postproc: option to wdba-minimize only when sure

Fixes #15.

* spot/twaalgos/minimize.hh, spot/twaalgos/minimize.cc
(minimize_obligation_garanteed_to_work): New function.
* spot/twaalgos/postproc.hh, spot/twaalgos/postproc.cc: Use it if
wdba-minimize=1.  Handle new default for wdba-minimize.
* NEWS, bin/spot-x.cc: Document those changes.
* tests/core/ltl2tgba2.test: Add some test cases.
* tests/core/genltl.test: Improve expected results.

spot/twaalgos/minimize.cc

@@ -1,5 +1,5 @@
 // -*- coding: utf-8 -*-
-// Copyright (C) 2010-2019 Laboratoire de Recherche et Développement
+// Copyright (C) 2010-2020 Laboratoire de Recherche et Développement
 // de l'Epita (LRDE).
 //
 // This file is part of Spot, a model checking library.
@@ -567,6 +567,16 @@ namespace spot
     return product(min_aut, aut_neg)->is_empty();
   }
 
+  bool minimize_obligation_garanteed_to_work(const const_twa_graph_ptr& aut_f,
+                                             formula f)
+  {
+    // WDBA-minimization necessarily work for obligations
+    return ((f && f.is_syntactic_obligation())
+            // Weak deterministic automata are obligations
+            || (aut_f->prop_weak().is_true() && is_deterministic(aut_f))
+            // Guarantee automata are obligations as well.
+            || is_terminal_automaton(aut_f));
+  }
 
   twa_graph_ptr
   minimize_obligation(const const_twa_graph_ptr& aut_f,
@@ -580,12 +590,7 @@ namespace spot
         ("minimize_obligation() does not support alternation");
 
     bool minimization_will_be_correct = false;
-    // WDBA-minimization necessarily work for obligations
-    if ((f && f.is_syntactic_obligation())
-        // Weak deterministic automata are obligations
-        || (aut_f->prop_weak() && is_deterministic(aut_f))
-        // Guarantee automata are obligations as well.
-        || is_terminal_automaton(aut_f))
+    if (minimize_obligation_garanteed_to_work(aut_f, f))
       {
         minimization_will_be_correct = true;
       }

spot/twaalgos/minimize.hh

@@ -1,6 +1,6 @@
 // -*- coding: utf-8 -*-
-// Copyright (C) 2009, 2010, 2011, 2012, 2013, 2014, 2015, 2016, 2018, 2019
-// Laboratoire de Recherche et Développement de l'Epita (LRDE).
+// Copyright (C) 2009-2016, 2018-2020 Laboratoire de Recherche et
+// Développement de l'Epita (LRDE).
 //
 // This file is part of Spot, a model checking library.
 //
@@ -116,5 +116,23 @@ namespace spot
                       const_twa_graph_ptr aut_neg_f = nullptr,
                       bool reject_bigger = false,
                       const output_aborter* aborter = nullptr);
+
+  /// \brief Whether calling minimize_obligation is sure to work
+  ///
+  /// This checks whether \a f is a syntactic obligation, or if \a
+  /// aut_f obviously corresponds to an obligation (for instance if
+  /// this is a terminal automaton, or if it is both weak and
+  /// deterministic).  In this case, calling minimize_obligation()
+  /// should not be a waste of time, as it will return a new
+  /// automaton.
+  ///
+  /// If this function returns false, the input property might still
+  /// be a pathological obligation.  The only way to know is to call
+  /// minimize_obligation(), but as it is less likely, you might
+  /// decide to save time.
+  SPOT_API
+  bool minimize_obligation_garanteed_to_work(const const_twa_graph_ptr& aut_f,
+                                             formula f = nullptr);
+
   /// @}
 }

spot/twaalgos/postproc.cc

@@ -83,7 +83,7 @@ namespace spot
         sat_acc_ = opt->get("sat-acc", 0);
         sat_states_ = opt->get("sat-states", 0);
         state_based_ = opt->get("state-based", 0);
-        wdba_minimize_ = opt->get("wdba-minimize", 1);
+        wdba_minimize_ = opt->get("wdba-minimize", -1);
         gen_reduce_parity_ = opt->get("gen-reduce-parity", 1);
 
         if (sat_acc_ && sat_minimize_ == 0)
@@ -371,9 +371,19 @@ namespace spot
     output_aborter* aborter =
       (det_max_states_ >= 0 || det_max_edges_ >= 0) ? &aborter_ : nullptr;
 
-    // (Small,Low) is the only configuration where we do not run
-    // WDBA-minimization.
-    if ((PREF_ != Small || level_ != Low) && wdba_minimize_)
+    int wdba_minimize = wdba_minimize_;
+    if (wdba_minimize == -1)
+      {
+        if (level_ == High)
+          wdba_minimize = 1;
+        else if (level_ == Medium || PREF_ == Deterministic)
+          wdba_minimize = 2;
+        else
+          wdba_minimize = 0;
+      }
+    if (wdba_minimize == 2)
+      wdba_minimize = minimize_obligation_garanteed_to_work(a, f);
+    if (wdba_minimize)
       {
         bool reject_bigger = (PREF_ == Small) && (level_ <= Medium);
         dba = minimize_obligation(a, f, nullptr, reject_bigger, aborter);

spot/twaalgos/postproc.hh

@@ -253,7 +253,7 @@ namespace spot
     int sat_states_ = 0;
     int gen_reduce_parity_ = 1;
     bool state_based_ = false;
-    bool wdba_minimize_ = true;
+    int wdba_minimize_ = -1;
   };
   /// @}
 }