/*
 * Copyright 2013-present Facebook, Inc.
 *
 * 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 <atomic>
#include <cassert>
#include <chrono>
#include <limits>
#include <type_traits>

#include <folly/portability/Unistd.h>

namespace folly { namespace detail {

enum class FutexResult {
  VALUE_CHANGED, /* futex value didn't match expected */
  AWOKEN,        /* wakeup by matching futex wake, or spurious wakeup */
  INTERRUPTED,   /* wakeup by interrupting signal */
  TIMEDOUT,      /* wakeup by expiring deadline */
};

/**
 * Futex is an atomic 32 bit unsigned integer that provides access to the
 * futex() syscall on that value.  It is templated in such a way that it
 * can interact properly with DeterministicSchedule testing.
 *
 * If you don't know how to use futex(), you probably shouldn't be using
 * this class.  Even if you do know how, you should have a good reason
 * (and benchmarks to back you up).
 */
template <template <typename> class Atom = std::atomic>
struct Futex : Atom<uint32_t> {
  using Atom<uint32_t>::Atom;

  /** Puts the thread to sleep if this->load() == expected.  Returns true when
   *  it is returning because it has consumed a wake() event, false for any
   *  other return (signal, this->load() != expected, or spurious wakeup). */
  FutexResult futexWait(uint32_t expected, uint32_t waitMask = -1) {
    auto rv = futexWaitImpl(expected, nullptr, nullptr, waitMask);
    assert(rv != FutexResult::TIMEDOUT);
    return rv;
  }

  /** Similar to futexWait but also accepts a deadline until when the wait call
   *  may block.
   *
   *  Optimal clock types: std::chrono::system_clock, std::chrono::steady_clock.
   *  NOTE: On some systems steady_clock is just an alias for system_clock,
   *  and is not actually steady.
   *
   *  For any other clock type, now() will be invoked twice. */
  template <class Clock, class Duration = typename Clock::duration>
  FutexResult futexWaitUntil(
      uint32_t expected,
      std::chrono::time_point<Clock, Duration> const& deadline,
      uint32_t waitMask = -1) {
    using Target = typename std::conditional<
        Clock::is_steady,
        std::chrono::steady_clock,
        std::chrono::system_clock>::type;
    auto const converted = time_point_conv<Target>(deadline);
    return converted == Target::time_point::max()
        ? futexWaitImpl(expected, nullptr, nullptr, waitMask)
        : futexWaitImpl(expected, converted, waitMask);
  }

  /** Wakens up to count waiters where (waitMask & wakeMask) !=
   *  0, returning the number of awoken threads, or -1 if an error
   *  occurred.  Note that when constructing a concurrency primitive
   *  that can guard its own destruction, it is likely that you will
   *  want to ignore EINVAL here (as well as making sure that you
   *  never touch the object after performing the memory store that
   *  is the linearization point for unlock or control handoff).
   *  See https://sourceware.org/bugzilla/show_bug.cgi?id=13690 */
  int futexWake(int count = std::numeric_limits<int>::max(),
                uint32_t wakeMask = -1);

 private:
  /** Optimal when TargetClock is the same type as Clock.
   *
   *  Otherwise, both Clock::now() and TargetClock::now() must be invoked. */
  template <typename TargetClock, typename Clock, typename Duration>
  static typename TargetClock::time_point time_point_conv(
      std::chrono::time_point<Clock, Duration> const& time) {
    using std::chrono::duration_cast;
    using TimePoint = std::chrono::time_point<Clock, Duration>;
    using TargetDuration = typename TargetClock::duration;
    using TargetTimePoint = typename TargetClock::time_point;
    if (time == TimePoint::max()) {
      return TargetTimePoint::max();
    } else if (std::is_same<Clock, TargetClock>::value) {
      // in place of time_point_cast, which cannot compile without if-constexpr
      auto const delta = time.time_since_epoch();
      return TargetTimePoint(duration_cast<TargetDuration>(delta));
    } else {
      // different clocks with different epochs, so non-optimal case
      auto const delta = time - Clock::now();
      return TargetClock::now() + duration_cast<TargetDuration>(delta);
    }
  }

  template <typename Deadline>
  typename std::enable_if<Deadline::clock::is_steady, FutexResult>::type
  futexWaitImpl(
      uint32_t expected,
      Deadline const& deadline,
      uint32_t waitMask) {
    return futexWaitImpl(expected, nullptr, &deadline, waitMask);
  }

  template <typename Deadline>
  typename std::enable_if<!Deadline::clock::is_steady, FutexResult>::type
  futexWaitImpl(
      uint32_t expected,
      Deadline const& deadline,
      uint32_t waitMask) {
    return futexWaitImpl(expected, &deadline, nullptr, waitMask);
  }

  /** Underlying implementation of futexWait and futexWaitUntil.
   *  At most one of absSystemTime and absSteadyTime should be non-null.
   *  Timeouts are separated into separate parameters to allow the
   *  implementations to be elsewhere without templating on the clock
   *  type, which is otherwise complicated by the fact that steady_clock
   *  is the same as system_clock on some platforms. */
  FutexResult futexWaitImpl(
      uint32_t expected,
      std::chrono::system_clock::time_point const* absSystemTime,
      std::chrono::steady_clock::time_point const* absSteadyTime,
      uint32_t waitMask);
};

/** A std::atomic subclass that can be used to force Futex to emulate
 *  the underlying futex() syscall.  This is primarily useful to test or
 *  benchmark the emulated implementation on systems that don't need it. */
template <typename T>
struct EmulatedFutexAtomic : public std::atomic<T> {
  EmulatedFutexAtomic() noexcept = default;
  constexpr /* implicit */ EmulatedFutexAtomic(T init) noexcept
      : std::atomic<T>(init) {}
  // It doesn't copy or move
  EmulatedFutexAtomic(EmulatedFutexAtomic&& rhs) = delete;
};

/* Available specializations, with definitions elsewhere */

template <>
int Futex<std::atomic>::futexWake(int count, uint32_t wakeMask);

template <>
FutexResult Futex<std::atomic>::futexWaitImpl(
    uint32_t expected,
    std::chrono::system_clock::time_point const* absSystemTime,
    std::chrono::steady_clock::time_point const* absSteadyTime,
    uint32_t waitMask);

template <>
int Futex<EmulatedFutexAtomic>::futexWake(int count, uint32_t wakeMask);

template <>
FutexResult Futex<EmulatedFutexAtomic>::futexWaitImpl(
    uint32_t expected,
    std::chrono::system_clock::time_point const* absSystemTime,
    std::chrono::steady_clock::time_point const* absSteadyTime,
    uint32_t waitMask);

} // namespace detail
} // namespace folly
