On Tuesday, August 19, 2014 7:39:18 PM UTC+2, Peter Bruin wrote: > > Hi Jean-Pierre, > > I'm currently wrapping FLINT fq modules into new Sage classes for finite >> fiels and got strange coercion errors when running non trivial code. >> Indeed, the polynomial ring constructors use a dictionary and so relies >> on the hash of finite fields (the parents). >> By some chance, the hashes of two previous implementations of finite >> fields using PARI where different because some variable name was replaced >> by something else in some object and this prevented this problem to occur. >> >> I read >> http://www.sagemath.org/doc/developer/coding_in_python.html#the-hash-special-method >> >> but would welcome any advice. >> >> Though (non-prime) finite fields with the same order and defining >> polynomial are considered equal, I'd say the hashes should be different. >> So, let's add the string of the implementation of the finite field into >> it's hash (which is cached by the way). >> Does that seem sensible? >> >> Or should I modify the finite field code so that these hashes are equal >> whatever the implementation is? >> Then we'd need coercion between all of the implementations (which is not >> currently the case but should be done anyway), but that would surely yield >> a non-negligible speed penalty, and you'll never know what the >> implementation of the base ring of the polynomial ring will be as it will >> depend on the order finite fields and their polynomial rings are created. >> >> Or should we modify the caching system of the polynomial rings not to use >> hashes but ids? >> > > I'm not sure I understand correctly; do you mean that the problem is > caused by fields comparing equal even if the implementations are > different? I think we should in any > Not sure. I seem to be able to produce things with different hashes but equal... sage: GF(13**5, 'a', impl='pari_ffelt') == GF(13**5, 'a', impl='flint_fq') True Though I have: sage: GF(13**5, 'a', impl='pari_ffelt') == GF(13**5, 'a', impl='pari_mod') False Maybe that's because of the __cmp__ functions I copied/pasted/slightly modified. Or maybe I forgot to implement some methods as the new classes are cdefed. I'll check that.
Even if it boils down to a different matter, I don't feel that the current other __hash__ method are good enough as well to tell between different finite field. It mostly work by chance. And indeed whe copy/paste/modify'ing it, it gave me similar hashes for different implems. Not sure for what __hash__ is used directly. In most cases the EqualityById stuff might do the trick. -- You received this message because you are subscribed to the Google Groups "sage-devel" group. To unsubscribe from this group and stop receiving emails from it, send an email to sage-devel+unsubscr...@googlegroups.com. To post to this group, send email to sage-devel@googlegroups.com. Visit this group at http://groups.google.com/group/sage-devel. For more options, visit https://groups.google.com/d/optout.
