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    #![allow(clippy::unit_arg)]

    use crate::signal::os::{OsExtraData, OsStorage};

    use crate::sync::mpsc::Sender;

    use once_cell::sync::Lazy;
    use std::ops;
    use std::pin::Pin;
    use std::sync::atomic::{AtomicBool, Ordering};
    use std::sync::Mutex;

    pub(crate) type EventId = usize;

    /// State for a specific event, whether a notification is pending delivery,
    /// and what listeners are registered.
    #[derive(Default, Debug)]
    pub(crate) struct EventInfo {
        pending: AtomicBool,
        recipients: Mutex<Vec<Sender<()>>>,
    }

    /// An interface for retrieving the `EventInfo` for a particular eventId.
    pub(crate) trait Storage {
        /// Gets the `EventInfo` for `id` if it exists.
        fn event_info(&self, id: EventId) -> Option<&EventInfo>;

        /// Invokes `f` once for each defined `EventInfo` in this storage.
        fn for_each<'a, F>(&'a self, f: F)
        where
            F: FnMut(&'a EventInfo);
    }

    impl Storage for Vec<EventInfo> {
        fn event_info(&self, id: EventId) -> Option<&EventInfo> {
            self.get(id)
        }

        fn for_each<'a, F>(&'a self, f: F)
        where
            F: FnMut(&'a EventInfo),
        {
            self.iter().for_each(f)
        }
    }

    /// An interface for initializing a type. Useful for situations where we cannot
    /// inject a configured instance in the constructor of another type.
    pub(crate) trait Init {
        fn init() -> Self;
    }

    /// Manages and distributes event notifications to any registered listeners.
    ///
    /// Generic over the underlying storage to allow for domain specific
    /// optimizations (e.g. eventIds may or may not be contiguous).
    #[derive(Debug)]
    pub(crate) struct Registry<S> {
        storage: S,
    }

    impl<S> Registry<S> {
        fn new(storage: S) -> Self {
            Self { storage }
        }
    }

    impl<S: Storage> Registry<S> {
        /// Registers a new listener for `event_id`.
        fn register_listener(&self, event_id: EventId, listener: Sender<()>) {
            self.storage
                .event_info(event_id)
                .unwrap_or_else(|| panic!("invalid event_id: {}", event_id))
                .recipients
                .lock()
                .unwrap()
                .push(listener);
        }

        /// Marks `event_id` as having been delivered, without broadcasting it to
        /// any listeners.
        fn record_event(&self, event_id: EventId) {
            if let Some(event_info) = self.storage.event_info(event_id) {
                event_info.pending.store(true, Ordering::SeqCst)
            }
        }

        /// Broadcasts all previously recorded events to their respective listeners.
        ///
        /// Returns `true` if an event was delivered to at least one listener.
        fn broadcast(&self) -> bool {
            use crate::sync::mpsc::error::TrySendError;

            let mut did_notify = false;
            self.storage.for_each(|event_info| {
                // Any signal of this kind arrived since we checked last?
                if !event_info.pending.swap(false, Ordering::SeqCst) {
                    return;
                }

                let mut recipients = event_info.recipients.lock().unwrap();

                // Notify all waiters on this signal that the signal has been
                // received. If we can't push a message into the queue then we don't
                // worry about it as everything is coalesced anyway. If the channel
                // has gone away then we can remove that slot.
                for i in (0..recipients.len()).rev() {
                    match recipients[i].try_send(()) {
                        Ok(()) => did_notify = true,
                        Err(TrySendError::Closed(..)) => {
                            recipients.swap_remove(i);
                        }

                        // Channel is full, ignore the error since the
                        // receiver has already been woken up
                        Err(_) => {}
                    }
                }
            });

            did_notify
        }
    }

    pub(crate) struct Globals {
        extra: OsExtraData,
        registry: Registry<OsStorage>,
    }

    impl ops::Deref for Globals {
        type Target = OsExtraData;

        fn deref(&self) -> &Self::Target {
            &self.extra
        }
    }

    impl Globals {
        /// Registers a new listener for `event_id`.
        pub(crate) fn register_listener(&self, event_id: EventId, listener: Sender<()>) {
            self.registry.register_listener(event_id, listener);
        }

        /// Marks `event_id` as having been delivered, without broadcasting it to
        /// any listeners.
        pub(crate) fn record_event(&self, event_id: EventId) {
            self.registry.record_event(event_id);
        }

        /// Broadcasts all previously recorded events to their respective listeners.
        ///
        /// Returns `true` if an event was delivered to at least one listener.
        pub(crate) fn broadcast(&self) -> bool {
            self.registry.broadcast()
        }

        #[cfg(unix)]
        pub(crate) fn storage(&self) -> &OsStorage {
            &self.registry.storage
        }
    }

    pub(crate) fn globals() -> Pin<&'static Globals>
    where
        OsExtraData: 'static + Send + Sync + Init,
        OsStorage: 'static + Send + Sync + Init,
    {
        static GLOBALS: Lazy<Pin<Box<Globals>>> = Lazy::new(|| {
            Box::pin(Globals {
                extra: OsExtraData::init(),
                registry: Registry::new(OsStorage::init()),
            })
        });

        GLOBALS.as_ref()
    }

    #[cfg(all(test, not(loom)))]
    mod tests {
        use super::*;
        use crate::runtime::{self, Runtime};
        use crate::sync::{mpsc, oneshot};

        use futures::future;

        #[test]
        fn smoke() {
            let rt = rt();
            rt.block_on(async move {
                let registry = Registry::new(vec![
                    EventInfo::default(),
                    EventInfo::default(),
                    EventInfo::default(),
                ]);

                let (first_tx, first_rx) = mpsc::channel(3);
                let (second_tx, second_rx) = mpsc::channel(3);
                let (third_tx, third_rx) = mpsc::channel(3);

                registry.register_listener(0, first_tx);
                registry.register_listener(1, second_tx);
                registry.register_listener(2, third_tx);

                let (fire, wait) = oneshot::channel();

                crate::spawn(async {
                    wait.await.expect("wait failed");

                    // Record some events which should get coalesced
                    registry.record_event(0);
                    registry.record_event(0);
                    registry.record_event(1);
                    registry.record_event(1);
                    registry.broadcast();

                    // Send subsequent signal
                    registry.record_event(0);
                    registry.broadcast();

                    drop(registry);
                });

                let _ = fire.send(());
                let all = future::join3(collect(first_rx), collect(second_rx), collect(third_rx));

                let (first_results, second_results, third_results) = all.await;
                assert_eq!(2, first_results.len());
                assert_eq!(1, second_results.len());
                assert_eq!(0, third_results.len());
            });
        }

        #[test]
        #[should_panic = "invalid event_id: 1"]
        fn register_panics_on_invalid_input() {
            let registry = Registry::new(vec![EventInfo::default()]);

            let (tx, _) = mpsc::channel(1);
            registry.register_listener(1, tx);
        }

        #[test]
        fn record_invalid_event_does_nothing() {
            let registry = Registry::new(vec![EventInfo::default()]);
            registry.record_event(42);
        }

        #[test]
        fn broadcast_cleans_up_disconnected_listeners() {
            let rt = Runtime::new().unwrap();

            rt.block_on(async {
                let registry = Registry::new(vec![EventInfo::default()]);

                let (first_tx, first_rx) = mpsc::channel(1);
                let (second_tx, second_rx) = mpsc::channel(1);
                let (third_tx, third_rx) = mpsc::channel(1);

                registry.register_listener(0, first_tx);
                registry.register_listener(0, second_tx);
                registry.register_listener(0, third_tx);

                drop(first_rx);
                drop(second_rx);

                let (fire, wait) = oneshot::channel();

                crate::spawn(async {
                    wait.await.expect("wait failed");

                    registry.record_event(0);
                    registry.broadcast();

                    assert_eq!(1, registry.storage[0].recipients.lock().unwrap().len());
                    drop(registry);
                });

                let _ = fire.send(());
                let results = collect(third_rx).await;

                assert_eq!(1, results.len());
            });
        }

        #[test]
        fn broadcast_returns_if_at_least_one_event_fired() {
            let registry = Registry::new(vec![EventInfo::default()]);

            registry.record_event(0);
            assert_eq!(false, registry.broadcast());

            let (first_tx, first_rx) = mpsc::channel(1);
            let (second_tx, second_rx) = mpsc::channel(1);

            registry.register_listener(0, first_tx);
            registry.register_listener(0, second_tx);

            registry.record_event(0);
            assert_eq!(true, registry.broadcast());

            drop(first_rx);
            registry.record_event(0);
            assert_eq!(false, registry.broadcast());

            drop(second_rx);
        }

        fn rt() -> Runtime {
            runtime::Builder::new_current_thread().build().unwrap()
        }

        async fn collect(mut rx: crate::sync::mpsc::Receiver<()>) -> Vec<()> {
            let mut ret = vec![];

            while let Some(v) = rx.recv().await {
                ret.push(v);
            }

            ret
        }
    }