/*********************
 * from wikipedia's quicksort article, 
 * the pseudo code for the "complex version" (in place) is as follows :

  function partition(array, left, right, pivotIndex)
     pivotValue := array[pivotIndex]
     swap array[pivotIndex] and array[right] // Move pivot to end
     storeIndex := left 
     for i  from  left to right - 1 // left ≤ i < right 
         if array[i] ≤ pivotValue
             swap array[i] and array[storeIndex]
             storeIndex := storeIndex + 1
     swap array[storeIndex] and array[right] // Move pivot to its final place
     return storeIndex

   procedure quicksort(array, left, right)
     if right > left
         select a pivot index //(e.g. pivotIndex := left + (right - left)/2)
         pivotNewIndex := partition(array, left, right, pivotIndex)
         quicksort(array, left, pivotNewIndex - 1)
         quicksort(array, pivotNewIndex + 1, right)

 * Here's what it might look like in C.
 *
 * Jim M | Feb 2011 | GPL
 *************/

#include <stdio.h>

// Swap two values in an array.
void swap(int array[], int i, int j){
  int tmp;
  tmp = array[i];
  array[i] = array[j];
  array[j] = tmp;
}


// See comments above.
int partition(int array[], int left, int right, int pivotIndex){
  int storeIndex, tmp, i, pivotValue;
  pivotValue = array[pivotIndex];
  swap(array, pivotIndex, right);       // move pivot to end
  storeIndex = left;
  for (i = left; i < right; i++){
    if (array[i] <= pivotValue){
      swap(array, i, storeIndex);
      storeIndex++;
    }
  }
  swap(array, storeIndex, right);       // move pivot to final place
  return storeIndex;
}

// See comments above.
void quicksort(int array[], int left, int right){
  int pivotIndex, pivotNewIndex;
  if (right > left){
    // select a pivot index; can be anything in left to right range.
    pivotIndex = left + (right - left)/2;
    pivotNewIndex = partition(array, left, right, pivotIndex);
    quicksort(array, left, pivotNewIndex - 1);
    quicksort(array, pivotNewIndex + 1, right);
  }
}

void print_array(int n, int array[]){
  // Print array[0,...(n-1)], followed by a newline.
  int i;
  printf("[");
  for (i=0; i<n; i++){
    printf("%i, ", array[i]);
  }
  printf("]\n");
}

int main(){
  int n = 10;    //  1  2  3  4  5  6   7  8  9  10
  int numbers[] =   {3, 5, 6, 8, 5, 17, 2, 0, 1, 4};
  printf("-- quicksort --\n");

  printf(" orginal: ");
  print_array(n, numbers);

  quicksort(numbers, 0, n-1);

  printf(" sorted: ");
  print_array(n, numbers);
}