merge_states: don't call defrag_states if unnecessary

* spot/twa/twagraph.cc (merge_states): Return the number
of removed states, and use that to decide if defrag_states
is needed.
* spot/twa/twagraph.hh, NEWS: Document that.
* tests/core/tgbagraph.test, tests/core/twagraph.cc: Adjust test case.

NEWS

@@ -228,6 +228,8 @@ New in spot 2.9.7.dev (not yet released)
     before attempting to detect mergeable states.  When merging states
     with identical outgoing transitions, it will now also consider
     self-looping transitions has having identical destinations.
+    Additionally, this function now returns the number of states that
+    have been merged (and therefore removed from the automaton).
 
   Python:
 

spot/twa/twagraph.cc

@@ -289,7 +289,7 @@ namespace spot
     g_.chain_edges_();
   }
 
-  void twa_graph::merge_states()
+  unsigned twa_graph::merge_states()
   {
     if (!is_existential())
       throw std::runtime_error(
@@ -379,7 +379,10 @@ namespace spot
       else
         s = -1U;
 
-    defrag_states(remap, st);
+    unsigned merged = num_states() - st;
+    if (merged)
+      defrag_states(remap, st);
+    return merged;
   }
 
   void twa_graph::purge_unreachable_states(shift_action* f, void* action_data)

spot/twa/twagraph.hh

@@ -579,8 +579,19 @@ namespace spot
     ///
     /// The implementation will sort the edges of the automaton to
     /// ease the comparison between two states.  This may miss some
-    /// self-loop based in non-deterministic automata.
+    /// self-loop equivalences in non-deterministic automata.
-    void merge_states();
+    ///
+    /// States whose input have been redirected as a consequence of a
+    /// merge are removed from the automaton.  This procedure
+    /// therefore renumber states.
+    ///
+    /// Merging states might create duplicate transitions in the
+    /// automaton.  For instance (1)-a->(2) and (1)-a->(3) will become
+    /// (1)-a->(1) and (1)-a->(1) if (1), (2) and (3) are merged into
+    /// (1).
+    ///
+    /// \return the number of states that have been merged and removed.
+    unsigned merge_states();
 
     /// \brief Remove all dead states
     ///

tests/core/tgbagraph.test

@@ -1,6 +1,6 @@
 #!/bin/sh
 # -*- coding: utf-8 -*-
-# Copyright (C) 2014-2018, 2020 Laboratoire de Recherche et
+# Copyright (C) 2014-2018, 2020, 2021 Laboratoire de Recherche et
 # Développement de l'Epita (LRDE).
 #
 # This file is part of Spot, a model checking library.
@@ -291,16 +291,20 @@ State: 2
 [t] 0&1
 --END--
 HOA: v1
-States: 1
+States: 2
-Start: 0
+Start: 1
 AP: 0
 acc-name: all
 Acceptance: 0 t
 properties: trans-labels explicit-labels state-acc complete
-properties: deterministic
 --BODY--
 State: 0
 [t] 0
+State: 1
+[t] 0
+[t] 0
+[t] 0
+[t] 0
 --END--
 EOF
 

tests/core/twagraph.cc

@@ -1,5 +1,5 @@
 // -*- coding: utf-8 -*-
-// Copyright (C) 2014-2018 Laboratoire de Recherche et Développement
+// Copyright (C) 2014-2019, 2021 Laboratoire de Recherche et Développement
 // de l'Epita (LRDE).
 //
 // This file is part of Spot, a model checking library.
@@ -167,14 +167,22 @@ static void f5()
   auto s1 = tg->new_state();
   auto s2 = tg->new_state();
   auto s3 = tg->new_state();
+  auto s4 = tg->new_state();
+  auto s5 = tg->new_state();
 
-  tg->set_init_state(s3);
+  tg->set_init_state(s5);
   tg->new_edge(s1, s2, bddtrue);
   tg->new_edge(s2, s2, bddtrue);
   tg->new_edge(s3, s2, bddtrue);
+  tg->new_edge(s4, s4, bddtrue);
+  tg->new_edge(s5, s1, bddtrue);
+  tg->new_edge(s5, s2, bddtrue);
+  tg->new_edge(s5, s3, bddtrue);
+  tg->new_edge(s5, s4, bddtrue);
 
-  tg->merge_states();
+  unsigned out = tg->merge_states();
-
+  std::cerr << out << '\n';
+  assert(out == 3);
   spot::print_hoa(std::cout, tg) << '\n';
 }