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    use crate::future::poll_fn;
    use crate::loom::sync::Mutex;
    use crate::park::{Park, Unpark};
    use crate::runtime::task::{self, JoinHandle, Schedule, Task};
    use crate::sync::notify::Notify;
    use crate::util::linked_list::{Link, LinkedList};
    use crate::util::{waker_ref, Wake, WakerRef};

    use std::cell::RefCell;
    use std::collections::VecDeque;
    use std::fmt;
    use std::future::Future;
    use std::sync::Arc;
    use std::task::Poll::{Pending, Ready};
    use std::time::Duration;

    /// Executes tasks on the current thread
    pub(crate) struct BasicScheduler<P: Park> {
        /// Inner state guarded by a mutex that is shared
        /// between all `block_on` calls.
        inner: Mutex<Option<Inner<P>>>,

        /// Notifier for waking up other threads to steal the
        /// parker.
        notify: Notify,

        /// Sendable task spawner
        spawner: Spawner,
    }

    /// The inner scheduler that owns the task queue and the main parker P.
    struct Inner<P: Park> {
        /// Scheduler run queue
        ///
        /// When the scheduler is executed, the queue is removed from `self` and
        /// moved into `Context`.
        ///
        /// This indirection is to allow `BasicScheduler` to be `Send`.
        tasks: Option<Tasks>,

        /// Sendable task spawner
        spawner: Spawner,

        /// Current tick
        tick: u8,

        /// Thread park handle
        park: P,
    }

    #[derive(Clone)]
    pub(crate) struct Spawner {
        shared: Arc<Shared>,
    }

    struct Tasks {
        /// Collection of all active tasks spawned onto this executor.
        owned: LinkedList<Task<Arc<Shared>>, <Task<Arc<Shared>> as Link>::Target>,

        /// Local run queue.
        ///
        /// Tasks notified from the current thread are pushed into this queue.
        queue: VecDeque<task::Notified<Arc<Shared>>>,
    }

    /// Scheduler state shared between threads.
    struct Shared {
        /// Remote run queue
        queue: Mutex<VecDeque<task::Notified<Arc<Shared>>>>,

        /// Unpark the blocked thread
        unpark: Box<dyn Unpark>,
    }

    /// Thread-local context.
    struct Context {
        /// Shared scheduler state
        shared: Arc<Shared>,

        /// Local queue
        tasks: RefCell<Tasks>,
    }

    /// Initial queue capacity.
    const INITIAL_CAPACITY: usize = 64;

    /// Max number of tasks to poll per tick.
    const MAX_TASKS_PER_TICK: usize = 61;

    /// How often to check the remote queue first.
    const REMOTE_FIRST_INTERVAL: u8 = 31;

    // Tracks the current BasicScheduler.
    scoped_thread_local!(static CURRENT: Context);

    impl<P: Park> BasicScheduler<P> {
        pub(crate) fn new(park: P) -> BasicScheduler<P> {
            let unpark = Box::new(park.unpark());

            let spawner = Spawner {
                shared: Arc::new(Shared {
                    queue: Mutex::new(VecDeque::with_capacity(INITIAL_CAPACITY)),
                    unpark: unpark as Box<dyn Unpark>,
                }),
            };

            let inner = Mutex::new(Some(Inner {
                tasks: Some(Tasks {
                    owned: LinkedList::new(),
                    queue: VecDeque::with_capacity(INITIAL_CAPACITY),
                }),
                spawner: spawner.clone(),
                tick: 0,
                park,
            }));

            BasicScheduler {
                inner,
                notify: Notify::new(),
                spawner,
            }
        }

        pub(crate) fn spawner(&self) -> &Spawner {
            &self.spawner
        }

        pub(crate) fn block_on<F: Future>(&self, future: F) -> F::Output {
            pin!(future);

            // Attempt to steal the dedicated parker and block_on the future if we can there,
            // otherwise, lets select on a notification that the parker is available
            // or the future is complete.
            loop {
                if let Some(inner) = &mut self.take_inner() {
                    return inner.block_on(future);
                } else {
                    let mut enter = crate::runtime::enter(false);

                    let notified = self.notify.notified();
                    pin!(notified);

                    if let Some(out) = enter
                        .block_on(poll_fn(|cx| {
                            if notified.as_mut().poll(cx).is_ready() {
                                return Ready(None);
                            }

                            if let Ready(out) = future.as_mut().poll(cx) {
                                return Ready(Some(out));
                            }

                            Pending
                        }))
                        .expect("Failed to `Enter::block_on`")
                    {
                        return out;
                    }
                }
            }
        }

        fn take_inner(&self) -> Option<InnerGuard<'_, P>> {
            let inner = self.inner.lock().take()?;

            Some(InnerGuard {
                inner: Some(inner),
                basic_scheduler: &self,
            })
        }
    }

    impl<P: Park> Inner<P> {
        /// Block on the future provided and drive the runtime's driver.
        fn block_on<F: Future>(&mut self, future: F) -> F::Output {
            enter(self, |scheduler, context| {
                let _enter = crate::runtime::enter(false);
                let waker = scheduler.spawner.waker_ref();
                let mut cx = std::task::Context::from_waker(&waker);

                pin!(future);

                'outer: loop {
                    if let Ready(v) = crate::coop::budget(|| future.as_mut().poll(&mut cx)) {
                        return v;
                    }

                    for _ in 0..MAX_TASKS_PER_TICK {
                        // Get and increment the current tick
                        let tick = scheduler.tick;
                        scheduler.tick = scheduler.tick.wrapping_add(1);

                        let next = if tick % REMOTE_FIRST_INTERVAL == 0 {
                            scheduler
                                .spawner
                                .pop()
                                .or_else(|| context.tasks.borrow_mut().queue.pop_front())
                        } else {
                            context
                                .tasks
                                .borrow_mut()
                                .queue
                                .pop_front()
                                .or_else(|| scheduler.spawner.pop())
                        };

                        match next {
                            Some(task) => crate::coop::budget(|| task.run()),
                            None => {
                                // Park until the thread is signaled
                                scheduler.park.park().ok().expect("failed to park");

                                // Try polling the `block_on` future next
                                continue 'outer;
                            }
                        }
                    }

                    // Yield to the park, this drives the timer and pulls any pending
                    // I/O events.
                    scheduler
                        .park
                        .park_timeout(Duration::from_millis(0))
                        .ok()
                        .expect("failed to park");
                }
            })
        }
    }

    /// Enter the scheduler context. This sets the queue and other necessary
    /// scheduler state in the thread-local
    fn enter<F, R, P>(scheduler: &mut Inner<P>, f: F) -> R
    where
        F: FnOnce(&mut Inner<P>, &Context) -> R,
        P: Park,
    {
        // Ensures the run queue is placed back in the `BasicScheduler` instance
        // once `block_on` returns.`
        struct Guard<'a, P: Park> {
            context: Option<Context>,
            scheduler: &'a mut Inner<P>,
        }

        impl<P: Park> Drop for Guard<'_, P> {
            fn drop(&mut self) {
                let Context { tasks, .. } = self.context.take().expect("context missing");
                self.scheduler.tasks = Some(tasks.into_inner());
            }
        }

        // Remove `tasks` from `self` and place it in a `Context`.
        let tasks = scheduler.tasks.take().expect("invalid state");

        let guard = Guard {
            context: Some(Context {
                shared: scheduler.spawner.shared.clone(),
                tasks: RefCell::new(tasks),
            }),
            scheduler,
        };

        let context = guard.context.as_ref().unwrap();
        let scheduler = &mut *guard.scheduler;

        CURRENT.set(context, || f(scheduler, context))
    }

    impl<P: Park> Drop for BasicScheduler<P> {
        fn drop(&mut self) {
            // Avoid a double panic if we are currently panicking and
            // the lock may be poisoned.

            let mut inner = match self.inner.lock().take() {
                Some(inner) => inner,
                None if std::thread::panicking() => return,
                None => panic!("Oh no! We never placed the Inner state back, this is a bug!"),
            };

            enter(&mut inner, |scheduler, context| {
                // Loop required here to ensure borrow is dropped between iterations
                #[allow(clippy::while_let_loop)]
                loop {
                    let task = match context.tasks.borrow_mut().owned.pop_back() {
                        Some(task) => task,
                        None => break,
                    };

                    task.shutdown();
                }

                // Drain local queue
                for task in context.tasks.borrow_mut().queue.drain(..) {
                    task.shutdown();
                }

                // Drain remote queue
                for task in scheduler.spawner.shared.queue.lock().drain(..) {
                    task.shutdown();
                }

                assert!(context.tasks.borrow().owned.is_empty());
            });
        }
    }

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

    // ===== impl Spawner =====

    impl Spawner {
        /// Spawns a future onto the thread pool
        pub(crate) fn spawn<F>(&self, future: F) -> JoinHandle<F::Output>
        where
            F: Future + Send + 'static,
            F::Output: Send + 'static,
        {
            let (task, handle) = task::joinable(future);
            self.shared.schedule(task);
            handle
        }

        fn pop(&self) -> Option<task::Notified<Arc<Shared>>> {
            self.shared.queue.lock().pop_front()
        }

        fn waker_ref(&self) -> WakerRef<'_> {
            waker_ref(&self.shared)
        }
    }

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

    // ===== impl Shared =====

    impl Schedule for Arc<Shared> {
        fn bind(task: Task<Self>) -> Arc<Shared> {
            CURRENT.with(|maybe_cx| {
                let cx = maybe_cx.expect("scheduler context missing");
                cx.tasks.borrow_mut().owned.push_front(task);
                cx.shared.clone()
            })
        }

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

            CURRENT.with(|maybe_cx| {
                let cx = maybe_cx.expect("scheduler context missing");

                // safety: the task is inserted in the list in `bind`.
                unsafe {
                    let ptr = NonNull::from(task.header());
                    cx.tasks.borrow_mut().owned.remove(ptr)
                }
            })
        }

        fn schedule(&self, task: task::Notified<Self>) {
            CURRENT.with(|maybe_cx| match maybe_cx {
                Some(cx) if Arc::ptr_eq(self, &cx.shared) => {
                    cx.tasks.borrow_mut().queue.push_back(task);
                }
                _ => {
                    self.queue.lock().push_back(task);
                    self.unpark.unpark();
                }
            });
        }
    }

    impl Wake for Shared {
        fn wake(self: Arc<Self>) {
            Wake::wake_by_ref(&self)
        }

        /// Wake by reference
        fn wake_by_ref(arc_self: &Arc<Self>) {
            arc_self.unpark.unpark();
        }
    }

    // ===== InnerGuard =====

    /// Used to ensure we always place the Inner value
    /// back into its slot in `BasicScheduler`, even if the
    /// future panics.
    struct InnerGuard<'a, P: Park> {
        inner: Option<Inner<P>>,
        basic_scheduler: &'a BasicScheduler<P>,
    }

    impl<P: Park> InnerGuard<'_, P> {
        fn block_on<F: Future>(&mut self, future: F) -> F::Output {
            // The only time inner gets set to `None` is if we have dropped
            // already so this unwrap is safe.
            self.inner.as_mut().unwrap().block_on(future)
        }
    }

    impl<P: Park> Drop for InnerGuard<'_, P> {
        fn drop(&mut self) {
            if let Some(scheduler) = self.inner.take() {
                let mut lock = self.basic_scheduler.inner.lock();

                // Replace old scheduler back into the state to allow
                // other threads to pick it up and drive it.
                lock.replace(scheduler);

                // Wake up other possible threads that could steal
                // the dedicated parker P.
                self.basic_scheduler.notify.notify_one()
            }
        }
    }