postproc: fix issue #402

* spot/twaalgos/postproc.cc, spot/twaalgos/postproc.hh,
spot/twaalgos/translate.cc: Introduce a gen-reduce-parity option and
use it on sub-automata built by ltl-split.
* bin/spot-x.cc: Document it.
* tests/core/ltl2tgba2.test: Add test case reported by Juraj Major.

bin/spot-x.cc

@@ -1,5 +1,5 @@
 // -*- coding: utf-8 -*-
-// Copyright (C) 2013-2019 Laboratoire de Recherche et Développement
+// Copyright (C) 2013-2020 Laboratoire de Recherche et Développement
 // de l'Epita (LRDE).
 //
 // This file is part of Spot, a model checking library.
@@ -117,6 +117,15 @@ the determinization algorithm.") },
 the determinization algorithm.") },
     { DOC("det-stutter", "Set to 0 to disable optimizations based on \
 the stutter-invariance in the determinization algorithm.") },
+    { DOC("gen-reduce-parity", "When the postprocessor routines are \
+configured to output automata with any kind of acceptance condition, \
+but they happen to process an automaton with parity acceptance, they \
+call a function to minimize the number of colors needed.  This option \
+controls what happen when this reduction does not reduce the number of \
+colors: when set (the default) the output of the reduction is returned, \
+this means the colors in the automaton may have changed slightly, and in \
+particular, there is no transition with more than one color; when unset, \
+the original automaton is returned.") },
     { DOC("gf-guarantee", "Set to 0 to disable alternate constructions \
 for GF(guarantee)->[D]BA and FG(safety)->DCA.  Those constructions \
 are from an LICS'18 paper by J. Esparza, J. Křentínský, and S. Sickert.  \

spot/twaalgos/postproc.cc

@@ -84,6 +84,7 @@ namespace spot
         sat_states_ = opt->get("sat-states", 0);
         state_based_ = opt->get("state-based", 0);
         wdba_minimize_ = opt->get("wdba-minimize", 1);
+        gen_reduce_parity_ = opt->get("gen-reduce-parity", 1);
 
         if (sat_acc_ && sat_minimize_ == 0)
           sat_minimize_ = 1;        // Dicho.
@@ -254,15 +255,25 @@ namespace spot
         bool isparity = in->acc().is_parity();
         if (isparity && in->is_existential()
             && (type_ == Generic || want_parity))
-          return reduce_parity(in);
-        if (!(want_parity && isparity))
           {
-            if (level_ == High)
-              return simplify_acceptance(in);
-            else
-              return cleanup_acceptance(in);
+            auto res = reduce_parity(in);
+            if (want_parity || gen_reduce_parity_)
+              return res;
+            // In case type_ == Generic and gen_reduce_parity_ == 0,
+            // we only return the result of reduce_parity() if it can
+            // lower the number of colors.  Otherwise,
+            // simplify_acceptance() will not do better and we return
+            // the input unchanged.  The reason for not always using
+            // the output of reduce_parity() is that is may hide
+            // symmetries between automata, as in issue #402.
+            return (res->num_sets() < in->num_sets()) ? res : in;
           }
-        return cleanup_parity(in);
+        if (want_parity && isparity)
+          return cleanup_parity(in);
+        if (level_ == High)
+          return simplify_acceptance(in);
+        else
+          return cleanup_acceptance(in);
       };
     a = simplify_acc(a);
 

spot/twaalgos/postproc.hh

@@ -1,5 +1,5 @@
 // -*- coding: utf-8 -*-
-// Copyright (C) 2012-2019 Laboratoire de Recherche et Développement
+// Copyright (C) 2012-2020 Laboratoire de Recherche et Développement
 // de l'Epita (LRDE).
 //
 // This file is part of Spot, a model checking library.
@@ -251,6 +251,7 @@ namespace spot
     bool sat_langmap_ = false;
     int sat_acc_ = 0;
     int sat_states_ = 0;
+    int gen_reduce_parity_ = 1;
     bool state_based_ = false;
     bool wdba_minimize_ = true;
   };

spot/twaalgos/translate.cc

@@ -1,5 +1,5 @@
 // -*- coding: utf-8 -*-
-// Copyright (C) 2013-2018 Laboratoire de Recherche et Développement
+// Copyright (C) 2013-2018, 2020 Laboratoire de Recherche et Développement
 // de l'Epita (LRDE).
 //
 // This file is part of Spot, a model checking library.
@@ -226,22 +226,32 @@ namespace spot
         if (susp.empty() && (type_ == TGBA || type_ == BA))
           goto nosplit;
 
-        option_map om;
+        option_map om_wos;
+        option_map om_ws;
         if (opt_)
-          om = *opt_;
-        om.set("ltl-split", 0);
-        translator translate_without_split(simpl_, &om);
+          om_ws = *opt_;
+        // Don't blindingly apply reduce_parity() in the
+        // generic case, for issue #402.
+        om_ws.set("gen-reduce-parity", 0);
+        om_wos = om_ws;
+        om_wos.set("ltl-split", 0);
+        translator translate_without_split(simpl_, &om_wos);
         // Never force colored automata at intermediate steps.
         // This is best added at the very end.
         translate_without_split.set_pref(pref_ & ~Colored);
         translate_without_split.set_level(level_);
         translate_without_split.set_type(type_);
+        translator translate_with_split(simpl_, &om_ws);
+        translate_with_split.set_pref(pref_ & ~Colored);
+        translate_with_split.set_level(level_);
+        translate_with_split.set_type(type_);
+
         auto transrun = [&](formula f)
           {
             if (f == r2)
               return translate_without_split.run(f);
             else
-              return run(f);
+              return translate_with_split.run(f);
           };
 
         // std::cerr << "splitting\n";

tests/core/ltl2tgba2.test

@@ -419,7 +419,6 @@ f=$f'& (!b | X(b | (a R !b))) & (!d | X(d | (c R !d))) & F(a | !b) & F(c | !d))'
 test '8,1' = `ltl2tgba "$f" --stats=%s,%d`
 test '8,1' = `ltl2tgba -GD "$f" --stats=%s,%d`
 
-
 # Two formulas for which ltl2tgba 2.7.3 was raising an error with -GDS
 # Reported by David Müller.
 cat >in <<EOF
@@ -432,8 +431,7 @@ cat >expected <<EOF
 8,(Fin(1) | Inf(0)) & (Fin(2) | Inf(1)) & (Fin(0) | Inf(2))
 EOF
 
-# This formulas used to produce translation with false label.
-
+# These formulas used to produce translation with false label.
 # Reported by Florian Renkin
 ltl2tgba -D -G 'F(G(a | !a) & ((b <-> c) W d))' | grep '\[f\]' && exit 1
 # Reported by Jens Kreber
@@ -448,4 +446,9 @@ ltl2tgba 'X!c & X(b & c & d & a U e)' --stats=%w | grep 0 && exit 1
 # would in turn cause the HOA printer to choke.
 ltlcross -f 'G(F(Gb ^ Fa) & FG!a)' 'ltl2tgba -G -D'
 
+# Issue #402, reported by Juraj Major.
+f='(GFp0 | FGp1) & (GF!p1 | FGp2) & (GF!p2 | FG!p0)'
+test 1,8,3 = `ltl2tgba -G -D "$f" --stats=%s,%e,%a`
+test 1,3,2 = `ltl2tgba -G -D "(GFp0 | FGp1)" --stats=%s,%e,%a`
+
 :