Hints for writing bit-twiddling code in Python

[Peter Otten]

Steven D'Aprano wrote:

> I have some bit-twiddling code written in Java which I am trying to port
> to Python. I'm not getting the same results though, and I think the
> problem is due to differences between Java's signed byte/int/long types,
> and Python's unified long integer type. E.g. Java's >>> is not exactly
> the same as Python's >> operator, and a character coerced to a byte in
> Java is not the same as ord(char) in Python. (The Java byte is in the
> range -128...127, I think, while the ord in Python is in 0...255.)
> 
> Can anyone point me to some good resources to help me port the Java code
> to Python?
> 
> If it helps, the Java code includes bits like this:
> 
> long newSeed = (seed & 0xFFFFFFFFL) * 0x41A7L;
> while (newSeed >= 0x80000000L) {
>     newSeed = (newSeed & 0x7FFFFFFFL) + (newSeed >>> 31L);
>     }
> seed = (newSeed == 0x7FFFFFFFL) ? 0 : (int)newSeed;
> 
> 
> which I've translated into:
> 
> newseed = (seed & 0xFFFFFFFF)*0x41A7
> while (newseed >= 0x80000000):
>     newseed = (newseed & 0x7FFFFFFF) + (newseed >> 31)
> seed = 0 if newseed == 0x7FFFFFFF else newseed & 0xFFFFFFFF

I think you need to take negative ints into account. Try adding

if seed & 0x80000000: # 2**31
    seed -= 0x100000000 # 2**32, two's complement