PolyhedralFan: fix for trivial fans (#2951)

oscar-system · Oct 25, 2023 · 4918a7a · 4918a7a
1 parent d5e7b73
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Showing 2 changed files with 22 additions and 6 deletions.
diff --git a/src/PolyhedralGeometry/PolyhedralFan/constructors.jl b/src/PolyhedralGeometry/PolyhedralFan/constructors.jl
@@ -101,19 +101,24 @@ Get the underlying polymake object, which can be used via Polymake.jl.
 """
 pm_object(PF::PolyhedralFan) = PF.pm_fan
 
-polyhedral_fan(itr::AbstractVector{Cone{T}}) where T<:scalar_types = PolyhedralFan{T}(Polymake.fan.check_fan_objects(pm_object.(itr)...), coefficient_field(iterate(itr)[1]))
+function polyhedral_fan(itr::AbstractVector{Cone{T}}) where T<:scalar_types
+  @req length(itr) > 0 "list of cones must be non-empty"
+  pmfan = Polymake.fan.check_fan_objects(pm_object.(itr)...)
+  if pmfan.N_MAXIMAL_CONES == 0
+    # if check fan returns no cones then the rays are empty and we have just one trivial cone (maybe with lineality)
+    C = itr[1]
+    return polyhedral_fan(coefficient_field(C), rays(C), lineality_space(C), IncidenceMatrix(1,0); non_redundant=true)
+  end
+  return PolyhedralFan{T}(pmfan, coefficient_field(iterate(itr)[1]))
+end
 
 #Same construction for when the user gives Matrix{Bool} as incidence matrix
 polyhedral_fan(f::scalar_type_or_field, Rays::AbstractCollection[RayVector], LS::AbstractCollection[RayVector], Incidence::Matrix{Bool}) =
   polyhedral_fan(f, Rays, LS, IncidenceMatrix(Polymake.IncidenceMatrix(Incidence)))
 polyhedral_fan(f::scalar_type_or_field, Rays::AbstractCollection[RayVector], Incidence::Matrix{Bool}) =
   polyhedral_fan(f, Rays, IncidenceMatrix(Polymake.IncidenceMatrix(Incidence)))
 
-
-function polyhedral_fan(C::Cone{T}) where T<:scalar_types
-    pmfan = Polymake.fan.check_fan_objects(pm_object(C))
-    return PolyhedralFan{T}(pmfan, coefficient_field(C))
-end
+polyhedral_fan(C::Cone{T}) where T<:scalar_types = polyhedral_fan([C])
 
 ###############################################################################
 ###############################################################################

diff --git a/test/PolyhedralGeometry/polyhedral_fan.jl b/test/PolyhedralGeometry/polyhedral_fan.jl
@@ -80,6 +80,17 @@
     @test lineality_space(F2NR) == collect(eachrow(L))
     @test ray_indices(maximal_cones(F2NR)) == incidence2
     @test IncidenceMatrix(maximal_cones(F2NR)) == incidence2
+
+    C = positive_hull(f, identity_matrix(ZZ, 0))
+    pf = polyhedral_fan(C)
+    @test n_maximal_cones(pf) == 1
+    @test dim(pf) == 0
+    pfc = polyhedral_fan(maximal_cones(pf))
+    @test n_maximal_cones(pfc) == 1
+
+    C = positive_hull(f, vcat(identity_matrix(ZZ, 4), -identity_matrix(ZZ,4)))
+    pf = polyhedral_fan(C)
+    @test n_maximal_cones(pf) == 1
   end
 
 end