/**
 * Copyright (c) Facebook, Inc. and its affiliates.
 *
 * This source code is licensed under the MIT license found in the
 * LICENSE file in the root directory of this source tree.
 */

#pragma once

#include <condition_variable>
#include <deque>
#include <functional>
#include <future>
#include <thread>

namespace faiss {

class WorkerThread {
   public:
    WorkerThread();

    /// Stops and waits for the worker thread to exit, flushing all
    /// pending lambdas
    ~WorkerThread();

    /// Request that the worker thread stop itself
    void stop();

    /// Blocking waits in the current thread for the worker thread to
    /// stop
    void waitForThreadExit();

    /// Adds a lambda to run on the worker thread; returns a future that
    /// can be used to block on its completion.
    /// Future status is `true` if the lambda was run in the worker
    /// thread; `false` if it was not run, because the worker thread is
    /// exiting or has exited.
    std::future<bool> add(std::function<void()> f);

   private:
    void startThread();
    void threadMain();
    void threadLoop();

    /// Thread that all queued lambdas are run on
    std::thread thread_;

    /// Mutex for the queue and exit status
    std::mutex mutex_;

    /// Monitor for the exit status and the queue
    std::condition_variable monitor_;

    /// Whether or not we want the thread to exit
    bool wantStop_;

    /// Queue of pending lambdas to call
    std::deque<std::pair<std::function<void()>, std::promise<bool>>> queue_;
};

} // namespace faiss
