scc_filter_states: also remove useless acceptance marks

* src/twaalgos/sccfilter.hh,
src/twaalgos/sccfilter.cc (scc_filter_states): Remove useless acceptance
marks while preserving state-based acceptance.  Add a new argument
to specify if all useless mark have to be removed, like for scc_filter.
* src/twaalgos/simulation.cc: Use the new parameter.
* src/twaalgos/postproc.cc: Likewise.  Also call do_scc_filter even
after WDBA simplification to cleanup trivial SCCs.  Preserve state-based
acceptance for weak automata.
* src/tests/readsave.test: Add one test.
* src/tests/dstar.test, src/tests/prodor.test, src/tests/remfin.test,
src/tests/sim3.test, wrap/python/tests/automata.ipynb,
wrap/python/tests/piperead.ipynb: Adjust expected output.
* NEWS: Mention the change.

src/tests/dstar.test

@@ -244,7 +244,7 @@ digraph G {
   node [shape="circle"]
   I [label="", style=invis, width=0]
   I -> 0
-  0 [label="0", peripheries=2]
+  0 [label="0"]
   0 -> 1 [label="!a & !b"]
   0 -> 2 [label="a & !b"]
   1 [label="1", peripheries=2]

src/tests/prodor.test

@@ -96,15 +96,15 @@ Acceptance: 2 Inf(0) | Inf(1)
 properties: trans-labels explicit-labels state-acc complete
 properties: deterministic
 --BODY--
-State: 0 {0 1}
+State: 0 {1}
 [0] 1
 [!0] 2
-State: 1 {0 1}
+State: 1 {1}
 [0&1] 3
 [!0&1] 4
 [0&!1] 5
 [!0&!1] 6
-State: 2 {0}
+State: 2
 [0&1] 3
 [!0&1] 4
 [0&!1] 5

src/tests/readsave.test

@@ -700,3 +700,40 @@ diff output3 expect3
 
 $autfilt -Hk input 2>stderr && exit 1
 grep 'print_hoa.*k' stderr
+
+cat >input4 <<EOF
+HOA: v1
+States: 3
+Start: 0
+AP: 2 "a" "b"
+Acceptance: 2 Inf(0) & Inf(1)
+--BODY--
+State: 0 {0}   [0] 1
+State: 1 {1}   [1] 2
+State: 2 {0 1} [0] 2
+--END--
+EOF
+
+$autfilt -H --small --high input4 >output4
+cat output4
+
+cat >expect4<<EOF
+HOA: v1
+States: 3
+Start: 1
+AP: 2 "a" "b"
+acc-name: Buchi
+Acceptance: 1 Inf(0)
+properties: trans-labels explicit-labels state-acc deterministic
+properties: inherently-weak
+--BODY--
+State: 0
+[1] 2
+State: 1
+[0] 0
+State: 2 {0}
+[0] 2
+--END--
+EOF
+
+diff output4 expect4

src/tests/remfin.test

@@ -663,7 +663,7 @@ State: 6
 [0] 0
 [!0] 6
 State: 7
-[0] 4 {0}
+[0] 4
 [!0] 6
 State: 8
 [0] 8 {0}
@@ -679,22 +679,22 @@ properties: trans-labels explicit-labels trans-acc
 State: 0
 [0&1] 0 {0}
 [!0&1] 1 {0}
-[0&!1] 11 {0}
+[0&!1] 11
 [!0&!1] 12
 State: 1
 [0&1] 0 {0}
 [!0&1] 1 {0}
-[0&!1] 11 {0}
+[0&!1] 11
 [!0&!1] 12
 State: 2
 [!0&1] 2 {0}
 [0&1] 9
-[!0&!1] 10 {0}
-[0&!1] 11 {0}
+[!0&!1] 10
+[0&!1] 11
 State: 3
-[!0&1] 1 {0}
+[!0&1] 1
 [0&1] 3 {0}
-[0&!1] 11 {0}
+[0&!1] 11
 [!0&!1] 12
 State: 4
 [!0&1] 4 {0}
@@ -706,7 +706,7 @@ State: 5
 [!0&!1] 10 {0}
 [0&!1] 12
 State: 6
-[!0&1] 4 {0}
+[!0&1] 4
 [0&1] 6 {0}
 [0&!1] 11 {0}
 [!0&!1] 12
@@ -750,23 +750,23 @@ Start: 0
 AP: 2 "p1" "p0"
 acc-name: Buchi
 Acceptance: 1 Inf(0)
-properties: trans-labels explicit-labels state-acc complete
+properties: trans-labels explicit-labels trans-acc complete
 properties: deterministic
 --BODY--
 State: 0
 [!1] 0
 [0&1] 2
 [!0&1] 3
-State: 1 {0}
-[!1] 1
-[1] 2
-State: 2 {0}
-[!1] 1
-[1] 2
-State: 3 {0}
-[!1] 0
+State: 1
+[!1] 1 {0}
+[1] 2 {0}
+State: 2
+[!1] 1 {0}
+[1] 2 {0}
+State: 3
+[!1] 0 {0}
 [0&1] 2
-[!0&1] 3
+[!0&1] 3 {0}
 --END--
 HOA: v1
 States: 5
@@ -782,7 +782,7 @@ State: 1
 [t] 1 {0}
 State: 2
 [!0&1] 0
-[0] 1 {0}
+[0] 1
 [!0&!1] 2 {0}
 State: 3
 [1] 0

src/tests/sim3.test

@@ -60,7 +60,7 @@ Acceptance: 4 (Fin(0) & Inf(1)) | (Fin(2) & Inf(3))
 properties: trans-labels explicit-labels state-acc complete
 properties: deterministic
 --BODY--
-State: 0 {0}
+State: 0
 [!1] 1
 [1] 3
 State: 1 {1 3}

src/twaalgos/postproc.cc

@@ -144,8 +144,10 @@ namespace spot
   {
     if (scc_filter_ == 0)
       return a;
-    if (state_based_ && a->has_state_based_acc())
-      return scc_filter_states(a);
+    // If the automaton is weak, using transition-based acceptance
+    // won't help, so let's preserve it.
+    if ((state_based_ || a->is_inherently_weak()) && a->has_state_based_acc())
+      return scc_filter_states(a, arg);
     else
       return scc_filter(a, arg);
   }
@@ -186,7 +188,7 @@ namespace spot
 	  || (type_ == Monitor && a->num_sets() == 0))
 	{
 	  if (tgb_used)
-	    a = do_scc_filter(a);
+	    a = do_scc_filter(a, true);
 	  if (COMP_)
 	    a = complete(a);
 	  if (SBACC_)
@@ -202,7 +204,7 @@ namespace spot
       // ignored.
       a = scc_filter_states(a);
     else
-      a = do_scc_filter(a);
+      a = do_scc_filter(a, (PREF_ == Any));
 
     if (type_ == Monitor)
       {
@@ -445,10 +447,12 @@ namespace spot
 	  sim = nullptr;
       }
 
-    if (type_ == TGBA && level_ == High && scc_filter_ != 0)
+    if (level_ == High && scc_filter_ != 0)
       {
-	if (dba && !dba_is_minimal) // WDBA is already clean.
+	if (dba)
 	  {
+	    // Do that even for WDBA, to remove marks from transitions
+	    // leaving trivial SCCs.
 	    dba = do_scc_filter(dba, true);
 	    assert(!sim);
 	  }

src/twaalgos/sccfilter.cc

@@ -126,8 +126,10 @@ namespace spot
 
     // Remove acceptance conditions from all edges outside of
     // non-accepting SCCs.  If "RemoveAll" is false, keep those on
-    // transitions entering accepting SCCs.
-    template <bool RemoveAll, class next_filter = id_filter>
+    // transitions entering accepting SCCs.  If "PreserveSBA", is set
+    // only touch a transition if all its neighbor siblings can be
+    // touched as well.
+    template <bool RemoveAll, bool PreserveSBA, class next_filter = id_filter>
     struct acc_filter_mask: next_filter
     {
       acc_cond::mark_t accmask;
@@ -154,16 +156,47 @@ namespace spot
 
 	if (keep)
 	  {
-	    unsigned u = this->si->scc_of(dst);
-	    // If the edge is between two SCCs, we can simplify
-	    // remove the acceptance sets.  If the SCC is non-accepting,
-	    // we can only remove the Inf sets.
-	    if (RemoveAll && u != this->si->scc_of(src))
-	      acc = 0U;
-	    else if (this->si->is_rejecting_scc(u))
-	      acc &= accmask;
+	    unsigned u = this->si->scc_of(src);
+	    unsigned v = this->si->scc_of(dst);
+	    // The basic rules are as follows:
+	    //
+	    // - If an edge is between two SCCs, is OK to remove
+	    //   all acceptance sets, as this edge cannot be part
+	    //   of any loop.
+	    // - If an edge is in an non-accepting SCC, we can only
+	    //   remove the Inf sets, as removinf the Fin sets
+	    //   might make the SCC accepting.
+	    //
+	    // The above rules are made more complex with two flags:
+	    //
+	    // - If PreserveSBA is set, we have to tree a transition
+	    //   leaving an SCC as other transitions inside the SCC,
+	    //   otherwise we will break the property that all
+	    //   transitions leaving the same state have identical set
+	    //   membership.
+	    // - If RemoveAll is false, we like to keep the membership
+	    //   of transitions entering an SCC.  This can only be
+	    //   done if PreserveSBA is unset, unfortunately.
+	    if (u == v)
+	      {
+		if (this->si->is_rejecting_scc(u))
+		  acc &= accmask;
+	      }
+	    else if (PreserveSBA && this->si->is_rejecting_scc(u))
+	      {
+		if (!this->si->is_trivial(u))
+		  acc &= accmask; // No choice.
+		else if (RemoveAll)
+		  acc = 0U;
+	      }
+	    else if (!PreserveSBA)
+	      {
+		if (RemoveAll)
+		  acc = 0U;
+		else if (this->si->is_rejecting_scc(v))
+		  acc &= accmask;
+	      }
 	  }
-
 	return filtered_trans(keep, cond, acc);
       }
     };
@@ -186,7 +219,7 @@ namespace spot
 	auto& acc = this->si->get_aut()->acc();
 	if (!acc.is_generalized_buchi())
 	  throw std::runtime_error
-	    ("simplification of SCC acceptance work only with "
+	    ("simplification of SCC acceptance works only with "
 	     "generalized Büchi acceptance");
 
 	unsigned scc_count = this->si->scc_count();
@@ -243,10 +276,11 @@ namespace spot
     twa_graph_ptr scc_filter_apply(const_twa_graph_ptr aut,
 				   scc_info* given_si, Args&&... args)
     {
+      unsigned in_n = aut->num_states();
+      if (in_n == 0)			 // nothing to filter.
+	return make_twa_graph(aut, twa::prop_set::all());
+
       twa_graph_ptr filtered = make_twa_graph(aut->get_dict());
-      unsigned in_n = aut->num_states(); // Number of input states.
-      if (in_n == 0)			 // Nothing to filter.
-	return filtered;
       filtered->copy_ap_of(aut);
 
       // Compute scc_info if not supplied.
@@ -302,9 +336,16 @@ namespace spot
   }
 
   twa_graph_ptr
-  scc_filter_states(const const_twa_graph_ptr& aut, scc_info* given_si)
+  scc_filter_states(const const_twa_graph_ptr& aut, bool remove_all_useless,
+		    scc_info* given_si)
   {
-    auto res = scc_filter_apply<state_filter<>>(aut, given_si);
+    twa_graph_ptr res;
+    if (remove_all_useless)
+      res = scc_filter_apply<state_filter
+			     <acc_filter_mask<true, true>>>(aut, given_si);
+    else
+      res = scc_filter_apply<state_filter
+			     <acc_filter_mask<false, true>>>(aut, given_si);
     res->prop_copy(aut, { true, true, true, true });
     return res;
   }
@@ -321,21 +362,25 @@ namespace spot
 	  res =
 	    scc_filter_apply<state_filter
 			     <acc_filter_mask
-			      <true, acc_filter_simplify<>>>>(aut, given_si);
+			      <true, false,
+			       acc_filter_simplify<>>>>(aut, given_si);
 	else
 	  res =
 	    scc_filter_apply<state_filter
 			     <acc_filter_mask
-			      <false, acc_filter_simplify<>>>>(aut, given_si);
+			      <false, false,
+			       acc_filter_simplify<>>>>(aut, given_si);
       }
     else
       {
 	if (remove_all_useless)
 	  res = scc_filter_apply<state_filter
-				 <acc_filter_mask<true>>>(aut, given_si);
+				 <acc_filter_mask
+				  <true, false>>>(aut, given_si);
 	else
 	  res = scc_filter_apply<state_filter
-				 <acc_filter_mask<false>>>(aut, given_si);
+				 <acc_filter_mask
+				  <false, false>>>(aut, given_si);
       }
     res->merge_edges();
     res->prop_copy(aut,
@@ -357,18 +402,20 @@ namespace spot
       res = scc_filter_apply<susp_filter
 			     <state_filter
 			      <acc_filter_mask
-			       <true, acc_filter_simplify<>>>>>(aut, given_si,
-								suspvars,
-								ignoredvars,
-								early_susp);
+			       <true, false,
+				acc_filter_simplify<>>>>>(aut, given_si,
+							  suspvars,
+							  ignoredvars,
+							  early_susp);
     else
       res = scc_filter_apply<susp_filter
 			     <state_filter
 			      <acc_filter_mask
-			       <false, acc_filter_simplify<>>>>>(aut, given_si,
-								 suspvars,
-								 ignoredvars,
-								 early_susp);
+			       <false, false,
+				acc_filter_simplify<>>>>>(aut, given_si,
+							  suspvars,
+							  ignoredvars,
+							  early_susp);
     res->merge_edges();
     res->prop_copy(aut,
 		   { false,  // state-based acceptance is not preserved

src/twaalgos/sccfilter.hh

@@ -63,14 +63,14 @@ namespace spot
 
   /// \brief Prune unaccepting SCCs.
   ///
-  /// This is an abridged version of scc_filter(), that only removes
-  /// useless states, without touching at the acceptance conditions.
-  ///
-  /// Especially, if the input TωA has the SBA property, (i.e.,
-  /// transitions leaving accepting states are all marked as
-  /// accepting), then the output TGBA will also have that property.
+  /// This is an abridged version of scc_filter(), that preserves
+  /// state-based acceptance.  I.e., if the input TωA has the SBA
+  /// property, (i.e., transitions leaving accepting states are all
+  /// marked as accepting), then the output TωA will also have that
+  /// property.
   SPOT_API twa_graph_ptr
   scc_filter_states(const const_twa_graph_ptr& aut,
+		    bool remove_all_useless = false,
 		    scc_info* given_si = nullptr);
 
   /// \brief Prune unaccepting SCCs, superfluous acceptance

src/twaalgos/simulation.cc

@@ -777,7 +777,7 @@ namespace spot
 	res = cosimul.run();
 
 	if (Sba)
-	  res = scc_filter_states(res);
+	  res = scc_filter_states(res, false);
 	else
 	  res = scc_filter(res, false);