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Chips-n-Salsa is a Java library of customizable, hybridizable, iterative, parallel, stochastic, and self-adaptive local search algorithms. The library includes implementations of several stochastic local search algorithms, including simulated annealing, hill climbers, as well as constructive search algorithms such as stochastic sampling. Chips-n-Salsa now also includes genetic algorithms as well as evolutionary algorithms more generally. The library very extensively supports simulated annealing. It includes several classes for representing solutions to a variety of optimization problems. For example, the library includes a BitVector class that implements vectors of bits, as well as classes for representing solutions to problems where we are searching for an optimal vector of integers or reals. For each of the built-in representations, the library provides the most common mutation operators for generating random neighbors of candidate solutions, as well as common crossover operators for use with evolutionary algorithms. Additionally, the library provides extensive support for permutation optimization problems, including implementations of many different mutation operators for permutations, and utilizing the efficiently implemented Permutation class of the JavaPermutationTools (JPT) library. Chips-n-Salsa is customizable, making extensive use of Java's generic types, enabling using the library to optimize other types of representations beyond what is provided in the library. It is hybridizable, providing support for integrating multiple forms of local search (e.g., using a hill climber on a solution generated by simulated annealing), creating hybrid mutation operators (e.g., local search using multiple mutation operators), as well as support for running more than one type of search for the same problem concurrently using multiple threads as a form of algorithm portfolio. Chips-n-Salsa is iterative, with support for multistart metaheuristics, including implementations of several restart schedules for varying the run lengths across the restarts. It also supports parallel execution of multiple instances of the same, or different, stochastic local search algorithms for an instance of a problem to accelerate the search process. The library supports self-adaptive search in a variety of ways, such as including implementations of adaptive annealing schedules for simulated annealing, such as the Modified Lam schedule, implementations of the simpler annealing schedules but which self-tune the initial temperature and other parameters, and restart schedules that adapt to run length.

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/*
 * Chips-n-Salsa: A library of parallel self-adaptive local search algorithms.
 * Copyright (C) 2002-2022 Vincent A. Cicirello
 *
 * This file is part of Chips-n-Salsa (https://chips-n-salsa.cicirello.org/).
 *
 * Chips-n-Salsa 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.
 *
 * Chips-n-Salsa 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 this program.  If not, see .
 */

package org.cicirello.search.operators.permutations;

import org.cicirello.math.rand.RandomIndexer;
import org.cicirello.permutations.Permutation;
import org.cicirello.permutations.PermutationUnaryOperator;
import org.cicirello.search.operators.UndoableMutationOperator;

/**
 * This class implements a scramble mutation on permutations, where one mutation consists in
 * randomizing the order of a randomly selected subpermutation. This version of scramble mutation
 * also supports the undo method. The pair of indexes that indicate the subpermutation to scramble
 * is chosen uniformly at random from among all n(n-1)/2 possible pairs of indexes, where n is the
 * length of the permutation.
 *
 * The runtime (worst case and average case) of both the {@link #mutate(Permutation) mutate} and
 * {@link #undo(Permutation) undo} methods is O(n), where n is the length of the permutation. The
 * worst case runtime occurs when the random indexes are the two end points. On average, a scramble
 * mutation moves approximately n/3 elements.
 *
 * If you don't need the {@link #undo(Permutation) undo} method, then it is recommended that you
 * instead use the {@link ScrambleMutation} class instead to avoid the O(n) extra memory required to
 * store the prior permutation state, as well as the time associated with copying that state prior
 * to mutation.
 *
 * @author Vincent A. Cicirello, https://www.cicirello.org/
 */
public class UndoableScrambleMutation
    implements UndoableMutationOperator, PermutationUnaryOperator {

  private int[] last;
  private final int[] indexes;

  /** Constructs an UndoableScrambleMutation mutation operator. */
  public UndoableScrambleMutation() {
    indexes = new int[2];
  }

  @Override
  public final void mutate(Permutation c) {
    if (c.length() >= 2) {
      last = c.toArray();
      generateIndexes(c.length(), indexes);
      c.scramble(indexes[0], indexes[1]);
    }
  }

  @Override
  public final void undo(Permutation c) {
    if (c.length() >= 2) {
      c.apply(this);
    }
  }

  @Override
  public UndoableScrambleMutation split() {
    return new UndoableScrambleMutation();
  }

  /**
   * See {@link PermutationUnaryOperator} for details of this method. This method is not intended
   * for direct usage. Use the {@link #undo} method instead.
   *
   * @param perm The raw representation of the permutation for undoing the most recent mutation.
   */
  @Override
  public final void apply(int[] perm) {
    if (indexes[0] < indexes[1]) {
      System.arraycopy(last, indexes[0], perm, indexes[0], indexes[1] - indexes[0] + 1);
    } else {
      System.arraycopy(last, indexes[1], perm, indexes[1], indexes[0] - indexes[1] + 1);
    }
  }

  /*
   * This package access method allows the window limited version
   * implemented as a subclass to change how indexes are generated
   * without modifying the mutate method.
   */
  void generateIndexes(int n, int[] indexes) {
    RandomIndexer.nextIntPair(n, indexes);
  }
}