translate: add support for -x exprop

This helped confirming a behavior observed in #298.

* spot/twaalgos/translate.cc, spot/twaalgos/translate.hh: Add support
for -x exprop.
* bin/spot-x.cc, NEWS: Document it.
* tests/core/ltl2tgba2.test: Test it.

NEWS

@@ -135,11 +135,21 @@ New in spot 2.9.5.dev (not yet released)
     automaton, but we should not waste too much space and time trying
     that.
 
-    spot::translator additionally honor the following new variable:
+    spot::translator additionally honor the following new variables:
 
-     tls-max-states  Maximum number of states of automata involved in
-                     automata-based implication checks for formula
-                     simplifications.  Defaults to 64.
+      tls-max-states  Maximum number of states of automata involved in
+                      automata-based implication checks for formula
+                      simplifications.  Defaults to 64.
+      exprop          When set, this causes the core LTL translation to
+                      explicitly iterate over all possible valuations of
+                      atomic propositions when considering the successors
+                      of a BDD-encoded state, instead of discovering
+                      possible successors by rewriting the BDD as a sum of
+                      product.  This is enabled by default for --high,
+                      and disabled by default otherwise.  When unambiguous
+                      automata are required, this option forced and
+                      cannot be disabled.  (This feature is not new, but
+                      was not tunable before.)
 
   - tgba_powerset() now takes an extra optional argument to specify a
     list of accepting sinks states if some are known.  Doing so can

bin/spot-x.cc

@@ -58,6 +58,12 @@ default) to disable.") },
 the transitions that enter accepting SCCs, and not only on the transitions \
 inside accepting SCCs.  This option defaults to 0, and is only used when \
 comp-susp=1.") },
+    { DOC("exprop", "When set, this causes the core LTL translation to \
+explicitly iterate over all possible valuations of atomic propositions when \
+considering the successors of a BDD-encoded state, instead of discovering \
+possible successors by rewriting the BDD as a sum of product.  This is enabled \
+by default for --high, and disabled by default otherwise.  When unambiguous \
+automata are required, this option forced and cannot be disabled.") },
     { DOC("skel-simul", "Default to 1.  Set to 0 to disable simulation \
 on the skeleton automaton during compositional suspension. Only used when \
 comp-susp=1.") },

spot/twaalgos/translate.cc

@@ -39,6 +39,7 @@ namespace spot
     relabel_bool_ = 4;
     tls_impl_ = -1;
     ltl_split_ = true;
+    exprop_ = -1;
 
     opt_ = opt;
     if (!opt)
@@ -62,6 +63,7 @@ namespace spot
     ltl_split_ = opt->get("ltl-split", 1);
     int tls_max_states = opt->get("tls-max-states", 64);
     tls_max_states_ = std::max(0, tls_max_states);
+    exprop_ = opt->get("exprop", -1);
   }
 
   void translator::build_simplifier(const bdd_dict_ptr& dict)
@@ -396,7 +398,9 @@ namespace spot
                   }
               }
           }
-        bool exprop = unambiguous || level_ == postprocessor::High;
+        bool exprop = unambiguous
+          || (level_ == postprocessor::High && exprop_ != 0)
+          || exprop_ > 0;
         aut = ltl_to_tgba_fm(r, simpl_->get_dict(), exprop,
                              true, false, false, nullptr, nullptr,
                              unambiguous);

spot/twaalgos/translate.hh

@@ -155,6 +155,7 @@ namespace spot
     bool gf_guarantee_set_ = false;
     bool ltl_split_;
     unsigned tls_max_states_ = 0;
+    int exprop_;
     const option_map* opt_;
   };
   /// @}

tests/core/ltl2tgba2.test

@@ -485,3 +485,21 @@ f='(!(G({(a)} |=> {(b)[*12]})))'
 test 14,1 = `ltl2tgba -B --stats=%s,%d "$f"`
 f='(!(G({(a)} |=> {(b)[*13]})))'
 test 15,0 = `ltl2tgba -B --stats=%s,%d "$f"`
+
+# Related to Issue #298.  The difference between the translation of
+# this formula with --med and --high is just a different ordering of
+# the states caused by the use of exprop.
+opts="-x scc-filter=0,tls-impl=0,gf-guarantee=0,wdba-minimize=0"
+f='G(p0 -> F(p1 & !p2 & X(!p2 U p3)))'
+ltl2tgba $opts --med "$f" >med.hoa
+ltl2tgba $opts,exprop=0 "$f" > high.hoa
+diff med.hoa high.hoa
+ltl2tgba $opts "$f" > high.hoa
+test $(wc -l < med.hoa) = $(wc -l < high.hoa)
+cmp med.hoa high.hoa && exit 1
+autfilt --stats=%F:%s,%e med.hoa high.hoa >out
+cat >exp <<EOF
+med.hoa:4,14
+high.hoa:4,14
+EOF
+diff exp out