protected int partition(double[] array, int[] index, int l, int r, 
                        final int indexStart) {
    
    double pivot = array[(l + r) / 2];
    int help;

    while (l < r) {
      while ((array[l] < pivot) && (l < r)) {
        l++;
      }
      while ((array[r] > pivot) && (l < r)) {
        r--;
      }
      if (l < r) {
        help = index[indexStart+l];
        index[indexStart+l] = index[indexStart+r];
        index[indexStart+r] = help;
        l++;
        r--;
      }
    }
    if ((l == r) && (array[r] > pivot)) {
      r--;
    } 

    return r;
  }