Merge pull request #4336 from paemurru/ep/fix_empty_cones

Fix zero-dimensional cone in `cones` in PolyhedralGeometry

src/PolyhedralGeometry/PolyhedralFan/properties.jl

@@ -212,8 +212,21 @@ julia> cones(PF, 2)
 """
 function cones(PF::_FanLikeType, cone_dim::Int)
   l = cone_dim - length(lineality_space(PF))
-  l < 1 && return nothing
-  return SubObjectIterator{Cone{_get_scalar_type(PF)}}(
+  t = Cone{_get_scalar_type(PF)}
+  (l < 0 || dim(PF) == -1) && return _empty_subobjectiterator(t, PF)
+
+  if l == 0
+    return SubObjectIterator{t}(
+      PF,
+      (_, _, _) -> positive_hull(
+        coefficient_field(PF), zeros(Int, ambient_dim(PF)), lineality_space(PF)
+      ),
+      1,
+      NamedTuple(),
+    )
+  end
+
+  return SubObjectIterator{t}(
     PF, _cone_of_dim, size(Polymake.fan.cones_of_dim(pm_object(PF), l), 1), (c_dim=l,)
   )
 end

test/PolyhedralGeometry/polyhedral_fan.jl

@@ -58,11 +58,18 @@
     @test size(cones(F2, 2)) == (2,)
     @test lineality_space(cones(F2, 2)[1]) == [[0, 1, 0]]
     @test rays.(cones(F2, 2)) == [[], []]
-    @test isnothing(cones(F2, 1))
     @test _check_im_perm_rows(ray_indices(cones(F1, 2)), incidence1)
     @test _check_im_perm_rows(incidence_matrix(cones(F1, 2)), incidence1)
     @test _check_im_perm_rows(cones(IncidenceMatrix, F1, 2), incidence1)
 
+    A3 = affine_space(NormalToricVariety, 3)
+    @test length(cones(A3, 1)) == 3
+    @test length(cones(A3, 0)) == 1
+    @test length(cones(A3, -1)) == 0
+    @test cones(A3, 1) isa SubObjectIterator{Cone{QQFieldElem}}
+    @test cones(A3, 0) isa SubObjectIterator{Cone{QQFieldElem}}
+    @test cones(A3, -1) isa SubObjectIterator{Cone{QQFieldElem}}
+
     II = ray_indices(maximal_cones(NFsquare))
     NF0 = polyhedral_fan(II, rays(NFsquare))
     @test n_rays(NF0) == 4