///! This manages the discovery and management of peers.
pub(crate) mod enr;
pub mod enr_ext;

// Allow external use of the lighthouse ENR builder
pub use enr::{build_enr, create_enr_builder_from_config, use_or_load_enr, CombinedKey, Eth2Enr};
pub use enr_ext::{CombinedKeyExt, EnrExt};
pub use libp2p::core::identity::Keypair;

use crate::metrics;
use crate::{error, Enr, NetworkConfig, NetworkGlobals, SubnetDiscovery};
use discv5::{enr::NodeId, Discv5, Discv5Event};
use enr::{BITFIELD_ENR_KEY, ETH2_ENR_KEY};
use futures::prelude::*;
use futures::stream::FuturesUnordered;
use libp2p::core::PeerId;
use lru::LruCache;
use slog::{crit, debug, error, info, warn};
use ssz::{Decode, Encode};
use ssz_types::BitVector;
use std::{
    collections::{HashMap, VecDeque},
    net::SocketAddr,
    path::Path,
    pin::Pin,
    sync::Arc,
    task::{Context, Poll},
    time::{Duration, Instant},
};
use tokio::sync::mpsc;
use types::{EnrForkId, EthSpec, SubnetId};

mod subnet_predicate;
pub use subnet_predicate::subnet_predicate;

/// Local ENR storage filename.
pub const ENR_FILENAME: &str = "enr.dat";
/// Target number of peers we'd like to have connected to a given long-lived subnet.
pub const TARGET_SUBNET_PEERS: usize = 3;
/// Target number of peers to search for given a grouped subnet query.
const TARGET_PEERS_FOR_GROUPED_QUERY: usize = 6;
/// Number of times to attempt a discovery request.
const MAX_DISCOVERY_RETRY: usize = 3;
/// The maximum number of concurrent discovery queries.
const MAX_CONCURRENT_QUERIES: usize = 2;
/// The max number of subnets to search for in a single subnet discovery query.
const MAX_SUBNETS_IN_QUERY: usize = 3;
/// The number of closest peers to search for when doing a regular peer search.
///
/// We could reduce this constant to speed up queries however at the cost of security. It will
/// make it easier to peers to eclipse this node. Kademlia suggests a value of 16.
const FIND_NODE_QUERY_CLOSEST_PEERS: usize = 16;
/// The threshold for updating `min_ttl` on a connected peer.
const DURATION_DIFFERENCE: Duration = Duration::from_millis(1);

/// The events emitted by polling discovery.
pub enum DiscoveryEvent {
    /// A query has completed. This result contains a mapping of discovered peer IDs to the `min_ttl`
    /// of the peer if it is specified.
    QueryResult(HashMap<PeerId, Option<Instant>>),
    /// This indicates that our local UDP socketaddr has been updated and we should inform libp2p.
    SocketUpdated(SocketAddr),
}

#[derive(Debug, Clone, PartialEq)]
struct SubnetQuery {
    subnet_id: SubnetId,
    min_ttl: Option<Instant>,
    retries: usize,
}

#[derive(Debug, Clone, PartialEq)]
enum QueryType {
    /// We are searching for subnet peers.
    Subnet(SubnetQuery),
    /// We are searching for more peers without ENR or time constraints.
    FindPeers,
}

#[derive(Debug, Clone, PartialEq)]
enum GroupedQueryType {
    /// We are searching for peers on one of a few subnets.
    Subnet(Vec<SubnetQuery>),
    /// We are searching for more peers without ENR or time constraints.
    FindPeers,
}

impl QueryType {
    /// Returns true if this query has expired.
    pub fn expired(&self) -> bool {
        match self {
            Self::FindPeers => false,
            Self::Subnet(subnet_query) => {
                if let Some(ttl) = subnet_query.min_ttl {
                    ttl < Instant::now()
                } else {
                    true
                }
            }
        }
    }
}

/// The result of a query.
struct QueryResult(GroupedQueryType, Result<Vec<Enr>, discv5::QueryError>);

// Awaiting the event stream future
enum EventStream {
    /// Awaiting an event stream to be generated. This is required due to the poll nature of
    /// `Discovery`
    Awaiting(
        Pin<
            Box<
                dyn Future<Output = Result<mpsc::Receiver<Discv5Event>, discv5::Discv5Error>>
                    + Send,
            >,
        >,
    ),
    /// The future has completed.
    Present(mpsc::Receiver<Discv5Event>),
    // The future has failed or discv5 has been disabled. There are no events from discv5.
    InActive,
}

/// The main discovery service. This can be disabled via CLI arguements. When disabled the
/// underlying processes are not started, but this struct still maintains our current ENR.
pub struct Discovery<TSpec: EthSpec> {
    /// A collection of seen live ENRs for quick lookup and to map peer-id's to ENRs.
    cached_enrs: LruCache<PeerId, Enr>,

    /// The directory where the ENR is stored.
    enr_dir: String,

    /// The handle for the underlying discv5 Server.
    ///
    /// This is behind a Reference counter to allow for futures to be spawned and polled with a
    /// static lifetime.
    discv5: Discv5,

    /// A collection of network constants that can be read from other threads.
    network_globals: Arc<NetworkGlobals<TSpec>>,

    /// Indicates if we are actively searching for peers. We only allow a single FindPeers query at
    /// a time, regardless of the query concurrency.
    find_peer_active: bool,

    /// A queue of discovery queries to be processed.
    queued_queries: VecDeque<QueryType>,

    /// Active discovery queries.
    active_queries: FuturesUnordered<std::pin::Pin<Box<dyn Future<Output = QueryResult> + Send>>>,

    /// The discv5 event stream.
    event_stream: EventStream,

    /// Indicates if the discovery service has been started. When the service is disabled, this is
    /// always false.
    started: bool,

    /// Logger for the discovery behaviour.
    log: slog::Logger,
}

impl<TSpec: EthSpec> Discovery<TSpec> {
    /// NOTE: Creating discovery requires running within a tokio execution environment.
    pub async fn new(
        local_key: &Keypair,
        config: &NetworkConfig,
        network_globals: Arc<NetworkGlobals<TSpec>>,
        log: &slog::Logger,
    ) -> error::Result<Self> {
        let log = log.clone();

        let enr_dir = match config.network_dir.to_str() {
            Some(path) => String::from(path),
            None => String::from(""),
        };

        let local_enr = network_globals.local_enr.read().clone();

        info!(log, "ENR Initialised"; "enr" => local_enr.to_base64(), "seq" => local_enr.seq(), "id"=> format!("{}",local_enr.node_id()), "ip" => format!("{:?}", local_enr.ip()), "udp"=> format!("{:?}", local_enr.udp()), "tcp" => format!("{:?}", local_enr.tcp()));

        let listen_socket = SocketAddr::new(config.listen_address, config.discovery_port);

        // convert the keypair into an ENR key
        let enr_key: CombinedKey = CombinedKey::from_libp2p(&local_key)?;

        let mut discv5 = Discv5::new(local_enr, enr_key, config.discv5_config.clone())
            .map_err(|e| format!("Discv5 service failed. Error: {:?}", e))?;

        // Add bootnodes to routing table
        for bootnode_enr in config.boot_nodes_enr.clone() {
            debug!(
                log,
                "Adding node to routing table";
                "node_id" => bootnode_enr.node_id().to_string(),
                "peer_id" => bootnode_enr.peer_id().to_string(),
                "ip" => format!("{:?}", bootnode_enr.ip()),
                "udp" => format!("{:?}", bootnode_enr.udp()),
                "tcp" => format!("{:?}", bootnode_enr.tcp())
            );
            let repr = bootnode_enr.to_string();
            let _ = discv5.add_enr(bootnode_enr).map_err(|e| {
                error!(
                    log,
                    "Could not add peer to the local routing table";
                    "addr" => repr,
                    "error" => e.to_string(),
                )
            });
        }

        // Start the discv5 service and obtain an event stream
        let event_stream = if !config.disable_discovery {
            discv5.start(listen_socket).map_err(|e| e.to_string())?;
            debug!(log, "Discovery service started");
            EventStream::Awaiting(Box::pin(discv5.event_stream()))
        } else {
            EventStream::InActive
        };

        if !config.boot_nodes_multiaddr.is_empty() {
            info!(log, "Contacting Multiaddr boot-nodes for their ENR");
        }

        // get futures for requesting the Enrs associated to these multiaddr and wait for their
        // completion
        let mut fut_coll = config
            .boot_nodes_multiaddr
            .iter()
            .map(|addr| addr.to_string())
            // request the ENR for this multiaddr and keep the original for logging
            .map(|addr| {
                futures::future::join(
                    discv5.request_enr(addr.clone()),
                    futures::future::ready(addr),
                )
            })
            .collect::<FuturesUnordered<_>>();

        while let Some((result, original_addr)) = fut_coll.next().await {
            match result {
                Ok(Some(enr)) => {
                    debug!(
                        log,
                        "Adding node to routing table";
                        "node_id" => enr.node_id().to_string(),
                        "peer_id" => enr.peer_id().to_string(),
                        "ip" => format!("{:?}", enr.ip()),
                        "udp" => format!("{:?}", enr.udp()),
                        "tcp" => format!("{:?}", enr.tcp())
                    );
                    let _ = discv5.add_enr(enr).map_err(|e| {
                        error!(
                            log,
                            "Could not add peer to the local routing table";
                            "addr" => original_addr.to_string(),
                            "error" => e.to_string(),
                        )
                    });
                }
                Ok(None) => {
                    error!(log, "No ENR found for MultiAddr"; "addr" => original_addr.to_string())
                }
                Err(e) => {
                    error!(log, "Error getting mapping to ENR"; "multiaddr" => original_addr.to_string(), "error" => e.to_string())
                }
            }
        }

        Ok(Self {
            cached_enrs: LruCache::new(50),
            network_globals,
            find_peer_active: false,
            queued_queries: VecDeque::with_capacity(10),
            active_queries: FuturesUnordered::new(),
            discv5,
            event_stream,
            started: !config.disable_discovery,
            log,
            enr_dir,
        })
    }

    /// Return the nodes local ENR.
    pub fn local_enr(&self) -> Enr {
        self.discv5.local_enr()
    }

    /// Return the cached enrs.
    pub fn cached_enrs(&self) -> impl Iterator<Item = (&PeerId, &Enr)> {
        self.cached_enrs.iter()
    }

    /// This adds a new `FindPeers` query to the queue if one doesn't already exist.
    pub fn discover_peers(&mut self) {
        // If the discv5 service isn't running or we are in the process of a query, don't bother queuing a new one.
        if !self.started || self.find_peer_active {
            return;
        }

        // If there is not already a find peer's query queued, add one
        let query = QueryType::FindPeers;
        if !self.queued_queries.contains(&query) {
            debug!(self.log, "Queuing a peer discovery request");
            self.queued_queries.push_back(query);
            // update the metrics
            metrics::set_gauge(&metrics::DISCOVERY_QUEUE, self.queued_queries.len() as i64);
        }
    }

    /// Processes a request to search for more peers on a subnet.
    pub fn discover_subnet_peers(&mut self, subnets_to_discover: Vec<SubnetDiscovery>) {
        // If the discv5 service isn't running, ignore queries
        if !self.started {
            return;
        }
        debug!(
            self.log,
            "Making discovery query for subnets";
            "subnets" => format!("{:?}", subnets_to_discover.iter().map(|s| s.subnet_id).collect::<Vec<_>>())
        );
        for subnet in subnets_to_discover {
            self.add_subnet_query(subnet.subnet_id, subnet.min_ttl, 0);
        }
    }

    /// Add an ENR to the routing table of the discovery mechanism.
    pub fn add_enr(&mut self, enr: Enr) {
        // add the enr to seen caches
        self.cached_enrs.put(enr.peer_id(), enr.clone());

        if let Err(e) = self.discv5.add_enr(enr) {
            debug!(
                self.log,
                "Could not add peer to the local routing table";
                "error" => e.to_string()
            )
        }
    }

    /// Returns an iterator over all enr entries in the DHT.
    pub fn table_entries_enr(&mut self) -> Vec<Enr> {
        self.discv5.table_entries_enr()
    }

    /// Returns the ENR of a known peer if it exists.
    pub fn enr_of_peer(&mut self, peer_id: &PeerId) -> Option<Enr> {
        // first search the local cache
        if let Some(enr) = self.cached_enrs.get(peer_id) {
            return Some(enr.clone());
        }
        // not in the local cache, look in the routing table
        if let Ok(node_id) = enr_ext::peer_id_to_node_id(peer_id) {
            self.discv5.find_enr(&node_id)
        } else {
            None
        }
    }

    /// Adds/Removes a subnet from the ENR Bitfield
    pub fn update_enr_bitfield(&mut self, subnet_id: SubnetId, value: bool) -> Result<(), String> {
        let id = *subnet_id as usize;

        let local_enr = self.discv5.local_enr();
        let mut current_bitfield = local_enr.bitfield::<TSpec>()?;

        if id >= current_bitfield.len() {
            return Err(format!(
                "Subnet id: {} is outside the ENR bitfield length: {}",
                id,
                current_bitfield.len()
            ));
        }

        if current_bitfield
            .get(id)
            .map_err(|_| String::from("Subnet ID out of bounds"))?
            == value
        {
            return Err(format!(
                "Subnet id: {} already in the local ENR already has value: {}",
                id, value
            ));
        }

        // set the subnet bitfield in the ENR
        current_bitfield
            .set(id, value)
            .map_err(|_| String::from("Subnet ID out of bounds, could not set subnet ID"))?;

        // insert the bitfield into the ENR record
        let _ = self
            .discv5
            .enr_insert(BITFIELD_ENR_KEY, current_bitfield.as_ssz_bytes());

        // replace the global version
        *self.network_globals.local_enr.write() = self.discv5.local_enr();

        // persist modified enr to disk
        enr::save_enr_to_disk(Path::new(&self.enr_dir), &self.local_enr(), &self.log);
        Ok(())
    }

    /// Updates the `eth2` field of our local ENR.
    pub fn update_eth2_enr(&mut self, enr_fork_id: EnrForkId) {
        // to avoid having a reference to the spec constant, for the logging we assume
        // FAR_FUTURE_EPOCH is u64::max_value()
        let next_fork_epoch_log = if enr_fork_id.next_fork_epoch == u64::max_value() {
            String::from("No other fork")
        } else {
            format!("{:?}", enr_fork_id.next_fork_epoch)
        };

        info!(self.log, "Updating the ENR fork version";
            "fork_digest" => format!("{:?}", enr_fork_id.fork_digest),
            "next_fork_version" => format!("{:?}", enr_fork_id.next_fork_version),
            "next_fork_epoch" => next_fork_epoch_log,
        );

        let _ = self
            .discv5
            .enr_insert(ETH2_ENR_KEY, enr_fork_id.as_ssz_bytes())
            .map_err(|e| {
                warn!(
                    self.log,
                    "Could not update eth2 ENR field";
                    "error" => format!("{:?}", e)
                )
            });

        // replace the global version with discovery version
        *self.network_globals.local_enr.write() = self.discv5.local_enr();

        // persist modified enr to disk
        enr::save_enr_to_disk(Path::new(&self.enr_dir), &self.local_enr(), &self.log);
    }

    /* Internal Functions */

    /// Adds a subnet query if one doesn't exist. If a subnet query already exists, this
    /// updates the min_ttl field.
    fn add_subnet_query(&mut self, subnet_id: SubnetId, min_ttl: Option<Instant>, retries: usize) {
        // remove the entry and complete the query if greater than the maximum search count
        if retries > MAX_DISCOVERY_RETRY {
            debug!(
                self.log,
                "Subnet peer discovery did not find sufficient peers. Reached max retry limit"
            );
            return;
        }

        // Search through any queued requests and update the timeout if a query for this subnet
        // already exists
        let mut found = false;
        for query in self.queued_queries.iter_mut() {
            if let QueryType::Subnet(ref mut subnet_query) = query {
                if subnet_query.subnet_id == subnet_id {
                    if subnet_query.min_ttl < min_ttl {
                        subnet_query.min_ttl = min_ttl;
                    }
                    // update the number of retries
                    subnet_query.retries = retries;
                    // mimic an `Iter::Find()` and short-circuit the loop
                    found = true;
                    break;
                }
            }
        }
        if !found {
            // Set up the query and add it to the queue
            let query = QueryType::Subnet(SubnetQuery {
                subnet_id,
                min_ttl,
                retries,
            });
            // update the metrics and insert into the queue.
            debug!(self.log, "Queuing subnet query"; "subnet" => *subnet_id, "retries" => retries);
            self.queued_queries.push_back(query);
            metrics::set_gauge(&metrics::DISCOVERY_QUEUE, self.queued_queries.len() as i64);
        }
    }

    /// Consume the discovery queue and initiate queries when applicable.
    ///
    /// This also sanitizes the queue removing out-dated queries.
    fn process_queue(&mut self) {
        // Sanitize the queue, removing any out-dated subnet queries
        self.queued_queries.retain(|query| !query.expired());

        // use this to group subnet queries together for a single discovery request
        let mut subnet_queries: Vec<SubnetQuery> = Vec::new();

        // Check that we are within our query concurrency limit
        while !self.at_capacity() && !self.queued_queries.is_empty() {
            // consume and process the query queue
            match self.queued_queries.pop_front() {
                Some(QueryType::FindPeers) => {
                    // Only start a find peers query if it is the last message in the queue.
                    // We want to prioritize subnet queries, so we don't miss attestations.
                    if self.queued_queries.is_empty() {
                        // This is a regular request to find additional peers
                        debug!(self.log, "Discovery query started");
                        self.find_peer_active = true;
                        self.start_query(
                            GroupedQueryType::FindPeers,
                            FIND_NODE_QUERY_CLOSEST_PEERS,
                            |_| true,
                        );
                    } else {
                        self.queued_queries.push_back(QueryType::FindPeers);
                    }
                }
                Some(QueryType::Subnet(subnet_query)) => {
                    subnet_queries.push(subnet_query);

                    // We want to start a grouped subnet query if:
                    //  1. We've grouped MAX_SUBNETS_IN_QUERY subnets together.
                    //  2. There are no more messages in the queue.
                    //  3. There is exactly one message in the queue and it is FindPeers.
                    if subnet_queries.len() == MAX_SUBNETS_IN_QUERY
                        || self.queued_queries.is_empty()
                        || (self.queued_queries.front() == Some(&QueryType::FindPeers)
                            && self.queued_queries.len() == 1)
                    {
                        // This query is for searching for peers of a particular subnet
                        // Drain subnet_queries so we can re-use it as we continue to process the queue
                        let grouped_queries: Vec<SubnetQuery> = subnet_queries.drain(..).collect();
                        debug!(
                            self.log,
                            "Starting grouped subnet query";
                            "subnets" => format!("{:?}", grouped_queries.iter().map(|q| q.subnet_id).collect::<Vec<_>>()),
                        );
                        self.start_subnet_query(grouped_queries);
                    }
                }
                None => {} // Queue is empty
            }
        }
        // Update the queue metric
        metrics::set_gauge(&metrics::DISCOVERY_QUEUE, self.queued_queries.len() as i64);
    }

    // Returns a boolean indicating if we are currently processing the maximum number of
    // concurrent queries or not.
    fn at_capacity(&self) -> bool {
        self.active_queries.len() >= MAX_CONCURRENT_QUERIES
    }

    /// Runs a discovery request for a given group of subnets.
    fn start_subnet_query(&mut self, subnet_queries: Vec<SubnetQuery>) {
        let mut filtered_subnet_ids: Vec<SubnetId> = Vec::new();

        // find subnet queries that are still necessary
        let filtered_subnet_queries: Vec<SubnetQuery> = subnet_queries
            .into_iter()
            .filter(|subnet_query| {
                // Determine if we have sufficient peers, which may make this discovery unnecessary.
                let peers_on_subnet = self
                    .network_globals
                    .peers
                    .read()
                    .peers_on_subnet(subnet_query.subnet_id)
                    .count();

                if peers_on_subnet >= TARGET_SUBNET_PEERS {
                    debug!(self.log, "Discovery ignored";
                        "reason" => "Already connected to desired peers",
                        "connected_peers_on_subnet" => peers_on_subnet,
                        "target_subnet_peers" => TARGET_SUBNET_PEERS,
                    );
                    return false;
                }

                let target_peers = TARGET_SUBNET_PEERS - peers_on_subnet;
                debug!(self.log, "Discovery query started for subnet";
                    "subnet_id" => *subnet_query.subnet_id,
                    "connected_peers_on_subnet" => peers_on_subnet,
                    "target_subnet_peers" => TARGET_SUBNET_PEERS,
                    "peers_to_find" => target_peers,
                    "attempt" => subnet_query.retries,
                    "min_ttl" => format!("{:?}", subnet_query.min_ttl),
                );

                filtered_subnet_ids.push(subnet_query.subnet_id);
                true
            })
            .collect();

        // Only start a discovery query if we have a subnet to look for.
        if !filtered_subnet_queries.is_empty() {
            // build the subnet predicate as a combination of the eth2_fork_predicate and the subnet predicate
            let subnet_predicate = subnet_predicate::<TSpec>(filtered_subnet_ids, &self.log);

            self.start_query(
                GroupedQueryType::Subnet(filtered_subnet_queries),
                TARGET_PEERS_FOR_GROUPED_QUERY,
                subnet_predicate,
            );
        }
    }

    /// Search for a specified number of new peers using the underlying discovery mechanism.
    ///
    /// This can optionally search for peers for a given predicate. Regardless of the predicate
    /// given, this will only search for peers on the same enr_fork_id as specified in the local
    /// ENR.
    fn start_query(
        &mut self,
        grouped_query: GroupedQueryType,
        target_peers: usize,
        additional_predicate: impl Fn(&Enr) -> bool + Send + 'static,
    ) {
        // Make sure there are subnet queries included
        let contains_queries = match &grouped_query {
            GroupedQueryType::Subnet(queries) => !queries.is_empty(),
            GroupedQueryType::FindPeers => true,
        };

        if !contains_queries {
            debug!(
                self.log,
                "No subnets included in this request. Skipping discovery request."
            );
            return;
        }

        // Generate a random target node id.
        let random_node = NodeId::random();

        let enr_fork_id = match self.local_enr().eth2() {
            Ok(v) => v,
            Err(e) => {
                crit!(self.log, "Local ENR has no fork id"; "error" => e);
                return;
            }
        };
        // predicate for finding nodes with a matching fork
        let eth2_fork_predicate = move |enr: &Enr| enr.eth2() == Ok(enr_fork_id.clone());

        // General predicate
        let predicate: Box<dyn Fn(&Enr) -> bool + Send> =
            Box::new(move |enr: &Enr| eth2_fork_predicate(enr) && additional_predicate(enr));

        // Build the future
        let query_future = self
            .discv5
            .find_node_predicate(random_node, predicate, target_peers)
            .map(|v| QueryResult(grouped_query, v));

        // Add the future to active queries, to be executed.
        self.active_queries.push(Box::pin(query_future));
    }

    /// Drives the queries returning any results from completed queries.
    fn poll_queries(&mut self, cx: &mut Context) -> Option<HashMap<PeerId, Option<Instant>>> {
        while let Poll::Ready(Some(query_future)) = self.active_queries.poll_next_unpin(cx) {
            match query_future.0 {
                GroupedQueryType::FindPeers => {
                    self.find_peer_active = false;
                    match query_future.1 {
                        Ok(r) if r.is_empty() => {
                            debug!(self.log, "Discovery query yielded no results.");
                        }
                        Ok(r) => {
                            debug!(self.log, "Discovery query completed"; "peers_found" => r.len());
                            let mut results: HashMap<PeerId, Option<Instant>> = HashMap::new();
                            r.iter().for_each(|enr| {
                                // cache the found ENR's
                                self.cached_enrs.put(enr.peer_id(), enr.clone());
                                results.insert(enr.peer_id(), None);
                            });
                            return Some(results);
                        }
                        Err(e) => {
                            warn!(self.log, "Discovery query failed"; "error" => e.to_string());
                        }
                    }
                }
                GroupedQueryType::Subnet(queries) => {
                    let subnets_searched_for: Vec<SubnetId> =
                        queries.iter().map(|query| query.subnet_id).collect();
                    match query_future.1 {
                        Ok(r) if r.is_empty() => {
                            debug!(self.log, "Grouped subnet discovery query yielded no results."; "subnets_searched_for" => format!("{:?}",subnets_searched_for));
                        }
                        Ok(r) => {
                            debug!(self.log, "Peer grouped subnet discovery request completed"; "peers_found" => r.len(), "subnets_searched_for" => format!("{:?}",subnets_searched_for));

                            let mut mapped_results: HashMap<PeerId, Option<Instant>> =
                                HashMap::new();

                            // cache the found ENR's
                            for enr in r.iter().cloned() {
                                self.cached_enrs.put(enr.peer_id(), enr);
                            }

                            // Map each subnet query's min_ttl to the set of ENR's returned for that subnet.
                            queries.iter().for_each(|query| {
                                // A subnet query has completed. Add back to the queue, incrementing retries.
                                self.add_subnet_query(
                                    query.subnet_id,
                                    query.min_ttl,
                                    query.retries + 1,
                                );

                                // Check the specific subnet against the enr
                                let subnet_predicate =
                                    subnet_predicate::<TSpec>(vec![query.subnet_id], &self.log);

                                r.iter()
                                    .filter(|enr| subnet_predicate(enr))
                                    .map(|enr| enr.peer_id())
                                    .for_each(|peer_id| {
                                        let other_min_ttl = mapped_results.get_mut(&peer_id);

                                        // map peer IDs to the min_ttl furthest in the future
                                        match (query.min_ttl, other_min_ttl) {
                                            // update the mapping if the min_ttl is greater
                                            (
                                                Some(min_ttl_instant),
                                                Some(Some(other_min_ttl_instant)),
                                            ) => {
                                                if min_ttl_instant.saturating_duration_since(
                                                    *other_min_ttl_instant,
                                                ) > DURATION_DIFFERENCE
                                                {
                                                    *other_min_ttl_instant = min_ttl_instant;
                                                }
                                            }
                                            // update the mapping if we have a specified min_ttl
                                            (Some(min_ttl), Some(None)) => {
                                                mapped_results.insert(peer_id, Some(min_ttl));
                                            }
                                            // first seen min_ttl for this enr
                                            (Some(min_ttl), None) => {
                                                mapped_results.insert(peer_id, Some(min_ttl));
                                            }
                                            // first seen min_ttl for this enr
                                            (None, None) => {
                                                mapped_results.insert(peer_id, None);
                                            }
                                            (None, Some(Some(_))) => {} // Don't replace the existing specific min_ttl
                                            (None, Some(None)) => {} // No-op because this is a duplicate
                                        }
                                    });
                            });

                            if mapped_results.is_empty() {
                                return None;
                            } else {
                                return Some(mapped_results);
                            }
                        }
                        Err(e) => {
                            warn!(self.log,"Grouped subnet discovery query failed"; "subnets_searched_for" => format!("{:?}",subnets_searched_for), "error" => e.to_string());
                        }
                    }
                }
            }
        }
        None
    }

    // Main execution loop to be driven by the peer manager.
    pub fn poll(&mut self, cx: &mut Context) -> Poll<DiscoveryEvent> {
        if !self.started {
            return Poll::Pending;
        }

        // Process the query queue
        self.process_queue();

        // Drive the queries and return any results from completed queries
        if let Some(results) = self.poll_queries(cx) {
            // return the result to the peer manager
            return Poll::Ready(DiscoveryEvent::QueryResult(results));
        }

        // Process the server event stream
        match self.event_stream {
            EventStream::Awaiting(ref mut fut) => {
                // Still awaiting the event stream, poll it
                if let Poll::Ready(event_stream) = fut.poll_unpin(cx) {
                    match event_stream {
                        Ok(stream) => self.event_stream = EventStream::Present(stream),
                        Err(e) => {
                            slog::crit!(self.log, "Discv5 event stream failed"; "error" => e.to_string());
                            self.event_stream = EventStream::InActive;
                        }
                    }
                }
            }
            EventStream::InActive => {} // ignore checking the stream
            EventStream::Present(ref mut stream) => {
                while let Ok(event) = stream.try_recv() {
                    match event {
                        // We filter out unwanted discv5 events here and only propagate useful results to
                        // the peer manager.
                        Discv5Event::Discovered(_enr) => {
                            // Peers that get discovered during a query but are not contactable or
                            // don't match a predicate can end up here. For debugging purposes we
                            // log these to see if we are unnecessarily dropping discovered peers
                            /*
                            if enr.eth2() == self.local_enr().eth2() {
                                trace!(self.log, "Peer found in process of query"; "peer_id" => format!("{}", enr.peer_id()), "tcp_socket" => enr.tcp_socket());
                            } else {
                            // this is temporary warning for debugging the DHT
                            warn!(self.log, "Found peer during discovery not on correct fork"; "peer_id" => format!("{}", enr.peer_id()), "tcp_socket" => enr.tcp_socket());
                            }
                            */
                        }
                        Discv5Event::SocketUpdated(socket) => {
                            info!(self.log, "Address updated"; "ip" => format!("{}",socket.ip()), "udp_port" => format!("{}", socket.port()));
                            metrics::inc_counter(&metrics::ADDRESS_UPDATE_COUNT);
                            // Discv5 will have updated our local ENR. We save the updated version
                            // to disk.
                            let enr = self.discv5.local_enr();
                            enr::save_enr_to_disk(Path::new(&self.enr_dir), &enr, &self.log);
                            // update  network globals
                            *self.network_globals.local_enr.write() = enr;
                            return Poll::Ready(DiscoveryEvent::SocketUpdated(socket));
                        }
                        _ => {} // Ignore all other discv5 server events
                    }
                }
            }
        }
        Poll::Pending
    }
}
