// cudadecoder/thread-pool.h
  // Source:  https://github.com/progschj/ThreadPool
  // Modified to add a priority queue 
  // Ubtained under this license:
  /*
  Copyright (c) 2012 Jakob Progsch, Václav Zeman
  
  This software is provided 'as-is', without any express or implied
  warranty. In no event will the authors be held liable for any damages
  arising from the use of this software.
  
  Permission is granted to anyone to use this software for any purpose,
  including commercial applications, and to alter it and redistribute it
  freely, subject to the following restrictions:
  
     1. The origin of this software must not be misrepresented; you must not
     claim that you wrote the original software. If you use this software
     in a product, an acknowledgment in the product documentation would be
     appreciated but is not required.
  
     2. Altered source versions must be plainly marked as such, and must not be
     misrepresented as being the original software.
  
     3. This notice may not be removed or altered from any source
     distribution.
  */
  
  #ifndef KALDI_CUDA_DECODER_THREAD_POOL_H_
  #define KALDI_CUDA_DECODER_THREAD_POOL_H_
  
  #include <condition_variable>
  #include <functional>
  #include <future>
  #include <memory>
  #include <mutex>
  #include <queue>
  #include <stdexcept>
  #include <thread>
  #include <vector>
  
  namespace kaldi {
  namespace cuda_decoder {
  
  // C++ indexes enum 0,1,2...
  enum ThreadPoolPriority  { THREAD_POOL_LOW_PRIORITY, THREAD_POOL_NORMAL_PRIORITY, THREAD_POOL_HIGH_PRIORITY };
  
  class ThreadPool {
  public:
    ThreadPool(size_t);
    template <class F, class... Args>
    auto enqueue(ThreadPoolPriority priority, F &&f, Args &&... args)
        -> std::future<typename std::result_of<F(Args...)>::type>;
    template <class F, class... Args>
    auto enqueue(F &&f, Args &&... args)
        -> std::future<typename std::result_of<F(Args...)>::type>;
    ~ThreadPool();
  
   private:
    // need to keep track of threads so we can join them
    std::vector<std::thread> workers;
    // the task queue
    struct Task {
  	  std::function<void()> func;
            // Ordered first by priority, then FIFO order
            // tasks created first will have a higher priority_with_fifo.second
            std::pair<ThreadPoolPriority, long long> priority_with_fifo;
    };
    friend bool operator<(const ThreadPool::Task &lhs,
                          const ThreadPool::Task &rhs);
  
    std::priority_queue<Task> tasks;
    long long task_counter;
  
    // synchronization
    std::mutex queue_mutex;
    std::condition_variable condition;
  
    bool stop;
  };
  
  inline bool operator<(const ThreadPool::Task &lhs,
                        const ThreadPool::Task &rhs) {
    return lhs.priority_with_fifo < rhs.priority_with_fifo;
  }
  
  // the constructor just launches some amount of workers
  inline ThreadPool::ThreadPool(size_t threads)
      : task_counter(LONG_MAX), stop(false) {
    for (size_t i = 0; i < threads; ++i)
      workers.emplace_back([this] {
        for (;;) {
          Task task;
  
  	{
            std::unique_lock<std::mutex> lock(this->queue_mutex);
            this->condition.wait(
                lock, [this] { return this->stop || !this->tasks.empty(); });
            if (this->stop && this->tasks.empty()) return;
            if (!tasks.empty()) {
              task = std::move(this->tasks.top());
              this->tasks.pop();
          }
  	}
          task.func();
        }
      });
  }
  
  // add new work item to the pool : normal priority
  template <class F, class... Args>
  auto ThreadPool::enqueue(F &&f, Args &&... args)
      -> std::future<typename std::result_of<F(Args...)>::type> {
    return enqueue(THREAD_POOL_NORMAL_PRIORITY, std::forward<F>(f), std::forward<Args>(args)...);
  }
  
  // add new work item to the pool
  template <class F, class... Args>
  auto ThreadPool::enqueue(ThreadPoolPriority priority, F &&f, Args &&... args)
      -> std::future<typename std::result_of<F(Args...)>::type> {
    using return_type = typename std::result_of<F(Args...)>::type;
  
    auto func = std::make_shared<std::packaged_task<return_type()>>(
        std::bind(std::forward<F>(f), std::forward<Args>(args)...));
  
    std::future<return_type> res = func->get_future();
    {
      std::unique_lock<std::mutex> lock(queue_mutex);
  
      // don't allow enqueueing after stopping the pool
      if (stop)
        throw std::runtime_error("enqueue on stopped ThreadPool");
      Task task;
      task.func = [func]() { (*func)(); };
      long long task_fifo_id = task_counter--;
      // The following if will temporarly break the FIFO order
      // (leading to a perf drop for a few seconds)
      // But it should trigger in ~50 million years
      if (task_counter == 0) task_counter = LONG_MAX;
      task.priority_with_fifo = {priority, task_fifo_id};
      tasks.push(std::move(task));
    }
    condition.notify_one();
    return res;
  }
  
  // the destructor joins all threads
  inline ThreadPool::~ThreadPool() {
    {
      std::unique_lock<std::mutex> lock(queue_mutex);
      stop = true;
    }
    condition.notify_all();
    for (std::thread &worker : workers)
      worker.join();
  }
  
  }  // end namespace cuda_decoder
  }  // end namespace kaldi
  
  
  #endif  // KALDI_CUDA_DECODER_THREAD_POOL_H_