/*
  This file is part of Raul.
  Copyright 2007-2012 David Robillard <http://drobilla.net>

  Raul 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 any later version.

  Raul 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 Raul.  If not, see <http://www.gnu.org/licenses/>.
*/

#ifndef RAUL_DOUBLE_BUFFER_HPP
#define RAUL_DOUBLE_BUFFER_HPP

#include "raul/AtomicInt.hpp"
#include "raul/AtomicPtr.hpp"

namespace Raul {

/** Double buffer.
 *
 * Can be thought of as a wrapper class to make a non-atomic type atomically
 * settable (with no locking).
 *
 * Read/Write realtime safe, many writers safe - but set calls may fail.
 *
 * Space:  2*sizeof(T) + sizeof(int) + sizeof(void*)
 * \ingroup raul
 */
template<typename T>
class DoubleBuffer {
public:

	inline DoubleBuffer(T val)
		: _state(RAUL_DB_READ_WRITE)
	{
		_vals[0] = val;
		_read_val = &_vals[0];
	}

	inline DoubleBuffer(const DoubleBuffer& copy)
		: _state(RAUL_DB_READ_WRITE)
	{
		T val = copy.get();
		_vals[0] = val;
		_read_val = &_vals[0];
	}

	inline T& get() const {
		return *_read_val.get();
	}

	inline bool set(T new_val) {
		if (_state.compare_and_exchange(RAUL_DB_READ_WRITE, RAUL_DB_READ_LOCK)) {

			// locked _vals[1] for write
			_vals[1] = new_val;
			_read_val = &_vals[1];
			_state = RAUL_DB_WRITE_READ;
			return true;

			// concurrent calls here are fine.  good, actually - caught
			// the WRITE_READ state immediately after it was set above

		} else if (_state.compare_and_exchange(RAUL_DB_WRITE_READ, RAUL_DB_LOCK_READ)) {

			// locked _vals[0] for write
			_vals[0] = new_val;
			_read_val = &_vals[0];
			_state = RAUL_DB_READ_WRITE;
			return true;

		} else {

			return false;

		}
	}

private:
	enum States {
		// vals[0] state _ vals[1] state
		RAUL_DB_READ_WRITE = 0,
		RAUL_DB_READ_LOCK,
		RAUL_DB_WRITE_READ,
		RAUL_DB_LOCK_READ
	};

	AtomicInt    _state;
	AtomicPtr<T> _read_val;
	T            _vals[2];
};

} // namespace Raul

#endif // RAUL_DOUBLE_BUFFER_HPP