Simulating int arithmetic with wrap-around

[Random832]

On Fri, Dec 30, 2016, at 09:47, Steve D'Aprano wrote:
> Again, assume both operands are in range for an N-bit signed integer.
> What's
> a good way to efficiently, or at least not too inefficiently, do the
> calculations in Python?

I'd do something like:

bit_mask = (1 << bits) - 1 # 0xFFFF sign_bit = 1 << (bits - 1) # 0x8000 
sign_ext = ~bit_mask # ...FFFFF0000

def signed(value):
    if value & sign_bit:
        return value | sign_ext
    else:
        return value & bit_mask

def unsigned(value):
    return value & bit_mask

And also avoid doing it on intermediate steps where it can be shown to not 
affect the result or allow the value to grow too large.