Improve purge_unreachable_states()

* NEWS: document it
* spot/twa/twagraph.hh, spot/twa/twagraph.cc: implement it
* tests/core/tgbagraph.test, tests/core/twagraph.cc: test it

spot/twa/twagraph.cc

@@ -231,7 +231,7 @@ namespace spot
     g_.chain_edges_();
   }
 
-  void twa_graph::purge_unreachable_states()
+  void twa_graph::purge_unreachable_states(shift_action* f, void* action_data)
   {
     unsigned num_states = g_.num_states();
     // The TODO vector serves two purposes:
@@ -280,6 +280,9 @@ namespace spot
     if (prop_complete().is_false())
       prop_complete(trival::maybe());
 
+    if (f)
+      (*f)(todo, action_data);
+
     defrag_states(std::move(todo), current);
   }
 

spot/twa/twagraph.hh

@@ -576,8 +576,16 @@ namespace spot
     /// purge_dead_states() will remove more states than
     /// purge_unreachable_states(), but it is more costly.
     ///
+    /// You can pass a function to this method, which will be invoked
+    /// with a vector indicating the renumbering of states.
+    /// newst[i] == -1U means that state i is unreachable and thus deleted.
+    /// Otherwise, state i is renumbered newst[i].
+    ///
     /// \see purge_dead_states
-    void purge_unreachable_states();
+    typedef void (*shift_action)(const std::vector<unsigned>& newst,
+                                 void* action_data);
+    void purge_unreachable_states(shift_action* f = nullptr,
+                                  void* action_data = nullptr);
 
     /// \brief Remove unused atomic propositions
     ///