implement is_liveness() and is_liveness_automaton()

* spot/twaalgos/strength.cc, spot/twaalgos/strength.hh,
spot/tl/hierarchy.cc, spot/tl/hierarchy.hh: Here.
* bin/ltlfilt.cc (--liveness): New filter.
* NEWS: Mention those.
* tests/core/ltlfilt.test, tests/python/ltlsimple.py: Add test cases.

NEWS

@@ -15,11 +15,8 @@ New in spot 2.6.2.dev (not yet released)
     Zielonka algorithm. Calude's quasi-polynomial time algorithm has
     been dropped as it was not used.
 
-  Build:
+  - ltlfilt learned --liveness to match formulas representing liveness
-
+    properties.
-  - We no longer distribute the Python-based CGI script + javascript
-    code for the online translator.  Its replacement has its own
-    repository: https://gitlab.lrde.epita.fr/spot/spot-web-app/
 
   Library:
 
@@ -64,6 +61,10 @@ New in spot 2.6.2.dev (not yet released)
     with arbitrary acceptance condition into a parity automaton,
     based on a last-appearance record (LAR) construction.
 
+  - The new function is_liveness() and is_liveness_automaton() can be
+    used to check whether a formula or an automaton represents a
+    liveness property.
+
   Bugs fixed:
 
   - Running "ltl2tgba -B" on formulas of the type FG(safety) would

bin/ltlfilt.cc

@@ -700,11 +700,12 @@ namespace
 
       if (matched && (obligation || recurrence || persistence
                       || !opt->acc_words.empty()
-                      || !opt->rej_words.empty()))
+                      || !opt->rej_words.empty()
+                      || liveness))
         {
           spot::twa_graph_ptr aut = nullptr;
 
-          if (!opt->acc_words.empty() || !opt->rej_words.empty())
+          if (!opt->acc_words.empty() || !opt->rej_words.empty() || liveness)
             {
               aut = ltl_to_tgba_fm(f, simpl.get_dict(), true);
 
@@ -724,6 +725,7 @@ namespace
                       break;
                     }
 
+              matched &= !liveness || is_liveness_automaton(aut);
             }
 
           // Some combinations of options can be simplified.

spot/tl/hierarchy.cc

@@ -469,4 +469,9 @@ namespace spot
     const op* vd = v.data();
     return nesting_depth(f, vd, vd + v.size());
   }
+
+  bool is_liveness(formula f)
+  {
+    return is_liveness_automaton(ltl_to_tgba_fm(f, spot::make_bdd_dict()));
+  }
 }

spot/tl/hierarchy.hh

@@ -1,6 +1,6 @@
 // -*- coding: utf-8 -*-
-// Copyright (C) 2017 Laboratoire de Recherche et Développement de
+// Copyright (C) 2017, 2018 Laboratoire de Recherche et Développement
-// l'Epita (LRDE)
+// de l'Epita (LRDE)
 //
 // This file is part of Spot, a model checking library.
 //
@@ -184,4 +184,14 @@ namespace spot
   ///
   /// The string should be terminated by '\0' or ']'.
   SPOT_API unsigned nesting_depth(formula f, const char* opers);
+
+
+  /// \brief Check whether a formula represents a liveness property.
+  ///
+  /// A formula represents a liveness property if any finite prefix
+  /// can be extended into a word accepted by the formula.
+  ///
+  /// The test is done by conversion to automaton.  If you already
+  /// have an automaton, use spot::is_liveness_automaton() instead.
+  SPOT_API bool is_liveness(formula f);
 }

spot/twaalgos/strength.cc

@@ -22,6 +22,9 @@
 #include <spot/misc/hash.hh>
 #include <spot/twaalgos/isweakscc.hh>
 #include <spot/twaalgos/mask.hh>
+#include <spot/twaalgos/minimize.hh>
+#include <spot/twaalgos/isdet.hh>
+#include <spot/twaalgos/sccfilter.hh>
 
 using namespace std::string_literals;
 
@@ -401,4 +404,13 @@ namespace spot
     std::string num = std::to_string(scc_num);
     return decompose_scc(sm, (accepting ? ('a' + num) : num).c_str());
   }
+
+  bool
+  is_liveness_automaton(const const_twa_graph_ptr& aut)
+  {
+    auto mon = minimize_monitor(scc_filter_states(aut));
+    return mon->num_states() == 1 && is_complete(mon);
+  }
+
+
 }

spot/twaalgos/strength.hh

@@ -1,5 +1,5 @@
 // -*- coding: utf-8 -*-
-// Copyright (C) 2010, 2011, 2013, 2014, 2015, 2016, 2017 Laboratoire
+// Copyright (C) 2010, 2011, 2013, 2014, 2015, 2016, 2017, 2018 Laboratoire
 // de Recherche et Développement de l'Epita (LRDE)
 //
 // This file is part of Spot, a model checking library.
@@ -104,7 +104,7 @@ namespace spot
   /// SCCs).
   ///
   /// A minimized WDBA (as returned by a successful run of
-  /// minimize_obligation()) represent safety property if it is a
+  /// minimize_obligation()) represents safety property if it is a
   /// safety automaton.
   ///
   /// \param aut the automaton to check
@@ -115,6 +115,15 @@ namespace spot
   is_safety_automaton(const const_twa_graph_ptr& aut,
                       scc_info* sm = nullptr);
 
+  /// \brief Whether the automaton represents a liveness property.
+  ///
+  /// An automaton represents a liveness property if after forcing the
+  /// acceptance condition to true, the resulting automaton accepts
+  /// all words.  In other words, there is no prefix that cannot be
+  /// extended into an accepting word.
+  SPOT_API bool
+  is_liveness_automaton(const const_twa_graph_ptr& aut);
+
   /// \brief Check whether an automaton is weak or terminal.
   ///
   /// This sets the "inherently weak", "weak", "very-weak" and

tests/core/ltlfilt.test

@@ -127,6 +127,16 @@ Fb
 F(a & X(!a & b))
 EOF
 
+checkopt --liveness <<EOF
+GFa | FGb
+F(GFa | Gb)
+F(b W GFa)
+GFa | Gb
+b W GFa
+a U Fb
+F(a & !Xa & Xb)
+EOF
+
 checkopt --safety <<EOF
 !({a;b[*];c}!)
 G(a & Xb)

tests/python/ltlsimple.py

@@ -142,3 +142,6 @@ def myparse(input):
 # This used to fail, because myparse would return a pointer
 # to pf.f inside the destroyed pf.
 assert myparse('a U b') == spot.formula('a U b')
+
+assert spot.is_liveness('a <-> GFb')
+assert not spot.is_liveness('a & GFb')