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

Class BlockInterchangeMutation

java.lang.Object
org.cicirello.search.operators.permutations.BlockInterchangeMutation
All Implemented Interfaces:
Splittable<MutationOperator<Permutation>>, IterableMutationOperator<Permutation>, MutationOperator<Permutation>, UndoableMutationOperator<Permutation>
public final class BlockInterchangeMutation extends Object implements UndoableMutationOperator<Permutation>, IterableMutationOperator<Permutation>
This class implements a block interchange mutation on permutations, where one mutation consists in swapping two randomly chosen non-overlapping "blocks" (i.e., subsequences). The block interchange is chosen uniformly at random from among all possible block interchanges. The two random blocks are not required to be of the same length, but are required to be non-overlapping (i.e., cannot share elements).

As an example, consider the permutation: p1 = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]. If a block interchange swaps the blocks [1, 2] and [5, 6, 7, 8], the result is: [0, 5, 6, 7, 8, 3, 4, 1, 2, 9]. This mutation operator is related to the BlockMoveMutation, which swaps a pair of randomly selected adjacent blocks.

The runtime (worst case and average case) of both the mutate and undo methods is O(n), where n is the length of the permutation. There are a variety of ways of demonstrating the worst case behavior, one of which is if the exchanged blocks are at opposite ends of the permutation and are of differing lengths, which would result in movement of every permutation element. On average, approximately 0.6 n elements are moved by this mutation operator.

  • Constructor Details

    • BlockInterchangeMutation

      public BlockInterchangeMutation()
      Constructs a BlockInterchangeMutation mutation operator.

  • Method Details

    • mutate

      public void mutate(Permutation c)
      Description copied from interface: MutationOperator
      Mutates a candidate solution to a problem, by randomly modifying its state. The mutant that is produced is in the local neighborhood of the original candidate solution.
      Specified by:
      mutate in interface MutationOperator<Permutation>
      Parameters:
      c - The candidate solution subject to the mutation. This method changes the state of c.

    • undo

      public void undo(Permutation c)
      Description copied from interface: UndoableMutationOperator
      Returns a candidate solution to its previous state prior to the most recent mutation performed.

      For example, consider the following. Let c' be the current state of c. Let c'' be the state of c after mutate(c); If we then call undo(c), the state of c should revert back to c'.

      The behavior of undo is undefined if c is altered by some other process between the calls to mutate and undo. The behavior is also undefined if a different candidate is given to undo then the last given to mutate. For example, if the following two statements are executed, mutate(c); undo(d);, the effect on d is undefined as it wasn't the most recently mutated candidate solution.

      Specified by:
      undo in interface UndoableMutationOperator<Permutation>
      Parameters:
      c - The candidate solution to revert.

    • split

      public BlockInterchangeMutation 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 IterableMutationOperator<Permutation>
      Specified by:
      split in interface Splittable<MutationOperator<Permutation>>
      Specified by:
      split in interface UndoableMutationOperator<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.

    • iterator

      public MutationIterator iterator(Permutation p)
      Creates and returns a MutationIterator that can be used to systematically iterate over all of the direct neighbors (i.e., a single mutation step away) of a candidate solution, as one might do in a hill climber.

      The worst case runtime of the MutationIterator.hasNext() and the MutationIterator.setSavepoint() methods of the MutationIterator created by this method is O(1). The worst case runtime of the MutationIterator.nextMutant() and MutationIterator.rollback() methods is O(n), where n is the length of the Permutation.

      Specified by:
      iterator in interface IterableMutationOperator<Permutation>
      Parameters:
      p - The candidate solution subject to the mutation. Calling methods of the MutationIterator that is returned changes the state of that candidate solution. See the documentation of those methods for details of how such changes may occur.
      Returns:
      A MutationIterator for iterating over the direct neighbors of a candidate solution.