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    // Currently, rust warns when an unsafe fn contains an unsafe {} block. However,
    // in the future, this will change to the reverse. For now, suppress this
    // warning and generally stick with being explicit about unsafety.
    #![allow(unused_unsafe)]
    #![cfg_attr(not(feature = "rt"), allow(dead_code))]

    //! Time driver

    mod entry;
    pub(self) use self::entry::{EntryList, TimerEntry, TimerHandle, TimerShared};

    mod handle;
    pub(crate) use self::handle::Handle;

    mod wheel;

    pub(super) mod sleep;

    use crate::loom::sync::{Arc, Mutex};
    use crate::park::{Park, Unpark};
    use crate::time::error::Error;
    use crate::time::{Clock, Duration, Instant};

    use std::convert::TryInto;
    use std::fmt;
    use std::{num::NonZeroU64, ptr::NonNull, task::Waker};

    /// Time implementation that drives [`Sleep`][sleep], [`Interval`][interval], and [`Timeout`][timeout].
    ///
    /// A `Driver` instance tracks the state necessary for managing time and
    /// notifying the [`Sleep`][sleep] instances once their deadlines are reached.
    ///
    /// It is expected that a single instance manages many individual [`Sleep`][sleep]
    /// instances. The `Driver` implementation is thread-safe and, as such, is able
    /// to handle callers from across threads.
    ///
    /// After creating the `Driver` instance, the caller must repeatedly call `park`
    /// or `park_timeout`. The time driver will perform no work unless `park` or
    /// `park_timeout` is called repeatedly.
    ///
    /// The driver has a resolution of one millisecond. Any unit of time that falls
    /// between milliseconds are rounded up to the next millisecond.
    ///
    /// When an instance is dropped, any outstanding [`Sleep`][sleep] instance that has not
    /// elapsed will be notified with an error. At this point, calling `poll` on the
    /// [`Sleep`][sleep] instance will result in panic.
    ///
    /// # Implementation
    ///
    /// The time driver is based on the [paper by Varghese and Lauck][paper].
    ///
    /// A hashed timing wheel is a vector of slots, where each slot handles a time
    /// slice. As time progresses, the timer walks over the slot for the current
    /// instant, and processes each entry for that slot. When the timer reaches the
    /// end of the wheel, it starts again at the beginning.
    ///
    /// The implementation maintains six wheels arranged in a set of levels. As the
    /// levels go up, the slots of the associated wheel represent larger intervals
    /// of time. At each level, the wheel has 64 slots. Each slot covers a range of
    /// time equal to the wheel at the lower level. At level zero, each slot
    /// represents one millisecond of time.
    ///
    /// The wheels are:
    ///
    /// * Level 0: 64 x 1 millisecond slots.
    /// * Level 1: 64 x 64 millisecond slots.
    /// * Level 2: 64 x ~4 second slots.
    /// * Level 3: 64 x ~4 minute slots.
    /// * Level 4: 64 x ~4 hour slots.
    /// * Level 5: 64 x ~12 day slots.
    ///
    /// When the timer processes entries at level zero, it will notify all the
    /// `Sleep` instances as their deadlines have been reached. For all higher
    /// levels, all entries will be redistributed across the wheel at the next level
    /// down. Eventually, as time progresses, entries with [`Sleep`][sleep] instances will
    /// either be canceled (dropped) or their associated entries will reach level
    /// zero and be notified.
    ///
    /// [paper]: http://www.cs.columbia.edu/~nahum/w6998/papers/ton97-timing-wheels.pdf
    /// [sleep]: crate::time::Sleep
    /// [timeout]: crate::time::Timeout
    /// [interval]: crate::time::Interval
    #[derive(Debug)]
    pub(crate) struct Driver<P: Park + 'static> {
        /// Timing backend in use
        time_source: ClockTime,

        /// Shared state
        inner: Handle,

        /// Parker to delegate to
        park: P,
    }

    /// A structure which handles conversion from Instants to u64 timestamps.
    #[derive(Debug, Clone)]
    pub(self) struct ClockTime {
        clock: super::clock::Clock,
        start_time: Instant,
    }

    impl ClockTime {
        pub(self) fn new(clock: Clock) -> Self {
            Self {
                clock,
                start_time: super::clock::now(),
            }
        }

        pub(self) fn deadline_to_tick(&self, t: Instant) -> u64 {
            // Round up to the end of a ms
            self.instant_to_tick(t + Duration::from_nanos(999_999))
        }

        pub(self) fn instant_to_tick(&self, t: Instant) -> u64 {
            // round up
            let dur: Duration = t
                .checked_duration_since(self.start_time)
                .unwrap_or_else(|| Duration::from_secs(0));
            let ms = dur.as_millis();

            ms.try_into().expect("Duration too far into the future")
        }

        pub(self) fn tick_to_duration(&self, t: u64) -> Duration {
            Duration::from_millis(t)
        }

        pub(self) fn now(&self) -> u64 {
            self.instant_to_tick(self.clock.now())
        }
    }

    /// Timer state shared between `Driver`, `Handle`, and `Registration`.
    pub(self) struct Inner {
        /// Timing backend in use
        time_source: ClockTime,

        /// The last published timer `elapsed` value.
        elapsed: u64,

        /// The earliest time at which we promise to wake up without unparking
        next_wake: Option<NonZeroU64>,

        /// Timer wheel
        wheel: wheel::Wheel,

        /// True if the driver is being shutdown
        is_shutdown: bool,

        /// Unparker that can be used to wake the time driver
        unpark: Box<dyn Unpark>,
    }

    // ===== impl Driver =====

    impl<P> Driver<P>
    where
        P: Park + 'static,
    {
        /// Creates a new `Driver` instance that uses `park` to block the current
        /// thread and `time_source` to get the current time and convert to ticks.
        ///
        /// Specifying the source of time is useful when testing.
        pub(crate) fn new(park: P, clock: Clock) -> Driver<P> {
            let time_source = ClockTime::new(clock);

            let inner = Inner::new(time_source.clone(), Box::new(park.unpark()));

            Driver {
                time_source,
                inner: Handle::new(Arc::new(Mutex::new(inner))),
                park,
            }
        }

        /// Returns a handle to the timer.
        ///
        /// The `Handle` is how `Sleep` instances are created. The `Sleep` instances
        /// can either be created directly or the `Handle` instance can be passed to
        /// `with_default`, setting the timer as the default timer for the execution
        /// context.
        pub(crate) fn handle(&self) -> Handle {
            self.inner.clone()
        }

        fn park_internal(&mut self, limit: Option<Duration>) -> Result<(), P::Error> {
            let clock = &self.time_source.clock;

            let mut lock = self.inner.lock();

            assert!(!lock.is_shutdown);

            let next_wake = lock.wheel.next_expiration_time();
            lock.next_wake =
                next_wake.map(|t| NonZeroU64::new(t).unwrap_or_else(|| NonZeroU64::new(1).unwrap()));

            drop(lock);

            match next_wake {
                Some(when) => {
                    let now = self.time_source.now();
                    // Note that we effectively round up to 1ms here - this avoids
                    // very short-duration microsecond-resolution sleeps that the OS
                    // might treat as zero-length.
                    let mut duration = self.time_source.tick_to_duration(when.saturating_sub(now));

                    if duration > Duration::from_millis(0) {
                        if let Some(limit) = limit {
                            duration = std::cmp::min(limit, duration);
                        }

                        if clock.is_paused() {
                            self.park.park_timeout(Duration::from_secs(0))?;

                            // Simulate advancing time
                            clock.advance(duration);
                        } else {
                            self.park.park_timeout(duration)?;
                        }
                    } else {
                        self.park.park_timeout(Duration::from_secs(0))?;
                    }
                }
                None => {
                    if let Some(duration) = limit {
                        if clock.is_paused() {
                            self.park.park_timeout(Duration::from_secs(0))?;
                            clock.advance(duration);
                        } else {
                            self.park.park_timeout(duration)?;
                        }
                    } else {
                        self.park.park()?;
                    }
                }
            }

            // Process pending timers after waking up
            self.inner.process();

            Ok(())
        }
    }

    impl Handle {
        /// Runs timer related logic, and returns the next wakeup time
        pub(self) fn process(&self) {
            let now = self.time_source().now();

            self.process_at_time(now)
        }

        pub(self) fn process_at_time(&self, now: u64) {
            let mut waker_list: [Option<Waker>; 32] = Default::default();
            let mut waker_idx = 0;

            let mut lock = self.lock();

            assert!(now >= lock.elapsed);

            while let Some(entry) = lock.wheel.poll(now) {
                debug_assert!(unsafe { entry.is_pending() });

                // SAFETY: We hold the driver lock, and just removed the entry from any linked lists.
                if let Some(waker) = unsafe { entry.fire(Ok(())) } {
                    waker_list[waker_idx] = Some(waker);

                    waker_idx += 1;

                    if waker_idx == waker_list.len() {
                        // Wake a batch of wakers. To avoid deadlock, we must do this with the lock temporarily dropped.
                        drop(lock);

                        for waker in waker_list.iter_mut() {
                            waker.take().unwrap().wake();
                        }

                        waker_idx = 0;

                        lock = self.lock();
                    }
                }
            }

            // Update the elapsed cache
            lock.elapsed = lock.wheel.elapsed();
            lock.next_wake = lock
                .wheel
                .poll_at()
                .map(|t| NonZeroU64::new(t).unwrap_or_else(|| NonZeroU64::new(1).unwrap()));

            drop(lock);

            for waker in waker_list[0..waker_idx].iter_mut() {
                waker.take().unwrap().wake();
            }
        }

        /// Removes a registered timer from the driver.
        ///
        /// The timer will be moved to the cancelled state. Wakers will _not_ be
        /// invoked. If the timer is already completed, this function is a no-op.
        ///
        /// This function always acquires the driver lock, even if the entry does
        /// not appear to be registered.
        ///
        /// SAFETY: The timer must not be registered with some other driver, and
        /// `add_entry` must not be called concurrently.
        pub(self) unsafe fn clear_entry(&self, entry: NonNull<TimerShared>) {
            unsafe {
                let mut lock = self.lock();

                if entry.as_ref().might_be_registered() {
                    lock.wheel.remove(entry);
                }

                entry.as_ref().handle().fire(Ok(()));
            }
        }

        /// Removes and re-adds an entry to the driver.
        ///
        /// SAFETY: The timer must be either unregistered, or registered with this
        /// driver. No other threads are allowed to concurrently manipulate the
        /// timer at all (the current thread should hold an exclusive reference to
        /// the `TimerEntry`)
        pub(self) unsafe fn reregister(&self, new_tick: u64, entry: NonNull<TimerShared>) {
            let waker = unsafe {
                let mut lock = self.lock();

                // We may have raced with a firing/deregistration, so check before
                // deregistering.
                if unsafe { entry.as_ref().might_be_registered() } {
                    lock.wheel.remove(entry);
                }

                // Now that we have exclusive control of this entry, mint a handle to reinsert it.
                let entry = entry.as_ref().handle();

                if lock.is_shutdown {
                    unsafe { entry.fire(Err(crate::time::error::Error::shutdown())) }
                } else {
                    entry.set_expiration(new_tick);

                    // Note: We don't have to worry about racing with some other resetting
                    // thread, because add_entry and reregister require exclusive control of
                    // the timer entry.
                    match unsafe { lock.wheel.insert(entry) } {
                        Ok(when) => {
                            if lock
                                .next_wake
                                .map(|next_wake| when < next_wake.get())
                                .unwrap_or(true)
                            {
                                lock.unpark.unpark();
                            }

                            None
                        }
                        Err((entry, super::error::InsertError::Elapsed)) => unsafe {
                            entry.fire(Ok(()))
                        },
                    }
                }

                // Must release lock before invoking waker to avoid the risk of deadlock.
            };

            // The timer was fired synchronously as a result of the reregistration.
            // Wake the waker; this is needed because we might reset _after_ a poll,
            // and otherwise the task won't be awoken to poll again.
            if let Some(waker) = waker {
                waker.wake();
            }
        }
    }

    impl<P> Park for Driver<P>
    where
        P: Park + 'static,
    {
        type Unpark = P::Unpark;
        type Error = P::Error;

        fn unpark(&self) -> Self::Unpark {
            self.park.unpark()
        }

        fn park(&mut self) -> Result<(), Self::Error> {
            self.park_internal(None)
        }

        fn park_timeout(&mut self, duration: Duration) -> Result<(), Self::Error> {
            self.park_internal(Some(duration))
        }

        fn shutdown(&mut self) {
            let mut lock = self.inner.lock();

            if lock.is_shutdown {
                return;
            }

            lock.is_shutdown = true;

            drop(lock);

            // Advance time forward to the end of time.

            self.inner.process_at_time(u64::MAX);

            self.park.shutdown();
        }
    }

    impl<P> Drop for Driver<P>
    where
        P: Park + 'static,
    {
        fn drop(&mut self) {
            self.shutdown();
        }
    }

    // ===== impl Inner =====

    impl Inner {
        pub(self) fn new(time_source: ClockTime, unpark: Box<dyn Unpark>) -> Self {
            Inner {
                time_source,
                elapsed: 0,
                next_wake: None,
                unpark,
                wheel: wheel::Wheel::new(),
                is_shutdown: false,
            }
        }
    }

    impl fmt::Debug for Inner {
        fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
            fmt.debug_struct("Inner").finish()
        }
    }

    #[cfg(test)]
    mod tests;