hoa: add option 'b' to build an alias-based basis for all labels

Related to issue #563.

* spot/twaalgos/hoa.hh (create_alias_basis): New function.
* spot/twaalgos/hoa.cc (create_alias_basis): New function.
(print_hoa): Add support for option 'b' and create_alias_basis
in this case.
* bin/common_aoutput.cc, NEWS: Document -Hb.
* tests/core/readsave.test, tests/python/aliases.py: Add test cases.

NEWS

@@ -12,6 +12,13 @@ New in spot 2.11.6.dev  (not yet released)
 
         autfilt input.hoa -o output-%l.hoa
 
+  - For tools that produce automata, using -Hb or --hoa=b will produce
+    an HOA file in which aliases are used to form a basis for the
+    whole set of labels.  Those aliases are only used when more than
+    one atomic proposition is used (otherwise, the atomic proposition
+    and its negation is already a basis).  This can help reducing the
+    size of large HOA files.
+
   - ltlfilt has a new option --relabel-overlapping-bool=abc|pnn that
     will replace boolean subformulas by fresh atomic propositions even
     if those subformulas share atomic propositions.

bin/common_aoutput.cc

@@ -132,6 +132,7 @@ static const argp_option options[] =
     { "hoaf", 'H', "1.1|i|k|l|m|s|t|v", OPTION_ARG_OPTIONAL,
       "Output the automaton in HOA format (default).  Add letters to select "
       "(1.1) version 1.1 of the format, "
+      "(b) create an alias basis if >=2 AP are used, "
       "(i) use implicit labels for complete deterministic automata, "
       "(s) prefer state-based acceptance when possible [default], "
       "(t) force transition-based acceptance, "

spot/twaalgos/hoa.cc

@@ -20,6 +20,7 @@
 #include <ostream>
 #include <sstream>
 #include <cstring>
+#include <algorithm>
 #include <map>
 #include <spot/twa/twa.hh>
 #include <spot/twa/twagraph.hh>
@@ -30,6 +31,7 @@
 #include <spot/tl/formula.hh>
 #include <spot/kripke/fairkripke.hh>
 #include <spot/kripke/kripkegraph.hh>
+#include <spot/twaalgos/split.hh>
 
 using namespace std::string_literals;
 
@@ -70,7 +72,8 @@ namespace spot
         if (bdd_is_cube(a))
           alias_cubes_.emplace_back(a, i);
         bdd neg = !a;
-        aliases_map_[neg.id()] = i;
+        // do not overwrite an existing alias with a negation
+        aliases_map_.emplace(neg.id(), i);
         if (bdd_is_cube(neg))
           alias_cubes_.emplace_back(neg, i);
       }
@@ -464,6 +467,7 @@ namespace spot
     bool verbose = false;
     bool state_labels = false;
     bool v1_1 = false;
+    bool alias_basis = false;
 
     if (opt)
       while (*opt)
@@ -486,6 +490,9 @@ namespace spot
                   v1_1 = false;
                 }
               break;
+            case 'b':
+              alias_basis = true;
+              break;
             case 'i':
               implicit_labels = true;
               break;
@@ -520,6 +527,27 @@ namespace spot
       throw std::runtime_error("print_hoa(): automaton is declared not weak, "
                                "but the acceptance makes this impossible");
 
+    // If we were asked to create an alias basis, make sure we save
+    // existing aliases, so we can restore it before we exit this
+    // function.
+    std::vector<std::pair<std::string, bdd>> old_aliases;
+    if (aut->ap().size() <= 1)
+      alias_basis = false;
+    if (alias_basis)
+      {
+        if (auto* aliases = get_aliases(aut))
+          old_aliases = *aliases;
+        create_alias_basis(std::const_pointer_cast<twa_graph>(aut));
+      }
+    // restore the old aliases using a unique_ptr-based scope guard,
+    // because there are too many ways to exit this function.
+    auto restore_aliases = [&old_aliases, alias_basis, aut](void*) {
+      if (alias_basis)
+        set_aliases(std::const_pointer_cast<twa_graph>(aut), old_aliases);
+    };
+    std::unique_ptr<void, decltype(restore_aliases)>
+      restore_aliases_guard((void*)1, restore_aliases);
+
     metadata md(aut, implicit_labels, state_labels);
 
     if (acceptance == Hoa_Acceptance_States && !md.has_state_acc)
@@ -1013,7 +1041,8 @@ namespace spot
   }
 
   void
-  set_aliases(twa_ptr g, std::vector<std::pair<std::string, bdd>> aliases)
+  set_aliases(twa_ptr g,
+              const std::vector<std::pair<std::string, bdd>>& aliases)
   {
     if (aliases.empty())
       {
@@ -1027,4 +1056,17 @@ namespace spot
       }
   }
 
+  void
+  create_alias_basis(const twa_graph_ptr& aut)
+  {
+    edge_separator es;
+    es.add_to_basis(aut);
+    std::vector<std::pair<std::string, bdd>> aliases;
+    unsigned n = 0;
+    for (bdd b: es.basis())
+      aliases.emplace_back(std::to_string(n++), b);
+    std::reverse(aliases.begin(), aliases.end());
+    set_aliases(aut, aliases);
+  }
+
 }

spot/twaalgos/hoa.hh

@@ -35,7 +35,8 @@ namespace spot
   /// \param os The output stream to print on.
   /// \param g The automaton to output.
   /// \param opt a set of characters each corresponding to a possible
-  ///        option: (i) implicit labels for complete and
+  ///        option: (b) create an alias basis if more >=2 AP
+  ///        are used, (i) implicit labels for complete and
   ///        deterministic automata, (k) state labels when possible,
   ///        (s) state-based acceptance when possible, (t)
   ///        transition-based acceptance, (m) mixed acceptance, (l)
@@ -62,7 +63,8 @@ namespace spot
   ///
   /// Pass an empty vector to remove existing aliases.
   SPOT_API void
-  set_aliases(twa_ptr g, std::vector<std::pair<std::string, bdd>> aliases);
+  set_aliases(twa_ptr g,
+              const std::vector<std::pair<std::string, bdd>>& aliases);
 
   /// \ingroup twa_io
   /// \brief Help printing BDDs as text, using aliases.
@@ -164,4 +166,18 @@ namespace spot
     }
   };
 
+  /// \ingroup twa_io
+  /// \brief Create an alias basis
+  ///
+  /// This use spot::edge_separator to build a set of alias that can
+  /// be used as a basis for all labels of the automaton.
+  ///
+  /// Such a basis can be used to shorten the size of an output file
+  /// when printing in HOA format (actually, calling print_hoa() with
+  /// option 'b' will call this function).  Such a basis may also be
+  /// useful to help visualize an automaton (using spot::print_dot's
+  /// `@` option) when its labels are too large.
+  SPOT_API void
+  create_alias_basis(const twa_graph_ptr& aut);
+
 }

tests/core/readsave.test

@@ -685,7 +685,7 @@ EOF
 
 diff output2 expect2
 
-SPOT_DEFAULT_FORMAT=hoa=k autfilt expect2 >output2b
+SPOT_DEFAULT_FORMAT=hoa=kb autfilt expect2 >output2b
 
 cat >expect2b <<EOF
 HOA: v1
@@ -695,12 +695,15 @@ AP: 2 "p0" "p1"
 acc-name: all
 Acceptance: 0 t
 properties: state-labels explicit-labels state-acc deterministic weak
+Alias: @0 !0&1 | 0&!1
+Alias: @1 !0&!1
+Alias: @2 !@1&!@0
 --BODY--
-State: [!0&!1] 0
+State: [@1] 0
 1
-State: [0&1] 1
+State: [@2] 1
 2
-State: [!0&!1] 2
+State: [@1] 2
 0
 --END--
 EOF
@@ -732,7 +735,7 @@ State: 5
 EOF
 
 autfilt -H --remove-unreach input >output3
-autfilt -H --remove-dead input >>output3
+autfilt -Hb --remove-dead input >>output3
 
 cat >expect3 <<EOF
 HOA: v1
@@ -759,13 +762,16 @@ AP: 2 "p0" "p1"
 acc-name: all
 Acceptance: 0 t
 properties: trans-labels explicit-labels state-acc deterministic weak
+Alias: @0 !0&1 | 0&!1
+Alias: @1 !0&!1
+Alias: @2 !@1&!@0
 --BODY--
 State: 0
-[!0&!1] 1
+[@1] 1
 State: 1
-[0&1] 2
+[@2] 2
 State: 2
-[!0&!1] 0
+[@1] 0
 --END--
 EOF
 

tests/python/aliases.py

@@ -108,6 +108,51 @@ State: 0
 [@p0&2 | @p1&2] 0
 --END--""")
 
+s2b = aut.to_str('hoa', 'b')
+tc.assertTrue(spot.are_equivalent(aut, spot.automaton(s2b)))
+tc.assertEqual(s2b, """HOA: v1
+States: 1
+Start: 0
+AP: 3 "x" "y" "z"
+acc-name: all
+Acceptance: 0 t
+properties: trans-labels explicit-labels state-acc complete very-weak
+Alias: @0 !0&!1&!2
+Alias: @1 0&!1&!2
+Alias: @2 !0&1&!2
+Alias: @3 0&1&!2
+Alias: @4 !0&!1&2
+Alias: @5 0&1&2
+Alias: @6 0&!1&2
+Alias: @7 !@6&!@5&!@4&!@3&!@2&!@1&!@0
+--BODY--
+State: 0
+[@6 | @5 | @3 | @1] 0
+[@7 | @4 | @2 | @0] 0
+[@7 | @5 | @3 | @2] 0
+[@6 | @4 | @1 | @0] 0
+[@4 | @0] 0
+[@7 | @6 | @5 | @3 | @2 | @1] 0
+[@7 | @2] 0
+[@6 | @5 | @4 | @3 | @1 | @0] 0
+[@6 | @1] 0
+[@7 | @5 | @4 | @3 | @2 | @0] 0
+[@7 | @4 | @2 | @0] 0
+[@6 | @4 | @1 | @0] 0
+[@7 | @6 | @2 | @1] 0
+[@7 | @6 | @4 | @2 | @1 | @0] 0
+[t] 0
+[t] 0
+[t] 0
+[@5 | @3] 0
+[@6 | @5 | @3 | @1] 0
+[@7 | @5 | @3 | @2] 0
+[@5 | @4 | @3 | @0] 0
+[@5 | @4] 0
+[@4] 0
+[@7 | @6 | @5] 0
+--END--""")
+
 # Check what happens to aliases when an AP has been removed, but
 # the aliases have been preserved...
 rem = spot.remove_ap()