Automata with no state are no longer allowed.

* NEWS, spot/twa/twa.hh: Document the change.
* spot/twa/twagraph.hh, spot/kripke/kripkegraph.hh:
  Add an exception in get_init_state_number().
  get_init_state() now calls get_init_state_number().
* spot/twa/twagraph.cc, spot/twaalgos/simulation.cc,
  spot/twaalgos/powerset.cc, spot/twaalgos/complete.cc,
  spot/twaalgos/sccfilter.cc: Remove now useless tests.
* spot/twaalgos/hoa.cc: Remove now useless comment.
* spot/twaalgos/minimize.cc: Never return an automaton with no state.

NEWS

@@ -1,5 +1,9 @@
 New in spot 2.2.1.dev (Not yet released)
 
+  Library:
+
+  * A twa is required to have at least one state, the initial state.
+
   Build:
 
   * If the system has an installed libltdl library, use it instead of

spot/kripke/kripkegraph.hh

@@ -177,16 +177,15 @@ namespace spot
 
     graph_t::state get_init_state_number() const
     {
+      // If the kripke has no state, it has no initial state.
       if (num_states() == 0)
-        const_cast<graph_t&>(g_).new_state();
+        throw std::runtime_error("kripke has no state at all");
       return init_number_;
     }
 
     virtual const kripke_graph_state* get_init_state() const override
     {
-      if (num_states() == 0)
-        const_cast<graph_t&>(g_).new_state();
-      return state_from_number(init_number_);
+      return state_from_number(get_init_state_number());
     }
 
     /// \brief Allow to get an iterator on the state we passed in

spot/twa/twa.hh

@@ -595,7 +595,8 @@ namespace spot
   /// Browsing a TωA is usually achieved using two methods: \c
   /// get_init_state(), and succ().  The former returns the initial
   /// state while the latter allows iterating over the outgoing edges
-  /// of any given state.
+  /// of any given state. A TωA is always assumed to have at least
+  /// one state, the initial one.
   ///
   /// Note that although this is a transition-based automata, we never
   /// represent edges in the API.  Information about edges can be
@@ -605,7 +606,7 @@ namespace spot
   /// The interface presented here is what we call the on-the-fly
   /// interface of automata, because the TωA class can be subclassed
   /// to implement an object that computes its successors on-the-fly.
-  /// The down-side is that all these methods are virtual, so you you
+  /// The down-side is that all these methods are virtual, so you
   /// pay the cost of virtual calls when iterating over automata
   /// constructed on-the-fly.  Also the interface assumes that each
   /// successor state is a new object whose memory management is the

spot/twa/twagraph.cc

@@ -154,8 +154,6 @@ namespace spot
   void twa_graph::purge_unreachable_states()
   {
     unsigned num_states = g_.num_states();
-    if (SPOT_UNLIKELY(num_states == 0))
-      return;
     // The TODO vector serves two purposes:
     // - it is a stack of state to process,
     // - it is a set of processed states.
@@ -166,7 +164,7 @@ namespace spot
     std::vector<unsigned> todo(num_states, 0);
     const unsigned seen = 1 << (sizeof(unsigned)*8-1);
     const unsigned mask = seen - 1;
-    todo[0] = init_number_;
+    todo[0] = get_init_state_number();
     todo[init_number_] |= seen;
     unsigned todo_pos = 1;
     do
@@ -197,16 +195,13 @@ namespace spot
   void twa_graph::purge_dead_states()
   {
     unsigned num_states = g_.num_states();
-    if (num_states == 0)
-      return;
-
     std::vector<unsigned> useful(num_states, 0);
 
     // Make a DFS to compute a topological order.
     std::vector<unsigned> order;
     order.reserve(num_states);
     std::vector<std::pair<unsigned, unsigned>> todo; // state, trans
-    useful[init_number_] = 1;
+    useful[get_init_state_number()] = 1;
     todo.emplace_back(init_number_, g_.state_storage(init_number_).succ);
     do
       {

spot/twa/twagraph.hh

@@ -264,16 +264,15 @@ namespace spot
 
     state_num get_init_state_number() const
     {
+      // If the automaton has no state, it has no initial state.
       if (num_states() == 0)
-        const_cast<graph_t&>(g_).new_state();
+        throw std::runtime_error("automaton has no state at all");
       return init_number_;
     }
 
     virtual const twa_graph_state* get_init_state() const override
     {
-      if (num_states() == 0)
-        const_cast<graph_t&>(g_).new_state();
-      return state_from_number(init_number_);
+      return state_from_number(get_init_state_number());
     }
 
     virtual twa_succ_iterator*

spot/twaalgos/complete.cc

@@ -23,11 +23,6 @@ namespace spot
 {
   unsigned complete_here(twa_graph_ptr aut)
   {
-    // We do not use the initial state, but calling
-    // get_init_state_number() may create it and change the number of
-    // states.  This has to be done before calling aut->num_states().
-    unsigned init = aut->get_init_state_number();
-
     unsigned n = aut->num_states();
     unsigned sink = -1U;
 
@@ -87,7 +82,7 @@ namespace spot
 
     // If the automaton is empty, pretend that state 0 is a sink.
     if (t == 0)
-      sink = init;
+      sink = aut->get_init_state_number();
 
     // Now complete all states (excluding any newly added the sink).
     for (unsigned i = 0; i < n; ++i)

spot/twaalgos/hoa.cc

@@ -312,16 +312,13 @@ namespace spot
             }
         }
 
-    // Calling get_init_state_number() may add a state to empty
-    // automata, so it has to be done first.
-    unsigned init = aut->get_init_state_number();
-
     metadata md(aut, implicit_labels, state_labels);
 
     if (acceptance == Hoa_Acceptance_States && !md.has_state_acc)
       acceptance = Hoa_Acceptance_Transitions;
 
     unsigned num_states = aut->num_states();
+    unsigned init = aut->get_init_state_number();
 
     const char nl = newline ? '\n' : ' ';
     os << (v1_1 ? "HOA: v1.1" : "HOA: v1") << nl;

spot/twaalgos/minimize.cc

@@ -171,6 +171,10 @@ namespace spot
           init_state->destroy();
           res->set_init_state(init_num);
         }
+      else
+        {
+          res->set_init_state(res->new_state());
+        }
       return res;
     }
 

spot/twaalgos/powerset.cc

@@ -1,6 +1,6 @@
 // -*- coding: utf-8 -*-
-// Copyright (C) 2009, 2010, 2011, 2013, 2014, 2015 Laboratoire de
-// Recherche et Développement de l'Epita (LRDE).
+// Copyright (C) 2009-2011, 2013-2016 Laboratoire de Recherche et
+// Développement de l'Epita (LRDE).
 // Copyright (C) 2004 Laboratoire d'Informatique de Paris 6 (LIP6),
 // département Systèmes Répartis Coopératifs (SRC), Université Pierre
 // et Marie Curie.
@@ -101,11 +101,8 @@ namespace spot
 
     unsigned nap = number_of_variables(allap);
 
-    // Call this before aut->num_states(), since it might add a state.
     unsigned init_num = aut->get_init_state_number();
-
     unsigned ns = aut->num_states();
-    assert(ns > 0);
 
     if ((-1UL / ns) >> nap == 0)
       throw std::runtime_error("too many atomic propositions (or states)");

spot/twaalgos/sccfilter.cc

@@ -277,8 +277,6 @@ namespace spot
                                    scc_info* given_si, Args&&... args)
     {
       unsigned in_n = aut->num_states();
-      if (in_n == 0)                         // nothing to filter.
-        return make_twa_graph(aut, twa::prop_set::all());
 
       twa_graph_ptr filtered = make_twa_graph(aut->get_dict());
       filtered->copy_ap_of(aut);

spot/twaalgos/simulation.cc

@@ -161,14 +161,11 @@ namespace spot
           throw std::runtime_error
             ("direct_simulation() requires separate Inf and Fin sets");
 
-        // Call get_init_state_number() before anything else as it
-        // might add a state.
-        unsigned init_state_number = in->get_init_state_number();
         scc_info_.reset(new scc_info(in));
 
         unsigned ns = in->num_states();
-        assert(ns > 0);
         size_a_ = ns;
+        unsigned init_state_number = in->get_init_state_number();
 
         auto all_inf = in->get_acceptance().used_inf_fin_sets().first;
         all_inf_ = all_inf;