// Licensed to the Apache Software Foundation (ASF) under one
// or more contributor license agreements.  See the NOTICE file
// distributed with this work for additional information
// regarding copyright ownership.  The ASF licenses this file
// to you 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 <vector>
#include <queue>
#include <memory>
#include <thread>
#include <mutex>
#include <condition_variable>
#include <future>
#include <functional>
#include <stdexcept>


#define MAX_THREADS_NUM     32

namespace zilliz {
namespace milvus {

class ThreadPool {
public:
    ThreadPool(size_t threads, size_t queue_size = 1000);

    template<class F, class... Args>
    auto enqueue(F &&f, Args &&... args)
    -> std::future<typename std::result_of<F(Args...)>::type>;

    ~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;

    size_t max_queue_size;

    // 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, size_t queue_size)
        : max_queue_size(queue_size), 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(); });
                            if (this->stop && this->tasks.empty())
                                return;
                            task = std::move(this->tasks.front());
                            this->tasks.pop();
                        }
                        this->condition.notify_all();

                        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);
        this->condition.wait(lock,
                             [this] { return this->tasks.size() < max_queue_size; });
        // don't allow enqueueing after stopping the pool
        if (stop)
            throw std::runtime_error("enqueue on stopped ThreadPool");

        tasks.emplace([task]() { (*task)(); });
    }
    condition.notify_all();
    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();
}

}
}