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

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

use super::repo::{
    channels::{Id as ChannelId, Provider as _},
    messages::{BroadcastMessage, Provider as _},
};
use crate::{clock::DateTime, error::BoxedError, login::repo::logins::Login};

pub struct Channels<'a> {
    db: &'a SqlitePool,
    broadcaster: &'a Broadcaster,
}

impl<'a> Channels<'a> {
    pub const fn new(db: &'a SqlitePool, broadcaster: &'a Broadcaster) -> Self {
        Self { db, broadcaster }
    }

    pub async fn create(&self, name: &str) -> Result<(), BoxedError> {
        let mut tx = self.db.begin().await?;
        let channel = tx.channels().create(name).await?;
        self.broadcaster.register_channel(&channel);
        tx.commit().await?;

        Ok(())
    }

    pub async fn send(
        &self,
        login: &Login,
        channel: &ChannelId,
        body: &str,
        sent_at: &DateTime,
    ) -> Result<(), BoxedError> {
        let mut tx = self.db.begin().await?;
        let message = tx
            .messages()
            .create(&login.id, channel, body, sent_at)
            .await?;
        tx.commit().await?;

        self.broadcaster.broadcast(channel, message);
        Ok(())
    }

    pub async fn events(
        &self,
        channel: &ChannelId,
    ) -> Result<impl Stream<Item = Result<BroadcastMessage, BoxedError>>, BoxedError> {
        let live_messages = self.broadcaster.listen(channel).map_err(BoxedError::from);

        let mut tx = self.db.begin().await?;
        let stored_messages = tx.messages().for_replay(channel).await?;
        tx.commit().await?;

        Ok(stream::iter(stored_messages).map(Ok).chain(live_messages))
    }
}

// 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<ChannelId, Sender<BroadcastMessage>>>>,
}

impl Broadcaster {
    pub async fn from_database(db: &SqlitePool) -> Result<Self, BoxedError> {
        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 ChannelId>) -> 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: &ChannelId) {
        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: &ChannelId, message: BroadcastMessage) {
        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.
        let _ = tx.send(message);
    }

    // panic: if ``channel`` has not been previously registered, and was not
    // part of the initial set of channels.
    pub fn listen(&self, channel: &ChannelId) -> BroadcastStream<BroadcastMessage> {
        let rx = self.sender(channel).subscribe();

        BroadcastStream::from(rx)
    }

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

    fn senders(&self) -> MutexGuard<HashMap<ChannelId, Sender<BroadcastMessage>>> {
        self.senders.lock().unwrap() // propagate panics when mutex is poisoned
    }

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