#ifndef IL_THREAD_POOL_H
#define IL_THREAD_POOL_H

#include <vector>
#include <queue>
#include <memory>
#include <thread>
#include <mutex>
#include <condition_variable>
#include <future>
#include <functional>
#include <stdexcept>

class iLThreadPool {
public:
    iLThreadPool(size_t);
    ~iLThreadPool();

    template<class F, class... Args>
    auto enqueue(F&& f, Args&&... args) 
        -> std::future<typename std::result_of<F(Args...)>::type>;
private:
    // need to keep track of threads so we can join them
    std::vector< std::thread > workers;
    // the task queue
    std::queue< std::function<void()> > tasks;
    
    // synchronization
    std::mutex queue_mutex;
    std::condition_variable condition;
    bool stop;
};
 
// the constructor just launches some amount of workers
inline iLThreadPool::iLThreadPool(size_t threads)
    :   stop(false)
{
  for (size_t i = 0; i < threads; ++i)
  {
    workers.emplace_back(
      [this]
      {
        for (;;)
        {
          std::function<void()> 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;
            task = std::move(this->tasks.front());
            this->tasks.pop();
          }
          task();
        }
      });
  }
}

// the destructor joins all threads
inline iLThreadPool::~iLThreadPool()
{
  {
    std::unique_lock<std::mutex> lock(queue_mutex);
    stop = true;
  }
  condition.notify_all();
  for (std::thread &worker : workers)
    worker.join();
}

// add new work item to the pool
template<class F, class... Args>
auto iLThreadPool::enqueue(F&& f, Args&&... args) 
    -> std::future<typename std::result_of<F(Args...)>::type>
{
  using return_type = typename std::result_of<F(Args...)>::type;
  auto task = std::make_shared< std::packaged_task<return_type()> >(
      std::bind(std::forward<F>(f), std::forward<Args>(args)...)
      );

  std::future<return_type> res = task->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");
    tasks.emplace([task](){ (*task)(); });
  }
  condition.notify_one();
  return res;
}

#endif // IL_THREAD_POOL_H