Module org.cicirello.chips_n_salsa
Package org.cicirello.search.operators.permutations

Class NonWrappingOrderCrossover

java.lang.Object
org.cicirello.search.operators.permutations.NonWrappingOrderCrossover
All Implemented Interfaces:
PermutationBinaryOperator, Splittable<CrossoverOperator<Permutation>>, CrossoverOperator<Permutation>
public final class NonWrappingOrderCrossover extends Object implements CrossoverOperator<Permutation>, PermutationBinaryOperator
Implementation of non-wrapping order crossover (NWOX). NWOX selects a random subsection similar to a 2-point crossover for bit-strings. One child gets the positions of the elements in the chosen subsection from parent 1, and the relative order of the remainder of the elements from parent 2. The other child gets the positions of the elements in the chosen subsection from parent 2, and the relative order of the remainder of the elements from parent 1. The relative ordered elements begin at index 0 (unless that index is part of the random segment) continue until the cross segment is reached, jump the cross region, and continue until the right end of the permutation.

For example, consider the permutation p1 = [0, 1, 2, 3, 4, 5, 6, 7] and the permutation p2 = [1, 2, 0, 5, 6, 7, 4, 3]. Consider that the random subsection of the permutations begins at index 2 and ends at index 4, inclusive. Thus, one child will get the positions of 2, 3, 4 from p1, and the relative ordering of the rest from p2. First, fill in the 2, 3, 4 from p1 to get c1 = [x, x, 2, 3, 4, x, x, x]. c1 will get the relative order of the rest of the elements from p2, namely 1, 0, 5, 6, 7. Fill these in that order into c1 beginning at the left-most index, and jumping over the cross segment, and continuing to the right end to get c1 = [1, 0, 2, 3, 4, 5, 6, 7]. In a similar way, c2 begins with the positions of 0, 5, 6 from p2, such that c2 = [x, x, 0, 5, 6, x, x, x]. c2 then gets the relative order of the remainder of the elements from p1, namely 1, 2, 3, 4, 7. After filling these in the given order, from the left-most open index to the right-most, we end up with c2 = [1, 2, 0, 5, 6, 3, 4, 7].

The worst case runtime of a call to cross is O(n), where n is the length of the permutations.

NWOX was introduced in the following paper:

Vincent A. Cicirello. Non-Wrapping Order Crossover: An Order Preserving Crossover Operator that Respects Absolute Position. Proceedings of the Genetic and Evolutionary Computation Conference (GECCO'06), volume 2, pages 1125-1131. ACM Press, July 2006. doi:10.1145/1143997.1144177. [PDF] [BIB] [From the ACM Digital Library]

  • Constructor Details

    • NonWrappingOrderCrossover

      public NonWrappingOrderCrossover()
      Constructs a non-wrapping order crossover (NWOX) operator.

  • Method Details

    • cross

      public void cross(Permutation c1, Permutation c2)
      Description copied from interface: CrossoverOperator
      Performs a crossover for an evolutionary algorithm, such that crossover forms two children from two parents. Implementations of this method modify the parameters, transforming the parents into the children.
      Specified by:
      cross in interface CrossoverOperator<Permutation>
      Parameters:
      c1 - A candidate solution subject to the crossover. This method changes the state of c1.
      c2 - A candidate solution subject to the crossover. This method changes the state of c2.

    • apply

      public void apply(int[] raw1, int[] raw2)
      See PermutationBinaryOperator for details of this method. This method is not intended for direct usage. Use the cross(org.cicirello.permutations.Permutation, org.cicirello.permutations.Permutation) method instead.
      Specified by:
      apply in interface PermutationBinaryOperator
      Parameters:
      raw1 - The raw representation of the first permutation.
      raw2 - The raw representation of the second permutation.

    • split

      public NonWrappingOrderCrossover split()
      Description copied from interface: Splittable
      Generates a functionally identical copy of this object, for use in multithreaded implementations of search algorithms. The state of the object that is returned may or may not be identical to that of the original. Thus, this is a distinct concept from the functionality of the Copyable interface. Classes that implement this interface must ensure that the object returned performs the same functionality, and that it does not share any state data that would be either unsafe or inefficient for concurrent access by multiple threads. The split method is allowed to simply return the this reference, provided that it is both safe and efficient for multiple threads to share a single copy of the Splittable object. The intention is to provide a multithreaded search with the capability to provide spawned threads with their own distinct search operators. Such multithreaded algorithms can call the split method for each thread it spawns to generate a functionally identical copy of the operator, but with independent state.
      Specified by:
      split in interface Splittable<CrossoverOperator<Permutation>>
      Returns:
      A functionally identical copy of the object, or a reference to this if it is both safe and efficient for multiple threads to share a single instance of this Splittable object.