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Minor fixes for nmod #78

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merged 1 commit into from
Sep 9, 2023
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Minor fixes for nmod #78

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8 changes: 8 additions & 0 deletions src/flint/test/test.py
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Original file line number Diff line number Diff line change
Expand Up @@ -1279,6 +1279,7 @@ def test_nmod():
assert G(1,2) != G(0,2)
assert G(0,2) != G(0,3)
assert G(3,5) == G(8,5)
assert isinstance(hash(G(3, 5)), int)
assert raises(lambda: G([], 3), TypeError)
#assert G(3,5) == 8 # do we want this?
#assert 8 == G(3,5)
Expand All @@ -1300,9 +1301,14 @@ def test_nmod():
assert G(3,17) / G(2,17) == G(10,17)
assert G(3,17) / 2 == G(10,17)
assert 3 / G(2,17) == G(10,17)
assert G(0,3) / G(1,3) == G(0,3)
assert G(3,17) * flint.fmpq(11,5) == G(10,17)
assert G(3,17) / flint.fmpq(11,5) == G(6,17)
assert G(1,3) ** 2 == G(1,3)
assert G(2,3) ** flint.fmpz(2) == G(1,3)
assert G(flint.fmpq(2, 3), 5) == G(4,5)
assert raises(lambda: G(2,5) ** G(2,5), TypeError)
assert raises(lambda: flint.fmpz(2) ** G(2,5), TypeError)
assert raises(lambda: G(flint.fmpq(2, 3), 3), ZeroDivisionError)
assert raises(lambda: G(2,5) / G(0,5), ZeroDivisionError)
assert raises(lambda: G(2,5) / 0, ZeroDivisionError)
Expand All @@ -1314,10 +1320,12 @@ def test_nmod():
assert raises(lambda: G(2,5) - [], TypeError)
assert raises(lambda: G(2,5) * [], TypeError)
assert raises(lambda: G(2,5) / [], TypeError)
assert raises(lambda: G(2,5) ** [], TypeError)
assert raises(lambda: [] + G(2,5), TypeError)
assert raises(lambda: [] - G(2,5), TypeError)
assert raises(lambda: [] * G(2,5), TypeError)
assert raises(lambda: [] / G(2,5), TypeError)
assert raises(lambda: [] ** G(2,5), TypeError)
assert G(3,17).modulus() == 17
assert str(G(3,5)) == "3"
assert G(3,5).repr() == "nmod(3, 5)"
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20 changes: 20 additions & 0 deletions src/flint/types/nmod.pyx
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Expand Up @@ -6,6 +6,7 @@ from flint.types.fmpz cimport fmpz
from flint.types.fmpq cimport fmpq

from flint.flintlib.fmpz cimport fmpz_t
from flint.flintlib.nmod cimport nmod_pow_fmpz, nmod_inv
from flint.flintlib.nmod_vec cimport *
from flint.flintlib.fmpz cimport fmpz_fdiv_ui, fmpz_init, fmpz_clear
from flint.flintlib.fmpz cimport fmpz_set_ui, fmpz_get_ui
Expand Down Expand Up @@ -89,6 +90,9 @@ cdef class nmod(flint_scalar):
return not res
return NotImplemented

def __hash__(self):
return hash((int(self.val), self.modulus))

def __nonzero__(self):
return self.val != 0

Expand Down Expand Up @@ -178,6 +182,8 @@ cdef class nmod(flint_scalar):
return NotImplemented
if tval == 0:
raise ZeroDivisionError("%s is not invertible mod %s" % (tval, mod.n))
if not s:
return s
# XXX: check invertibility?
x = nmod_div(sval, tval, mod)
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This change makes it so that nmod(0, p) / nmod(a, p) gives nmod(0, p).

About invertibility: in the context of SymPy's use the modulus here will always be prime. Otherwise though if tval and mod are not coprime then this gives a core dump:

In [3]: flint.nmod(1, 6) / flint.nmod(3, 6)
Flint exception (Impossible inverse):
    Cannot invert modulo 3*0
Aborted (core dumped)

That is not very friendly in Python land but I am not sure if adding checks here for is worth it: you would need to compute gcd(tval, mod) and that calculation would always be unnecessary for prime mod. Ideally Flint should provide an efficient function that could return an error code rather than aborting.

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There is such a function: n_gcdinv.

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Thanks. I've added this in #79

if x == 0:
Expand All @@ -195,3 +201,17 @@ cdef class nmod(flint_scalar):

def __invert__(self):
return (1 / self) # XXX: speed up

def __pow__(self, exp):
cdef nmod r
e = any_as_fmpz(exp)
if e is NotImplemented:
return NotImplemented
r = nmod.__new__(nmod)
r.mod = self.mod
r.val = self.val
if e < 0:
r.val = nmod_inv(r.val, self.mod)
e = -e
r.val = nmod_pow_fmpz(r.val, (<fmpz>e).val, self.mod)
return r