twa_run: add a highlight method

* spot/twaalgos/emptiness.hh, spot/twaalgos/emptiness.cc: Add
the method.
* tests/python/highlighting.ipynb: Add a small test.

spot/twaalgos/emptiness.cc

@@ -21,6 +21,7 @@
 // along with this program.  If not, see <http://www.gnu.org/licenses/>.
 
 #include <sstream>
+#include <memory>
 #include <spot/twaalgos/emptiness.hh>
 #include <spot/twaalgos/bfssteps.hh>
 #include <spot/twaalgos/gtec/gtec.hh>
@@ -658,6 +659,52 @@ namespace spot
     return true;
   }
 
+  /// Note that this works only if the automaton is a twa_graph_ptr.
+  void twa_run::highlight(unsigned color)
+  {
+    auto a = std::dynamic_pointer_cast<twa_graph>
+      (std::const_pointer_cast<twa>(aut));
+    if (!a)
+      throw std::runtime_error("highlight() only work for twa_graph");
+
+    auto h = new std::map<unsigned, unsigned>; // highlighted edges
+
+    unsigned src = a->get_init_state_number();
+    auto l = prefix.empty() ? &cycle : &prefix;
+    auto e = l->end();
+    for (auto i = l->begin(); i != e;)
+      {
+	bdd label = i->label;
+	acc_cond::mark_t acc = i->acc;
+	unsigned dst;
+	++i;
+	if (i != e)
+	  {
+	    dst = a->state_number(i->s);
+	  }
+	else if (l == &prefix)
+	  {
+	    l = &cycle;
+	    i = l->begin();
+	    e = l->end();
+	    dst = a->state_number(i->s);
+	  }
+	else
+	  {
+	    dst = a->state_number(l->begin()->s);
+	  }
+
+	for (auto& t: a->out(src))
+	  if (t.dst == dst && t.cond == label && t.acc == acc)
+	    {
+	      (*h)[a->get_graph().index_of_edge(t)] = color;
+	      break;
+	    }
+	src = dst;
+      }
+    a->set_named_prop("highlight-edges", h);
+  }
+
   twa_graph_ptr
   twa_run::as_twa() const
   {

spot/twaalgos/emptiness.hh

@@ -1,5 +1,5 @@
 // -*- coding: utf-8 -*-
-// Copyright (C) 2011, 2013, 2014, 2015 Laboratoire de Recherche et
+// Copyright (C) 2011, 2013, 2014, 2015, 2016 Laboratoire de Recherche et
 // Developpement de l'Epita (LRDE).
 // Copyright (C) 2004, 2005 Laboratoire d'Informatique de Paris 6 (LIP6),
 // département Systèmes Répartis Coopératifs (SRC), Université Pierre
@@ -312,6 +312,11 @@ namespace spot
     /// \return true iff the run could be completed
     bool replay(std::ostream& os, bool debug = false) const;
 
+    /// \brief Highlight the accepting run on the automaton.
+    ///
+    /// Note that this works only if the automaton is a twa_graph_ptr.
+    void highlight(unsigned color);
+
     /// \brief Return a twa_graph_ptr corresponding to \a run
     ///
     /// Identical states are merged.