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    use crate::time::driver::{TimerHandle, TimerShared};
    use crate::time::error::InsertError;

    mod level;
    pub(crate) use self::level::Expiration;
    use self::level::Level;

    use std::ptr::NonNull;

    use super::EntryList;

    /// Timing wheel implementation.
    ///
    /// This type provides the hashed timing wheel implementation that backs `Timer`
    /// and `DelayQueue`.
    ///
    /// The structure is generic over `T: Stack`. This allows handling timeout data
    /// being stored on the heap or in a slab. In order to support the latter case,
    /// the slab must be passed into each function allowing the implementation to
    /// lookup timer entries.
    ///
    /// See `Timer` documentation for some implementation notes.
    #[derive(Debug)]
    pub(crate) struct Wheel {
        /// The number of milliseconds elapsed since the wheel started.
        elapsed: u64,

        /// Timer wheel.
        ///
        /// Levels:
        ///
        /// * 1 ms slots / 64 ms range
        /// * 64 ms slots / ~ 4 sec range
        /// * ~ 4 sec slots / ~ 4 min range
        /// * ~ 4 min slots / ~ 4 hr range
        /// * ~ 4 hr slots / ~ 12 day range
        /// * ~ 12 day slots / ~ 2 yr range
        levels: Vec<Level>,

        /// Entries queued for firing
        pending: EntryList,
    }

    /// Number of levels. Each level has 64 slots. By using 6 levels with 64 slots
    /// each, the timer is able to track time up to 2 years into the future with a
    /// precision of 1 millisecond.
    const NUM_LEVELS: usize = 6;

    /// The maximum duration of a `Sleep`
    pub(super) const MAX_DURATION: u64 = (1 << (6 * NUM_LEVELS)) - 1;

    impl Wheel {
        /// Create a new timing wheel
        pub(crate) fn new() -> Wheel {
            let levels = (0..NUM_LEVELS).map(Level::new).collect();

            Wheel {
                elapsed: 0,
                levels,
                pending: EntryList::new(),
            }
        }

        /// Return the number of milliseconds that have elapsed since the timing
        /// wheel's creation.
        pub(crate) fn elapsed(&self) -> u64 {
            self.elapsed
        }

        /// Insert an entry into the timing wheel.
        ///
        /// # Arguments
        ///
        /// * `item`: The item to insert into the wheel.
        ///
        /// # Return
        ///
        /// Returns `Ok` when the item is successfully inserted, `Err` otherwise.
        ///
        /// `Err(Elapsed)` indicates that `when` represents an instant that has
        /// already passed. In this case, the caller should fire the timeout
        /// immediately.
        ///
        /// `Err(Invalid)` indicates an invalid `when` argument as been supplied.
        ///
        /// # Safety
        ///
        /// This function registers item into an intrusive linked list. The caller
        /// must ensure that `item` is pinned and will not be dropped without first
        /// being deregistered.
        pub(crate) unsafe fn insert(
            &mut self,
            item: TimerHandle,
        ) -> Result<u64, (TimerHandle, InsertError)> {
            let when = item.sync_when();

            if when <= self.elapsed {
                return Err((item, InsertError::Elapsed));
            }

            // Get the level at which the entry should be stored
            let level = self.level_for(when);

            unsafe {
                self.levels[level].add_entry(item);
            }

            debug_assert!({
                self.levels[level]
                    .next_expiration(self.elapsed)
                    .map(|e| e.deadline >= self.elapsed)
                    .unwrap_or(true)
            });

            Ok(when)
        }

        /// Remove `item` from the timing wheel.
        pub(crate) unsafe fn remove(&mut self, item: NonNull<TimerShared>) {
            unsafe {
                let when = item.as_ref().cached_when();
                if when == u64::max_value() {
                    self.pending.remove(item);
                } else {
                    let level = self.level_for(when);

                    self.levels[level].remove_entry(item);
                }
            }
        }

        /// Instant at which to poll
        pub(crate) fn poll_at(&self) -> Option<u64> {
            self.next_expiration().map(|expiration| expiration.deadline)
        }

        /// Advances the timer up to the instant represented by `now`.
        pub(crate) fn poll(&mut self, now: u64) -> Option<TimerHandle> {
            loop {
                if let Some(handle) = self.pending.pop_back() {
                    return Some(handle);
                }

                // under what circumstances is poll.expiration Some vs. None?
                let expiration = self.next_expiration().and_then(|expiration| {
                    if expiration.deadline > now {
                        None
                    } else {
                        Some(expiration)
                    }
                });

                match expiration {
                    Some(ref expiration) if expiration.deadline > now => return None,
                    Some(ref expiration) => {
                        self.process_expiration(expiration);

                        self.set_elapsed(expiration.deadline);
                    }
                    None => {
                        // in this case the poll did not indicate an expiration
                        // _and_ we were not able to find a next expiration in
                        // the current list of timers.  advance to the poll's
                        // current time and do nothing else.
                        self.set_elapsed(now);
                        break;
                    }
                }
            }

            self.pending.pop_back()
        }

        /// Returns the instant at which the next timeout expires.
        fn next_expiration(&self) -> Option<Expiration> {
            if !self.pending.is_empty() {
                // Expire immediately as we have things pending firing
                return Some(Expiration {
                    level: 0,
                    slot: 0,
                    deadline: self.elapsed,
                });
            }

            // Check all levels
            for level in 0..NUM_LEVELS {
                if let Some(expiration) = self.levels[level].next_expiration(self.elapsed) {
                    // There cannot be any expirations at a higher level that happen
                    // before this one.
                    debug_assert!(self.no_expirations_before(level + 1, expiration.deadline));

                    return Some(expiration);
                }
            }

            None
        }

        /// Returns the tick at which this timer wheel next needs to perform some
        /// processing, or None if there are no timers registered.
        pub(super) fn next_expiration_time(&self) -> Option<u64> {
            self.next_expiration().map(|ex| ex.deadline)
        }

        /// Used for debug assertions
        fn no_expirations_before(&self, start_level: usize, before: u64) -> bool {
            let mut res = true;

            for l2 in start_level..NUM_LEVELS {
                if let Some(e2) = self.levels[l2].next_expiration(self.elapsed) {
                    if e2.deadline < before {
                        res = false;
                    }
                }
            }

            res
        }

        /// iteratively find entries that are between the wheel's current
        /// time and the expiration time.  for each in that population either
        /// queue it for notification (in the case of the last level) or tier
        /// it down to the next level (in all other cases).
        pub(crate) fn process_expiration(&mut self, expiration: &Expiration) {
            // Note that we need to take _all_ of the entries off the list before
            // processing any of them. This is important because it's possible that
            // those entries might need to be reinserted into the same slot.
            //
            // This happens only on the highest level, when an entry is inserted
            // more than MAX_DURATION into the future. When this happens, we wrap
            // around, and process some entries a multiple of MAX_DURATION before
            // they actually need to be dropped down a level. We then reinsert them
            // back into the same position; we must make sure we don't then process
            // those entries again or we'll end up in an infinite loop.
            let mut entries = self.take_entries(expiration);

            while let Some(item) = entries.pop_back() {
                if expiration.level == 0 {
                    debug_assert_eq!(unsafe { item.cached_when() }, expiration.deadline);
                }

                // Try to expire the entry; this is cheap (doesn't synchronize) if
                // the timer is not expired, and updates cached_when.
                match unsafe { item.mark_pending(expiration.deadline) } {
                    Ok(()) => {
                        // Item was expired
                        self.pending.push_front(item);
                    }
                    Err(expiration_tick) => {
                        let level = level_for(expiration.deadline, expiration_tick);
                        unsafe {
                            self.levels[level].add_entry(item);
                        }
                    }
                }
            }
        }

        fn set_elapsed(&mut self, when: u64) {
            assert!(
                self.elapsed <= when,
                "elapsed={:?}; when={:?}",
                self.elapsed,
                when
            );

            if when > self.elapsed {
                self.elapsed = when;
            }
        }

        /// Obtains the list of entries that need processing for the given expiration.
        ///
        fn take_entries(&mut self, expiration: &Expiration) -> EntryList {
            self.levels[expiration.level].take_slot(expiration.slot)
        }

        fn level_for(&self, when: u64) -> usize {
            level_for(self.elapsed, when)
        }
    }

    fn level_for(elapsed: u64, when: u64) -> usize {
        let mut masked = elapsed ^ when;

        if masked >= MAX_DURATION {
            // Fudge the timer into the top level
            masked = MAX_DURATION - 1;
        }

        assert!(masked != 0, "elapsed={}; when={}", elapsed, when);

        let leading_zeros = masked.leading_zeros() as usize;
        let significant = 63 - leading_zeros;

        significant / 6
    }

    #[cfg(all(test, not(loom)))]
    mod test {
        use super::*;

        #[test]
        fn test_level_for() {
            for pos in 1..64 {
                assert_eq!(
                    0,
                    level_for(0, pos),
                    "level_for({}) -- binary = {:b}",
                    pos,
                    pos
                );
            }

            for level in 1..5 {
                for pos in level..64 {
                    let a = pos * 64_usize.pow(level as u32);
                    assert_eq!(
                        level,
                        level_for(0, a as u64),
                        "level_for({}) -- binary = {:b}",
                        a,
                        a
                    );

                    if pos > level {
                        let a = a - 1;
                        assert_eq!(
                            level,
                            level_for(0, a as u64),
                            "level_for({}) -- binary = {:b}",
                            a,
                            a
                        );
                    }

                    if pos < 64 {
                        let a = a + 1;
                        assert_eq!(
                            level,
                            level_for(0, a as u64),
                            "level_for({}) -- binary = {:b}",
                            a,
                            a
                        );
                    }
                }
            }
        }
    }