readwritespinlock.h

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// The DAGFrameScheduler is a Multi-Threaded lock free and wait free scheduling library.
// © Copyright 2010 - 2016 BlackTopp Studios Inc.
/* This file is part of The DAGFrameScheduler.

    The DAGFrameScheduler is free software: you can redistribute it and/or modify
    it under the terms of the GNU General Public License as published by
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    GNU General Public License for more details.

    You should have received a copy of the GNU General Public License
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*/
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   'doc' folder. See 'gpl.txt'
*/
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*/
#ifndef _readwritespinlock_h
#define _readwritespinlock_h

#include "datatypes.h"
#include "lockguard.h"
#include "spinlock.h"

/// @file
/// @brief Declares a Mutex, Mutex tools, and at least one MutexLike object.

namespace Mezzanine
{
    namespace Threading
    {
        /// @brief A mutex like construct that supports multiple readsingle writer semantics and never makes a system call and uses CPU instructions instead.
        /// @details This should be used when delay is likely to be measured in CPUs cycles and almost
        /// certainly a short while. For pauses of unknown length use a Mutex so that the OS is informed it
        /// could schedule another thread.
        /// @n @n
        /// Provided readers and writers call the correct functions this will only allow access to a single writer
        /// or a group of readers at one time
        class MEZZ_LIB ReadWriteSpinLock
        {
            private:
                /// @brief Used to synchronize access to Locked, to indicate either the current count of Read locks or the existence of write locks
                SpinLock CountGaurd;

                /// @internal
                /// @brief 0 if unlocked, A positive amount is amount of locking readers and a negative value is writed locked.
                Int32 Locked;

            public:
                ///@brief Constructor, creates an unlocked mutex
                ReadWriteSpinLock();
                ///@brief Destructor.
                ~ReadWriteSpinLock();

                /// @brief Lock the ReadWriteSpinLock for Reading
                /// @details The method will block the calling thread until a read lock
                /// be obtained. This remains locked until @c UnlockRead() is called.
                /// @n @n
                /// Multiple read locks can be obtained. If this is locked multiple this
                /// will keep count until @c UnlockRead() decrements that count.
                void LockForRead();
                /// @brief Try to lock this for reading.
                /// @details The method will try to lock the ReadWriteSpinLock for Reading. If
                /// it fails, the function will return immediately (non-blocking).
                /// @return @c true if the lock was acquired, or @c false if the lock could
                /// not be acquired.
                bool TryLockForRead();
                /// @brief Unlock this .
                /// @details Decrement the reader count and If zero any threads that are waiting
                /// for the LockWrite method on this mutex, one of them will be unblocked. If
                /// locked for write, this does nothing.
                void UnlockRead();

                /// @brief Lock the ReadWriteSpinLock for Writing.
                /// @details The method will block the calling thread until a write lock
                /// be obtained. This remains locked until @c UnlockWrite() is called.
                /// @n @n
                /// Only a single write lock can exist and it can co-exist with no read locks.
                void LockForWrite();
                /// @brief Try to lock this for writing.
                /// @details The method will try to lock the ReadWriteSpinLock for writing. If
                /// it fails, the function will return immediately (non-blocking).
                /// @return @c true if the lock was acquired, or @c false if the lock could
                /// not be acquired.
                bool TryLockForWrite();
                /// @brief Unlock this for writing.
                /// @details If any threads are waiting for the read or write lock on this,
                /// one of them willbe unblocked. If locked for read this doe nothing.
                void UnlockWrite();

                /// @brief Simply calls LockForWrite() for compatibility with lock_guard
                void lock()
                    { LockForWrite(); }
                /// @brief Simply calls UnlockWrite() for compatibility with lock_guard
                void unlock()
                    { UnlockWrite(); }
        };//Mutex



        /// @brief Read only lock guard.
        /// @details The constructor locks the mutex, and the destructor unlocks the mutex, so
        /// the mutex will automatically be unlocked when the lock guard goes out of
        /// scope. Example usage:
        /// @code
        /// ReadWriteSpinLock m;
        /// ClassThatIsWrittenInAnotherThread n;
        ///
        /// ClassThatIsWrittenInAnotherThread GetCurrent()
        /// {
        ///   ReadOnlyLockGuard<ReadWriteSpinLock> guard(m);
        ///   return n; // returns a copy guaranteed to be valid because
        ///             // the lock (which is attempted in the constructor
        ///             // of the guard) could not be acquired until only
        ///             // readers want it.
        /// }
        /// @endcode
        template <class T>
        class ReadOnlyLockGuard
        {
            public:
                /// @brief This allows other code to use the type of this mutex in a more safe way.
                typedef T mutex_type;

            private:
                /// @internal
                /// @brief A non-owning pointer to the mutex.
                mutex_type* mMutex;

            public:
                /// @brief The constructor locks the mutex.
                /// @param aMutex Any mutex which implements lock() and unlock().
                explicit ReadOnlyLockGuard(mutex_type& aMutex)
                    : mMutex(&aMutex)
                    { mMutex->LockForRead(); }

                /// @brief The destructor unlocks the mutex.
                ~ReadOnlyLockGuard()
                    { mMutex->UnlockRead(); }
        };//lock_guard

        /// @brief ReadWrite lock guard.
        /// @details The constructor locks the mutex, and the destructor unlocks the mutex, so
        /// the mutex will automatically be unlocked when the lock guard goes out of
        /// scope. Example usage:
        /// @code
        /// ReadWriteSpinLock m;
        /// int counter;
        ///
        /// int Increment()
        /// {
        ///   ReadWriteLockGuard<ReadWriteSpinLock> guard(m);
        ///   return counter++;
        /// }
        /// @endcode
        template <class T>
        class ReadWriteLockGuard
        {
            public:
                /// @brief This allows other code to use the type of this mutex in a more safe way.
                typedef T mutex_type;

            private:
                /// @internal
                /// @brief A non-owning pointer to the mutex.
                mutex_type* mMutex;

            public:
                /// @brief The constructor locks the mutex.
                /// @param aMutex Any mutex which implements lock() and unlock().
                explicit ReadWriteLockGuard(mutex_type& aMutex)
                    : mMutex(&aMutex)
                    { mMutex->LockForWrite(); }

                /// @brief The destructor unlocks the mutex.
                ~ReadWriteLockGuard()
                    { mMutex->UnlockWrite(); }
        };//lock_guard

    }//Threading
}//Mezzanine

#endif