stein's gcd algorithm

gcd.py

#!/usr/bin/env python

def is_even(x):
    return x & 1

def gcd(x,y):
    factors_of_2 = 0
    while true:
        if x==0:
            return y << factors_of_2
        elif y==0:
            return x << factors_of_2
        elif is_even(x) and is_even(y):
            factors_of_2 += 1
            x >>= 1
            y >>= 1
        elif is_even(x):
            x >>= 1
        elif is_even(y):
            y>>=1
        else:
            mx = max(x,y)
            mn = min(x,y)
            (x,y) = (mn, mx-mn)