Class CycleMutation

  • All Implemented Interfaces:
    Splittable<MutationOperator<Permutation>>, MutationOperator<Permutation>, UndoableMutationOperator<Permutation>

    public final class CycleMutation
    extends Object
    implements UndoableMutationOperator<Permutation>

    This class implements a cycle mutation on permutations, where one mutation generates a random permutation cycle. Given the original parent permutation and its mutant, a permutation cycle can be defined as follows. Imagine a graph with n vertexes, where n is the permutation length. Now consider that for each index i, we define an edge in that graph between vertex parent[i] and vertex mutant[i]. A permutation cycle consists of all of the elements from one of the cycles in that graph. The length of a cycle is the number of elements in it. Consider an example permutation, p1 = [0, 1, 2, 3, 4], and another permutation, p2 = [0, 3, 2, 1, 4]. This pair of permutations has a 2-cycle (i.e., a cycle of length 2) consisting of elements 1 and 3. Consider a second example, p1 = [0, 1, 2, 3, 4], and p2 = [0, 4, 2, 1, 3]. This example has a 3-cycle consisting of elements 1, 3, and 4. Notice that position 1 has elements 1 and 4, position 4 has elements 4 and 3, and position 3 has elements 3 and 1, so in the hypothetical graph described above, there would be an edges from 1 to 4, 4 to 3, and 3 to 1, a cycle of length 3.

    This mutation operator is configured with a parameter to specify the maximum cycle size. A call to the mutate method chooses the cycle size k uniformly at random from [2, max], and then creates a random k-element cycle. The combination of k elements is chosen uniformly at random from all possible combinations of k elements. Note that a 2-cycle is simply a swap.

    The runtime of the mutate method is O(min(n, max2)), and derives from the combination of algorithms utilized by the RandomIndexer class in sampling k random integers. For small values of max, the runtime is essentially constant. The runtime of the undo method is O(max).

    • Constructor Detail

      • CycleMutation

        public CycleMutation​(int maxCycleLength)
        Constructs an CycleMutation mutation operator.
        Parameters:
        maxCycleLength - The maximum length cycle to generate, which must be at least 2.
        Throws:
        IllegalArgumentException - if maxCycleLength < 2.
    • Method Detail

      • mutate

        public final 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 final 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 CycleMutation 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<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.