Re: How to best dynamically make use of cypari2?

[Vincent Delecroix <vincent.delecroix@u-bordeaux.fr>]

A more pythonic way is

try:
   import cypari2
except ImportError:
   cypari2 = None

if cypari2 is None:
    # cypari2 not available
    ...
else:
    # cypari2 is available
    ...

Le 13/06/2023 à 20:51, hermann@stamm-wilbrandt.de a écrit :
> I want to add a function to a RSA_numbers_factored python lib.
>
> Determining sum of squares from "sqrt(-1) (mod p)" takes 42min with
> Python "gcd()" on gaussian integers, but only few milliseconds for
> 388342-digit prime with Pari "halfgcd()" not available in Python.
>
> So I want to use "halfgcd()" when cpyari2 is available, and
> if not, I want to use Python "gcd()" on gaussian integers.
>
> Here is demonstration without cypari2:
>
> Type "help", "copyright", "credits" or "license" for more information.
> > > > from dyn_cypari2 import *
> > > > to_sum2sqs(12, 29)
> (2, 5)
> > > >
>
> Here with cypari2, imported before importing dyn_cypari2:
>
> Type "help", "copyright", "credits" or "license" for more information.
> > > > import cypari2
> > > > from dyn_cypari2 import *
> > > > to_sum2sqs(12, 29)
> pari
> (5, -2)
> > > >
>
>
> I did it this way, is that OK?
> Are there alternatives?
>
> $ cat dyn_cypari2.py
> from sys import modules
>
> if "cypari2" in modules:
>      import cypari2
>      pari = cypari2.Pari()
>
>      def to_sum2sqs(sqrtm1, p):
>          print("pari")
>          [M,V] = pari.halfgcd(sqrtm1, p)
>          return V[1], M[1,0]
> else:
>      from sympy import gcd, I
>
>      def to_sum2sqs(sqrtm1, p):
>          return gcd(p, sqrtm1+I).as_real_imag()
>
> def to_sqrtm1(xy, p):
>      return xy[0] * pow(xy[1], -1, p) % p
> $
>
>
> Regards,
>
> Hermann.