def prev_pow_two(n):
    """Gets the power of two that is less than or equal to the given input
    """
    if ((n & -n) == n):
        return n
    n -= 1
    n |= n >> 1
    n |= n >> 2
    n |= n >> 4
    n |= n >> 8
    n |= n >> 16
    n += 1
    return n//2

def crazy_fib(n):
    """Crazy fast fibonacci number calculation
    """
    pow_two = prev_pow_two(n)

    q = r = i = 1
    s = 0

    while i < pow_two:
        i *= 2
        q, r, s = q*q + r*r, r * (q + s), (r*r + s*s)

    while i < n:
        i += 1
        q, r, s = q+r, q, r

    return q