#pragma once

#include <mapbox/std/weak.hpp>

#include <functional>
#include <memory>

namespace mbgl {

class Mailbox;

/**
    A `Scheduler` is responsible for coordinating the processing of messages by
    one or more actors via their mailboxes. It's an abstract interface. Currently,
    the following concrete implementations exist:

    * `ThreadPool` can coordinate an unlimited number of actors over any number of
      threads via a pool, preserving the following behaviors:

      - Messages from each individual mailbox are processed in order
      - Only a single message from a mailbox is processed at a time; there is no
        concurrency within a mailbox

      Subject to these constraints, processing can happen on whatever thread in the
      pool is available.

    * `Scheduler::GetCurrent()` is typically used to create a mailbox and `ActorRef`
      for an object that lives on the main thread and is not itself wrapped an
      `Actor`. The underlying implementation of this Scheduler should usually be
      a `RunLoop`
        auto mailbox = std::make_shared<Mailbox>(*Scheduler::Get());
        Actor<Worker> worker(threadPool, ActorRef<Foo>(*this, mailbox));
*/
class Scheduler {
public:
    virtual ~Scheduler() = default;

    /// Enqueues a function for execution.
    virtual void schedule(std::function<void()>) = 0;
    /// Makes a weak pointer to this Scheduler.
    virtual mapbox::base::WeakPtr<Scheduler> makeWeakPtr() = 0;

    /// Returns a closure wrapping the given one.
    ///
    /// When the returned closure is invoked for the first time, it schedules
    /// the given closure to this scheduler, the consequent calls of the
    /// returned closure are ignored.
    ///
    /// If this scheduler is already deleted by the time the returnded closure
    /// is first invoked, the call is ignored.
    std::function<void()> bindOnce(std::function<void()>);

    /// Enqueues the given |task| for execution into this scheduler's task queue
    /// and then enqueues the |reply| with the captured task result to the
    /// current task queue.
    ///
    /// The |TaskFn| return type must be compatible with the |ReplyFn| argument
    /// type. Note: the task result is copied and passed by value.
    template <typename TaskFn, typename ReplyFn>
    void scheduleAndReplyValue(const TaskFn& task, const ReplyFn& reply) {
        assert(GetCurrent());
        scheduleAndReplyValue(task, reply, GetCurrent()->makeWeakPtr());
    }

    /// Set/Get the current Scheduler for this thread
    static Scheduler* GetCurrent();
    static void SetCurrent(Scheduler*);

    /// Get the scheduler for asynchronous tasks. This method
    /// will lazily initialize a shared worker pool when ran
    /// from the first time.
    /// The scheduled tasks might run in parallel on different
    /// threads.
    /// TODO : Rename to GetPool()
    [[nodiscard]] static std::shared_ptr<Scheduler> GetBackground();

    /// Get the *sequenced* scheduler for asynchronous tasks.
    /// Unlike the method above, the returned scheduler
    /// (once stored) represents a single thread, thus each
    /// newly scheduled task is guarantied to run after the
    /// previously scheduled one.
    ///
    /// Sequenced scheduler can be used for running tasks
    /// on the same thread-unsafe object.
    [[nodiscard]] static std::shared_ptr<Scheduler> GetSequenced();

protected:
    template <typename TaskFn, typename ReplyFn>
    void scheduleAndReplyValue(const TaskFn& task,
                               const ReplyFn& reply,
                               mapbox::base::WeakPtr<Scheduler> replyScheduler) {
        auto scheduled = [replyScheduler = std::move(replyScheduler), task, reply] {
            auto lock = replyScheduler.lock();
            if (!replyScheduler) return;
            auto scheduledReply = [reply, result = task()] {
                reply(result);
            };
            replyScheduler->schedule(std::move(scheduledReply));
        };

        schedule(std::move(scheduled));
    }
};

} // namespace mbgl