// Copyright (c) 2012 Jakob Progsch, Václav Zeman

// This software is provided 'as-is', without any express or implied
// warranty. In no event will the authors be held liable for any damages
// arising from the use of this software.

// Permission is granted to anyone to use this software for any purpose,
// including commercial applications, and to alter it and redistribute it
// freely, subject to the following restrictions:

//   1. The origin of this software must not be misrepresented; you must not
//   claim that you wrote the original software. If you use this software
//   in a product, an acknowledgment in the product documentation would be
//   appreciated but is not required.

//   2. Altered source versions must be plainly marked as such, and must not be
//   misrepresented as being the original software.

//   3. This notice may not be removed or altered from any source
//   distribution.
// this code is from https://github.com/progschj/ThreadPool

#ifndef BASE_THREAD_POOL_H
#define BASE_THREAD_POOL_H

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

class ThreadPool {
  public:
    explicit 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;
};

// 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::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>
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;
}

// 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
