/* Copyright (c) 2018 PaddlePaddle Authors. All Rights Reserved.

Licensed under the Apache License, Version 2.0 (the "License");
you may not use this file except in compliance with the License.
You may obtain a copy of the License at

    http://www.apache.org/licenses/LICENSE-2.0

Unless required by applicable law or agreed to in writing, software
distributed under the License is distributed on an "AS IS" BASIS,
WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
See the License for the specific language governing permissions and
limitations under the License. */

#pragma once

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

class ThreadPool {
 public:
  static ThreadPool& getThreadPool();
  static int getThreadPoolThreadId();
  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();
  int getTid(const std::thread::id& id) {
    for (int i = 0; i < workers.size(); i++) {
      if (workers[i].get_id() == id) {
        return i;
      }
    }
    return -1;
  }

 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(); });
          // for (;;) {
          //     if (this->stop || !this->tasks.empty()) {
          //         break;
          //     }
          //     lock.unlock();
          //     lock.lock();
          // }
          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();
}

ThreadPool& ThreadPool::getThreadPool() {
  static ThreadPool threadPool(3);
  return threadPool;
}

int ThreadPool::getThreadPoolThreadId() {
  return getThreadPool().getTid(std::this_thread::get_id());
}