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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) 2002-2021  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.sa;

import java.util.Arrays;

/**
 * An AcceptanceTracker can be used to extract fine-grained information about the behavior of an
 * annealing schedule across several runs of simulated annealing. Specifically, it can be used to
 * compute the rate of neighbor acceptance, across a set of runs of SA, as it changes from the
 * beginning of the run to the end of the run.
 *
 * @author Vincent A. Cicirello, https://www.cicirello.org/
 * @version 7.30.2021
 */
public final class AcceptanceTracker implements AnnealingSchedule {

  private final AnnealingSchedule schedule;
  private int[] acceptanceCounts;
  private int[] numRuns;
  private int iteration;

  /**
   * Constructs the AcceptanceTracker.
   *
   * @param schedule The AnnealingSchedule object.
   */
  public AcceptanceTracker(AnnealingSchedule schedule) {
    this.schedule = schedule;
  }

  /**
   * Computes the acceptance rate for a specific iteration number computed across all runs since
   * either the last call to {@link #reset} or since the last run of simulated annealing with a
   * different run length.
   *
   * @param iterationIndex The iteration number of interest, which must be in the interval: 0 ≤
   *     iterationIndex < maxEvals, where maxEvals is the run length of simulation annealing.
   * @return The acceptance rate across all runs since the last call to reset or since the last
   *     change in run length, computed at iteration iterationIndex.
   * @throws ArrayIndexOutOfBoundsException if iterationIndex is negative or too high.
   * @throws NullPointerException if reset has not been called and no runs of simulated annealing
   *     have been performed.
   */
  public double getAcceptanceRate(int iterationIndex) {
    if (acceptanceCounts[iterationIndex] == 0) {
      return 0;
    } else {
      return acceptanceCounts[iterationIndex] / ((double) numRuns[iterationIndex]);
    }
  }

  /**
   * Resets the AcceptanceTracker.
   *
   * @param maxEvals The length of the simulated annealing run.
   * @throws IllegalArgumentException if maxEvals ≤ 0.
   */
  public void reset(int maxEvals) {
    if (maxEvals <= 0) throw new IllegalArgumentException("maxEvals must be positive");
    if (acceptanceCounts == null || acceptanceCounts.length != maxEvals) {
      acceptanceCounts = new int[maxEvals];
      numRuns = new int[maxEvals];
    } else {
      Arrays.fill(acceptanceCounts, 0);
      Arrays.fill(numRuns, 0);
    }
  }

  @Override
  public void init(int maxEvals) {
    schedule.init(maxEvals);
    if (acceptanceCounts == null || acceptanceCounts.length != maxEvals) {
      reset(maxEvals);
    }
    iteration = 0;
  }

  @Override
  public boolean accept(double neighborCost, double currentCost) {
    boolean didAccept = schedule.accept(neighborCost, currentCost);
    if (iteration < acceptanceCounts.length) {
      if (didAccept) {
        acceptanceCounts[iteration]++;
      }
      numRuns[iteration]++;
      iteration++;
    }
    return didAccept;
  }

  @Override
  public AcceptanceTracker split() {
    return new AcceptanceTracker(schedule.split());
  }
}