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use collections::{BTreeMap, BTreeSet};
use parking_lot::Mutex;
use std::{fmt::Debug, mem, sync::Arc};
use util::post_inc;

pub(crate) struct SubscriberSet<EmitterKey, Callback>(
    Arc<Mutex<SubscriberSetState<EmitterKey, Callback>>>,
);

impl<EmitterKey, Callback> Clone for SubscriberSet<EmitterKey, Callback> {
    fn clone(&self) -> Self {
        SubscriberSet(self.0.clone())
    }
}

struct SubscriberSetState<EmitterKey, Callback> {
    subscribers: BTreeMap<EmitterKey, Option<BTreeMap<usize, Callback>>>,
    dropped_subscribers: BTreeSet<(EmitterKey, usize)>,
    next_subscriber_id: usize,
}

impl<EmitterKey, Callback> SubscriberSet<EmitterKey, Callback>
where
    EmitterKey: 'static + Ord + Clone + Debug,
    Callback: 'static,
{
    pub fn new() -> Self {
        Self(Arc::new(Mutex::new(SubscriberSetState {
            subscribers: Default::default(),
            dropped_subscribers: Default::default(),
            next_subscriber_id: 0,
        })))
    }

    pub fn insert(&self, emitter_key: EmitterKey, callback: Callback) -> Subscription {
        let mut lock = self.0.lock();
        let subscriber_id = post_inc(&mut lock.next_subscriber_id);
        lock.subscribers
            .entry(emitter_key.clone())
            .or_default()
            .get_or_insert_with(|| Default::default())
            .insert(subscriber_id, callback);
        let this = self.0.clone();
        Subscription {
            unsubscribe: Some(Box::new(move || {
                let mut lock = this.lock();
                let Some(subscribers) = lock.subscribers.get_mut(&emitter_key) else {
                    // remove was called with this emitter_key
                    return;
                };

                if let Some(subscribers) = subscribers {
                    subscribers.remove(&subscriber_id);
                    if subscribers.is_empty() {
                        lock.subscribers.remove(&emitter_key);
                    }
                    return;
                }

                // We didn't manage to remove the subscription, which means it was dropped
                // while invoking the callback. Mark it as dropped so that we can remove it
                // later.
                lock.dropped_subscribers
                    .insert((emitter_key, subscriber_id));
            })),
        }
    }

    pub fn remove(&self, emitter: &EmitterKey) -> impl IntoIterator<Item = Callback> {
        let subscribers = self.0.lock().subscribers.remove(&emitter);
        subscribers
            .unwrap_or_default()
            .map(|s| s.into_values())
            .into_iter()
            .flatten()
    }

    /// Call the given callback for each subscriber to the given emitter.
    /// If the callback returns false, the subscriber is removed.
    pub fn retain<F>(&self, emitter: &EmitterKey, mut f: F)
    where
        F: FnMut(&mut Callback) -> bool,
    {
        let Some(mut subscribers) = self
            .0
            .lock()
            .subscribers
            .get_mut(emitter)
            .and_then(|s| s.take())
        else {
            return;
        };

        subscribers.retain(|_, callback| f(callback));
        let mut lock = self.0.lock();

        // Add any new subscribers that were added while invoking the callback.
        if let Some(Some(new_subscribers)) = lock.subscribers.remove(&emitter) {
            subscribers.extend(new_subscribers);
        }

        // Remove any dropped subscriptions that were dropped while invoking the callback.
        for (dropped_emitter, dropped_subscription_id) in mem::take(&mut lock.dropped_subscribers) {
            debug_assert_eq!(*emitter, dropped_emitter);
            subscribers.remove(&dropped_subscription_id);
        }

        if !subscribers.is_empty() {
            lock.subscribers.insert(emitter.clone(), Some(subscribers));
        }
    }
}

#[must_use]
pub struct Subscription {
    unsubscribe: Option<Box<dyn FnOnce() + 'static>>,
}

impl Subscription {
    pub fn detach(mut self) {
        self.unsubscribe.take();
    }
}

impl Drop for Subscription {
    fn drop(&mut self) {
        if let Some(unsubscribe) = self.unsubscribe.take() {
            unsubscribe();
        }
    }
}