threadpool.h

#pragma once
#include <vector>
#include <queue>
#include <thread>
#include <mutex>
#include <condition_variable>
#include <future>
#include <functional>
#include <stdexcept>
#include <algorithm>
#include <memory>
#include <map>
#include <type_traits>

namespace cpplib {
    class ThreadPool {
    public:
        ThreadPool(size_t threads) : stop(false) {
            newThreads(threads);
        }

        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;
        }

        void removeThreads(size_t threads) {
            if (workers.size() < threads) throw std::runtime_error("attempted to stop more then the amount of threads");
            size_t threadsStopped = 0;
            while (threadsStopped < threads) {
                auto it = workers.begin();
                while (it != workers.end() && threadsStopped < threads) {
                    {
                        std::lock_guard<std::mutex> state_lock(it->second->state_mutex);
                        it->second->state = ThreadState::Stopped;
                    }
                    condition.notify_all();
                    it = workers.erase(it);
                    ++threadsStopped;
                }
            }
        }

        void addThreads(size_t threads) { newThreads(threads); }

        size_t getThreadCount() const { return workers.size(); }
        [[deprecated("Use getThreadCount() instead")]]
        size_t gethreadCount() const { return getThreadCount(); }

        ~ThreadPool() {
            {
                std::unique_lock<std::mutex> lock(queue_mutex);
                stop = true;
            }
            condition.notify_all();
        }

    private:
        enum class ThreadState {
            Waiting,
            Running,
            Stopped
        };

        class Worker {
        public:
            Worker(std::thread&& thread) : thread(std::move(thread)), state(ThreadState::Waiting) {}
            Worker() : state(ThreadState::Stopped) {}
            ~Worker() { if (thread.joinable()) thread.join(); }
            std::thread thread;
            ThreadState state;
            std::mutex state_mutex;
        };

        void newThreads(size_t threads) {
            for (size_t i = 0; i < threads; ++i, threadID++) {
                workers.emplace(threadID, std::make_unique<Worker>());
                Worker* worker = workers[this->threadID].get();
                worker->thread = std::thread([this, worker] {
                    {
                        std::lock_guard<std::mutex> state_lock(worker->state_mutex);
                        worker->state = ThreadState::Waiting;
                    }
                    for (;;) {
                        std::function<void()> task;
                        {
                            std::unique_lock<std::mutex> lock(this->queue_mutex);
                            this->condition.wait(lock, [this, worker] {
                                std::lock_guard<std::mutex> state_guard(worker->state_mutex);
                                return this->stop || worker->state == ThreadState::Stopped || !this->tasks.empty(); 
                            });
                            std::lock_guard<std::mutex> state_guard(worker->state_mutex);
                            if ((this->stop && this->tasks.empty()) || worker->state == ThreadState::Stopped) {
                                return;
                            }
                            else {
                                task = std::move(this->tasks.front());
                                this->tasks.pop();
                            }
                        }
                        if (task) {
                            {
                                std::lock_guard<std::mutex> state_lock(worker->state_mutex);
                                if (worker->state != ThreadState::Stopped) {
                                    worker->state = ThreadState::Running;
                                }
                            }
                            task();
                            {
                                std::lock_guard<std::mutex> state_lock(worker->state_mutex);
                                if (worker->state != ThreadState::Stopped) {
                                    worker->state = ThreadState::Waiting;
                                }
                            }
                        }
                    }
                });
            }
        }
        std::map<size_t, std::unique_ptr<Worker>> workers;
        std::queue<std::function<void()>> tasks;
        std::mutex queue_mutex;
        std::condition_variable condition;
        bool stop;
        size_t threadID = 0;
    };
}