IsPrime (C#)

public static bool IsPrime(ulong N)
{
    if ((N & 1) == 0) return N == 2;
    if (N % 3 == 0) return N == 3;
    if (N == 1) return false;
    if (N > 0xfffffff9ffffffc5) throw new OverflowException();

    ulong p = 1, x = 2, max = (ulong)Math.Sqrt(N);
    while ((p += (x ^= 6)) <= max)
        if (N % p == 0)
            return false;
    return true;
}