//
// experimental/promise.hpp
// ~~~~~~~~~~~~~~~~~~~~~~~~
//
// Copyright (c) 2021-2022 Klemens D. Morgenstern
//                         (klemens dot morgenstern at gmx dot net)
//
// Distributed under the Boost Software License, Version 1.0. (See accompanying
// file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
//

#ifndef ASIO_EXPERIMENTAL_PROMISE_HPP
#define ASIO_EXPERIMENTAL_PROMISE_HPP

#if defined(_MSC_VER) && (_MSC_VER >= 1200)
# pragma once
#endif // defined(_MSC_VER) && (_MSC_VER >= 1200)

#include "asio/detail/config.hpp"
#include "asio/detail/type_traits.hpp"
#include "asio/any_io_executor.hpp"
#include "asio/associated_cancellation_slot.hpp"
#include "asio/associated_executor.hpp"
#include "asio/bind_executor.hpp"
#include "asio/cancellation_signal.hpp"
#include "asio/dispatch.hpp"
#include "asio/experimental/impl/promise.hpp"
#include "asio/post.hpp"

#include <algorithm>

#include "asio/detail/push_options.hpp"

namespace asio {
namespace experimental {

template <typename T>
struct is_promise : std::false_type {};

template <typename ... Ts>
struct is_promise<promise<Ts...>> : std::true_type {};

template <typename T>
constexpr bool is_promise_v = is_promise<T>::value;

template <typename ... Ts>
struct promise_value_type
{
  using type = std::tuple<Ts...>;
};

template <typename T>
struct promise_value_type<T>
{
  using type = T;
};

template <>
struct promise_value_type<>
{
  using type = std::tuple<>;
};

#if defined(GENERATING_DOCUMENTATION)
/// A disposable handle for an eager operation.
/**
 * @tparam Signature The signature of the operation.
 *
 * @tparam Executor The executor to be used by the promise (taken from the
 * operation).
 *
 * @tparam Allocator The allocator used for the promise. Can be set through
 * use_allocator.
 *
 * A promise can be used to initiate an asynchronous option that can be
 * completed later. If the promise gets destroyed before completion, the
 * operation gets a cancel signal and the result is ignored.
 *
 * A promise fulfills the requirements of async_operation.
 *
 * @par Examples
 * Reading and writing from one coroutine.
 * @code
 * awaitable<void> read_write_some(asio::ip::tcp::socket & sock,
 *     asio::mutable_buffer read_buf, asio::const_buffer to_write)
 * {
 *   auto p = asio::async_read(read_buf, asio::use_awaitable);
 *   co_await asio::async_write_some(to_write, asio::deferred);
 *   co_await p;
 * }
 * @endcode
 */
template<typename Signature = void(),
    typename Executor = asio::any_io_executor,
    typename Allocator = std::allocator<void>>
struct promise
#else
template <typename ... Ts, typename Executor, typename Allocator>
struct promise<void(Ts...), Executor,  Allocator>
#endif // defined(GENERATING_DOCUMENTATION)
{
  /// The value that's returned by the promise.
  using value_type = typename promise_value_type<Ts...>::type;

  /// Cancel the promise. Usually done through the destructor.
  void cancel(cancellation_type level = cancellation_type::all)
  {
    if (impl_ && !impl_->done)
    {
      asio::dispatch(impl_->executor,
          [level, impl = impl_]{ impl->cancel.emit(level); });
    }
  }

  /// Check if the promise is completed already.
  bool completed() const noexcept
  {
    return impl_ && impl_->done;
  }

  /// Wait for the promise to become ready.
  template <ASIO_COMPLETION_TOKEN_FOR(void(Ts...)) CompletionToken>
  inline ASIO_INITFN_AUTO_RESULT_TYPE(CompletionToken, void(Ts...))
  operator()(CompletionToken&& token)
  {
    assert(impl_);

    return async_initiate<CompletionToken, void(Ts...)>(
        initiate_async_wait{impl_}, token);
  }

  promise() = delete;
  promise(const promise& ) = delete;
  promise(promise&& ) noexcept = default;

  /// Destruct the promise and cancel the operation.
  /**
   * It is safe to destruct a promise of a promise that didn't complete.
   */
  ~promise() { cancel(); }


private:
#if !defined(GENERATING_DOCUMENTATION)
  template <typename, typename, typename> friend struct promise;
  friend struct detail::promise_handler<void(Ts...), Executor, Allocator>;
#endif // !defined(GENERATING_DOCUMENTATION)

  std::shared_ptr<detail::promise_impl<
    void(Ts...), Executor, Allocator>> impl_;

  promise(
      std::shared_ptr<detail::promise_impl<
        void(Ts...), Executor, Allocator>> impl)
    : impl_(impl)
  {
  }

  struct initiate_async_wait
  {
    std::shared_ptr<detail::promise_impl<
      void(Ts...), Executor, Allocator>> self_;

    template <typename WaitHandler>
    void operator()(WaitHandler&& handler) const
    {
      const auto alloc = get_associated_allocator(
          handler, self_->get_allocator());

      auto cancel = get_associated_cancellation_slot(handler);

      if (self_->done)
      {
        auto exec = asio::get_associated_executor(
            handler, self_->get_executor());

        asio::post(exec,
            [self = std::move(self_),
              handler = std::forward<WaitHandler>(handler)]() mutable
            {
              self->apply(std::move(handler));
            });
      }
      else
      {
        if (cancel.is_connected())
        {
          struct cancel_handler
          {
            std::weak_ptr<detail::promise_impl<
              void(Ts...), Executor, Allocator>> self;

            cancel_handler(
                std::weak_ptr<detail::promise_impl<
                  void(Ts...), Executor, Allocator>> self)
              : self(std::move(self))
            {
            }

            void operator()(cancellation_type level) const
            {
              if (auto p = self.lock())
              {
                p->cancel.emit(level);
                p->cancel_();
              }
            }
          };
          cancel.template emplace<cancel_handler>(self_);
        }

        self_->set_completion(alloc, std::forward<WaitHandler>(handler));
      }
    }
  };
};

} // namespace experimental

} // namespace asio

#include "asio/detail/pop_options.hpp"

#endif // ASIO_EXPERIMENTAL_PROMISE_HPP