JavaAlgorithms/Sorts/ShellSort.java

package Sorts;

import static Sorts.SortUtils.*;

public class ShellSort implements SortAlgorithm {

  /**
   * Implements generic shell sort.
   *
   * @param array the array to be sorted.
   * @param <T> the type of elements in the array.
   * @return the sorted array.
   */
  @Override
  public <T extends Comparable<T>> T[] sort(T[] array) {
    int length = array.length;
    int gap = 1;

    /* Calculate gap for optimization purpose */
    while (gap < length / 3) {
      gap = 3 * gap + 1;
    }

    for (; gap > 0; gap /= 3) {
      for (int i = gap; i < length; i++) {
        int j;
        T temp = array[i];
        for (j = i; j >= gap && less(temp, array[j - gap]); j -= gap) {
          array[j] = array[j - gap];
        }
        array[j] = temp;
      }
    }
    return array;
  }

  /* Driver Code */
  public static void main(String[] args) {
    Integer[] toSort = {4, 23, 6, 78, 1, 54, 231, 9, 12};

    ShellSort sort = new ShellSort();
    sort.sort(toSort);
    for (int i = 0; i < toSort.length - 1; ++i) {
      assert toSort[i] <= toSort[i + 1];
    }
    print(toSort);
  }
}