FM Pocketstation IR code

def f(a):
    xs = []
    for i in range(4):
        xs.append(a & 0xFF)
        a >>= 8

    R0 = sum(xs)
    R1 = xs[0] * xs[3]
    R2 = xs[1] * xs[2]
    R4 = xs[2] << 8
    R3 = xs[0] + 256 * xs[1]

    numb = R0 + R1 + R2 + R3 + R4
    return numb

def card(a):
    numb = f(a) & 0xFFFF
    code = (numb % 720) - (numb // 720)
    if (code < 0):
        code = numb % 720
    return code + 1