def gcd(a, b):
    """ computes the greatest common divisor of a and b.
    """
    if a == 0 and b == 0:   ##this throws a cream pie in the caller's face.
        raise ValueError
    if a == 0:
        return b
    if b == 0:
        return a
    d = 2
    out = 1
    n = min(a,b)
    while d <= n:
        if b%d == 0 and a%d ==0:
            out = d
        d += 1
    return out

print(gcd(12,15)== 3)
print(gcd(1332, 185) == 37)
print(gcd(1048576, 7776) == 32)
print(gcd(13244123124125, 32421421212625))