#ifndef SRC_CALLBACK_QUEUE_H_

#define SRC_CALLBACK_QUEUE_H_

#if defined(NODE_WANT_INTERNALS) && NODE_WANT_INTERNALS

#include <atomic>

namespace node {

namespace CallbackFlags {

enum Flags {

  kUnrefed = 0,

  kRefed = 1,

};

}

// A queue of C++ functions that take Args... as arguments and return R

// (this is similar to the signature of std::function).

// New entries are added using `CreateCallback()`/`Push()`, and removed using

// `Shift()`.

// The `refed` flag is left for easier use in situations in which some of these

// should be run even if nothing else is keeping the event loop alive.

template <typename R, typename... Args>

class CallbackQueue {

 public:

  class Callback {

   public:

    explicit inline Callback(CallbackFlags::Flags flags);

    virtual R Call(Args... args) = 0;

    inline CallbackFlags::Flags flags() const;

   private:

    inline std::unique_ptr<Callback> get_next();

    inline void set_next(std::unique_ptr<Callback> next);

    CallbackFlags::Flags flags_;

    std::unique_ptr<Callback> next_;

    friend class CallbackQueue;

  };

  template <typename Fn>

  inline std::unique_ptr<Callback> CreateCallback(

      Fn&& fn, CallbackFlags::Flags);

  inline std::unique_ptr<Callback> Shift();

  inline void Push(std::unique_ptr<Callback> cb);

  // ConcatMove adds elements from 'other' to the end of this list, and clears

  // 'other' afterwards.

  inline void ConcatMove(CallbackQueue&& other);

  // size() is atomic and may be called from any thread.

  inline size_t size() const;

 private:

  template <typename Fn>

  class CallbackImpl final : public Callback {

   public:

    CallbackImpl(Fn&& callback, CallbackFlags::Flags flags);

    R Call(Args... args) override;

   private:

    Fn callback_;

  };

  std::atomic<size_t> size_ {0};

  std::unique_ptr<Callback> head_;

  Callback* tail_ = nullptr;

};

}  // namespace node

#endif  // defined(NODE_WANT_INTERNALS) && NODE_WANT_INTERNALS

#endif  // SRC_CALLBACK_QUEUE_H_