twa: add support for very-weak property

* spot/twa/twa.hh: Implement the property.
* spot/parseaut/parseaut.yy, spot/twaalgos/hoa.cc: Add input
and output for it.
* spot/twaalgos/strength.cc,
spot/twaalgos/strength.hh (is_very_weak_automaton): New function.
* tests/core/alternating.test: Add a test for --check=strength
on an alternating automaton.
* tests/core/strength.test, tests/core/parseaut.test: Adjust expected
output.
* NEWS, doc/org/hoa.org, doc/org/concepts.org: Document it.

NEWS

@@ -14,6 +14,10 @@ New in spot 2.2.2.dev (Not yet released)
   * The new version of the Couvreur emptiness check is now the default
     one, used by is_empty() and accepting_run().
 
+  * twa objects have a new property, very-weak, that can be set or
+    retrieved via twa::prop_very_weak(), and that can be tested by
+    is_very_weak_automaton().
+
   Build:
 
   * The configure script has a new option --enable-c++14, to compile in

doc/org/concepts.org

@@ -995,6 +995,7 @@ automaton, and that can be queried or set by algorithms:
 | =state_acc=         | automaton should be considered has having state-based acceptance                             |
 | =inherently_weak=   | accepting and rejecting cycles cannot be mixed in the same SCC                               |
 | =weak=              | transitions of an SCC all belong to the same acceptance sets                                 |
+| =very-weak=         | weak automaton where all SCCs have size 1                                                    |
 | =terminal=          | automaton is weak, accepting SCCs are complete, accepting edges may not go to rejecting SCCs |
 | =deterministic=     | there is at most one run *recognizing* a word, but not necessarily accepting it              |
 | =unambiguous=       | there is at most one run *accepting* a word (but it might be recognized several time)        |

doc/org/hoa.org

@@ -603,10 +603,10 @@ body of the file, and then return and error if what has been declared
 does not correspond to the reality.
 
 Some supported properties (like =weak=, =inherently-weak=,
-=unambiguous=, or =stutter-invariant=) are not double-checked, because
+=very-weak=, =terminal=, =unambiguous=, or =stutter-invariant=) are
-that would require more operations.  Command-line tools that
+not double-checked, because that would require more operations.
-read HOA files all take a =--trust-hoa=no= option to ignore properties
+Command-line tools that read HOA files all take a =--trust-hoa=no=
-that are not double-checked by the parser.
+option to ignore properties that are not double-checked by the parser.
 
 It should be noted that each property can take three values: true,
 false, or maybe.  So actually two bits are used per property.  For
@@ -652,24 +652,25 @@ particular:
   to detect deterministic automata that have been output by algorithms
   that do not try to output deterministic automata.
 
-| property            | parser  | stored | printer                                     | notes                                                            |
+| property            | parser  | stored | printer                                                 | notes                                                            |
-|---------------------+---------+--------+---------------------------------------------+------------------------------------------------------------------|
+|---------------------+---------+--------+---------------------------------------------------------+------------------------------------------------------------------|
-| =state-labels=      | checked | no     | checked if =-Hk=                            | state labels are converted to transition labels when reading TωA |
+| =state-labels=      | checked | no     | checked if =-Hk=                                        | state labels are converted to transition labels when reading TωA |
-| =trans-labels=      | checked | no     | always, unless =-Hi= or =-Hk=               |                                                                  |
+| =trans-labels=      | checked | no     | always, unless =-Hi= or =-Hk=                           |                                                                  |
-| =implicit-labels=   | checked | no     | if =-Hi=                                    | =-Hi= only works for deterministic automata                      |
+| =implicit-labels=   | checked | no     | if =-Hi=                                                | =-Hi= only works for deterministic automata                      |
-| =explicit-labels=   | checked | no     | always, unless =-Hi=                        |                                                                  |
+| =explicit-labels=   | checked | no     | always, unless =-Hi=                                    |                                                                  |
-| =state-acc=         | checked | yes    | checked, unless =-Ht= or =-Hm=              |                                                                  |
+| =state-acc=         | checked | yes    | checked, unless =-Ht= or =-Hm=                          |                                                                  |
-| =trans-acc=         | checked | no     | if not =state-acc= and not =-Hm=            |                                                                  |
+| =trans-acc=         | checked | no     | if not =state-acc= and not =-Hm=                        |                                                                  |
-| =no-univ-branch=    | ignored | no     | only if =-Hv=                               |                                                                  |
+| =no-univ-branch=    | ignored | no     | only if =-Hv=                                           |                                                                  |
-| =deterministic=     | checked | yes    | checked                                     |                                                                  |
+| =deterministic=     | checked | yes    | checked                                                 |                                                                  |
-| =complete=          | checked | no     | checked                                     |                                                                  |
+| =complete=          | checked | no     | checked                                                 |                                                                  |
-| =unambiguous=       | trusted | yes    | as stored if (=-Hv= or not =deterministic=) | can be checked with =--check=unambiguous=                        |
+| =unambiguous=       | trusted | yes    | as stored if (=-Hv= or not =deterministic=)             | can be checked with =--check=unambiguous=                        |
-| =stutter-invariant= | trusted | yes    | as stored                                   | can be checked with =--check=stuttering=                         |
+| =stutter-invariant= | trusted | yes    | as stored                                               | can be checked with =--check=stuttering=                         |
-| =stutter-sensitive= | trusted | yes    | as stored (opposite of =stutter-invariant=) | can be checked with =--check=stuttering=                         |
+| =stutter-sensitive= | trusted | yes    | as stored (opposite of =stutter-invariant=)             | can be checked with =--check=stuttering=                         |
-| =terminal=          | trusted | yes    | as stored                                   | can be checked with =--check=strength=                           |
+| =terminal=          | trusted | yes    | as stored                                               | can be checked with =--check=strength=                           |
-| =weak=              | trusted | yes    | as stored if (=-Hv= or not =terminal=)      | can be checked with =--check=strength=                           |
+| =very-weak=         | trusted | yes    | as stored if (=-Hv= or not =terminal=)                  | can be checked with =--check=strength=                           |
-| =inherently-weak=   | trusted | yes    | as stored if (=-Hv= or not =weak=)          | can be checked with =--check=strength=                           |
+| =weak=              | trusted | yes    | as stored if (=-Hv= or not (=terminal= or =very-weak=)) | can be checked with =--check=strength=                           |
-| =colored=           | ignored | no     | checked                                     |                                                                  |
+| =inherently-weak=   | trusted | yes    | as stored if (=-Hv= or not =weak=)                      | can be checked with =--check=strength=                           |
+| =colored=           | ignored | no     | checked                                                 |                                                                  |
 
 The above table is for version 1 of the format.  When version 1.1 is
 selected (using =-H1.1=), some negated properties may be output.  In

spot/parseaut/parseaut.yy

@@ -526,8 +526,27 @@ header: format-version header-items
 		    a->prop_stutter_invariant(!ss->second.val);
 		}
 	      auto iw = p.find("inherently-weak");
+	      auto vw = p.find("very-weak");
 	      auto w = p.find("weak");
 	      auto t = p.find("terminal");
+              if (vw != e)
+                {
+                  a->prop_very_weak(vw->second.val);
+                  if (w != e && !w->second.val && vw->second.val)
+                    {
+		      error(w->second.loc,
+                            "'properties: !weak' contradicts...");
+		      error(vw->second.loc,
+			    "... 'properties: very-weak' given here");
+                    }
+                  if (iw != e && !iw->second.val && vw->second.val)
+                    {
+		      error(iw->second.loc,
+                            "'properties: !inherently-weak' contradicts...");
+		      error(vw->second.loc,
+			    "... 'properties: very-weak' given here");
+                    }
+                }
 	      if (iw != e)
 		{
 		  a->prop_inherently_weak(iw->second.val);

spot/twa/twa.hh

@@ -990,6 +990,7 @@ namespace spot
       trival::repr_t deterministic:2;     // Deterministic automaton.
       trival::repr_t unambiguous:2;       // Unambiguous automaton.
       trival::repr_t stutter_invariant:2; // Stutter invariant language.
+      trival::repr_t very_weak:2;         // very-weak, or 1-weak
     };
     union
     {
@@ -1167,7 +1168,7 @@ namespace spot
     /// \brief Set the "inherently weak" proeprty.
     ///
     /// Setting "inherently weak" to false automatically
-    /// disables "terminal" and "weak".
+    /// disables "terminal", "very weak", and "weak".
     ///
     /// \see prop_weak()
     /// \see prop_terminal()
@@ -1175,7 +1176,7 @@ namespace spot
     {
       is.inherently_weak = val.val();
       if (!val)
-        is.terminal = is.weak = val.val();
+        is.very_weak = is.terminal = is.weak = val.val();
     }
 
     /// \brief Whether the automaton is terminal.
@@ -1234,9 +1235,38 @@ namespace spot
       if (val)
         is.inherently_weak = val.val();
       if (!val)
-        is.terminal = val.val();
+        is.very_weak = is.terminal = val.val();
     }
 
+    /// \brief Whether the automaton is very-weak.
+    ///
+    /// An automaton is very-weak if it is weak (inside each strongly connected
+    /// component, all transitions belong to the same acceptance sets)
+    /// and each SCC contains only one state.
+    ///
+    /// \see prop_terminal()
+    /// \see prop_weak()
+    trival prop_very_weak() const
+    {
+      return is.very_weak;
+    }
+
+    /// \brief Set the very-weak property.
+    ///
+    /// Marking an automaton as "very-weak" automatically marks it as
+    /// "weak" and "inherently weak".
+    ///
+    /// \see prop_terminal()
+    /// \see prop_weak()
+    void prop_very_weak(trival val)
+    {
+      is.very_weak = val.val();
+      if (val)
+        is.weak = is.inherently_weak = val.val();
+    }
+
+
+
     /// \brief Whether the automaton is deterministic.
     ///
     /// An automaton is deterministic if the conjunction between the
@@ -1400,6 +1430,7 @@ namespace spot
         {
           prop_terminal(other->prop_terminal());
           prop_weak(other->prop_weak());
+          prop_very_weak(other->prop_very_weak());
           prop_inherently_weak(other->prop_inherently_weak());
         }
       if (p.deterministic)

spot/twaalgos/hoa.cc

@@ -500,12 +500,18 @@ namespace spot
       }
     if (aut->prop_terminal())
       prop(" terminal");
-    if (aut->prop_weak() && (verbose || aut->prop_terminal() != true))
+    if (aut->prop_very_weak() && (verbose || aut->prop_terminal() != true))
+      prop(" very-weak");
+    if (aut->prop_weak() && (verbose || (aut->prop_terminal() != true &&
+                                         aut->prop_very_weak() != true)))
       prop(" weak");
     if (aut->prop_inherently_weak() && (verbose || aut->prop_weak() != true))
       prop(" inherently-weak");
     if (v1_1 && !aut->prop_terminal() && (verbose || aut->prop_weak() != false))
       prop(" !terminal");
+    if (v1_1 && !aut->prop_very_weak() && (verbose
+                                           || aut->prop_weak() != false))
+      prop(" !very-weak");
     if (v1_1 && !aut->prop_weak() && (verbose ||
                                       aut->prop_inherently_weak() != false))
       prop(" !weak");

spot/twaalgos/strength.cc

@@ -34,22 +34,29 @@ namespace spot
       bool need_si = !si;
       if (need_si)
         si = new scc_info(aut);
-      si->determine_unknown_acceptance();
+      if (inweak)
+        si->determine_unknown_acceptance();
 
       bool is_inweak = true;
       bool is_weak = true;
+      bool is_single_state_scc = true;
       bool is_term = true;
       unsigned n = si->scc_count();
       for (unsigned i = 0; i < n; ++i)
         {
           if (si->is_trivial(i))
             continue;
+          if (si->states_of(i).size() > 1)
+            is_single_state_scc = false;
           bool first = true;
           acc_cond::mark_t m = 0U;
           if (is_weak)
             for (auto src: si->states_of(i))
               for (auto& t: aut->out(src))
-                if (si->scc_of(t.dst) == i)
+                // In case of a universal edge we only need to check
+                // the first destination of an inside the SCC, because
+                // the other have the same t.acc.
+                if (si->scc_of(*aut->univ_dests(t.dst).begin()) == i)
                   {
                     if (first)
                       {
@@ -98,6 +105,7 @@ namespace spot
           if (terminal)
             aut->prop_terminal(is_term && is_weak);
           aut->prop_weak(is_weak);
+          aut->prop_very_weak(is_single_state_scc && is_weak);
           if (inweak)
             aut->prop_inherently_weak(is_inweak);
         }
@@ -131,6 +139,17 @@ namespace spot
     return res;
   }
 
+  bool
+  is_very_weak_automaton(const const_twa_graph_ptr& aut, scc_info* si)
+  {
+    trival v = aut->prop_very_weak();
+    if (v.is_known())
+      return v.is_true();
+    is_type_automaton<false, false, true>
+      (std::const_pointer_cast<twa_graph>(aut), si);
+    return aut->prop_very_weak().is_true();
+  }
+
   bool
   is_inherently_weak_automaton(const const_twa_graph_ptr& aut, scc_info* si)
   {
@@ -145,7 +164,10 @@ namespace spot
 
   void check_strength(const twa_graph_ptr& aut, scc_info* si)
   {
-    is_type_automaton<true, true, true>(aut, si);
+    if (aut->is_alternating())
+      is_type_automaton<false, false, true>(aut, si);
+    else
+      is_type_automaton<true, true, true>(aut, si);
   }
 
   bool is_safety_mwdba(const const_twa_graph_ptr& aut)

spot/twaalgos/strength.hh

@@ -60,6 +60,24 @@ namespace spot
   SPOT_API bool
   is_weak_automaton(const const_twa_graph_ptr& aut, scc_info* sm = nullptr);
 
+  /// \brief Check whether an automaton is very-weak.
+  ///
+  /// An automaton is very-weak if in any given SCC, all transitions
+  /// belong to the same acceptance sets, and the SCC has only one
+  /// state.
+  ///
+  /// \param aut the automaton to check
+  ///
+  /// \param sm an scc_info object for the automaton if available (it
+  /// will be built otherwise).
+  ///
+  /// In addition to returning the result as a Boolean, this will set
+  /// the prop_very_weak() and prop_weak() properties of the automaton
+  /// as a side-effect, so further calls will return in constant-time.
+  SPOT_API bool
+  is_very_weak_automaton(const const_twa_graph_ptr& aut,
+                         scc_info* sm = nullptr);
+
   /// \brief Check whether an automaton is inherently weak.
   ///
   /// An automaton is inherently weak if in any given SCC, there
@@ -90,7 +108,8 @@ namespace spot
 
   /// \brief Check whether an automaton is weak or terminal.
   ///
-  /// This sets the "weak" and "terminal" property as appropriate.
+  /// This sets the "inherently weak", "weak", "very-weak" and
+  /// "terminal" properties as appropriate.
   ///
   /// \param aut the automaton to check
   ///

tests/core/alternating.test

@@ -126,3 +126,10 @@ digraph G {
 EOF
 
 diff expect.dot alt.dot
+
+autfilt --trust=no --check=strength alt.hoa | grep properties: >output
+cat >expected <<EOF
+properties: univ-branch trans-labels explicit-labels state-acc
+properties: very-weak
+EOF
+diff output expected

tests/core/parseaut.test

@@ -1928,6 +1928,7 @@ AP: 4 "p2" "c1" "p1" "c2"
 acc-name: co-Buchi
 Acceptance: 1 Fin(0)
 properties: univ-branch trans-labels explicit-labels state-acc
+properties: very-weak
 --BODY--
 State: 0 "X (G(c2))"
 [t] 1
@@ -2403,6 +2404,7 @@ AP: 2 "b" "a"
 acc-name: co-Buchi
 Acceptance: 1 Fin(0)
 properties: univ-branch trans-labels explicit-labels state-acc
+properties: very-weak
 --BODY--
 State: 0 "((((a) U (b)) && GF(b)) && FG(a))"
 [0] 3&1
@@ -2608,6 +2610,7 @@ AP: 2 "b" "a"
 acc-name: Buchi
 Acceptance: 1 Inf(0)
 properties: univ-branch trans-labels explicit-labels state-acc
+properties: very-weak
 --BODY--
 State: 0 "((((a) U (b)) && GF(b)) && FG(a))"
 [0] 3&1

tests/core/strength.test

@@ -182,7 +182,7 @@ AP: 1 "a"
 acc-name: Buchi
 Acceptance: 1 Inf(0)
 properties: trans-labels explicit-labels state-acc colored
-properties: deterministic weak
+properties: deterministic very-weak
 --BODY--
 State: 0 {0}
 [t] 1
@@ -644,7 +644,7 @@ AP: 1 "a"
 acc-name: Buchi
 Acceptance: 1 Inf(0)
 properties: trans-labels explicit-labels trans-acc complete
-properties: deterministic weak
+properties: deterministic very-weak
 --BODY--
 State: 0
 [t] 0
@@ -659,7 +659,7 @@ AP: 1 "a"
 acc-name: co-Buchi
 Acceptance: 1 Fin(0)
 properties: trans-labels explicit-labels trans-acc complete
-properties: deterministic weak
+properties: deterministic very-weak
 --BODY--
 State: 0
 [t] 0