twa: add accepting_run() and accepting_word() methods

Fixes #153.

* spot/twa/twa.cc, spot/twa/twa.hh: Add the methods.
* bin/autfilt.cc, bin/common_aoutput.hh, bin/ltlcross.cc,
tests/python/highlighting.ipynb, tests/python/word.ipynb: Use
them to simplify the code.
* NEWS: Mention them.

spot/twa/twa.cc

@@ -23,6 +23,7 @@
 #include <spot/twa/twa.hh>
 #include <spot/twa/twagraph.hh>
 #include <spot/twaalgos/gtec/gtec.hh>
+#include <spot/twaalgos/word.hh>
 #include <spot/twaalgos/remfin.hh>
 #include <utility>
 
@@ -69,6 +70,42 @@ namespace spot
     return !couvreur99(a)->check();
   }
 
+  twa_run_ptr
+  twa::accepting_run() const
+  {
+    if (acc().uses_fin_acceptance())
+      throw std::runtime_error("twa::accepting_run() does not work with "
+                               "Fin acceptance (but twa:is_empty() and "
+                               "twa::accepting_run() can)");
+    auto res = couvreur99(shared_from_this())->check();
+    if (!res)
+      return nullptr;
+    return res->accepting_run();
+  }
+
+  twa_word_ptr
+  twa::accepting_word() const
+  {
+    auto a = shared_from_this();
+    if (a->acc().uses_fin_acceptance())
+      {
+        auto aa = std::dynamic_pointer_cast<const twa_graph>(a);
+        if (!aa)
+          aa = make_twa_graph(a, prop_set::all());
+        a = remove_fin(aa);
+      }
+    if (auto run = a->accepting_run())
+      {
+        auto w = make_twa_word(run->reduce());
+        w->simplify();
+        return w;
+      }
+    else
+      {
+        return nullptr;
+      }
+  }
+
   void
   twa::set_named_prop(std::string s, std::nullptr_t)
   {

spot/twa/twa.hh

@@ -39,6 +39,12 @@
 
 namespace spot
 {
+  struct twa_run;
+  typedef std::shared_ptr<twa_run> twa_run_ptr;
+
+  struct twa_word;
+  typedef std::shared_ptr<twa_word> twa_word_ptr;
+
   /// \ingroup twa_essentials
   /// \brief Abstract class for states.
   class SPOT_API state
@@ -821,9 +827,28 @@ namespace spot
     }
     ///@}
 
-    /// Check whether the language of the automaton is empty.
+    /// \brief Check whether the language of the automaton is empty.
     virtual bool is_empty() const;
 
+    /// \brief Return an accepting run if one exists.
+    ///
+    /// Note that this method currently one works for Fin-less
+    /// acceptance.  For acceptance conditions that contain Fin
+    /// acceptance, you can either rely on is_empty() and not use any
+    /// accepting run, or remove Fin acceptance using remove_fin() and
+    /// compute an accepting run on that larger automaton.
+    ///
+    /// Return nullptr if no accepting run were found.
+    virtual twa_run_ptr accepting_run() const;
+
+    /// \brief Return an accepting word if one exists.
+    ///
+    /// Note that this method DO works with Fin
+    /// acceptance.
+    ///
+    /// Return nullptr if no accepting word were found.
+    virtual twa_word_ptr accepting_word() const;
+
   private:
     acc_cond acc_;