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    //! Timer optimized for I/O related operations

    #![allow(deprecated, missing_debug_implementations)]

    use {convert, io, Ready, Poll, PollOpt, Registration, SetReadiness, Token};
    use event::Evented;
    use lazycell::LazyCell;
    use slab::Slab;
    use std::{cmp, error, fmt, u64, usize, iter, thread};
    use std::sync::Arc;
    use std::sync::atomic::{AtomicUsize, Ordering};
    use std::time::{Duration, Instant};

    use self::TimerErrorKind::TimerOverflow;

    pub struct Timer<T> {
        // Size of each tick in milliseconds
        tick_ms: u64,
        // Slab of timeout entries
        entries: Slab<Entry<T>>,
        // Timeout wheel. Each tick, the timer will look at the next slot for
        // timeouts that match the current tick.
        wheel: Vec<WheelEntry>,
        // Tick 0's time instant
        start: Instant,
        // The current tick
        tick: Tick,
        // The next entry to possibly timeout
        next: Token,
        // Masks the target tick to get the slot
        mask: u64,
        // Set on registration with Poll
        inner: LazyCell<Inner>,
    }

    pub struct Builder {
        // Approximate duration of each tick
        tick: Duration,
        // Number of slots in the timer wheel
        num_slots: usize,
        // Max number of timeouts that can be in flight at a given time.
        capacity: usize,
    }

    #[derive(Clone, Debug)]
    pub struct Timeout {
        // Reference into the timer entry slab
        token: Token,
        // Tick that it should match up with
        tick: u64,
    }

    struct Inner {
        registration: Registration,
        set_readiness: SetReadiness,
        wakeup_state: WakeupState,
        wakeup_thread: thread::JoinHandle<()>,
    }

    impl Drop for Inner {
        fn drop(&mut self) {
            // 1. Set wakeup state to TERMINATE_THREAD (https://github.com/carllerche/mio/blob/master/src/timer.rs#L451)
            self.wakeup_state.store(TERMINATE_THREAD, Ordering::Release);
            // 2. Wake him up
            self.wakeup_thread.thread().unpark();
        }
    }

    #[derive(Copy, Clone, Debug)]
    struct WheelEntry {
        next_tick: Tick,
        head: Token,
    }

    // Doubly linked list of timer entries. Allows for efficient insertion /
    // removal of timeouts.
    struct Entry<T> {
        state: T,
        links: EntryLinks,
    }

    #[derive(Copy, Clone)]
    struct EntryLinks {
        tick: Tick,
        prev: Token,
        next: Token
    }

    type Tick = u64;

    const TICK_MAX: Tick = u64::MAX;

    // Manages communication with wakeup thread
    type WakeupState = Arc<AtomicUsize>;

    pub type Result<T> = ::std::result::Result<T, TimerError>;
    // TODO: remove
    pub type TimerResult<T> = Result<T>;

    /// Deprecated and unused.
    #[derive(Debug)]
    pub struct TimerError;

    /// Deprecated and unused.
    #[derive(Debug)]
    pub enum TimerErrorKind {
        TimerOverflow,
    }

    // TODO: Remove
    pub type OldTimerResult<T> = Result<T>;

    const TERMINATE_THREAD: usize = 0;
    const EMPTY: Token = Token(usize::MAX);

    impl Builder {
        pub fn tick_duration(mut self, duration: Duration) -> Builder {
            self.tick = duration;
            self
        }

        pub fn num_slots(mut self, num_slots: usize) -> Builder {
            self.num_slots = num_slots;
            self
        }

        pub fn capacity(mut self, capacity: usize) -> Builder {
            self.capacity = capacity;
            self
        }

        pub fn build<T>(self) -> Timer<T> {
            Timer::new(convert::millis(self.tick), self.num_slots, self.capacity, Instant::now())
        }
    }

    impl Default for Builder {
        fn default() -> Builder {
            Builder {
                tick: Duration::from_millis(100),
                num_slots: 256,
                capacity: 65_536,
            }
        }
    }

    impl<T> Timer<T> {
        fn new(tick_ms: u64, num_slots: usize, capacity: usize, start: Instant) -> Timer<T> {
            let num_slots = num_slots.next_power_of_two();
            let capacity = capacity.next_power_of_two();
            let mask = (num_slots as u64) - 1;
            let wheel = iter::repeat(WheelEntry { next_tick: TICK_MAX, head: EMPTY })
                .take(num_slots).collect();

            Timer {
                tick_ms,
                entries: Slab::with_capacity(capacity),
                wheel,
                start,
                tick: 0,
                next: EMPTY,
                mask,
                inner: LazyCell::new(),
            }
        }

        pub fn set_timeout(&mut self, delay_from_now: Duration, state: T) -> Result<Timeout> {
            let delay_from_start = self.start.elapsed() + delay_from_now;
            self.set_timeout_at(delay_from_start, state)
        }

        fn set_timeout_at(&mut self, delay_from_start: Duration, state: T) -> Result<Timeout> {
            let mut tick = duration_to_tick(delay_from_start, self.tick_ms);
            trace!("setting timeout; delay={:?}; tick={:?}; current-tick={:?}", delay_from_start, tick, self.tick);

            // Always target at least 1 tick in the future
            if tick <= self.tick {
                tick = self.tick + 1;
            }

            self.insert(tick, state)
        }

        fn insert(&mut self, tick: Tick, state: T) -> Result<Timeout> {
            // Get the slot for the requested tick
            let slot = (tick & self.mask) as usize;
            let curr = self.wheel[slot];

            // Insert the new entry
            let entry = Entry::new(state, tick, curr.head);
            let token = Token(self.entries.insert(entry));

            if curr.head != EMPTY {
                // If there was a previous entry, set its prev pointer to the new
                // entry
                self.entries[curr.head.into()].links.prev = token;
            }

            // Update the head slot
            self.wheel[slot] = WheelEntry {
                next_tick: cmp::min(tick, curr.next_tick),
                head: token,
            };

            self.schedule_readiness(tick);

            trace!("inserted timeout; slot={}; token={:?}", slot, token);

            // Return the new timeout
            Ok(Timeout {
                token,
                tick
            })
        }

        pub fn cancel_timeout(&mut self, timeout: &Timeout) -> Option<T> {
            let links = match self.entries.get(timeout.token.into()) {
                Some(e) => e.links,
                None => return None
            };

            // Sanity check
            if links.tick != timeout.tick {
                return None;
            }

            self.unlink(&links, timeout.token);
            Some(self.entries.remove(timeout.token.into()).state)
        }

        pub fn poll(&mut self) -> Option<T> {
            let target_tick = current_tick(self.start, self.tick_ms);
            self.poll_to(target_tick)
        }

        fn poll_to(&mut self, mut target_tick: Tick) -> Option<T> {
            trace!("tick_to; target_tick={}; current_tick={}", target_tick, self.tick);

            if target_tick < self.tick {
                target_tick = self.tick;
            }

            while self.tick <= target_tick {
                let curr = self.next;

                trace!("ticking; curr={:?}", curr);

                if curr == EMPTY {
                    self.tick += 1;

                    let slot = self.slot_for(self.tick);
                    self.next = self.wheel[slot].head;

                    // Handle the case when a slot has a single timeout which gets
                    // canceled before the timeout expires. In this case, the
                    // slot's head is EMPTY but there is a value for next_tick. Not
                    // resetting next_tick here causes the timer to get stuck in a
                    // loop.
                    if self.next == EMPTY {
                        self.wheel[slot].next_tick = TICK_MAX;
                    }
                } else {
                    let slot = self.slot_for(self.tick);

                    if curr == self.wheel[slot].head {
                        self.wheel[slot].next_tick = TICK_MAX;
                    }

                    let links = self.entries[curr.into()].links;

                    if links.tick <= self.tick {
                        trace!("triggering; token={:?}", curr);

                        // Unlink will also advance self.next
                        self.unlink(&links, curr);

                        // Remove and return the token
                        return Some(self.entries.remove(curr.into()).state);
                    } else {
                        let next_tick = self.wheel[slot].next_tick;
                        self.wheel[slot].next_tick = cmp::min(next_tick, links.tick);
                        self.next = links.next;
                    }
                }
            }

            // No more timeouts to poll
            if let Some(inner) = self.inner.borrow() {
                trace!("unsetting readiness");
                let _ = inner.set_readiness.set_readiness(Ready::empty());

                if let Some(tick) = self.next_tick() {
                    self.schedule_readiness(tick);
                }
            }

            None
        }

        fn unlink(&mut self, links: &EntryLinks, token: Token) {
           trace!("unlinking timeout; slot={}; token={:?}",
                   self.slot_for(links.tick), token);

            if links.prev == EMPTY {
                let slot = self.slot_for(links.tick);
                self.wheel[slot].head = links.next;
            } else {
                self.entries[links.prev.into()].links.next = links.next;
            }

            if links.next != EMPTY {
                self.entries[links.next.into()].links.prev = links.prev;

                if token == self.next {
                    self.next = links.next;
                }
            } else if token == self.next {
                self.next = EMPTY;
            }
        }

        fn schedule_readiness(&self, tick: Tick) {
            if let Some(inner) = self.inner.borrow() {
                // Coordinate setting readiness w/ the wakeup thread
                let mut curr = inner.wakeup_state.load(Ordering::Acquire);

                loop {
                    if curr as Tick <= tick {
                        // Nothing to do, wakeup is already scheduled
                        return;
                    }

                    // Attempt to move the wakeup time forward
                    trace!("advancing the wakeup time; target={}; curr={}", tick, curr);
                    let actual = inner.wakeup_state.compare_and_swap(curr, tick as usize, Ordering::Release);

                    if actual == curr {
                        // Signal to the wakeup thread that the wakeup time has
                        // been changed.
                        trace!("unparking wakeup thread");
                        inner.wakeup_thread.thread().unpark();
                        return;
                    }

                    curr = actual;
                }
            }
        }

        // Next tick containing a timeout
        fn next_tick(&self) -> Option<Tick> {
            if self.next != EMPTY {
                let slot = self.slot_for(self.entries[self.next.into()].links.tick);

                if self.wheel[slot].next_tick == self.tick {
                    // There is data ready right now
                    return Some(self.tick);
                }
            }

            self.wheel.iter().map(|e| e.next_tick).min()
        }

        fn slot_for(&self, tick: Tick) -> usize {
            (self.mask & tick) as usize
        }
    }

    impl<T> Default for Timer<T> {
        fn default() -> Timer<T> {
            Builder::default().build()
        }
    }

    impl<T> Evented for Timer<T> {
        fn register(&self, poll: &Poll, token: Token, interest: Ready, opts: PollOpt) -> io::Result<()> {
            if self.inner.borrow().is_some() {
                return Err(io::Error::new(io::ErrorKind::Other, "timer already registered"));
            }

            let (registration, set_readiness) = Registration::new(poll, token, interest, opts);
            let wakeup_state = Arc::new(AtomicUsize::new(usize::MAX));
            let thread_handle = spawn_wakeup_thread(
                wakeup_state.clone(),
                set_readiness.clone(),
                self.start, self.tick_ms);

            self.inner.fill(Inner {
                registration,
                set_readiness,
                wakeup_state,
                wakeup_thread: thread_handle,
            }).expect("timer already registered");

            if let Some(next_tick) = self.next_tick() {
                self.schedule_readiness(next_tick);
            }

            Ok(())
        }

        fn reregister(&self, poll: &Poll, token: Token, interest: Ready, opts: PollOpt) -> io::Result<()> {
            match self.inner.borrow() {
                Some(inner) => inner.registration.update(poll, token, interest, opts),
                None => Err(io::Error::new(io::ErrorKind::Other, "receiver not registered")),
            }
        }

        fn deregister(&self, poll: &Poll) -> io::Result<()> {
            match self.inner.borrow() {
                Some(inner) => inner.registration.deregister(poll),
                None => Err(io::Error::new(io::ErrorKind::Other, "receiver not registered")),
            }
        }
    }

    impl fmt::Debug for Inner {
        fn fmt(&self, fmt: &mut fmt::Formatter) -> fmt::Result {
            fmt.debug_struct("Inner")
                .field("registration", &self.registration)
                .field("wakeup_state", &self.wakeup_state.load(Ordering::Relaxed))
                .finish()
        }
    }

    fn spawn_wakeup_thread(state: WakeupState, set_readiness: SetReadiness, start: Instant, tick_ms: u64) -> thread::JoinHandle<()> {
        thread::spawn(move || {
            let mut sleep_until_tick = state.load(Ordering::Acquire) as Tick;

            loop {
                if sleep_until_tick == TERMINATE_THREAD as Tick {
                    return;
                }

                let now_tick = current_tick(start, tick_ms);

                trace!("wakeup thread: sleep_until_tick={:?}; now_tick={:?}", sleep_until_tick, now_tick);

                if now_tick < sleep_until_tick {
                    // Calling park_timeout with u64::MAX leads to undefined
                    // behavior in pthread, causing the park to return immediately
                    // and causing the thread to tightly spin. Instead of u64::MAX
                    // on large values, simply use a blocking park.
                    match tick_ms.checked_mul(sleep_until_tick - now_tick) {
                        Some(sleep_duration) => {
                            trace!("sleeping; tick_ms={}; now_tick={}; sleep_until_tick={}; duration={:?}",
                                   tick_ms, now_tick, sleep_until_tick, sleep_duration);
                            thread::park_timeout(Duration::from_millis(sleep_duration));
                        }
                        None => {
                            trace!("sleeping; tick_ms={}; now_tick={}; blocking sleep",
                                   tick_ms, now_tick);
                            thread::park();
                        }
                    }
                    sleep_until_tick = state.load(Ordering::Acquire) as Tick;
                } else {
                    let actual = state.compare_and_swap(sleep_until_tick as usize, usize::MAX, Ordering::AcqRel) as Tick;

                    if actual == sleep_until_tick {
                        trace!("setting readiness from wakeup thread");
                        let _ = set_readiness.set_readiness(Ready::readable());
                        sleep_until_tick = usize::MAX as Tick;
                    } else {
                        sleep_until_tick = actual as Tick;
                    }
                }
            }
        })
    }

    fn duration_to_tick(elapsed: Duration, tick_ms: u64) -> Tick {
        // Calculate tick rounding up to the closest one
        let elapsed_ms = convert::millis(elapsed);
        elapsed_ms.saturating_add(tick_ms / 2) / tick_ms
    }

    fn current_tick(start: Instant, tick_ms: u64) -> Tick {
        duration_to_tick(start.elapsed(), tick_ms)
    }

    impl<T> Entry<T> {
        fn new(state: T, tick: u64, next: Token) -> Entry<T> {
            Entry {
                state,
                links: EntryLinks {
                    tick,
                    prev: EMPTY,
                    next,
                },
            }
        }
    }

    impl fmt::Display for TimerError {
        fn fmt(&self, _: &mut fmt::Formatter) -> fmt::Result {
            // `TimerError` will never be constructed.
            unreachable!();
        }
    }

    impl error::Error for TimerError {
        fn description(&self) -> &str {
            // `TimerError` will never be constructed.
            unreachable!();
        }
    }

    impl fmt::Display for TimerErrorKind {
        fn fmt(&self, fmt: &mut fmt::Formatter) -> fmt::Result {
            match *self {
                TimerOverflow => write!(fmt, "TimerOverflow"),
            }
        }
    }

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