From: William Djaja Tjokroaminata <billtj@...>
Date: 2002-10-16T14:02:23+09:00
Subject: Re: Porting a CRC function to ruby

Hi Matthew,

The problem is not signed vs unsigned.  The problem is, given an int, the
Ruby Fixnum is obtained by left shifting the int by one (and setting the
least significant bit to 1).  Therefore, before you do your CRC, you have
to right shift the Fixnum.  (Just be careful that the right shifted
version of the Fixnum may not be a valid Ruby object, so "don't interact
with Ruby" using the right shifted version.)

The problem is probably more complicated if the data type used in the C
function is unsigned int or unsigned long.  In this case probably you have
to use Bignum (and know how Bignum is implemented internally in
Ruby).  However, as Ruby performs conversion between Bignum and Fixnum
automatically, you have to be careful too.

Finally, because when you are dealing with communication packets the Ruby
Fixnum is not really "clean", probably it is better and easier to use Ruby
BitVector instead of Fixnum.

Regards,

Bill
===========================================================================
Matthew Miller <namille2@vt.edu> wrote:
> Hello,

> I'm a bit new to Ruby and I'm having trouble porting a CRC function from C++
> to Ruby. The problem seems to be that Fixnums are signed while the data type
> used in the C function is an unsigned short. I guess I could compile the C++
> function into a shared object and load that in Ruby, but I would like to
> avoid that if possible as it is currently over my head.