#pragma once

#include <vector>
#include <queue>
#include <thread>
#include <mutex>
#include <condition_variable>
#include <functional>
#include <atomic>
#include <future>

class ThreadPool {
private:
    std::vector<std::thread> workers_;
    std::queue<std::function<void()>> tasks_;
    std::mutex queue_mutex_;
    std::condition_variable condition_;
    std::atomic<bool> stop_{false};
    
public:
    explicit ThreadPool(size_t num_threads = std::thread::hardware_concurrency()) {
        for (size_t i = 0; i < num_threads; ++i) {
            workers_.emplace_back([this] {
                for (;;) {
                    std::function<void()> task;
                    
                    {
                        std::unique_lock<std::mutex> lock(queue_mutex_);
                        condition_.wait(lock, [this] { return stop_ || !tasks_.empty(); });
                        
                        if (stop_ && tasks_.empty()) return;
                        
                        task = std::move(tasks_.front());
                        tasks_.pop();
                    }
                    
                    task();
                }
            });
        }
    }
    
    template<class F, class... Args>
    auto enqueue(F&& f, Args&&... args) -> std::future<typename std::invoke_result<F, Args...>::type> {
        using return_type = typename std::invoke_result<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_);
            if (stop_) {
                throw std::runtime_error("enqueue on stopped ThreadPool");
            }
            
            tasks_.emplace([task](){ (*task)(); });
        }
        
        condition_.notify_one();
        return res;
    }
    
    size_t pending_tasks() const {
        std::lock_guard<std::mutex> lock(const_cast<std::mutex&>(queue_mutex_));
        return tasks_.size();
    }
    
    ~ThreadPool() {
        stop_ = true;
        condition_.notify_all();
        
        for (std::thread& worker : workers_) {
            worker.join();
        }
    }
};