Fix direction handling and incorrect EulerianDigraph requirement in FacialWalks

doc/attr.xml

@@ -1535,28 +1535,27 @@ gap> DigraphAllChordlessCycles(D);
   <Oper Name="FacialWalks" Arg="digraph, list"/>
   <Returns>A list of lists of vertices.</Returns>
   <Description>
-    If <A>digraph</A> is an Eulerian digraph and <A>list</A> is a rotation system of <A>digraph</A>, 
+    If <A>digraph</A> is a digraph and <A>list</A> is a rotation system of <A>digraph</A>, 
     then <C>FacialWalks</C> returns a list of the <E>facial walks</E> in <A>digraph</A>. <P/>
 
-    A rotation system defines for each vertex the ordering of the out-neighbours.
+    A rotation system defines for each vertex of a symmetric digraph the ordering of the neighbours.
     For example, the method <Ref Subsect="PlanarEmbedding" Style="Number" /> computes for a
     planar digraph <A>D</A> the rotation system of a planar embedding of <A>D</A>.
     The facial walks of <A>digraph</A> are closed walks and they are defined by the rotation system <A>list</A>. 
     They describe the boundaries of the faces of the embedding of <A>digraph</A> given
     by the rotation system <A>list</A>.
 
-    The operation <C>FacialWalks</C> ignores
-    multiple edges and loops.<P/>
+    The operation <C>FacialWalks</C> ignores multiple edges, loops and the direction of the edges.<P/>
     Here are some examples for planar embeddings:
     <Example><![CDATA[
 gap> D1 := CycleDigraph(4);;
 gap> planar := PlanarEmbedding(D1);
-[ [ 2 ], [ 3 ], [ 4 ], [ 1 ] ]
+[ [ 2, 4 ], [ 3, 1 ], [ 4, 2 ], [ 1, 3 ] ]
 gap> FacialWalks(D1, planar);
-[ [ 1, 2, 3, 4 ] ]
+[ [ 1, 2, 3, 4 ], [ 1, 4, 3, 2 ] ]
 gap> nonPlanar := [[2, 4], [1, 3], [2, 4], [1, 3]];;
 gap> FacialWalks(D1, nonPlanar);
-[ [ 1, 2, 3, 4 ] ]
+[ [ 1, 2, 3, 4 ], [ 1, 4, 3, 2 ] ]
 gap> D2 := CompleteMultipartiteDigraph([2, 2, 2]);;
 gap> rotationSystem := PlanarEmbedding(D2);
 [ [ 3, 5, 4, 6 ], [ 6, 4, 5, 3 ], [ 6, 2, 5, 1 ], [ 1, 5, 2, 6 ],

doc/planar.xml

@@ -258,12 +258,12 @@ gap> KuratowskiOuterPlanarSubdigraph(D);
     in <Cite Key="BM06"/>.
 
 <Example><![CDATA[
-gap> D := Digraph([[3, 5, 10], [8, 9, 10], [1, 4], [3, 6], 
+gap> D := Digraph([[3, 5, 10], [8, 9, 10], [1, 4], [3, 6],
 > [1, 7, 11], [4, 7], [6, 8], [2, 7], [2, 11], [1, 2], [5, 9]]);
 <immutable digraph with 11 vertices, 25 edges>
 gap> PlanarEmbedding(D);
 [ [ 3, 10, 5 ], [ 10, 8, 9 ], [ 4, 1 ], [ 6, 3 ], [ 1, 11, 7 ], 
-  [ 7, 4 ], [ 8, 6 ], [ 7, 2 ], [ 2, 11 ], [ 1, 2 ], [ 9, 5 ] ]
+  [ 7, 4 ], [ 5, 8, 6 ], [ 7, 2 ], [ 2, 11 ], [ 1, 2 ], [ 9, 5 ] ]
 gap> D := Digraph([[2, 4, 7, 9, 10], [1, 3, 4, 6, 9, 10], [6, 10], 
 > [2, 5, 8, 9], [1, 2, 3, 4, 6, 7, 9, 10], [3, 4, 5, 7, 9, 10], 
 > [3, 4, 5, 6, 9, 10], [3, 4, 5, 7, 9], [2, 3, 5, 6, 7, 8], [3, 5]]);
@@ -274,8 +274,8 @@ gap> D := Digraph(IsMutableDigraph, [[3, 5, 10], [8, 9, 10], [1, 4],
 > [3, 6], [1, 7, 11], [4, 7], [6, 8], [2, 7], [2, 11], [1, 2], [5, 9]]);
 <mutable digraph with 11 vertices, 25 edges>
 gap> PlanarEmbedding(D);
-[ [ 3, 10, 5 ], [ 10, 8, 9 ], [ 4, 1 ], [ 6, 3 ], [ 1, 11, 7 ], 
-  [ 7, 4 ], [ 8, 6 ], [ 7, 2 ], [ 2, 11 ], [ 1, 2 ], [ 9, 5 ] ]
+[ [ 3, 10, 5 ], [ 10, 8, 9 ], [ 4, 1 ], [ 6, 3 ], [ 1, 11, 7 ],
+  [ 7, 4 ], [ 5, 8, 6 ], [ 7, 2 ], [ 2, 11 ], [ 1, 2 ], [ 9, 5 ] ]
 gap> D := Digraph(IsMutableDigraph, [[2, 4, 7, 9, 10],
 > [1, 3, 4, 6, 9, 10], [6, 10], [2, 5, 8, 9],
 > [1, 2, 3, 4, 6, 7, 9, 10], [3, 4, 5, 7, 9, 10],
@@ -433,13 +433,15 @@ fail
   <Attr Name="DualPlanarGraph" Arg="digraph"/>
   <Returns>A digraph or <K>fail</K>.</Returns>
   <Description>
-    If <A>digraph</A> is a planar digraph, then <Ref Attr="DualPlanarGraph"/> returns the the dual graph of <A>digraph</A>.
+    If <A>digraph</A> is a planar digraph, then <Ref Attr="DualPlanarGraph"/> returns the symmetric dual graph
+    of <A>digraph</A>.
     If <A>digraph</A> is not planar, then <K>fail</K> is returned.<P/>
 
     The dual graph of a planar digraph <A>digraph</A> has a vertex for each face of <A>digraph</A> and an edge for 
     each pair of faces that are separated by an edge from each other.
     Vertex <A>i</A> of the dual graph corresponds to the facial walk at the <A>i</A>-th position calling 
     <Ref Oper="FacialWalks"/> of <A>digraph</A> with the rotation system returned by <Ref Attr="PlanarEmbedding"/>.
+    The directions and multiplicities of any edges in digraph are ignored by <Ref Attr="DualPlanarGraph"/>.
     <P/>
 
     Note that <Ref Attr="PlanarEmbedding"/>, and therefore

gap/attr.gi

@@ -1859,10 +1859,6 @@ InstallMethod(FacialWalks, "for a digraph and a dense list",
 function(D, rotationSystem)
   local FacialWalk, facialWalks, remEdges, cycle;
 
-  if not IsEulerianDigraph(D) then
-    ErrorNoReturn("the 1st argument (digraph <D>) must be Eulerian");
-  fi;
-
   if Length(rotationSystem) <> DigraphNrVertices(D)
       or not ForAll(rotationSystem, IsList) then
     ErrorNoReturn("the 2nd argument (dense list <rotationSystem>) is not a ",
@@ -1911,8 +1907,8 @@ function(D, rotationSystem)
     return cycle;
   end;
 
-  D := DigraphRemoveLoops(DigraphRemoveAllMultipleEdges(
-       DigraphMutableCopyIfMutable(D)));
+  D := DigraphSymmetricClosure(DigraphRemoveLoops(DigraphRemoveAllMultipleEdges(
+       DigraphMutableCopyIfMutable(D))));
 
   facialWalks := [];
   remEdges := ShallowCopy(DigraphEdges(D));

gap/planar.gi

@@ -44,6 +44,7 @@ end);
 
 InstallMethod(PlanarEmbedding, "for a digraph", [IsDigraph],
 function(D)
+  D := DigraphSymmetricClosure(DigraphMutableCopyIfMutable(D));
   if DIGRAPHS_HasTrivialRotationSystem(D) then;
     return OutNeighbors(D);
   fi;

tst/standard/attr.tst

@@ -1097,8 +1097,6 @@ gap> DigraphAllUndirectedSimpleCircuits(g);
   [ 9, 5, 6, 10 ], [ 9, 5, 7, 8, 6, 10 ] ]
 
 # FacialCycles
-gap> FacialWalks(ChainDigraph(3), []);
-Error, the 1st argument (digraph <D>) must be Eulerian
 gap> FacialWalks(CycleDigraph(3), []);
 Error, the 2nd argument (dense list <rotationSystem>) is not a rotation system\
  for the 1st argument (digraph <D>), expected a list of 3 lists,
@@ -1109,6 +1107,9 @@ gap> FacialWalks(CycleDigraph(3), [[4], [1], [3]]);
 Error, the 2nd argument (dense list <rotationSystem>) is not a rotation system\
  for the 1st argument (digraph <D>), expected its union to be the vertices of \
 <D>,
+gap> g := ChainDigraph(3);;
+gap> FacialWalks(g, PlanarEmbedding(g));
+[ [ 1, 2, 3, 2 ] ]
 gap> g := Digraph([]);;
 gap> rotationSy := [];;
 gap> FacialWalks(g, rotationSy);
@@ -1119,12 +1120,12 @@ gap> FacialWalks(g, rotationSy);
 [ [ 1, 2, 3, 4, 3, 2 ] ]
 gap> g := CycleDigraph(4);;
 gap> planar := PlanarEmbedding(g);
-[ [ 2 ], [ 3 ], [ 4 ], [ 1 ] ]
+[ [ 2, 4 ], [ 3, 1 ], [ 4, 2 ], [ 1, 3 ] ]
 gap> FacialWalks(g, planar);
-[ [ 1, 2, 3, 4 ] ]
+[ [ 1, 2, 3, 4 ], [ 1, 4, 3, 2 ] ]
 gap> nonPlanar := [[2, 4], [1, 3], [2, 4], [1, 3]];;
 gap> FacialWalks(g, nonPlanar);
-[ [ 1, 2, 3, 4 ] ]
+[ [ 1, 2, 3, 4 ], [ 1, 4, 3, 2 ] ]
 gap> g := CompleteMultipartiteDigraph([2, 2, 2]);;
 gap> rotationSystem := PlanarEmbedding(g);
 [ [ 3, 5, 4, 6 ], [ 6, 4, 5, 3 ], [ 6, 2, 5, 1 ], [ 1, 5, 2, 6 ], 

tst/standard/planar.tst

@@ -90,7 +90,7 @@ gap> D := Digraph([[3, 5, 10], [8, 9, 10], [1, 4], [3, 6], [1, 7, 11], [4, 7],
 <immutable digraph with 11 vertices, 25 edges>
 gap> PlanarEmbedding(D);
 [ [ 3, 10, 5 ], [ 10, 8, 9 ], [ 4, 1 ], [ 6, 3 ], [ 1, 11, 7 ], [ 7, 4 ], 
-  [ 8, 6 ], [ 7, 2 ], [ 2, 11 ], [ 1, 2 ], [ 9, 5 ] ]
+  [ 5, 8, 6 ], [ 7, 2 ], [ 2, 11 ], [ 1, 2 ], [ 9, 5 ] ]
 gap> D := Digraph([[2, 4, 7, 9, 10], [1, 3, 4, 6, 9, 10], [6, 10], 
 > [2, 5, 8, 9], [1, 2, 3, 4, 6, 7, 9, 10], [3, 4, 5, 7, 9, 10], 
 > [3, 4, 5, 6, 9, 10], [3, 4, 5, 7, 9], [2, 3, 5, 6, 7, 8], [3, 5]]);