text: /*
   * Copyright (c) Facebook, Inc. and its affiliates.
   *
   * Licensed under the Apache License, Version 2.0 (the "License");
   * you may not use this file except in compliance with the License.
   * You may obtain a copy of the License at
   *
   *     http://www.apache.org/licenses/LICENSE-2.0
   *
   * Unless required by applicable law or agreed to in writing, software
   * distributed under the License is distributed on an "AS IS" BASIS,
   * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
   * See the License for the specific language governing permissions and
   * limitations under the License.
   */
  
  #pragma once
  
  #include <condition_variable>
  #include <cstdint>
  
  #include <folly/detail/Futex.h>
  #include <folly/synchronization/ParkingLot.h>
  
  namespace folly {
  namespace detail {
  namespace atomic_notification {
  /**
   * We use Futex<std::atomic> as the alias that has the lowest performance
   * overhead with respect to atomic notifications.  Assert that
   * atomic_uint_fast_wait_t is the same as Futex<std::atomic>
   */
  static_assert(std::is_same<atomic_uint_fast_wait_t, Futex<std::atomic>>{}, "");
  
  /**
   * Implementation and specializations for the atomic_wait() family of
   * functions
   */
  inline std::cv_status toCvStatus(FutexResult result) {
    return (result == FutexResult::TIMEDOUT) ? std::cv_status::timeout
                                             : std::cv_status::no_timeout;
  }
  inline std::cv_status toCvStatus(ParkResult result) {
    return (result == ParkResult::Timeout) ? std::cv_status::timeout
                                           : std::cv_status::no_timeout;
  }
  
  // ParkingLot instantiation for futex management
  extern ParkingLot<std::uint32_t> parkingLot;
  
  template <template <typename...> class Atom, typename... Args>
  void atomic_wait_impl(
      const Atom<std::uint32_t, Args...>* atomic,
      std::uint32_t expected) {
    futexWait(atomic, expected);
    return;
  }
  
  template <template <typename...> class Atom, typename Integer, typename... Args>
  void atomic_wait_impl(const Atom<Integer, Args...>* atomic, Integer expected) {
    static_assert(!std::is_same<Integer, std::uint32_t>{}, "");
    parkingLot.park(
        atomic, -1, [&] { return atomic->load() == expected; }, [] {});
  }
  
  template <
      template <typename...> class Atom,
      typename... Args,
      typename Clock,
      typename Duration>
  std::cv_status atomic_wait_until_impl(
      const Atom<std::uint32_t, Args...>* atomic,
      std::uint32_t expected,
      const std::chrono::time_point<Clock, Duration>& deadline) {
    return toCvStatus(futexWaitUntil(atomic, expected, deadline));
  }
  
  template <
      template <typename...> class Atom,
      typename Integer,
      typename... Args,
      typename Clock,
      typename Duration>
  std::cv_status atomic_wait_until_impl(
      const Atom<Integer, Args...>* atomic,
      Integer expected,
      const std::chrono::time_point<Clock, Duration>& deadline) {
    static_assert(!std::is_same<Integer, std::uint32_t>{}, "");
    return toCvStatus(parkingLot.park_until(
        atomic, -1, [&] { return atomic->load() == expected; }, [] {}, deadline));
  }
  
  template <template <typename...> class Atom, typename... Args>
  void atomic_notify_one_impl(const Atom<std::uint32_t, Args...>* atomic) {
    futexWake(atomic, 1);
    return;
  }
  
  template <template <typename...> class Atom, typename Integer, typename... Args>
  void atomic_notify_one_impl(const Atom<Integer, Args...>* atomic) {
    static_assert(!std::is_same<Integer, std::uint32_t>{}, "");
    parkingLot.unpark(atomic, [&](const auto& data) {
      FOLLY_SAFE_DCHECK(data == std::numeric_limits<std::uint32_t>::max(), "");
      return UnparkControl::RemoveBreak;
    });
  }
  
  template <template <typename...> class Atom, typename... Args>
  void atomic_notify_all_impl(const Atom<std::uint32_t, Args...>* atomic) {
    futexWake(atomic);
    return;
  }
  
  template <template <typename...> class Atom, typename Integer, typename... Args>
  void atomic_notify_all_impl(const Atom<Integer, Args...>* atomic) {
    static_assert(!std::is_same<Integer, std::uint32_t>{}, "");
    parkingLot.unpark(atomic, [&](const auto& data) {
      FOLLY_SAFE_DCHECK(data == std::numeric_limits<std::uint32_t>::max(), "");
      return UnparkControl::RemoveContinue;
    });
  }
  } // namespace atomic_notification
  } // namespace detail
  
  template <typename Integer>
  void atomic_wait(const std::atomic<Integer>* atomic, Integer expected) {
    detail::atomic_notification::atomic_wait_impl(atomic, expected);
  }
  
  template <typename Integer, typename Clock, typename Duration>
  std::cv_status atomic_wait_until(
      const std::atomic<Integer>* atomic,
      Integer expected,
      const std::chrono::time_point<Clock, Duration>& deadline) {
    return detail::atomic_notification::atomic_wait_until_impl(
        atomic, expected, deadline);
  }
  
  template <typename Integer>
  void atomic_notify_one(const std::atomic<Integer>* atomic) {
    detail::atomic_notification::atomic_notify_one_impl(atomic);
  }
  
  template <typename Integer>
  void atomic_notify_all(const std::atomic<Integer>* atomic) {
    detail::atomic_notification::atomic_notify_all_impl(atomic);
  }
  
  } // namespace folly
  