readerwriterqueue.h

// ©2013-2016 Cameron Desrochers.
// Distributed under the simplified BSD license (see the license file that
// should have come with this header).

#pragma once

#include "atomicops.h"
#include <type_traits>
#include <utility>
#include <cassert>
#include <stdexcept>
#include <cstdint>
#include <cstdlib>      // For malloc/free/abort & size_t


// A lock-free queue for a single-consumer, single-producer architecture.
// The queue is also wait-free in the common path (except if more memory
// needs to be allocated, in which case malloc is called).
// Allocates memory sparingly (O(lg(n) times, amortized), and only once if
// the original maximum size estimate is never exceeded.
// Tested on x86/x64 processors, but semantics should be correct for all
// architectures (given the right implementations in atomicops.h), provided
// that aligned integer and pointer accesses are naturally atomic.
// Note that there should only be one consumer thread and producer thread;
// Switching roles of the threads, or using multiple consecutive threads for
// one role, is not safe unless properly synchronized.
// Using the queue exclusively from one thread is fine, though a bit silly.

#ifndef MOODYCAMEL_CACHE_LINE_SIZE
#define MOODYCAMEL_CACHE_LINE_SIZE 64
#endif

#ifndef MOODYCAMEL_EXCEPTIONS_ENABLED
#if (defined(_MSC_VER) && defined(_CPPUNWIND)) || (defined(__GNUC__) && defined(__EXCEPTIONS)) || (!defined(_MSC_VER) && !defined(__GNUC__))
#define MOODYCAMEL_EXCEPTIONS_ENABLED
#endif
#endif

#ifdef AE_VCPP
#pragma warning(push)
#pragma warning(disable: 4324)  // structure was padded due to __declspec(align())
#pragma warning(disable: 4820)  // padding was added
#pragma warning(disable: 4127)  // conditional expression is constant
#endif

namespace moodycamel {

template<typename T, size_t MAX_BLOCK_SIZE = 512>
class ReaderWriterQueue
{
    // Design: Based on a queue-of-queues. The low-level queues are just
    // circular buffers with front and tail indices indicating where the
    // next element to dequeue is and where the next element can be enqueued,
    // respectively. Each low-level queue is called a "block". Each block
    // wastes exactly one element's worth of space to keep the design simple
    // (if front == tail then the queue is empty, and can't be full).
    // The high-level queue is a circular linked list of blocks; again there
    // is a front and tail, but this time they are pointers to the blocks.
    // The front block is where the next element to be dequeued is, provided
    // the block is not empty. The back block is where elements are to be
    // enqueued, provided the block is not full.
    // The producer thread owns all the tail indices/pointers. The consumer
    // thread owns all the front indices/pointers. Both threads read each
    // other's variables, but only the owning thread updates them. E.g. After
    // the consumer reads the producer's tail, the tail may change before the
    // consumer is done dequeuing an object, but the consumer knows the tail
    // will never go backwards, only forwards.
    // If there is no room to enqueue an object, an additional block (of
    // equal size to the last block) is added. Blocks are never removed.

public:
    // Constructs a queue that can hold maxSize elements without further
    // allocations. If more than MAX_BLOCK_SIZE elements are requested,
    // then several blocks of MAX_BLOCK_SIZE each are reserved (including
    // at least one extra buffer block).
    explicit ReaderWriterQueue(size_t maxSize = 15)
#ifndef NDEBUG
        : enqueuing(false)
        ,dequeuing(false)
#endif
    {
        assert(maxSize > 0);
        assert(MAX_BLOCK_SIZE == ceilToPow2(MAX_BLOCK_SIZE) && "MAX_BLOCK_SIZE must be a power of 2");
        assert(MAX_BLOCK_SIZE >= 2 && "MAX_BLOCK_SIZE must be at least 2");
        
        Block* firstBlock = nullptr;
        
        largestBlockSize = ceilToPow2(maxSize + 1);     // We need a spare slot to fit maxSize elements in the block
        if (largestBlockSize > MAX_BLOCK_SIZE * 2) {
            // We need a spare block in case the producer is writing to a different block the consumer is reading from, and
            // wants to enqueue the maximum number of elements. We also need a spare element in each block to avoid the ambiguity
            // between front == tail meaning "empty" and "full".
            // So the effective number of slots that are guaranteed to be usable at any time is the block size - 1 times the
            // number of blocks - 1. Solving for maxSize and applying a ceiling to the division gives us (after simplifying):
            size_t initialBlockCount = (maxSize + MAX_BLOCK_SIZE * 2 - 3) / (MAX_BLOCK_SIZE - 1);
            largestBlockSize = MAX_BLOCK_SIZE;
            Block* lastBlock = nullptr;
            for (size_t i = 0; i != initialBlockCount; ++i) {
                auto block = make_block(largestBlockSize);
                if (block == nullptr) {
#ifdef MOODYCAMEL_EXCEPTIONS_ENABLED
                    throw std::bad_alloc();
#else
                    abort();
#endif
                }
                if (firstBlock == nullptr) {
                    firstBlock = block;
                }
                else {
                    lastBlock->next = block;
                }
                lastBlock = block;
                block->next = firstBlock;
            }
        }
        else {
            firstBlock = make_block(largestBlockSize);
            if (firstBlock == nullptr) {
#ifdef MOODYCAMEL_EXCEPTIONS_ENABLED
                throw std::bad_alloc();
#else
                abort();
#endif
            }
            firstBlock->next = firstBlock;
        }
        frontBlock = firstBlock;
        tailBlock = firstBlock;
        
        // Make sure the reader/writer threads will have the initialized memory setup above:
        fence(memory_order_sync);
    }

    // Note: The queue should not be accessed concurrently while it's
    // being deleted. It's up to the user to synchronize this.
    ~ReaderWriterQueue()
    {
        // Make sure we get the latest version of all variables from other CPUs:
        fence(memory_order_sync);

        // Destroy any remaining objects in queue and free memory
        Block* frontBlock_ = frontBlock;
        Block* block = frontBlock_;
        do {
            Block* nextBlock = block->next;
            size_t blockFront = block->front;
            size_t blockTail = block->tail;

            for (size_t i = blockFront; i != blockTail; i = (i + 1) & block->sizeMask) {
                auto element = reinterpret_cast<T*>(block->data + i * sizeof(T));
                element->~T();
                (void)element;
            }
            
            auto rawBlock = block->rawThis;
            block->~Block();
            std::free(rawBlock);
            block = nextBlock;
        } while (block != frontBlock_);
    }


    // Enqueues a copy of element if there is room in the queue.
    // Returns true if the element was enqueued, false otherwise.
    // Does not allocate memory.
    AE_FORCEINLINE bool try_enqueue(T const& element)
    {
        return inner_enqueue<CannotAlloc>(element);
    }

    // Enqueues a moved copy of element if there is room in the queue.
    // Returns true if the element was enqueued, false otherwise.
    // Does not allocate memory.
    AE_FORCEINLINE bool try_enqueue(T&& element)
    {
        return inner_enqueue<CannotAlloc>(std::forward<T>(element));
    }


    // Enqueues a copy of element on the queue.
    // Allocates an additional block of memory if needed.
    // Only fails (returns false) if memory allocation fails.
    AE_FORCEINLINE bool enqueue(T const& element)
    {
        return inner_enqueue<CanAlloc>(element);
    }

    // Enqueues a moved copy of element on the queue.
    // Allocates an additional block of memory if needed.
    // Only fails (returns false) if memory allocation fails.
    AE_FORCEINLINE bool enqueue(T&& element)
    {
        return inner_enqueue<CanAlloc>(std::forward<T>(element));
    }


    // Attempts to dequeue an element; if the queue is empty,
    // returns false instead. If the queue has at least one element,
    // moves front to result using operator=, then returns true.
    template<typename U>
    bool try_dequeue(U& result)
    {
#ifndef NDEBUG
        ReentrantGuard guard(this->dequeuing);
#endif

        // High-level pseudocode:
        // Remember where the tail block is
        // If the front block has an element in it, dequeue it
        // Else
        //     If front block was the tail block when we entered the function, return false
        //     Else advance to next block and dequeue the item there

        // Note that we have to use the value of the tail block from before we check if the front
        // block is full or not, in case the front block is empty and then, before we check if the
        // tail block is at the front block or not, the producer fills up the front block *and
        // moves on*, which would make us skip a filled block. Seems unlikely, but was consistently
        // reproducible in practice.
        // In order to avoid overhead in the common case, though, we do a double-checked pattern
        // where we have the fast path if the front block is not empty, then read the tail block,
        // then re-read the front block and check if it's not empty again, then check if the tail
        // block has advanced.
        
        Block* frontBlock_ = frontBlock.load();
        size_t blockTail = frontBlock_->localTail;
        size_t blockFront = frontBlock_->front.load();
        
        if (blockFront != blockTail || blockFront != (frontBlock_->localTail = frontBlock_->tail.load())) {
            fence(memory_order_acquire);
            
        non_empty_front_block:
            // Front block not empty, dequeue from here
            auto element = reinterpret_cast<T*>(frontBlock_->data + blockFront * sizeof(T));
            result = std::move(*element);
            element->~T();

            blockFront = (blockFront + 1) & frontBlock_->sizeMask;

            fence(memory_order_release);
            frontBlock_->front = blockFront;
        }
        else if (frontBlock_ != tailBlock.load()) {
            fence(memory_order_acquire);

            frontBlock_ = frontBlock.load();
            blockTail = frontBlock_->localTail = frontBlock_->tail.load();
            blockFront = frontBlock_->front.load();
            fence(memory_order_acquire);
            
            if (blockFront != blockTail) {
                // Oh look, the front block isn't empty after all
                goto non_empty_front_block;
            }
            
            // Front block is empty but there's another block ahead, advance to it
            Block* nextBlock = frontBlock_->next;
            // Don't need an acquire fence here since next can only ever be set on the tailBlock,
            // and we're not the tailBlock, and we did an acquire earlier after reading tailBlock which
            // ensures next is up-to-date on this CPU in case we recently were at tailBlock.

            size_t nextBlockFront = nextBlock->front.load();
            size_t nextBlockTail = nextBlock->localTail = nextBlock->tail.load();
            fence(memory_order_acquire);

            // Since the tailBlock is only ever advanced after being written to,
            // we know there's for sure an element to dequeue on it
            assert(nextBlockFront != nextBlockTail);
            AE_UNUSED(nextBlockTail);

            // We're done with this block, let the producer use it if it needs
            fence(memory_order_release);        // Expose possibly pending changes to frontBlock->front from last dequeue
            frontBlock = frontBlock_ = nextBlock;

            compiler_fence(memory_order_release);   // Not strictly needed

            auto element = reinterpret_cast<T*>(frontBlock_->data + nextBlockFront * sizeof(T));
            
            result = std::move(*element);
            element->~T();

            nextBlockFront = (nextBlockFront + 1) & frontBlock_->sizeMask;
            
            fence(memory_order_release);
            frontBlock_->front = nextBlockFront;
        }
        else {
            // No elements in current block and no other block to advance to
            return false;
        }

        return true;
    }


    // Returns a pointer to the front element in the queue (the one that
    // would be removed next by a call to `try_dequeue` or `pop`). If the
    // queue appears empty at the time the method is called, nullptr is
    // returned instead.
    // Must be called only from the consumer thread.
    T* peek()
    {
#ifndef NDEBUG
        ReentrantGuard guard(this->dequeuing);
#endif
        // See try_dequeue() for reasoning

        Block* frontBlock_ = frontBlock.load();
        size_t blockTail = frontBlock_->localTail;
        size_t blockFront = frontBlock_->front.load();
        
        if (blockFront != blockTail || blockFront != (frontBlock_->localTail = frontBlock_->tail.load())) {
            fence(memory_order_acquire);
        non_empty_front_block:
            return reinterpret_cast<T*>(frontBlock_->data + blockFront * sizeof(T));
        }
        else if (frontBlock_ != tailBlock.load()) {
            fence(memory_order_acquire);
            frontBlock_ = frontBlock.load();
            blockTail = frontBlock_->localTail = frontBlock_->tail.load();
            blockFront = frontBlock_->front.load();
            fence(memory_order_acquire);
            
            if (blockFront != blockTail) {
                goto non_empty_front_block;
            }
            
            Block* nextBlock = frontBlock_->next;
            
            size_t nextBlockFront = nextBlock->front.load();
            fence(memory_order_acquire);

            assert(nextBlockFront != nextBlock->tail.load());
            return reinterpret_cast<T*>(nextBlock->data + nextBlockFront * sizeof(T));
        }
        
        return nullptr;
    }
    
    // Removes the front element from the queue, if any, without returning it.
    // Returns true on success, or false if the queue appeared empty at the time
    // `pop` was called.
    bool pop()
    {
#ifndef NDEBUG
        ReentrantGuard guard(this->dequeuing);
#endif
        // See try_dequeue() for reasoning
        
        Block* frontBlock_ = frontBlock.load();
        size_t blockTail = frontBlock_->localTail;
        size_t blockFront = frontBlock_->front.load();
        
        if (blockFront != blockTail || blockFront != (frontBlock_->localTail = frontBlock_->tail.load())) {
            fence(memory_order_acquire);
            
        non_empty_front_block:
            auto element = reinterpret_cast<T*>(frontBlock_->data + blockFront * sizeof(T));
            element->~T();

            blockFront = (blockFront + 1) & frontBlock_->sizeMask;

            fence(memory_order_release);
            frontBlock_->front = blockFront;
        }
        else if (frontBlock_ != tailBlock.load()) {
            fence(memory_order_acquire);
            frontBlock_ = frontBlock.load();
            blockTail = frontBlock_->localTail = frontBlock_->tail.load();
            blockFront = frontBlock_->front.load();
            fence(memory_order_acquire);
            
            if (blockFront != blockTail) {
                goto non_empty_front_block;
            }
            
            // Front block is empty but there's another block ahead, advance to it
            Block* nextBlock = frontBlock_->next;
            
            size_t nextBlockFront = nextBlock->front.load();
            size_t nextBlockTail = nextBlock->localTail = nextBlock->tail.load();
            fence(memory_order_acquire);

            assert(nextBlockFront != nextBlockTail);
            AE_UNUSED(nextBlockTail);

            fence(memory_order_release);
            frontBlock = frontBlock_ = nextBlock;

            compiler_fence(memory_order_release);

            auto element = reinterpret_cast<T*>(frontBlock_->data + nextBlockFront * sizeof(T));
            element->~T();

            nextBlockFront = (nextBlockFront + 1) & frontBlock_->sizeMask;
            
            fence(memory_order_release);
            frontBlock_->front = nextBlockFront;
        }
        else {
            // No elements in current block and no other block to advance to
            return false;
        }

        return true;
    }
    
    // Returns the approximate number of items currently in the queue.
    // Safe to call from both the producer and consumer threads.
    inline size_t size_approx() const
    {
        size_t result = 0;
        Block* frontBlock_ = frontBlock.load();
        Block* block = frontBlock_;
        do {
            fence(memory_order_acquire);
            size_t blockFront = block->front.load();
            size_t blockTail = block->tail.load();
            result += (blockTail - blockFront) & block->sizeMask;
            block = block->next.load();
        } while (block != frontBlock_);
        return result;
    }


private:
    enum AllocationMode { CanAlloc, CannotAlloc };

    template<AllocationMode canAlloc, typename U>
    bool inner_enqueue(U&& element)
    {
#ifndef NDEBUG
        ReentrantGuard guard(this->enqueuing);
#endif

        // High-level pseudocode (assuming we're allowed to alloc a new block):
        // If room in tail block, add to tail
        // Else check next block
        //     If next block is not the head block, enqueue on next block
        //     Else create a new block and enqueue there
        //     Advance tail to the block we just enqueued to

        Block* tailBlock_ = tailBlock.load();
        size_t blockFront = tailBlock_->localFront;
        size_t blockTail = tailBlock_->tail.load();

        size_t nextBlockTail = (blockTail + 1) & tailBlock_->sizeMask;
        if (nextBlockTail != blockFront || nextBlockTail != (tailBlock_->localFront = tailBlock_->front.load())) {
            fence(memory_order_acquire);
            // This block has room for at least one more element
            char* location = tailBlock_->data + blockTail * sizeof(T);
            new (location) T(std::forward<U>(element));

            fence(memory_order_release);
            tailBlock_->tail = nextBlockTail;
        }
        else {
            fence(memory_order_acquire);
            if (tailBlock_->next.load() != frontBlock) {
                // Note that the reason we can't advance to the frontBlock and start adding new entries there
                // is because if we did, then dequeue would stay in that block, eventually reading the new values,
                // instead of advancing to the next full block (whose values were enqueued first and so should be
                // consumed first).
                
                fence(memory_order_acquire);        // Ensure we get latest writes if we got the latest frontBlock

                // tailBlock is full, but there's a free block ahead, use it
                Block* tailBlockNext = tailBlock_->next.load();
                size_t nextBlockFront = tailBlockNext->localFront = tailBlockNext->front.load();
                nextBlockTail = tailBlockNext->tail.load();
                fence(memory_order_acquire);

                // This block must be empty since it's not the head block and we
                // go through the blocks in a circle
                assert(nextBlockFront == nextBlockTail);
                tailBlockNext->localFront = nextBlockFront;

                char* location = tailBlockNext->data + nextBlockTail * sizeof(T);
                new (location) T(std::forward<U>(element));

                tailBlockNext->tail = (nextBlockTail + 1) & tailBlockNext->sizeMask;

                fence(memory_order_release);
                tailBlock = tailBlockNext;
            }
            else if (canAlloc == CanAlloc) {
                // tailBlock is full and there's no free block ahead; create a new block
                auto newBlockSize = largestBlockSize >= MAX_BLOCK_SIZE ? largestBlockSize : largestBlockSize * 2;
                auto newBlock = make_block(newBlockSize);
                if (newBlock == nullptr) {
                    // Could not allocate a block!
                    return false;
                }
                largestBlockSize = newBlockSize;

                new (newBlock->data) T(std::forward<U>(element));

                assert(newBlock->front == 0);
                newBlock->tail = newBlock->localTail = 1;

                newBlock->next = tailBlock_->next.load();
                tailBlock_->next = newBlock;

                // Might be possible for the dequeue thread to see the new tailBlock->next
                // *without* seeing the new tailBlock value, but this is OK since it can't
                // advance to the next block until tailBlock is set anyway (because the only
                // case where it could try to read the next is if it's already at the tailBlock,
                // and it won't advance past tailBlock in any circumstance).
                
                fence(memory_order_release);
                tailBlock = newBlock;
            }
            else if (canAlloc == CannotAlloc) {
                // Would have had to allocate a new block to enqueue, but not allowed
                return false;
            }
            else {
                assert(false && "Should be unreachable code");
                return false;
            }
        }

        return true;
    }


    // Disable copying
    ReaderWriterQueue(ReaderWriterQueue const&) {  }

    // Disable assignment
    ReaderWriterQueue& operator=(ReaderWriterQueue const&) {  }



    AE_FORCEINLINE static size_t ceilToPow2(size_t x)
    {
        // From http://graphics.stanford.edu/~seander/bithacks.html#RoundUpPowerOf2
        --x;
        x |= x >> 1;
        x |= x >> 2;
        x |= x >> 4;
        for (size_t i = 1; i < sizeof(size_t); i <<= 1) {
            x |= x >> (i << 3);
        }
        ++x;
        return x;
    }
    
    template<typename U>
    static AE_FORCEINLINE char* align_for(char* ptr)
    {
        const std::size_t alignment = std::alignment_of<U>::value;
        return ptr + (alignment - (reinterpret_cast<std::uintptr_t>(ptr) % alignment)) % alignment;
    }
private:
#ifndef NDEBUG
    struct ReentrantGuard
    {
        ReentrantGuard(bool& _inSection)
            : inSection(_inSection)
        {
            assert(!inSection && "ReaderWriterQueue does not support enqueuing or dequeuing elements from other elements' ctors and dtors");
            inSection = true;
        }

        ~ReentrantGuard() { inSection = false; }

    private:
        ReentrantGuard& operator=(ReentrantGuard const&);

    private:
        bool& inSection;
    };
#endif

    struct Block
    {
        // Avoid false-sharing by putting highly contended variables on their own cache lines
        weak_atomic<size_t> front;  // (Atomic) Elements are read from here
        size_t localTail;           // An uncontended shadow copy of tail, owned by the consumer
        
        char cachelineFiller0[MOODYCAMEL_CACHE_LINE_SIZE - sizeof(weak_atomic<size_t>) - sizeof(size_t)];
        weak_atomic<size_t> tail;   // (Atomic) Elements are enqueued here
        size_t localFront;
        
        char cachelineFiller1[MOODYCAMEL_CACHE_LINE_SIZE - sizeof(weak_atomic<size_t>) - sizeof(size_t)];   // next isn't very contended, but we don't want it on the same cache line as tail (which is)
        weak_atomic<Block*> next;   // (Atomic)
        
        char* data;     // Contents (on heap) are aligned to T's alignment

        const size_t sizeMask;


        // size must be a power of two (and greater than 0)
        Block(size_t const& _size, char* _rawThis, char* _data)
            : front(0), localTail(0), tail(0), localFront(0), next(nullptr), data(_data), sizeMask(_size - 1), rawThis(_rawThis)
        {
        }

    private:
        // C4512 - Assignment operator could not be generated
        Block& operator=(Block const&);

    public:
        char* rawThis;
    };
    
    
    static Block* make_block(size_t capacity)
    {
        // Allocate enough memory for the block itself, as well as all the elements it will contain
        auto size = sizeof(Block) + std::alignment_of<Block>::value - 1;
        size += sizeof(T) * capacity + std::alignment_of<T>::value - 1;
        auto newBlockRaw = static_cast<char*>(std::malloc(size));
        if (newBlockRaw == nullptr) {
            return nullptr;
        }
        
        auto newBlockAligned = align_for<Block>(newBlockRaw);
        auto newBlockData = align_for<T>(newBlockAligned + sizeof(Block));
        return new (newBlockAligned) Block(capacity, newBlockRaw, newBlockData);
    }

private:
    weak_atomic<Block*> frontBlock;     // (Atomic) Elements are enqueued to this block
    
    char cachelineFiller[MOODYCAMEL_CACHE_LINE_SIZE - sizeof(weak_atomic<Block*>)];
    weak_atomic<Block*> tailBlock;      // (Atomic) Elements are dequeued from this block

    size_t largestBlockSize;

#ifndef NDEBUG
    bool enqueuing;
    bool dequeuing;
#endif
};

// Like ReaderWriterQueue, but also providees blocking operations
template<typename T, size_t MAX_BLOCK_SIZE = 512>
class BlockingReaderWriterQueue
{
private:
    typedef ::moodycamel::ReaderWriterQueue<T, MAX_BLOCK_SIZE> ReaderWriterQueue;
    
public:
    explicit BlockingReaderWriterQueue(size_t maxSize = 15)
        : inner(maxSize)
    { }

    
    // Enqueues a copy of element if there is room in the queue.
    // Returns true if the element was enqueued, false otherwise.
    // Does not allocate memory.
    AE_FORCEINLINE bool try_enqueue(T const& element)
    {
        if (inner.try_enqueue(element)) {
            sema.signal();
            return true;
        }
        return false;
    }

    // Enqueues a moved copy of element if there is room in the queue.
    // Returns true if the element was enqueued, false otherwise.
    // Does not allocate memory.
    AE_FORCEINLINE bool try_enqueue(T&& element)
    {
        if (inner.try_enqueue(std::forward<T>(element))) {
            sema.signal();
            return true;
        }
        return false;
    }


    // Enqueues a copy of element on the queue.
    // Allocates an additional block of memory if needed.
    // Only fails (returns false) if memory allocation fails.
    AE_FORCEINLINE bool enqueue(T const& element)
    {
        if (inner.enqueue(element)) {
            sema.signal();
            return true;
        }
        return false;
    }

    // Enqueues a moved copy of element on the queue.
    // Allocates an additional block of memory if needed.
    // Only fails (returns false) if memory allocation fails.
    AE_FORCEINLINE bool enqueue(T&& element)
    {
        if (inner.enqueue(std::forward<T>(element))) {
            sema.signal();
            return true;
        }
        return false;
    }


    // Attempts to dequeue an element; if the queue is empty,
    // returns false instead. If the queue has at least one element,
    // moves front to result using operator=, then returns true.
    template<typename U>
    bool try_dequeue(U& result)
    {
        if (sema.tryWait()) {
            bool success = inner.try_dequeue(result);
            assert(success);
            AE_UNUSED(success);
            return true;
        }
        return false;
    }
    
    
    // Attempts to dequeue an element; if the queue is empty,
    // waits until an element is available, then dequeues it.
    template<typename U>
    void wait_dequeue(U& result)
    {
        sema.wait();
        bool success = inner.try_dequeue(result);
        AE_UNUSED(result);
        assert(success);
        AE_UNUSED(success);
    }


    // Returns a pointer to the front element in the queue (the one that
    // would be removed next by a call to `try_dequeue` or `pop`). If the
    // queue appears empty at the time the method is called, nullptr is
    // returned instead.
    // Must be called only from the consumer thread.
    AE_FORCEINLINE T* peek()
    {
        return inner.peek();
    }
    
    // Removes the front element from the queue, if any, without returning it.
    // Returns true on success, or false if the queue appeared empty at the time
    // `pop` was called.
    AE_FORCEINLINE bool pop()
    {
        if (sema.tryWait()) {
            bool result = inner.pop();
            assert(result);
            AE_UNUSED(result);
            return true;
        }
        return false;
    }
    
    // Returns the approximate number of items currently in the queue.
    // Safe to call from both the producer and consumer threads.
    AE_FORCEINLINE size_t size_approx() const
    {
        return sema.availableApprox();
    }


private:
    // Disable copying & assignment
    BlockingReaderWriterQueue(ReaderWriterQueue const&) {  }
    BlockingReaderWriterQueue& operator=(ReaderWriterQueue const&) {  }
    
private:
    ReaderWriterQueue inner;
    spsc_sema::LightweightSemaphore sema;
};

}    // end namespace moodycamel

#ifdef AE_VCPP
#pragma warning(pop)
#endif