concat.hpp

//                  ReactivePlusPlus library
//
//          Copyright Aleksey Loginov 2023 - present.
// Distributed under the Boost Software License, Version 1.0.
//    (See accompanying file LICENSE_1_0.txt or copy at
//          https://www.boost.org/LICENSE_1_0.txt)
//
// Project home: https://github.com/victimsnino/ReactivePlusPlus
//
 
#pragma once
 
#include <rpp/operators/fwd.hpp>
 
#include <rpp/disposables/composite_disposable.hpp>
#include <rpp/operators/details/strategy.hpp>
#include <rpp/utils/utils.hpp>
 
#include <array>
#include <cassert>
#include <queue>
 
 
namespace rpp::operators::details
{
    template<rpp::constraint::observable TObservable, rpp::constraint::observer TObserver>
    struct concat_inner_observer_strategy;
 
    enum class ConcatStage : uint8_t
    {
        None                   = 0,
        Draining               = 1,
        CompletedWhileDraining = 2,
        Processing             = 3,
    };
 
    template<rpp::constraint::observable TObservable, rpp::constraint::observer TObserver>
    class concat_disposable final : public rpp::details::base_disposable
        , public rpp::details::enable_wrapper_from_this<concat_disposable<TObservable, TObserver>>
    {
    public:
        concat_disposable(TObserver&& observer)
            : m_observer{std::move(observer)}
        {
        }
 
        rpp::utils::pointer_under_lock<TObserver>               get_observer() { return m_observer; }
        rpp::utils::pointer_under_lock<std::queue<TObservable>> get_queue() { return m_queue; }
 
        std::atomic<ConcatStage>& stage() { return m_stage; }
 
        void drain()
        {
            while (!is_disposed())
            {
                const auto observable = get_observable();
                if (!observable)
                {
                    stage().store(ConcatStage::None, std::memory_order::relaxed);
                    if (get_base_child_disposable().is_disposed())
                        get_observer()->on_completed();
                    return;
                }
 
                if (handle_observable_impl(observable.value()))
                    return;
            }
        }
 
        void handle_observable(const rpp::constraint::decayed_same_as<TObservable> auto& observable)
        {
            if (handle_observable_impl(observable))
                return;
 
            drain();
        }
 
        rpp::composite_disposable& get_base_child_disposable() { return m_child_disposables[0]; }
        rpp::composite_disposable& get_inner_child_disposable() { return m_child_disposables[1]; }
 
    private:
        bool handle_observable_impl(const rpp::constraint::decayed_same_as<TObservable> auto& observable)
        {
            stage().store(ConcatStage::Draining, std::memory_order::relaxed);
            observable.subscribe(concat_inner_observer_strategy<TObservable, TObserver>{disposable_wrapper_impl<concat_disposable>{this->wrapper_from_this()}.lock()});
 
            ConcatStage current = ConcatStage::Draining;
            return stage().compare_exchange_strong(current, ConcatStage::Processing, std::memory_order::seq_cst);
        }
 
        void base_dispose_impl(interface_disposable::Mode) noexcept override
        {
            for (auto& d : m_child_disposables)
                d.dispose();
        }
 
        std::optional<TObservable> get_observable()
        {
            auto queue = get_queue();
            if (queue->empty())
                return std::nullopt;
            auto observable = queue->front();
            queue->pop();
            return observable;
        }
 
    private:
        rpp::utils::value_with_mutex<TObserver>               m_observer;
        rpp::utils::value_with_mutex<std::queue<TObservable>> m_queue;
        std::atomic<ConcatStage>                              m_stage{};
 
        std::array<rpp::composite_disposable, 2> m_child_disposables{};
    };
 
    template<rpp::constraint::observable TObservable, rpp::constraint::observer TObserver>
    struct concat_inner_observer_strategy
    {
        static constexpr auto preferred_disposables_mode = rpp::details::observers::disposables_mode::Boolean;
 
        std::shared_ptr<concat_disposable<TObservable, TObserver>> disposable{};
 
        template<typename T>
        void on_next(T&& v) const
        {
            disposable->get_observer()->on_next(std::forward<T>(v));
        }
 
        void on_error(const std::exception_ptr& err) const
        {
            disposable->get_observer()->on_error(err);
        }
 
        void on_completed() const
        {
            disposable->get_inner_child_disposable().clear();
 
            ConcatStage current{ConcatStage::Draining};
            if (disposable->stage().compare_exchange_strong(current, ConcatStage::CompletedWhileDraining, std::memory_order::seq_cst))
                return;
 
            assert(current == ConcatStage::Processing);
 
            disposable->drain();
        }
 
        void set_upstream(const disposable_wrapper& d) const { disposable->get_inner_child_disposable().add(d); }
 
        bool is_disposed() const { return disposable->get_inner_child_disposable().is_disposed(); }
    };
 
    template<rpp::constraint::observable TObservable, rpp::constraint::observer TObserver>
    struct concat_observer_strategy
    {
        static constexpr auto preferred_disposables_mode = rpp::details::observers::disposables_mode::None;
 
        std::shared_ptr<concat_disposable<TObservable, TObserver>> disposable;
 
        concat_observer_strategy(TObserver&& observer)
            : disposable{init_state(std::move(observer))}
        {
        }
 
        template<typename T>
        void on_next(T&& v) const
        {
            ConcatStage current = ConcatStage::None;
            if (disposable->stage().compare_exchange_strong(current, ConcatStage::Draining, std::memory_order::seq_cst))
                disposable->handle_observable(std::forward<T>(v));
            else
                disposable->get_queue()->push(std::forward<T>(v));
        }
 
        void on_error(const std::exception_ptr& err) const
        {
            disposable->get_observer()->on_error(err);
        }
 
        void on_completed() const
        {
            disposable->get_base_child_disposable().dispose();
            if (disposable->stage() == ConcatStage::None)
                disposable->get_observer()->on_completed();
        }
 
        void set_upstream(const disposable_wrapper& d) const { disposable->get_base_child_disposable().add(d); }
 
        bool is_disposed() const { return disposable->get_base_child_disposable().is_disposed(); }
 
    private:
        static std::shared_ptr<concat_disposable<TObservable, TObserver>> init_state(TObserver&& observer)
        {
            const auto d   = disposable_wrapper_impl<concat_disposable<TObservable, TObserver>>::make(std::move(observer));
            auto       ptr = d.lock();
            ptr->get_observer()->set_upstream(d.as_weak());
            return ptr;
        }
    };
 
    struct concat_t : lift_operator<concat_t>
    {
        using lift_operator<concat_t>::lift_operator;
 
        template<rpp::constraint::decayed_type T>
        struct operator_traits
        {
            static_assert(rpp::constraint::observable<T>, "T is not observable");
 
            using result_type = rpp::utils::extract_observable_type_t<T>;
 
            template<rpp::constraint::observer_of_type<result_type> TObserver>
            using observer_strategy = concat_observer_strategy<T, TObserver>;
        };
 
        template<rpp::details::observables::constraint::disposables_strategy Prev>
        using updated_optimal_disposables_strategy = rpp::details::observables::fixed_disposables_strategy<1>;
    };
} // namespace rpp::operators::details
 
 
namespace rpp::operators
{
     * @note `#include <rpp/operators/concat.hpp>`
     *
     * @par Example
     * @snippet concat.cpp concat_as_operator
     *
     * @ingroup creational_operators
     * @see https://reactivex.io/documentation/operators/concat.html
     */
    inline auto concat()
    {
        return details::concat_t{};
    }
} // namespace rpp::operators