distinct_until_changed.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/AlexInLog/ReactivePlusPlus
//
 
#pragma once
 
#include <rpp/operators/fwd.hpp>
 
#include <rpp/defs.hpp>
#include <rpp/operators/details/strategy.hpp>
 
#include <type_traits>
 
namespace rpp::operators::details
{
    template<rpp::constraint::decayed_type Type, rpp::constraint::observer TObserver, rpp::constraint::decayed_type EqualityFn>
    struct distinct_until_changed_observer_strategy
    {
        static constexpr auto preferred_disposables_mode = rpp::details::observers::disposables_mode::None;
 
        RPP_NO_UNIQUE_ADDRESS TObserver  observer;
        RPP_NO_UNIQUE_ADDRESS EqualityFn comparator;
        mutable std::optional<Type>      last_value{};
 
        template<typename T>
        void on_next(T&& v) const
        {
            if (last_value.has_value() && comparator(utils::as_const(last_value.value()), rpp::utils::as_const(v)))
                return;
 
            last_value.emplace(std::forward<T>(v));
            observer.on_next(utils::as_const(last_value.value()));
        }
 
        void on_error(const std::exception_ptr& err) const { observer.on_error(err); }
 
        void on_completed() const { observer.on_completed(); }
 
        void set_upstream(const disposable_wrapper& d) { observer.set_upstream(d); }
 
        bool is_disposed() const { return observer.is_disposed(); }
    };
 
    template<rpp::constraint::decayed_type EqualityFn>
    struct distinct_until_changed_t : public operators::details::lift_operator<distinct_until_changed_t<EqualityFn>, EqualityFn>
    {
        using operators::details::lift_operator<distinct_until_changed_t<EqualityFn>, EqualityFn>::lift_operator;
 
        template<rpp::constraint::decayed_type T>
        struct operator_traits
        {
            static_assert(rpp::constraint::invocable_r_v<bool, EqualityFn, T, T>, "EqualityFn is not invocable with T and T returning bool");
 
            using result_type = T;
 
            template<rpp::constraint::observer_of_type<result_type> TObserver>
            using observer_strategy = distinct_until_changed_observer_strategy<T, TObserver, EqualityFn>;
        };
 
        template<rpp::details::observables::constraint::disposables_strategy Prev>
        using updated_optimal_disposables_strategy = Prev;
    };
} // namespace rpp::operators::details
 
namespace rpp::operators
{
     * @see https://reactivex.io/documentation/operators/distinct.html
     */
    template<typename EqualityFn>
        requires (!utils::is_not_template_callable<EqualityFn> || std::same_as<bool, std::invoke_result_t<EqualityFn, rpp::utils::convertible_to_any, rpp::utils::convertible_to_any>>)
    auto distinct_until_changed(EqualityFn&& equality_fn)
    {
        return details::distinct_until_changed_t<std::decay_t<EqualityFn>>{std::forward<EqualityFn>(equality_fn)};
    }
} // namespace rpp::operators