Perl and integer overflows

[Shlomi Fish]

On Sunday 20 Jun 2010 13:25:00 Shachar Shemesh wrote:
> Hi all,
> 
> I'm trying to calculate the result of a string hash function in perl. I
> need an explicit function, as the perl output is used to create a C
> program, that will then use that very same hash function. Please, do not
> direct me to perl's excellent hash handling. I know it's there. I use it
> where applicable. This is not one of those cases.
> 
> The hash function I'm using is sdbm hash function
> (http://www.cse.yorku.ca/~oz/hash.html). To calculate it, I need to
> perform integer arithmetics, knowing full well it may overflow. It is
> okay for it to overflow, as I only need the bottom 32 bits. In fact, my
> code ANDs the result with 0xffffffff, to make sure that, on 64 bit
> platforms, I only get the bottom 32 bits.
> 
> Here's the problem. When the integer gets too big, Perl replaces it with
> MAXINT. On 64 bit platforms, that's not too bad - I can simply truncate
> to 32 bits after each operation. On 32 bit platforms, I'm a bit at a
> loss as to what to do.

You can compile Perl with 64-bit integers, even on 32-bit platforms. That or 
use a big-integer module such as Math::BigInt :

http://perldoc.perl.org/Math/BigInt.html

See its lib => 'GMP' option if you want much better speed.

Regards,

	Shlomi Fish

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