Re: ellgroup hangs for certain input on Version 2.6.2 (development git-0f10e3b)

[Karim Belabas <Karim.Belabas@math.u-bordeaux1.fr>]

* Richard Heylen [2013-11-07 11:47]:
> I believe it's policy that functions don't bother to check for bad
> input values in order to keep things fast. However, when a function
> hangs using 100% CPU on bad input then I think it needs to be fixed
> because this would be bad as part of a long running cpu intensive pari
> script.
> 
> Cheers!
> 
> Richard
> 
> ? aa=ffgen(Mod(1,3)*x^2+Mod(2,3)*x+Mod(1,3),'a)
> %1 = a
> ? e=ellinit([aa^3,0],aa);
> ? ellgroup(e)

Just don't use reducible polynomials as input to ffgen :-)

Prompted by your mail, I included a test for irreducibility in ffgen but it's
far too expensive in general:

? P=ffinit(nextprime(2^500),1009);
time = 120 ms.


Before:
? ffgen(P);
time = 0ms.

After:
? ffgen(P);   \\ runs out of memory if parisize < 800MB
time = 5min, 23,092 ms.

So I removed that new check.

I kept a check for squarefree-ness, which is still usually more
expensive than producing P itsel, but not much more; and does not use too much
memory. But I still feel it's a waste...

Cheers,

    K.B.
--
Karim Belabas, IMB (UMR 5251)  Tel: (+33) (0)5 40 00 26 17
Universite Bordeaux 1          Fax: (+33) (0)5 40 00 69 50
351, cours de la Liberation    http://www.math.u-bordeaux1.fr/~kbelabas/
F-33405 Talence (France)       http://pari.math.u-bordeaux1.fr/  [PARI/GP]
`