text: // Adapted from https://github.com/facebook/folly/blob/master/folly/ProducerConsumerQueue.h
  /*
   * Copyright 2013 Facebook, Inc.
   *
   * Licensed under the Apache License, Version 2.0 (the "License");
   * you may not use this file except in compliance with the License.
   * You may obtain a copy of the License at
   *
   *   http://www.apache.org/licenses/LICENSE-2.0
   *
   * Unless required by applicable law or agreed to in writing, software
   * distributed under the License is distributed on an "AS IS" BASIS,
   * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
   * See the License for the specific language governing permissions and
   * limitations under the License.
   */
  
  // @author Bo Hu (bhu@fb.com)
  // @author Jordan DeLong (delong.j@fb.com)
  
  #ifndef PRODUCER_CONSUMER_QUEUE_H_
  #define PRODUCER_CONSUMER_QUEUE_H_
  
  #include <new>
  #include <atomic>
  #include <cassert>
  #include <cstdlib>
  #include <stdexcept>
  #include <type_traits>
  #include <utility>
  //#include <boost/noncopyable.hpp>
  
  namespace folly {
  
  /*
   * ProducerConsumerQueue is a one producer and one consumer queue
   * without locks.
   */
  template<class T>
  struct ProducerConsumerQueue {
    typedef T value_type;
  
    // size must be >= 1.
    explicit ProducerConsumerQueue(uint32_t size)
      : size_(size + 1)    // +1 because one slot is always empty
      , records_(static_cast<T*>(std::malloc(sizeof(T) * (size + 1))))
      , readIndex_(0)
      , writeIndex_(0)
    {
      assert(size >= 1);
      if (!records_) {
        throw std::bad_alloc();
      }
    }
  
    ~ProducerConsumerQueue() {
      // We need to destruct anything that may still exist in our queue.
      // (No real synchronization needed at destructor time: only one
      // thread can be doing this.)
      if (!std::is_trivially_destructible<T>::value) {
        int read = readIndex_;
        int end = writeIndex_;
        while (read != end) {
          records_[read].~T();
          if (++read == size_) {
            read = 0;
          }
        }
      }
  
      std::free(records_);
    }
  
    template<class ...Args>
    bool enqueue(Args&&... recordArgs) {
      auto const currentWrite = writeIndex_.load(std::memory_order_relaxed);
      auto nextRecord = currentWrite + 1;
      if (nextRecord == size_) {
        nextRecord = 0;
      }
      if (nextRecord != readIndex_.load(std::memory_order_acquire)) {
        new (&records_[currentWrite]) T(std::forward<Args>(recordArgs)...);
        writeIndex_.store(nextRecord, std::memory_order_release);
        return true;
      }
  
      // queue is full
      return false;
    }
  
    // move (or copy) the value at the front of the queue to given variable
    bool try_dequeue(T& record) {
      auto const currentRead = readIndex_.load(std::memory_order_relaxed);
      if (currentRead == writeIndex_.load(std::memory_order_acquire)) {
        // queue is empty
        return false;
      }
  
      auto nextRecord = currentRead + 1;
      if (nextRecord == size_) {
        nextRecord = 0;
      }
      record = std::move(records_[currentRead]);
      records_[currentRead].~T();
      readIndex_.store(nextRecord, std::memory_order_release);
      return true;
    }
  
    // pointer to the value at the front of the queue (for use in-place) or
    // nullptr if empty.
    T* frontPtr() {
      auto const currentRead = readIndex_.load(std::memory_order_relaxed);
      if (currentRead == writeIndex_.load(std::memory_order_acquire)) {
        // queue is empty
        return nullptr;
      }
      return &records_[currentRead];
    }
  
    // queue must not be empty
    void popFront() {
      auto const currentRead = readIndex_.load(std::memory_order_relaxed);
      assert(currentRead != writeIndex_.load(std::memory_order_acquire));
  
      auto nextRecord = currentRead + 1;
      if (nextRecord == size_) {
        nextRecord = 0;
      }
      records_[currentRead].~T();
      readIndex_.store(nextRecord, std::memory_order_release);
    }
  
    bool isEmpty() const {
     return readIndex_.load(std::memory_order_consume) ==
           writeIndex_.load(std::memory_order_consume);
    }
  
    bool isFull() const {
      auto nextRecord = writeIndex_.load(std::memory_order_consume) + 1;
      if (nextRecord == size_) {
        nextRecord = 0;
      }
      if (nextRecord != readIndex_.load(std::memory_order_consume)) {
        return false;
      }
      // queue is full
      return true;
    }
  
    // * If called by consumer, then true size may be more (because producer may
    //   be adding items concurrently).
    // * If called by producer, then true size may be less (because consumer may
    //   be removing items concurrently).
    // * It is undefined to call this from any other thread.
    size_t sizeGuess() const {
      int ret = writeIndex_.load(std::memory_order_consume) -
                readIndex_.load(std::memory_order_consume);
      if (ret < 0) {
        ret += size_;
      }
      return ret;
    }
  
  private:
    const uint32_t size_;
    T* const records_;
  
    std::atomic<int> readIndex_;
    std::atomic<int> writeIndex_;
  };
  
  }
  
  #endif
  