#ifndef THREAD_POOL_HPP
#define THREAD_POOL_HPP

#include <functional>
#include <future>
#include <queue>

class ThreadPool {
public:
    explicit ThreadPool(size_t);
    template<class F, class... Args>
    decltype(auto) enqueue(F&& f, Args&&... args);
    // the destructor joins all threads
    ~ThreadPool() {
        {
            std::unique_lock<std::mutex> lock(queue_mutex);
            stop = true;
        }
        condition.notify_all();
        for (std::thread& worker : workers) {
            worker.join();
        }
    }
private:
    // need to keep track of threads so we can join them
    std::vector< std::thread > workers;
    // the task queue
    std::queue< std::packaged_task<void()> > tasks;

    // synchronization
    std::mutex queue_mutex;
    std::condition_variable condition;
    bool stop;
};

// the constructor just launches some amount of workers
inline ThreadPool::ThreadPool(size_t threads)
    :   stop(false)
{
    for(size_t i = 0;i<threads;++i)
        workers.emplace_back(
            [this]
            {
                for(;;)
                {
                    std::packaged_task<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();
                }
            }
        );
}

// add new work item to the pool
template<class F, class... Args>
decltype(auto) ThreadPool::enqueue(F&& f, Args&&... args)
{
    using return_type = std::invoke_result_t<F, Args...>;

    std::packaged_task<return_type()> task(
            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(std::move(task));
    }
    condition.notify_one();
    return res;
}

#endif