//! Handles query execution

use crate::{
    error::{LookupError, LookupResult},
    resolver::Resolver,
    sync::ResolveKind,
    tree::{DnsEntry, LinkEntry, TreeRootEntry},
};
use enr::EnrKeyUnambiguous;
use reth_tokio_util::ratelimit::{Rate, RateLimit};
use std::{
    collections::VecDeque,
    future::Future,
    num::NonZeroUsize,
    pin::Pin,
    sync::Arc,
    task::{ready, Context, Poll},
    time::Duration,
};

/// The `QueryPool` provides an aggregate state machine for driving queries to completion.
pub(crate) struct QueryPool<R: Resolver, K: EnrKeyUnambiguous> {
    /// The [Resolver] that's used to lookup queries.
    resolver: Arc<R>,
    /// Buffered queries
    queued_queries: VecDeque<Query<K>>,
    /// All active queries
    active_queries: Vec<Query<K>>,
    /// buffered results
    queued_outcomes: VecDeque<QueryOutcome<K>>,
    /// Rate limit for DNS requests
    rate_limit: RateLimit,
    /// Timeout for DNS lookups.
    lookup_timeout: Duration,
}

// === impl QueryPool ===

impl<R: Resolver, K: EnrKeyUnambiguous> QueryPool<R, K> {
    pub(crate) fn new(
        resolver: Arc<R>,
        max_requests_per_sec: NonZeroUsize,
        lookup_timeout: Duration,
    ) -> Self {
        Self {
            resolver,
            queued_queries: Default::default(),
            active_queries: vec![],
            queued_outcomes: Default::default(),
            rate_limit: RateLimit::new(Rate::new(
                max_requests_per_sec.get() as u64,
                Duration::from_secs(1),
            )),
            lookup_timeout,
        }
    }

    /// Resolves the root the link's domain references
    pub(crate) fn resolve_root(&mut self, link: LinkEntry<K>) {
        let resolver = Arc::clone(&self.resolver);
        let timeout = self.lookup_timeout;
        self.queued_queries.push_back(Query::Root(Box::pin(resolve_root(resolver, link, timeout))))
    }

    /// Resolves the [`DnsEntry`] for `<hash.domain>`
    pub(crate) fn resolve_entry(&mut self, link: LinkEntry<K>, hash: String, kind: ResolveKind) {
        let resolver = Arc::clone(&self.resolver);
        let timeout = self.lookup_timeout;
        self.queued_queries
            .push_back(Query::Entry(Box::pin(resolve_entry(resolver, link, hash, kind, timeout))))
    }

    /// Advances the state of the queries
    pub(crate) fn poll(&mut self, cx: &mut Context<'_>) -> Poll<QueryOutcome<K>> {
        loop {
            // drain buffered events first
            if let Some(event) = self.queued_outcomes.pop_front() {
                return Poll::Ready(event)
            }

            // queue in new queries if we have capacity
            'queries: while self.active_queries.len() < self.rate_limit.limit() as usize {
                if self.rate_limit.poll_ready(cx).is_ready() {
                    if let Some(query) = self.queued_queries.pop_front() {
                        self.rate_limit.tick();
                        self.active_queries.push(query);
                        continue 'queries
                    }
                }
                break
            }

            // advance all queries
            for idx in (0..self.active_queries.len()).rev() {
                let mut query = self.active_queries.swap_remove(idx);
                if let Poll::Ready(outcome) = query.poll(cx) {
                    self.queued_outcomes.push_back(outcome);
                } else {
                    // still pending
                    self.active_queries.push(query);
                }
            }

            if self.queued_outcomes.is_empty() {
                return Poll::Pending
            }
        }
    }
}

// === Various future/type alias ===

pub(crate) struct ResolveEntryResult<K: EnrKeyUnambiguous> {
    pub(crate) entry: Option<LookupResult<DnsEntry<K>>>,
    pub(crate) link: LinkEntry<K>,
    pub(crate) hash: String,
    pub(crate) kind: ResolveKind,
}

pub(crate) type ResolveRootResult<K> =
    Result<(TreeRootEntry, LinkEntry<K>), (LookupError, LinkEntry<K>)>;

type ResolveRootFuture<K> = Pin<Box<dyn Future<Output = ResolveRootResult<K>> + Send>>;

type ResolveEntryFuture<K> = Pin<Box<dyn Future<Output = ResolveEntryResult<K>> + Send>>;

enum Query<K: EnrKeyUnambiguous> {
    Root(ResolveRootFuture<K>),
    Entry(ResolveEntryFuture<K>),
}

// === impl Query ===

impl<K: EnrKeyUnambiguous> Query<K> {
    /// Advances the query
    fn poll(&mut self, cx: &mut Context<'_>) -> Poll<QueryOutcome<K>> {
        match self {
            Self::Root(ref mut query) => {
                let outcome = ready!(query.as_mut().poll(cx));
                Poll::Ready(QueryOutcome::Root(outcome))
            }
            Self::Entry(ref mut query) => {
                let outcome = ready!(query.as_mut().poll(cx));
                Poll::Ready(QueryOutcome::Entry(outcome))
            }
        }
    }
}

/// The output the queries return
pub(crate) enum QueryOutcome<K: EnrKeyUnambiguous> {
    Root(ResolveRootResult<K>),
    Entry(ResolveEntryResult<K>),
}

/// Retrieves the [`DnsEntry`]
async fn resolve_entry<K: EnrKeyUnambiguous, R: Resolver>(
    resolver: Arc<R>,
    link: LinkEntry<K>,
    hash: String,
    kind: ResolveKind,
    timeout: Duration,
) -> ResolveEntryResult<K> {
    let fqn = format!("{hash}.{}", link.domain);
    let mut resp = ResolveEntryResult { entry: None, link, hash, kind };
    match lookup_with_timeout::<R>(&resolver, &fqn, timeout).await {
        Ok(Some(entry)) => {
            resp.entry = Some(entry.parse::<DnsEntry<K>>().map_err(|err| err.into()))
        }
        Err(err) => resp.entry = Some(Err(err)),
        Ok(None) => {}
    }
    resp
}

/// Retrieves the root entry the link points to and returns the verified entry
///
/// Returns an error if the record could be retrieved but is not a root entry or failed to be
/// verified.
async fn resolve_root<K: EnrKeyUnambiguous, R: Resolver>(
    resolver: Arc<R>,
    link: LinkEntry<K>,
    timeout: Duration,
) -> ResolveRootResult<K> {
    let root = match lookup_with_timeout::<R>(&resolver, &link.domain, timeout).await {
        Ok(Some(root)) => root,
        Ok(_) => return Err((LookupError::EntryNotFound, link)),
        Err(err) => return Err((err, link)),
    };

    match root.parse::<TreeRootEntry>() {
        Ok(root) => {
            if root.verify::<K>(&link.pubkey) {
                Ok((root, link))
            } else {
                Err((LookupError::InvalidRoot(root), link))
            }
        }
        Err(err) => Err((err.into(), link)),
    }
}

async fn lookup_with_timeout<R: Resolver>(
    r: &R,
    query: &str,
    timeout: Duration,
) -> LookupResult<Option<String>> {
    tokio::time::timeout(timeout, r.lookup_txt(query))
        .await
        .map_err(|_| LookupError::RequestTimedOut)
}

#[cfg(test)]
mod tests {
    use super::*;
    use crate::{resolver::TimeoutResolver, DnsDiscoveryConfig, MapResolver};
    use std::future::poll_fn;

    #[tokio::test]
    async fn test_rate_limit() {
        let resolver = Arc::new(MapResolver::default());
        let config = DnsDiscoveryConfig::default();
        let mut pool = QueryPool::new(resolver, config.max_requests_per_sec, config.lookup_timeout);

        let s = "enrtree://AM5FCQLWIZX2QFPNJAP7VUERCCRNGRHWZG3YYHIUV7BVDQ5FDPRT2@nodes.example.org";
        let entry: LinkEntry = s.parse().unwrap();

        for _n in 0..config.max_requests_per_sec.get() {
            poll_fn(|cx| {
                pool.resolve_root(entry.clone());
                assert_eq!(pool.queued_queries.len(), 1);
                assert!(pool.rate_limit.poll_ready(cx).is_ready());
                let _ = pool.poll(cx);
                assert_eq!(pool.queued_queries.len(), 0);
                Poll::Ready(())
            })
            .await;
        }

        pool.resolve_root(entry.clone());
        assert_eq!(pool.queued_queries.len(), 1);
        poll_fn(|cx| {
            assert!(pool.rate_limit.poll_ready(cx).is_pending());
            let _ = pool.poll(cx);
            assert_eq!(pool.queued_queries.len(), 1);
            Poll::Ready(())
        })
        .await;
    }

    #[tokio::test]
    async fn test_timeouts() {
        let config =
            DnsDiscoveryConfig { lookup_timeout: Duration::from_millis(500), ..Default::default() };
        let resolver = Arc::new(TimeoutResolver(config.lookup_timeout * 2));
        let mut pool = QueryPool::new(resolver, config.max_requests_per_sec, config.lookup_timeout);

        let s = "enrtree://AM5FCQLWIZX2QFPNJAP7VUERCCRNGRHWZG3YYHIUV7BVDQ5FDPRT2@nodes.example.org";
        let entry: LinkEntry = s.parse().unwrap();
        pool.resolve_root(entry);

        let outcome = poll_fn(|cx| pool.poll(cx)).await;

        match outcome {
            QueryOutcome::Root(res) => {
                let res = res.unwrap_err().0;
                match res {
                    LookupError::RequestTimedOut => {}
                    _ => unreachable!(),
                }
            }
            QueryOutcome::Entry(_) => {
                unreachable!()
            }
        }
    }
}