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    //! A scheduler is initialized with a fixed number of workers. Each worker is
    //! driven by a thread. Each worker has a "core" which contains data such as the
    //! run queue and other state. When `block_in_place` is called, the worker's
    //! "core" is handed off to a new thread allowing the scheduler to continue to
    //! make progress while the originating thread blocks.

    use crate::coop;
    use crate::loom::rand::seed;
    use crate::loom::sync::{Arc, Mutex};
    use crate::park::{Park, Unpark};
    use crate::runtime;
    use crate::runtime::enter::EnterContext;
    use crate::runtime::park::{Parker, Unparker};
    use crate::runtime::thread_pool::{AtomicCell, Idle};
    use crate::runtime::{queue, task};
    use crate::util::linked_list::{Link, LinkedList};
    use crate::util::FastRand;

    use std::cell::RefCell;
    use std::time::Duration;

    /// A scheduler worker
    pub(super) struct Worker {
        /// Reference to shared state
        shared: Arc<Shared>,

        /// Index holding this worker's remote state
        index: usize,

        /// Used to hand-off a worker's core to another thread.
        core: AtomicCell<Core>,
    }

    /// Core data
    struct Core {
        /// Used to schedule bookkeeping tasks every so often.
        tick: u8,

        /// When a task is scheduled from a worker, it is stored in this slot. The
        /// worker will check this slot for a task **before** checking the run
        /// queue. This effectively results in the **last** scheduled task to be run
        /// next (LIFO). This is an optimization for message passing patterns and
        /// helps to reduce latency.
        lifo_slot: Option<Notified>,

        /// The worker-local run queue.
        run_queue: queue::Local<Arc<Worker>>,

        /// True if the worker is currently searching for more work. Searching
        /// involves attempting to steal from other workers.
        is_searching: bool,

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

        /// Tasks owned by the core
        tasks: LinkedList<Task, <Task as Link>::Target>,

        /// Parker
        ///
        /// Stored in an `Option` as the parker is added / removed to make the
        /// borrow checker happy.
        park: Option<Parker>,

        /// Fast random number generator.
        rand: FastRand,
    }

    /// State shared across all workers
    pub(super) struct Shared {
        /// Per-worker remote state. All other workers have access to this and is
        /// how they communicate between each other.
        remotes: Box<[Remote]>,

        /// Submit work to the scheduler while **not** currently on a worker thread.
        inject: queue::Inject<Arc<Worker>>,

        /// Coordinates idle workers
        idle: Idle,

        /// Cores that have observed the shutdown signal
        ///
        /// The core is **not** placed back in the worker to avoid it from being
        /// stolen by a thread that was spawned as part of `block_in_place`.
        #[allow(clippy::vec_box)] // we're moving an already-boxed value
        shutdown_cores: Mutex<Vec<Box<Core>>>,
    }

    /// Used to communicate with a worker from other threads.
    struct Remote {
        /// Steal tasks from this worker.
        steal: queue::Steal<Arc<Worker>>,

        /// Transfers tasks to be released. Any worker pushes tasks, only the owning
        /// worker pops.
        pending_drop: task::TransferStack<Arc<Worker>>,

        /// Unparks the associated worker thread
        unpark: Unparker,
    }

    /// Thread-local context
    struct Context {
        /// Worker
        worker: Arc<Worker>,

        /// Core data
        core: RefCell<Option<Box<Core>>>,
    }

    /// Starts the workers
    pub(crate) struct Launch(Vec<Arc<Worker>>);

    /// Running a task may consume the core. If the core is still available when
    /// running the task completes, it is returned. Otherwise, the worker will need
    /// to stop processing.
    type RunResult = Result<Box<Core>, ()>;

    /// A task handle
    type Task = task::Task<Arc<Worker>>;

    /// A notified task handle
    type Notified = task::Notified<Arc<Worker>>;

    // Tracks thread-local state
    scoped_thread_local!(static CURRENT: Context);

    pub(super) fn create(size: usize, park: Parker) -> (Arc<Shared>, Launch) {
        let mut cores = vec![];
        let mut remotes = vec![];

        // Create the local queues
        for _ in 0..size {
            let (steal, run_queue) = queue::local();

            let park = park.clone();
            let unpark = park.unpark();

            cores.push(Box::new(Core {
                tick: 0,
                lifo_slot: None,
                run_queue,
                is_searching: false,
                is_shutdown: false,
                tasks: LinkedList::new(),
                park: Some(park),
                rand: FastRand::new(seed()),
            }));

            remotes.push(Remote {
                steal,
                pending_drop: task::TransferStack::new(),
                unpark,
            });
        }

        let shared = Arc::new(Shared {
            remotes: remotes.into_boxed_slice(),
            inject: queue::Inject::new(),
            idle: Idle::new(size),
            shutdown_cores: Mutex::new(vec![]),
        });

        let mut launch = Launch(vec![]);

        for (index, core) in cores.drain(..).enumerate() {
            launch.0.push(Arc::new(Worker {
                shared: shared.clone(),
                index,
                core: AtomicCell::new(Some(core)),
            }));
        }

        (shared, launch)
    }

    pub(crate) fn block_in_place<F, R>(f: F) -> R
    where
        F: FnOnce() -> R,
    {
        // Try to steal the worker core back
        struct Reset(coop::Budget);

        impl Drop for Reset {
            fn drop(&mut self) {
                CURRENT.with(|maybe_cx| {
                    if let Some(cx) = maybe_cx {
                        let core = cx.worker.core.take();
                        let mut cx_core = cx.core.borrow_mut();
                        assert!(cx_core.is_none());
                        *cx_core = core;

                        // Reset the task budget as we are re-entering the
                        // runtime.
                        coop::set(self.0);
                    }
                });
            }
        }

        let mut had_entered = false;

        CURRENT.with(|maybe_cx| {
            match (crate::runtime::enter::context(), maybe_cx.is_some()) {
                (EnterContext::Entered { .. }, true) => {
                    // We are on a thread pool runtime thread, so we just need to set up blocking.
                    had_entered = true;
                }
                (EnterContext::Entered { allow_blocking }, false) => {
                    // We are on an executor, but _not_ on the thread pool.
                    // That is _only_ okay if we are in a thread pool runtime's block_on method:
                    if allow_blocking {
                        had_entered = true;
                        return;
                    } else {
                        // This probably means we are on the basic_scheduler or in a LocalSet,
                        // where it is _not_ okay to block.
                        panic!("can call blocking only when running on the multi-threaded runtime");
                    }
                }
                (EnterContext::NotEntered, true) => {
                    // This is a nested call to block_in_place (we already exited).
                    // All the necessary setup has already been done.
                    return;
                }
                (EnterContext::NotEntered, false) => {
                    // We are outside of the tokio runtime, so blocking is fine.
                    // We can also skip all of the thread pool blocking setup steps.
                    return;
                }
            }

            let cx = maybe_cx.expect("no .is_some() == false cases above should lead here");

            // Get the worker core. If none is set, then blocking is fine!
            let core = match cx.core.borrow_mut().take() {
                Some(core) => core,
                None => return,
            };

            // The parker should be set here
            assert!(core.park.is_some());

            // In order to block, the core must be sent to another thread for
            // execution.
            //
            // First, move the core back into the worker's shared core slot.
            cx.worker.core.set(core);

            // Next, clone the worker handle and send it to a new thread for
            // processing.
            //
            // Once the blocking task is done executing, we will attempt to
            // steal the core back.
            let worker = cx.worker.clone();
            runtime::spawn_blocking(move || run(worker));
        });

        if had_entered {
            // Unset the current task's budget. Blocking sections are not
            // constrained by task budgets.
            let _reset = Reset(coop::stop());

            crate::runtime::enter::exit(f)
        } else {
            f()
        }
    }

    /// After how many ticks is the global queue polled. This helps to ensure
    /// fairness.
    ///
    /// The number is fairly arbitrary. I believe this value was copied from golang.
    const GLOBAL_POLL_INTERVAL: u8 = 61;

    impl Launch {
        pub(crate) fn launch(mut self) {
            for worker in self.0.drain(..) {
                runtime::spawn_blocking(move || run(worker));
            }
        }
    }

    fn run(worker: Arc<Worker>) {
        // Acquire a core. If this fails, then another thread is running this
        // worker and there is nothing further to do.
        let core = match worker.core.take() {
            Some(core) => core,
            None => return,
        };

        // Set the worker context.
        let cx = Context {
            worker,
            core: RefCell::new(None),
        };

        let _enter = crate::runtime::enter(true);

        CURRENT.set(&cx, || {
            // This should always be an error. It only returns a `Result` to support
            // using `?` to short circuit.
            assert!(cx.run(core).is_err());
        });
    }

    impl Context {
        fn run(&self, mut core: Box<Core>) -> RunResult {
            while !core.is_shutdown {
                // Increment the tick
                core.tick();

                // Run maintenance, if needed
                core = self.maintenance(core);

                // First, check work available to the current worker.
                if let Some(task) = core.next_task(&self.worker) {
                    core = self.run_task(task, core)?;
                    continue;
                }

                // There is no more **local** work to process, try to steal work
                // from other workers.
                if let Some(task) = core.steal_work(&self.worker) {
                    core = self.run_task(task, core)?;
                } else {
                    // Wait for work
                    core = self.park(core);
                }
            }

            core.pre_shutdown(&self.worker);

            // Signal shutdown
            self.worker.shared.shutdown(core);
            Err(())
        }

        fn run_task(&self, task: Notified, mut core: Box<Core>) -> RunResult {
            // Make sure the worker is not in the **searching** state. This enables
            // another idle worker to try to steal work.
            core.transition_from_searching(&self.worker);

            // Make the core available to the runtime context
            *self.core.borrow_mut() = Some(core);

            // Run the task
            coop::budget(|| {
                task.run();

                // As long as there is budget remaining and a task exists in the
                // `lifo_slot`, then keep running.
                loop {
                    // Check if we still have the core. If not, the core was stolen
                    // by another worker.
                    let mut core = match self.core.borrow_mut().take() {
                        Some(core) => core,
                        None => return Err(()),
                    };

                    // Check for a task in the LIFO slot
                    let task = match core.lifo_slot.take() {
                        Some(task) => task,
                        None => return Ok(core),
                    };

                    if coop::has_budget_remaining() {
                        // Run the LIFO task, then loop
                        *self.core.borrow_mut() = Some(core);
                        task.run();
                    } else {
                        // Not enough budget left to run the LIFO task, push it to
                        // the back of the queue and return.
                        core.run_queue.push_back(task, self.worker.inject());
                        return Ok(core);
                    }
                }
            })
        }

        fn maintenance(&self, mut core: Box<Core>) -> Box<Core> {
            if core.tick % GLOBAL_POLL_INTERVAL == 0 {
                // Call `park` with a 0 timeout. This enables the I/O driver, timer, ...
                // to run without actually putting the thread to sleep.
                core = self.park_timeout(core, Some(Duration::from_millis(0)));

                // Run regularly scheduled maintenance
                core.maintenance(&self.worker);
            }

            core
        }

        fn park(&self, mut core: Box<Core>) -> Box<Core> {
            core.transition_to_parked(&self.worker);

            while !core.is_shutdown {
                core = self.park_timeout(core, None);

                // Run regularly scheduled maintenance
                core.maintenance(&self.worker);

                if core.transition_from_parked(&self.worker) {
                    return core;
                }
            }

            core
        }

        fn park_timeout(&self, mut core: Box<Core>, duration: Option<Duration>) -> Box<Core> {
            // Take the parker out of core
            let mut park = core.park.take().expect("park missing");

            // Store `core` in context
            *self.core.borrow_mut() = Some(core);

            // Park thread
            if let Some(timeout) = duration {
                park.park_timeout(timeout).expect("park failed");
            } else {
                park.park().expect("park failed");
            }

            // Remove `core` from context
            core = self.core.borrow_mut().take().expect("core missing");

            // Place `park` back in `core`
            core.park = Some(park);

            // If there are tasks available to steal, notify a worker
            if core.run_queue.is_stealable() {
                self.worker.shared.notify_parked();
            }

            core
        }
    }

    impl Core {
        /// Increment the tick
        fn tick(&mut self) {
            self.tick = self.tick.wrapping_add(1);
        }

        /// Return the next notified task available to this worker.
        fn next_task(&mut self, worker: &Worker) -> Option<Notified> {
            if self.tick % GLOBAL_POLL_INTERVAL == 0 {
                worker.inject().pop().or_else(|| self.next_local_task())
            } else {
                self.next_local_task().or_else(|| worker.inject().pop())
            }
        }

        fn next_local_task(&mut self) -> Option<Notified> {
            self.lifo_slot.take().or_else(|| self.run_queue.pop())
        }

        fn steal_work(&mut self, worker: &Worker) -> Option<Notified> {
            if !self.transition_to_searching(worker) {
                return None;
            }

            let num = worker.shared.remotes.len();
            // Start from a random worker
            let start = self.rand.fastrand_n(num as u32) as usize;

            for i in 0..num {
                let i = (start + i) % num;

                // Don't steal from ourself! We know we don't have work.
                if i == worker.index {
                    continue;
                }

                let target = &worker.shared.remotes[i];
                if let Some(task) = target.steal.steal_into(&mut self.run_queue) {
                    return Some(task);
                }
            }

            // Fallback on checking the global queue
            worker.shared.inject.pop()
        }

        fn transition_to_searching(&mut self, worker: &Worker) -> bool {
            if !self.is_searching {
                self.is_searching = worker.shared.idle.transition_worker_to_searching();
            }

            self.is_searching
        }

        fn transition_from_searching(&mut self, worker: &Worker) {
            if !self.is_searching {
                return;
            }

            self.is_searching = false;
            worker.shared.transition_worker_from_searching();
        }

        /// Prepare the worker state for parking
        fn transition_to_parked(&mut self, worker: &Worker) {
            // When the final worker transitions **out** of searching to parked, it
            // must check all the queues one last time in case work materialized
            // between the last work scan and transitioning out of searching.
            let is_last_searcher = worker
                .shared
                .idle
                .transition_worker_to_parked(worker.index, self.is_searching);

            // The worker is no longer searching. Setting this is the local cache
            // only.
            self.is_searching = false;

            if is_last_searcher {
                worker.shared.notify_if_work_pending();
            }
        }

        /// Returns `true` if the transition happened.
        fn transition_from_parked(&mut self, worker: &Worker) -> bool {
            // If a task is in the lifo slot, then we must unpark regardless of
            // being notified
            if self.lifo_slot.is_some() {
                worker.shared.idle.unpark_worker_by_id(worker.index);
                self.is_searching = true;
                return true;
            }

            if worker.shared.idle.is_parked(worker.index) {
                return false;
            }

            // When unparked, the worker is in the searching state.
            self.is_searching = true;
            true
        }

        /// Runs maintenance work such as free pending tasks and check the pool's
        /// state.
        fn maintenance(&mut self, worker: &Worker) {
            self.drain_pending_drop(worker);

            if !self.is_shutdown {
                // Check if the scheduler has been shutdown
                self.is_shutdown = worker.inject().is_closed();
            }
        }

        // Signals all tasks to shut down, and waits for them to complete. Must run
        // before we enter the single-threaded phase of shutdown processing.
        fn pre_shutdown(&mut self, worker: &Worker) {
            // Signal to all tasks to shut down.
            for header in self.tasks.iter() {
                header.shutdown();
            }

            loop {
                self.drain_pending_drop(worker);

                if self.tasks.is_empty() {
                    break;
                }

                // Wait until signalled
                let park = self.park.as_mut().expect("park missing");
                park.park().expect("park failed");
            }
        }

        // Shutdown the core
        fn shutdown(&mut self) {
            assert!(self.tasks.is_empty());

            // Take the core
            let mut park = self.park.take().expect("park missing");

            // Drain the queue
            while self.next_local_task().is_some() {}

            park.shutdown();
        }

        fn drain_pending_drop(&mut self, worker: &Worker) {
            use std::mem::ManuallyDrop;

            for task in worker.remote().pending_drop.drain() {
                let task = ManuallyDrop::new(task);

                // safety: tasks are only pushed into the `pending_drop` stacks that
                // are associated with the list they are inserted into. When a task
                // is pushed into `pending_drop`, the ref-inc is skipped, so we must
                // not ref-dec here.
                //
                // See `bind` and `release` implementations.
                unsafe {
                    self.tasks.remove(task.header().into());
                }
            }
        }
    }

    impl Worker {
        /// Returns a reference to the scheduler's injection queue
        fn inject(&self) -> &queue::Inject<Arc<Worker>> {
            &self.shared.inject
        }

        /// Return a reference to this worker's remote data
        fn remote(&self) -> &Remote {
            &self.shared.remotes[self.index]
        }

        fn eq(&self, other: &Worker) -> bool {
            self.shared.ptr_eq(&other.shared) && self.index == other.index
        }
    }

    impl task::Schedule for Arc<Worker> {
        fn bind(task: Task) -> Arc<Worker> {
            CURRENT.with(|maybe_cx| {
                let cx = maybe_cx.expect("scheduler context missing");

                // Track the task
                cx.core
                    .borrow_mut()
                    .as_mut()
                    .expect("scheduler core missing")
                    .tasks
                    .push_front(task);

                // Return a clone of the worker
                cx.worker.clone()
            })
        }

        fn release(&self, task: &Task) -> Option<Task> {
            use std::ptr::NonNull;

            enum Immediate {
                // Task has been synchronously removed from the Core owned by the
                // current thread
                Removed(Option<Task>),
                // Task is owned by another thread, so we need to notify it to clean
                // up the task later.
                MaybeRemote,
            }

            let immediate = CURRENT.with(|maybe_cx| {
                let cx = match maybe_cx {
                    Some(cx) => cx,
                    None => return Immediate::MaybeRemote,
                };

                if !self.eq(&cx.worker) {
                    // Task owned by another core, so we need to notify it.
                    return Immediate::MaybeRemote;
                }

                let mut maybe_core = cx.core.borrow_mut();

                if let Some(core) = &mut *maybe_core {
                    // Directly remove the task
                    //
                    // safety: the task is inserted in the list in `bind`.
                    unsafe {
                        let ptr = NonNull::from(task.header());
                        return Immediate::Removed(core.tasks.remove(ptr));
                    }
                }

                Immediate::MaybeRemote
            });

            // Checks if we were called from within a worker, allowing for immediate
            // removal of a scheduled task. Else we have to go through the slower
            // process below where we remotely mark a task as dropped.
            match immediate {
                Immediate::Removed(task) => return task,
                Immediate::MaybeRemote => (),
            };

            // Track the task to be released by the worker that owns it
            //
            // Safety: We get a new handle without incrementing the ref-count.
            // A ref-count is held by the "owned" linked list and it is only
            // ever removed from that list as part of the release process: this
            // method or popping the task from `pending_drop`. Thus, we can rely
            // on the ref-count held by the linked-list to keep the memory
            // alive.
            //
            // When the task is removed from the stack, it is forgotten instead
            // of dropped.
            let task = unsafe { Task::from_raw(task.header().into()) };

            self.remote().pending_drop.push(task);

            // The worker core has been handed off to another thread. In the
            // event that the scheduler is currently shutting down, the thread
            // that owns the task may be waiting on the release to complete
            // shutdown.
            if self.inject().is_closed() {
                self.remote().unpark.unpark();
            }

            None
        }

        fn schedule(&self, task: Notified) {
            self.shared.schedule(task, false);
        }

        fn yield_now(&self, task: Notified) {
            self.shared.schedule(task, true);
        }
    }

    impl Shared {
        pub(super) fn schedule(&self, task: Notified, is_yield: bool) {
            CURRENT.with(|maybe_cx| {
                if let Some(cx) = maybe_cx {
                    // Make sure the task is part of the **current** scheduler.
                    if self.ptr_eq(&cx.worker.shared) {
                        // And the current thread still holds a core
                        if let Some(core) = cx.core.borrow_mut().as_mut() {
                            self.schedule_local(core, task, is_yield);
                            return;
                        }
                    }
                }

                // Otherwise, use the inject queue
                self.inject.push(task);
                self.notify_parked();
            });
        }

        fn schedule_local(&self, core: &mut Core, task: Notified, is_yield: bool) {
            // Spawning from the worker thread. If scheduling a "yield" then the
            // task must always be pushed to the back of the queue, enabling other
            // tasks to be executed. If **not** a yield, then there is more
            // flexibility and the task may go to the front of the queue.
            let should_notify = if is_yield {
                core.run_queue.push_back(task, &self.inject);
                true
            } else {
                // Push to the LIFO slot
                let prev = core.lifo_slot.take();
                let ret = prev.is_some();

                if let Some(prev) = prev {
                    core.run_queue.push_back(prev, &self.inject);
                }

                core.lifo_slot = Some(task);

                ret
            };

            // Only notify if not currently parked. If `park` is `None`, then the
            // scheduling is from a resource driver. As notifications often come in
            // batches, the notification is delayed until the park is complete.
            if should_notify && core.park.is_some() {
                self.notify_parked();
            }
        }

        pub(super) fn close(&self) {
            if self.inject.close() {
                self.notify_all();
            }
        }

        fn notify_parked(&self) {
            if let Some(index) = self.idle.worker_to_notify() {
                self.remotes[index].unpark.unpark();
            }
        }

        fn notify_all(&self) {
            for remote in &self.remotes[..] {
                remote.unpark.unpark();
            }
        }

        fn notify_if_work_pending(&self) {
            for remote in &self.remotes[..] {
                if !remote.steal.is_empty() {
                    self.notify_parked();
                    return;
                }
            }

            if !self.inject.is_empty() {
                self.notify_parked();
            }
        }

        fn transition_worker_from_searching(&self) {
            if self.idle.transition_worker_from_searching() {
                // We are the final searching worker. Because work was found, we
                // need to notify another worker.
                self.notify_parked();
            }
        }

        /// Signals that a worker has observed the shutdown signal and has replaced
        /// its core back into its handle.
        ///
        /// If all workers have reached this point, the final cleanup is performed.
        fn shutdown(&self, core: Box<Core>) {
            let mut cores = self.shutdown_cores.lock();
            cores.push(core);

            if cores.len() != self.remotes.len() {
                return;
            }

            for mut core in cores.drain(..) {
                core.shutdown();
            }

            // Drain the injection queue
            while self.inject.pop().is_some() {}
        }

        fn ptr_eq(&self, other: &Shared) -> bool {
            self as *const _ == other as *const _
        }
    }