JavaAlgorithms/Sorts/TimSort.java

package Sorts;

import java.util.Random;

/**
 * @author [Hemanth Kotagiri](https://github.com/hemanth-kotagiri)
 * @see [Tim Sort](https://en.wikipedia.org/wiki/Tim_sort)
 */
class TimSort {
  int array[];
  int array_length;
  int RUN = 32;

  /**
   * @brief A constructor which takes in the array specified by the user.
   * @param array : Array given by the user.
   */
  public TimSort(int[] array) {
    this.array = array;
    this.array_length = array.length;
  }

  /**
   * @brief A constructor which takes in an array length and randomly initializes an array.
   * @param array_length length given by the user.
   */
  public TimSort(int array_length) {
    Random rand = new Random();

    this.array_length = array_length;
    this.array = new int[this.array_length];

    for (int i = 0; i < this.array_length; i++) {
      int random_number = rand.nextInt(1000);
      this.array[i] = random_number;
    }
  }

  /**
   * @brief A method to change the size of the run.
   * @param run : Value specified by the user to change the run.
   */
  public void change_run(int run) {
    this.RUN = run;
  }

  /**
   * @brief A default constructor when no parameters are given. Initializes the array length to be
   *     100. Generates a random number array of size 100.
   */
  public TimSort() {
    this.array_length = 100;
    this.array = new int[this.array_length];

    Random rand = new Random();
    for (int i = 0; i < this.array_length; i++) {
      int random_number = rand.nextInt(1000);
      this.array[i] = random_number;
    }
  }

  /**
   * @brief Performs Insertion Sort Algorithm on given array with bounded indices.
   * @param array: The array on which the algorithm is to be performed.
   * @param start_idx: The starting index from which the algorithm is to be performed.
   * @param end_idx: The ending index at which the algorithm needs to stop sorting.
   */
  public void insertion_sort(int[] array, int start_idx, int end_idx) {
    for (int i = 0; i < array.length; i++) {
      int current_element = array[i];
      int j = i - 1;
      while (j >= 0 && array[j] > current_element) {
        array[j + 1] = array[j];
        j--;
      }
      array[j + 1] = current_element;
    }
  }

  /**
   * @brief A method to merge two runs(chunks of array).
   * @param array: The origin array which is to be sorted.
   * @param start: Starting index of the first run(chunk).
   * @param mid: The ending index of the first run(chunk).
   * @param end: Ending index of the second run(chunk).
   */
  public void merge_runs(int array[], int start, int mid, int end) {

    int first_array_size = mid - start + 1, second_array_size = end - mid;
    int array1[] = new int[first_array_size], array2[] = new int[second_array_size];
    int i = 0, j = 0, k = 0;

    // Building the two sub arrays from the array to merge later
    for (i = 0; i < first_array_size; i++) array1[i] = array[start + i];
    for (i = 0; i < second_array_size; i++) array2[i] = array[mid + 1 + i];

    i = 0;
    j = 0;
    k = start;

    while (i < first_array_size && j < second_array_size) {
      if (array1[i] <= array2[j]) {
        array[k] = array1[i];
        i++;
      } else {
        array[k] = array2[j];
        j++;
      }
      k++;
    }

    while (i < first_array_size) {
      array[k] = array1[i];
      k++;
      i++;
    }

    while (j < second_array_size) {
      array[k] = array2[j];
      k++;
      j++;
    }
  }

  /** @brief Tim Sort Algorithm method. */
  public void algorithm() {
    // Before Sorting
    System.out.println("Before sorting the array: ");
    this.showArrayElements();
    System.out.println();

    // Applying insertion sort on RUNS.
    for (int i = 0; i < this.array_length; i += this.RUN)
      this.insertion_sort(this.array, i, Math.min(i + this.RUN, (this.array_length - 1)));

    for (int split = this.RUN; split < this.array_length; split = 2 * split) {
      for (int start_idx = 0; start_idx < this.array_length; start_idx += 2 * split) {
        int mid = start_idx + split - 1;
        int end_idx = Math.min((start_idx + 2 * split - 1), (this.array_length - 1));

        this.merge_runs(this.array, start_idx, mid, end_idx);
      }
    }
    // After sorting
    System.out.println("After sorting the array: ");
    this.showArrayElements();
    System.out.println();
  }

  /** @brief A method to show the elements inside the array. */
  public void showArrayElements() {
    for (int i = 0; i < this.array.length; i++) {
      System.out.print(this.array[i] + " ");
    }
    System.out.println();
  }

  /** @brief A method to test the sorting algorithm */
  static void test() {
    int[] array = {4, 1, 3, 17, 12, 11, 8};
    TimSort sorterObj1 = new TimSort();
    TimSort sorterObj2 = new TimSort(50);
    TimSort sorterObj3 = new TimSort(array);

    sorterObj1.algorithm();
    sorterObj2.algorithm();
    sorterObj3.algorithm();

    // Testing the first array
    for (int i = 0; i < sorterObj1.array_length - 1; i++) {
      assert ((sorterObj1.array[i] <= sorterObj1.array[i + 1])) : "Array is not sorted";
    }

    // Testing the second array.
    for (int i = 0; i < sorterObj2.array_length - 1; i++) {
      assert ((sorterObj2.array[i] <= sorterObj2.array[i + 1])) : "Array is not sorted";
    }

    // Testing the third array.
    for (int i = 0; i < sorterObj3.array_length - 1; i++) {
      assert ((sorterObj3.array[i] <= sorterObj3.array[i + 1])) : "Array is not sorted";
    }
  }

  public static void main(String[] args) {
    test();
  }
}