_ak_fifo_queue_8h

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 #ifndef _AKFIFOQUEUE_H
 #define _AKFIFOQUEUE_H
  
 #include <AK/SoundEngine/Common/AkTypes.h>
 #include <AK/SoundEngine/Common/AkAtomic.h>
 #include <AK/Tools/Common/AkArray.h>
  
 /// AkFifoQueue is a lock-less, thread-safe, multi-producer-multi-consumer queue data structure.
 /// It is designed to hold copyable values.
 template<typename T, T TDEFAULT, class TAlloc = ArrayPoolDefault>
 struct AkFifoQueue : public TAlloc
 {
 public:
  
     AkFifoQueue()
         : m_buffer(nullptr)
         , m_uQueueIndexMask(0)
         , m_readPos(0)
         , m_writePos(0)
     {
     }
  
     ~AkFifoQueue()
     {
         Term();
     }
  
     /// Initializes the FifoQueue and allocates memory for the specified number of entries.
     /// The number of entries is not growable after initialization.
     AKRESULT Init(
         AkUInt32 in_uMaxEntries      ///< The number of entries. Must be a power of two.
         )
     {
         // check that maxentries is a power of 2
         AKASSERT((in_uMaxEntries & (in_uMaxEntries - 1)) == 0);
  
         m_uQueueIndexMask = in_uMaxEntries - 1;
         m_writePos = 0;
         m_readPos = 0;
  
         m_buffer = (FifoQueueEntry*)TAlloc::Alloc(sizeof(FifoQueueEntry) * in_uMaxEntries);
         if (m_buffer == nullptr)
         {
             return AK_InsufficientMemory;
         }
  
         AkZeroMemLarge(m_buffer, sizeof(FifoQueueEntry) * in_uMaxEntries);
         for (AkUInt32 i = 0; i < in_uMaxEntries; ++i)
         {
             m_buffer[i].value = TDEFAULT;
             AkAtomicStore64(&m_buffer[i].uSequence, i);
         }
  
         return AK_Success;
     }
  
     /// Free memory reserved for the queue and reset internal state
     /// The queue MUST be empty when this is called!
     void Term()
     {
         if (m_buffer)
         {
             AKASSERT(m_readPos == m_writePos);
             TAlloc::Free(m_buffer);
  
             m_buffer = nullptr;
         }
         m_readPos = 0;
         m_writePos = 0;
     }
  
     /// Enqueues the provided value. The value will be copied to the queue's internal buffer.
     /// Returns true if the enqueue was performed successfully.
     /// Returns false if the enqueue could not be performed. This can happen if the queue is "full", and some dequeue operations have to occur.
     AK_NODISCARD bool Enqueue(T in_value)
     {
         const AkUInt64 uQueueIndexMask = m_uQueueIndexMask;
         FifoQueueEntry* pBuffer = m_buffer;
  
         AkInt64 writePos = AkAtomicLoad64(&m_writePos);
         do {
             // see where we are in the sequence, relative to where we can write data
             AkInt64 sequenceDelta = AkAtomicLoad64(&pBuffer[writePos & uQueueIndexMask].uSequence) - writePos;
             // if we're in the right spot, and we can successfully write an updated write position, break out and write the handle into the queue
             if (sequenceDelta == 0)
             {
                 if (AkAtomicCas64(&m_writePos, writePos + 1, writePos))
                 {
                     break;
                 }
             }
             else if (sequenceDelta < 0)
             {
                 // we would have over-enqueued if we tried to write the position in. Return false; the user needs to decide how to handle things
                 return false;
             }
             else
             {
                 // if it didn't work, reload writePos: someone else must have written to the sequence and we need to get caught up
                 writePos = AkAtomicLoad64(&m_writePos);
             }
         } while (true);
  
         // advance the sequence by one so that it can be dequeued
         pBuffer[writePos & uQueueIndexMask].value = in_value;
         AkAtomicStore64(&pBuffer[writePos & uQueueIndexMask].uSequence, writePos + 1);
         return true;
     }
  
     /// Dequeues a value from the specified queue, copying it to io_value
     /// \return true if a value was successfully dequeued, false otherwise (if false, io_value will not be written to)
     bool Dequeue(T& io_value)
     {
         const AkInt64 uQueueIndexMask = m_uQueueIndexMask;
         FifoQueueEntry* pBuffer = m_buffer;
  
         AkInt64 readPos = AkAtomicLoad64(&m_readPos);
         do {
             // see where we are in the sequence relative to where we can write data
             AkInt64 sequenceDelta = AkAtomicLoad64(&pBuffer[readPos & uQueueIndexMask].uSequence) - (readPos + 1);
             // if we're in the right spot, and we can successfully write an updated read position, break out and read the entry
             if (sequenceDelta == 0)
             {
                 if (AkAtomicCas64(&m_readPos, readPos + 1, readPos))
                 {
                     break;
                 }
             }
             // if an entry has yet to be written, bail out
             else if (sequenceDelta < 0)
             {
                 return false;
             }
             else
             {
                 // if it didn't work, reload readPos
                 readPos = AkAtomicLoad64(&m_readPos);
             }
         } while (true);
  
         // update the acceptable sequence value for this entry
         io_value = pBuffer[readPos & uQueueIndexMask].value;
         AkAtomicStore64(&pBuffer[readPos & uQueueIndexMask].uSequence, readPos + m_uQueueIndexMask + 1);
  
         return true;
     }
  
     /// Checks if there is a value available to be dequeued
     bool Empty()
     {
         AkInt64 readPos = AkAtomicLoad64(&m_readPos);
         AkInt64 sequenceDelta = AkAtomicLoad64(&m_buffer[readPos & m_uQueueIndexMask].uSequence) - (readPos + 1);
         return sequenceDelta < 0;
     }
  
 private:
     struct FifoQueueEntry
     {
         // Value actually contained in the queue
         T value;
         // Global index of the queue entry in the sequence, to detect when we are at a valid read or write pos
         AkAtomic64 uSequence;
     };
  
     // Buffer of QueueEntries
     FifoQueueEntry* m_buffer;
     // Mask to apply to the read/write position to clamp it to array bounds
     AkInt64 m_uQueueIndexMask;
  
     // readIndex of where we are in the sequence
     AkAtomic64 m_readPos;
     // writeIndex of where we are in the sequence
     AkAtomic64 m_writePos;
 };
  
 #endif // _AKFIFOQUEUE_H