// -*- coding: utf-8 -*-
// Copyright (C) 2009, 2010, 2012, 2013, 2014 Laboratoire de Recherche
// et Développement de l'Epita (LRDE).
// Copyright (C) 2003, 2004 Laboratoire d'Informatique de Paris 6 (LIP6),
//
// 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/>.

/// \file ltlast/constant.hh
/// \brief LTL constants
#ifndef SPOT_LTLAST_CONSTANT_HH
# define SPOT_LTLAST_CONSTANT_HH

#include "formula.hh"

namespace spot
{
  namespace ltl
  {

    /// \ingroup ltl_ast
    /// \brief A constant (True or False)
    class SPOT_API constant final : public formula
    {
    public:
      enum type { False, True, EmptyWord };
      virtual void accept(visitor& v) const override;

      /// Return the value of the constant.
      type val() const
      {
	return val_;
      }

      /// Return the value of the constant as a string.
      const char* val_name() const;

      virtual std::string dump() const override;

      /// Get the sole instance of spot::ltl::constant::constant(True).
      static constant* true_instance() { return &true_instance_; }
      /// Get the sole instance of spot::ltl::constant::constant(False).
      static constant* false_instance() { return &false_instance_; }
      /// Get the sole instance of spot::ltl::constant::constant(EmptyWord).
      static constant* empty_word_instance() { return &empty_word_instance_; }

    protected:
      constant(type val);
      virtual ~constant();

    private:
      type val_;

      static constant true_instance_;
      static constant false_instance_;
      static constant empty_word_instance_;
      // If you add new constants here, be sure to update the
      // formula::formula() constructor.
    };


    /// \brief Cast \a f into a constant.
    ///
    /// Cast \a f into a constant iff it is a constant instance.
    /// Return 0 otherwise.  This is faster than \c dynamic_cast.
    inline
    const constant*
    is_constant(const formula* f)
    {
      if (f->kind() != formula::Constant)
	return 0;
      return static_cast<const constant*>(f);
    }
  }
}

#endif // SPOT_LTLAST_CONSTANT_HH
