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

class ThreadPool {
public:
  ThreadPool(size_t);
  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
  std::queue< std::function<void()> > tasks;
  
  // synchronization
  std::mutex queue_mutex;
  std::condition_variable condition;
  bool stop;
};

inline ThreadPool::ThreadPool(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 ThreadPool::~ThreadPool()
{
  {
    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 ThreadPool::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;
}

void DataProcessing(std::string& data, const std::string& operation) {
  data += operation;
  std::this_thread::sleep_for(std::chrono::seconds(1));
}

int main(int argc, char** argv) {
  ThreadPool thread_pool(4);
  std::string data = "raw";
  auto time1 = std::chrono::system_clock::now();
  std::vector<std::future<void>> futures;
  for (int i = 0; i < 4; ++i) {
    futures.emplace_back(thread_pool.enqueue(std::bind(DataProcessing, std::ref(data), std::to_string(i))));
  }
  for (auto& future : futures) {
    try {
      if (future.valid()) {
        future.get();
      } else {
        std::cerr << "Future is invalid";
      }
    } catch (const std::future_error& ex) {
      std::cerr << "Caught a future error with code[" << ex.code() << "] message[" << ex.what() << "].";
      throw ex;
    }
  }
  auto time2 = std::chrono::system_clock::now();
  std::chrono::duration<double> diff = time2 - time1;
  std::cout << data << std::endl;
  std::cout << "Time Diff = " << diff.count() * 1000<< " msec." << std::endl;
  return 0;
}