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    use crate::loom::sync::atomic::AtomicUsize;

    use std::fmt;
    use std::sync::atomic::Ordering::{AcqRel, Acquire, Release};
    use std::usize;

    pub(super) struct State {
        val: AtomicUsize,
    }

    /// Current state value
    #[derive(Copy, Clone)]
    pub(super) struct Snapshot(usize);

    type UpdateResult = Result<Snapshot, Snapshot>;

    /// The task is currently being run.
    const RUNNING: usize = 0b0001;

    /// The task is complete.
    ///
    /// Once this bit is set, it is never unset
    const COMPLETE: usize = 0b0010;

    /// Extracts the task's lifecycle value from the state
    const LIFECYCLE_MASK: usize = 0b11;

    /// Flag tracking if the task has been pushed into a run queue.
    const NOTIFIED: usize = 0b100;

    /// The join handle is still around
    const JOIN_INTEREST: usize = 0b1_000;

    /// A join handle waker has been set
    const JOIN_WAKER: usize = 0b10_000;

    /// The task has been forcibly cancelled.
    const CANCELLED: usize = 0b100_000;

    /// All bits
    const STATE_MASK: usize = LIFECYCLE_MASK | NOTIFIED | JOIN_INTEREST | JOIN_WAKER | CANCELLED;

    /// Bits used by the ref count portion of the state.
    const REF_COUNT_MASK: usize = !STATE_MASK;

    /// Number of positions to shift the ref count
    const REF_COUNT_SHIFT: usize = REF_COUNT_MASK.count_zeros() as usize;

    /// One ref count
    const REF_ONE: usize = 1 << REF_COUNT_SHIFT;

    /// State a task is initialized with
    ///
    /// A task is initialized with two references: one for the scheduler and one for
    /// the `JoinHandle`. As the task starts with a `JoinHandle`, `JOIN_INTERST` is
    /// set. A new task is immediately pushed into the run queue for execution and
    /// starts with the `NOTIFIED` flag set.
    const INITIAL_STATE: usize = (REF_ONE * 2) | JOIN_INTEREST | NOTIFIED;

    /// All transitions are performed via RMW operations. This establishes an
    /// unambiguous modification order.
    impl State {
        /// Return a task's initial state
        pub(super) fn new() -> State {
            // A task is initialized with three references: one for the scheduler,
            // one for the `JoinHandle`, one for the task handle made available in
            // release. As the task starts with a `JoinHandle`, `JOIN_INTERST` is
            // set. A new task is immediately pushed into the run queue for
            // execution and starts with the `NOTIFIED` flag set.
            State {
                val: AtomicUsize::new(INITIAL_STATE),
            }
        }

        /// Loads the current state, establishes `Acquire` ordering.
        pub(super) fn load(&self) -> Snapshot {
            Snapshot(self.val.load(Acquire))
        }

        /// Attempt to transition the lifecycle to `Running`.
        ///
        /// If `ref_inc` is set, the reference count is also incremented.
        ///
        /// The `NOTIFIED` bit is always unset.
        pub(super) fn transition_to_running(&self, ref_inc: bool) -> UpdateResult {
            self.fetch_update(|curr| {
                assert!(curr.is_notified());

                let mut next = curr;

                if !next.is_idle() {
                    return None;
                }

                if ref_inc {
                    next.ref_inc();
                }

                next.set_running();
                next.unset_notified();
                Some(next)
            })
        }

        /// Transitions the task from `Running` -> `Idle`.
        ///
        /// Returns `Ok` if the transition to `Idle` is successful, `Err` otherwise.
        /// In both cases, a snapshot of the state from **after** the transition is
        /// returned.
        ///
        /// The transition to `Idle` fails if the task has been flagged to be
        /// cancelled.
        pub(super) fn transition_to_idle(&self) -> UpdateResult {
            self.fetch_update(|curr| {
                assert!(curr.is_running());

                if curr.is_cancelled() {
                    return None;
                }

                let mut next = curr;
                next.unset_running();

                if next.is_notified() {
                    // The caller needs to schedule the task. To do this, it needs a
                    // waker. The waker requires a ref count.
                    next.ref_inc();
                }

                Some(next)
            })
        }

        /// Transitions the task from `Running` -> `Complete`.
        pub(super) fn transition_to_complete(&self) -> Snapshot {
            const DELTA: usize = RUNNING | COMPLETE;

            let prev = Snapshot(self.val.fetch_xor(DELTA, AcqRel));
            assert!(prev.is_running());
            assert!(!prev.is_complete());

            Snapshot(prev.0 ^ DELTA)
        }

        /// Transition from `Complete` -> `Terminal`, decrementing the reference
        /// count by 1.
        ///
        /// When `ref_dec` is set, an additional ref count decrement is performed.
        /// This is used to batch atomic ops when possible.
        pub(super) fn transition_to_terminal(&self, complete: bool, ref_dec: bool) -> Snapshot {
            self.fetch_update(|mut snapshot| {
                if complete {
                    snapshot.set_complete();
                } else {
                    assert!(snapshot.is_complete());
                }

                // Decrement the primary handle
                snapshot.ref_dec();

                if ref_dec {
                    // Decrement a second time
                    snapshot.ref_dec();
                }

                Some(snapshot)
            })
            .unwrap()
        }

        /// Transitions the state to `NOTIFIED`.
        ///
        /// Returns `true` if the task needs to be submitted to the pool for
        /// execution
        pub(super) fn transition_to_notified(&self) -> bool {
            let prev = Snapshot(self.val.fetch_or(NOTIFIED, AcqRel));
            prev.will_need_queueing()
        }

        /// Set the `CANCELLED` bit and attempt to transition to `Running`.
        ///
        /// Returns `true` if the transition to `Running` succeeded.
        pub(super) fn transition_to_shutdown(&self) -> bool {
            let mut prev = Snapshot(0);

            let _ = self.fetch_update(|mut snapshot| {
                prev = snapshot;

                if snapshot.is_idle() {
                    snapshot.set_running();

                    if snapshot.is_notified() {
                        // If the task is idle and notified, this indicates the task is
                        // in the run queue and is considered owned by the scheduler.
                        // The shutdown operation claims ownership of the task, which
                        // means we need to assign an additional ref-count to the task
                        // in the queue.
                        snapshot.ref_inc();
                    }
                }

                snapshot.set_cancelled();
                Some(snapshot)
            });

            prev.is_idle()
        }

        /// Optimistically tries to swap the state assuming the join handle is
        /// __immediately__ dropped on spawn
        pub(super) fn drop_join_handle_fast(&self) -> Result<(), ()> {
            use std::sync::atomic::Ordering::Relaxed;

            // Relaxed is acceptable as if this function is called and succeeds,
            // then nothing has been done w/ the join handle.
            //
            // The moment the join handle is used (polled), the `JOIN_WAKER` flag is
            // set, at which point the CAS will fail.
            //
            // Given this, there is no risk if this operation is reordered.
            self.val
                .compare_exchange_weak(
                    INITIAL_STATE,
                    (INITIAL_STATE - REF_ONE) & !JOIN_INTEREST,
                    Release,
                    Relaxed,
                )
                .map(|_| ())
                .map_err(|_| ())
        }

        /// Try to unset the JOIN_INTEREST flag.
        ///
        /// Returns `Ok` if the operation happens before the task transitions to a
        /// completed state, `Err` otherwise.
        pub(super) fn unset_join_interested(&self) -> UpdateResult {
            self.fetch_update(|curr| {
                assert!(curr.is_join_interested());

                if curr.is_complete() {
                    return None;
                }

                let mut next = curr;
                next.unset_join_interested();

                Some(next)
            })
        }

        /// Set the `JOIN_WAKER` bit.
        ///
        /// Returns `Ok` if the bit is set, `Err` otherwise. This operation fails if
        /// the task has completed.
        pub(super) fn set_join_waker(&self) -> UpdateResult {
            self.fetch_update(|curr| {
                assert!(curr.is_join_interested());
                assert!(!curr.has_join_waker());

                if curr.is_complete() {
                    return None;
                }

                let mut next = curr;
                next.set_join_waker();

                Some(next)
            })
        }

        /// Unsets the `JOIN_WAKER` bit.
        ///
        /// Returns `Ok` has been unset, `Err` otherwise. This operation fails if
        /// the task has completed.
        pub(super) fn unset_waker(&self) -> UpdateResult {
            self.fetch_update(|curr| {
                assert!(curr.is_join_interested());
                assert!(curr.has_join_waker());

                if curr.is_complete() {
                    return None;
                }

                let mut next = curr;
                next.unset_join_waker();

                Some(next)
            })
        }

        pub(super) fn ref_inc(&self) {
            use std::process;
            use std::sync::atomic::Ordering::Relaxed;

            // Using a relaxed ordering is alright here, as knowledge of the
            // original reference prevents other threads from erroneously deleting
            // the object.
            //
            // As explained in the [Boost documentation][1], Increasing the
            // reference counter can always be done with memory_order_relaxed: New
            // references to an object can only be formed from an existing
            // reference, and passing an existing reference from one thread to
            // another must already provide any required synchronization.
            //
            // [1]: (www.boost.org/doc/libs/1_55_0/doc/html/atomic/usage_examples.html)
            let prev = self.val.fetch_add(REF_ONE, Relaxed);

            // If the reference count overflowed, abort.
            if prev > isize::max_value() as usize {
                process::abort();
            }
        }

        /// Returns `true` if the task should be released.
        pub(super) fn ref_dec(&self) -> bool {
            let prev = Snapshot(self.val.fetch_sub(REF_ONE, AcqRel));
            prev.ref_count() == 1
        }

        fn fetch_update<F>(&self, mut f: F) -> Result<Snapshot, Snapshot>
        where
            F: FnMut(Snapshot) -> Option<Snapshot>,
        {
            let mut curr = self.load();

            loop {
                let next = match f(curr) {
                    Some(next) => next,
                    None => return Err(curr),
                };

                let res = self.val.compare_exchange(curr.0, next.0, AcqRel, Acquire);

                match res {
                    Ok(_) => return Ok(next),
                    Err(actual) => curr = Snapshot(actual),
                }
            }
        }
    }

    // ===== impl Snapshot =====

    impl Snapshot {
        /// Returns `true` if the task is in an idle state.
        pub(super) fn is_idle(self) -> bool {
            self.0 & (RUNNING | COMPLETE) == 0
        }

        /// Returns `true` if the task has been flagged as notified.
        pub(super) fn is_notified(self) -> bool {
            self.0 & NOTIFIED == NOTIFIED
        }

        fn unset_notified(&mut self) {
            self.0 &= !NOTIFIED
        }

        pub(super) fn is_running(self) -> bool {
            self.0 & RUNNING == RUNNING
        }

        fn set_running(&mut self) {
            self.0 |= RUNNING;
        }

        fn unset_running(&mut self) {
            self.0 &= !RUNNING;
        }

        pub(super) fn is_cancelled(self) -> bool {
            self.0 & CANCELLED == CANCELLED
        }

        fn set_cancelled(&mut self) {
            self.0 |= CANCELLED;
        }

        fn set_complete(&mut self) {
            self.0 |= COMPLETE;
        }

        /// Returns `true` if the task's future has completed execution.
        pub(super) fn is_complete(self) -> bool {
            self.0 & COMPLETE == COMPLETE
        }

        pub(super) fn is_join_interested(self) -> bool {
            self.0 & JOIN_INTEREST == JOIN_INTEREST
        }

        fn unset_join_interested(&mut self) {
            self.0 &= !JOIN_INTEREST
        }

        pub(super) fn has_join_waker(self) -> bool {
            self.0 & JOIN_WAKER == JOIN_WAKER
        }

        fn set_join_waker(&mut self) {
            self.0 |= JOIN_WAKER;
        }

        fn unset_join_waker(&mut self) {
            self.0 &= !JOIN_WAKER
        }

        pub(super) fn ref_count(self) -> usize {
            (self.0 & REF_COUNT_MASK) >> REF_COUNT_SHIFT
        }

        fn ref_inc(&mut self) {
            assert!(self.0 <= isize::max_value() as usize);
            self.0 += REF_ONE;
        }

        pub(super) fn ref_dec(&mut self) {
            assert!(self.ref_count() > 0);
            self.0 -= REF_ONE
        }

        fn will_need_queueing(self) -> bool {
            !self.is_notified() && self.is_idle()
        }
    }

    impl fmt::Debug for State {
        fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
            let snapshot = self.load();
            snapshot.fmt(fmt)
        }
    }

    impl fmt::Debug for Snapshot {
        fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
            fmt.debug_struct("Snapshot")
                .field("is_running", &self.is_running())
                .field("is_complete", &self.is_complete())
                .field("is_notified", &self.is_notified())
                .field("is_cancelled", &self.is_cancelled())
                .field("is_join_interested", &self.is_join_interested())
                .field("has_join_waker", &self.has_join_waker())
                .field("ref_count", &self.ref_count())
                .finish()
        }
    }