use futures::{
    Stream, future,
    stream::{self, StreamExt as _},
};
use itertools::Itertools as _;
use sqlx::sqlite::SqlitePool;

use super::{Event, Sequence, Sequenced, broadcaster::Broadcaster};
use crate::{
    conversation::{self, repo::Provider as _},
    db::{self, NotFound as _},
    error::failed::{Failed, ResultExt as _},
    message::{self, repo::Provider as _},
    user::{self, repo::Provider as _},
    vapid,
    vapid::repo::Provider as _,
};

pub struct Events {
    db: SqlitePool,
    events: Broadcaster,
}

impl Events {
    pub const fn new(db: SqlitePool, events: Broadcaster) -> Self {
        Self { db, events }
    }

    pub async fn subscribe(
        &self,
        resume_at: Sequence,
    ) -> Result<impl Stream<Item = Event> + std::fmt::Debug + use<>, Failed> {
        // Subscribe before retrieving, to catch messages broadcast while we're
        // querying the DB. We'll prune out duplicates later.
        let live_messages = self.events.subscribe();

        let mut tx = self.db.begin().await.fail(db::failed::BEGIN)?;

        let users = tx
            .users()
            .replay(resume_at)
            .await
            .fail("Failed to load user events")?;
        let user_events = users
            .iter()
            .map(user::History::events)
            .kmerge_by(Sequence::merge)
            .filter(Sequence::after(resume_at))
            .map(Event::from);

        let conversations = tx
            .conversations()
            .replay(resume_at)
            .await
            .fail("Failed to load conversation events")?;
        let conversation_events = conversations
            .iter()
            .map(conversation::History::events)
            .kmerge_by(Sequence::merge)
            .filter(Sequence::after(resume_at))
            .map(Event::from);

        let messages = tx
            .messages()
            .replay(resume_at)
            .await
            .fail("Failed to load message events")?;
        let message_events = messages
            .iter()
            .map(message::History::events)
            .kmerge_by(Sequence::merge)
            .filter(Sequence::after(resume_at))
            .map(Event::from);

        let vapid = tx
            .vapid()
            .current()
            .await
            .optional()
            .fail("Failed to load VAPID key events")?;
        let vapid_events = vapid
            .iter()
            .flat_map(vapid::History::events)
            .filter(Sequence::after(resume_at))
            .map(Event::from);

        let replay_events = user_events
            .merge_by(conversation_events, Sequence::merge)
            .merge_by(message_events, Sequence::merge)
            .merge_by(vapid_events, Sequence::merge)
            .collect::<Vec<_>>();
        let resume_live_at = replay_events.last().map_or(resume_at, Sequenced::sequence);

        let replay = stream::iter(replay_events);

        let live_messages = live_messages
            // Filtering on the broadcast resume point filters out messages
            // before resume_at, and filters out messages duplicated from
            // `replay_events`.
            .flat_map(stream::iter)
            .filter(Self::resume(resume_live_at));

        Ok(replay.chain(live_messages))
    }

    fn resume(resume_at: Sequence) -> impl for<'m> FnMut(&'m Event) -> future::Ready<bool> + use<> {
        let filter = Sequence::after(resume_at);
        move |event| future::ready(filter(event))
    }
}