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    use crate::time::driver::TimerHandle;

    use crate::time::driver::{EntryList, TimerShared};

    use std::{fmt, ptr::NonNull};

    /// Wheel for a single level in the timer. This wheel contains 64 slots.
    pub(crate) struct Level {
        level: usize,

        /// Bit field tracking which slots currently contain entries.
        ///
        /// Using a bit field to track slots that contain entries allows avoiding a
        /// scan to find entries. This field is updated when entries are added or
        /// removed from a slot.
        ///
        /// The least-significant bit represents slot zero.
        occupied: u64,

        /// Slots. We access these via the EntryInner `current_list` as well, so this needs to be an UnsafeCell.
        slot: [EntryList; LEVEL_MULT],
    }

    /// Indicates when a slot must be processed next.
    #[derive(Debug)]
    pub(crate) struct Expiration {
        /// The level containing the slot.
        pub(crate) level: usize,

        /// The slot index.
        pub(crate) slot: usize,

        /// The instant at which the slot needs to be processed.
        pub(crate) deadline: u64,
    }

    /// Level multiplier.
    ///
    /// Being a power of 2 is very important.
    const LEVEL_MULT: usize = 64;

    impl Level {
        pub(crate) fn new(level: usize) -> Level {
            // A value has to be Copy in order to use syntax like:
            //     let stack = Stack::default();
            //     ...
            //     slots: [stack; 64],
            //
            // Alternatively, since Stack is Default one can
            // use syntax like:
            //     let slots: [Stack; 64] = Default::default();
            //
            // However, that is only supported for arrays of size
            // 32 or fewer.  So in our case we have to explicitly
            // invoke the constructor for each array element.
            let ctor = || EntryList::default();

            Level {
                level,
                occupied: 0,
                slot: [
                    ctor(),
                    ctor(),
                    ctor(),
                    ctor(),
                    ctor(),
                    ctor(),
                    ctor(),
                    ctor(),
                    ctor(),
                    ctor(),
                    ctor(),
                    ctor(),
                    ctor(),
                    ctor(),
                    ctor(),
                    ctor(),
                    ctor(),
                    ctor(),
                    ctor(),
                    ctor(),
                    ctor(),
                    ctor(),
                    ctor(),
                    ctor(),
                    ctor(),
                    ctor(),
                    ctor(),
                    ctor(),
                    ctor(),
                    ctor(),
                    ctor(),
                    ctor(),
                    ctor(),
                    ctor(),
                    ctor(),
                    ctor(),
                    ctor(),
                    ctor(),
                    ctor(),
                    ctor(),
                    ctor(),
                    ctor(),
                    ctor(),
                    ctor(),
                    ctor(),
                    ctor(),
                    ctor(),
                    ctor(),
                    ctor(),
                    ctor(),
                    ctor(),
                    ctor(),
                    ctor(),
                    ctor(),
                    ctor(),
                    ctor(),
                    ctor(),
                    ctor(),
                    ctor(),
                    ctor(),
                    ctor(),
                    ctor(),
                    ctor(),
                    ctor(),
                ],
            }
        }

        /// Finds the slot that needs to be processed next and returns the slot and
        /// `Instant` at which this slot must be processed.
        pub(crate) fn next_expiration(&self, now: u64) -> Option<Expiration> {
            // Use the `occupied` bit field to get the index of the next slot that
            // needs to be processed.
            let slot = match self.next_occupied_slot(now) {
                Some(slot) => slot,
                None => return None,
            };

            // From the slot index, calculate the `Instant` at which it needs to be
            // processed. This value *must* be in the future with respect to `now`.

            let level_range = level_range(self.level);
            let slot_range = slot_range(self.level);

            // TODO: This can probably be simplified w/ power of 2 math
            let level_start = now - (now % level_range);
            let mut deadline = level_start + slot as u64 * slot_range;

            if deadline <= now {
                // A timer is in a slot "prior" to the current time. This can occur
                // because we do not have an infinite hierarchy of timer levels, and
                // eventually a timer scheduled for a very distant time might end up
                // being placed in a slot that is beyond the end of all of the
                // arrays.
                //
                // To deal with this, we first limit timers to being scheduled no
                // more than MAX_DURATION ticks in the future; that is, they're at
                // most one rotation of the top level away. Then, we force timers
                // that logically would go into the top+1 level, to instead go into
                // the top level's slots.
                //
                // What this means is that the top level's slots act as a
                // pseudo-ring buffer, and we rotate around them indefinitely. If we
                // compute a deadline before now, and it's the top level, it
                // therefore means we're actually looking at a slot in the future.
                debug_assert_eq!(self.level, super::NUM_LEVELS - 1);

                deadline += level_range;
            }

            debug_assert!(
                deadline >= now,
                "deadline={:016X}; now={:016X}; level={}; lr={:016X}, sr={:016X}, slot={}; occupied={:b}",
                deadline,
                now,
                self.level,
                level_range,
                slot_range,
                slot,
                self.occupied
            );

            Some(Expiration {
                level: self.level,
                slot,
                deadline,
            })
        }

        fn next_occupied_slot(&self, now: u64) -> Option<usize> {
            if self.occupied == 0 {
                return None;
            }

            // Get the slot for now using Maths
            let now_slot = (now / slot_range(self.level)) as usize;
            let occupied = self.occupied.rotate_right(now_slot as u32);
            let zeros = occupied.trailing_zeros() as usize;
            let slot = (zeros + now_slot) % 64;

            Some(slot)
        }

        pub(crate) unsafe fn add_entry(&mut self, item: TimerHandle) {
            let slot = slot_for(item.cached_when(), self.level);

            self.slot[slot].push_front(item);

            self.occupied |= occupied_bit(slot);
        }

        pub(crate) unsafe fn remove_entry(&mut self, item: NonNull<TimerShared>) {
            let slot = slot_for(unsafe { item.as_ref().cached_when() }, self.level);

            unsafe { self.slot[slot].remove(item) };
            if self.slot[slot].is_empty() {
                // The bit is currently set
                debug_assert!(self.occupied & occupied_bit(slot) != 0);

                // Unset the bit
                self.occupied ^= occupied_bit(slot);
            }
        }

        pub(crate) fn take_slot(&mut self, slot: usize) -> EntryList {
            self.occupied &= !occupied_bit(slot);

            std::mem::take(&mut self.slot[slot])
        }
    }

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

    fn occupied_bit(slot: usize) -> u64 {
        1 << slot
    }

    fn slot_range(level: usize) -> u64 {
        LEVEL_MULT.pow(level as u32) as u64
    }

    fn level_range(level: usize) -> u64 {
        LEVEL_MULT as u64 * slot_range(level)
    }

    /// Convert a duration (milliseconds) and a level to a slot position
    fn slot_for(duration: u64, level: usize) -> usize {
        ((duration >> (level * 6)) % LEVEL_MULT as u64) as usize
    }

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

        #[test]
        fn test_slot_for() {
            for pos in 1..64 {
                assert_eq!(pos as usize, slot_for(pos, 0));
            }

            for level in 1..5 {
                for pos in level..64 {
                    let a = pos * 64_usize.pow(level as u32);
                    assert_eq!(pos as usize, slot_for(a as u64, level));
                }
            }
        }
    }
    */