trac #17474 adding an increasing option to digraph of polytope

src/sage/geometry/polyhedron/base.py

@@ -3277,7 +3277,7 @@ def vertex_graph(self):
 
     graph = vertex_graph
 
-    def vertex_digraph(self, f):
+    def vertex_digraph(self, f, increasing=False):
         """
         Return the directed graph of the polyhedron according to a linear form.
 
@@ -3287,7 +3287,10 @@ def vertex_digraph(self, f):
 
         - `f` -- a linear form (as a tuple, list or vector)
 
-        An edge is oriented from `v` to `w` if `f(v-w) > 0`.
+	- ``increasing`` -- boolean (default ``False``) wether to orient 
+	  edges in the increasing direction instead.
+
+        By default, an edge is oriented from `v` to `w` if `f(v-w) > 0`.
 
         EXAMPLES::
 
@@ -3301,6 +3304,8 @@ def vertex_digraph(self, f):
         """
         from sage.graphs.digraph import DiGraph
         dg = DiGraph()
+	if increasing:
+	    f = -vector(f)
         for j in range(self.n_vertices()):
             vj = self.Vrepresentation(j)
             for vi in vj.neighbors():