/* track-fn.h */
#pragma once

enum TrackConsts { track_no_limit=-1, track_start_line=0, track_end_line=-1 };
enum TrackResultCodes { track_rc_done=0, track_rc_active=1, track_rc_int=2 };

typedef struct Track { int line,limit; } Track;

#define TRACK_RESET(v) { Track *_track=(v); _track->line=0; _track->limit=1; }
#define TRACK_SET_LIMIT(track,n) { (track)->limit=n; }

#define TRACK_BEGIN(v) { Track *_track=(v); track_begin: \
    switch(_track->line) { default: case 0: TRACK_POINT
#define TRACK_POINT { case __LINE__: _track->line=__LINE__; \
    if (_track->limit>=0 && !_track->limit--) { _track->limit=1; return 1; } }
#define TRACK_END   track_end: case -1: _track->line=-1; return 0; } \
    track_interrupt: return 2; }

#define TRACK_END_R track_end: _track->line=-1; _track->limit=1; return 0; } \
    track_interrupt: return 2; }

#define TRACK_REPEAT_LAST goto track_begin;
#define TRACK_LEAVE       goto track_end;
#define TRACK_INTERRUPT   goto track_interrupt;

#define TRACK_CALL(fn,state) { int rc; \
    for(fn##_reset(state);0!=(rc=fn(state));) { \
        if (rc==track_rc_int) { TRACK_INTERRUPT } else { TRACK_POINT } \
    }}

// g++-10 -std=c++20 -fcoroutines cpp20.cpp && ./a.out
#include <coroutine>
#include <iostream>

struct resumable {
    struct promise_type {
        using coro_handle = std::coroutine_handle<promise_type>;
        auto get_return_object() { return coro_handle::from_promise(*this); }
        auto initial_suspend() { return std::suspend_always{}; }
        auto final_suspend() noexcept { return std::suspend_always{}; }
        void return_void() {}
        void unhandled_exception() { std::terminate(); }
    };
    using coro_handle = std::coroutine_handle<promise_type>;
    resumable(coro_handle handle) : handle(handle) {}
    bool resume() { if (!handle.done()) handle.resume(); return !handle.done(); }
    ~resumable() { if (handle) handle.destroy(); }
private:
    coro_handle handle;
};

resumable fn1() {
    std::cout<<"Hello"<<std::endl;
    co_await std::suspend_always{};
    std::cout<<"World"<<std::endl;
    co_await std::suspend_always{};
    std::cout<<"!!!"<<std::endl;
}

int main(int argc, char const *argv[]) {
    auto t=fn1();
    t.resume();
    t.resume();
    t.resume();
    return 0;
}