CoRoutineThreadPool.hpp

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/*
    MIT License
 
    DiscordCoreAPI, A bot library for Discord, written in C++, and featuring explicit multithreading through the usage of custom, asynchronous C++ CoRoutines.
 
    Copyright 2022, 2023 Chris M. (RealTimeChris)
 
    Permission is hereby granted, free of charge, to any person obtaining a copy
    of this software and associated documentation files (the "Software"), to deal
    in the Software without restriction, including without limitation the rights
    to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
    copies of the Software, and to permit persons to whom the Software is
    furnished to do so, subject to the following conditions:
 
    The above copyright notice and this permission notice shall be included in all
    copies or substantial portions of the Software.
 
    THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
    IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
    FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
    AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
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*/
/// CoRoutineThreadPool.hpp - Header for the "co_routine_thread_pool" related stuff.
/// Dec 18, 2021
/// https://discordcoreapi.com
/// \file CoRoutineThreadPool.hpp
#pragma once
 
#include <discordcoreapi/Utilities/UnboundedMessageBlock.hpp>
#include <coroutine>
#include <deque>
 
using namespace std::literals;
 
namespace discord_core_api {
 
    namespace discord_core_internal {
 
        /**
        * \addtogroup discord_core_internal
        * @{
        */
 
        /// @brief A struct representing a worker thread for coroutine-based tasks.
        struct worker_thread {
            inline worker_thread() = default;
 
            inline worker_thread& operator=(worker_thread&& other) noexcept {
                this->areWeCurrentlyWorking.store(other.areWeCurrentlyWorking.load(std::memory_order_acquire), std::memory_order_release);
                std::swap(this->thread, other.thread);
                tasks = std::move(other.tasks);
                return *this;
            }
 
            inline worker_thread(worker_thread&& other) noexcept {
                *this = std::move(other);
            }
 
            inline ~worker_thread() = default;
 
            unbounded_message_block<std::coroutine_handle<>> tasks{};///< Queue of coroutine tasks.
            std::atomic_bool areWeCurrentlyWorking{};///< Atomic flag indicating if the thread is working.
            std::jthread thread{};///< Joinable thread.
        };
 
        /// @brief A class representing a coroutine-based thread pool.
        class co_routine_thread_pool : protected discord_core_api::unordered_map<uint64_t, unique_ptr<worker_thread>> {
          public:
            using map_type = discord_core_api::unordered_map<uint64_t, unique_ptr<worker_thread>>;
 
            /// @brief Constructor to create a coroutine thread pool. initializes the worker threads.
            inline co_routine_thread_pool() : threadCount(std::thread::hardware_concurrency()) {
                for (uint32_t x = 0; x < threadCount; ++x) {
                    currentIndex.fetch_add(1, std::memory_order_release);
                    currentCount.fetch_add(1, std::memory_order_release);
                    uint64_t indexNew = currentIndex.load(std::memory_order_acquire);
                    getMap().emplace(indexNew, makeUnique<worker_thread>());
                    getMap()[indexNew]->thread = std::jthread([=, this](std::stop_token tokenNew) mutable {
                        threadFunction(getMap().at(indexNew).get(), tokenNew);
                    });
                }
            }
 
            /// @brief Submit a coroutine task to the thread pool.
            /// @param coro the coroutine handle to submit.
            inline void submitTask(std::coroutine_handle<> coro) {
                bool areWeAllBusy{ true };
                uint64_t currentLowestValue{ std::numeric_limits<uint64_t>::max() };
                uint64_t currentLowestIndex{ std::numeric_limits<uint64_t>::max() };
                std::shared_lock lock01{ workerAccessMutex };
                for (auto& [key, value]: getMap()) {
                    if (!value->areWeCurrentlyWorking.load(std::memory_order_acquire)) {
                        areWeAllBusy = false;
                        if (value->tasks.size() < currentLowestValue) {
                            currentLowestValue = value->tasks.size();
                            currentLowestIndex = key;
                        }
                        break;
                    }
                }
                if (areWeAllBusy) {
                    currentIndex.fetch_add(1, std::memory_order_release);
                    currentCount.fetch_add(1, std::memory_order_release);
                    uint64_t indexNew = currentIndex.load(std::memory_order_acquire);
                    lock01.unlock();
                    std::unique_lock lock02{ workerAccessMutex };
                    getMap().emplace(indexNew, makeUnique<worker_thread>());
                    getMap()[indexNew]->thread = std::jthread([=, this](std::stop_token tokenNew) mutable {
                        threadFunction(getMap().at(indexNew).get(), tokenNew);
                    });
                    lock02.unlock();
                } else {
                    getMap()[currentLowestIndex]->tasks.send(std::move(coro));
                }
            }
 
            ~co_routine_thread_pool() {
                doWeQuit.store(true, std::memory_order_release);
            }
 
          protected:
            std::shared_mutex workerAccessMutex{};///< Shared mutex for worker thread access.
            std::atomic_uint64_t currentCount{};///< current count of worker threads.
            std::atomic_uint64_t currentIndex{};///< current index of worker threads.
            std::atomic_bool doWeQuit{ false };///< Whether or not we're quitting.
            const uint64_t threadCount{};///< Total thread count.
 
            /// @brief Thread function for each worker thread.
            /// @param thread A pointer to the current thread of execution.
            /// @param tokenNew The stop token for the thread.
            inline void threadFunction(worker_thread* thread, std::stop_token tokenNew) {
                while (!doWeQuit.load(std::memory_order_acquire) && !tokenNew.stop_requested()) {
                    std::coroutine_handle<> coroHandle{};
                    if (thread->tasks.tryReceive(coroHandle)) {
                        thread->areWeCurrentlyWorking.store(true, std::memory_order_release);
                        try {
                            coroHandle();
                            while (!coroHandle.done()) {
                                std::this_thread::sleep_for(1ms);
                            }
                        } catch (const std::runtime_error& error) {
                            message_printer::printError<print_message_type::general>(error.what());
                        }
                        thread->areWeCurrentlyWorking.store(false, std::memory_order_release);
                    }
                    if (currentCount.load(std::memory_order_acquire) > threadCount) {
                        uint64_t extraWorkers{ currentCount.load(std::memory_order_acquire) - threadCount };
                        while (extraWorkers > 0) {
                            --extraWorkers;
                            size_type currentHighestIndex{};
                            for (const auto& [key, value]: *this) {
                                if (key > currentHighestIndex) {
                                    currentHighestIndex = key;
                                }
                            }
                            std::unique_lock lock{ workerAccessMutex };
                            auto oldThread = begin() + currentHighestIndex;
                            if (oldThread->second->thread.joinable()) {
                                oldThread->second->thread.request_stop();
                                oldThread->second->thread.detach();
                                currentCount.fetch_sub(1, std::memory_order_release);
                                getMap().erase(oldThread->first);
                            }
                        }
                    }
                    std::this_thread::sleep_for(std::chrono::nanoseconds{ 100000 });
                }
            }
 
            inline map_type& getMap() {
                return *this;
            }
        };
 
        /**@}*/
    }
}