Add a decompose_scc() function

See #172.
While at it, fix typo in doxygen comment.

* spot/twaalgos/strength.cc, spot/twaalgos/strength.hh: New function.
* tests/python/decompose_scc.py, tests/Makefile.am: Test python
binding.

* spot/twaalgos/mask.hh: Fix typo.

spot/twaalgos/mask.hh

@@ -29,7 +29,7 @@ namespace spot
   /// \a cpy, creating new states at the same time.  The argument \a
   /// trans should behave as a function with the following prototype:
   /// <code>
-  ///   void (*trans) (unsigned srcbdd& cond, acc_cond::mark_t& acc,
+  ///   void (*trans) (unsigned src, bdd& cond, acc_cond::mark_t& acc,
   ///   unsigned dst)
   /// </code>
   /// It can modify either the condition or the acceptance sets of

spot/twaalgos/strength.cc

@@ -21,7 +21,6 @@
 #include <spot/misc/hash.hh>
 #include <spot/twaalgos/isweakscc.hh>
 #include <spot/twaalgos/mask.hh>
-#include <deque>
 
 namespace spot
 {
@@ -338,4 +337,58 @@ namespace spot
       }
     return res;
   }
+
+  twa_graph_ptr
+  decompose_scc(scc_info& sm, unsigned scc_num)
+  {
+    unsigned n = sm.scc_count();
+
+    if (n <= scc_num)
+      throw std::invalid_argument
+        (std::string("decompose_scc(): requested SCC index is out of bounds"));
+
+    std::vector<bool> want(n, false);
+    want[scc_num] = true;
+
+    // mark all the SCCs that can reach scc_num as wanted
+    for (unsigned i = scc_num + 1; i < n; ++i)
+      for (unsigned succ : sm.succ(i))
+        if (want[succ])
+          {
+            want[i] = true;
+            break;
+          }
+
+    const_twa_graph_ptr aut = sm.get_aut();
+    twa_graph_ptr res = make_twa_graph(aut->get_dict());
+    res->copy_ap_of(aut);
+    res->prop_copy(aut, { true, false, false, true, false });
+    res->copy_acceptance_of(aut);
+
+    auto um = aut->acc().unsat_mark();
+
+    // If aut has an unsatisfying mark, we are going to use it to remove the
+    // acceptance of some transitions. If it doesn't, we make res a rejecting
+    // Büchi automaton, and get back an accepting mark that we are going to set
+    // on the transitions of the SCC we selected.
+    auto new_mark = um.first ? um.second : res->set_buchi();
+    auto fun = [sm, &want, um, new_mark, scc_num]
+      (unsigned src, bdd& cond, acc_cond::mark_t& acc, unsigned dst)
+      {
+        if (!want[sm.scc_of(dst)])
+          {
+            cond = bddfalse;
+            return;
+          }
+        // no need to check if src is wanted, we already know dst is.
+        // if res is accepting, make only the upstream SCCs rejecting
+        // if res is rejecting, make only the requested SCC accepting
+        if (um.first != (sm.scc_of(src) == scc_num))
+          acc = new_mark;
+      };
+
+    transform_accessible(aut, res, fun);
+
+    return res;
+  }
 }

spot/twaalgos/strength.hh

@@ -171,4 +171,14 @@ namespace spot
   /// \param keep a string specifying the strengths to keep: it should
   SPOT_API twa_graph_ptr
   decompose_strength(const const_twa_graph_ptr& aut, const char* keep);
+
+  /// \brief Extract a sub-automaton of a SCC
+  ///
+  /// This algorithm returns a subautomaton that contains the requested SCC,
+  /// plus any upstream SCC (but adjusted not to be accepting).
+  ///
+  /// \param sm the SCC info map of the automaton
+  /// \param scc_num the index in the map of the SCC to keep
+  SPOT_API twa_graph_ptr
+  decompose_scc(scc_info& sm, unsigned scc_num);
 }

tests/Makefile.am

@@ -332,6 +332,7 @@ TESTS_python = \
   python/bddnqueen.py \
   python/bugdet.py \
   python/sccinfo.py \
+  python/decompose_scc.py \
   python/implies.py \
   python/interdep.py \
   python/ltl2tgba.test \

tests/python/decompose_scc.py

@@ -0,0 +1,51 @@
+# -*- mode: python; coding: utf-8 -*-
+# Copyright (C) 2017 Laboratoire de Recherche et
+# Développement de l'Epita
+#
+# This file is part of Spot, a model checking library.
+#
+# Spot is free software; you can redistribute it and/or modify it
+# under the terms of the GNU General Public License as published by
+# the Free Software Foundation; either version 3 of the License, or
+# (at your option) any later version.
+#
+# Spot is distributed in the hope that it will be useful, but WITHOUT
+# ANY WARRANTY; without even the implied warranty of MERCHANTABILITY
+# or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public
+# License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+import spot
+
+aut = spot.translate('(Ga -> Gb) W c')
+
+si = spot.scc_info(aut)
+
+assert (spot.decompose_scc(si, 2).to_str('hoa', '1.1') == """HOA: v1.1
+States: 3
+Start: 0
+AP: 3 "b" "a" "c"
+acc-name: Buchi
+Acceptance: 1 Inf(0)
+properties: trans-labels explicit-labels state-acc !complete
+properties: deterministic
+--BODY--
+State: 0
+[!1&!2] 0
+[1&!2] 1
+State: 1
+[!1&!2] 0
+[1&!2] 1
+[1&2] 2
+State: 2
+[1] 2
+--END--""")
+
+try:
+    spot.decompose_scc(si, 4)
+except ValueError:
+    pass
+else:
+    raise AssertionError