threadpool.hpp

#ifndef MUSH_THREADPOOL
#define MUSH_THREADPOOL

#include <vector>
#include <mutex>
#include <condition_variable>
#include <future>
#include <algorithm>
#include <functional>
#include <atomic>

#include "shared_queue.hpp"
#include "monadic_error.hpp"

namespace mush
{
    class thread_pool
    {
        public:
            template<typename Function, typename... Args>
            using enqueued_future = std::future<typename std::result_of<Function(Args...)>::type>;

            thread_pool(size_t thread_count = std::max(std::thread::hardware_concurrency(), 2u));
           ~thread_pool();

            thread_pool(const thread_pool&) = delete;
            thread_pool& operator=(const thread_pool&) = delete;

            size_t size() const;
            size_t active_threads() const;

            template<typename Function, typename... Args>
            Result<enqueued_future<Function, Args...>> enqueue(Function&& f, Args&&... args);

        private:
            std::vector<std::thread>            threads;
            shared_queue<std::function<void()>> tasks;

            std::mutex                          queue_mutex;
            std::condition_variable             condition;

            std::atomic<bool>                   end = false;
            std::atomic<size_t>                 running = 0;
    };

    inline thread_pool::thread_pool(size_t number_of_threads)
    {
        for (size_t i = 0; i < number_of_threads; ++i)
        {
            threads.emplace_back([&]{
                while(true)
                {
                    std::function<void()> task;

                    {
                        std::unique_lock<std::mutex> lock(queue_mutex);
                        condition.wait(lock, [&]{return end || tasks.empty(); });

                        if (end && tasks.empty())
                            return;

                        task = std::move(tasks.front());
                        tasks.pop();
                    }

                    running++;
                    task();
                    running--;
                }
            });
        }
    }

    inline thread_pool::~thread_pool()
    {
        {
            std::unique_lock<std::mutex> lock(queue_mutex);
            end = true;
        }
        condition.notify_all();

        for(auto& thread : threads)
            thread.join();
    }

    inline size_t thread_pool::size() const
    {
        return threads.size();
    }

    inline size_t thread_pool::active_threads() const
    {
        return running;
    }

    template <typename Function, typename... Args>
    Result<thread_pool::enqueued_future<Function, Args...>> thread_pool::enqueue(Function&& f, Args&&... args)
    {
        using return_type = typename std::result_of<Function(Args...)>::type;

        auto task = std::make_shared<std::packaged_task<return_type>>(
                        std::bind(std::forward<Function>(f), std::forward<Args>(args)...)
                    );

        std::future<return_type> result = task->get_future();
        {
            std::unique_lock<std::mutex> lock(queue_mutex);

            if (end)
                return Error("submitting to stopped threadpool");

            tasks.push([task](){(*task)();});
        }

        condition.notify_one();

        return result;
    }
}
#endif