Re: Word length and number of bits in an integer

[Bill Allombert <Bill.Allombert@math.u-bordeaux1.fr>]

On Fri, Oct 19, 2012 at 05:58:11PM +0200, Dirk Laurie wrote:
> I need a routine for the exact smallest value B such that x>>B is zero,
> where x is a t_INT.  SInce a may be very large indeed, I coded it thus:
> 
> { bitsin(x) =
>    my(w=32*(length(x)-1)); x>>=w;
>       while(x, x>>=1; w+=1);
>    w
> }
> 
> I have two queries:
> 
> 1. Am I missing some useful builtin function?

Yes, but I do not thing it is available in GP.
you can do
install(expi,lG);
bitsin(x)=expi(x)+1

Maybe we could add a function logint().

> 2. Can I rely on the words making up x to be 32-bit?
>     If not, how do I find out the word length from inside GP?

You should not try :)

Cheers,
Bill.