Trac #15491: directed immutable graphs report twice too many edges

As reported on #15278 :

{{{
sage: g=digraphs.RandomDirectedGNP(10,.3)
sage: gi=DiGraph(g,data_structure="static_sparse")
sage: print gi.size(), len(gi.edges())
68 34
}}}

Simplest bug eve : Sage was taught to return the wrong thing. The sum of
all arcs in the digraph, plus the sum of all arcs in the reversed
digraph. That's clearly more than necessary `:-P`

Sorryyyyyyyyyyyyyyyyyyyy !!

Nathann

URL: http://trac.sagemath.org/15491
Reported by: ncohen
Ticket author(s): Nathann Cohen
Reviewer(s): Simon King

src/sage/graphs/base/static_sparse_backend.pyx

@@ -676,23 +676,40 @@ class StaticSparseBackend(CGraphBackend):
         INPUT:
 
         - ``directed`` (boolean) -- whether to consider the graph as directed or
-          not (
+          not.
 
         TEST::
 
             sage: from sage.graphs.base.static_sparse_backend import StaticSparseBackend
             sage: g = StaticSparseBackend(graphs.PetersenGraph())
             sage: g.num_edges(False)
             15
+
+        Testing the exception::
+
+            sage: g = StaticSparseBackend(digraphs.Circuit(4))
+            sage: g.num_edges(False)
+            Traceback (most recent call last):
+            ...
+            NotImplementedError: Sorry, I have no idea what is expected in this situation. I don't think that it is well-defined either, especially for multigraphs.
+
+        :trac:`15491`::
+
+            sage: g=digraphs.RandomDirectedGNP(10,.3)
+            sage: gi=DiGraph(g,data_structure="static_sparse")
+            sage: gi.size() == len(gi.edges())
+            True
         """
         cdef StaticSparseCGraph cg = <StaticSparseCGraph> self._cg
-        cdef unsigned int m
+
         if directed:
             if cg.directed:
                 # Returns the real number of directed arcs
-                return int(cg.g.m+cg.g_rev.m)
+                return int(cg.g.m)
             else:
-                # Returns twice the number of edges, minus the number of loops
+                # Returns twice the number of edges, minus the number of
+                # loops. This is actually equal to the index of
+                # cg.g.neighbors[cg.g.n] in the array `cg.g.edges`
                 return int(cg.g.neighbors[cg.g.n]-cg.g.edges)
         else:
             if cg.directed: