// Copyright 2022 Huawei Cloud Computing Technology Co., Ltd.
//
// 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.

#ifndef INCLUDED_SRC_BUILDTOOL_MULTITHREADING_TASK_SYSTEM_HPP
#define INCLUDED_SRC_BUILDTOOL_MULTITHREADING_TASK_SYSTEM_HPP

#include <algorithm>
#include <atomic>
#include <cstddef>
#include <thread>
#include <utility>  // std::forward
#include <vector>

#include "src/buildtool/multithreading/notification_queue.hpp"

class TaskSystem {
  public:
    // Constructors create as many threads as specified (or
    // std::thread::hardware_concurrency() many if not specified) running
    // `TaskSystem::Run(index)` on them, where `index` is their position in
    // `threads_`
    TaskSystem();
    explicit TaskSystem(std::size_t number_of_threads);

    TaskSystem(TaskSystem const&) = delete;
    TaskSystem(TaskSystem&&) = delete;
    auto operator=(TaskSystem const&) -> TaskSystem& = delete;
    auto operator=(TaskSystem&&) -> TaskSystem& = delete;

    // Destructor calls sets to "done" all notification queues and joins the
    // threads. Note that joining the threads will wait until the Run method
    // they are running is finished
    ~TaskSystem();

    // Queue a task. Task will be added to the first notification queue that is
    // found to be unlocked or, if none is found (after kNumberOfAttemps
    // iterations), to the one in `index+1` position waiting until it's
    // unlocked.
    template <typename FunctionType>
    void QueueTask(FunctionType&& f) noexcept {
        auto idx = index_++;

        for (std::size_t i = 0; i < thread_count_ * kNumberOfAttempts; ++i) {
            if (queues_[(idx + i) % thread_count_].try_push(
                    std::forward<FunctionType>(f))) {
                return;
            }
        }
        queues_[idx % thread_count_].push(std::forward<FunctionType>(f));
    }

    [[nodiscard]] auto NumberOfThreads() const noexcept -> std::size_t {
        return thread_count_;
    }

    // Initiate shutdown, skip execution of pending tasks
    void Shutdown() noexcept;

    // Wait for all queues to become empty _and_ all tasks to finish.
    void Finish() noexcept;

  private:
    std::size_t const thread_count_{
        std::max(1U, std::thread::hardware_concurrency())};
    std::vector<std::thread> threads_;
    std::vector<NotificationQueue> queues_;
    std::atomic<std::size_t> index_{0};
    std::atomic<bool> shutdown_ = false;
    WaitableZeroCounter total_workload_;

    static constexpr std::size_t kNumberOfAttempts = 5;

    void Run(std::size_t idx);
};

#endif  // INCLUDED_SRC_BUILDTOOL_MULTITHREADING_TASK_SYSTEM_HPP