Fast factorial implementation (using 1/2 iterations) in C

ffact.c

typedef unsigned long uli;

uli fact(uli n) {
	if (n < 2) return 1;
	uli r = (n % 2) ? 1+n/2 : 1; // if odd...
	uli p = n, a = n, b = n-2;
	while (b>1) {
		p *= a + b;
		a += b; 
		b -= 2;
	}
	return p * r; // ...multiply by mid
}