blockingconcurrentqueue.h

// Provides an efficient blocking version of moodycamel::ConcurrentQueue.
// ©2015-2016 Cameron Desrochers. Distributed under the terms of the simplified
// BSD license, available at the top of concurrentqueue.h.
// Uses Jeff Preshing's semaphore implementation (under the terms of its
// separate zlib license, embedded below).

#pragma once

#include "concurrentqueue.h"
#include <type_traits>
#include <cerrno>
#include <memory>
#include <chrono>
#include <ctime>

#if defined(_WIN32)
// Avoid including windows.h in a header; we only need a handful of
// items, so we'll redeclare them here (this is relatively safe since
// the API generally has to remain stable between Windows versions).
// I know this is an ugly hack but it still beats polluting the global
// namespace with thousands of generic names or adding a .cpp for nothing.
extern "C" {
	struct _SECURITY_ATTRIBUTES;
	__declspec(dllimport) void* __stdcall CreateSemaphoreW(_SECURITY_ATTRIBUTES* lpSemaphoreAttributes, long lInitialCount, long lMaximumCount, const wchar_t* lpName);
	__declspec(dllimport) int __stdcall CloseHandle(void* hObject);
	__declspec(dllimport) unsigned long __stdcall WaitForSingleObject(void* hHandle, unsigned long dwMilliseconds);
	__declspec(dllimport) int __stdcall ReleaseSemaphore(void* hSemaphore, long lReleaseCount, long* lpPreviousCount);
}
#elif defined(__MACH__)
#include <mach/mach.h>
#elif defined(__unix__)
#include <semaphore.h>
#endif

namespace moodycamel
{
namespace details
{
	// Code in the mpmc_sema namespace below is an adaptation of Jeff Preshing's
	// portable + lightweight semaphore implementations, originally from
	// https://github.com/preshing/cpp11-on-multicore/blob/master/common/sema.h
	// LICENSE:
	// Copyright (c) 2015 Jeff Preshing
	//
	// This software is provided 'as-is', without any express or implied
	// warranty. In no event will the authors be held liable for any damages
	// arising from the use of this software.
	//
	// Permission is granted to anyone to use this software for any purpose,
	// including commercial applications, and to alter it and redistribute it
	// freely, subject to the following restrictions:
	//
	// 1. The origin of this software must not be misrepresented; you must not
	//	claim that you wrote the original software. If you use this software
	//	in a product, an acknowledgement in the product documentation would be
	//	appreciated but is not required.
	// 2. Altered source versions must be plainly marked as such, and must not be
	//	misrepresented as being the original software.
	// 3. This notice may not be removed or altered from any source distribution.
	namespace mpmc_sema
	{
#if defined(_WIN32)
		class Semaphore
		{
		private:
			void* m_hSema;
			
			Semaphore(const Semaphore& other) MOODYCAMEL_DELETE_FUNCTION;
			Semaphore& operator=(const Semaphore& other) MOODYCAMEL_DELETE_FUNCTION;

		public:
			Semaphore(int initialCount = 0)
			{
				assert(initialCount >= 0);
				const long maxLong = 0x7fffffff;
				m_hSema = CreateSemaphoreW(nullptr, initialCount, maxLong, nullptr);
			}

			~Semaphore()
			{
				CloseHandle(m_hSema);
			}

			void wait()
			{
				const unsigned long infinite = 0xffffffff;
				WaitForSingleObject(m_hSema, infinite);
			}
			
			bool try_wait()
			{
				const unsigned long RC_WAIT_TIMEOUT = 0x00000102;
				return WaitForSingleObject(m_hSema, 0) != RC_WAIT_TIMEOUT;
			}
			
			bool timed_wait(std::uint64_t usecs)
			{
				const unsigned long RC_WAIT_TIMEOUT = 0x00000102;
				return WaitForSingleObject(m_hSema, (unsigned long)(usecs / 1000)) != RC_WAIT_TIMEOUT;
			}

			void signal(int count = 1)
			{
				ReleaseSemaphore(m_hSema, count, nullptr);
			}
		};
#elif defined(__MACH__)
		//---------------------------------------------------------
		// Semaphore (Apple iOS and OSX)
		// Can't use POSIX semaphores due to http://lists.apple.com/archives/darwin-kernel/2009/Apr/msg00010.html
		//---------------------------------------------------------
		class Semaphore
		{
		private:
			semaphore_t m_sema;

			Semaphore(const Semaphore& other) MOODYCAMEL_DELETE_FUNCTION;
			Semaphore& operator=(const Semaphore& other) MOODYCAMEL_DELETE_FUNCTION;

		public:
			Semaphore(int initialCount = 0)
			{
				assert(initialCount >= 0);
				semaphore_create(mach_task_self(), &m_sema, SYNC_POLICY_FIFO, initialCount);
			}

			~Semaphore()
			{
				semaphore_destroy(mach_task_self(), m_sema);
			}

			void wait()
			{
				semaphore_wait(m_sema);
			}
			
			bool try_wait()
			{
				return timed_wait(0);
			}
			
			bool timed_wait(std::uint64_t timeout_usecs)
			{
				mach_timespec_t ts;
				ts.tv_sec = static_cast<unsigned int>(timeout_usecs / 1000000);
				ts.tv_nsec = (timeout_usecs % 1000000) * 1000;

				// added in OSX 10.10: https://developer.apple.com/library/prerelease/mac/documentation/General/Reference/APIDiffsMacOSX10_10SeedDiff/modules/Darwin.html
				kern_return_t rc = semaphore_timedwait(m_sema, ts);

				return rc != KERN_OPERATION_TIMED_OUT && rc != KERN_ABORTED;
			}

			void signal()
			{
				semaphore_signal(m_sema);
			}

			void signal(int count)
			{
				while (count-- > 0)
				{
					semaphore_signal(m_sema);
				}
			}
		};
#elif defined(__unix__)
		//---------------------------------------------------------
		// Semaphore (POSIX, Linux)
		//---------------------------------------------------------
		class Semaphore
		{
		private:
			sem_t m_sema;

			Semaphore(const Semaphore& other) MOODYCAMEL_DELETE_FUNCTION;
			Semaphore& operator=(const Semaphore& other) MOODYCAMEL_DELETE_FUNCTION;

		public:
			Semaphore(int initialCount = 0)
			{
				assert(initialCount >= 0);
				sem_init(&m_sema, 0, initialCount);
			}

			~Semaphore()
			{
				sem_destroy(&m_sema);
			}

			void wait()
			{
				// http://stackoverflow.com/questions/2013181/gdb-causes-sem-wait-to-fail-with-eintr-error
				int rc;
				do {
					rc = sem_wait(&m_sema);
				} while (rc == -1 && errno == EINTR);
			}

			bool try_wait()
			{
				int rc;
				do {
					rc = sem_trywait(&m_sema);
				} while (rc == -1 && errno == EINTR);
				return !(rc == -1 && errno == EAGAIN);
			}

			bool timed_wait(std::uint64_t usecs)
			{
				struct timespec ts;
				const int usecs_in_1_sec = 1000000;
				const int nsecs_in_1_sec = 1000000000;
				clock_gettime(CLOCK_REALTIME, &ts);
				ts.tv_sec += usecs / usecs_in_1_sec;
				ts.tv_nsec += (usecs % usecs_in_1_sec) * 1000;
				// sem_timedwait bombs if you have more than 1e9 in tv_nsec
				// so we have to clean things up before passing it in
				if (ts.tv_nsec >= nsecs_in_1_sec) {
					ts.tv_nsec -= nsecs_in_1_sec;
					++ts.tv_sec;
				}

				int rc;
				do {
					rc = sem_timedwait(&m_sema, &ts);
				} while (rc == -1 && errno == EINTR);
				return !(rc == -1 && errno == ETIMEDOUT);
			}

			void signal()
			{
				sem_post(&m_sema);
			}

			void signal(int count)
			{
				while (count-- > 0)
				{
					sem_post(&m_sema);
				}
			}
		};
#else
#error Unsupported platform! (No semaphore wrapper available)
#endif

		//---------------------------------------------------------
		// LightweightSemaphore
		//---------------------------------------------------------
		class LightweightSemaphore
		{
		public:
			typedef std::make_signed<std::size_t>::type ssize_t;

		private:
			std::atomic<ssize_t> m_count;
			Semaphore m_sema;

			bool waitWithPartialSpinning(std::int64_t timeout_usecs = -1)
			{
				ssize_t oldCount;
				// Is there a better way to set the initial spin count?
				// If we lower it to 1000, testBenaphore becomes 15x slower on my Core i7-5930K Windows PC,
				// as threads start hitting the kernel semaphore.
				int spin = 10000;
				while (--spin >= 0)
				{
					oldCount = m_count.load(std::memory_order_relaxed);
					if ((oldCount > 0) && m_count.compare_exchange_strong(oldCount, oldCount - 1, std::memory_order_acquire, std::memory_order_relaxed))
						return true;
					std::atomic_signal_fence(std::memory_order_acquire);	 // Prevent the compiler from collapsing the loop.
				}
				oldCount = m_count.fetch_sub(1, std::memory_order_acquire);
				if (oldCount > 0)
					return true;
				if (timeout_usecs < 0)
				{
					m_sema.wait();
					return true;
				}
				if (m_sema.timed_wait((std::uint64_t)timeout_usecs))
					return true;
				// At this point, we've timed out waiting for the semaphore, but the
				// count is still decremented indicating we may still be waiting on
				// it. So we have to re-adjust the count, but only if the semaphore
				// wasn't signaled enough times for us too since then. If it was, we
				// need to release the semaphore too.
				while (true)
				{
					oldCount = m_count.load(std::memory_order_acquire);
					if (oldCount >= 0 && m_sema.try_wait())
						return true;
					if (oldCount < 0 && m_count.compare_exchange_strong(oldCount, oldCount + 1, std::memory_order_relaxed, std::memory_order_relaxed))
						return false;
				}
			}

			ssize_t waitManyWithPartialSpinning(ssize_t max, std::int64_t timeout_usecs = -1)
			{
				assert(max > 0);
				ssize_t oldCount;
				int spin = 10000;
				while (--spin >= 0)
				{
					oldCount = m_count.load(std::memory_order_relaxed);
					if (oldCount > 0)
					{
						ssize_t newCount = oldCount > max ? oldCount - max : 0;
						if (m_count.compare_exchange_strong(oldCount, newCount, std::memory_order_acquire, std::memory_order_relaxed))
							return oldCount - newCount;
					}
					std::atomic_signal_fence(std::memory_order_acquire);
				}
				oldCount = m_count.fetch_sub(1, std::memory_order_acquire);
				if (oldCount <= 0)
				{
					if (timeout_usecs < 0)
						m_sema.wait();
					else if (!m_sema.timed_wait((std::uint64_t)timeout_usecs))
					{
						while (true)
						{
							oldCount = m_count.load(std::memory_order_acquire);
							if (oldCount >= 0 && m_sema.try_wait())
								break;
							if (oldCount < 0 && m_count.compare_exchange_strong(oldCount, oldCount + 1, std::memory_order_relaxed, std::memory_order_relaxed))
								return 0;
						}
					}
				}
				if (max > 1)
					return 1 + tryWaitMany(max - 1);
				return 1;
			}

		public:
			LightweightSemaphore(ssize_t initialCount = 0) : m_count(initialCount)
			{
				assert(initialCount >= 0);
			}

			bool tryWait()
			{
				ssize_t oldCount = m_count.load(std::memory_order_relaxed);
				while (oldCount > 0)
				{
					if (m_count.compare_exchange_weak(oldCount, oldCount - 1, std::memory_order_acquire, std::memory_order_relaxed))
						return true;
				}
				return false;
			}

			void wait()
			{
				if (!tryWait())
					waitWithPartialSpinning();
			}

			bool wait(std::int64_t timeout_usecs)
			{
				return tryWait() || waitWithPartialSpinning(timeout_usecs);
			}

			// Acquires between 0 and (greedily) max, inclusive
			ssize_t tryWaitMany(ssize_t max)
			{
				assert(max >= 0);
				ssize_t oldCount = m_count.load(std::memory_order_relaxed);
				while (oldCount > 0)
				{
					ssize_t newCount = oldCount > max ? oldCount - max : 0;
					if (m_count.compare_exchange_weak(oldCount, newCount, std::memory_order_acquire, std::memory_order_relaxed))
						return oldCount - newCount;
				}
				return 0;
			}

			// Acquires at least one, and (greedily) at most max
			ssize_t waitMany(ssize_t max, std::int64_t timeout_usecs)
			{
				assert(max >= 0);
				ssize_t result = tryWaitMany(max);
				if (result == 0 && max > 0)
					result = waitManyWithPartialSpinning(max, timeout_usecs);
				return result;
			}
			
			ssize_t waitMany(ssize_t max)
			{
				ssize_t result = waitMany(max, -1);
				assert(result > 0);
				return result;
			}

			void signal(ssize_t count = 1)
			{
				assert(count >= 0);
				ssize_t oldCount = m_count.fetch_add(count, std::memory_order_release);
				ssize_t toRelease = -oldCount < count ? -oldCount : count;
				if (toRelease > 0)
				{
					m_sema.signal((int)toRelease);
				}
			}
			
			ssize_t availableApprox() const
			{
				ssize_t count = m_count.load(std::memory_order_relaxed);
				return count > 0 ? count : 0;
			}
		};
	}	// end namespace mpmc_sema
}	// end namespace details


// This is a blocking version of the queue. It has an almost identical interface to
// the normal non-blocking version, with the addition of various wait_dequeue() methods
// and the removal of producer-specific dequeue methods.
template<typename T, typename Traits = ConcurrentQueueDefaultTraits>
class BlockingConcurrentQueue
{
private:
	typedef ::moodycamel::ConcurrentQueue<T, Traits> ConcurrentQueue;
	typedef details::mpmc_sema::LightweightSemaphore LightweightSemaphore;

public:
	typedef typename ConcurrentQueue::producer_token_t producer_token_t;
	typedef typename ConcurrentQueue::consumer_token_t consumer_token_t;
	
	typedef typename ConcurrentQueue::index_t index_t;
	typedef typename ConcurrentQueue::size_t size_t;
	typedef typename std::make_signed<size_t>::type ssize_t;
	
	static const size_t BLOCK_SIZE = ConcurrentQueue::BLOCK_SIZE;
	static const size_t EXPLICIT_BLOCK_EMPTY_COUNTER_THRESHOLD = ConcurrentQueue::EXPLICIT_BLOCK_EMPTY_COUNTER_THRESHOLD;
	static const size_t EXPLICIT_INITIAL_INDEX_SIZE = ConcurrentQueue::EXPLICIT_INITIAL_INDEX_SIZE;
	static const size_t IMPLICIT_INITIAL_INDEX_SIZE = ConcurrentQueue::IMPLICIT_INITIAL_INDEX_SIZE;
	static const size_t INITIAL_IMPLICIT_PRODUCER_HASH_SIZE = ConcurrentQueue::INITIAL_IMPLICIT_PRODUCER_HASH_SIZE;
	static const std::uint32_t EXPLICIT_CONSUMER_CONSUMPTION_QUOTA_BEFORE_ROTATE = ConcurrentQueue::EXPLICIT_CONSUMER_CONSUMPTION_QUOTA_BEFORE_ROTATE;
	static const size_t MAX_SUBQUEUE_SIZE = ConcurrentQueue::MAX_SUBQUEUE_SIZE;
	
public:
	// Creates a queue with at least `capacity` element slots; note that the
	// actual number of elements that can be inserted without additional memory
	// allocation depends on the number of producers and the block size (e.g. if
	// the block size is equal to `capacity`, only a single block will be allocated
	// up-front, which means only a single producer will be able to enqueue elements
	// without an extra allocation -- blocks aren't shared between producers).
	// This method is not thread safe -- it is up to the user to ensure that the
	// queue is fully constructed before it starts being used by other threads (this
	// includes making the memory effects of construction visible, possibly with a
	// memory barrier).
	explicit BlockingConcurrentQueue(size_t capacity = 6 * BLOCK_SIZE)
		: inner(capacity), sema(create<LightweightSemaphore>(), &BlockingConcurrentQueue::template destroy<LightweightSemaphore>)
	{
		assert(reinterpret_cast<ConcurrentQueue*>((BlockingConcurrentQueue*)1) == &((BlockingConcurrentQueue*)1)->inner && "BlockingConcurrentQueue must have ConcurrentQueue as its first member");
		if (!sema) {
			MOODYCAMEL_THROW(std::bad_alloc());
		}
	}
	
	BlockingConcurrentQueue(size_t minCapacity, size_t maxExplicitProducers, size_t maxImplicitProducers)
		: inner(minCapacity, maxExplicitProducers, maxImplicitProducers), sema(create<LightweightSemaphore>(), &BlockingConcurrentQueue::template destroy<LightweightSemaphore>)
	{
		assert(reinterpret_cast<ConcurrentQueue*>((BlockingConcurrentQueue*)1) == &((BlockingConcurrentQueue*)1)->inner && "BlockingConcurrentQueue must have ConcurrentQueue as its first member");
		if (!sema) {
			MOODYCAMEL_THROW(std::bad_alloc());
		}
	}
	
	// Disable copying and copy assignment
	BlockingConcurrentQueue(BlockingConcurrentQueue const&) MOODYCAMEL_DELETE_FUNCTION;
	BlockingConcurrentQueue& operator=(BlockingConcurrentQueue const&) MOODYCAMEL_DELETE_FUNCTION;
	
	// Moving is supported, but note that it is *not* a thread-safe operation.
	// Nobody can use the queue while it's being moved, and the memory effects
	// of that move must be propagated to other threads before they can use it.
	// Note: When a queue is moved, its tokens are still valid but can only be
	// used with the destination queue (i.e. semantically they are moved along
	// with the queue itself).
	BlockingConcurrentQueue(BlockingConcurrentQueue&& other) MOODYCAMEL_NOEXCEPT
		: inner(std::move(other.inner)), sema(std::move(other.sema))
	{ }
	
	inline BlockingConcurrentQueue& operator=(BlockingConcurrentQueue&& other) MOODYCAMEL_NOEXCEPT
	{
		return swap_internal(other);
	}
	
	// Swaps this queue's state with the other's. Not thread-safe.
	// Swapping two queues does not invalidate their tokens, however
	// the tokens that were created for one queue must be used with
	// only the swapped queue (i.e. the tokens are tied to the
	// queue's movable state, not the object itself).
	inline void swap(BlockingConcurrentQueue& other) MOODYCAMEL_NOEXCEPT
	{
		swap_internal(other);
	}
	
private:
	BlockingConcurrentQueue& swap_internal(BlockingConcurrentQueue& other)
	{
		if (this == &other) {
			return *this;
		}
		
		inner.swap(other.inner);
		sema.swap(other.sema);
		return *this;
	}
	
public:
	// Enqueues a single item (by copying it).
	// Allocates memory if required. Only fails if memory allocation fails (or implicit
	// production is disabled because Traits::INITIAL_IMPLICIT_PRODUCER_HASH_SIZE is 0,
	// or Traits::MAX_SUBQUEUE_SIZE has been defined and would be surpassed).
	// Thread-safe.
	inline bool enqueue(T const& item)
	{
		if ((details::likely)(inner.enqueue(item))) {
			sema->signal();
			return true;
		}
		return false;
	}
	
	// Enqueues a single item (by moving it, if possible).
	// Allocates memory if required. Only fails if memory allocation fails (or implicit
	// production is disabled because Traits::INITIAL_IMPLICIT_PRODUCER_HASH_SIZE is 0,
	// or Traits::MAX_SUBQUEUE_SIZE has been defined and would be surpassed).
	// Thread-safe.
	inline bool enqueue(T&& item)
	{
		if ((details::likely)(inner.enqueue(std::move(item)))) {
			sema->signal();
			return true;
		}
		return false;
	}
	
	// Enqueues a single item (by copying it) using an explicit producer token.
	// Allocates memory if required. Only fails if memory allocation fails (or
	// Traits::MAX_SUBQUEUE_SIZE has been defined and would be surpassed).
	// Thread-safe.
	inline bool enqueue(producer_token_t const& token, T const& item)
	{
		if ((details::likely)(inner.enqueue(token, item))) {
			sema->signal();
			return true;
		}
		return false;
	}
	
	// Enqueues a single item (by moving it, if possible) using an explicit producer token.
	// Allocates memory if required. Only fails if memory allocation fails (or
	// Traits::MAX_SUBQUEUE_SIZE has been defined and would be surpassed).
	// Thread-safe.
	inline bool enqueue(producer_token_t const& token, T&& item)
	{
		if ((details::likely)(inner.enqueue(token, std::move(item)))) {
			sema->signal();
			return true;
		}
		return false;
	}
	
	// Enqueues several items.
	// Allocates memory if required. Only fails if memory allocation fails (or
	// implicit production is disabled because Traits::INITIAL_IMPLICIT_PRODUCER_HASH_SIZE
	// is 0, or Traits::MAX_SUBQUEUE_SIZE has been defined and would be surpassed).
	// Note: Use std::make_move_iterator if the elements should be moved instead of copied.
	// Thread-safe.
	template<typename It>
	inline bool enqueue_bulk(It itemFirst, size_t count)
	{
		if ((details::likely)(inner.enqueue_bulk(std::forward<It>(itemFirst), count))) {
			sema->signal((LightweightSemaphore::ssize_t)(ssize_t)count);
			return true;
		}
		return false;
	}
	
	// Enqueues several items using an explicit producer token.
	// Allocates memory if required. Only fails if memory allocation fails
	// (or Traits::MAX_SUBQUEUE_SIZE has been defined and would be surpassed).
	// Note: Use std::make_move_iterator if the elements should be moved
	// instead of copied.
	// Thread-safe.
	template<typename It>
	inline bool enqueue_bulk(producer_token_t const& token, It itemFirst, size_t count)
	{
		if ((details::likely)(inner.enqueue_bulk(token, std::forward<It>(itemFirst), count))) {
			sema->signal((LightweightSemaphore::ssize_t)(ssize_t)count);
			return true;
		}
		return false;
	}
	
	// Enqueues a single item (by copying it).
	// Does not allocate memory. Fails if not enough room to enqueue (or implicit
	// production is disabled because Traits::INITIAL_IMPLICIT_PRODUCER_HASH_SIZE
	// is 0).
	// Thread-safe.
	inline bool try_enqueue(T const& item)
	{
		if (inner.try_enqueue(item)) {
			sema->signal();
			return true;
		}
		return false;
	}
	
	// Enqueues a single item (by moving it, if possible).
	// Does not allocate memory (except for one-time implicit producer).
	// Fails if not enough room to enqueue (or implicit production is
	// disabled because Traits::INITIAL_IMPLICIT_PRODUCER_HASH_SIZE is 0).
	// Thread-safe.
	inline bool try_enqueue(T&& item)
	{
		if (inner.try_enqueue(std::move(item))) {
			sema->signal();
			return true;
		}
		return false;
	}
	
	// Enqueues a single item (by copying it) using an explicit producer token.
	// Does not allocate memory. Fails if not enough room to enqueue.
	// Thread-safe.
	inline bool try_enqueue(producer_token_t const& token, T const& item)
	{
		if (inner.try_enqueue(token, item)) {
			sema->signal();
			return true;
		}
		return false;
	}
	
	// Enqueues a single item (by moving it, if possible) using an explicit producer token.
	// Does not allocate memory. Fails if not enough room to enqueue.
	// Thread-safe.
	inline bool try_enqueue(producer_token_t const& token, T&& item)
	{
		if (inner.try_enqueue(token, std::move(item))) {
			sema->signal();
			return true;
		}
		return false;
	}
	
	// Enqueues several items.
	// Does not allocate memory (except for one-time implicit producer).
	// Fails if not enough room to enqueue (or implicit production is
	// disabled because Traits::INITIAL_IMPLICIT_PRODUCER_HASH_SIZE is 0).
	// Note: Use std::make_move_iterator if the elements should be moved
	// instead of copied.
	// Thread-safe.
	template<typename It>
	inline bool try_enqueue_bulk(It itemFirst, size_t count)
	{
		if (inner.try_enqueue_bulk(std::forward<It>(itemFirst), count)) {
			sema->signal((LightweightSemaphore::ssize_t)(ssize_t)count);
			return true;
		}
		return false;
	}
	
	// Enqueues several items using an explicit producer token.
	// Does not allocate memory. Fails if not enough room to enqueue.
	// Note: Use std::make_move_iterator if the elements should be moved
	// instead of copied.
	// Thread-safe.
	template<typename It>
	inline bool try_enqueue_bulk(producer_token_t const& token, It itemFirst, size_t count)
	{
		if (inner.try_enqueue_bulk(token, std::forward<It>(itemFirst), count)) {
			sema->signal((LightweightSemaphore::ssize_t)(ssize_t)count);
			return true;
		}
		return false;
	}
	
	
	// Attempts to dequeue from the queue.
	// Returns false if all producer streams appeared empty at the time they
	// were checked (so, the queue is likely but not guaranteed to be empty).
	// Never allocates. Thread-safe.
	template<typename U>
	inline bool try_dequeue(U& item)
	{
		if (sema->tryWait()) {
			while (!inner.try_dequeue(item)) {
				continue;
			}
			return true;
		}
		return false;
	}
	
	// Attempts to dequeue from the queue using an explicit consumer token.
	// Returns false if all producer streams appeared empty at the time they
	// were checked (so, the queue is likely but not guaranteed to be empty).
	// Never allocates. Thread-safe.
	template<typename U>
	inline bool try_dequeue(consumer_token_t& token, U& item)
	{
		if (sema->tryWait()) {
			while (!inner.try_dequeue(token, item)) {
				continue;
			}
			return true;
		}
		return false;
	}
	
	// Attempts to dequeue several elements from the queue.
	// Returns the number of items actually dequeued.
	// Returns 0 if all producer streams appeared empty at the time they
	// were checked (so, the queue is likely but not guaranteed to be empty).
	// Never allocates. Thread-safe.
	template<typename It>
	inline size_t try_dequeue_bulk(It itemFirst, size_t max)
	{
		size_t count = 0;
		max = (size_t)sema->tryWaitMany((LightweightSemaphore::ssize_t)(ssize_t)max);
		while (count != max) {
			count += inner.template try_dequeue_bulk<It&>(itemFirst, max - count);
		}
		return count;
	}
	
	// Attempts to dequeue several elements from the queue using an explicit consumer token.
	// Returns the number of items actually dequeued.
	// Returns 0 if all producer streams appeared empty at the time they
	// were checked (so, the queue is likely but not guaranteed to be empty).
	// Never allocates. Thread-safe.
	template<typename It>
	inline size_t try_dequeue_bulk(consumer_token_t& token, It itemFirst, size_t max)
	{
		size_t count = 0;
		max = (size_t)sema->tryWaitMany((LightweightSemaphore::ssize_t)(ssize_t)max);
		while (count != max) {
			count += inner.template try_dequeue_bulk<It&>(token, itemFirst, max - count);
		}
		return count;
	}
	
	
	
	// Blocks the current thread until there's something to dequeue, then
	// dequeues it.
	// Never allocates. Thread-safe.
	template<typename U>
	inline void wait_dequeue(U& item)
	{
		sema->wait();
		while (!inner.try_dequeue(item)) {
			continue;
		}
	}

	// Blocks the current thread until either there's something to dequeue
	// or the timeout (specified in microseconds) expires. Returns false
	// without setting `item` if the timeout expires, otherwise assigns
	// to `item` and returns true.
	// Using a negative timeout indicates an indefinite timeout,
	// and is thus functionally equivalent to calling wait_dequeue.
	// Never allocates. Thread-safe.
	template<typename U>
	inline bool wait_dequeue_timed(U& item, std::int64_t timeout_usecs)
	{
		if (!sema->wait(timeout_usecs)) {
			return false;
		}
		while (!inner.try_dequeue(item)) {
			continue;
		}
		return true;
	}
    
    // Blocks the current thread until either there's something to dequeue
	// or the timeout expires. Returns false without setting `item` if the
    // timeout expires, otherwise assigns to `item` and returns true.
	// Never allocates. Thread-safe.
	template<typename U, typename Rep, typename Period>
	inline bool wait_dequeue_timed(U& item, std::chrono::duration<Rep, Period> const& timeout)
    {
        return wait_dequeue_timed(item, std::chrono::duration_cast<std::chrono::microseconds>(timeout).count());
    }
	
	// Blocks the current thread until there's something to dequeue, then
	// dequeues it using an explicit consumer token.
	// Never allocates. Thread-safe.
	template<typename U>
	inline void wait_dequeue(consumer_token_t& token, U& item)
	{
		sema->wait();
		while (!inner.try_dequeue(token, item)) {
			continue;
		}
	}
	
	// Blocks the current thread until either there's something to dequeue
	// or the timeout (specified in microseconds) expires. Returns false
	// without setting `item` if the timeout expires, otherwise assigns
	// to `item` and returns true.
	// Using a negative timeout indicates an indefinite timeout,
	// and is thus functionally equivalent to calling wait_dequeue.
	// Never allocates. Thread-safe.
	template<typename U>
	inline bool wait_dequeue_timed(consumer_token_t& token, U& item, std::int64_t timeout_usecs)
	{
		if (!sema->wait(timeout_usecs)) {
			return false;
		}
		while (!inner.try_dequeue(token, item)) {
			continue;
		}
		return true;
	}
    
    // Blocks the current thread until either there's something to dequeue
	// or the timeout expires. Returns false without setting `item` if the
    // timeout expires, otherwise assigns to `item` and returns true.
	// Never allocates. Thread-safe.
	template<typename U, typename Rep, typename Period>
	inline bool wait_dequeue_timed(consumer_token_t& token, U& item, std::chrono::duration<Rep, Period> const& timeout)
    {
        return wait_dequeue_timed(token, item, std::chrono::duration_cast<std::chrono::microseconds>(timeout).count());
    }
	
	// Attempts to dequeue several elements from the queue.
	// Returns the number of items actually dequeued, which will
	// always be at least one (this method blocks until the queue
	// is non-empty) and at most max.
	// Never allocates. Thread-safe.
	template<typename It>
	inline size_t wait_dequeue_bulk(It itemFirst, size_t max)
	{
		size_t count = 0;
		max = (size_t)sema->waitMany((LightweightSemaphore::ssize_t)(ssize_t)max);
		while (count != max) {
			count += inner.template try_dequeue_bulk<It&>(itemFirst, max - count);
		}
		return count;
	}
	
	// Attempts to dequeue several elements from the queue.
	// Returns the number of items actually dequeued, which can
	// be 0 if the timeout expires while waiting for elements,
	// and at most max.
	// Using a negative timeout indicates an indefinite timeout,
	// and is thus functionally equivalent to calling wait_dequeue_bulk.
	// Never allocates. Thread-safe.
	template<typename It>
	inline size_t wait_dequeue_bulk_timed(It itemFirst, size_t max, std::int64_t timeout_usecs)
	{
		size_t count = 0;
		max = (size_t)sema->waitMany((LightweightSemaphore::ssize_t)(ssize_t)max, timeout_usecs);
		while (count != max) {
			count += inner.template try_dequeue_bulk<It&>(itemFirst, max - count);
		}
		return count;
	}
    
    // Attempts to dequeue several elements from the queue.
	// Returns the number of items actually dequeued, which can
	// be 0 if the timeout expires while waiting for elements,
	// and at most max.
	// Never allocates. Thread-safe.
	template<typename It, typename Rep, typename Period>
	inline size_t wait_dequeue_bulk_timed(It itemFirst, size_t max, std::chrono::duration<Rep, Period> const& timeout)
    {
        return wait_dequeue_bulk_timed<It&>(itemFirst, max, std::chrono::duration_cast<std::chrono::microseconds>(timeout).count());
    }
	
	// Attempts to dequeue several elements from the queue using an explicit consumer token.
	// Returns the number of items actually dequeued, which will
	// always be at least one (this method blocks until the queue
	// is non-empty) and at most max.
	// Never allocates. Thread-safe.
	template<typename It>
	inline size_t wait_dequeue_bulk(consumer_token_t& token, It itemFirst, size_t max)
	{
		size_t count = 0;
		max = (size_t)sema->waitMany((LightweightSemaphore::ssize_t)(ssize_t)max);
		while (count != max) {
			count += inner.template try_dequeue_bulk<It&>(token, itemFirst, max - count);
		}
		return count;
	}
	
	// Attempts to dequeue several elements from the queue using an explicit consumer token.
	// Returns the number of items actually dequeued, which can
	// be 0 if the timeout expires while waiting for elements,
	// and at most max.
	// Using a negative timeout indicates an indefinite timeout,
	// and is thus functionally equivalent to calling wait_dequeue_bulk.
	// Never allocates. Thread-safe.
	template<typename It>
	inline size_t wait_dequeue_bulk_timed(consumer_token_t& token, It itemFirst, size_t max, std::int64_t timeout_usecs)
	{
		size_t count = 0;
		max = (size_t)sema->waitMany((LightweightSemaphore::ssize_t)(ssize_t)max, timeout_usecs);
		while (count != max) {
			count += inner.template try_dequeue_bulk<It&>(token, itemFirst, max - count);
		}
		return count;
	}
	
	// Attempts to dequeue several elements from the queue using an explicit consumer token.
	// Returns the number of items actually dequeued, which can
	// be 0 if the timeout expires while waiting for elements,
	// and at most max.
	// Never allocates. Thread-safe.
	template<typename It, typename Rep, typename Period>
	inline size_t wait_dequeue_bulk_timed(consumer_token_t& token, It itemFirst, size_t max, std::chrono::duration<Rep, Period> const& timeout)
    {
        return wait_dequeue_bulk_timed<It&>(token, itemFirst, max, std::chrono::duration_cast<std::chrono::microseconds>(timeout).count());
    }
	
	
	// Returns an estimate of the total number of elements currently in the queue. This
	// estimate is only accurate if the queue has completely stabilized before it is called
	// (i.e. all enqueue and dequeue operations have completed and their memory effects are
	// visible on the calling thread, and no further operations start while this method is
	// being called).
	// Thread-safe.
	inline size_t size_approx() const
	{
		return (size_t)sema->availableApprox();
	}
	
	
	// Returns true if the underlying atomic variables used by
	// the queue are lock-free (they should be on most platforms).
	// Thread-safe.
	static bool is_lock_free()
	{
		return ConcurrentQueue::is_lock_free();
	}
	

private:
	template<typename U>
	static inline U* create()
	{
		auto p = (Traits::malloc)(sizeof(U));
		return p != nullptr ? new (p) U : nullptr;
	}
	
	template<typename U, typename A1>
	static inline U* create(A1&& a1)
	{
		auto p = (Traits::malloc)(sizeof(U));
		return p != nullptr ? new (p) U(std::forward<A1>(a1)) : nullptr;
	}
	
	template<typename U>
	static inline void destroy(U* p)
	{
		if (p != nullptr) {
			p->~U();
		}
		(Traits::free)(p);
	}
	
private:
	ConcurrentQueue inner;
	std::unique_ptr<LightweightSemaphore, void (*)(LightweightSemaphore*)> sema;
};


template<typename T, typename Traits>
inline void swap(BlockingConcurrentQueue<T, Traits>& a, BlockingConcurrentQueue<T, Traits>& b) MOODYCAMEL_NOEXCEPT
{
	a.swap(b);
}

}	// end namespace moodycamel