typedef std::function<void (WorkItemPtr)> AppCallback;

OrderedWorkerPool(const AppCallback& callback, uint32_t pool_size);

~OrderedWorkerPool();

void Add(WorkItemPtr work_item);

void DoWork();

void DoDelivery();

void ReadyForDelivery(WorkItemPtr work_item);

const AppCallback callback_;

// Consumer threads and their queue

std::queue<WorkItemPtr> queue_;

std::vector<std::thread> threads_;

std::mutex mutex_;

std::condition_variable cond_;

bool finished_;

// Delivery thread and its queue

std::thread delivery_thread_;

std::queue<WorkItemPtr> delivery_queue_;

std::mutex delivery_mutex_;

std::condition_variable delivery_cond_;

bool delivery_finished_;