/*
 * copyright: 2014-2016
 * name : Francis Banyikwa
 * email: mhogomchungu@gmail.com
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
 * are met:
 *
 * 1. Redistributions of source code must retain the above copyright
 *    notice, this list of conditions and the following disclaimer.
 * 2. Redistributions in binary form must reproduce the above copyright
 *    notice, this list of conditions and the following disclaimer in
 *    the documentation and/or other materials provided with the
 *    distribution.
 *
 * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
 * ``AS IS'' AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
 * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS
 * FOR A PARTICULAR PURPOSE ARE DISCLAIMED.  IN NO EVENT SHALL THE
 * COPYRIGHT HOLDERS OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT,
 * INCIDENTAL, SPECIAL, EXEMPLARY OR CONSEQUENTIAL DAMAGES (INCLUDING,
 * BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;
 * LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED
 * AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
 * OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT
 * OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
 * SUCH DAMAGE.
 */

#ifndef __TASK_HH_INCLUDED__
#define __TASK_HH_INCLUDED__

#include <utility>
#include <future>
#include <functional>
#include <QThread>
#include <QEventLoop>

/*
 *
 * Examples on how to use the library are at the end of this file.
 *
 */

/*
 * This library wraps a function into a future where the result of the function
 * can be retrieved through the future's 3 public methods:
 *
 * 1. .get()   This method runs the wrapped function on the current thread
 *             and could block the thread and hang GUI.
 *
 * 2. .then()  This method does three things:
 *
 *             1. Registers a method to be called when a wrapped function finish running.
 *
 *             2. Runs the wrapped function on a background thread.
 *
 *             3. Runs the registered method on the current thread when the wrapped function finish
 *                running.
 *
 * 3. .await() This method does three things:
 *
 *             1. Suspends the current thread at a point where this method is called.
 *
 *             2. Creates a background thread and then runs the wrapped function in the background
 *                thread.
 *
 *             3. Unsuspends the current thread when the wrapped function finish and let the
 *                current thread continue normally.
 *
 *             The suspension at step 1 is done without blocking the thread and hence the suspension
 *             can be done in the GUI thread and the GUI will remain responsive.
 *
 * The future is of type "Task::future<T>&" and "std::reference_wrapper"[1]
 * class can be used if they are to be managed in a container that can not handle references.
 *
 * [1] http://en.cppreference.com/w/cpp/utility/functional/reference_wrapper
 */
namespace LXQt
{

namespace Wallet
{

namespace Task
{
	class Thread : public QThread
	{
		Q_OBJECT
	public:
		Thread()
		{
			#if QT_VERSION < QT_VERSION_CHECK( 5,0,0 )
				connect( this,SIGNAL( finished() ),this,SLOT( deleteLater() ) ) ;
			#else
				connect( this,&QThread::finished,this,&QThread::deleteLater ) ;
			#endif
		}
	protected:
		virtual ~Thread()
		{
		}
	private:
		virtual void run()
		{
		}
	};

        template< typename T >
        class future
        {
        public:
                future( QThread * e,
                        std::function< void() >&& start,
                        std::function< void() >&& cancel,
                        std::function< void( T& ) >&& get ) :
                        m_thread( e ),
                        m_start ( std::move( start ) ),
                        m_cancel( std::move( cancel ) ),
                        m_get   ( std::move( get ) )
                {
                }
                void then( std::function< void( T ) > function )
                {
                        m_function = std::move( function ) ;
                        this->start() ;
                }
                T get()
                {
                        T r ;
                        m_get( r ) ;
                        return r ;
                }
                T await()
                {
                        QEventLoop p ;

                        T q ;

                        m_function = [ & ]( T&& r ){ q = std::move( r ) ; p.exit() ; } ;

                        this->start() ;

                        p.exec() ;

                        return q ;
                }
                QThread& thread()
                {
                        return *m_thread ;
                }
                void start()
                {
                        m_start() ;
                }
                void cancel()
                {
                        m_cancel() ;
                }
                void run( T&& r )
                {
                        m_function( std::move( r ) ) ;
                }
        private:
                QThread * m_thread ;
		std::function< void( T ) > m_function = []( T&& t ){ Q_UNUSED( t ) } ;
                std::function< void() > m_start ;
                std::function< void() > m_cancel ;
                std::function< void( T& ) > m_get ;
        };

	template< typename T >
	class ThreadHelper : public Thread
	{
	public:
		ThreadHelper( std::function< T() >&& function ) :
			m_function( std::move( function ) ),
			m_future( this,
				  [ this ](){ this->start() ; },
				  [ this ](){ this->deleteLater() ; },
                                  [ this ]( T& r ){ r = m_function() ; this->deleteLater() ; } )
		{
		}
                future<T>& Future()
		{
                        return m_future ;
		}
	private:
		~ThreadHelper()
                {
			m_future.run( std::move( m_result ) ) ;
		}
		void run()
		{
			m_result = m_function() ;
		}
		std::function< T() > m_function ;
                future<T> m_future ;
                T m_result ;
        };

	template<>
	class future< void >
	{
	public:
		future(	QThread * e ,
			std::function< void() >&& start,
			std::function< void() >&& cancel,
			std::function< void() >&& get ) :
			m_thread( e ),
			m_start ( std::move( start ) ),
			m_cancel( std::move( cancel ) ),
			m_get   ( std::move( get ) )
		{
                }
		void then( std::function< void() > function )
		{
			m_function = std::move( function ) ;
			this->start() ;
		}
		void get()
		{
                        m_get() ;
		}
		void await()
		{
			QEventLoop p ;

			m_function = [ & ](){ p.exit() ; } ;

			this->start() ;

			p.exec() ;
		}
                QThread& thread()
		{
			return *m_thread ;
		}
		void start()
		{
                        m_start() ;
		}
		void run()
		{
			m_function() ;
		}
		void cancel()
		{
			m_cancel() ;
		}
	private:
		QThread * m_thread ;
		std::function< void() > m_function = [](){} ;
		std::function< void() > m_start ;
		std::function< void() > m_cancel ;
                std::function< void() > m_get ;
	};

	template<>
	class ThreadHelper< void > : public Thread
	{
	public:
		ThreadHelper( std::function< void() >&& function ) :
			m_function( std::move( function ) ),
			m_future( this,
				  [ this ](){ this->start() ; },
				  [ this ](){ this->deleteLater() ; },
				  [ this ](){ m_function() ; this->deleteLater() ; } )
		{
		}
                future< void >& Future()
		{
                        return m_future ;
		}
	private:
		~ThreadHelper()
		{
			m_future.run() ;
		}
		void run()
		{
			m_function() ;
		}
		std::function< void() > m_function ;
		future< void > m_future ;
	};

	/*
	 *
	 * Below APIs wrappes a function around a future and then returns the future.
	 *
	 */

	template< typename T >
        future<T>& run( std::function< T() > function )
	{
		return ( new ThreadHelper<T>( std::move( function ) ) )->Future() ;
	}

	template< typename T,typename ... Args >
        future<T>& run( std::function< T( Args ... ) > function,Args ... args )
	{
		return Task::run<T>( std::bind( std::move( function ),std::move( args ) ... ) ) ;
	}

        static inline future< void >& run( std::function< void() > function )
	{
		return Task::run< void >( std::move( function ) ) ;
	}

	template< typename ... Args >
        future< void >& run( std::function< void( Args ... ) > function,Args ... args )
	{
		return Task::run< void >( std::bind( std::move( function ),std::move( args ) ... ) ) ;
	}

	/*
	 *
	 * A few useful helper functions
	 *
	 */

	template< typename T >
	T await( std::function< T() > function )
	{
		return Task::run<T>( std::move( function ) ).await() ;
	}

	template< typename T,typename ... Args >
	T await( std::function< T( Args ... ) > function,Args ... args )
	{
		return Task::await<T>( std::bind( std::move( function ),std::move( args ) ... ) ) ;
	}

	static inline void await( std::function< void() > function )
	{
		Task::await< void >( std::move( function ) ) ;
	}

	template< typename T >
        T await( Task::future<T>& e )
	{
		return e.await() ;
	}

	template< typename T >
	T await( std::future<T> t )
	{
		return Task::await<T>( [ & ](){ return t.get() ; } ) ;
	}

	/*
	 * These methods run their arguments in a separate thread and does not offer
	 * continuation feature.Useful when wanting to just run a function in a
	 * different thread.
	 */
	static inline void exec( std::function< void() > function )
	{
		Task::run( std::move( function ) ).start() ;
	}

	template< typename T,typename ... Args >
	void exec( std::function< T( Args ... ) > function,Args ... args )
	{
		Task::exec( std::bind( std::move( function ),std::move( args ) ... ) ) ;
	}
}

}

}

#if 0 // start example block

Examples on how to use the library

**********************************************************
* Example use cases on how to use Task::run().then() API
**********************************************************

templated version that passes a return value of one function to another function
---------------------------------------------------------------------------------

int foo
{
	/*
	 * This task will run on a different thread
	 * This tasks returns a result
	 */
	return 0 ;
}

void bar( int r )
{
	/*
	 * This task will run on the original thread.
	 * This tasks takes an argument returned by task _a
	 */
}

Task::run<int>( foo ).then( bar ) ;

alternatively,

Task::future<int>& e = Task::run<int>( foo ) ;

e.then( bar ) ;


Non templated version that does not pass around return value
----------------------------------------------------------------
void foo_1()
{
	/*
	 * This task will run on a different thread
	 * This tasks returns with no result
	 */
}

void bar_1()
{
	/*
	 * This task will run on the original thread.
	 * This tasks takes no argument
	 */
}

Task::run( foo_1 ).then( bar_1 ) ;

alternatively,

Task::future<void>& e = Task::run( foo_1 ) ;

e.then( bar_1 ) ;

**********************************************************
* Example use cases on how to use Task::run().await() API
**********************************************************

int r = Task::await<int>( foo ) ;

alternatively,

Task::future<int>& e = Task::run<int>( foo ) ;

int r = e.await() ;

alternatively,

int r = Task::run<int>( foo ).await() ;

*******************************************************************
* Example use cases on how to use lambda that requires an argument
*******************************************************************

/*
 * declaring "foo_2" with an auto keyword will not be sufficient here
 * and the full std::function<blablabla> is required.
 *
 * For the same reason,just plugging in a lambda that requires arguments
 * into Task::run() will not be sufficent and the plugged in lambda must
 * be casted to std::function<blablabla> for it to compile.
 *
 * Why the above restriction? No idea but i suspect it has to do with
 * variadic template type deduction failing to see something.
 */

std::function< int( int ) > foo_2 = []( int x ){

	return x + 1 ;
} ;

Task::run( foo_2,6 ).then( []( int r ){

	qDebug() << r ;
} ) ;

alternatively,

r = Task::await( foo_2,6 ) ;

#endif //end example block

#endif //__TASK_HH_INCLUDED__
