streett_to_generalized_buchi: fix incorrect algorithm

Fixes #284, reported by Juraj Major.

* spot/twaalgos/totgba.cc: Fix the algorithm.
* spot/twa/acc.hh: More doc for future generations.
* tests/core/scc.test: More test cases.
* NEWS: Mention the issues.

NEWS

@@ -70,11 +70,6 @@ New in spot 2.4.0.dev (not yet released)
     will work either on f or its negation.
     (see https://spot.lrde.epita.fr/hierarchy.html for details).
 
-  Bugs fixed:
-
-  - spot::scc_info::determine_unknown_acceptance() incorrectly
-    considered some rejecting SCC as accepting.
-
   Deprecation notices:
 
   (These functions still work but compilers emit warnings.)
@@ -86,6 +81,16 @@ New in spot 2.4.0.dev (not yet released)
 
   Bugs fixed:
 
+  - spot::scc_info::determine_unknown_acceptance() incorrectly
+    considered some rejecting SCC as accepting.
+
+  - spot::streett_to_generalized_buchi() could generate automata with
+    empty language if some Fin set did not intersect all accepting
+    SCCs.  As a consequence, some Streett-like automata were
+    considered empty even though they were not.  Also, the same
+    function could crash on input that had a Streett-like acceptance
+    not using all declared sets.
+
   - The twa_graph::mege_edges() function relied on BDD IDs to sort
     edges.  This in turn caused some algorithm (like the
     degeneralization) to produce slighltly different outputs (but

spot/twa/acc.hh

@@ -1051,6 +1051,15 @@ namespace spot
     // Returns a number of pairs (>=0) if Streett, or -1 else.
     int is_streett() const;
 
+    /// \brief Rabin/streett pairs used by is_rabin_like and is_streett_like.
+    ///
+    /// These pairs hold two marks which can each contain one or no set.
+    ///
+    /// For instance is_streett_like() rewrites  Inf(0)&(Inf(2)|Fin(1))&Fin(3)
+    /// as three pairs: [(fin={},inf={0}),(fin={1},inf={2}),(fin={3},inf={})].
+    ///
+    /// Empty marks should be interpreted in a way that makes them
+    /// false in Streett, and true in Rabin.
     struct SPOT_API rs_pair
     {
       rs_pair() = default;

spot/twaalgos/totgba.cc

@@ -533,7 +533,9 @@ namespace spot
     acc_cond::mark_t fin;
     std::tie(inf, fin) = in->get_acceptance().used_inf_fin_sets();
     unsigned p = inf.count();
-    acc_cond::mark_t fin_not_inf = fin - inf;
+    // At some point we will remove anything that is not used as Inf.
+    acc_cond::mark_t to_strip = in->acc().all_sets() - inf;
+    acc_cond::mark_t inf_alone = 0U;
 
     if (!p)
       return remove_fin(in);
@@ -543,8 +545,11 @@ namespace spot
     std::vector<acc_cond::mark_t> inf_to_finpairs(numsets, 0U);
     for (auto pair: pairs)
       {
+        if (pair.fin)
           for (unsigned mark: pair.fin.sets())
             fin_to_infpairs[mark] |= pair.inf;
+        else
+          inf_alone |= pair.inf;
 
         for (unsigned mark: pair.inf.sets())
           inf_to_finpairs[mark] |= pair.fin;
@@ -634,11 +639,14 @@ namespace spot
                 for (unsigned mark: (t.acc & fin).sets())
                   pend |= fin_to_infpairs[mark];
 
+                // If we see some Inf set immediately, they are not
+                // pending anymore.
                 pend -= t.acc & inf;
+
                 // Label this transition with all non-pending
                 // inf sets.  The strip will shift everything
                 // to the correct numbers in the targets.
-                acc = (inf - pend).strip(fin - inf);
+                acc = (inf - pend).strip(to_strip);
                 // Adjust the pending sets to what will be necessary
                 // required on the destination state.
                 if (sbacc)
@@ -651,14 +659,14 @@ namespace spot
 
                     pend -= a & inf;
                   }
-                pend |= scc_inf_wo_fin;
+                pend |= inf_alone;
               }
             else if (no_fin && maybe_acc)
               {
                 // If the acceptance is (Fin(0) | Inf(1)) & Inf(2)
-                // but we do not see any Fin set in this SCC, an
+                // but we do not see any Fin set in this SCC, a
                 // mark {2} should become {1,2} before striping.
-                acc = (t.acc | (inf - scc_inf_wo_fin)).strip(fin_not_inf);
+                acc = (t.acc | (inf - scc_inf_wo_fin)).strip(to_strip);
               }
             assert((acc & out->acc().all_sets()) == acc);
 
@@ -694,7 +702,7 @@ namespace spot
 
                     stpend -= a & inf;
                   }
-                st2gba_state d(t.dst, stpend | scc_inf_wo_fin);
+                st2gba_state d(t.dst, stpend | inf_alone);
                 // Have we already seen this destination?
                 unsigned dest;
                 auto dres = bs2num.emplace(d, 0);

tests/core/scc.test

@@ -139,3 +139,56 @@ State: 3 {1}
 --END--
 EOF
 test 2 = `autfilt --stats=%[a]c in.hoa`
+
+# From issue #284, reported by Juraj Major.
+cat >in.hoa <<EOF
+HOA: v1
+States: 4
+Start: 0
+AP: 1 "a"
+Acceptance: 8 Fin(5) & Fin(3) & (Inf(6) | Fin(7)) &
+              (Inf(2) | Fin(1)) & (Inf(4) | Fin(0))
+properties: trans-labels explicit-labels trans-acc complete
+spot.highlight.edges: 3 5 11 5 14 5
+--BODY--
+State: 0
+[t] 0 {0}
+[0] 1 {4}
+[!0] 2 {4}
+[0] 0 {2 3}
+[!0] 0 {1}
+State: 1
+[!0] 1 {0}
+[!0] 3 {4}
+[!0] 1 {1}
+[0] 1 {4 5}
+[0] 1 {2 3 5}
+State: 2
+[0] 2 {0 6}
+[0] 3 {4 6}
+[0] 2 {2 3 6}
+[!0] 2 {4 7}
+State: 3
+[0] 3 {4 5 6}
+[0] 3 {2 3 5 6}
+[!0] 3 {4 7}
+--END--
+/* this variant also cause some issues while fixing the above */
+HOA: v1
+States: 1
+Start: 0
+AP: 1 "a"
+Acceptance: 5 Fin(0) & (Inf(1) | Fin(2)) & (Inf(4))
+--BODY--
+State: 0
+[0] 0 {1}
+[0] 0 {0 1}
+[!0] 0 {4 2}
+--END--
+EOF
+autfilt --stats='%[a]c' in.hoa >out
+cat >exp <<EOF
+1
+1
+EOF
+diff out exp