permutations in lexicographic order

permutation.c

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

int numbers[4] = { 1, 2, 3, 4};

int asc (const void * a, const void * b)
{
  if ( *(int*)a <  *(int*)b ) return -1;
  if ( *(int*)a == *(int*)b ) return 0;
  if ( *(int*)a >  *(int*)b ) return 1;
}

bool next_permutation(int* numbers, int length)
{
  const int invalid = -1;
  int k = invalid;
  int l = invalid;
  for(int i = 0; i < length-1; ++i)
  {
    if (numbers[i] < numbers[i+1] && i > k)
    {
      k = i;
    }
  }
  for(int i = 0; i < length; ++i)
  {
    if (numbers[k] < numbers[i] && i > l)
    {
      l = i;
    }
  }
  if (k != invalid && l != invalid)
  {
    int tmp = numbers[l];
    numbers[l] = numbers[k];
    numbers[k] = tmp;
  }

  int s = k + 1;
  int e = length;
  int d = e - s;

  for(int i = 0; i < d/2; ++i)
  {
    int tmp = numbers[s + i];
    numbers[s + i] = numbers[e - i - 1];
    numbers[e - i - 1] = tmp;
  }
  return k != invalid;
}

int main() {
  qsort (numbers, 4, sizeof(int), asc);
  printf("%d %d %d %d\n", numbers[0], numbers[1], numbers[2], numbers[3]);
  while(next_permutation(numbers, 4))
  {
      printf("%d %d %d %d\n", numbers[0], numbers[1], numbers[2], numbers[3]);
  }
}