bug in rings/fast_arith.pyx

[was]

This chunk of code in fast_arith.pyx:

    cdef public long long c_inverse_mod_longlong(self, long long a, long long m) except -1:
        cdef long long g, s, t
        g = self.c_xgcd_longlong(a,m, &s, &t)
        s = s % m
        if s < 0:
            s = s + m
        return s

should raise an exception if g != 1, so should be

    cdef public long long c_inverse_mod_longlong(self, long long a, long long m) except -1:
        cdef long long g, s, t
        g = self.c_xgcd_longlong(a,m, &s, &t)
        if g != 1: raise ZeroDivisionError
        s = s % m
        if s < 0:
            s = s + m
        return s

Apply only trac_10217_rebased.patch

[aapitzsch]

If g!=1 then Arithmetic Error is raised like in c_inverse_mod_int

[zimmerma]

I started to look at this. I had difficulties trying to find a test that could hit the case g != 1. I found the following:

from sage.ext.multi_modular import MultiModularBasis_base
mm = MultiModularBasis_base([6, 10])

which raises another problem, since the new exception is not trapped:

sage: mm.crt([1,2])
13

This is wrong since 13 is not equal to 2 mod 10...

Paul

[mhansen]

MultiModularBasis?_base requires that it's inputs actually be prime so mm = MultiModularBasis_base([6, 10]) shouldn't produce a valid object.

[mhansen]

I've added some code which adds a doctest for this functionality.

[aapitzsch]

I've replaced the deprecated raise syntax by a Python 3 compatible one.

[mhansen]

Looks good to me. If you're okay with my changes, could you mark it as positive review?

[mhansen]

Apply only trac_10217_merged.patch

[jdemeyer]

This should be rebased to sage-5.0.beta12:

applying trac_10217_merged.patch
patching file sage/ext/multi_modular.pyx
Hunk #1 FAILED at 156
1 out of 2 hunks FAILED -- saving rejects to file sage/ext/multi_modular.pyx.rej
patch failed, unable to continue (try -v)
patch failed, rejects left in working dir
errors during apply, please fix and refresh trac_10217_merged.patch

Also the Reviewer field needs to be filled in.

[aapitzsch]

rebased to sage-5.0.beta12

[aapitzsch]

Apply only trac_10217_rebased.patch