remove_fin: never return acceptance "f"

Fixes #333.

* spot/twaalgos/remfin.cc, spot/twaalgos/remfin.hh,
spot/twaalgos/totgba.cc: Adjust.  The assert() added
to remove_fin() triggered a lot of failure in the test
suite before the different functions were fixed.
* tests/core/remfin.test, tests/python/tra2tba.py:
Adjust expected result.
* NEWS: Mention the bug.

NEWS

@@ -6,6 +6,9 @@ New in spot 2.5.1.dev (not yet released)
     acc_cond::is_generalized_streett() would segfault on weird
     acceptance conditions such as "3 t" or "3 f".
 
+  - remove_fin(), streett_to_generalized_buchi() should never
+    return automata with "f" acceptance.
+
 New in spot 2.5.1 (2018-02-20)
 
   Library:

spot/twaalgos/remfin.cc

@@ -464,6 +464,20 @@ namespace spot
 
     twa_graph_ptr trivial_strategy(const const_twa_graph_ptr& aut)
     {
+      if (aut->acc().is_f())
+        {
+          // The original acceptance was equivalent to
+          // "f". Simply return an empty automaton with "t"
+          // acceptance.
+          auto res = make_twa_graph(aut->get_dict());
+          res->set_generalized_buchi(0);
+          res->set_init_state(res->new_state());
+          res->prop_stutter_invariant(true);
+          res->prop_weak(true);
+          res->prop_complete(false);
+          return res;
+        }
+
       return (!aut->acc().uses_fin_acceptance())
              ? std::const_pointer_cast<twa_graph>(aut)
              : nullptr;
@@ -514,10 +528,14 @@ namespace spot
             if (acccode.is_f())
               {
                 // The original acceptance was equivalent to
-                // "f". Simply return an empty automaton.
+                // "f". Simply return an empty automaton with "t"
+                // acceptance.
                 auto res = make_twa_graph(aut->get_dict());
-                res->set_acceptance(0, acccode);
+                res->set_generalized_buchi(0);
                 res->set_init_state(res->new_state());
+                res->prop_stutter_invariant(true);
+                res->prop_weak(true);
+                res->prop_complete(false);
                 return res;
               }
           }
@@ -746,6 +764,18 @@ namespace spot
       trace << "before cleanup: " << res->get_acceptance() << '\n';
       cleanup_acceptance_here(res);
       trace << "after cleanup: " << res->get_acceptance() << '\n';
+      if (res->acc().is_f())
+        {
+          // "f" is not generalized-Büchi.  Just return an
+          // empty automaton instead.
+          auto res2 = make_twa_graph(res->get_dict());
+          res2->set_generalized_buchi(0);
+          res2->set_init_state(res2->new_state());
+          res2->prop_stutter_invariant(true);
+          res2->prop_weak(true);
+          res2->prop_complete(false);
+          return res2;
+        }
       res->merge_edges();
       return res;
     }
@@ -822,6 +852,9 @@ namespace spot
 
   twa_graph_ptr remove_fin(const const_twa_graph_ptr& aut)
   {
-    return remove_fin_impl(aut);
+    twa_graph_ptr res = remove_fin_impl(aut);
+    assert(!res->acc().uses_fin_acceptance());
+    assert(!res->acc().is_f());
+    return res;
   }
 }

spot/twaalgos/remfin.hh

@@ -48,12 +48,16 @@ namespace spot
   rabin_to_buchi_maybe(const const_twa_graph_ptr& aut);
 
   /// \ingroup twa_acc_transform
-  /// \brief Rewrite an automaton without Fin acceptance.
+  /// \brief Rewrite an automaton without Fin or f acceptance.
   ///
   /// This algorithm dispatches between many strategies.  It has
   /// dedicated algorithms for weak automata, automata with Rabin-like
   /// acceptance, automata with Streett-like acceptance, and some
   /// generic code that will work on any kind of acceptance condition.
+  ///
+  /// In Spot "f" acceptance is not considered Fin-less, because
+  /// it can be seen as a case of generalized co-Büchi with 0 sets.
+  /// Just like "t" corresponds generalized Büchi with 0 sets.)
   SPOT_API twa_graph_ptr
   remove_fin(const const_twa_graph_ptr& aut);
 }

spot/twaalgos/totgba.cc

@@ -753,6 +753,18 @@ namespace spot
           }
       }
     simplify_acceptance_here(out);
+    if (out->acc().is_f())
+      {
+        // "f" is not generalized-Büchi.  Just return an
+        // empty automaton instead.
+        auto res = make_twa_graph(out->get_dict());
+        res->set_generalized_buchi(0);
+        res->set_init_state(res->new_state());
+        res->prop_stutter_invariant(true);
+        res->prop_weak(true);
+        res->prop_complete(false);
+        return res;
+      }
     return out;
   }
 

tests/core/remfin.test

@@ -363,9 +363,10 @@ HOA: v1
 States: 1
 Start: 0
 AP: 0
-acc-name: none
+acc-name: all
-Acceptance: 0 f
+Acceptance: 0 t
 properties: trans-labels explicit-labels state-acc deterministic
+properties: stutter-invariant weak
 --BODY--
 State: 0
 --END--

tests/python/tra2tba.py

@@ -91,18 +91,15 @@ State: 1
 """)
 
 exp = """HOA: v1
-States: 2
+States: 1
 Start: 0
-AP: 2 "a" "b"
+AP: 0
-acc-name: none
+acc-name: all
-Acceptance: 0 f
+Acceptance: 0 t
 properties: trans-labels explicit-labels state-acc deterministic
+properties: stutter-invariant weak
 --BODY--
 State: 0
-[!0] 0
-[0] 1
-State: 1
-[0] 1
 --END--"""
 
 res = spot.remove_fin(aut)