#17717: Failure to reduce int modulo large prime
----------------------------+--------------------------------------
Reporter: xander.faber | Owner:
Type: defect | Status: new
Priority: minor | Milestone: sage-6.5
Component: finite rings | Keywords: modular arithmetic
Merged in: | Authors:
Reviewers: | Report Upstream: N/A
Work issues: | Branch:
Commit: | Dependencies:
Stopgaps: |
----------------------------+--------------------------------------
{{{
sage: p = next_prime(2^32)
sage: p
4294967311
sage: GF(p)(int(p+1))
4294967312
sage: GF(p)(p+1)
1
}}}
The issue arises in
sage/rings/finite_rings/integer_mod.pyx
class method
IntegerMod_gmp.set_from_long
On line 1732, the code reduces value (mod modulus) when value < 0 or (
value < modulus ). The latter condition is backward. It should be replaced
with ( modulus < value).
Replace line 1732
`if value < 0 or mpz_cmp_si(self.__modulus.sageInteger.value,
value) >= 0:`
with
`if value < 0 or mpz_cmp_si(value,
self.__modulus.sageInteger.value) >= 0:`
--
Ticket URL: <http://trac.sagemath.org/ticket/17717>
Sage <http://www.sagemath.org>
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