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

use super::Broadcaster;
use crate::{
    clock::DateTime,
    events::repo::broadcast::{self, Provider as _},
    repo::{
        channel::{self, Provider as _},
        error::NotFound as _,
        login::Login,
    },
};

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

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

    pub async fn send(
        &self,
        login: &Login,
        channel: &channel::Id,
        body: &str,
        sent_at: &DateTime,
    ) -> Result<broadcast::Message, EventsError> {
        let mut tx = self.db.begin().await?;
        let channel = tx
            .channels()
            .by_id(channel)
            .await
            .not_found(|| EventsError::ChannelNotFound(channel.clone()))?;
        let message = tx
            .broadcast()
            .create(login, &channel, body, sent_at)
            .await?;
        tx.commit().await?;

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

    pub async fn subscribe(
        &self,
        channel: &channel::Id,
        subscribed_at: &DateTime,
        resume_at: Option<broadcast::Sequence>,
    ) -> Result<impl Stream<Item = broadcast::Message> + std::fmt::Debug, EventsError> {
        // Somewhat arbitrarily, expire after 90 days.
        let expire_at = subscribed_at.to_owned() - TimeDelta::days(90);

        let mut tx = self.db.begin().await?;
        let channel = tx
            .channels()
            .by_id(channel)
            .await
            .not_found(|| EventsError::ChannelNotFound(channel.clone()))?;

        // Subscribe before retrieving, to catch messages broadcast while we're
        // querying the DB. We'll prune out duplicates later.
        let live_messages = self.broadcaster.subscribe(&channel.id);

        tx.broadcast().expire(&expire_at).await?;
        let stored_messages = tx.broadcast().replay(&channel, resume_at).await?;
        tx.commit().await?;

        let resume_broadcast_at = stored_messages
            .last()
            .map(|message| message.sequence)
            .or(resume_at);

        // This should always be the case, up to integer rollover, primarily
        // because every message in stored_messages has a sequence not less
        // than `resume_at`, or `resume_at` is None. We use the last message
        // (if any) to decide when to resume the `live_messages` stream.
        //
        // It probably simplifies to assert!(resume_at <= resume_broadcast_at), but
        // this form captures more of the reasoning.
        assert!(
            (resume_at.is_none() && resume_broadcast_at.is_none())
                || (stored_messages.is_empty() && resume_at == resume_broadcast_at)
                || resume_at < resume_broadcast_at
        );

        // no skip_expired or resume transforms for stored_messages, as it's
        // constructed not to contain messages meeting either criterion.
        //
        // * skip_expired is redundant with the `tx.broadcasts().expire(…)` call;
        // * resume is redundant with the resume_at argument to
        //   `tx.broadcasts().replay(…)`.
        let stored_messages = stream::iter(stored_messages);
        let live_messages = live_messages
            // Sure, it's temporally improbable that we'll ever skip a message
            // that's 90 days old, but there's no reason not to be thorough.
            .filter(Self::skip_expired(&expire_at))
            // Filtering on the broadcast resume point filters out messages
            // before resume_at, and filters out messages duplicated from
            // stored_messages.
            .filter(Self::resume(resume_broadcast_at));

        Ok(stored_messages.chain(live_messages))
    }

    fn resume(
        resume_at: Option<broadcast::Sequence>,
    ) -> impl for<'m> FnMut(&'m broadcast::Message) -> future::Ready<bool> {
        move |msg| {
            future::ready(match resume_at {
                None => true,
                Some(resume_at) => msg.sequence > resume_at,
            })
        }
    }
    fn skip_expired(
        expire_at: &DateTime,
    ) -> impl for<'m> FnMut(&'m broadcast::Message) -> future::Ready<bool> {
        let expire_at = expire_at.to_owned();
        move |msg| future::ready(msg.sent_at > expire_at)
    }
}

#[derive(Debug, thiserror::Error)]
pub enum EventsError {
    #[error("channel {0} not found")]
    ChannelNotFound(channel::Id),
    #[error(transparent)]
    DatabaseError(#[from] sqlx::Error),
}