Serialize random state/seed?

[Pedro Gimeno]

Jan Wielemaker wrote:
> I'm using the GMP random functions in SWI-Prolog. I'm trying to emulate
> the API of an old but de-facto standard random library for Prolog. One
> of the operations there is to fetch the state of the random generator
> and later restore it. The state is an arbitrary Prolog term, so you can
> store it, or read/write it to a file.

There's no provision for exporting/importing the state in the current
version. I've pushed for it but it didn't seem to be favourably received.

See for example:
http://gmplib.org/list-archives/gmp-discuss/2010-September/004283.html
but I seem to remember older discussion.

> which half-way (the remark about _mp_d is suspicious) suggests to fetch
> the _mp_seed member. Alas, when handled as a mpz_t, it is considered 0
> because _mp_size is always 0.
> 
> I can set _mp_size to _mp_alloc in the seed to get a nice big number and
> write a function to set this back using mpz_set on the _mp_seed member.

That would be tying your application to the internals of the generator,
which is not a very good idea. That structure is inherited from the
first GMP random generation functions, which had to be reused to
accommodate a more flexible random number generation infrastructure, and
should be changed as soon as binary compatibility is decided to be
broken, if that ever happens, together with other legacy stuff.

The state is considered opaque, to allow for other kinds of generators
in future. My initial idea was to provide for import/export functions as
part of each generator (see the message above), but I did not submit it
as part of the initial patch, and now it would seem like an unwanted
addition.

But just in case I am wrong in my assessment, I have to ask again: would
a patch to implement functionality for importing/exporting the random
state of each generator, be accepted?

If not, or in the meanwhile, you can try to learn about the structure of
each generator and write your own reader for the internal storage format
of the state depending on the generator type.

-- Pedro Gimeno