tgbagraph: add a merge_transitions() method.

* src/graph/graph.hh: Add some framework to erase transitions, and
defrag the resulting transitions_ vector on demand.  Also remove
the nb_states() and nb_transitions() because num_states() and
num_transitions() already exist.
* src/graphtest/graph.cc, src/graphtest/ngraph.cc: Adjust to
use num_states().
* src/tgba/tgbagraph.hh (merge_transitions): New method.
* src/misc/hash.hh: Add a pair_hash class, needed by
merge_transitions().
* src/graphtest/tgbagraph.cc, src/graphtest/tgbagraph.test: Add states
for transitions removal and merge_transitions().

src/tgba/tgbagraph.hh

@@ -233,7 +233,7 @@ namespace spot
     {
       auto s = down_cast<const typename graph_t::state_storage_t*>(st);
       assert(s);
-      assert(s->succ < g_.num_transitions());
+      assert(!s->succ || g_.valid_trans(s->succ));
 
       if (this->iter_cache_)
 	{
@@ -308,6 +308,47 @@ namespace spot
     {
       return bddtrue;
     }
+
+    /// Iterate over all transitions, and merge those with compatible
+    /// extremities and acceptance.
+    void merge_transitions()
+    {
+      for (auto& s: g_.states())
+	{
+	  // Map a pair (dest state, acc) to the first transition seen
+	  // with such characteristic.
+
+	  typedef std::pair<graph_t::state, int> key_t;
+	  std::unordered_map<key_t, graph_t::transition, pair_hash> trmap;
+
+	  auto t = g_.out_iteraser(s);
+	  while (t)
+	    {
+	      // Simply skip false transitions.
+	      if (t->cond == bddfalse)
+		{
+		  t.erase();
+		  continue;
+		}
+
+	      key_t k(t->dst, t->acc.id());
+	      auto p = trmap.insert(make_pair(k, t.trans()));
+	      if (!p.second)
+		{
+		  // A previous transitions exist for k, merge the
+		  // condition, and schedule the transition for
+		  // removal.
+		  g_.trans_data(p.first->second).cond |= t->cond;
+		  t.erase();
+		}
+	      else
+		{
+		  ++t;
+		}
+	    }
+	}
+      g_.defrag();
+    }
   };
 
 }