fix: make invmod more robust for non-prime moduli

(before it was giving silently wrong results)
Nemocas · Dec 20, 2024 · 7a51b60 · 7a51b60
1 parent 58071cd
commit 7a51b60
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diff --git a/src/HeckeMoreStuff.jl b/src/HeckeMoreStuff.jl
@@ -415,7 +415,8 @@ end
 function invmod(f::ZZModPolyRingElem, M::ZZModPolyRingElem)
   if !is_unit(f)
     r = parent(f)()
-    i = @ccall libflint.fmpz_mod_poly_invmod(r::Ref{ZZModPolyRingElem}, f::Ref{ZZModPolyRingElem}, M::Ref{ZZModPolyRingElem}, f.parent.base_ring.ninv::Ref{fmpz_mod_ctx_struct})::Int
+    ff = ZZ()
+    i = @ccall libflint.fmpz_mod_poly_invmod_f(ff::Ref{ZZRingElem}, r::Ref{ZZModPolyRingElem}, f::Ref{ZZModPolyRingElem}, M::Ref{ZZModPolyRingElem}, f.parent.base_ring.ninv::Ref{fmpz_mod_ctx_struct})::Int
     if iszero(i)
       error("not yet implemented")
     else

diff --git a/test/flint/fmpz_mod_poly-test.jl b/test/flint/fmpz_mod_poly-test.jl
@@ -343,6 +343,12 @@ end
   @test powermod(f, -10, g) == 78305338116088931412*x+91239060941924718463
 
   @test powermod(f, -ZZRingElem(10), g) == 78305338116088931412*x+91239060941924718463
+
+  R, = residue_ring(ZZ, ZZ(2809))
+  Rx, x = R[:x]
+  f = 32*x^9 + 28*x^8 + 2497*x^7 + 2443*x^6 + 708*x^5 + 996*x^4 + 2557*x^3 + 2104*x^2 + 2517*x + 2752
+  g = x^10 + 2798*x^8 + 2806*x^7 + 37*x^6 + 14*x^5 + 2761*x^4 + 2787*x^3 + 20*x^2 + 12*x + 1
+  @test_throws ErrorException invmod(f, g)
 end
 
 @testset "ZZModPolyRingElem.euclidean_division" begin