mpz_limbs interface

[Niels Möller]

Zimmermann Paul <Paul.Zimmermann at inria.fr> writes:

> we have looked at the mpz_limbs interface, 

Thanks for the review.

> it seems it won't be sufficient for
> our needs in MPFR. Consider for example the following function, which
> generates nbits random bits into mp[]:

I see. In this particular case, I think the right gmp interface change
is to add mpn_urandomb and mpn_rrandomb (similar to current mpn_random
and mpn_random2, but with a randstate argument). If I understand this
correctly, the main obstacle is that random number internals use mpz
functions, which is an undesirable dependency for mpn functions.

Any other problematic cases?

>   MPFR_ASSERTN (nbits >= 1);
>   /* To be sure to avoid the potential allocation of mpz_urandomb */
>   ALLOC(z) = SIZ(z) = MPFR_PREC2LIMBS (nbits);
>   PTR(z)   = mp;
[...]
>   mpz_urandomb (z, rstate, nbits);

I think this is brittle, and I don't think this style should be
officially supported in the public gmp interface. mpz_urandomb is not
included in mini-gmp, but if you, e.g., try the same with other
functions, it will most likely break with mini-gmp which assigns many of
the output parameters using mpz_swap.

Regards,
/Niels

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