Merge pull request #55 from ederc/zero-elements

Handling zero elements in input generators

Author: ederc

src/algorithms/groebner-bases.jl

@@ -147,7 +147,8 @@ function _core_groebner_basis(
         error("At the moment we only supports finite fields.")
     end
 
-    lens, cfs, exps = _convert_to_msolve(F)
+    # nr_gens might change if F contains zero polynomials
+    lens, cfs, exps, nr_gens = _convert_to_msolve(F)
 
     gb_ld  = Ref(Cint(0))
     gb_len = Ref(Ptr{Cint}(0))

src/interfaces/nemo.jl

@@ -21,6 +21,8 @@ function _convert_to_msolve(
     if field_char > 2^31 || degree(base_ring(R)) != 1
         error("At the moment we only support prime fields up to prime characteristic < 2^31.")
     end
+
+    lens = Int32[]
     # get coefficients
     if field_char == 0
         cfs = BigInt[]
@@ -29,15 +31,21 @@ function _convert_to_msolve(
     end
     if field_char == 0
         for i in 1:nr_gens
-            for cf in coefficients(F[i])
-                push!(cfs, BigInt(numerator(cf)))
-                push!(cfs, BigInt(denominator(cf)))
+            if F[i] != R(0)
+                for cf in coefficients(F[i])
+                    push!(cfs, BigInt(numerator(cf)))
+                    push!(cfs, BigInt(denominator(cf)))
+                end
+                push!(lens, length(F[i]))
             end
         end
     else
         for i in 1:nr_gens
-            for cf in coefficients(F[i])
-                push!(cfs, Int32(lift(Nemo.ZZ, cf)))
+            if F[i] != R(0)
+                for cf in coefficients(F[i])
+                    push!(cfs, Int32(lift(Nemo.ZZ, cf)))
+                end
+                push!(lens, length(F[i]))
             end
         end
     end
@@ -50,7 +58,7 @@ function _convert_to_msolve(
         end
     end
 
-    return lens, cfs, exps
+    return lens, cfs, exps, length(lens)
 end
 
 @doc Markdown.doc"""

test/interfaces/nemo.jl

@@ -1,8 +1,13 @@
 @testset "Interfaces -> Nemo" begin
     R, (x,y,z) = polynomial_ring(QQ,["x","y","z"], internal_ordering=:degrevlex)
     F = [x^2+1-3, x*y-z, x*z^2-3*y^2]
-    cmp = (Int32[2, 2, 2], BigInt[1, 1, -2, 1, 1, 1, -1, 1, 1, 1, -3, 1], Int32[2, 0, 0, 0, 0, 0, 1, 1, 0, 0, 0, 1, 1, 0, 2, 0, 2, 0])
+    cmp = (Int32[2, 2, 2], BigInt[1, 1, -2, 1, 1, 1, -1, 1, 1, 1, -3, 1], Int32[2, 0, 0, 0, 0, 0, 1, 1, 0, 0, 0, 1, 1, 0, 2, 0, 2, 0], 3)
     @test AlgebraicSolving._convert_to_msolve(F) == cmp
+    # issue #54
+    R, (x1, x2) = polynomial_ring(GF(17), ["x1", "x2"])
+    I = Ideal([x1, R(0)])
+    cmp = (Int32[1], BigInt[1], Int32[1, 0], 1) 
+    @test AlgebraicSolving._convert_to_msolve(I.gens) == cmp
     for _GF in [GF, AlgebraicSolving.Nemo.Native.GF]
       R, (x,y,z) = polynomial_ring(_GF(2147483659),["x","y","z"], internal_ordering=:degrevlex)
       F = [x^2+1-3, x*y-z, x*z^2-3*y^2]
@@ -11,6 +16,6 @@
       R, (x,y,z) = polynomial_ring(_GF(101),["x","y","z"], internal_ordering=:degrevlex)
       F = [x^2+1-3, x*y-z, x*z^2-3*y^2]
       res = AlgebraicSolving._convert_to_msolve(F)
-      @test AlgebraicSolving._convert_finite_field_array_to_abstract_algebra(Int32(3), res..., R) == F
+      @test AlgebraicSolving._convert_finite_field_array_to_abstract_algebra(Int32(3), res[1], res[2], res[3], R) == F
     end
 end