text: /** Credits to Jakob Progsch (@progschj)
   https://github.com/progschj/ThreadPool
  
   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.
  
   Modified by Binrui Li at Alibaba Group, 2021.
  */
  
  #ifndef GRAPE_UTILS_THREAD_POOL_H_
  #define GRAPE_UTILS_THREAD_POOL_H_
  
  #include <glog/logging.h>
  
  #include <condition_variable>
  #include <functional>
  #include <future>
  #include <memory>
  #include <mutex>
  #include <queue>
  #include <stdexcept>
  #include <thread>
  #include <vector>
  
  #include "grape/parallel/parallel_engine_spec.h"
  
  #if __linux__
  #ifndef _GNU_SOURCE
  #define _GNU_SOURCE
  #endif
  #include <sched.h>
  #endif
  
  class ThreadPool {
   public:
    ThreadPool(ThreadPool const&) = delete;
    ThreadPool& operator=(ThreadPool const&) = delete;
    ThreadPool() {}
    inline void InitThreadPool(const grape::ParallelEngineSpec&);
  
    template <class F, class... Args>
    auto enqueue(F&& f, Args&&... args)
        -> std::future<typename std::result_of<F(Args...)>::type>;
    inline int GetThreadNum() { return thread_num_; }
    void WaitEnd(std::vector<std::future<void>>& results);
    ~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{false};
    size_t thread_num_{1};
  };
  
  // the constructor just launches some amount of workers
  inline void ThreadPool::InitThreadPool(const grape::ParallelEngineSpec& spec) {
    bool affinity = false;
    affinity = spec.affinity && (!spec.cpu_list.empty());
    thread_num_ = spec.thread_num;
    for (size_t i = 0; i < thread_num_; ++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();
        }
      });
  #if __linux__
      if (affinity) {
        cpu_set_t cpuset;
        CPU_ZERO(&cpuset);
        CPU_SET(spec.cpu_list[i], &cpuset);
        pthread_setaffinity_np(workers[i].native_handle(), sizeof(cpu_set_t),
                               &cpuset);
        VLOG(2) << "bind thread " << i << " to " << spec.cpu_list[i] << std::endl;
      }
  #endif
    }
  }
  
  // use to wait for all tasks end
  inline void ThreadPool::WaitEnd(std::vector<std::future<void>>& results) {
    for (size_t tid = 0; tid < thread_num_; ++tid)
      results[tid].get();
  }
  
  // add new task 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  // GRAPE_UTILS_THREAD_POOL_H_
  