use std::collections::{hash_map::Entry, HashMap};
use std::sync::{Arc, Mutex, MutexGuard};

use futures::{future, stream::StreamExt as _, Stream};
use sqlx::sqlite::SqlitePool;
use tokio::sync::broadcast::{channel, Sender};
use tokio_stream::wrappers::{errors::BroadcastStreamRecvError, BroadcastStream};

use crate::{
    events::repo::broadcast,
    repo::channel::{self, Provider as _},
};

// Clones will share the same senders collection.
#[derive(Clone)]
pub struct Broadcaster {
    // The use of std::sync::Mutex, and not tokio::sync::Mutex, follows Tokio's
    // own advice: <https://tokio.rs/tokio/tutorial/shared-state>. Methods that
    // lock it must be sync.
    senders: Arc<Mutex<HashMap<channel::Id, Sender<broadcast::Message>>>>,
}

impl Broadcaster {
    pub async fn from_database(db: &SqlitePool) -> Result<Self, sqlx::Error> {
        let mut tx = db.begin().await?;
        let channels = tx.channels().all().await?;
        tx.commit().await?;

        let channels = channels.iter().map(|c| &c.id);
        let broadcaster = Self::new(channels);
        Ok(broadcaster)
    }

    fn new<'i>(channels: impl IntoIterator<Item = &'i channel::Id>) -> Self {
        let senders: HashMap<_, _> = channels
            .into_iter()
            .cloned()
            .map(|id| (id, Self::make_sender()))
            .collect();

        Self {
            senders: Arc::new(Mutex::new(senders)),
        }
    }

    // panic: if ``channel`` is already registered.
    pub fn register_channel(&self, channel: &channel::Id) {
        match self.senders().entry(channel.clone()) {
            // This ever happening indicates a serious logic error.
            Entry::Occupied(_) => panic!("duplicate channel registration for channel {channel}"),
            Entry::Vacant(entry) => {
                entry.insert(Self::make_sender());
            }
        }
    }

    // panic: if ``channel`` has not been previously registered, and was not
    // part of the initial set of channels.
    pub fn broadcast(&self, channel: &channel::Id, message: &broadcast::Message) {
        let tx = self.sender(channel);

        // Per the Tokio docs, the returned error is only used to indicate that
        // there are no receivers. In this use case, that's fine; a lack of
        // listening consumers (chat clients) when a message is sent isn't an
        // error.
        //
        // The successful return value, which includes the number of active
        // receivers, also isn't that interesting to us.
        let _ = tx.send(message.clone());
    }

    // panic: if ``channel`` has not been previously registered, and was not
    // part of the initial set of channels.
    pub fn subscribe(
        &self,
        channel: &channel::Id,
    ) -> impl Stream<Item = broadcast::Message> + std::fmt::Debug {
        let rx = self.sender(channel).subscribe();

        BroadcastStream::from(rx).scan((), |(), r| {
            future::ready(match r {
                Ok(message) => Some(message),
                // Stop the stream here. This will disconnect SSE clients
                // (see `routes.rs`), who will then resume from
                // `Last-Event-ID`, allowing them to catch up by reading
                // the skipped messages from the database.
                //
                // See also:
                // <https://users.rust-lang.org/t/taking-from-stream-while-ok/48854>
                Err(BroadcastStreamRecvError::Lagged(_)) => None,
            })
        })
    }

    // panic: if ``channel`` has not been previously registered, and was not
    // part of the initial set of channels.
    fn sender(&self, channel: &channel::Id) -> Sender<broadcast::Message> {
        self.senders()[channel].clone()
    }

    fn senders(&self) -> MutexGuard<HashMap<channel::Id, Sender<broadcast::Message>>> {
        self.senders.lock().unwrap() // propagate panics when mutex is poisoned
    }

    fn make_sender() -> Sender<broadcast::Message> {
        // Queue depth of 16 chosen entirely arbitrarily. Don't read too much
        // into it.
        let (tx, _) = channel(16);
        tx
    }
}