//
// Created by krislyy on 2018/11/14.
//

#ifndef ALGORITHM_THREADPOOL_H
#define ALGORITHM_THREADPOOL_H

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

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

    template<class F, class... Args>
    std::future<typename std::result_of<F(Args...)>::type> enqueue(F&& f, Args&&... args);

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 ThreadPool::ThreadPool(size_t threads)
        :   stop(false) {
    for (size_t i = 0; i < threads; ++i) {
        workers.emplace_back(
            [this] () ->void {
                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();
                }
            }
        );
    }
}

// add new work item to the pool
template<class F, class... Args>
std::future<typename std::result_of<F(Args...)>::type> ThreadPool::enqueue(F&& f, Args&&... args)
{
    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;
}

// 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();
}

#endif //ALGORITHM_THREADPOOL_H

//测试代码

cout << "///////////////////////////////////" << endl;
    ThreadPool pool(4);
    std::vector< std::future<int> > results;

    for(int i = 0; i < 8; ++i) {
        results.emplace_back(
                pool.enqueue([i](){
                    cout << "thread id: " << std::this_thread::get_id() << endl;
                    std::this_thread::sleep_for(std::chrono::milliseconds(500));
                    return i*i;
                })
        );
    }

    for(auto && result: results)
        std::cout << result.get() << ' ';