Function, childIndex);
        foreach (IFunction c in allAllowedChildren) {
          if (TreeGardener.IsTerminal(c)) allowedTerminals.Add(c);
        }
      }
      // selecting from the terminals should always work since the current child was also a terminal
      // so in the worst case we will just create a new terminal of the same type again.
      return gardener.CreateRandomTree(allowedTerminals, 1, 1);
    }

    private IFunctionTree ChangeFunctionType(IFunctionTree parent, IFunctionTree child, int childIndex, TreeGardener gardener, MersenneTwister random,
      out List<IFunctionTree> uninitializedBranches) {
      // since there are subtrees, we have to check which 
      // and how many of the existing subtrees we can reuse.
      // first let's choose the function we want to use instead of the old child. For this we have to determine the
      // pool of allowed functions based on constraints of the parent if there is one.
      List<IFunction> allowedFunctions = new List<IFunction>(gardener.GetAllowedSubFunctions(parent != null ? parent.Function : null, childIndex));
      // try to make a tree with the same arity as the old child.
      int actualArity = child.SubTrees.Count;
      // create a new tree-node for a randomly selected function
      IFunction selectedFunction = allowedFunctions[random.Next(allowedFunctions.Count)];
      // arity of the selected operator
      int minArity = selectedFunction.MinSubTrees;
      int maxArity = selectedFunction.MaxSubTrees;
      // if the old child had too many sub-trees then the new child should keep as many sub-trees as possible
      if (actualArity > maxArity)
        actualArity = maxArity;
      if (actualArity < minArity)
        actualArity = minArity;
      // create a list that holds old sub-trees that we can reuse in the new tree
      List<IFunctionTree> availableSubTrees = new List<IFunctionTree>(child.SubTrees);
      List<IFunctionTree> freshSubTrees = new List<IFunctionTree>();
      IFunctionTree newTree = selectedFunction.GetTreeNode();
      // randomly select the sub-trees that we keep
      for (int i = 0; i < actualArity; i++) {
        // fill all sub-tree slots of the new tree
        // if for a given slot i there are multiple existing sub-trees that can be used in that slot
        // then use a random existing sub-tree. When there are no existing sub-trees
        // that fit in the given slot then create a new random tree and use it for the slot
        ICollection<IFunction> allowedSubFunctions = gardener.GetAllowedSubFunctions(selectedFunction, i);
        var matchingSubTrees = availableSubTrees.Where(subTree => allowedSubFunctions.Contains(subTree.Function));
        if (matchingSubTrees.Count() > 0) {
          IFunctionTree selectedSubTree = matchingSubTrees.ElementAt(random.Next(matchingSubTrees.Count()));
          // we can just add it as subtree
          newTree.InsertSubTree(i, selectedSubTree);
          availableSubTrees.Remove(selectedSubTree); // the branch shouldn't be available for the following slots
        } else {
          // no existing matching tree found => create a new tree of minimal size
          IFunctionTree freshTree = gardener.CreateRandomTree(allowedSubFunctions, 1, 1);
          freshSubTrees.AddRange(TreeGardener.GetAllSubTrees(freshTree));
          newTree.InsertSubTree(i, freshTree);
        }
      }
      freshSubTrees.Add(newTree);
      uninitializedBranches = freshSubTrees;
      return newTree;
    }
  }
}