/** 
 * @file llsingleton.h
 *
 * $LicenseInfo:firstyear=2002&license=viewerlgpl$
 * Second Life Viewer Source Code
 * Copyright (C) 2010, Linden Research, Inc.
 * 
 * This library is free software; you can redistribute it and/or
 * modify it under the terms of the GNU Lesser General Public
 * License as published by the Free Software Foundation;
 * version 2.1 of the License only.
 * 
 * This library 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
 * Lesser General Public License for more details.
 * 
 * You should have received a copy of the GNU Lesser General Public
 * License along with this library; if not, write to the Free Software
 * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301  USA
 * 
 * Linden Research, Inc., 945 Battery Street, San Francisco, CA  94111  USA
 * $/LicenseInfo$
 */
#ifndef LLSINGLETON_H
#define LLSINGLETON_H

#include "llerror.h"	// *TODO: eliminate this

#include <typeinfo>
#include <boost/noncopyable.hpp>

// LLSingleton implements the getInstance() method part of the Singleton
// pattern. It can't make the derived class constructors protected, though, so
// you have to do that yourself.
//
// There are two ways to use LLSingleton. The first way is to inherit from it
// while using the typename that you'd like to be static as the template
// parameter, like so:
//
//   class Foo: public LLSingleton<Foo>{};
//
//   Foo& instance = Foo::instance();
//
// The second way is to use the singleton class directly, without inheritance:
//
//   typedef LLSingleton<Foo> FooSingleton;
//
//   Foo& instance = FooSingleton::instance();
//
// In this case, the class being managed as a singleton needs to provide an
// initSingleton() method since the LLSingleton virtual method won't be
// available
//
// As currently written, it is not thread-safe.

template <typename DERIVED_TYPE>
class LLSingleton : private boost::noncopyable
{
	
private:
	typedef enum e_init_state
	{
		UNINITIALIZED,
		CONSTRUCTING,
		INITIALIZING,
		INITIALIZED,
		DELETED
	} EInitState;
	
	// stores pointer to singleton instance
	struct SingletonLifetimeManager
	{
		SingletonLifetimeManager()
		{
			construct();
		}

		static void construct()
		{
			sData.mInitState = CONSTRUCTING;
			sData.mInstance = new DERIVED_TYPE(); 
			sData.mInitState = INITIALIZING;
		}

		~SingletonLifetimeManager()
		{
			if (sData.mInitState != DELETED)
			{
				deleteSingleton();
			}
		}
	};
	
public:
	virtual ~LLSingleton()
	{
		sData.mInstance = NULL;
		sData.mInitState = DELETED;
	}

	/**
	 * @brief Immediately delete the singleton.
	 *
	 * A subsequent call to LLProxy::getInstance() will construct a new
	 * instance of the class.
	 *
	 * LLSingletons are normally destroyed after main() has exited and the C++
	 * runtime is cleaning up statically-constructed objects. Some classes
	 * derived from LLSingleton have objects that are part of a runtime system
	 * that is terminated before main() exits. Calling the destructor of those
	 * objects after the termination of their respective systems can cause
	 * crashes and other problems during termination of the project. Using this
	 * method to destroy the singleton early can prevent these crashes.
	 *
	 * An example where this is needed is for a LLSingleton that has an APR
	 * object as a member that makes APR calls on destruction. The APR system is
	 * shut down explicitly before main() exits. This causes a crash on exit.
	 * Using this method before the call to apr_terminate() and NOT calling
	 * getInstance() again will prevent the crash.
	 */
	static void deleteSingleton()
	{
		delete sData.mInstance;
		sData.mInstance = NULL;
		sData.mInitState = DELETED;
	}


	static DERIVED_TYPE* getInstance()
	{
		static SingletonLifetimeManager sLifeTimeMgr;

		switch (sData.mInitState)
		{
		case UNINITIALIZED:
			// should never be uninitialized at this point
			llassert(false);
			return NULL;
		case CONSTRUCTING:
			llerrs << "Tried to access singleton " << typeid(DERIVED_TYPE).name() << " from singleton constructor!" << llendl;
			return NULL;
		case INITIALIZING:
			// go ahead and flag ourselves as initialized so we can be reentrant during initialization
			sData.mInitState = INITIALIZED;	
			// initialize singleton after constructing it so that it can reference other singletons which in turn depend on it,
			// thus breaking cyclic dependencies
			sData.mInstance->initSingleton(); 
			return sData.mInstance;
		case INITIALIZED:
			return sData.mInstance;
		case DELETED:
			llwarns << "Trying to access deleted singleton " << typeid(DERIVED_TYPE).name() << " creating new instance" << llendl;
			SingletonLifetimeManager::construct();
			// same as first time construction
			sData.mInitState = INITIALIZED;	
			sData.mInstance->initSingleton(); 
			return sData.mInstance;
		}

		return NULL;
	}

	static DERIVED_TYPE* getIfExists()
	{
		return sData.mInstance;
	}

	// Reference version of getInstance()
	// Preferred over getInstance() as it disallows checking for NULL
	static DERIVED_TYPE& instance()
	{
		return *getInstance();
	}
	
	// Has this singleton been created uet?
	// Use this to avoid accessing singletons before the can safely be constructed
	static bool instanceExists()
	{
		return sData.mInitState == INITIALIZED;
	}
	
	// Has this singleton already been deleted?
	// Use this to avoid accessing singletons from a static object's destructor
	static bool destroyed()
	{
		return sData.mInitState == DELETED;
	}

private:

	virtual void initSingleton() {}

	struct SingletonData
	{
		// explicitly has a default constructor so that member variables are zero initialized in BSS
		// and only changed by singleton logic, not constructor running during startup
		EInitState		mInitState;
		DERIVED_TYPE*	mInstance;
	};
	static SingletonData sData;
};

template<typename T>
typename LLSingleton<T>::SingletonData LLSingleton<T>::sData;

#endif