JavaAlgorithms/Sorts/BitonicSort.java

package Sorts;

/* Java program for Bitonic Sort. Note that this program
works only when size of input is a power of 2. */
public class BitonicSort {
  /* The parameter dir indicates the sorting direction,
  ASCENDING or DESCENDING; if (a[i] > a[j]) agrees
  with the direction, then a[i] and a[j] are
  interchanged. */
  void compAndSwap(int a[], int i, int j, int dir) {
    if ((a[i] > a[j] && dir == 1) || (a[i] < a[j] && dir == 0)) {
      // Swapping elements
      int temp = a[i];
      a[i] = a[j];
      a[j] = temp;
    }
  }

  /* It recursively sorts a bitonic sequence in ascending
  order, if dir = 1, and in descending order otherwise
  (means dir=0). The sequence to be sorted starts at
  index position low, the parameter cnt is the number
  of elements to be sorted.*/
  void bitonicMerge(int a[], int low, int cnt, int dir) {
    if (cnt > 1) {
      int k = cnt / 2;
      for (int i = low; i < low + k; i++) compAndSwap(a, i, i + k, dir);
      bitonicMerge(a, low, k, dir);
      bitonicMerge(a, low + k, k, dir);
    }
  }

  /* This funcion first produces a bitonic sequence by
  recursively sorting its two halves in opposite sorting
  orders, and then calls bitonicMerge to make them in
  the same order */
  void bitonicSort(int a[], int low, int cnt, int dir) {
    if (cnt > 1) {
      int k = cnt / 2;

      // sort in ascending order since dir here is 1
      bitonicSort(a, low, k, 1);

      // sort in descending order since dir here is 0
      bitonicSort(a, low + k, k, 0);

      // Will merge whole sequence in ascending order
      // since dir=1.
      bitonicMerge(a, low, cnt, dir);
    }
  }

  /*Caller of bitonicSort for sorting the entire array
  of length N in ASCENDING order */
  void sort(int a[], int N, int up) {
    bitonicSort(a, 0, N, up);
  }

  /* A utility function to print array of size n */
  static void printArray(int arr[]) {
    int n = arr.length;
    for (int i = 0; i < n; ++i) System.out.print(arr[i] + " ");
    System.out.println();
  }

  public static void main(String args[]) {
    int a[] = {3, 7, 4, 8, 6, 2, 1, 5};
    int up = 1;
    BitonicSort ob = new BitonicSort();
    ob.sort(a, a.length, up);
    System.out.println("\nSorted array");
    printArray(a);
  }
}