concurrentqueue.h

// Provides a C++11 implementation of a multi-producer, multi-consumer lock-free queue.
// An overview, including benchmark results, is provided here:
//     http://moodycamel.com/blog/2014/a-fast-general-purpose-lock-free-queue-for-c++
// The full design is also described in excruciating detail at:
//    http://moodycamel.com/blog/2014/detailed-design-of-a-lock-free-queue
 
// Simplified BSD license:
// Copyright (c) 2013-2020, Cameron Desrochers.
// All rights reserved.
//
// Redistribution and use in source and binary forms, with or without modification,
// are permitted provided that the following conditions are met:
//
// - Redistributions of source code must retain the above copyright notice, this list of
// conditions and the following disclaimer.
// - 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 HOLDER 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.
 
// Also dual-licensed under the Boost Software License (see LICENSE.md)
 
#pragma once
 
#if defined(__GNUC__) && !defined(__INTEL_COMPILER)
// Disable -Wconversion warnings (spuriously triggered when Traits::size_t and
// Traits::index_t are set to < 32 bits, causing integer promotion, causing warnings
// upon assigning any computed values)
#pragma GCC diagnostic push
#pragma GCC diagnostic ignored "-Wconversion"
 
#ifdef MCDBGQ_USE_RELACY
#pragma GCC diagnostic ignored "-Wint-to-pointer-cast"
#endif
#endif
 
#if defined(_MSC_VER) && (!defined(_HAS_CXX17) || !_HAS_CXX17)
// VS2019 with /W4 warns about constant conditional expressions but unless /std=c++17 or higher
// does not support `if constexpr`, so we have no choice but to simply disable the warning
#pragma warning(push)
#pragma warning(disable : 4127) // conditional expression is constant
#endif
 
#if defined(__APPLE__)
#include "TargetConditionals.h"
#endif
 
#ifdef MCDBGQ_USE_RELACY
#include "relacy/relacy_std.hpp"
#include "relacy_shims.h"
// We only use malloc/free anyway, and the delete macro messes up `= delete` method declarations.
// We'll override the default trait malloc ourselves without a macro.
#undef new
#undef delete
#undef malloc
#undef free
#else
#include <atomic> // Requires C++11. Sorry VS2010.
#include <cassert>
#endif
#include <cstddef> // for max_align_t
#include <cstdint>
#include <cstdlib>
#include <type_traits>
#include <algorithm>
#include <utility>
#include <limits>
#include <climits> // for CHAR_BIT
#include <array>
#include <thread> // partly for __WINPTHREADS_VERSION if on MinGW-w64 w/ POSIX threading
#include <mutex>  // used for thread exit synchronization
 
// Platform-specific definitions of a numeric thread ID type and an invalid value
namespace moodycamel {
namespace details {
template <typename thread_id_t> struct thread_id_converter {
    typedef thread_id_t thread_id_numeric_size_t;
    typedef thread_id_t thread_id_hash_t;
    static thread_id_hash_t prehash(thread_id_t const& x) { return x; }
};
} // namespace details
} // namespace moodycamel
#if defined(MCDBGQ_USE_RELACY)
namespace moodycamel {
namespace details {
typedef std::uint32_t thread_id_t;
static const thread_id_t invalid_thread_id = 0xFFFFFFFFU;
static const thread_id_t invalid_thread_id2 = 0xFFFFFFFEU;
static inline thread_id_t thread_id()
{
    return rl::thread_index();
}
} // namespace details
} // namespace moodycamel
#elif defined(_WIN32) || defined(__WINDOWS__) || defined(__WIN32__)
// No sense pulling in windows.h in a header, we'll manually declare the function
// we use and rely on backwards-compatibility for this not to break
extern "C" __declspec(dllimport) unsigned long __stdcall GetCurrentThreadId(void);
namespace moodycamel {
namespace details {
static_assert(sizeof(unsigned long) == sizeof(std::uint32_t),
              "Expected size of unsigned long to be 32 bits on Windows");
typedef std::uint32_t thread_id_t;
static const thread_id_t invalid_thread_id = 0; // See http://blogs.msdn.com/b/oldnewthing/archive/2004/02/23/78395.aspx
static const thread_id_t invalid_thread_id2 =
    0xFFFFFFFFU; // Not technically guaranteed to be invalid, but is never used in practice. Note that all Win32 thread
                 // IDs are presently multiples of 4.
static inline thread_id_t thread_id()
{
    return static_cast<thread_id_t>(::GetCurrentThreadId());
}
} // namespace details
} // namespace moodycamel
#elif defined(__arm__) || defined(_M_ARM) || defined(__aarch64__) || (defined(__APPLE__) && TARGET_OS_IPHONE) ||       \
    defined(MOODYCAMEL_NO_THREAD_LOCAL)
namespace moodycamel {
namespace details {
static_assert(sizeof(std::thread::id) == 4 || sizeof(std::thread::id) == 8,
              "std::thread::id is expected to be either 4 or 8 bytes");
 
typedef std::thread::id thread_id_t;
static const thread_id_t invalid_thread_id; // Default ctor creates invalid ID
 
// Note we don't define a invalid_thread_id2 since std::thread::id doesn't have one; it's
// only used if MOODYCAMEL_CPP11_THREAD_LOCAL_SUPPORTED is defined anyway, which it won't
// be.
static inline thread_id_t thread_id()
{
    return std::this_thread::get_id();
}
 
template <std::size_t> struct thread_id_size {};
template <> struct thread_id_size<4> {
    typedef std::uint32_t numeric_t;
};
template <> struct thread_id_size<8> {
    typedef std::uint64_t numeric_t;
};
 
template <> struct thread_id_converter<thread_id_t> {
    typedef thread_id_size<sizeof(thread_id_t)>::numeric_t thread_id_numeric_size_t;
#ifndef __APPLE__
    typedef std::size_t thread_id_hash_t;
#else
    typedef thread_id_numeric_size_t thread_id_hash_t;
#endif
 
    static thread_id_hash_t prehash(thread_id_t const& x)
    {
#ifndef __APPLE__
        return std::hash<std::thread::id>()(x);
#else
        return *reinterpret_cast<thread_id_hash_t const*>(&x);
#endif
    }
};
}
}
#else
// Use a nice trick from this answer: http://stackoverflow.com/a/8438730/21475
// In order to get a numeric thread ID in a platform-independent way, we use a thread-local
// static variable's address as a thread identifier :-)
#if defined(__GNUC__) || defined(__INTEL_COMPILER)
#define MOODYCAMEL_THREADLOCAL __thread
#elif defined(_MSC_VER)
#define MOODYCAMEL_THREADLOCAL __declspec(thread)
#else
// Assume C++11 compliant compiler
#define MOODYCAMEL_THREADLOCAL thread_local
#endif
namespace moodycamel {
namespace details {
typedef std::uintptr_t thread_id_t;
static const thread_id_t invalid_thread_id = 0; // Address can't be nullptr
static const thread_id_t invalid_thread_id2 =
    1; // Member accesses off a null pointer are also generally invalid. Plus it's not aligned.
inline thread_id_t thread_id()
{
    static MOODYCAMEL_THREADLOCAL int x;
    return reinterpret_cast<thread_id_t>(&x);
}
}
}
#endif
 
// Constexpr if
#ifndef MOODYCAMEL_CONSTEXPR_IF
#if (defined(_MSC_VER) && defined(_HAS_CXX17) && _HAS_CXX17) || __cplusplus > 201402L
#define MOODYCAMEL_CONSTEXPR_IF if constexpr
#define MOODYCAMEL_MAYBE_UNUSED [[maybe_unused]]
#else
#define MOODYCAMEL_CONSTEXPR_IF if
#define MOODYCAMEL_MAYBE_UNUSED
#endif
#endif
 
// Exceptions
#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 MOODYCAMEL_EXCEPTIONS_ENABLED
#define MOODYCAMEL_TRY try
#define MOODYCAMEL_CATCH(...) catch (__VA_ARGS__)
#define MOODYCAMEL_RETHROW throw
#define MOODYCAMEL_THROW(expr) throw(expr)
#else
#define MOODYCAMEL_TRY MOODYCAMEL_CONSTEXPR_IF(true)
#define MOODYCAMEL_CATCH(...) else MOODYCAMEL_CONSTEXPR_IF(false)
#define MOODYCAMEL_RETHROW
#define MOODYCAMEL_THROW(expr)
#endif
 
#ifndef MOODYCAMEL_NOEXCEPT
#if !defined(MOODYCAMEL_EXCEPTIONS_ENABLED)
#define MOODYCAMEL_NOEXCEPT
#define MOODYCAMEL_NOEXCEPT_CTOR(type, valueType, expr) true
#define MOODYCAMEL_NOEXCEPT_ASSIGN(type, valueType, expr) true
#elif defined(_MSC_VER) && defined(_NOEXCEPT) && _MSC_VER < 1800
// VS2012's std::is_nothrow_[move_]constructible is broken and returns true when it shouldn't :-(
// We have to assume *all* non-trivial constructors may throw on VS2012!
#define MOODYCAMEL_NOEXCEPT _NOEXCEPT
#define MOODYCAMEL_NOEXCEPT_CTOR(type, valueType, expr)                                                                \
    (std::is_rvalue_reference<valueType>::value && std::is_move_constructible<type>::value                             \
         ? std::is_trivially_move_constructible<type>::value                                                           \
         : std::is_trivially_copy_constructible<type>::value)
#define MOODYCAMEL_NOEXCEPT_ASSIGN(type, valueType, expr)                                                              \
    ((std::is_rvalue_reference<valueType>::value && std::is_move_assignable<type>::value                               \
          ? std::is_trivially_move_assignable<type>::value || std::is_nothrow_move_assignable<type>::value             \
          : std::is_trivially_copy_assignable<type>::value || std::is_nothrow_copy_assignable<type>::value) &&         \
     MOODYCAMEL_NOEXCEPT_CTOR(type, valueType, expr))
#elif defined(_MSC_VER) && defined(_NOEXCEPT) && _MSC_VER < 1900
#define MOODYCAMEL_NOEXCEPT _NOEXCEPT
#define MOODYCAMEL_NOEXCEPT_CTOR(type, valueType, expr)                                                                \
    (std::is_rvalue_reference<valueType>::value && std::is_move_constructible<type>::value                             \
         ? std::is_trivially_move_constructible<type>::value || std::is_nothrow_move_constructible<type>::value        \
         : std::is_trivially_copy_constructible<type>::value || std::is_nothrow_copy_constructible<type>::value)
#define MOODYCAMEL_NOEXCEPT_ASSIGN(type, valueType, expr)                                                              \
    ((std::is_rvalue_reference<valueType>::value && std::is_move_assignable<type>::value                               \
          ? std::is_trivially_move_assignable<type>::value || std::is_nothrow_move_assignable<type>::value             \
          : std::is_trivially_copy_assignable<type>::value || std::is_nothrow_copy_assignable<type>::value) &&         \
     MOODYCAMEL_NOEXCEPT_CTOR(type, valueType, expr))
#else
#define MOODYCAMEL_NOEXCEPT noexcept
#define MOODYCAMEL_NOEXCEPT_CTOR(type, valueType, expr) noexcept(expr)
#define MOODYCAMEL_NOEXCEPT_ASSIGN(type, valueType, expr) noexcept(expr)
#endif
#endif
 
#ifndef MOODYCAMEL_CPP11_THREAD_LOCAL_SUPPORTED
#ifdef MCDBGQ_USE_RELACY
#define MOODYCAMEL_CPP11_THREAD_LOCAL_SUPPORTED
#else
// VS2013 doesn't support `thread_local`, and MinGW-w64 w/ POSIX threading has a crippling bug:
// http://sourceforge.net/p/mingw-w64/bugs/445 g++ <=4.7 doesn't support thread_local either. Finally, iOS/ARM doesn't
// have support for it either, and g++/ARM allows it to compile but it's unconfirmed to actually work
#if (!defined(_MSC_VER) || _MSC_VER >= 1900) &&                                                                        \
    (!defined(__MINGW32__) && !defined(__MINGW64__) || !defined(__WINPTHREADS_VERSION)) &&                             \
    (!defined(__GNUC__) || __GNUC__ > 4 || (__GNUC__ == 4 && __GNUC_MINOR__ >= 8)) &&                                  \
    (!defined(__APPLE__) || !TARGET_OS_IPHONE) && !defined(__arm__) && !defined(_M_ARM) && !defined(__aarch64__)
// Assume `thread_local` is fully supported in all other C++11 compilers/platforms
#define MOODYCAMEL_CPP11_THREAD_LOCAL_SUPPORTED // tentatively enabled for now; years ago several users report having
                                                // problems with it on
#endif
#endif
#endif
 
// VS2012 doesn't support deleted functions.
// In this case, we declare the function normally but don't define it. A link error will be generated if the function is
// called.
#ifndef MOODYCAMEL_DELETE_FUNCTION
#if defined(_MSC_VER) && _MSC_VER < 1800
#define MOODYCAMEL_DELETE_FUNCTION
#else
#define MOODYCAMEL_DELETE_FUNCTION = delete
#endif
#endif
 
namespace moodycamel {
namespace details {
#ifndef MOODYCAMEL_ALIGNAS
// VS2013 doesn't support alignas or alignof, and align() requires a constant literal
#if defined(_MSC_VER) && _MSC_VER <= 1800
#define MOODYCAMEL_ALIGNAS(alignment) __declspec(align(alignment))
#define MOODYCAMEL_ALIGNOF(obj) __alignof(obj)
#define MOODYCAMEL_ALIGNED_TYPE_LIKE(T, obj) typename details::Vs2013Aligned<std::alignment_of<obj>::value, T>::type
template <int Align, typename T> struct Vs2013Aligned {}; // default, unsupported alignment
template <typename T> struct Vs2013Aligned<1, T> {
    typedef __declspec(align(1)) T type;
};
template <typename T> struct Vs2013Aligned<2, T> {
    typedef __declspec(align(2)) T type;
};
template <typename T> struct Vs2013Aligned<4, T> {
    typedef __declspec(align(4)) T type;
};
template <typename T> struct Vs2013Aligned<8, T> {
    typedef __declspec(align(8)) T type;
};
template <typename T> struct Vs2013Aligned<16, T> {
    typedef __declspec(align(16)) T type;
};
template <typename T> struct Vs2013Aligned<32, T> {
    typedef __declspec(align(32)) T type;
};
template <typename T> struct Vs2013Aligned<64, T> {
    typedef __declspec(align(64)) T type;
};
template <typename T> struct Vs2013Aligned<128, T> {
    typedef __declspec(align(128)) T type;
};
template <typename T> struct Vs2013Aligned<256, T> {
    typedef __declspec(align(256)) T type;
};
#else
template <typename T> struct identity {
    typedef T type;
};
#define MOODYCAMEL_ALIGNAS(alignment) alignas(alignment)
#define MOODYCAMEL_ALIGNOF(obj) alignof(obj)
#define MOODYCAMEL_ALIGNED_TYPE_LIKE(T, obj) alignas(alignof(obj)) typename details::identity<T>::type
#endif
#endif
} // namespace details
} // namespace moodycamel
 
// TSAN can false report races in lock-free code.  To enable TSAN to be used from projects that use this one,
// we can apply per-function compile-time suppression.
// See https://clang.llvm.org/docs/ThreadSanitizer.html#has-feature-thread-sanitizer
#define MOODYCAMEL_NO_TSAN
#if defined(__has_feature)
#if __has_feature(thread_sanitizer)
#undef MOODYCAMEL_NO_TSAN
#define MOODYCAMEL_NO_TSAN __attribute__((no_sanitize("thread")))
#endif // TSAN
#endif // TSAN
 
// Compiler-specific likely/unlikely hints
namespace moodycamel {
namespace details {
#if defined(__GNUC__)
static inline bool(likely)(bool x)
{
    return __builtin_expect((x), true);
}
static inline bool(unlikely)(bool x)
{
    return __builtin_expect((x), false);
}
#else
static inline bool(likely)(bool x)
{
    return x;
}
static inline bool(unlikely)(bool x)
{
    return x;
}
#endif
} // namespace details
} // namespace moodycamel
 
#ifdef MOODYCAMEL_QUEUE_INTERNAL_DEBUG
#include "internal/concurrentqueue_internal_debug.h"
#endif
 
namespace moodycamel {
namespace details {
template <typename T> struct const_numeric_max {
    static_assert(std::is_integral<T>::value, "const_numeric_max can only be used with integers");
    static const T value = std::numeric_limits<T>::is_signed
                               ? (static_cast<T>(1) << (sizeof(T) * CHAR_BIT - 1)) - static_cast<T>(1)
                               : static_cast<T>(-1);
};
 
#if defined(__GLIBCXX__)
typedef ::max_align_t std_max_align_t; // libstdc++ forgot to add it to std:: for a while
#else
typedef std::max_align_t std_max_align_t; // Others (e.g. MSVC) insist it can *only* be accessed via std::
#endif
 
// Some platforms have incorrectly set max_align_t to a type with <8 bytes alignment even while supporting
// 8-byte aligned scalar values (*cough* 32-bit iOS). Work around this with our own union. See issue #64.
typedef union {
    std_max_align_t x;
    long long y;
    void* z;
} max_align_t;
} // namespace details
 
// Default traits for the ConcurrentQueue. To change some of the
// traits without re-implementing all of them, inherit from this
// struct and shadow the declarations you wish to be different;
// since the traits are used as a template type parameter, the
// shadowed declarations will be used where defined, and the defaults
// otherwise.
struct ConcurrentQueueDefaultTraits {
    // General-purpose size type. std::size_t is strongly recommended.
    typedef std::size_t size_t;
 
    // The type used for the enqueue and dequeue indices. Must be at least as
    // large as size_t. Should be significantly larger than the number of elements
    // you expect to hold at once, especially if you have a high turnover rate;
    // for example, on 32-bit x86, if you expect to have over a hundred million
    // elements or pump several million elements through your queue in a very
    // short space of time, using a 32-bit type *may* trigger a race condition.
    // A 64-bit int type is recommended in that case, and in practice will
    // prevent a race condition no matter the usage of the queue. Note that
    // whether the queue is lock-free with a 64-int type depends on the whether
    // std::atomic<std::uint64_t> is lock-free, which is platform-specific.
    typedef std::size_t index_t;
 
    // Internally, all elements are enqueued and dequeued from multi-element
    // blocks; this is the smallest controllable unit. If you expect few elements
    // but many producers, a smaller block size should be favoured. For few producers
    // and/or many elements, a larger block size is preferred. A sane default
    // is provided. Must be a power of 2.
    static const size_t BLOCK_SIZE = 32;
 
    // For explicit producers (i.e. when using a producer token), the block is
    // checked for being empty by iterating through a list of flags, one per element.
    // For large block sizes, this is too inefficient, and switching to an atomic
    // counter-based approach is faster. The switch is made for block sizes strictly
    // larger than this threshold.
    static const size_t EXPLICIT_BLOCK_EMPTY_COUNTER_THRESHOLD = 32;
 
    // How many full blocks can be expected for a single explicit producer? This should
    // reflect that number's maximum for optimal performance. Must be a power of 2.
    static const size_t EXPLICIT_INITIAL_INDEX_SIZE = 32;
 
    // How many full blocks can be expected for a single implicit producer? This should
    // reflect that number's maximum for optimal performance. Must be a power of 2.
    static const size_t IMPLICIT_INITIAL_INDEX_SIZE = 32;
 
    // The initial size of the hash table mapping thread IDs to implicit producers.
    // Note that the hash is resized every time it becomes half full.
    // Must be a power of two, and either 0 or at least 1. If 0, implicit production
    // (using the enqueue methods without an explicit producer token) is disabled.
    static const size_t INITIAL_IMPLICIT_PRODUCER_HASH_SIZE = 32;
 
    // Controls the number of items that an explicit consumer (i.e. one with a token)
    // must consume before it causes all consumers to rotate and move on to the next
    // internal queue.
    static const std::uint32_t EXPLICIT_CONSUMER_CONSUMPTION_QUOTA_BEFORE_ROTATE = 256;
 
    // The maximum number of elements (inclusive) that can be enqueued to a sub-queue.
    // Enqueue operations that would cause this limit to be surpassed will fail. Note
    // that this limit is enforced at the block level (for performance reasons), i.e.
    // it's rounded up to the nearest block size.
    static const size_t MAX_SUBQUEUE_SIZE = details::const_numeric_max<size_t>::value;
 
    // The number of times to spin before sleeping when waiting on a semaphore.
    // Recommended values are on the order of 1000-10000 unless the number of
    // consumer threads exceeds the number of idle cores (in which case try 0-100).
    // Only affects instances of the BlockingConcurrentQueue.
    static const int MAX_SEMA_SPINS = 10000;
 
    // Whether to recycle dynamically-allocated blocks into an internal free list or
    // not. If false, only pre-allocated blocks (controlled by the constructor
    // arguments) will be recycled, and all others will be `free`d back to the heap.
    // Note that blocks consumed by explicit producers are only freed on destruction
    // of the queue (not following destruction of the token) regardless of this trait.
    static const bool RECYCLE_ALLOCATED_BLOCKS = false;
 
#ifndef MCDBGQ_USE_RELACY
    // Memory allocation can be customized if needed.
    // malloc should return nullptr on failure, and handle alignment like std::malloc.
#if defined(malloc) || defined(free)
    // Gah, this is 2015, stop defining macros that break standard code already!
    // Work around malloc/free being special macros:
    static inline void* WORKAROUND_malloc(size_t size) { return malloc(size); }
    static inline void WORKAROUND_free(void* ptr) { return free(ptr); }
    static inline void*(malloc)(size_t size) { return WORKAROUND_malloc(size); }
    static inline void(free)(void* ptr) { return WORKAROUND_free(ptr); }
#else
    static inline void* malloc(size_t size) { return std::malloc(size); }
    static inline void free(void* ptr) { return std::free(ptr); }
#endif
#else
    // Debug versions when running under the Relacy race detector (ignore
    // these in user code)
    static inline void* malloc(size_t size) { return rl::rl_malloc(size, $); }
    static inline void free(void* ptr) { return rl::rl_free(ptr, $); }
#endif
};
 
// When producing or consuming many elements, the most efficient way is to:
//    1) Use one of the bulk-operation methods of the queue with a token
//    2) Failing that, use the bulk-operation methods without a token
//    3) Failing that, create a token and use that with the single-item methods
//    4) Failing that, use the single-parameter methods of the queue
// Having said that, don't create tokens willy-nilly -- ideally there should be
// a maximum of one token per thread (of each kind).
struct ProducerToken;
struct ConsumerToken;
 
template <typename T, typename Traits> class ConcurrentQueue;
template <typename T, typename Traits> class BlockingConcurrentQueue;
class ConcurrentQueueTests;
 
namespace details {
struct ConcurrentQueueProducerTypelessBase {
    ConcurrentQueueProducerTypelessBase* next;
    std::atomic<bool> inactive;
    ProducerToken* token;
 
    ConcurrentQueueProducerTypelessBase()
        : next(nullptr)
        , inactive(false)
        , token(nullptr)
    {}
};
 
template <bool use32> struct _hash_32_or_64 {
    static inline std::uint32_t hash(std::uint32_t h)
    {
        // MurmurHash3 finalizer -- see https://code.google.com/p/smhasher/source/browse/trunk/MurmurHash3.cpp
        // Since the thread ID is already unique, all we really want to do is propagate that
        // uniqueness evenly across all the bits, so that we can use a subset of the bits while
        // reducing collisions significantly
        h ^= h >> 16;
        h *= 0x85ebca6b;
        h ^= h >> 13;
        h *= 0xc2b2ae35;
        return h ^ (h >> 16);
    }
};
template <> struct _hash_32_or_64<1> {
    static inline std::uint64_t hash(std::uint64_t h)
    {
        h ^= h >> 33;
        h *= 0xff51afd7ed558ccd;
        h ^= h >> 33;
        h *= 0xc4ceb9fe1a85ec53;
        return h ^ (h >> 33);
    }
};
template <std::size_t size> struct hash_32_or_64 : public _hash_32_or_64<(size > 4)> {};
 
static inline size_t hash_thread_id(thread_id_t id)
{
    static_assert(sizeof(thread_id_t) <= 8, "Expected a platform where thread IDs are at most 64-bit values");
    return static_cast<size_t>(hash_32_or_64<sizeof(thread_id_converter<thread_id_t>::thread_id_hash_t)>::hash(
        thread_id_converter<thread_id_t>::prehash(id)));
}
 
template <typename T> static inline bool circular_less_than(T a, T b)
{
    static_assert(std::is_integral<T>::value && !std::numeric_limits<T>::is_signed,
                  "circular_less_than is intended to be used only with unsigned integer types");
    return static_cast<T>(a - b) > static_cast<T>(static_cast<T>(1) << (static_cast<T>(sizeof(T) * CHAR_BIT - 1)));
    // Note: extra parens around rhs of operator<< is MSVC bug:
    // https://developercommunity2.visualstudio.com/t/C4554-triggers-when-both-lhs-and-rhs-is/10034931
    //       silencing the bug requires #pragma warning(disable: 4554) around the calling code and has no effect when
    //       done here.
}
 
template <typename U> static inline 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;
}
 
template <typename T> static inline T ceil_to_pow_2(T x)
{
    static_assert(std::is_integral<T>::value && !std::numeric_limits<T>::is_signed,
                  "ceil_to_pow_2 is intended to be used only with unsigned integer types");
 
    // Adapted from http://graphics.stanford.edu/~seander/bithacks.html#RoundUpPowerOf2
    --x;
    x |= x >> 1;
    x |= x >> 2;
    x |= x >> 4;
    for (std::size_t i = 1; i < sizeof(T); i <<= 1) {
        x |= x >> (i << 3);
    }
    ++x;
    return x;
}
 
template <typename T> static inline void swap_relaxed(std::atomic<T>& left, std::atomic<T>& right)
{
    T temp = std::move(left.load(std::memory_order_relaxed));
    left.store(std::move(right.load(std::memory_order_relaxed)), std::memory_order_relaxed);
    right.store(std::move(temp), std::memory_order_relaxed);
}
 
template <typename T> static inline T const& nomove(T const& x)
{
    return x;
}
 
template <bool Enable> struct nomove_if {
    template <typename T> static inline T const& eval(T const& x) { return x; }
};
 
template <> struct nomove_if<false> {
    template <typename U> static inline auto eval(U&& x) -> decltype(std::forward<U>(x)) { return std::forward<U>(x); }
};
 
template <typename It> static inline auto deref_noexcept(It& it) MOODYCAMEL_NOEXCEPT->decltype(*it)
{
    return *it;
}
 
#if defined(__clang__) || !defined(__GNUC__) || __GNUC__ > 4 || (__GNUC__ == 4 && __GNUC_MINOR__ >= 8)
template <typename T> struct is_trivially_destructible : std::is_trivially_destructible<T> {};
#else
template <typename T> struct is_trivially_destructible : std::has_trivial_destructor<T> {};
#endif
 
#ifdef MOODYCAMEL_CPP11_THREAD_LOCAL_SUPPORTED
#ifdef MCDBGQ_USE_RELACY
typedef RelacyThreadExitListener ThreadExitListener;
typedef RelacyThreadExitNotifier ThreadExitNotifier;
#else
class ThreadExitNotifier;
 
struct ThreadExitListener {
    typedef void (*callback_t)(void*);
    callback_t callback;
    void* userData;
 
    ThreadExitListener* next;  // reserved for use by the ThreadExitNotifier
    ThreadExitNotifier* chain; // reserved for use by the ThreadExitNotifier
};
 
class ThreadExitNotifier {
  public:
    static void subscribe(ThreadExitListener* listener)
    {
        auto& tlsInst = instance();
        std::lock_guard<std::mutex> guard(mutex());
        listener->next = tlsInst.tail;
        listener->chain = &tlsInst;
        tlsInst.tail = listener;
    }
 
    static void unsubscribe(ThreadExitListener* listener)
    {
        std::lock_guard<std::mutex> guard(mutex());
        if (!listener->chain) {
            return; // race with ~ThreadExitNotifier
        }
        auto& tlsInst = *listener->chain;
        listener->chain = nullptr;
        ThreadExitListener** prev = &tlsInst.tail;
        for (auto ptr = tlsInst.tail; ptr != nullptr; ptr = ptr->next) {
            if (ptr == listener) {
                *prev = ptr->next;
                break;
            }
            prev = &ptr->next;
        }
    }
 
  private:
    ThreadExitNotifier()
        : tail(nullptr)
    {}
    ThreadExitNotifier(ThreadExitNotifier const&) MOODYCAMEL_DELETE_FUNCTION;
    ThreadExitNotifier& operator=(ThreadExitNotifier const&) MOODYCAMEL_DELETE_FUNCTION;
 
    ~ThreadExitNotifier()
    {
        // This thread is about to exit, let everyone know!
        assert(this == &instance() &&
               "If this assert fails, you likely have a buggy compiler! Change the preprocessor conditions such that "
               "MOODYCAMEL_CPP11_THREAD_LOCAL_SUPPORTED is no longer defined.");
        std::lock_guard<std::mutex> guard(mutex());
        for (auto ptr = tail; ptr != nullptr; ptr = ptr->next) {
            ptr->chain = nullptr;
            ptr->callback(ptr->userData);
        }
    }
 
    // Thread-local
    static inline ThreadExitNotifier& instance()
    {
        static thread_local ThreadExitNotifier notifier;
        return notifier;
    }
 
    static inline std::mutex& mutex()
    {
        // Must be static because the ThreadExitNotifier could be destroyed while unsubscribe is called
        static std::mutex mutex;
        return mutex;
    }
 
  private:
    ThreadExitListener* tail;
};
#endif
#endif
 
template <typename T> struct static_is_lock_free_num {
    enum { value = 0 };
};
template <> struct static_is_lock_free_num<signed char> {
    enum { value = ATOMIC_CHAR_LOCK_FREE };
};
template <> struct static_is_lock_free_num<short> {
    enum { value = ATOMIC_SHORT_LOCK_FREE };
};
template <> struct static_is_lock_free_num<int> {
    enum { value = ATOMIC_INT_LOCK_FREE };
};
template <> struct static_is_lock_free_num<long> {
    enum { value = ATOMIC_LONG_LOCK_FREE };
};
template <> struct static_is_lock_free_num<long long> {
    enum { value = ATOMIC_LLONG_LOCK_FREE };
};
template <typename T> struct static_is_lock_free : static_is_lock_free_num<typename std::make_signed<T>::type> {};
template <> struct static_is_lock_free<bool> {
    enum { value = ATOMIC_BOOL_LOCK_FREE };
};
template <typename U> struct static_is_lock_free<U*> {
    enum { value = ATOMIC_POINTER_LOCK_FREE };
};
} // namespace details
 
struct ProducerToken {
    template <typename T, typename Traits> explicit ProducerToken(ConcurrentQueue<T, Traits>& queue);
 
    template <typename T, typename Traits> explicit ProducerToken(BlockingConcurrentQueue<T, Traits>& queue);
 
    ProducerToken(ProducerToken&& other) MOODYCAMEL_NOEXCEPT : producer(other.producer)
    {
        other.producer = nullptr;
        if (producer != nullptr) {
            producer->token = this;
        }
    }
 
    inline ProducerToken& operator=(ProducerToken&& other) MOODYCAMEL_NOEXCEPT
    {
        swap(other);
        return *this;
    }
 
    void swap(ProducerToken& other) MOODYCAMEL_NOEXCEPT
    {
        std::swap(producer, other.producer);
        if (producer != nullptr) {
            producer->token = this;
        }
        if (other.producer != nullptr) {
            other.producer->token = &other;
        }
    }
 
    // A token is always valid unless:
    //     1) Memory allocation failed during construction
    //     2) It was moved via the move constructor
    //        (Note: assignment does a swap, leaving both potentially valid)
    //     3) The associated queue was destroyed
    // Note that if valid() returns true, that only indicates
    // that the token is valid for use with a specific queue,
    // but not which one; that's up to the user to track.
    inline bool valid() const { return producer != nullptr; }
 
    ~ProducerToken()
    {
        if (producer != nullptr) {
            producer->token = nullptr;
            producer->inactive.store(true, std::memory_order_release);
        }
    }
 
    // Disable copying and assignment
    ProducerToken(ProducerToken const&) MOODYCAMEL_DELETE_FUNCTION;
    ProducerToken& operator=(ProducerToken const&) MOODYCAMEL_DELETE_FUNCTION;
 
  private:
    template <typename T, typename Traits> friend class ConcurrentQueue;
    friend class ConcurrentQueueTests;
 
  protected:
    details::ConcurrentQueueProducerTypelessBase* producer;
};
 
struct ConsumerToken {
    template <typename T, typename Traits> explicit ConsumerToken(ConcurrentQueue<T, Traits>& q);
 
    template <typename T, typename Traits> explicit ConsumerToken(BlockingConcurrentQueue<T, Traits>& q);
 
    ConsumerToken(ConsumerToken&& other) MOODYCAMEL_NOEXCEPT : initialOffset(other.initialOffset),
                                                               lastKnownGlobalOffset(other.lastKnownGlobalOffset),
                                                               itemsConsumedFromCurrent(other.itemsConsumedFromCurrent),
                                                               currentProducer(other.currentProducer),
                                                               desiredProducer(other.desiredProducer)
    {}
 
    inline ConsumerToken& operator=(ConsumerToken&& other) MOODYCAMEL_NOEXCEPT
    {
        swap(other);
        return *this;
    }
 
    void swap(ConsumerToken& other) MOODYCAMEL_NOEXCEPT
    {
        std::swap(initialOffset, other.initialOffset);
        std::swap(lastKnownGlobalOffset, other.lastKnownGlobalOffset);
        std::swap(itemsConsumedFromCurrent, other.itemsConsumedFromCurrent);
        std::swap(currentProducer, other.currentProducer);
        std::swap(desiredProducer, other.desiredProducer);
    }
 
    // Disable copying and assignment
    ConsumerToken(ConsumerToken const&) MOODYCAMEL_DELETE_FUNCTION;
    ConsumerToken& operator=(ConsumerToken const&) MOODYCAMEL_DELETE_FUNCTION;
 
  private:
    template <typename T, typename Traits> friend class ConcurrentQueue;
    friend class ConcurrentQueueTests;
 
  private: // but shared with ConcurrentQueue
    std::uint32_t initialOffset;
    std::uint32_t lastKnownGlobalOffset;
    std::uint32_t itemsConsumedFromCurrent;
    details::ConcurrentQueueProducerTypelessBase* currentProducer;
    details::ConcurrentQueueProducerTypelessBase* desiredProducer;
};
 
// Need to forward-declare this swap because it's in a namespace.
// See
// http://stackoverflow.com/questions/4492062/why-does-a-c-friend-class-need-a-forward-declaration-only-in-other-namespaces
template <typename T, typename Traits>
inline void swap(typename ConcurrentQueue<T, Traits>::ImplicitProducerKVP& a,
                 typename ConcurrentQueue<T, Traits>::ImplicitProducerKVP& b) MOODYCAMEL_NOEXCEPT;
 
template <typename T, typename Traits = ConcurrentQueueDefaultTraits> class ConcurrentQueue {
  public:
    typedef ::moodycamel::ProducerToken producer_token_t;
    typedef ::moodycamel::ConsumerToken consumer_token_t;
 
    typedef typename Traits::index_t index_t;
    typedef typename Traits::size_t size_t;
 
    static const size_t BLOCK_SIZE = static_cast<size_t>(Traits::BLOCK_SIZE);
    static const size_t EXPLICIT_BLOCK_EMPTY_COUNTER_THRESHOLD =
        static_cast<size_t>(Traits::EXPLICIT_BLOCK_EMPTY_COUNTER_THRESHOLD);
    static const size_t EXPLICIT_INITIAL_INDEX_SIZE = static_cast<size_t>(Traits::EXPLICIT_INITIAL_INDEX_SIZE);
    static const size_t IMPLICIT_INITIAL_INDEX_SIZE = static_cast<size_t>(Traits::IMPLICIT_INITIAL_INDEX_SIZE);
    static const size_t INITIAL_IMPLICIT_PRODUCER_HASH_SIZE =
        static_cast<size_t>(Traits::INITIAL_IMPLICIT_PRODUCER_HASH_SIZE);
    static const std::uint32_t EXPLICIT_CONSUMER_CONSUMPTION_QUOTA_BEFORE_ROTATE =
        static_cast<std::uint32_t>(Traits::EXPLICIT_CONSUMER_CONSUMPTION_QUOTA_BEFORE_ROTATE);
#ifdef _MSC_VER
#pragma warning(push)
#pragma warning(disable : 4307) // + integral constant overflow (that's what the ternary expression is for!)
#pragma warning(disable : 4309) // static_cast: Truncation of constant value
#endif
    static const size_t MAX_SUBQUEUE_SIZE =
        (details::const_numeric_max<size_t>::value - static_cast<size_t>(Traits::MAX_SUBQUEUE_SIZE) < BLOCK_SIZE)
            ? details::const_numeric_max<size_t>::value
            : ((static_cast<size_t>(Traits::MAX_SUBQUEUE_SIZE) + (BLOCK_SIZE - 1)) / BLOCK_SIZE * BLOCK_SIZE);
#ifdef _MSC_VER
#pragma warning(pop)
#endif
 
    static_assert(!std::numeric_limits<size_t>::is_signed && std::is_integral<size_t>::value,
                  "Traits::size_t must be an unsigned integral type");
    static_assert(!std::numeric_limits<index_t>::is_signed && std::is_integral<index_t>::value,
                  "Traits::index_t must be an unsigned integral type");
    static_assert(sizeof(index_t) >= sizeof(size_t), "Traits::index_t must be at least as wide as Traits::size_t");
    static_assert((BLOCK_SIZE > 1) && !(BLOCK_SIZE & (BLOCK_SIZE - 1)),
                  "Traits::BLOCK_SIZE must be a power of 2 (and at least 2)");
    static_assert((EXPLICIT_BLOCK_EMPTY_COUNTER_THRESHOLD > 1) &&
                      !(EXPLICIT_BLOCK_EMPTY_COUNTER_THRESHOLD & (EXPLICIT_BLOCK_EMPTY_COUNTER_THRESHOLD - 1)),
                  "Traits::EXPLICIT_BLOCK_EMPTY_COUNTER_THRESHOLD must be a power of 2 (and greater than 1)");
    static_assert((EXPLICIT_INITIAL_INDEX_SIZE > 1) &&
                      !(EXPLICIT_INITIAL_INDEX_SIZE & (EXPLICIT_INITIAL_INDEX_SIZE - 1)),
                  "Traits::EXPLICIT_INITIAL_INDEX_SIZE must be a power of 2 (and greater than 1)");
    static_assert((IMPLICIT_INITIAL_INDEX_SIZE > 1) &&
                      !(IMPLICIT_INITIAL_INDEX_SIZE & (IMPLICIT_INITIAL_INDEX_SIZE - 1)),
                  "Traits::IMPLICIT_INITIAL_INDEX_SIZE must be a power of 2 (and greater than 1)");
    static_assert((INITIAL_IMPLICIT_PRODUCER_HASH_SIZE == 0) ||
                      !(INITIAL_IMPLICIT_PRODUCER_HASH_SIZE & (INITIAL_IMPLICIT_PRODUCER_HASH_SIZE - 1)),
                  "Traits::INITIAL_IMPLICIT_PRODUCER_HASH_SIZE must be a power of 2");
    static_assert(
        INITIAL_IMPLICIT_PRODUCER_HASH_SIZE == 0 || INITIAL_IMPLICIT_PRODUCER_HASH_SIZE >= 1,
        "Traits::INITIAL_IMPLICIT_PRODUCER_HASH_SIZE must be at least 1 (or 0 to disable implicit enqueueing)");
 
  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 ConcurrentQueue(size_t capacity = 32 * BLOCK_SIZE)
        : producerListTail(nullptr)
        , producerCount(0)
        , initialBlockPoolIndex(0)
        , nextExplicitConsumerId(0)
        , globalExplicitConsumerOffset(0)
    {
        implicitProducerHashResizeInProgress.clear(std::memory_order_relaxed);
        populate_initial_implicit_producer_hash();
        populate_initial_block_list(capacity / BLOCK_SIZE + ((capacity & (BLOCK_SIZE - 1)) == 0 ? 0 : 1));
 
#ifdef MOODYCAMEL_QUEUE_INTERNAL_DEBUG
        // Track all the producers using a fully-resolved typed list for
        // each kind; this makes it possible to debug them starting from
        // the root queue object (otherwise wacky casts are needed that
        // don't compile in the debugger's expression evaluator).
        explicitProducers.store(nullptr, std::memory_order_relaxed);
        implicitProducers.store(nullptr, std::memory_order_relaxed);
#endif
    }
 
    // Computes the correct amount of pre-allocated blocks for you based
    // on the minimum number of elements you want available at any given
    // time, and the maximum concurrent number of each type of producer.
    ConcurrentQueue(size_t minCapacity, size_t maxExplicitProducers, size_t maxImplicitProducers)
        : producerListTail(nullptr)
        , producerCount(0)
        , initialBlockPoolIndex(0)
        , nextExplicitConsumerId(0)
        , globalExplicitConsumerOffset(0)
    {
        implicitProducerHashResizeInProgress.clear(std::memory_order_relaxed);
        populate_initial_implicit_producer_hash();
        size_t blocks = (((minCapacity + BLOCK_SIZE - 1) / BLOCK_SIZE) - 1) * (maxExplicitProducers + 1) +
                        2 * (maxExplicitProducers + maxImplicitProducers);
        populate_initial_block_list(blocks);
 
#ifdef MOODYCAMEL_QUEUE_INTERNAL_DEBUG
        explicitProducers.store(nullptr, std::memory_order_relaxed);
        implicitProducers.store(nullptr, std::memory_order_relaxed);
#endif
    }
 
    // Note: The queue should not be accessed concurrently while it's
    // being deleted. It's up to the user to synchronize this.
    // This method is not thread safe.
    ~ConcurrentQueue()
    {
        // Destroy producers
        auto ptr = producerListTail.load(std::memory_order_relaxed);
        while (ptr != nullptr) {
            auto next = ptr->next_prod();
            if (ptr->token != nullptr) {
                ptr->token->producer = nullptr;
            }
            destroy(ptr);
            ptr = next;
        }
 
        // Destroy implicit producer hash tables
        MOODYCAMEL_CONSTEXPR_IF(INITIAL_IMPLICIT_PRODUCER_HASH_SIZE != 0)
        {
            auto hash = implicitProducerHash.load(std::memory_order_relaxed);
            while (hash != nullptr) {
                auto prev = hash->prev;
                if (prev != nullptr) { // The last hash is part of this object and was not allocated dynamically
                    for (size_t i = 0; i != hash->capacity; ++i) {
                        hash->entries[i].~ImplicitProducerKVP();
                    }
                    hash->~ImplicitProducerHash();
                    (Traits::free)(hash);
                }
                hash = prev;
            }
        }
 
        // Destroy global free list
        auto block = freeList.head_unsafe();
        while (block != nullptr) {
            auto next = block->freeListNext.load(std::memory_order_relaxed);
            if (block->dynamicallyAllocated) {
                destroy(block);
            }
            block = next;
        }
 
        // Destroy initial free list
        destroy_array(initialBlockPool, initialBlockPoolSize);
    }
 
    // Disable copying and copy assignment
    ConcurrentQueue(ConcurrentQueue const&) MOODYCAMEL_DELETE_FUNCTION;
    ConcurrentQueue& operator=(ConcurrentQueue 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).
    ConcurrentQueue(ConcurrentQueue&& other) MOODYCAMEL_NOEXCEPT
        : producerListTail(other.producerListTail.load(std::memory_order_relaxed)),
          producerCount(other.producerCount.load(std::memory_order_relaxed)),
          initialBlockPoolIndex(other.initialBlockPoolIndex.load(std::memory_order_relaxed)),
          initialBlockPool(other.initialBlockPool),
          initialBlockPoolSize(other.initialBlockPoolSize),
          freeList(std::move(other.freeList)),
          nextExplicitConsumerId(other.nextExplicitConsumerId.load(std::memory_order_relaxed)),
          globalExplicitConsumerOffset(other.globalExplicitConsumerOffset.load(std::memory_order_relaxed))
    {
        // Move the other one into this, and leave the other one as an empty queue
        implicitProducerHashResizeInProgress.clear(std::memory_order_relaxed);
        populate_initial_implicit_producer_hash();
        swap_implicit_producer_hashes(other);
 
        other.producerListTail.store(nullptr, std::memory_order_relaxed);
        other.producerCount.store(0, std::memory_order_relaxed);
        other.nextExplicitConsumerId.store(0, std::memory_order_relaxed);
        other.globalExplicitConsumerOffset.store(0, std::memory_order_relaxed);
 
#ifdef MOODYCAMEL_QUEUE_INTERNAL_DEBUG
        explicitProducers.store(other.explicitProducers.load(std::memory_order_relaxed), std::memory_order_relaxed);
        other.explicitProducers.store(nullptr, std::memory_order_relaxed);
        implicitProducers.store(other.implicitProducers.load(std::memory_order_relaxed), std::memory_order_relaxed);
        other.implicitProducers.store(nullptr, std::memory_order_relaxed);
#endif
 
        other.initialBlockPoolIndex.store(0, std::memory_order_relaxed);
        other.initialBlockPoolSize = 0;
        other.initialBlockPool = nullptr;
 
        reown_producers();
    }
 
    inline ConcurrentQueue& operator=(ConcurrentQueue&& 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(ConcurrentQueue& other) MOODYCAMEL_NOEXCEPT { swap_internal(other); }
 
  private:
    ConcurrentQueue& swap_internal(ConcurrentQueue& other)
    {
        if (this == &other) {
            return *this;
        }
 
        details::swap_relaxed(producerListTail, other.producerListTail);
        details::swap_relaxed(producerCount, other.producerCount);
        details::swap_relaxed(initialBlockPoolIndex, other.initialBlockPoolIndex);
        std::swap(initialBlockPool, other.initialBlockPool);
        std::swap(initialBlockPoolSize, other.initialBlockPoolSize);
        freeList.swap(other.freeList);
        details::swap_relaxed(nextExplicitConsumerId, other.nextExplicitConsumerId);
        details::swap_relaxed(globalExplicitConsumerOffset, other.globalExplicitConsumerOffset);
 
        swap_implicit_producer_hashes(other);
 
        reown_producers();
        other.reown_producers();
 
#ifdef MOODYCAMEL_QUEUE_INTERNAL_DEBUG
        details::swap_relaxed(explicitProducers, other.explicitProducers);
        details::swap_relaxed(implicitProducers, other.implicitProducers);
#endif
 
        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)
    {
        MOODYCAMEL_CONSTEXPR_IF(INITIAL_IMPLICIT_PRODUCER_HASH_SIZE == 0) return false;
        else return inner_enqueue<CanAlloc>(item);
    }
 
    // 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)
    {
        MOODYCAMEL_CONSTEXPR_IF(INITIAL_IMPLICIT_PRODUCER_HASH_SIZE == 0) return false;
        else return inner_enqueue<CanAlloc>(std::move(item));
    }
 
    // 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) { return inner_enqueue<CanAlloc>(token, item); }
 
    // 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)
    {
        return inner_enqueue<CanAlloc>(token, std::move(item));
    }
 
    // 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> bool enqueue_bulk(It itemFirst, size_t count)
    {
        MOODYCAMEL_CONSTEXPR_IF(INITIAL_IMPLICIT_PRODUCER_HASH_SIZE == 0) return false;
        else return inner_enqueue_bulk<CanAlloc>(itemFirst, count);
    }
 
    // 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> bool enqueue_bulk(producer_token_t const& token, It itemFirst, size_t count)
    {
        return inner_enqueue_bulk<CanAlloc>(token, itemFirst, count);
    }
 
    // 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)
    {
        MOODYCAMEL_CONSTEXPR_IF(INITIAL_IMPLICIT_PRODUCER_HASH_SIZE == 0) return false;
        else return inner_enqueue<CannotAlloc>(item);
    }
 
    // 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)
    {
        MOODYCAMEL_CONSTEXPR_IF(INITIAL_IMPLICIT_PRODUCER_HASH_SIZE == 0) return false;
        else return inner_enqueue<CannotAlloc>(std::move(item));
    }
 
    // 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)
    {
        return inner_enqueue<CannotAlloc>(token, item);
    }
 
    // 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)
    {
        return inner_enqueue<CannotAlloc>(token, std::move(item));
    }
 
    // 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> bool try_enqueue_bulk(It itemFirst, size_t count)
    {
        MOODYCAMEL_CONSTEXPR_IF(INITIAL_IMPLICIT_PRODUCER_HASH_SIZE == 0) return false;
        else return inner_enqueue_bulk<CannotAlloc>(itemFirst, count);
    }
 
    // 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> bool try_enqueue_bulk(producer_token_t const& token, It itemFirst, size_t count)
    {
        return inner_enqueue_bulk<CannotAlloc>(token, itemFirst, count);
    }
 
    // 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> bool try_dequeue(U& item)
    {
        // Instead of simply trying each producer in turn (which could cause needless contention on the first
        // producer), we score them heuristically.
        size_t nonEmptyCount = 0;
        ProducerBase* best = nullptr;
        size_t bestSize = 0;
        for (auto ptr = producerListTail.load(std::memory_order_acquire); nonEmptyCount < 3 && ptr != nullptr;
             ptr = ptr->next_prod()) {
            auto size = ptr->size_approx();
            if (size > 0) {
                if (size > bestSize) {
                    bestSize = size;
                    best = ptr;
                }
                ++nonEmptyCount;
            }
        }
 
        // If there was at least one non-empty queue but it appears empty at the time
        // we try to dequeue from it, we need to make sure every queue's been tried
        if (nonEmptyCount > 0) {
            if ((details::likely)(best->dequeue(item))) {
                return true;
            }
            for (auto ptr = producerListTail.load(std::memory_order_acquire); ptr != nullptr; ptr = ptr->next_prod()) {
                if (ptr != best && ptr->dequeue(item)) {
                    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).
    // This differs from the try_dequeue(item) method in that this one does
    // not attempt to reduce contention by interleaving the order that producer
    // streams are dequeued from. So, using this method can reduce overall throughput
    // under contention, but will give more predictable results in single-threaded
    // consumer scenarios. This is mostly only useful for internal unit tests.
    // Never allocates. Thread-safe.
    template <typename U> bool try_dequeue_non_interleaved(U& item)
    {
        for (auto ptr = producerListTail.load(std::memory_order_acquire); ptr != nullptr; ptr = ptr->next_prod()) {
            if (ptr->dequeue(item)) {
                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> bool try_dequeue(consumer_token_t& token, U& item)
    {
        // The idea is roughly as follows:
        // Every 256 items from one producer, make everyone rotate (increase the global offset) -> this means the
        // highest efficiency consumer dictates the rotation speed of everyone else, more or less If you see that the
        // global offset has changed, you must reset your consumption counter and move to your designated place If
        // there's no items where you're supposed to be, keep moving until you find a producer with some items If the
        // global offset has not changed but you've run out of items to consume, move over from your current position
        // until you find an producer with something in it
 
        if (token.desiredProducer == nullptr ||
            token.lastKnownGlobalOffset != globalExplicitConsumerOffset.load(std::memory_order_relaxed)) {
            if (!update_current_producer_after_rotation(token)) {
                return false;
            }
        }
 
        // If there was at least one non-empty queue but it appears empty at the time
        // we try to dequeue from it, we need to make sure every queue's been tried
        if (static_cast<ProducerBase*>(token.currentProducer)->dequeue(item)) {
            if (++token.itemsConsumedFromCurrent == EXPLICIT_CONSUMER_CONSUMPTION_QUOTA_BEFORE_ROTATE) {
                globalExplicitConsumerOffset.fetch_add(1, std::memory_order_relaxed);
            }
            return true;
        }
 
        auto tail = producerListTail.load(std::memory_order_acquire);
        auto ptr = static_cast<ProducerBase*>(token.currentProducer)->next_prod();
        if (ptr == nullptr) {
            ptr = tail;
        }
        while (ptr != static_cast<ProducerBase*>(token.currentProducer)) {
            if (ptr->dequeue(item)) {
                token.currentProducer = ptr;
                token.itemsConsumedFromCurrent = 1;
                return true;
            }
            ptr = ptr->next_prod();
            if (ptr == nullptr) {
                ptr = tail;
            }
        }
        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> size_t try_dequeue_bulk(It itemFirst, size_t max)
    {
        size_t count = 0;
        for (auto ptr = producerListTail.load(std::memory_order_acquire); ptr != nullptr; ptr = ptr->next_prod()) {
            count += ptr->dequeue_bulk(itemFirst, max - count);
            if (count == max) {
                break;
            }
        }
        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> size_t try_dequeue_bulk(consumer_token_t& token, It itemFirst, size_t max)
    {
        if (token.desiredProducer == nullptr ||
            token.lastKnownGlobalOffset != globalExplicitConsumerOffset.load(std::memory_order_relaxed)) {
            if (!update_current_producer_after_rotation(token)) {
                return 0;
            }
        }
 
        size_t count = static_cast<ProducerBase*>(token.currentProducer)->dequeue_bulk(itemFirst, max);
        if (count == max) {
            if ((token.itemsConsumedFromCurrent += static_cast<std::uint32_t>(max)) >=
                EXPLICIT_CONSUMER_CONSUMPTION_QUOTA_BEFORE_ROTATE) {
                globalExplicitConsumerOffset.fetch_add(1, std::memory_order_relaxed);
            }
            return max;
        }
        token.itemsConsumedFromCurrent += static_cast<std::uint32_t>(count);
        max -= count;
 
        auto tail = producerListTail.load(std::memory_order_acquire);
        auto ptr = static_cast<ProducerBase*>(token.currentProducer)->next_prod();
        if (ptr == nullptr) {
            ptr = tail;
        }
        while (ptr != static_cast<ProducerBase*>(token.currentProducer)) {
            auto dequeued = ptr->dequeue_bulk(itemFirst, max);
            count += dequeued;
            if (dequeued != 0) {
                token.currentProducer = ptr;
                token.itemsConsumedFromCurrent = static_cast<std::uint32_t>(dequeued);
            }
            if (dequeued == max) {
                break;
            }
            max -= dequeued;
            ptr = ptr->next_prod();
            if (ptr == nullptr) {
                ptr = tail;
            }
        }
        return count;
    }
 
    // Attempts to dequeue from a specific producer's inner queue.
    // If you happen to know which producer you want to dequeue from, this
    // is significantly faster than using the general-case try_dequeue methods.
    // Returns false if the producer's queue appeared empty at the time it
    // was checked (so, the queue is likely but not guaranteed to be empty).
    // Never allocates. Thread-safe.
    template <typename U> inline bool try_dequeue_from_producer(producer_token_t const& producer, U& item)
    {
        return static_cast<ExplicitProducer*>(producer.producer)->dequeue(item);
    }
 
    // Attempts to dequeue several elements from a specific producer's inner queue.
    // Returns the number of items actually dequeued.
    // If you happen to know which producer you want to dequeue from, this
    // is significantly faster than using the general-case try_dequeue methods.
    // Returns 0 if the producer's queue appeared empty at the time it
    // was 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_from_producer(producer_token_t const& producer, It itemFirst, size_t max)
    {
        return static_cast<ExplicitProducer*>(producer.producer)->dequeue_bulk(itemFirst, max);
    }
 
    // 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.
    size_t size_approx() const
    {
        size_t size = 0;
        for (auto ptr = producerListTail.load(std::memory_order_acquire); ptr != nullptr; ptr = ptr->next_prod()) {
            size += ptr->size_approx();
        }
        return size;
    }
 
    // Returns true if the underlying atomic variables used by
    // the queue are lock-free (they should be on most platforms).
    // Thread-safe.
    static constexpr bool is_lock_free()
    {
        return details::static_is_lock_free<bool>::value == 2 && details::static_is_lock_free<size_t>::value == 2 &&
               details::static_is_lock_free<std::uint32_t>::value == 2 &&
               details::static_is_lock_free<index_t>::value == 2 && details::static_is_lock_free<void*>::value == 2 &&
               details::static_is_lock_free<
                   typename details::thread_id_converter<details::thread_id_t>::thread_id_numeric_size_t>::value == 2;
    }
 
  private:
    friend struct ProducerToken;
    friend struct ConsumerToken;
    struct ExplicitProducer;
    friend struct ExplicitProducer;
    struct ImplicitProducer;
    friend struct ImplicitProducer;
    friend class ConcurrentQueueTests;
 
    enum AllocationMode { CanAlloc, CannotAlloc };
 
    // Queue methods
 
    template <AllocationMode canAlloc, typename U> inline bool inner_enqueue(producer_token_t const& token, U&& element)
    {
        return static_cast<ExplicitProducer*>(token.producer)
            ->ConcurrentQueue::ExplicitProducer::template enqueue<canAlloc>(std::forward<U>(element));
    }
 
    template <AllocationMode canAlloc, typename U> inline bool inner_enqueue(U&& element)
    {
        auto producer = get_or_add_implicit_producer();
        return producer == nullptr
                   ? false
                   : producer->ConcurrentQueue::ImplicitProducer::template enqueue<canAlloc>(std::forward<U>(element));
    }
 
    template <AllocationMode canAlloc, typename It>
    inline bool inner_enqueue_bulk(producer_token_t const& token, It itemFirst, size_t count)
    {
        return static_cast<ExplicitProducer*>(token.producer)
            ->ConcurrentQueue::ExplicitProducer::template enqueue_bulk<canAlloc>(itemFirst, count);
    }
 
    template <AllocationMode canAlloc, typename It> inline bool inner_enqueue_bulk(It itemFirst, size_t count)
    {
        auto producer = get_or_add_implicit_producer();
        return producer == nullptr
                   ? false
                   : producer->ConcurrentQueue::ImplicitProducer::template enqueue_bulk<canAlloc>(itemFirst, count);
    }
 
    inline bool update_current_producer_after_rotation(consumer_token_t& token)
    {
        // Ah, there's been a rotation, figure out where we should be!
        auto tail = producerListTail.load(std::memory_order_acquire);
        if (token.desiredProducer == nullptr && tail == nullptr) {
            return false;
        }
        auto prodCount = producerCount.load(std::memory_order_relaxed);
        auto globalOffset = globalExplicitConsumerOffset.load(std::memory_order_relaxed);
        if ((details::unlikely)(token.desiredProducer == nullptr)) {
            // Aha, first time we're dequeueing anything.
            // Figure out our local position
            // Note: offset is from start, not end, but we're traversing from end -- subtract from count first
            std::uint32_t offset = prodCount - 1 - (token.initialOffset % prodCount);
            token.desiredProducer = tail;
            for (std::uint32_t i = 0; i != offset; ++i) {
                token.desiredProducer = static_cast<ProducerBase*>(token.desiredProducer)->next_prod();
                if (token.desiredProducer == nullptr) {
                    token.desiredProducer = tail;
                }
            }
        }
 
        std::uint32_t delta = globalOffset - token.lastKnownGlobalOffset;
        if (delta >= prodCount) {
            delta = delta % prodCount;
        }
        for (std::uint32_t i = 0; i != delta; ++i) {
            token.desiredProducer = static_cast<ProducerBase*>(token.desiredProducer)->next_prod();
            if (token.desiredProducer == nullptr) {
                token.desiredProducer = tail;
            }
        }
 
        token.lastKnownGlobalOffset = globalOffset;
        token.currentProducer = token.desiredProducer;
        token.itemsConsumedFromCurrent = 0;
        return true;
    }
 
    // Free list
 
    template <typename N> struct FreeListNode {
        FreeListNode()
            : freeListRefs(0)
            , freeListNext(nullptr)
        {}
 
        std::atomic<std::uint32_t> freeListRefs;
        std::atomic<N*> freeListNext;
    };
 
    // A simple CAS-based lock-free free list. Not the fastest thing in the world under heavy contention, but
    // simple and correct (assuming nodes are never freed until after the free list is destroyed), and fairly
    // speedy under low contention.
    template <typename N> // N must inherit FreeListNode or have the same fields (and initialization of them)
    struct FreeList {
        FreeList()
            : freeListHead(nullptr)
        {}
        FreeList(FreeList&& other)
            : freeListHead(other.freeListHead.load(std::memory_order_relaxed))
        {
            other.freeListHead.store(nullptr, std::memory_order_relaxed);
        }
        void swap(FreeList& other) { details::swap_relaxed(freeListHead, other.freeListHead); }
 
        FreeList(FreeList const&) MOODYCAMEL_DELETE_FUNCTION;
        FreeList& operator=(FreeList const&) MOODYCAMEL_DELETE_FUNCTION;
 
        inline void add(N* node)
        {
#ifdef MCDBGQ_NOLOCKFREE_FREELIST
            debug::DebugLock lock(mutex);
#endif
            // We know that the should-be-on-freelist bit is 0 at this point, so it's safe to
            // set it using a fetch_add
            if (node->freeListRefs.fetch_add(SHOULD_BE_ON_FREELIST, std::memory_order_acq_rel) == 0) {
                // Oh look! We were the last ones referencing this node, and we know
                // we want to add it to the free list, so let's do it!
                add_knowing_refcount_is_zero(node);
            }
        }
 
        inline N* try_get()
        {
#ifdef MCDBGQ_NOLOCKFREE_FREELIST
            debug::DebugLock lock(mutex);
#endif
            auto head = freeListHead.load(std::memory_order_acquire);
            while (head != nullptr) {
                auto prevHead = head;
                auto refs = head->freeListRefs.load(std::memory_order_relaxed);
                if ((refs & REFS_MASK) == 0 ||
                    !head->freeListRefs.compare_exchange_strong(
                        refs, refs + 1, std::memory_order_acquire, std::memory_order_relaxed)) {
                    head = freeListHead.load(std::memory_order_acquire);
                    continue;
                }
 
                // Good, reference count has been incremented (it wasn't at zero), which means we can read the
                // next and not worry about it changing between now and the time we do the CAS
                auto next = head->freeListNext.load(std::memory_order_relaxed);
                if (freeListHead.compare_exchange_strong(
                        head, next, std::memory_order_acquire, std::memory_order_relaxed)) {
                    // Yay, got the node. This means it was on the list, which means shouldBeOnFreeList must be false no
                    // matter the refcount (because nobody else knows it's been taken off yet, it can't have been put
                    // back on).
                    assert((head->freeListRefs.load(std::memory_order_relaxed) & SHOULD_BE_ON_FREELIST) == 0);
 
                    // Decrease refcount twice, once for our ref, and once for the list's ref
                    head->freeListRefs.fetch_sub(2, std::memory_order_release);
                    return head;
                }
 
                // OK, the head must have changed on us, but we still need to decrease the refcount we increased.
                // Note that we don't need to release any memory effects, but we do need to ensure that the reference
                // count decrement happens-after the CAS on the head.
                refs = prevHead->freeListRefs.fetch_sub(1, std::memory_order_acq_rel);
                if (refs == SHOULD_BE_ON_FREELIST + 1) {
                    add_knowing_refcount_is_zero(prevHead);
                }
            }
 
            return nullptr;
        }
 
        // Useful for traversing the list when there's no contention (e.g. to destroy remaining nodes)
        N* head_unsafe() const { return freeListHead.load(std::memory_order_relaxed); }
 
      private:
        inline void add_knowing_refcount_is_zero(N* node)
        {
            // Since the refcount is zero, and nobody can increase it once it's zero (except us, and we run
            // only one copy of this method per node at a time, i.e. the single thread case), then we know
            // we can safely change the next pointer of the node; however, once the refcount is back above
            // zero, then other threads could increase it (happens under heavy contention, when the refcount
            // goes to zero in between a load and a refcount increment of a node in try_get, then back up to
            // something non-zero, then the refcount increment is done by the other thread) -- so, if the CAS
            // to add the node to the actual list fails, decrease the refcount and leave the add operation to
            // the next thread who puts the refcount back at zero (which could be us, hence the loop).
            auto head = freeListHead.load(std::memory_order_relaxed);
            while (true) {
                node->freeListNext.store(head, std::memory_order_relaxed);
                node->freeListRefs.store(1, std::memory_order_release);
                if (!freeListHead.compare_exchange_strong(
                        head, node, std::memory_order_release, std::memory_order_relaxed)) {
                    // Hmm, the add failed, but we can only try again when the refcount goes back to zero
                    if (node->freeListRefs.fetch_add(SHOULD_BE_ON_FREELIST - 1, std::memory_order_release) == 1) {
                        continue;
                    }
                }
                return;
            }
        }
 
      private:
        // Implemented like a stack, but where node order doesn't matter (nodes are inserted out of order under
        // contention)
        std::atomic<N*> freeListHead;
 
        static const std::uint32_t REFS_MASK = 0x7FFFFFFF;
        static const std::uint32_t SHOULD_BE_ON_FREELIST = 0x80000000;
 
#ifdef MCDBGQ_NOLOCKFREE_FREELIST
        debug::DebugMutex mutex;
#endif
    };
 
    // Block
 
    enum InnerQueueContext { implicit_context = 0, explicit_context = 1 };
 
    struct Block {
        Block()
            : next(nullptr)
            , elementsCompletelyDequeued(0)
            , freeListRefs(0)
            , freeListNext(nullptr)
            , dynamicallyAllocated(true)
        {
#ifdef MCDBGQ_TRACKMEM
            owner = nullptr;
#endif
        }
 
        template <InnerQueueContext context> inline bool is_empty() const
        {
            MOODYCAMEL_CONSTEXPR_IF(context == explicit_context && BLOCK_SIZE <= EXPLICIT_BLOCK_EMPTY_COUNTER_THRESHOLD)
            {
                // Check flags
                for (size_t i = 0; i < BLOCK_SIZE; ++i) {
                    if (!emptyFlags[i].load(std::memory_order_relaxed)) {
                        return false;
                    }
                }
 
                // Aha, empty; make sure we have all other memory effects that happened before the empty flags were set
                std::atomic_thread_fence(std::memory_order_acquire);
                return true;
            }
            else
            {
                // Check counter
                if (elementsCompletelyDequeued.load(std::memory_order_relaxed) == BLOCK_SIZE) {
                    std::atomic_thread_fence(std::memory_order_acquire);
                    return true;
                }
                assert(elementsCompletelyDequeued.load(std::memory_order_relaxed) <= BLOCK_SIZE);
                return false;
            }
        }
 
        // Returns true if the block is now empty (does not apply in explicit context)
        template <InnerQueueContext context> inline bool set_empty(MOODYCAMEL_MAYBE_UNUSED index_t i)
        {
            MOODYCAMEL_CONSTEXPR_IF(context == explicit_context && BLOCK_SIZE <= EXPLICIT_BLOCK_EMPTY_COUNTER_THRESHOLD)
            {
                // Set flag
                assert(!emptyFlags[BLOCK_SIZE - 1 - static_cast<size_t>(i & static_cast<index_t>(BLOCK_SIZE - 1))].load(
                    std::memory_order_relaxed));
                emptyFlags[BLOCK_SIZE - 1 - static_cast<size_t>(i & static_cast<index_t>(BLOCK_SIZE - 1))].store(
                    true, std::memory_order_release);
                return false;
            }
            else
            {
                // Increment counter
                auto prevVal = elementsCompletelyDequeued.fetch_add(1, std::memory_order_release);
                assert(prevVal < BLOCK_SIZE);
                return prevVal == BLOCK_SIZE - 1;
            }
        }
 
        // Sets multiple contiguous item statuses to 'empty' (assumes no wrapping and count > 0).
        // Returns true if the block is now empty (does not apply in explicit context).
        template <InnerQueueContext context> inline bool set_many_empty(MOODYCAMEL_MAYBE_UNUSED index_t i, size_t count)
        {
            MOODYCAMEL_CONSTEXPR_IF(context == explicit_context && BLOCK_SIZE <= EXPLICIT_BLOCK_EMPTY_COUNTER_THRESHOLD)
            {
                // Set flags
                std::atomic_thread_fence(std::memory_order_release);
                i = BLOCK_SIZE - 1 - static_cast<size_t>(i & static_cast<index_t>(BLOCK_SIZE - 1)) - count + 1;
                for (size_t j = 0; j != count; ++j) {
                    assert(!emptyFlags[i + j].load(std::memory_order_relaxed));
                    emptyFlags[i + j].store(true, std::memory_order_relaxed);
                }
                return false;
            }
            else
            {
                // Increment counter
                auto prevVal = elementsCompletelyDequeued.fetch_add(count, std::memory_order_release);
                assert(prevVal + count <= BLOCK_SIZE);
                return prevVal + count == BLOCK_SIZE;
            }
        }
 
        template <InnerQueueContext context> inline void set_all_empty()
        {
            MOODYCAMEL_CONSTEXPR_IF(context == explicit_context && BLOCK_SIZE <= EXPLICIT_BLOCK_EMPTY_COUNTER_THRESHOLD)
            {
                // Set all flags
                for (size_t i = 0; i != BLOCK_SIZE; ++i) {
                    emptyFlags[i].store(true, std::memory_order_relaxed);
                }
            }
            else
            {
                // Reset counter
                elementsCompletelyDequeued.store(BLOCK_SIZE, std::memory_order_relaxed);
            }
        }
 
        template <InnerQueueContext context> inline void reset_empty()
        {
            MOODYCAMEL_CONSTEXPR_IF(context == explicit_context && BLOCK_SIZE <= EXPLICIT_BLOCK_EMPTY_COUNTER_THRESHOLD)
            {
                // Reset flags
                for (size_t i = 0; i != BLOCK_SIZE; ++i) {
                    emptyFlags[i].store(false, std::memory_order_relaxed);
                }
            }
            else
            {
                // Reset counter
                elementsCompletelyDequeued.store(0, std::memory_order_relaxed);
            }
        }
 
        inline T* operator[](index_t idx) MOODYCAMEL_NOEXCEPT
        {
            return static_cast<T*>(static_cast<void*>(elements)) +
                   static_cast<size_t>(idx & static_cast<index_t>(BLOCK_SIZE - 1));
        }
        inline T const* operator[](index_t idx) const MOODYCAMEL_NOEXCEPT
        {
            return static_cast<T const*>(static_cast<void const*>(elements)) +
                   static_cast<size_t>(idx & static_cast<index_t>(BLOCK_SIZE - 1));
        }
 
      private:
        static_assert(std::alignment_of<T>::value <= sizeof(T),
                      "The queue does not support types with an alignment greater than their size at this time");
        MOODYCAMEL_ALIGNED_TYPE_LIKE(char[sizeof(T) * BLOCK_SIZE], T) elements;
 
      public:
        Block* next;
        std::atomic<size_t> elementsCompletelyDequeued;
        std::atomic<bool> emptyFlags[BLOCK_SIZE <= EXPLICIT_BLOCK_EMPTY_COUNTER_THRESHOLD ? BLOCK_SIZE : 1];
 
      public:
        std::atomic<std::uint32_t> freeListRefs;
        std::atomic<Block*> freeListNext;
        bool dynamicallyAllocated; // Perhaps a better name for this would be 'isNotPartOfInitialBlockPool'
 
#ifdef MCDBGQ_TRACKMEM
        void* owner;
#endif
    };
    static_assert(std::alignment_of<Block>::value >= std::alignment_of<T>::value,
                  "Internal error: Blocks must be at least as aligned as the type they are wrapping");
 
#ifdef MCDBGQ_TRACKMEM
  public:
    struct MemStats;
 
  private:
#endif
 
    // Producer base
 
    struct ProducerBase : public details::ConcurrentQueueProducerTypelessBase {
        ProducerBase(ConcurrentQueue* parent_, bool isExplicit_)
            : tailIndex(0)
            , headIndex(0)
            , dequeueOptimisticCount(0)
            , dequeueOvercommit(0)
            , tailBlock(nullptr)
            , isExplicit(isExplicit_)
            , parent(parent_)
        {}
 
        virtual ~ProducerBase() {}
 
        template <typename U> inline bool dequeue(U& element)
        {
            if (isExplicit) {
                return static_cast<ExplicitProducer*>(this)->dequeue(element);
            } else {
                return static_cast<ImplicitProducer*>(this)->dequeue(element);
            }
        }
 
        template <typename It> inline size_t dequeue_bulk(It& itemFirst, size_t max)
        {
            if (isExplicit) {
                return static_cast<ExplicitProducer*>(this)->dequeue_bulk(itemFirst, max);
            } else {
                return static_cast<ImplicitProducer*>(this)->dequeue_bulk(itemFirst, max);
            }
        }
 
        inline ProducerBase* next_prod() const { return static_cast<ProducerBase*>(next); }
 
        inline size_t size_approx() const
        {
            auto tail = tailIndex.load(std::memory_order_relaxed);
            auto head = headIndex.load(std::memory_order_relaxed);
            return details::circular_less_than(head, tail) ? static_cast<size_t>(tail - head) : 0;
        }
 
        inline index_t getTail() const { return tailIndex.load(std::memory_order_relaxed); }
 
      protected:
        std::atomic<index_t> tailIndex; // Where to enqueue to next
        std::atomic<index_t> headIndex; // Where to dequeue from next
 
        std::atomic<index_t> dequeueOptimisticCount;
        std::atomic<index_t> dequeueOvercommit;
 
        Block* tailBlock;
 
      public:
        bool isExplicit;
        ConcurrentQueue* parent;
 
      protected:
#ifdef MCDBGQ_TRACKMEM
        friend struct MemStats;
#endif
    };
 
    // Explicit queue
 
    struct ExplicitProducer : public ProducerBase {
        explicit ExplicitProducer(ConcurrentQueue* parent_)
            : ProducerBase(parent_, true)
            , blockIndex(nullptr)
            , pr_blockIndexSlotsUsed(0)
            , pr_blockIndexSize(EXPLICIT_INITIAL_INDEX_SIZE >> 1)
            , pr_blockIndexFront(0)
            , pr_blockIndexEntries(nullptr)
            , pr_blockIndexRaw(nullptr)
        {
            size_t poolBasedIndexSize = details::ceil_to_pow_2(parent_->initialBlockPoolSize) >> 1;
            if (poolBasedIndexSize > pr_blockIndexSize) {
                pr_blockIndexSize = poolBasedIndexSize;
            }
 
            new_block_index(
                0); // This creates an index with double the number of current entries, i.e. EXPLICIT_INITIAL_INDEX_SIZE
        }
 
        ~ExplicitProducer()
        {
            // Destruct any elements not yet dequeued.
            // Since we're in the destructor, we can assume all elements
            // are either completely dequeued or completely not (no halfways).
            if (this->tailBlock != nullptr) { // Note this means there must be a block index too
                // First find the block that's partially dequeued, if any
                Block* halfDequeuedBlock = nullptr;
                if ((this->headIndex.load(std::memory_order_relaxed) & static_cast<index_t>(BLOCK_SIZE - 1)) != 0) {
                    // The head's not on a block boundary, meaning a block somewhere is partially dequeued
                    // (or the head block is the tail block and was fully dequeued, but the head/tail are still not on a
                    // boundary)
                    size_t i = (pr_blockIndexFront - pr_blockIndexSlotsUsed) & (pr_blockIndexSize - 1);
                    while (details::circular_less_than<index_t>(pr_blockIndexEntries[i].base + BLOCK_SIZE,
                                                                this->headIndex.load(std::memory_order_relaxed))) {
                        i = (i + 1) & (pr_blockIndexSize - 1);
                    }
                    assert(details::circular_less_than<index_t>(pr_blockIndexEntries[i].base,
                                                                this->headIndex.load(std::memory_order_relaxed)));
                    halfDequeuedBlock = pr_blockIndexEntries[i].block;
                }
 
                // Start at the head block (note the first line in the loop gives us the head from the tail on the first
                // iteration)
                auto block = this->tailBlock;
                do {
                    block = block->next;
                    if (block->ConcurrentQueue::Block::template is_empty<explicit_context>()) {
                        continue;
                    }
 
                    size_t i = 0; // Offset into block
                    if (block == halfDequeuedBlock) {
                        i = static_cast<size_t>(this->headIndex.load(std::memory_order_relaxed) &
                                                static_cast<index_t>(BLOCK_SIZE - 1));
                    }
 
                    // Walk through all the items in the block; if this is the tail block, we need to stop when we reach
                    // the tail index
                    auto lastValidIndex =
                        (this->tailIndex.load(std::memory_order_relaxed) & static_cast<index_t>(BLOCK_SIZE - 1)) == 0
                            ? BLOCK_SIZE
                            : static_cast<size_t>(this->tailIndex.load(std::memory_order_relaxed) &
                                                  static_cast<index_t>(BLOCK_SIZE - 1));
                    while (i != BLOCK_SIZE && (block != this->tailBlock || i != lastValidIndex)) {
                        (*block)[i++]->~T();
                    }
                } while (block != this->tailBlock);
            }
 
            // Destroy all blocks that we own
            if (this->tailBlock != nullptr) {
                auto block = this->tailBlock;
                do {
                    auto nextBlock = block->next;
                    this->parent->add_block_to_free_list(block);
                    block = nextBlock;
                } while (block != this->tailBlock);
            }
 
            // Destroy the block indices
            auto header = static_cast<BlockIndexHeader*>(pr_blockIndexRaw);
            while (header != nullptr) {
                auto prev = static_cast<BlockIndexHeader*>(header->prev);
                header->~BlockIndexHeader();
                (Traits::free)(header);
                header = prev;
            }
        }
 
        template <AllocationMode allocMode, typename U> inline bool enqueue(U&& element)
        {
            index_t currentTailIndex = this->tailIndex.load(std::memory_order_relaxed);
            index_t newTailIndex = 1 + currentTailIndex;
            if ((currentTailIndex & static_cast<index_t>(BLOCK_SIZE - 1)) == 0) {
                // We reached the end of a block, start a new one
                auto startBlock = this->tailBlock;
                auto originalBlockIndexSlotsUsed = pr_blockIndexSlotsUsed;
                if (this->tailBlock != nullptr &&
                    this->tailBlock->next->ConcurrentQueue::Block::template is_empty<explicit_context>()) {
                    // We can re-use the block ahead of us, it's empty!
                    this->tailBlock = this->tailBlock->next;
                    this->tailBlock->ConcurrentQueue::Block::template reset_empty<explicit_context>();
 
                    // We'll put the block on the block index (guaranteed to be room since we're conceptually removing
                    // the last block from it first -- except instead of removing then adding, we can just overwrite).
                    // Note that there must be a valid block index here, since even if allocation failed in the ctor,
                    // it would have been re-attempted when adding the first block to the queue; since there is such
                    // a block, a block index must have been successfully allocated.
                } else {
                    // Whatever head value we see here is >= the last value we saw here (relatively),
                    // and <= its current value. Since we have the most recent tail, the head must be
                    // <= to it.
                    auto head = this->headIndex.load(std::memory_order_relaxed);
                    assert(!details::circular_less_than<index_t>(currentTailIndex, head));
                    if (!details::circular_less_than<index_t>(head, currentTailIndex + BLOCK_SIZE) ||
                        (MAX_SUBQUEUE_SIZE != details::const_numeric_max<size_t>::value &&
                         (MAX_SUBQUEUE_SIZE == 0 || MAX_SUBQUEUE_SIZE - BLOCK_SIZE < currentTailIndex - head))) {
                        // We can't enqueue in another block because there's not enough leeway -- the
                        // tail could surpass the head by the time the block fills up! (Or we'll exceed
                        // the size limit, if the second part of the condition was true.)
                        return false;
                    }
                    // We're going to need a new block; check that the block index has room
                    if (pr_blockIndexRaw == nullptr || pr_blockIndexSlotsUsed == pr_blockIndexSize) {
                        // Hmm, the circular block index is already full -- we'll need
                        // to allocate a new index. Note pr_blockIndexRaw can only be nullptr if
                        // the initial allocation failed in the constructor.
 
                        MOODYCAMEL_CONSTEXPR_IF(allocMode == CannotAlloc)
                        {
                            return false;
                        }
                        else if (!new_block_index(pr_blockIndexSlotsUsed))
                        {
                            return false;
                        }
                    }
 
                    // Insert a new block in the circular linked list
                    auto newBlock = this->parent->ConcurrentQueue::template requisition_block<allocMode>();
                    if (newBlock == nullptr) {
                        return false;
                    }
#ifdef MCDBGQ_TRACKMEM
                    newBlock->owner = this;
#endif
                    newBlock->ConcurrentQueue::Block::template reset_empty<explicit_context>();
                    if (this->tailBlock == nullptr) {
                        newBlock->next = newBlock;
                    } else {
                        newBlock->next = this->tailBlock->next;
                        this->tailBlock->next = newBlock;
                    }
                    this->tailBlock = newBlock;
                    ++pr_blockIndexSlotsUsed;
                }
 
                MOODYCAMEL_CONSTEXPR_IF(
                    !MOODYCAMEL_NOEXCEPT_CTOR(T, U, new (static_cast<T*>(nullptr)) T(std::forward<U>(element))))
                {
                    // The constructor may throw. We want the element not to appear in the queue in
                    // that case (without corrupting the queue):
                    MOODYCAMEL_TRY
                    {
                        new ((*this->tailBlock)[currentTailIndex]) T(std::forward<U>(element));
                    }
                    MOODYCAMEL_CATCH(...)
                    {
                        // Revert change to the current block, but leave the new block available
                        // for next time
                        pr_blockIndexSlotsUsed = originalBlockIndexSlotsUsed;
                        this->tailBlock = startBlock == nullptr ? this->tailBlock : startBlock;
                        MOODYCAMEL_RETHROW;
                    }
                }
                else
                {
                    (void)startBlock;
                    (void)originalBlockIndexSlotsUsed;
                }
 
                // Add block to block index
                auto& entry = blockIndex.load(std::memory_order_relaxed)->entries[pr_blockIndexFront];
                entry.base = currentTailIndex;
                entry.block = this->tailBlock;
                blockIndex.load(std::memory_order_relaxed)->front.store(pr_blockIndexFront, std::memory_order_release);
                pr_blockIndexFront = (pr_blockIndexFront + 1) & (pr_blockIndexSize - 1);
 
                MOODYCAMEL_CONSTEXPR_IF(
                    !MOODYCAMEL_NOEXCEPT_CTOR(T, U, new (static_cast<T*>(nullptr)) T(std::forward<U>(element))))
                {
                    this->tailIndex.store(newTailIndex, std::memory_order_release);
                    return true;
                }
            }
 
            // Enqueue
            new ((*this->tailBlock)[currentTailIndex]) T(std::forward<U>(element));
 
            this->tailIndex.store(newTailIndex, std::memory_order_release);
            return true;
        }
 
        template <typename U> bool dequeue(U& element)
        {
            auto tail = this->tailIndex.load(std::memory_order_relaxed);
            auto overcommit = this->dequeueOvercommit.load(std::memory_order_relaxed);
            if (details::circular_less_than<index_t>(
                    this->dequeueOptimisticCount.load(std::memory_order_relaxed) - overcommit, tail)) {
                // Might be something to dequeue, let's give it a try
 
                // Note that this if is purely for performance purposes in the common case when the queue is
                // empty and the values are eventually consistent -- we may enter here spuriously.
 
                // Note that whatever the values of overcommit and tail are, they are not going to change (unless we
                // change them) and must be the same value at this point (inside the if) as when the if condition was
                // evaluated.
 
                // We insert an acquire fence here to synchronize-with the release upon incrementing dequeueOvercommit
                // below. This ensures that whatever the value we got loaded into overcommit, the load of
                // dequeueOptisticCount in the fetch_add below will result in a value at least as recent as that (and
                // therefore at least as large). Note that I believe a compiler (signal) fence here would be sufficient
                // due to the nature of fetch_add (all read-modify-write operations are guaranteed to work on the latest
                // value in the modification order), but unfortunately that can't be shown to be correct using only the
                // C++11 standard. See
                // http://stackoverflow.com/questions/18223161/what-are-the-c11-memory-ordering-guarantees-in-this-corner-case
                std::atomic_thread_fence(std::memory_order_acquire);
 
                // Increment optimistic counter, then check if it went over the boundary
                auto myDequeueCount = this->dequeueOptimisticCount.fetch_add(1, std::memory_order_relaxed);
 
                // Note that since dequeueOvercommit must be <= dequeueOptimisticCount (because dequeueOvercommit is
                // only ever incremented after dequeueOptimisticCount -- this is enforced in the `else` block below),
                // and since we now have a version of dequeueOptimisticCount that is at least as recent as overcommit
                // (due to the release upon incrementing dequeueOvercommit and the acquire above that synchronizes with
                // it), overcommit <= myDequeueCount. However, we can't assert this since both dequeueOptimisticCount
                // and dequeueOvercommit may (independently) overflow; in such a case, though, the logic still holds
                // since the difference between the two is maintained.
 
                // Note that we reload tail here in case it changed; it will be the same value as before or greater,
                // since this load is sequenced after (happens after) the earlier load above. This is supported by
                // read-read coherency (as defined in the standard), explained here:
                // http://en.cppreference.com/w/cpp/atomic/memory_order
                tail = this->tailIndex.load(std::memory_order_acquire);
                if ((details::likely)(details::circular_less_than<index_t>(myDequeueCount - overcommit, tail))) {
                    // Guaranteed to be at least one element to dequeue!
 
                    // Get the index. Note that since there's guaranteed to be at least one element, this
                    // will never exceed tail. We need to do an acquire-release fence here since it's possible
                    // that whatever condition got us to this point was for an earlier enqueued element (that
                    // we already see the memory effects for), but that by the time we increment somebody else
                    // has incremented it, and we need to see the memory effects for *that* element, which is
                    // in such a case is necessarily visible on the thread that incremented it in the first
                    // place with the more current condition (they must have acquired a tail that is at least
                    // as recent).
                    auto index = this->headIndex.fetch_add(1, std::memory_order_acq_rel);
 
                    // Determine which block the element is in
 
                    auto localBlockIndex = blockIndex.load(std::memory_order_acquire);
                    auto localBlockIndexHead = localBlockIndex->front.load(std::memory_order_acquire);
 
                    // We need to be careful here about subtracting and dividing because of index wrap-around.
                    // When an index wraps, we need to preserve the sign of the offset when dividing it by the
                    // block size (in order to get a correct signed block count offset in all cases):
                    auto headBase = localBlockIndex->entries[localBlockIndexHead].base;
                    auto blockBaseIndex = index & ~static_cast<index_t>(BLOCK_SIZE - 1);
                    auto offset = static_cast<size_t>(
                        static_cast<typename std::make_signed<index_t>::type>(blockBaseIndex - headBase) /
                        static_cast<typename std::make_signed<index_t>::type>(BLOCK_SIZE));
                    auto block =
                        localBlockIndex->entries[(localBlockIndexHead + offset) & (localBlockIndex->size - 1)].block;
 
                    // Dequeue
                    auto& el = *((*block)[index]);
                    if (!MOODYCAMEL_NOEXCEPT_ASSIGN(T, T&&, element = std::move(el))) {
                        // Make sure the element is still fully dequeued and destroyed even if the assignment
                        // throws
                        struct Guard {
                            Block* block;
                            index_t index;
 
                            ~Guard()
                            {
                                (*block)[index]->~T();
                                block->ConcurrentQueue::Block::template set_empty<explicit_context>(index);
                            }
                        } guard = { block, index };
 
                        element = std::move(el); // NOLINT
                    } else {
                        element = std::move(el); // NOLINT
                        el.~T();                 // NOLINT
                        block->ConcurrentQueue::Block::template set_empty<explicit_context>(index);
                    }
 
                    return true;
                } else {
                    // Wasn't anything to dequeue after all; make the effective dequeue count eventually consistent
                    this->dequeueOvercommit.fetch_add(
                        1, std::memory_order_release); // Release so that the fetch_add on dequeueOptimisticCount is
                                                       // guaranteed to happen before this write
                }
            }
 
            return false;
        }
 
        template <AllocationMode allocMode, typename It>
        bool MOODYCAMEL_NO_TSAN enqueue_bulk(It itemFirst, size_t count)
        {
            // First, we need to make sure we have enough room to enqueue all of the elements;
            // this means pre-allocating blocks and putting them in the block index (but only if
            // all the allocations succeeded).
            index_t startTailIndex = this->tailIndex.load(std::memory_order_relaxed);
            auto startBlock = this->tailBlock;
            auto originalBlockIndexFront = pr_blockIndexFront;
            auto originalBlockIndexSlotsUsed = pr_blockIndexSlotsUsed;
 
            Block* firstAllocatedBlock = nullptr;
 
            // Figure out how many blocks we'll need to allocate, and do so
            size_t blockBaseDiff = ((startTailIndex + count - 1) & ~static_cast<index_t>(BLOCK_SIZE - 1)) -
                                   ((startTailIndex - 1) & ~static_cast<index_t>(BLOCK_SIZE - 1));
            index_t currentTailIndex = (startTailIndex - 1) & ~static_cast<index_t>(BLOCK_SIZE - 1);
            if (blockBaseDiff > 0) {
                // Allocate as many blocks as possible from ahead
                while (blockBaseDiff > 0 && this->tailBlock != nullptr &&
                       this->tailBlock->next != firstAllocatedBlock &&
                       this->tailBlock->next->ConcurrentQueue::Block::template is_empty<explicit_context>()) {
                    blockBaseDiff -= static_cast<index_t>(BLOCK_SIZE);
                    currentTailIndex += static_cast<index_t>(BLOCK_SIZE);
 
                    this->tailBlock = this->tailBlock->next;
                    firstAllocatedBlock = firstAllocatedBlock == nullptr ? this->tailBlock : firstAllocatedBlock;
 
                    auto& entry = blockIndex.load(std::memory_order_relaxed)->entries[pr_blockIndexFront];
                    entry.base = currentTailIndex;
                    entry.block = this->tailBlock;
                    pr_blockIndexFront = (pr_blockIndexFront + 1) & (pr_blockIndexSize - 1);
                }
 
                // Now allocate as many blocks as necessary from the block pool
                while (blockBaseDiff > 0) {
                    blockBaseDiff -= static_cast<index_t>(BLOCK_SIZE);
                    currentTailIndex += static_cast<index_t>(BLOCK_SIZE);
 
                    auto head = this->headIndex.load(std::memory_order_relaxed);
                    assert(!details::circular_less_than<index_t>(currentTailIndex, head));
                    bool full = !details::circular_less_than<index_t>(head, currentTailIndex + BLOCK_SIZE) ||
                                (MAX_SUBQUEUE_SIZE != details::const_numeric_max<size_t>::value &&
                                 (MAX_SUBQUEUE_SIZE == 0 || MAX_SUBQUEUE_SIZE - BLOCK_SIZE < currentTailIndex - head));
                    if (pr_blockIndexRaw == nullptr || pr_blockIndexSlotsUsed == pr_blockIndexSize || full) {
                        MOODYCAMEL_CONSTEXPR_IF(allocMode == CannotAlloc)
                        {
                            // Failed to allocate, undo changes (but keep injected blocks)
                            pr_blockIndexFront = originalBlockIndexFront;
                            pr_blockIndexSlotsUsed = originalBlockIndexSlotsUsed;
                            this->tailBlock = startBlock == nullptr ? firstAllocatedBlock : startBlock;
                            return false;
                        }
                        else if (full || !new_block_index(originalBlockIndexSlotsUsed))
                        {
                            // Failed to allocate, undo changes (but keep injected blocks)
                            pr_blockIndexFront = originalBlockIndexFront;
                            pr_blockIndexSlotsUsed = originalBlockIndexSlotsUsed;
                            this->tailBlock = startBlock == nullptr ? firstAllocatedBlock : startBlock;
                            return false;
                        }
 
                        // pr_blockIndexFront is updated inside new_block_index, so we need to
                        // update our fallback value too (since we keep the new index even if we
                        // later fail)
                        originalBlockIndexFront = originalBlockIndexSlotsUsed;
                    }
 
                    // Insert a new block in the circular linked list
                    auto newBlock = this->parent->ConcurrentQueue::template requisition_block<allocMode>();
                    if (newBlock == nullptr) {
                        pr_blockIndexFront = originalBlockIndexFront;
                        pr_blockIndexSlotsUsed = originalBlockIndexSlotsUsed;
                        this->tailBlock = startBlock == nullptr ? firstAllocatedBlock : startBlock;
                        return false;
                    }
 
#ifdef MCDBGQ_TRACKMEM
                    newBlock->owner = this;
#endif
                    newBlock->ConcurrentQueue::Block::template set_all_empty<explicit_context>();
                    if (this->tailBlock == nullptr) {
                        newBlock->next = newBlock;
                    } else {
                        newBlock->next = this->tailBlock->next;
                        this->tailBlock->next = newBlock;
                    }
                    this->tailBlock = newBlock;
                    firstAllocatedBlock = firstAllocatedBlock == nullptr ? this->tailBlock : firstAllocatedBlock;
 
                    ++pr_blockIndexSlotsUsed;
 
                    auto& entry = blockIndex.load(std::memory_order_relaxed)->entries[pr_blockIndexFront];
                    entry.base = currentTailIndex;
                    entry.block = this->tailBlock;
                    pr_blockIndexFront = (pr_blockIndexFront + 1) & (pr_blockIndexSize - 1);
                }
 
                // Excellent, all allocations succeeded. Reset each block's emptiness before we fill them up, and
                // publish the new block index front
                auto block = firstAllocatedBlock;
                while (true) {
                    block->ConcurrentQueue::Block::template reset_empty<explicit_context>();
                    if (block == this->tailBlock) {
                        break;
                    }
                    block = block->next;
                }
 
                MOODYCAMEL_CONSTEXPR_IF(MOODYCAMEL_NOEXCEPT_CTOR(
                    T, decltype(*itemFirst), new (static_cast<T*>(nullptr)) T(details::deref_noexcept(itemFirst))))
                {
                    blockIndex.load(std::memory_order_relaxed)
                        ->front.store((pr_blockIndexFront - 1) & (pr_blockIndexSize - 1), std::memory_order_release);
                }
            }
 
            // Enqueue, one block at a time
            index_t newTailIndex = startTailIndex + static_cast<index_t>(count);
            currentTailIndex = startTailIndex;
            auto endBlock = this->tailBlock;
            this->tailBlock = startBlock;
            assert((startTailIndex & static_cast<index_t>(BLOCK_SIZE - 1)) != 0 || firstAllocatedBlock != nullptr ||
                   count == 0);
            if ((startTailIndex & static_cast<index_t>(BLOCK_SIZE - 1)) == 0 && firstAllocatedBlock != nullptr) {
                this->tailBlock = firstAllocatedBlock;
            }
            while (true) {
                index_t stopIndex =
                    (currentTailIndex & ~static_cast<index_t>(BLOCK_SIZE - 1)) + static_cast<index_t>(BLOCK_SIZE);
                if (details::circular_less_than<index_t>(newTailIndex, stopIndex)) {
                    stopIndex = newTailIndex;
                }
                MOODYCAMEL_CONSTEXPR_IF(MOODYCAMEL_NOEXCEPT_CTOR(
                    T, decltype(*itemFirst), new (static_cast<T*>(nullptr)) T(details::deref_noexcept(itemFirst))))
                {
                    while (currentTailIndex != stopIndex) {
                        new ((*this->tailBlock)[currentTailIndex++]) T(*itemFirst++);
                    }
                }
                else
                {
                    MOODYCAMEL_TRY
                    {
                        while (currentTailIndex != stopIndex) {
                            // Must use copy constructor even if move constructor is available
                            // because we may have to revert if there's an exception.
                            // Sorry about the horrible templated next line, but it was the only way
                            // to disable moving *at compile time*, which is important because a type
                            // may only define a (noexcept) move constructor, and so calls to the
                            // cctor will not compile, even if they are in an if branch that will never
                            // be executed
                            new ((*this->tailBlock)[currentTailIndex])
                                T(details::nomove_if<!MOODYCAMEL_NOEXCEPT_CTOR(
                                      T,
                                      decltype(*itemFirst),
                                      new (static_cast<T*>(nullptr))
                                          T(details::deref_noexcept(itemFirst)))>::eval(*itemFirst));
                            ++currentTailIndex;
                            ++itemFirst;
                        }
                    }
                    MOODYCAMEL_CATCH(...)
                    {
                        // Oh dear, an exception's been thrown -- destroy the elements that
                        // were enqueued so far and revert the entire bulk operation (we'll keep
                        // any allocated blocks in our linked list for later, though).
                        auto constructedStopIndex = currentTailIndex;
                        auto lastBlockEnqueued = this->tailBlock;
 
                        pr_blockIndexFront = originalBlockIndexFront;
                        pr_blockIndexSlotsUsed = originalBlockIndexSlotsUsed;
                        this->tailBlock = startBlock == nullptr ? firstAllocatedBlock : startBlock;
 
                        if (!details::is_trivially_destructible<T>::value) {
                            auto block = startBlock;
                            if ((startTailIndex & static_cast<index_t>(BLOCK_SIZE - 1)) == 0) {
                                block = firstAllocatedBlock;
                            }
                            currentTailIndex = startTailIndex;
                            while (true) {
                                stopIndex = (currentTailIndex & ~static_cast<index_t>(BLOCK_SIZE - 1)) +
                                            static_cast<index_t>(BLOCK_SIZE);
                                if (details::circular_less_than<index_t>(constructedStopIndex, stopIndex)) {
                                    stopIndex = constructedStopIndex;
                                }
                                while (currentTailIndex != stopIndex) {
                                    (*block)[currentTailIndex++]->~T();
                                }
                                if (block == lastBlockEnqueued) {
                                    break;
                                }
                                block = block->next;
                            }
                        }
                        MOODYCAMEL_RETHROW;
                    }
                }
 
                if (this->tailBlock == endBlock) {
                    assert(currentTailIndex == newTailIndex);
                    break;
                }
                this->tailBlock = this->tailBlock->next;
            }
 
            MOODYCAMEL_CONSTEXPR_IF(!MOODYCAMEL_NOEXCEPT_CTOR(
                T, decltype(*itemFirst), new (static_cast<T*>(nullptr)) T(details::deref_noexcept(itemFirst))))
            {
                if (firstAllocatedBlock != nullptr)
                    blockIndex.load(std::memory_order_relaxed)
                        ->front.store((pr_blockIndexFront - 1) & (pr_blockIndexSize - 1), std::memory_order_release);
            }
 
            this->tailIndex.store(newTailIndex, std::memory_order_release);
            return true;
        }
 
        template <typename It> size_t dequeue_bulk(It& itemFirst, size_t max)
        {
            auto tail = this->tailIndex.load(std::memory_order_relaxed);
            auto overcommit = this->dequeueOvercommit.load(std::memory_order_relaxed);
            auto desiredCount =
                static_cast<size_t>(tail - (this->dequeueOptimisticCount.load(std::memory_order_relaxed) - overcommit));
            if (details::circular_less_than<size_t>(0, desiredCount)) {
                desiredCount = desiredCount < max ? desiredCount : max;
                std::atomic_thread_fence(std::memory_order_acquire);
 
                auto myDequeueCount = this->dequeueOptimisticCount.fetch_add(desiredCount, std::memory_order_relaxed);
 
                tail = this->tailIndex.load(std::memory_order_acquire);
                auto actualCount = static_cast<size_t>(tail - (myDequeueCount - overcommit));
                if (details::circular_less_than<size_t>(0, actualCount)) {
                    actualCount = desiredCount < actualCount ? desiredCount : actualCount;
                    if (actualCount < desiredCount) {
                        this->dequeueOvercommit.fetch_add(desiredCount - actualCount, std::memory_order_release);
                    }
 
                    // Get the first index. Note that since there's guaranteed to be at least actualCount elements, this
                    // will never exceed tail.
                    auto firstIndex = this->headIndex.fetch_add(actualCount, std::memory_order_acq_rel);
 
                    // Determine which block the first element is in
                    auto localBlockIndex = blockIndex.load(std::memory_order_acquire);
                    auto localBlockIndexHead = localBlockIndex->front.load(std::memory_order_acquire);
 
                    auto headBase = localBlockIndex->entries[localBlockIndexHead].base;
                    auto firstBlockBaseIndex = firstIndex & ~static_cast<index_t>(BLOCK_SIZE - 1);
                    auto offset = static_cast<size_t>(
                        static_cast<typename std::make_signed<index_t>::type>(firstBlockBaseIndex - headBase) /
                        static_cast<typename std::make_signed<index_t>::type>(BLOCK_SIZE));
                    auto indexIndex = (localBlockIndexHead + offset) & (localBlockIndex->size - 1);
 
                    // Iterate the blocks and dequeue
                    auto index = firstIndex;
                    do {
                        auto firstIndexInBlock = index;
                        index_t endIndex =
                            (index & ~static_cast<index_t>(BLOCK_SIZE - 1)) + static_cast<index_t>(BLOCK_SIZE);
                        endIndex = details::circular_less_than<index_t>(firstIndex + static_cast<index_t>(actualCount),
                                                                        endIndex)
                                       ? firstIndex + static_cast<index_t>(actualCount)
                                       : endIndex;
                        auto block = localBlockIndex->entries[indexIndex].block;
                        if (MOODYCAMEL_NOEXCEPT_ASSIGN(
                                T, T&&, details::deref_noexcept(itemFirst) = std::move((*(*block)[index])))) {
                            while (index != endIndex) {
                                auto& el = *((*block)[index]);
                                *itemFirst++ = std::move(el);
                                el.~T();
                                ++index;
                            }
                        } else {
                            MOODYCAMEL_TRY
                            {
                                while (index != endIndex) {
                                    auto& el = *((*block)[index]);
                                    *itemFirst = std::move(el);
                                    ++itemFirst;
                                    el.~T();
                                    ++index;
                                }
                            }
                            MOODYCAMEL_CATCH(...)
                            {
                                // It's too late to revert the dequeue, but we can make sure that all
                                // the dequeued objects are properly destroyed and the block index
                                // (and empty count) are properly updated before we propagate the exception
                                do {
                                    block = localBlockIndex->entries[indexIndex].block;
                                    while (index != endIndex) {
                                        (*block)[index++]->~T();
                                    }
                                    block->ConcurrentQueue::Block::template set_many_empty<explicit_context>(
                                        firstIndexInBlock, static_cast<size_t>(endIndex - firstIndexInBlock));
                                    indexIndex = (indexIndex + 1) & (localBlockIndex->size - 1);
 
                                    firstIndexInBlock = index;
                                    endIndex = (index & ~static_cast<index_t>(BLOCK_SIZE - 1)) +
                                               static_cast<index_t>(BLOCK_SIZE);
                                    endIndex = details::circular_less_than<index_t>(
                                                   firstIndex + static_cast<index_t>(actualCount), endIndex)
                                                   ? firstIndex + static_cast<index_t>(actualCount)
                                                   : endIndex;
                                } while (index != firstIndex + actualCount);
 
                                MOODYCAMEL_RETHROW;
                            }
                        }
                        block->ConcurrentQueue::Block::template set_many_empty<explicit_context>(
                            firstIndexInBlock, static_cast<size_t>(endIndex - firstIndexInBlock));
                        indexIndex = (indexIndex + 1) & (localBlockIndex->size - 1);
                    } while (index != firstIndex + actualCount);
 
                    return actualCount;
                } else {
                    // Wasn't anything to dequeue after all; make the effective dequeue count eventually consistent
                    this->dequeueOvercommit.fetch_add(desiredCount, std::memory_order_release);
                }
            }
 
            return 0;
        }
 
      private:
        struct BlockIndexEntry {
            index_t base;
            Block* block;
        };
 
        struct BlockIndexHeader {
            size_t size;
            std::atomic<size_t> front; // Current slot (not next, like pr_blockIndexFront)
            BlockIndexEntry* entries;
            void* prev;
        };
 
        bool new_block_index(size_t numberOfFilledSlotsToExpose)
        {
            auto prevBlockSizeMask = pr_blockIndexSize - 1;
 
            // Create the new block
            pr_blockIndexSize <<= 1;
            auto newRawPtr = static_cast<char*>((Traits::malloc)(sizeof(BlockIndexHeader) +
                                                                 std::alignment_of<BlockIndexEntry>::value - 1 +
                                                                 sizeof(BlockIndexEntry) * pr_blockIndexSize));
            if (newRawPtr == nullptr) {
                pr_blockIndexSize >>= 1; // Reset to allow graceful retry
                return false;
            }
 
            auto newBlockIndexEntries = reinterpret_cast<BlockIndexEntry*>(
                details::align_for<BlockIndexEntry>(newRawPtr + sizeof(BlockIndexHeader)));
 
            // Copy in all the old indices, if any
            size_t j = 0;
            if (pr_blockIndexSlotsUsed != 0) {
                auto i = (pr_blockIndexFront - pr_blockIndexSlotsUsed) & prevBlockSizeMask;
                do {
                    newBlockIndexEntries[j++] = pr_blockIndexEntries[i];
                    i = (i + 1) & prevBlockSizeMask;
                } while (i != pr_blockIndexFront);
            }
 
            // Update everything
            auto header = new (newRawPtr) BlockIndexHeader;
            header->size = pr_blockIndexSize;
            header->front.store(numberOfFilledSlotsToExpose - 1, std::memory_order_relaxed);
            header->entries = newBlockIndexEntries;
            header->prev = pr_blockIndexRaw; // we link the new block to the old one so we can free it later
 
            pr_blockIndexFront = j;
            pr_blockIndexEntries = newBlockIndexEntries;
            pr_blockIndexRaw = newRawPtr;
            blockIndex.store(header, std::memory_order_release);
 
            return true;
        }
 
      private:
        std::atomic<BlockIndexHeader*> blockIndex;
 
        // To be used by producer only -- consumer must use the ones in referenced by blockIndex
        size_t pr_blockIndexSlotsUsed;
        size_t pr_blockIndexSize;
        size_t pr_blockIndexFront; // Next slot (not current)
        BlockIndexEntry* pr_blockIndexEntries;
        void* pr_blockIndexRaw;
 
#ifdef MOODYCAMEL_QUEUE_INTERNAL_DEBUG
      public:
        ExplicitProducer* nextExplicitProducer;
 
      private:
#endif
 
#ifdef MCDBGQ_TRACKMEM
        friend struct MemStats;
#endif
    };
 
    // Implicit queue
 
    struct ImplicitProducer : public ProducerBase {
        ImplicitProducer(ConcurrentQueue* parent_)
            : ProducerBase(parent_, false)
            , nextBlockIndexCapacity(IMPLICIT_INITIAL_INDEX_SIZE)
            , blockIndex(nullptr)
        {
            new_block_index();
        }
 
        ~ImplicitProducer()
        {
            // Note that since we're in the destructor we can assume that all enqueue/dequeue operations
            // completed already; this means that all undequeued elements are placed contiguously across
            // contiguous blocks, and that only the first and last remaining blocks can be only partially
            // empty (all other remaining blocks must be completely full).
 
#ifdef MOODYCAMEL_CPP11_THREAD_LOCAL_SUPPORTED
            // Unregister ourselves for thread termination notification
            if (!this->inactive.load(std::memory_order_relaxed)) {
                details::ThreadExitNotifier::unsubscribe(&threadExitListener);
            }
#endif
 
            // Destroy all remaining elements!
            auto tail = this->tailIndex.load(std::memory_order_relaxed);
            auto index = this->headIndex.load(std::memory_order_relaxed);
            Block* block = nullptr;
            assert(index == tail || details::circular_less_than(index, tail));
            bool forceFreeLastBlock =
                index != tail; // If we enter the loop, then the last (tail) block will not be freed
            while (index != tail) {
                if ((index & static_cast<index_t>(BLOCK_SIZE - 1)) == 0 || block == nullptr) {
                    if (block != nullptr) {
                        // Free the old block
                        this->parent->add_block_to_free_list(block);
                    }
 
                    block = get_block_index_entry_for_index(index)->value.load(std::memory_order_relaxed);
                }
 
                ((*block)[index])->~T();
                ++index;
            }
            // Even if the queue is empty, there's still one block that's not on the free list
            // (unless the head index reached the end of it, in which case the tail will be poised
            // to create a new block).
            if (this->tailBlock != nullptr &&
                (forceFreeLastBlock || (tail & static_cast<index_t>(BLOCK_SIZE - 1)) != 0)) {
                this->parent->add_block_to_free_list(this->tailBlock);
            }
 
            // Destroy block index
            auto localBlockIndex = blockIndex.load(std::memory_order_relaxed);
            if (localBlockIndex != nullptr) {
                for (size_t i = 0; i != localBlockIndex->capacity; ++i) {
                    localBlockIndex->index[i]->~BlockIndexEntry();
                }
                do {
                    auto prev = localBlockIndex->prev;
                    localBlockIndex->~BlockIndexHeader();
                    (Traits::free)(localBlockIndex);
                    localBlockIndex = prev;
                } while (localBlockIndex != nullptr);
            }
        }
 
        template <AllocationMode allocMode, typename U> inline bool enqueue(U&& element)
        {
            index_t currentTailIndex = this->tailIndex.load(std::memory_order_relaxed);
            index_t newTailIndex = 1 + currentTailIndex;
            if ((currentTailIndex & static_cast<index_t>(BLOCK_SIZE - 1)) == 0) {
                // We reached the end of a block, start a new one
                auto head = this->headIndex.load(std::memory_order_relaxed);
                assert(!details::circular_less_than<index_t>(currentTailIndex, head));
                if (!details::circular_less_than<index_t>(head, currentTailIndex + BLOCK_SIZE) ||
                    (MAX_SUBQUEUE_SIZE != details::const_numeric_max<size_t>::value &&
                     (MAX_SUBQUEUE_SIZE == 0 || MAX_SUBQUEUE_SIZE - BLOCK_SIZE < currentTailIndex - head))) {
                    return false;
                }
#ifdef MCDBGQ_NOLOCKFREE_IMPLICITPRODBLOCKINDEX
                debug::DebugLock lock(mutex);
#endif
                // Find out where we'll be inserting this block in the block index
                BlockIndexEntry* idxEntry;
                if (!insert_block_index_entry<allocMode>(idxEntry, currentTailIndex)) {
                    return false;
                }
 
                // Get ahold of a new block
                auto newBlock = this->parent->ConcurrentQueue::template requisition_block<allocMode>();
                if (newBlock == nullptr) {
                    rewind_block_index_tail();
                    idxEntry->value.store(nullptr, std::memory_order_relaxed);
                    return false;
                }
#ifdef MCDBGQ_TRACKMEM
                newBlock->owner = this;
#endif
                newBlock->ConcurrentQueue::Block::template reset_empty<implicit_context>();
 
                MOODYCAMEL_CONSTEXPR_IF(
                    !MOODYCAMEL_NOEXCEPT_CTOR(T, U, new (static_cast<T*>(nullptr)) T(std::forward<U>(element))))
                {
                    // May throw, try to insert now before we publish the fact that we have this new block
                    MOODYCAMEL_TRY
                    {
                        new ((*newBlock)[currentTailIndex]) T(std::forward<U>(element));
                    }
                    MOODYCAMEL_CATCH(...)
                    {
                        rewind_block_index_tail();
                        idxEntry->value.store(nullptr, std::memory_order_relaxed);
                        this->parent->add_block_to_free_list(newBlock);
                        MOODYCAMEL_RETHROW;
                    }
                }
 
                // Insert the new block into the index
                idxEntry->value.store(newBlock, std::memory_order_relaxed);
 
                this->tailBlock = newBlock;
 
                MOODYCAMEL_CONSTEXPR_IF(
                    !MOODYCAMEL_NOEXCEPT_CTOR(T, U, new (static_cast<T*>(nullptr)) T(std::forward<U>(element))))
                {
                    this->tailIndex.store(newTailIndex, std::memory_order_release);
                    return true;
                }
            }
 
            // Enqueue
            new ((*this->tailBlock)[currentTailIndex]) T(std::forward<U>(element));
 
            this->tailIndex.store(newTailIndex, std::memory_order_release);
            return true;
        }
 
        template <typename U> bool dequeue(U& element)
        {
            // See ExplicitProducer::dequeue for rationale and explanation
            index_t tail = this->tailIndex.load(std::memory_order_relaxed);
            index_t overcommit = this->dequeueOvercommit.load(std::memory_order_relaxed);
            if (details::circular_less_than<index_t>(
                    this->dequeueOptimisticCount.load(std::memory_order_relaxed) - overcommit, tail)) {
                std::atomic_thread_fence(std::memory_order_acquire);
 
                index_t myDequeueCount = this->dequeueOptimisticCount.fetch_add(1, std::memory_order_relaxed);
                tail = this->tailIndex.load(std::memory_order_acquire);
                if ((details::likely)(details::circular_less_than<index_t>(myDequeueCount - overcommit, tail))) {
                    index_t index = this->headIndex.fetch_add(1, std::memory_order_acq_rel);
 
                    // Determine which block the element is in
                    auto entry = get_block_index_entry_for_index(index);
 
                    // Dequeue
                    auto block = entry->value.load(std::memory_order_relaxed);
                    auto& el = *((*block)[index]);
 
                    if (!MOODYCAMEL_NOEXCEPT_ASSIGN(T, T&&, element = std::move(el))) {
#ifdef MCDBGQ_NOLOCKFREE_IMPLICITPRODBLOCKINDEX
                        // Note: Acquiring the mutex with every dequeue instead of only when a block
                        // is released is very sub-optimal, but it is, after all, purely debug code.
                        debug::DebugLock lock(producer->mutex);
#endif
                        struct Guard {
                            Block* block;
                            index_t index;
                            BlockIndexEntry* entry;
                            ConcurrentQueue* parent;
 
                            ~Guard()
                            {
                                (*block)[index]->~T();
                                if (block->ConcurrentQueue::Block::template set_empty<implicit_context>(index)) {
                                    entry->value.store(nullptr, std::memory_order_relaxed);
                                    parent->add_block_to_free_list(block);
                                }
                            }
                        } guard = { block, index, entry, this->parent };
 
                        element = std::move(el); // NOLINT
                    } else {
                        element = std::move(el); // NOLINT
                        el.~T();                 // NOLINT
 
                        if (block->ConcurrentQueue::Block::template set_empty<implicit_context>(index)) {
                            {
#ifdef MCDBGQ_NOLOCKFREE_IMPLICITPRODBLOCKINDEX
                                debug::DebugLock lock(mutex);
#endif
                                // Add the block back into the global free pool (and remove from block index)
                                entry->value.store(nullptr, std::memory_order_relaxed);
                            }
                            this->parent->add_block_to_free_list(block); // releases the above store
                        }
                    }
 
                    return true;
                } else {
                    this->dequeueOvercommit.fetch_add(1, std::memory_order_release);
                }
            }
 
            return false;
        }
 
#ifdef _MSC_VER
#pragma warning(push)
#pragma warning(disable : 4706) // assignment within conditional expression
#endif
        template <AllocationMode allocMode, typename It> bool enqueue_bulk(It itemFirst, size_t count)
        {
            // First, we need to make sure we have enough room to enqueue all of the elements;
            // this means pre-allocating blocks and putting them in the block index (but only if
            // all the allocations succeeded).
 
            // Note that the tailBlock we start off with may not be owned by us any more;
            // this happens if it was filled up exactly to the top (setting tailIndex to
            // the first index of the next block which is not yet allocated), then dequeued
            // completely (putting it on the free list) before we enqueue again.
 
            index_t startTailIndex = this->tailIndex.load(std::memory_order_relaxed);
            auto startBlock = this->tailBlock;
            Block* firstAllocatedBlock = nullptr;
            auto endBlock = this->tailBlock;
 
            // Figure out how many blocks we'll need to allocate, and do so
            size_t blockBaseDiff = ((startTailIndex + count - 1) & ~static_cast<index_t>(BLOCK_SIZE - 1)) -
                                   ((startTailIndex - 1) & ~static_cast<index_t>(BLOCK_SIZE - 1));
            index_t currentTailIndex = (startTailIndex - 1) & ~static_cast<index_t>(BLOCK_SIZE - 1);
            if (blockBaseDiff > 0) {
#ifdef MCDBGQ_NOLOCKFREE_IMPLICITPRODBLOCKINDEX
                debug::DebugLock lock(mutex);
#endif
                do {
                    blockBaseDiff -= static_cast<index_t>(BLOCK_SIZE);
                    currentTailIndex += static_cast<index_t>(BLOCK_SIZE);
 
                    // Find out where we'll be inserting this block in the block index
                    BlockIndexEntry* idxEntry =
                        nullptr; // initialization here unnecessary but compiler can't always tell
                    Block* newBlock;
                    bool indexInserted = false;
                    auto head = this->headIndex.load(std::memory_order_relaxed);
                    assert(!details::circular_less_than<index_t>(currentTailIndex, head));
                    bool full = !details::circular_less_than<index_t>(head, currentTailIndex + BLOCK_SIZE) ||
                                (MAX_SUBQUEUE_SIZE != details::const_numeric_max<size_t>::value &&
                                 (MAX_SUBQUEUE_SIZE == 0 || MAX_SUBQUEUE_SIZE - BLOCK_SIZE < currentTailIndex - head));
 
                    if (full || !(indexInserted = insert_block_index_entry<allocMode>(idxEntry, currentTailIndex)) ||
                        (newBlock = this->parent->ConcurrentQueue::template requisition_block<allocMode>()) ==
                            nullptr) {
                        // Index allocation or block allocation failed; revert any other allocations
                        // and index insertions done so far for this operation
                        if (indexInserted) {
                            rewind_block_index_tail();
                            idxEntry->value.store(nullptr, std::memory_order_relaxed);
                        }
                        currentTailIndex = (startTailIndex - 1) & ~static_cast<index_t>(BLOCK_SIZE - 1);
                        for (auto block = firstAllocatedBlock; block != nullptr; block = block->next) {
                            currentTailIndex += static_cast<index_t>(BLOCK_SIZE);
                            idxEntry = get_block_index_entry_for_index(currentTailIndex);
                            idxEntry->value.store(nullptr, std::memory_order_relaxed);
                            rewind_block_index_tail();
                        }
                        this->parent->add_blocks_to_free_list(firstAllocatedBlock);
                        this->tailBlock = startBlock;
 
                        return false;
                    }
 
#ifdef MCDBGQ_TRACKMEM
                    newBlock->owner = this;
#endif
                    newBlock->ConcurrentQueue::Block::template reset_empty<implicit_context>();
                    newBlock->next = nullptr;
 
                    // Insert the new block into the index
                    idxEntry->value.store(newBlock, std::memory_order_relaxed);
 
                    // Store the chain of blocks so that we can undo if later allocations fail,
                    // and so that we can find the blocks when we do the actual enqueueing
                    if ((startTailIndex & static_cast<index_t>(BLOCK_SIZE - 1)) != 0 ||
                        firstAllocatedBlock != nullptr) {
                        assert(this->tailBlock != nullptr);
                        this->tailBlock->next = newBlock;
                    }
                    this->tailBlock = newBlock;
                    endBlock = newBlock;
                    firstAllocatedBlock = firstAllocatedBlock == nullptr ? newBlock : firstAllocatedBlock;
                } while (blockBaseDiff > 0);
            }
 
            // Enqueue, one block at a time
            index_t newTailIndex = startTailIndex + static_cast<index_t>(count);
            currentTailIndex = startTailIndex;
            this->tailBlock = startBlock;
            assert((startTailIndex & static_cast<index_t>(BLOCK_SIZE - 1)) != 0 || firstAllocatedBlock != nullptr ||
                   count == 0);
            if ((startTailIndex & static_cast<index_t>(BLOCK_SIZE - 1)) == 0 && firstAllocatedBlock != nullptr) {
                this->tailBlock = firstAllocatedBlock;
            }
            while (true) {
                index_t stopIndex =
                    (currentTailIndex & ~static_cast<index_t>(BLOCK_SIZE - 1)) + static_cast<index_t>(BLOCK_SIZE);
                if (details::circular_less_than<index_t>(newTailIndex, stopIndex)) {
                    stopIndex = newTailIndex;
                }
                MOODYCAMEL_CONSTEXPR_IF(MOODYCAMEL_NOEXCEPT_CTOR(
                    T, decltype(*itemFirst), new (static_cast<T*>(nullptr)) T(details::deref_noexcept(itemFirst))))
                {
                    while (currentTailIndex != stopIndex) {
                        new ((*this->tailBlock)[currentTailIndex++]) T(*itemFirst++);
                    }
                }
                else
                {
                    MOODYCAMEL_TRY
                    {
                        while (currentTailIndex != stopIndex) {
                            new ((*this->tailBlock)[currentTailIndex])
                                T(details::nomove_if<!MOODYCAMEL_NOEXCEPT_CTOR(
                                      T,
                                      decltype(*itemFirst),
                                      new (static_cast<T*>(nullptr))
                                          T(details::deref_noexcept(itemFirst)))>::eval(*itemFirst));
                            ++currentTailIndex;
                            ++itemFirst;
                        }
                    }
                    MOODYCAMEL_CATCH(...)
                    {
                        auto constructedStopIndex = currentTailIndex;
                        auto lastBlockEnqueued = this->tailBlock;
 
                        if (!details::is_trivially_destructible<T>::value) {
                            auto block = startBlock;
                            if ((startTailIndex & static_cast<index_t>(BLOCK_SIZE - 1)) == 0) {
                                block = firstAllocatedBlock;
                            }
                            currentTailIndex = startTailIndex;
                            while (true) {
                                stopIndex = (currentTailIndex & ~static_cast<index_t>(BLOCK_SIZE - 1)) +
                                            static_cast<index_t>(BLOCK_SIZE);
                                if (details::circular_less_than<index_t>(constructedStopIndex, stopIndex)) {
                                    stopIndex = constructedStopIndex;
                                }
                                while (currentTailIndex != stopIndex) {
                                    (*block)[currentTailIndex++]->~T();
                                }
                                if (block == lastBlockEnqueued) {
                                    break;
                                }
                                block = block->next;
                            }
                        }
 
                        currentTailIndex = (startTailIndex - 1) & ~static_cast<index_t>(BLOCK_SIZE - 1);
                        for (auto block = firstAllocatedBlock; block != nullptr; block = block->next) {
                            currentTailIndex += static_cast<index_t>(BLOCK_SIZE);
                            auto idxEntry = get_block_index_entry_for_index(currentTailIndex);
                            idxEntry->value.store(nullptr, std::memory_order_relaxed);
                            rewind_block_index_tail();
                        }
                        this->parent->add_blocks_to_free_list(firstAllocatedBlock);
                        this->tailBlock = startBlock;
                        MOODYCAMEL_RETHROW;
                    }
                }
 
                if (this->tailBlock == endBlock) {
                    assert(currentTailIndex == newTailIndex);
                    break;
                }
                this->tailBlock = this->tailBlock->next;
            }
            this->tailIndex.store(newTailIndex, std::memory_order_release);
            return true;
        }
#ifdef _MSC_VER
#pragma warning(pop)
#endif
 
        template <typename It> size_t dequeue_bulk(It& itemFirst, size_t max)
        {
            auto tail = this->tailIndex.load(std::memory_order_relaxed);
            auto overcommit = this->dequeueOvercommit.load(std::memory_order_relaxed);
            auto desiredCount =
                static_cast<size_t>(tail - (this->dequeueOptimisticCount.load(std::memory_order_relaxed) - overcommit));
            if (details::circular_less_than<size_t>(0, desiredCount)) {
                desiredCount = desiredCount < max ? desiredCount : max;
                std::atomic_thread_fence(std::memory_order_acquire);
 
                auto myDequeueCount = this->dequeueOptimisticCount.fetch_add(desiredCount, std::memory_order_relaxed);
 
                tail = this->tailIndex.load(std::memory_order_acquire);
                auto actualCount = static_cast<size_t>(tail - (myDequeueCount - overcommit));
                if (details::circular_less_than<size_t>(0, actualCount)) {
                    actualCount = desiredCount < actualCount ? desiredCount : actualCount;
                    if (actualCount < desiredCount) {
                        this->dequeueOvercommit.fetch_add(desiredCount - actualCount, std::memory_order_release);
                    }
 
                    // Get the first index. Note that since there's guaranteed to be at least actualCount elements, this
                    // will never exceed tail.
                    auto firstIndex = this->headIndex.fetch_add(actualCount, std::memory_order_acq_rel);
 
                    // Iterate the blocks and dequeue
                    auto index = firstIndex;
                    BlockIndexHeader* localBlockIndex;
                    auto indexIndex = get_block_index_index_for_index(index, localBlockIndex);
                    do {
                        auto blockStartIndex = index;
                        index_t endIndex =
                            (index & ~static_cast<index_t>(BLOCK_SIZE - 1)) + static_cast<index_t>(BLOCK_SIZE);
                        endIndex = details::circular_less_than<index_t>(firstIndex + static_cast<index_t>(actualCount),
                                                                        endIndex)
                                       ? firstIndex + static_cast<index_t>(actualCount)
                                       : endIndex;
 
                        auto entry = localBlockIndex->index[indexIndex];
                        auto block = entry->value.load(std::memory_order_relaxed);
                        if (MOODYCAMEL_NOEXCEPT_ASSIGN(
                                T, T&&, details::deref_noexcept(itemFirst) = std::move((*(*block)[index])))) {
                            while (index != endIndex) {
                                auto& el = *((*block)[index]);
                                *itemFirst++ = std::move(el);
                                el.~T();
                                ++index;
                            }
                        } else {
                            MOODYCAMEL_TRY
                            {
                                while (index != endIndex) {
                                    auto& el = *((*block)[index]);
                                    *itemFirst = std::move(el);
                                    ++itemFirst;
                                    el.~T();
                                    ++index;
                                }
                            }
                            MOODYCAMEL_CATCH(...)
                            {
                                do {
                                    entry = localBlockIndex->index[indexIndex];
                                    block = entry->value.load(std::memory_order_relaxed);
                                    while (index != endIndex) {
                                        (*block)[index++]->~T();
                                    }
 
                                    if (block->ConcurrentQueue::Block::template set_many_empty<implicit_context>(
                                            blockStartIndex, static_cast<size_t>(endIndex - blockStartIndex))) {
#ifdef MCDBGQ_NOLOCKFREE_IMPLICITPRODBLOCKINDEX
                                        debug::DebugLock lock(mutex);
#endif
                                        entry->value.store(nullptr, std::memory_order_relaxed);
                                        this->parent->add_block_to_free_list(block);
                                    }
                                    indexIndex = (indexIndex + 1) & (localBlockIndex->capacity - 1);
 
                                    blockStartIndex = index;
                                    endIndex = (index & ~static_cast<index_t>(BLOCK_SIZE - 1)) +
                                               static_cast<index_t>(BLOCK_SIZE);
                                    endIndex = details::circular_less_than<index_t>(
                                                   firstIndex + static_cast<index_t>(actualCount), endIndex)
                                                   ? firstIndex + static_cast<index_t>(actualCount)
                                                   : endIndex;
                                } while (index != firstIndex + actualCount);
 
                                MOODYCAMEL_RETHROW;
                            }
                        }
                        if (block->ConcurrentQueue::Block::template set_many_empty<implicit_context>(
                                blockStartIndex, static_cast<size_t>(endIndex - blockStartIndex))) {
                            {
#ifdef MCDBGQ_NOLOCKFREE_IMPLICITPRODBLOCKINDEX
                                debug::DebugLock lock(mutex);
#endif
                                // Note that the set_many_empty above did a release, meaning that anybody who acquires
                                // the block we're about to free can use it safely since our writes (and reads!) will
                                // have happened-before then.
                                entry->value.store(nullptr, std::memory_order_relaxed);
                            }
                            this->parent->add_block_to_free_list(block); // releases the above store
                        }
                        indexIndex = (indexIndex + 1) & (localBlockIndex->capacity - 1);
                    } while (index != firstIndex + actualCount);
 
                    return actualCount;
                } else {
                    this->dequeueOvercommit.fetch_add(desiredCount, std::memory_order_release);
                }
            }
 
            return 0;
        }
 
      private:
        // The block size must be > 1, so any number with the low bit set is an invalid block base index
        static const index_t INVALID_BLOCK_BASE = 1;
 
        struct BlockIndexEntry {
            std::atomic<index_t> key;
            std::atomic<Block*> value;
        };
 
        struct BlockIndexHeader {
            size_t capacity;
            std::atomic<size_t> tail;
            BlockIndexEntry* entries;
            BlockIndexEntry** index;
            BlockIndexHeader* prev;
        };
 
        template <AllocationMode allocMode>
        inline bool insert_block_index_entry(BlockIndexEntry*& idxEntry, index_t blockStartIndex)
        {
            auto localBlockIndex =
                blockIndex.load(std::memory_order_relaxed); // We're the only writer thread, relaxed is OK
            if (localBlockIndex == nullptr) {
                return false; // this can happen if new_block_index failed in the constructor
            }
            size_t newTail =
                (localBlockIndex->tail.load(std::memory_order_relaxed) + 1) & (localBlockIndex->capacity - 1);
            idxEntry = localBlockIndex->index[newTail];
            if (idxEntry->key.load(std::memory_order_relaxed) == INVALID_BLOCK_BASE ||
                idxEntry->value.load(std::memory_order_relaxed) == nullptr) {
 
                idxEntry->key.store(blockStartIndex, std::memory_order_relaxed);
                localBlockIndex->tail.store(newTail, std::memory_order_release);
                return true;
            }
 
            // No room in the old block index, try to allocate another one!
            MOODYCAMEL_CONSTEXPR_IF(allocMode == CannotAlloc)
            {
                return false;
            }
            else if (!new_block_index())
            {
                return false;
            }
            else
            {
                localBlockIndex = blockIndex.load(std::memory_order_relaxed);
                newTail = (localBlockIndex->tail.load(std::memory_order_relaxed) + 1) & (localBlockIndex->capacity - 1);
                idxEntry = localBlockIndex->index[newTail];
                assert(idxEntry->key.load(std::memory_order_relaxed) == INVALID_BLOCK_BASE);
                idxEntry->key.store(blockStartIndex, std::memory_order_relaxed);
                localBlockIndex->tail.store(newTail, std::memory_order_release);
                return true;
            }
        }
 
        inline void rewind_block_index_tail()
        {
            auto localBlockIndex = blockIndex.load(std::memory_order_relaxed);
            localBlockIndex->tail.store((localBlockIndex->tail.load(std::memory_order_relaxed) - 1) &
                                            (localBlockIndex->capacity - 1),
                                        std::memory_order_relaxed);
        }
 
        inline BlockIndexEntry* get_block_index_entry_for_index(index_t index) const
        {
            BlockIndexHeader* localBlockIndex;
            auto idx = get_block_index_index_for_index(index, localBlockIndex);
            return localBlockIndex->index[idx];
        }
 
        inline size_t get_block_index_index_for_index(index_t index, BlockIndexHeader*& localBlockIndex) const
        {
#ifdef MCDBGQ_NOLOCKFREE_IMPLICITPRODBLOCKINDEX
            debug::DebugLock lock(mutex);
#endif
            index &= ~static_cast<index_t>(BLOCK_SIZE - 1);
            localBlockIndex = blockIndex.load(std::memory_order_acquire);
            auto tail = localBlockIndex->tail.load(std::memory_order_acquire);
            auto tailBase = localBlockIndex->index[tail]->key.load(std::memory_order_relaxed);
            assert(tailBase != INVALID_BLOCK_BASE);
            // Note: Must use division instead of shift because the index may wrap around, causing a negative
            // offset, whose negativity we want to preserve
            auto offset = static_cast<size_t>(static_cast<typename std::make_signed<index_t>::type>(index - tailBase) /
                                              static_cast<typename std::make_signed<index_t>::type>(BLOCK_SIZE));
            size_t idx = (tail + offset) & (localBlockIndex->capacity - 1);
            assert(localBlockIndex->index[idx]->key.load(std::memory_order_relaxed) == index &&
                   localBlockIndex->index[idx]->value.load(std::memory_order_relaxed) != nullptr);
            return idx;
        }
 
        bool new_block_index()
        {
            auto prev = blockIndex.load(std::memory_order_relaxed);
            size_t prevCapacity = prev == nullptr ? 0 : prev->capacity;
            auto entryCount = prev == nullptr ? nextBlockIndexCapacity : prevCapacity;
            auto raw = static_cast<char*>(
                (Traits::malloc)(sizeof(BlockIndexHeader) + std::alignment_of<BlockIndexEntry>::value - 1 +
                                 sizeof(BlockIndexEntry) * entryCount + std::alignment_of<BlockIndexEntry*>::value - 1 +
                                 sizeof(BlockIndexEntry*) * nextBlockIndexCapacity));
            if (raw == nullptr) {
                return false;
            }
 
            auto header = new (raw) BlockIndexHeader;
            auto entries =
                reinterpret_cast<BlockIndexEntry*>(details::align_for<BlockIndexEntry>(raw + sizeof(BlockIndexHeader)));
            auto index = reinterpret_cast<BlockIndexEntry**>(details::align_for<BlockIndexEntry*>(
                reinterpret_cast<char*>(entries) + sizeof(BlockIndexEntry) * entryCount));
            if (prev != nullptr) {
                auto prevTail = prev->tail.load(std::memory_order_relaxed);
                auto prevPos = prevTail;
                size_t i = 0;
                do {
                    prevPos = (prevPos + 1) & (prev->capacity - 1);
                    index[i++] = prev->index[prevPos];
                } while (prevPos != prevTail);
                assert(i == prevCapacity);
            }
            for (size_t i = 0; i != entryCount; ++i) {
                new (entries + i) BlockIndexEntry;
                entries[i].key.store(INVALID_BLOCK_BASE, std::memory_order_relaxed);
                index[prevCapacity + i] = entries + i;
            }
            header->prev = prev;
            header->entries = entries;
            header->index = index;
            header->capacity = nextBlockIndexCapacity;
            header->tail.store((prevCapacity - 1) & (nextBlockIndexCapacity - 1), std::memory_order_relaxed);
 
            blockIndex.store(header, std::memory_order_release);
 
            nextBlockIndexCapacity <<= 1;
 
            return true;
        }
 
      private:
        size_t nextBlockIndexCapacity;
        std::atomic<BlockIndexHeader*> blockIndex;
 
#ifdef MOODYCAMEL_CPP11_THREAD_LOCAL_SUPPORTED
      public:
        details::ThreadExitListener threadExitListener;
 
      private:
#endif
 
#ifdef MOODYCAMEL_QUEUE_INTERNAL_DEBUG
      public:
        ImplicitProducer* nextImplicitProducer;
 
      private:
#endif
 
#ifdef MCDBGQ_NOLOCKFREE_IMPLICITPRODBLOCKINDEX
        mutable debug::DebugMutex mutex;
#endif
#ifdef MCDBGQ_TRACKMEM
        friend struct MemStats;
#endif
    };
 
    // Block pool manipulation
 
    void populate_initial_block_list(size_t blockCount)
    {
        initialBlockPoolSize = blockCount;
        if (initialBlockPoolSize == 0) {
            initialBlockPool = nullptr;
            return;
        }
 
        initialBlockPool = create_array<Block>(blockCount);
        if (initialBlockPool == nullptr) {
            initialBlockPoolSize = 0;
        }
        for (size_t i = 0; i < initialBlockPoolSize; ++i) {
            initialBlockPool[i].dynamicallyAllocated = false;
        }
    }
 
    inline Block* try_get_block_from_initial_pool()
    {
        if (initialBlockPoolIndex.load(std::memory_order_relaxed) >= initialBlockPoolSize) {
            return nullptr;
        }
 
        auto index = initialBlockPoolIndex.fetch_add(1, std::memory_order_relaxed);
 
        return index < initialBlockPoolSize ? (initialBlockPool + index) : nullptr;
    }
 
    inline void add_block_to_free_list(Block* block)
    {
#ifdef MCDBGQ_TRACKMEM
        block->owner = nullptr;
#endif
        if (!Traits::RECYCLE_ALLOCATED_BLOCKS && block->dynamicallyAllocated) {
            destroy(block);
        } else {
            freeList.add(block);
        }
    }
 
    inline void add_blocks_to_free_list(Block* block)
    {
        while (block != nullptr) {
            auto next = block->next;
            add_block_to_free_list(block);
            block = next;
        }
    }
 
    inline Block* try_get_block_from_free_list() { return freeList.try_get(); }
 
    // Gets a free block from one of the memory pools, or allocates a new one (if applicable)
    template <AllocationMode canAlloc> Block* requisition_block()
    {
        auto block = try_get_block_from_initial_pool();
        if (block != nullptr) {
            return block;
        }
 
        block = try_get_block_from_free_list();
        if (block != nullptr) {
            return block;
        }
 
        MOODYCAMEL_CONSTEXPR_IF(canAlloc == CanAlloc)
        {
            return create<Block>();
        }
        else
        {
            return nullptr;
        }
    }
 
#ifdef MCDBGQ_TRACKMEM
  public:
    struct MemStats {
        size_t allocatedBlocks;
        size_t usedBlocks;
        size_t freeBlocks;
        size_t ownedBlocksExplicit;
        size_t ownedBlocksImplicit;
        size_t implicitProducers;
        size_t explicitProducers;
        size_t elementsEnqueued;
        size_t blockClassBytes;
        size_t queueClassBytes;
        size_t implicitBlockIndexBytes;
        size_t explicitBlockIndexBytes;
 
        friend class ConcurrentQueue;
 
      private:
        static MemStats getFor(ConcurrentQueue* q)
        {
            MemStats stats = { 0 };
 
            stats.elementsEnqueued = q->size_approx();
 
            auto block = q->freeList.head_unsafe();
            while (block != nullptr) {
                ++stats.allocatedBlocks;
                ++stats.freeBlocks;
                block = block->freeListNext.load(std::memory_order_relaxed);
            }
 
            for (auto ptr = q->producerListTail.load(std::memory_order_acquire); ptr != nullptr;
                 ptr = ptr->next_prod()) {
                bool implicit = dynamic_cast<ImplicitProducer*>(ptr) != nullptr;
                stats.implicitProducers += implicit ? 1 : 0;
                stats.explicitProducers += implicit ? 0 : 1;
 
                if (implicit) {
                    auto prod = static_cast<ImplicitProducer*>(ptr);
                    stats.queueClassBytes += sizeof(ImplicitProducer);
                    auto head = prod->headIndex.load(std::memory_order_relaxed);
                    auto tail = prod->tailIndex.load(std::memory_order_relaxed);
                    auto hash = prod->blockIndex.load(std::memory_order_relaxed);
                    if (hash != nullptr) {
                        for (size_t i = 0; i != hash->capacity; ++i) {
                            if (hash->index[i]->key.load(std::memory_order_relaxed) !=
                                    ImplicitProducer::INVALID_BLOCK_BASE &&
                                hash->index[i]->value.load(std::memory_order_relaxed) != nullptr) {
                                ++stats.allocatedBlocks;
                                ++stats.ownedBlocksImplicit;
                            }
                        }
                        stats.implicitBlockIndexBytes +=
                            hash->capacity * sizeof(typename ImplicitProducer::BlockIndexEntry);
                        for (; hash != nullptr; hash = hash->prev) {
                            stats.implicitBlockIndexBytes +=
                                sizeof(typename ImplicitProducer::BlockIndexHeader) +
                                hash->capacity * sizeof(typename ImplicitProducer::BlockIndexEntry*);
                        }
                    }
                    for (; details::circular_less_than<index_t>(head, tail); head += BLOCK_SIZE) {
                        // auto block = prod->get_block_index_entry_for_index(head);
                        ++stats.usedBlocks;
                    }
                } else {
                    auto prod = static_cast<ExplicitProducer*>(ptr);
                    stats.queueClassBytes += sizeof(ExplicitProducer);
                    auto tailBlock = prod->tailBlock;
                    bool wasNonEmpty = false;
                    if (tailBlock != nullptr) {
                        auto block = tailBlock;
                        do {
                            ++stats.allocatedBlocks;
                            if (!block->ConcurrentQueue::Block::template is_empty<explicit_context>() || wasNonEmpty) {
                                ++stats.usedBlocks;
                                wasNonEmpty = wasNonEmpty || block != tailBlock;
                            }
                            ++stats.ownedBlocksExplicit;
                            block = block->next;
                        } while (block != tailBlock);
                    }
                    auto index = prod->blockIndex.load(std::memory_order_relaxed);
                    while (index != nullptr) {
                        stats.explicitBlockIndexBytes +=
                            sizeof(typename ExplicitProducer::BlockIndexHeader) +
                            index->size * sizeof(typename ExplicitProducer::BlockIndexEntry);
                        index = static_cast<typename ExplicitProducer::BlockIndexHeader*>(index->prev);
                    }
                }
            }
 
            auto freeOnInitialPool =
                q->initialBlockPoolIndex.load(std::memory_order_relaxed) >= q->initialBlockPoolSize
                    ? 0
                    : q->initialBlockPoolSize - q->initialBlockPoolIndex.load(std::memory_order_relaxed);
            stats.allocatedBlocks += freeOnInitialPool;
            stats.freeBlocks += freeOnInitialPool;
 
            stats.blockClassBytes = sizeof(Block) * stats.allocatedBlocks;
            stats.queueClassBytes += sizeof(ConcurrentQueue);
 
            return stats;
        }
    };
 
    // For debugging only. Not thread-safe.
    MemStats getMemStats() { return MemStats::getFor(this); }
 
  private:
    friend struct MemStats;
#endif
 
    // Producer list manipulation
 
    ProducerBase* recycle_or_create_producer(bool isExplicit)
    {
#ifdef MCDBGQ_NOLOCKFREE_IMPLICITPRODHASH
        debug::DebugLock lock(implicitProdMutex);
#endif
        // Try to re-use one first
        for (auto ptr = producerListTail.load(std::memory_order_acquire); ptr != nullptr; ptr = ptr->next_prod()) {
            if (ptr->inactive.load(std::memory_order_relaxed) && ptr->isExplicit == isExplicit) {
                bool expected = true;
                if (ptr->inactive.compare_exchange_strong(
                        expected, /* desired */ false, std::memory_order_acquire, std::memory_order_relaxed)) {
                    // We caught one! It's been marked as activated, the caller can have it
                    return ptr;
                }
            }
        }
 
        return add_producer(isExplicit ? static_cast<ProducerBase*>(create<ExplicitProducer>(this))
                                       : create<ImplicitProducer>(this));
    }
 
    ProducerBase* add_producer(ProducerBase* producer)
    {
        // Handle failed memory allocation
        if (producer == nullptr) {
            return nullptr;
        }
 
        producerCount.fetch_add(1, std::memory_order_relaxed);
 
        // Add it to the lock-free list
        auto prevTail = producerListTail.load(std::memory_order_relaxed);
        do {
            producer->next = prevTail;
        } while (!producerListTail.compare_exchange_weak(
            prevTail, producer, std::memory_order_release, std::memory_order_relaxed));
 
#ifdef MOODYCAMEL_QUEUE_INTERNAL_DEBUG
        if (producer->isExplicit) {
            auto prevTailExplicit = explicitProducers.load(std::memory_order_relaxed);
            do {
                static_cast<ExplicitProducer*>(producer)->nextExplicitProducer = prevTailExplicit;
            } while (!explicitProducers.compare_exchange_weak(prevTailExplicit,
                                                              static_cast<ExplicitProducer*>(producer),
                                                              std::memory_order_release,
                                                              std::memory_order_relaxed));
        } else {
            auto prevTailImplicit = implicitProducers.load(std::memory_order_relaxed);
            do {
                static_cast<ImplicitProducer*>(producer)->nextImplicitProducer = prevTailImplicit;
            } while (!implicitProducers.compare_exchange_weak(prevTailImplicit,
                                                              static_cast<ImplicitProducer*>(producer),
                                                              std::memory_order_release,
                                                              std::memory_order_relaxed));
        }
#endif
 
        return producer;
    }
 
    void reown_producers()
    {
        // After another instance is moved-into/swapped-with this one, all the
        // producers we stole still think their parents are the other queue.
        // So fix them up!
        for (auto ptr = producerListTail.load(std::memory_order_relaxed); ptr != nullptr; ptr = ptr->next_prod()) {
            ptr->parent = this;
        }
    }
 
    // Implicit producer hash
 
    struct ImplicitProducerKVP {
        std::atomic<details::thread_id_t> key;
        ImplicitProducer*
            value; // No need for atomicity since it's only read by the thread that sets it in the first place
 
        ImplicitProducerKVP()
            : value(nullptr)
        {}
 
        ImplicitProducerKVP(ImplicitProducerKVP&& other) MOODYCAMEL_NOEXCEPT
        {
            key.store(other.key.load(std::memory_order_relaxed), std::memory_order_relaxed);
            value = other.value;
        }
 
        inline ImplicitProducerKVP& operator=(ImplicitProducerKVP&& other) MOODYCAMEL_NOEXCEPT
        {
            swap(other);
            return *this;
        }
 
        inline void swap(ImplicitProducerKVP& other) MOODYCAMEL_NOEXCEPT
        {
            if (this != &other) {
                details::swap_relaxed(key, other.key);
                std::swap(value, other.value);
            }
        }
    };
 
    template <typename XT, typename XTraits>
    friend void moodycamel::swap(typename ConcurrentQueue<XT, XTraits>::ImplicitProducerKVP&,
                                 typename ConcurrentQueue<XT, XTraits>::ImplicitProducerKVP&) MOODYCAMEL_NOEXCEPT;
 
    struct ImplicitProducerHash {
        size_t capacity;
        ImplicitProducerKVP* entries;
        ImplicitProducerHash* prev;
    };
 
    inline void populate_initial_implicit_producer_hash()
    {
        MOODYCAMEL_CONSTEXPR_IF(INITIAL_IMPLICIT_PRODUCER_HASH_SIZE == 0)
        {
            return;
        }
        else
        {
            implicitProducerHashCount.store(0, std::memory_order_relaxed);
            auto hash = &initialImplicitProducerHash;
            hash->capacity = INITIAL_IMPLICIT_PRODUCER_HASH_SIZE;
            hash->entries = &initialImplicitProducerHashEntries[0];
            for (size_t i = 0; i != INITIAL_IMPLICIT_PRODUCER_HASH_SIZE; ++i) {
                initialImplicitProducerHashEntries[i].key.store(details::invalid_thread_id, std::memory_order_relaxed);
            }
            hash->prev = nullptr;
            implicitProducerHash.store(hash, std::memory_order_relaxed);
        }
    }
 
    void swap_implicit_producer_hashes(ConcurrentQueue& other)
    {
        MOODYCAMEL_CONSTEXPR_IF(INITIAL_IMPLICIT_PRODUCER_HASH_SIZE == 0)
        {
            return;
        }
        else
        {
            // Swap (assumes our implicit producer hash is initialized)
            initialImplicitProducerHashEntries.swap(other.initialImplicitProducerHashEntries);
            initialImplicitProducerHash.entries = &initialImplicitProducerHashEntries[0];
            other.initialImplicitProducerHash.entries = &other.initialImplicitProducerHashEntries[0];
 
            details::swap_relaxed(implicitProducerHashCount, other.implicitProducerHashCount);
 
            details::swap_relaxed(implicitProducerHash, other.implicitProducerHash);
            if (implicitProducerHash.load(std::memory_order_relaxed) == &other.initialImplicitProducerHash) {
                implicitProducerHash.store(&initialImplicitProducerHash, std::memory_order_relaxed);
            } else {
                ImplicitProducerHash* hash;
                for (hash = implicitProducerHash.load(std::memory_order_relaxed);
                     hash->prev != &other.initialImplicitProducerHash;
                     hash = hash->prev) {
                    continue;
                }
                hash->prev = &initialImplicitProducerHash;
            }
            if (other.implicitProducerHash.load(std::memory_order_relaxed) == &initialImplicitProducerHash) {
                other.implicitProducerHash.store(&other.initialImplicitProducerHash, std::memory_order_relaxed);
            } else {
                ImplicitProducerHash* hash;
                for (hash = other.implicitProducerHash.load(std::memory_order_relaxed);
                     hash->prev != &initialImplicitProducerHash;
                     hash = hash->prev) {
                    continue;
                }
                hash->prev = &other.initialImplicitProducerHash;
            }
        }
    }
 
    // Only fails (returns nullptr) if memory allocation fails
    ImplicitProducer* get_or_add_implicit_producer()
    {
        // Note that since the data is essentially thread-local (key is thread ID),
        // there's a reduced need for fences (memory ordering is already consistent
        // for any individual thread), except for the current table itself.
 
        // Start by looking for the thread ID in the current and all previous hash tables.
        // If it's not found, it must not be in there yet, since this same thread would
        // have added it previously to one of the tables that we traversed.
 
        // Code and algorithm adapted from http://preshing.com/20130605/the-worlds-simplest-lock-free-hash-table
 
#ifdef MCDBGQ_NOLOCKFREE_IMPLICITPRODHASH
        debug::DebugLock lock(implicitProdMutex);
#endif
 
        auto id = details::thread_id();
        auto hashedId = details::hash_thread_id(id);
 
        auto mainHash = implicitProducerHash.load(std::memory_order_acquire);
        assert(mainHash != nullptr); // silence clang-tidy and MSVC warnings (hash cannot be null)
        for (auto hash = mainHash; hash != nullptr; hash = hash->prev) {
            // Look for the id in this hash
            auto index = hashedId;
            while (true) { // Not an infinite loop because at least one slot is free in the hash table
                index &= hash->capacity - 1u;
 
                auto probedKey = hash->entries[index].key.load(std::memory_order_relaxed);
                if (probedKey == id) {
                    // Found it! If we had to search several hashes deep, though, we should lazily add it
                    // to the current main hash table to avoid the extended search next time.
                    // Note there's guaranteed to be room in the current hash table since every subsequent
                    // table implicitly reserves space for all previous tables (there's only one
                    // implicitProducerHashCount).
                    auto value = hash->entries[index].value;
                    if (hash != mainHash) {
                        index = hashedId;
                        while (true) {
                            index &= mainHash->capacity - 1u;
                            auto empty = details::invalid_thread_id;
#ifdef MOODYCAMEL_CPP11_THREAD_LOCAL_SUPPORTED
                            auto reusable = details::invalid_thread_id2;
                            if (mainHash->entries[index].key.compare_exchange_strong(
                                    empty, id, std::memory_order_seq_cst, std::memory_order_relaxed) ||
                                mainHash->entries[index].key.compare_exchange_strong(
                                    reusable, id, std::memory_order_seq_cst, std::memory_order_relaxed)) {
#else
                            if (mainHash->entries[index].key.compare_exchange_strong(
                                    empty, id, std::memory_order_seq_cst, std::memory_order_relaxed)) {
#endif
                                mainHash->entries[index].value = value;
                                break;
                            }
                            ++index;
                        }
                    }
 
                    return value;
                }
                if (probedKey == details::invalid_thread_id) {
                    break; // Not in this hash table
                }
                ++index;
            }
        }
 
        // Insert!
        auto newCount = 1 + implicitProducerHashCount.fetch_add(1, std::memory_order_relaxed);
        while (true) {
            // NOLINTNEXTLINE(clang-analyzer-core.NullDereference)
            if (newCount >= (mainHash->capacity >> 1) &&
                !implicitProducerHashResizeInProgress.test_and_set(std::memory_order_acquire)) {
                // We've acquired the resize lock, try to allocate a bigger hash table.
                // Note the acquire fence synchronizes with the release fence at the end of this block, and hence when
                // we reload implicitProducerHash it must be the most recent version (it only gets changed within this
                // locked block).
                mainHash = implicitProducerHash.load(std::memory_order_acquire);
                if (newCount >= (mainHash->capacity >> 1)) {
                    size_t newCapacity = mainHash->capacity << 1;
                    while (newCount >= (newCapacity >> 1)) {
                        newCapacity <<= 1;
                    }
                    auto raw = static_cast<char*>((Traits::malloc)(sizeof(ImplicitProducerHash) +
                                                                   std::alignment_of<ImplicitProducerKVP>::value - 1 +
                                                                   sizeof(ImplicitProducerKVP) * newCapacity));
                    if (raw == nullptr) {
                        // Allocation failed
                        implicitProducerHashCount.fetch_sub(1, std::memory_order_relaxed);
                        implicitProducerHashResizeInProgress.clear(std::memory_order_relaxed);
                        return nullptr;
                    }
 
                    auto newHash = new (raw) ImplicitProducerHash;
                    newHash->capacity = static_cast<size_t>(newCapacity);
                    newHash->entries = reinterpret_cast<ImplicitProducerKVP*>(
                        details::align_for<ImplicitProducerKVP>(raw + sizeof(ImplicitProducerHash)));
                    for (size_t i = 0; i != newCapacity; ++i) {
                        new (newHash->entries + i) ImplicitProducerKVP;
                        newHash->entries[i].key.store(details::invalid_thread_id, std::memory_order_relaxed);
                    }
                    newHash->prev = mainHash;
                    implicitProducerHash.store(newHash, std::memory_order_release);
                    implicitProducerHashResizeInProgress.clear(std::memory_order_release);
                    mainHash = newHash;
                } else {
                    implicitProducerHashResizeInProgress.clear(std::memory_order_release);
                }
            }
 
            // If it's < three-quarters full, add to the old one anyway so that we don't have to wait for the next table
            // to finish being allocated by another thread (and if we just finished allocating above, the condition will
            // always be true)
            if (newCount < (mainHash->capacity >> 1) + (mainHash->capacity >> 2)) {
                auto producer = static_cast<ImplicitProducer*>(recycle_or_create_producer(false));
                if (producer == nullptr) {
                    implicitProducerHashCount.fetch_sub(1, std::memory_order_relaxed);
                    return nullptr;
                }
 
#ifdef MOODYCAMEL_CPP11_THREAD_LOCAL_SUPPORTED
                producer->threadExitListener.callback = &ConcurrentQueue::implicit_producer_thread_exited_callback;
                producer->threadExitListener.userData = producer;
                details::ThreadExitNotifier::subscribe(&producer->threadExitListener);
#endif
 
                auto index = hashedId;
                while (true) {
                    index &= mainHash->capacity - 1u;
                    auto empty = details::invalid_thread_id;
#ifdef MOODYCAMEL_CPP11_THREAD_LOCAL_SUPPORTED
                    auto reusable = details::invalid_thread_id2;
                    if (mainHash->entries[index].key.compare_exchange_strong(
                            reusable, id, std::memory_order_seq_cst, std::memory_order_relaxed)) {
                        implicitProducerHashCount.fetch_sub(
                            1, std::memory_order_relaxed); // already counted as a used slot
                        mainHash->entries[index].value = producer;
                        break;
                    }
#endif
                    if (mainHash->entries[index].key.compare_exchange_strong(
                            empty, id, std::memory_order_seq_cst, std::memory_order_relaxed)) {
                        mainHash->entries[index].value = producer;
                        break;
                    }
                    ++index;
                }
                return producer;
            }
 
            // Hmm, the old hash is quite full and somebody else is busy allocating a new one.
            // We need to wait for the allocating thread to finish (if it succeeds, we add, if not,
            // we try to allocate ourselves).
            mainHash = implicitProducerHash.load(std::memory_order_acquire);
        }
    }
 
#ifdef MOODYCAMEL_CPP11_THREAD_LOCAL_SUPPORTED
    void implicit_producer_thread_exited(ImplicitProducer* producer)
    {
        // Remove from hash
#ifdef MCDBGQ_NOLOCKFREE_IMPLICITPRODHASH
        debug::DebugLock lock(implicitProdMutex);
#endif
        auto hash = implicitProducerHash.load(std::memory_order_acquire);
        assert(hash !=
               nullptr); // The thread exit listener is only registered if we were added to a hash in the first place
        auto id = details::thread_id();
        auto hashedId = details::hash_thread_id(id);
        details::thread_id_t probedKey;
 
        // We need to traverse all the hashes just in case other threads aren't on the current one yet and are
        // trying to add an entry thinking there's a free slot (because they reused a producer)
        for (; hash != nullptr; hash = hash->prev) {
            auto index = hashedId;
            do {
                index &= hash->capacity - 1u;
                probedKey = id;
                if (hash->entries[index].key.compare_exchange_strong(
                        probedKey, details::invalid_thread_id2, std::memory_order_seq_cst, std::memory_order_relaxed)) {
                    break;
                }
                ++index;
            } while (probedKey !=
                     details::invalid_thread_id); // Can happen if the hash has changed but we weren't put back in it
                                                  // yet, or if we weren't added to this hash in the first place
        }
 
        // Mark the queue as being recyclable
        producer->inactive.store(true, std::memory_order_release);
    }
 
    static void implicit_producer_thread_exited_callback(void* userData)
    {
        auto producer = static_cast<ImplicitProducer*>(userData);
        auto queue = producer->parent;
        queue->implicit_producer_thread_exited(producer);
    }
#endif
 
    // Utility functions
 
    template <typename TAlign> static inline void* aligned_malloc(size_t size)
    {
        MOODYCAMEL_CONSTEXPR_IF(std::alignment_of<TAlign>::value <= std::alignment_of<details::max_align_t>::value)
        return (Traits::malloc)(size);
        else
        {
            size_t alignment = std::alignment_of<TAlign>::value;
            void* raw = (Traits::malloc)(size + alignment - 1 + sizeof(void*));
            if (!raw)
                return nullptr;
            char* ptr = details::align_for<TAlign>(reinterpret_cast<char*>(raw) + sizeof(void*));
            *(reinterpret_cast<void**>(ptr) - 1) = raw;
            return ptr;
        }
    }
 
    template <typename TAlign> static inline void aligned_free(void* ptr)
    {
        MOODYCAMEL_CONSTEXPR_IF(std::alignment_of<TAlign>::value <= std::alignment_of<details::max_align_t>::value)
        return (Traits::free)(ptr);
        else(Traits::free)(ptr ? *(reinterpret_cast<void**>(ptr) - 1) : nullptr);
    }
 
    template <typename U> static inline U* create_array(size_t count)
    {
        assert(count > 0);
        U* p = static_cast<U*>(aligned_malloc<U>(sizeof(U) * count));
        if (p == nullptr)
            return nullptr;
 
        for (size_t i = 0; i != count; ++i)
            new (p + i) U();
        return p;
    }
 
    template <typename U> static inline void destroy_array(U* p, size_t count)
    {
        if (p != nullptr) {
            assert(count > 0);
            for (size_t i = count; i != 0;)
                (p + --i)->~U();
        }
        aligned_free<U>(p);
    }
 
    template <typename U> static inline U* create()
    {
        void* p = aligned_malloc<U>(sizeof(U));
        return p != nullptr ? new (p) U : nullptr;
    }
 
    template <typename U, typename A1> static inline U* create(A1&& a1)
    {
        void* p = aligned_malloc<U>(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();
        aligned_free<U>(p);
    }
 
  private:
    std::atomic<ProducerBase*> producerListTail;
    std::atomic<std::uint32_t> producerCount;
 
    std::atomic<size_t> initialBlockPoolIndex;
    Block* initialBlockPool;
    size_t initialBlockPoolSize;
 
#ifndef MCDBGQ_USEDEBUGFREELIST
    FreeList<Block> freeList;
#else
    debug::DebugFreeList<Block> freeList;
#endif
 
    std::atomic<ImplicitProducerHash*> implicitProducerHash;
    std::atomic<size_t> implicitProducerHashCount; // Number of slots logically used
    ImplicitProducerHash initialImplicitProducerHash;
    std::array<ImplicitProducerKVP, INITIAL_IMPLICIT_PRODUCER_HASH_SIZE> initialImplicitProducerHashEntries;
    std::atomic_flag implicitProducerHashResizeInProgress;
 
    std::atomic<std::uint32_t> nextExplicitConsumerId;
    std::atomic<std::uint32_t> globalExplicitConsumerOffset;
 
#ifdef MCDBGQ_NOLOCKFREE_IMPLICITPRODHASH
    debug::DebugMutex implicitProdMutex;
#endif
 
#ifdef MOODYCAMEL_QUEUE_INTERNAL_DEBUG
    std::atomic<ExplicitProducer*> explicitProducers;
    std::atomic<ImplicitProducer*> implicitProducers;
#endif
};
 
template <typename T, typename Traits>
ProducerToken::ProducerToken(ConcurrentQueue<T, Traits>& queue)
    : producer(queue.recycle_or_create_producer(true))
{
    if (producer != nullptr) {
        producer->token = this;
    }
}
 
template <typename T, typename Traits>
ProducerToken::ProducerToken(BlockingConcurrentQueue<T, Traits>& queue)
    : producer(reinterpret_cast<ConcurrentQueue<T, Traits>*>(&queue)->recycle_or_create_producer(true))
{
    if (producer != nullptr) {
        producer->token = this;
    }
}
 
template <typename T, typename Traits>
ConsumerToken::ConsumerToken(ConcurrentQueue<T, Traits>& queue)
    : itemsConsumedFromCurrent(0)
    , currentProducer(nullptr)
    , desiredProducer(nullptr)
{
    initialOffset = queue.nextExplicitConsumerId.fetch_add(1, std::memory_order_release);
    lastKnownGlobalOffset = static_cast<std::uint32_t>(-1);
}
 
template <typename T, typename Traits>
ConsumerToken::ConsumerToken(BlockingConcurrentQueue<T, Traits>& queue)
    : itemsConsumedFromCurrent(0)
    , currentProducer(nullptr)
    , desiredProducer(nullptr)
{
    initialOffset = reinterpret_cast<ConcurrentQueue<T, Traits>*>(&queue)->nextExplicitConsumerId.fetch_add(
        1, std::memory_order_release);
    lastKnownGlobalOffset = static_cast<std::uint32_t>(-1);
}
 
template <typename T, typename Traits>
inline void swap(ConcurrentQueue<T, Traits>& a, ConcurrentQueue<T, Traits>& b) MOODYCAMEL_NOEXCEPT
{
    a.swap(b);
}
 
inline void swap(ProducerToken& a, ProducerToken& b) MOODYCAMEL_NOEXCEPT
{
    a.swap(b);
}
 
inline void swap(ConsumerToken& a, ConsumerToken& b) MOODYCAMEL_NOEXCEPT
{
    a.swap(b);
}
 
template <typename T, typename Traits>
inline void swap(typename ConcurrentQueue<T, Traits>::ImplicitProducerKVP& a,
                 typename ConcurrentQueue<T, Traits>::ImplicitProducerKVP& b) MOODYCAMEL_NOEXCEPT
{
    a.swap(b);
}
 
} // namespace moodycamel
 
#if defined(_MSC_VER) && (!defined(_HAS_CXX17) || !_HAS_CXX17)
#pragma warning(pop)
#endif
 
#if defined(__GNUC__) && !defined(__INTEL_COMPILER)
#pragma GCC diagnostic pop
#endif