xo-umbrella2/xo-reactor/include/xo/reactor/LastReducer.hpp

/* @file LastReducer.hpp */

#pragma once

#include "Reducer.hpp"
#include <array>

namespace xo {
    namespace reactor {
        /* implementation record used in LastReducer.
         * LastReducer (see below) remembers a single event,
         * + will be updated on successive calls to
         * LastReducer.include_event()
         *
         * need to remember the _first_ (& therefore earliest)
         * event timestamp in such a wave,  since that establishes when simulator
         * should deliver the event -- even if event is subsequently
         * overwritten.
         *
         * once event is delivered,  timestamp can reset
         *
         * otherwise if upstream producer sends events with
         * future timestamps,  can get indefinite postponement
         * with simulation clock failing to catch up to event time.
         *
         */

        template<typename Event>
        class EventRecd {
        public:
            using utc_nanos = xo::time::utc_nanos;

        public:
            EventRecd() = default;
            EventRecd(utc_nanos tm, Event ev) : trigger_tm_{tm}, ev_{ev} {}
            EventRecd(utc_nanos tm, Event && ev) : trigger_tm_{tm}, ev_{std::move(ev)} {}

        public:
            /* if sim, deliver event when simulation clock reaches
             * .trigger_tm;  .trigger_tm can be earlier than .ev time
             */
            utc_nanos trigger_tm_;
            /* event to deliver */
            Event ev_;
        };

        /* reducer that just remembers the last event
         *
         * Require:
         * - Event is null-contructible
         * - Event is copyable
         *
         * LastReducer provides reentrancy support.  This support doesn't operate
         * if Event copy is not deep,  e.g. for Event = rp<Foo>n
         *
         *              .include_event()
         *   /-------\  -----------------> /------\
         *   | empty |                     | full |
         *   \-------/ <-----------------  \------/
         *      .         .annex_one()        .
         *      .                             .
         *   .is_empty()=true              .is_empty()=false
         */
        template<typename Event, typename EventTimeFn = StructEventTimeFn<Event>>
        class LastReducer : public ReducerBase<Event, EventTimeFn> {
        public:
            using utc_nanos = xo::time::utc_nanos;

        public:
            LastReducer() = default;
            LastReducer(EventTimeFn const & evtfn) : ReducerBase<Event, EventTimeFn>(evtfn) {}

            bool is_empty() const { return empty_flag_; }
            /* require: .is_empty() = false */
            utc_nanos next_tm() const {
                return this->last_ev_[this->last_ix_].trigger_tm_;
                //return this->event_tm(this->last_ev_[this->last_ix_]);
            }
            /* #of events stored in this reducer (0 or 1) */
            uint32_t n_event() const { return this->empty_flag_ ? 0 : 1; }

            Event const & last_annexed_ev() const {
                return this->last_ev_[1 - this->last_ix_].ev_;
            }

            EventRecd<Event> & include_event_aux(Event const & ev) {
                EventRecd<Event> & evr
                    = this->last_ev_[this->last_ix_];

                if (this->empty_flag_) {
                    /* evr.trigger_tm will be preserved across
                     * successive calls to .include_event();
                     * until .annex_one()
                     */
                    evr.trigger_tm_ = this->event_tm(ev);

                    this->empty_flag_ = false;
                }

                return evr;
            } /*include_event_aux*/

            void include_event(Event const & ev) {
                EventRecd<Event> & evr
                    = this->include_event_aux(ev);

                evr.ev_ = ev;
            } /*include_event*/

            void include_event(Event && ev) {
                EventRecd<Event> & evr
                    = this->include_event_aux(ev);

                evr.ev_ = std::move(ev);
            } /*include_event*/

            Event & annex_one() {
                std::uint32_t annexed_ix = this->last_ix_;

                /* since .empty_flag is true,
                 * next call to .include_event_aux() will
                 * capture new timestamp
                 */
                this->empty_flag_ = true;
                this->last_ix_ = (1 - this->last_ix_);

                return this->last_ev_[annexed_ix].ev_;
            } /*annex_one*/

            // ----- Inherited from ReducerBase -----

            //utc_nanos event_tm(Event const & ev) const { return this->event_tm_fn_(ev); }

        private:
            /* true when reducer contains 0 queued events,
             * not counting any annexed event
             */
            bool empty_flag_ = true;

            /* .last_ev[.last_ix] updated by .include_event()
             */
            std::uint32_t last_ix_ = 0;
            /* remember two events
             * (a) a single queued event  (updated by .include_event())
             * (b) a single removed event (reported by .annex_one())
             *
             * roles of .last_ev[0], .last_ev[1] reverse each time .annex_one() runs
             */
            std::array<EventRecd<Event>, 2> last_ev_;
        }; /*LastReducer*/
    } /*namespace reactor*/
} /*namespace xo*/

/* end LastReducer.hpp */