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

use super::{broadcaster::Broadcaster, Event, ResumePoint, Sequence, Sequenced};
use crate::{
    channel::{self, repo::Provider as _},
    login::{self, repo::Provider as _},
    message::{self, repo::Provider as _},
};

pub struct Events<'a> {
    db: &'a SqlitePool,
    events: &'a Broadcaster,
}

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

    pub async fn subscribe(
        &self,
        resume_at: impl Into<ResumePoint>,
    ) -> Result<impl Stream<Item = Event> + std::fmt::Debug, sqlx::Error> {
        let resume_at = resume_at.into();
        // 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?;

        let logins = tx.logins().replay(resume_at).await?;
        let login_events = logins
            .iter()
            .map(login::History::events)
            .kmerge_by(Sequence::merge)
            .filter(Sequence::after(resume_at))
            .map(Event::from);

        let channels = tx.channels().replay(resume_at).await?;
        let channel_events = channels
            .iter()
            .map(channel::History::events)
            .kmerge_by(Sequence::merge)
            .filter(Sequence::after(resume_at))
            .map(Event::from);

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

        let replay_events = login_events
            .merge_by(channel_events, Sequence::merge)
            .merge_by(message_events, Sequence::merge)
            .collect::<Vec<_>>();
        let resume_live_at = replay_events.last().map(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: ResumePoint) -> impl for<'m> FnMut(&'m Event) -> future::Ready<bool> {
        let filter = Sequence::after(resume_at);
        move |event| future::ready(filter(event))
    }
}