QUICKSORT.C 

#include <stdlib.h>
#include <stdio.h>

#define INSERTION_SORT_BOUND 16 /* boundary point to use insertion sort */

#define uint32 unsigned int

typedef int (*CMPFUN)(int, int);

/* explain function
 * Description:
 *   fixarray::Qsort() is an internal subroutine that implements quick sort.
 *
 * Return Value: none
 */
void Qsort(int This[], CMPFUN fun_ptr, uint32 first, uint32 last)
{
  uint32 stack_pointer = 0;
  int first_stack[32];
  int last_stack[32];

  for (;;)
  {
    if (last - first <= INSERTION_SORT_BOUND)
    {
      /* for small sort, use insertion sort */
      uint32 indx;
      int prev_val = This[first];
      int cur_val;

      for (indx = first + 1; indx <= last; ++indx)
      {
        cur_val = This[indx];
        if ((*fun_ptr)(prev_val, cur_val) > 0)
        {
          /* out of order: array[indx-1] > array[indx] */
          uint32 indx2;
          This[indx] = prev_val; /* move up the larger item first */

          /* find the insertion point for the smaller item */
          for (indx2 = indx - 1; indx2 > first; )
          {
            int temp_val = This[indx2 - 1];
            if ((*fun_ptr)(temp_val, cur_val) > 0)
            {
              This[indx2--] = temp_val;
              /* still out of order, move up 1 slot to make room */
            }
            else
              break;
          }
          This[indx2] = cur_val; /* insert the smaller item right here */
        }
        else
        {
          /* in order, advance to next element */
          prev_val = cur_val;
        }
      }
    }
    else
    {
      int pivot;

      /* try quick sort */
      {
        int temp;
        uint32 med = (first + last) >> 1;
        /* Choose pivot from first, last, and median position. */
        /* Sort the three elements. */
        temp = This[first];
        if ((*fun_ptr)(temp, This[last]) > 0)
        {
          This[first] = This[last]; This[last] = temp;
        }
        temp = This[med];
        if ((*fun_ptr)(This[first], temp) > 0)
        {
          This[med] = This[first]; This[first] = temp;
        }
        temp = This[last];
        if ((*fun_ptr)(This[med], temp) > 0)
        {
          This[last] = This[med]; This[med] = temp;
        }
        pivot = This[med];
      }
      {
        uint32 up;
        {
	  uint32 down;
          /* First and last element will be loop stopper. */
	  /* Split array into two partitions. */
	  down = first;
	  up = last;
	  for (;;)
	  {
	    do
	    {
	      ++down;
	    } while ((*fun_ptr)(pivot, This[down]) > 0);

	    do
	    {
	      --up;
	    } while ((*fun_ptr)(This[up], pivot) > 0);

	    if (up > down)
	    {
	      int temp;
	      /* interchange L[down] and L[up] */
	      temp = This[down]; This[down]= This[up]; This[up] = temp;
	    }
	    else
	      break;
	  }
	}
	{
	  uint32 len1; /* length of first segment */
	  uint32 len2; /* length of second segment */
	  len1 = up - first + 1;
	  len2 = last - up;
	  /* stack the partition that is larger */
	  if (len1 >= len2)
	  {
	    first_stack[stack_pointer] = first;
	    last_stack[stack_pointer++] = up;

	    first = up + 1;
	    /*  tail recursion elimination of
	     *  Qsort(This,fun_ptr,up + 1,last)
	     */
	  }
	  else
	  {
	    first_stack[stack_pointer] = up + 1;
	    last_stack[stack_pointer++] = last;

	    last = up;
	    /* tail recursion elimination of
	     * Qsort(This,fun_ptr,first,up)
	     */
	  }
	}
        continue;
      }
      /* end of quick sort */
    }
    if (stack_pointer > 0)
    {
      /* Sort segment from stack. */
      first = first_stack[--stack_pointer];
      last = last_stack[stack_pointer];
    }
    else
      break;
  } /* end for */
}


void ArraySort(int This[], CMPFUN fun_ptr, uint32 the_len)
{
  Qsort(This, fun_ptr, 0, the_len - 1);
}

#define ARRAY_SIZE 250000

int my_array[ARRAY_SIZE];

uint32 fill_array()
{
  int indx;
  uint32 checksum = 0;
  for (indx=0; indx < ARRAY_SIZE; ++indx)
  {
    checksum += my_array[indx] = rand();
  }
  return checksum;
}

int cmpfun(int a, int b)
{
  if (a > b)
    return 1;
  else if (a < b)
    return -1;
  else
    return 0;
}

int main()
{
  int indx;
  uint32 checksum1;
  uint32 checksum2 = 0;
  checksum1 = fill_array();

  ArraySort(my_array, cmpfun, ARRAY_SIZE);

  for (indx=1; indx < ARRAY_SIZE; ++indx)
  {
    if (my_array[indx - 1] > my_array[indx])
    {
      printf("bad sort\n");
      return(1);
    }
  }

  for (indx=0; indx < ARRAY_SIZE; ++indx)
  {
    checksum2 += my_array[indx];
  }

  if (checksum1 != checksum2)
  {
    printf("bad checksum %d %d\n", checksum1, checksum2);
    return(1);
  }

  return(0);
}