#include <concepts>
#include <coroutine>
#include <exception>
#include <iostream>
#include <vector>
#include <assert.h>
#include <chrono>

using namespace std;
using namespace std::chrono;

template<typename T>
struct Task {
  struct promise_type;

  using handle_type = std::coroutine_handle<promise_type>;

  struct promise_type {
    T value_;
    std::exception_ptr exception_;

    Task get_return_object() {
      return Task(handle_type::from_promise(*this));
    }

    std::suspend_always initial_suspend() {
      return {};
    }

    std::suspend_always final_suspend() noexcept {
      return {};
    }

    void unhandled_exception() {
      exception_ = std::current_exception();
    }

    template<std::convertible_to<T> From> // C++20 concept
    void return_value(From &&from) {
      value_ = std::forward<From>(from);
    }

    template<std::convertible_to<T> From> // C++20 concept
    std::suspend_always yield_value(From &&from) {
      value_ = std::forward<From>(from);
      return {};
    }

    void return_void() {}
  };

  handle_type h_;

  Task() {
  }

  Task(handle_type h) : h_(h) {
  }

  Task(const Task &t) : h_(t.h_) {
  }

  void destroy() {
    h_.destroy();
  }

  T operator()() {
    assert(!h_.done());
    call();
    return std::move(h_.promise().value_);
  }

  bool done() {
    return h_.done();
  }

  private:

  void call() {
    h_();

    if (h_.promise().exception_)
      std::rethrow_exception(h_.promise().exception_);
  }
};

#define pass() co_await std::suspend_always{}

Task<unsigned> task_test()
{
  pass();

  for (unsigned i = 0; i < 3; ++i)
      co_yield i;

  co_return 1000;
}

int main()
{

  const int task_count = 4;
  vector<Task<unsigned>> tasks;

  for(int i = 0; i < task_count; i++) {
    auto t = task_test();
    tasks.push_back(std::move(t));
  }

  int done_count = 0;
  while(done_count < task_count) {
    for(int i = 0; i < task_count; i++) {
      Task<unsigned> &t = tasks[i];

      if(t.done()) {
        t.destroy();
        done_count++;
      } else {
        auto res = t();
        cout << "T# " << i << " result "
         << res << endl;
      }
    }
  }
}