Re: integer representation in pari

[Sam Steingold <sds@gnu.org>]

On 5/28/10, Karim Belabas <Karim.Belabas@math.u-bordeaux1.fr> wrote:
> * Bill Allombert [2010-05-28 18:26]:
> > On Fri, May 28, 2010 at 11:13:11AM -0400, Sam Steingold wrote:
>  > > On 5/27/10, Bill Allombert <Bill.Allombert@math.u-bordeaux1.fr> wrote:
>  > > >
>  > > >  You can also check whether the C macro PARI_KERNEL_GMP is defined,
>  > > >  but usually it is better to use int_W() to acces the mantissa.
>  > >
>  > > thanks!
>  > > can I also use int_W to access the mantissa of a t_REAL?
>  > > (it never explicitly says that, but ...)
>  >
>  > No you cannot. t_REAL mantissa is always in HIGH bytes first,
>  > so this would not work with the GMP kernel. Note that
>  > t_REAL mantissa is normalized (i.e. first bit is one).
>
> All this may change (in fact, it *will* change : it is very annoying to
>  have different internal representations for t_INTs and t_REALs).

indeed.

>  In general, direct access to an object's internal representation is
>  strongly discouraged.

understandably so.

>  In the 2.4.* branch (2.5-stable-to-be), you can use trunc2nr() or
>  trun2nr_lg() to recover any part of the matissa as a t_INT.
>
>  Btw, why do you want to access words of a t_REAL mantissa in the first place ?

I want to be able to convert a PARI number (integer or real) to a Lisp number.

for that, I need something equivalent to the following API:

ulong get_integer_length (GEN gint); /* already present as lgef() */
void get_integer_data (GEN gint, ulong *data);

to be used like this:

ulong len = get_integer_length(gint);
ulong *data = (ulong*)alloca(len*sizeof(ulong));
get_integer_data(gint,data);

and a similar interface for reals:

ulong get_real_length (GEN greal); /* already present as lgef() */
ulong get_real_exponent (GEN greal); /* already present as expo() */
void get_real_mantissa (GEN greal, ulong* data);

to be used like this:

ulong len = get_real_length(greal);
ulong *data = (ulong*)alloca(len*sizeof(ulong));
get_real_mantissa(greal,data);

where the words in data and mantissa are returned in some fixed order
(platform- and library-independent).

-- 
Sam Steingold <http://sds.podval.org>