record_last.hpp

/*
 * This file is part of libtcspc
 * Copyright 2019-2026 Board of Regents of the University of Wisconsin System
 * SPDX-License-Identifier: MIT
 */
 
#pragma once
 
#include "context.hpp"
#include "introspect.hpp"
#include "processor.hpp"
 
#include <functional>
#include <optional>
#include <type_traits>
#include <utility>
 
namespace tcspc {

template <typename Event> class record_last_accessor {
    std::function<std::optional<Event>()> get_fn;
 
  public:
    template <typename F>
    explicit record_last_accessor(F get_func) : get_fn(get_func) {}

    auto get() -> std::optional<Event> { return get_fn(); }
};
 
namespace internal {
 
template <typename Event, typename Downstream> class record_last {
    std::optional<Event> last;
 
    Downstream downstream;
 
    // Cold data after downstream.
    access_tracker<record_last_accessor<Event>> trk;
 
  public:
    explicit record_last(access_tracker<record_last_accessor<Event>> &&tracker,
                         Downstream downstream)
        : downstream(std::move(downstream)), trk(std::move(tracker)) {
        trk.register_accessor_factory([](auto &tracker) {
            auto *self =
                LIBTCSPC_OBJECT_FROM_TRACKER(record_last, trk, tracker);
            return record_last_accessor<Event>([self] { return self->last; });
        });
    }
 
    [[nodiscard]] auto introspect_node() const -> processor_info {
        return processor_info(this, "record_last");
    }
 
    [[nodiscard]] auto introspect_graph() const -> processor_graph {
        return downstream.introspect_graph().push_entry_point(this);
    }
 
    template <typename E>
        requires handler_for<Downstream, std::remove_cvref_t<E>>
    void handle(E &&event) {
        if constexpr (std::is_same_v<std::remove_cvref_t<E>, Event>)
            last = event; // Retain a copy.
        downstream.handle(std::forward<E>(event));
    }
 
    void flush() { downstream.flush(); }
};
 
} // namespace internal

template <typename Event, typename Downstream>
auto record_last(access_tracker<record_last_accessor<Event>> &&tracker,
                 Downstream downstream) {
    return internal::record_last<Event, Downstream>(std::move(tracker),
                                                    std::move(downstream));
}
 
} // namespace tcspc