Trac #25994: Blocks_and_cut_vertices - bug with directed graphs and B…

…oost interface

While using the Boost interface to compute the blocks and cut vertices,
the output is wrong if the input is a directed graph.
{{{
sage: from sage.graphs.connectivity import blocks_and_cut_vertices
sage: rings = graphs.CycleGraph(10)
sage: rings.merge_vertices([0, 5])
sage: blocks_and_cut_vertices(rings, algorithm="Tarjan_Boost")
([[0, 1, 4, 2, 3], [0, 6, 9, 7, 8]], [0])
}}}
{{{
sage: from sage.graphs.connectivity import blocks_and_cut_vertices
sage: rings = graphs.CycleGraph(10)
sage: rings.merge_vertices([0, 5])
sage: rings = rings.to_directed()
sage: blocks_and_cut_vertices(rings, algorithm="Tarjan_Boost")
([[0, 1, 4, 2, 3, 6, 7, 8, 9], [0, 6, 9, 7, 8]], [0, 9, 8, 6, 7])
}}}

If the input graph is a directed graph, the blocks and cut vertices are
computed on the underlying simple graph.

URL: https://trac.sagemath.org/25994
Reported by: meghanamreddy
Ticket author(s): Meghana M Reddy
Reviewer(s): David Coudert

src/sage/graphs/base/boost_graph.pyx

@@ -748,7 +748,7 @@ cpdef blocks_and_cut_vertices(g):
     if not isinstance(g, GenericGraph):
         raise TypeError("the input must be a Sage graph")
 
-    if g.allows_loops() or g.allows_multiple_edges():
+    if g.allows_loops() or g.allows_multiple_edges() or g.is_directed():
         g = g.to_simple()
 
     cdef BoostVecGraph g_boost

src/sage/graphs/connectivity.pyx

@@ -399,6 +399,14 @@ def blocks_and_cut_vertices(G, algorithm="Tarjan_Boost"):
         sage: blocks_and_cut_vertices(g)
         ([[1, 2, 3], [5, 6], [5, 7], [5, 8], [4]], [5])
 
+    A directed graph with Boost's algorithm (:trac:`25994`)::
+
+        sage: rings = graphs.CycleGraph(10)
+        sage: rings.merge_vertices([0, 5])
+        sage: rings = rings.to_directed()
+        sage: blocks_and_cut_vertices(rings, algorithm="Tarjan_Boost")
+        ([[0, 1, 4, 2, 3], [0, 6, 9, 7, 8]], [0])
+
     TESTS::
 
         sage: blocks_and_cut_vertices(Graph(0))