/* Recursive Factorial */
long int rfact
  (long int x)
{
  long int rval;
  if (x <= 1)
    return 1;
  rval = rfact(x-1);
  return rval * x;
}

/* Tail recursive computation */
long int t_helper
  (long int x, long int val)
{
  if (x <= 1)
    return val;
  return
    t_helper(x-1, val*x);
}

/* Tail recursive version of factorial */
long int tfact(long int x)
{
  return t_helper(x, 1);
}

/* Binary partitioning helper */
long int r_prod
  (long int from, long int to)
{
  long int middle;
  long int prodA, prodB;
  if (from >= to)
    return from;
  middle = (from + to) >> 1;
  prodA = r_prod(from, middle);
  prodB = r_prod(middle+1, to);
  return prodA * prodB;
}

/* Binary splitting version of factorial */
int bfact
  (long int x)
{
  return r_prod(1,x);
}

/* Tail recursive updating computation */
void s_helper
  (long int x,
   long int *accum)
{
  if (x <= 1)
    return;
  else {
    *accum *= x;
    s_helper
      (x-1, accum);
  }
}

/* Storing version of factorial */
long int sfact
  (long int x)
{
  long int val = 1;
  s_helper(x, &val);
  return val;
}


long int left_prod(long int *, long int *);
long int right_prod(long int *, long int *);

long int left_prod
  (long int *leftp, long int *rightp)
{
  long int left = *leftp;
  if (left >= *rightp)
    return left;
  else {
    long int plus1 = left+1;
    return left *
      right_prod(&plus1, rightp);
  }
}

long int right_prod
  (long int *leftp, long int *rightp)
{
  long int right = *rightp;
  if (*leftp == right)
    return right;
  else {
    long int minus1 = right-1;
    return right *
      left_prod(leftp, &minus1);
  }
}

/* Factorial with mutual recursion */
long int lrfact(long int x)
{
  long int left = 1;
  return left_prod(&left, &x);
}

main () {
  lrfact(4);
}