/* An instance of this class can partition arrays in accord with a
** RED/BLUE classifier provided at construction.
** Author: R. McCloskey and ...
*/
public class RedBluePartitioner<T> {

   // instance variable
   // -----------------

   // to distinguish RED values from BLUE values
   protected RedBlueClassifier<T> rbClassifier;

   // constructor
   // -----------

   /* Establishes the given RedBlueClassifier as that to be used by
   ** this object in partitioning arrays.
   */
   public RedBluePartitioner(RedBlueClassifier<T> classifier) {
      this.rbClassifier = classifier;
   }


   // method of interest
   // ------------------
   
   /* Rearranges the elements of the given array b[] so that, for some k
   ** satisfying 0 <= k <= b.length, all RED elements are in b[0..k)
   ** and all BLUE elements are in b[k..b.length).  The value returned
   ** is k.
   */
   public int partition(T[] b) {
      int k = 0, m = b.length;
      // loop invariant: 0 <= k <= m <= b.length &&
      //                 all elements in b[0..k) are RED &&
      //                 all elements in b[m..b.length) are BLUE
      while (k != m) {
         if (b[k] is RED) { k++; }
         else if (b[m-1] is BLUE) { m--; }
         else {  // b[k] is BLUE and b[m-1] is RED
            swap(b, k, m-1);
            k++; m--;
         }
      }
      return k;
   }

   // private utility 
   // ---------------

   /* Swaps the elements at the specified locations of the specified array.
   */
   private static void swap(Object[] a, int p, int q) {
      Object temp = a[p];
      a[p] = a[q];
      a[q] = temp;
   }
   

}