/* 1 1 check primeness of small numbers

<i>isprime</i> uses an inefficient brute-force method, checking for divisibility by
all possible divisors.  It should only be used to check for primeness of
relatively small numbers.

*/
double nf_isprime(const Node *n, const Cell *c)
{
  Node *r = n->u.t.right;
  double d;
  int i, j, k;

  i = d = eval_tree( r, c);

  if( i != d || i < 0)
  {
    fprintf( out, "isprime: argument must be integer >= 0\n");
    return 0;
  }

  if( i < 2) return 0;

  if( i < 4) return 1;

  if( !(i & 1)) return 0; /* not odd */

  k = i/2;

  for( j = 3; j <= k; j += 2)
    if( i % j == 0) return 0;

  return 1;
}