use std::collections::{hash_map::Entry::Vacant, HashMap, VecDeque};
use std::net::SocketAddr;
use std::sync::atomic::{self, AtomicU64};
use std::sync::{Arc, RwLock};
use std::time::{Duration, Instant};

use arc_swap::ArcSwap;
use async_trait::async_trait;
use log::{debug, info, trace, warn};
use serde::{Deserialize, Serialize};

use tokio::sync::watch;

use sodiumoxide::crypto::hash;

use crate::endpoint::*;
use crate::netapp::*;
use crate::proto::*;
use crate::NodeID;

const CONN_RETRY_INTERVAL: Duration = Duration::from_secs(30);
const CONN_MAX_RETRIES: usize = 10;
const PING_INTERVAL: Duration = Duration::from_secs(10);
const LOOP_DELAY: Duration = Duration::from_secs(1);

// -- Protocol messages --

#[derive(Serialize, Deserialize)]
struct PingMessage {
	pub id: u64,
	pub peer_list_hash: hash::Digest,
}

impl Message for PingMessage {
	type Response = PingMessage;
}

#[derive(Serialize, Deserialize)]
struct PeerListMessage {
	pub list: Vec<(NodeID, SocketAddr)>,
}

impl Message for PeerListMessage {
	type Response = PeerListMessage;
}

// -- Algorithm data structures --

#[derive(Debug)]
struct PeerInfoInternal {
	addr: SocketAddr,
	state: PeerConnState,
	last_seen: Option<Instant>,
	ping: VecDeque<Duration>,
}

#[derive(Copy, Clone, Debug)]
pub struct PeerInfo {
	/// The node's identifier (its public key)
	pub id: NodeID,
	/// The node's network address
	pub addr: SocketAddr,
	/// The current status of our connection to this node
	pub state: PeerConnState,
	/// The last time at which the node was seen
	pub last_seen: Option<Instant>,
	/// The average ping to this node  on recent observations (if at least one ping value is known)
	pub avg_ping: Option<Duration>,
	/// The maximum observed ping to this node on recent observations (if at least one
	/// ping value is known)
	pub max_ping: Option<Duration>,
	/// The median ping to this node on recent observations (if at least one ping value
	/// is known)
	pub med_ping: Option<Duration>,
}

impl PeerInfo {
	/// Returns true if we can currently send requests to this peer
	pub fn is_up(&self) -> bool {
		self.state.is_up()
	}
}

/// PeerConnState: possible states for our tentative connections to given peer
/// This structure is only interested in recording connection info for outgoing
/// TCP connections
#[derive(Copy, Clone, Debug, PartialEq)]
pub enum PeerConnState {
	/// This entry represents ourself (the local node)
	Ourself,

	/// We currently have a connection to this peer
	Connected,

	/// Our next connection tentative (the nth, where n is the first value of the tuple)
	/// will be at given Instant
	Waiting(usize, Instant),

	/// A connection tentative is in progress (the nth, where n is the value stored)
	Trying(usize),

	/// We abandonned trying to connect to this peer (too many failed attempts)
	Abandonned,
}

impl PeerConnState {
	/// Returns true if we can currently send requests to this peer
	pub fn is_up(&self) -> bool {
		matches!(self, Self::Ourself | Self::Connected)
	}
}

struct KnownHosts {
	list: HashMap<NodeID, PeerInfoInternal>,
	hash: hash::Digest,
}

impl KnownHosts {
	fn new() -> Self {
		let list = HashMap::new();
		let hash = Self::calculate_hash(&list);
		Self { list, hash }
	}
	fn update_hash(&mut self) {
		self.hash = Self::calculate_hash(&self.list);
	}
	fn map_into_vec(input: &HashMap<NodeID, PeerInfoInternal>) -> Vec<(NodeID, SocketAddr)> {
		let mut list = Vec::with_capacity(input.len());
		for (id, peer) in input.iter() {
			if peer.state == PeerConnState::Connected || peer.state == PeerConnState::Ourself {
				list.push((*id, peer.addr));
			}
		}
		list
	}
	fn calculate_hash(input: &HashMap<NodeID, PeerInfoInternal>) -> hash::Digest {
		let mut list = Self::map_into_vec(input);
		list.sort();
		let mut hash_state = hash::State::new();
		for (id, addr) in list {
			hash_state.update(&id[..]);
			hash_state.update(&format!("{}\n", addr).into_bytes()[..]);
		}
		hash_state.finalize()
	}
}

/// A "Full Mesh" peering strategy is a peering strategy that tries
/// to establish and maintain a direct connection with all of the
/// known nodes in the network.
pub struct FullMeshPeeringStrategy {
	netapp: Arc<NetApp>,
	known_hosts: RwLock<KnownHosts>,
	public_peer_list: ArcSwap<Vec<PeerInfo>>,

	next_ping_id: AtomicU64,
	ping_endpoint: Arc<Endpoint<PingMessage, Self>>,
	peer_list_endpoint: Arc<Endpoint<PeerListMessage, Self>>,
}

impl FullMeshPeeringStrategy {
	/// Create a new Full Mesh peering strategy.
	/// The strategy will not be run until `.run()` is called and awaited.
	/// Once that happens, the peering strategy will try to connect
	/// to all of the nodes specified in the bootstrap list.
	pub fn new(
		netapp: Arc<NetApp>,
		bootstrap_list: Vec<(NodeID, SocketAddr)>,
		our_addr: Option<SocketAddr>,
	) -> Arc<Self> {
		let mut known_hosts = KnownHosts::new();
		for (id, addr) in bootstrap_list {
			if id != netapp.id {
				known_hosts.list.insert(
					id,
					PeerInfoInternal {
						addr,
						state: PeerConnState::Waiting(0, Instant::now()),
						last_seen: None,
						ping: VecDeque::new(),
					},
				);
			}
		}

		if let Some(addr) = our_addr {
			known_hosts.list.insert(
				netapp.id,
				PeerInfoInternal {
					addr,
					state: PeerConnState::Ourself,
					last_seen: None,
					ping: VecDeque::new(),
				},
			);
		}

		let strat = Arc::new(Self {
			netapp: netapp.clone(),
			known_hosts: RwLock::new(known_hosts),
			public_peer_list: ArcSwap::new(Arc::new(Vec::new())),
			next_ping_id: AtomicU64::new(42),
			ping_endpoint: netapp.endpoint("__netapp/peering/fullmesh.rs/Ping".into()),
			peer_list_endpoint: netapp.endpoint("__netapp/peering/fullmesh.rs/PeerList".into()),
		});

		strat.update_public_peer_list(&strat.known_hosts.read().unwrap());

		strat.ping_endpoint.set_handler(strat.clone());
		strat.peer_list_endpoint.set_handler(strat.clone());

		let strat2 = strat.clone();
		netapp.on_connected(move |id: NodeID, addr: SocketAddr, is_incoming: bool| {
			let strat2 = strat2.clone();
			strat2.on_connected(id, addr, is_incoming);
		});

		let strat2 = strat.clone();
		netapp.on_disconnected(move |id: NodeID, is_incoming: bool| {
			let strat2 = strat2.clone();
			strat2.on_disconnected(id, is_incoming);
		});

		strat
	}

	/// Run the full mesh peering strategy.
	/// This future exits when the `must_exit` watch becomes true.
	pub async fn run(self: Arc<Self>, must_exit: watch::Receiver<bool>) {
		while !*must_exit.borrow() {
			// 1. Read current state: get list of connected peers (ping them)
			let (to_ping, to_retry) = {
				let known_hosts = self.known_hosts.read().unwrap();
				trace!("known_hosts: {} peers", known_hosts.list.len());

				let mut to_ping = vec![];
				let mut to_retry = vec![];
				for (id, info) in known_hosts.list.iter() {
					trace!("{}, {:?}", hex::encode(&id[..8]), info);
					match info.state {
						PeerConnState::Connected => {
							let must_ping = match info.last_seen {
								None => true,
								Some(t) => Instant::now() - t > PING_INTERVAL,
							};
							if must_ping {
								to_ping.push(*id);
							}
						}
						PeerConnState::Waiting(_, t) => {
							if Instant::now() >= t {
								to_retry.push(*id);
							}
						}
						_ => (),
					}
				}
				(to_ping, to_retry)
			};

			// 2. Dispatch ping to hosts
			trace!("to_ping: {} peers", to_retry.len());
			for id in to_ping {
				tokio::spawn(self.clone().ping(id));
			}

			// 3. Try reconnects
			trace!("to_retry: {} peers", to_retry.len());
			if !to_retry.is_empty() {
				let mut known_hosts = self.known_hosts.write().unwrap();
				for id in to_retry {
					if let Some(h) = known_hosts.list.get_mut(&id) {
						if let PeerConnState::Waiting(i, _) = h.state {
							info!(
								"Retrying connection to {} at {} ({})",
								hex::encode(&id[..8]),
								h.addr,
								i + 1
							);
							h.state = PeerConnState::Trying(i);
							tokio::spawn(self.clone().try_connect(id, h.addr));
						}
					}
				}
				self.update_public_peer_list(&known_hosts);
			}

			// 4. Sleep before next loop iteration
			tokio::time::sleep(LOOP_DELAY).await;
		}
	}

	/// Returns a list of currently known peers in the network.
	pub fn get_peer_list(&self) -> Arc<Vec<PeerInfo>> {
		self.public_peer_list.load_full()
	}

	// -- internal stuff --

	fn update_public_peer_list(&self, known_hosts: &KnownHosts) {
		let mut pub_peer_list = Vec::with_capacity(known_hosts.list.len());
		for (id, info) in known_hosts.list.iter() {
			let mut pings = info.ping.iter().cloned().collect::<Vec<_>>();
			pings.sort();
			if !pings.is_empty() {
				pub_peer_list.push(PeerInfo {
					id: *id,
					addr: info.addr,
					state: info.state,
					last_seen: info.last_seen,
					avg_ping: Some(
						pings
							.iter()
							.fold(Duration::from_secs(0), |x, y| x + *y)
							.div_f64(pings.len() as f64),
					),
					max_ping: pings.last().cloned(),
					med_ping: Some(pings[pings.len() / 2]),
				});
			} else {
				pub_peer_list.push(PeerInfo {
					id: *id,
					addr: info.addr,
					state: info.state,
					last_seen: info.last_seen,
					avg_ping: None,
					max_ping: None,
					med_ping: None,
				});
			}
		}
		self.public_peer_list.store(Arc::new(pub_peer_list));
	}

	async fn ping(self: Arc<Self>, id: NodeID) {
		let peer_list_hash = self.known_hosts.read().unwrap().hash;
		let ping_id = self.next_ping_id.fetch_add(1u64, atomic::Ordering::Relaxed);
		let ping_time = Instant::now();
		let ping_msg = PingMessage {
			id: ping_id,
			peer_list_hash,
		};

		debug!(
			"Sending ping {} to {} at {:?}",
			ping_id,
			hex::encode(&id[..8]),
			ping_time
		);
		match self.ping_endpoint.call(&id, &ping_msg, PRIO_HIGH).await {
			Err(e) => warn!("Error pinging {}: {}", hex::encode(&id[..8]), e),
			Ok(ping_resp) => {
				let resp_time = Instant::now();
				debug!(
					"Got ping response from {} at {:?}",
					hex::encode(&id[..8]),
					resp_time
				);
				{
					let mut known_hosts = self.known_hosts.write().unwrap();
					if let Some(host) = known_hosts.list.get_mut(&id) {
						host.last_seen = Some(resp_time);
						host.ping.push_back(resp_time - ping_time);
						while host.ping.len() > 10 {
							host.ping.pop_front();
						}
						self.update_public_peer_list(&known_hosts);
					}
				}
				if ping_resp.peer_list_hash != peer_list_hash {
					self.exchange_peers(&id).await;
				}
			}
		}
	}

	async fn exchange_peers(self: Arc<Self>, id: &NodeID) {
		let peer_list = KnownHosts::map_into_vec(&self.known_hosts.read().unwrap().list);
		let pex_message = PeerListMessage { list: peer_list };
		match self
			.peer_list_endpoint
			.call(id, &pex_message, PRIO_BACKGROUND)
			.await
		{
			Err(e) => warn!("Error doing peer exchange: {}", e),
			Ok(resp) => {
				self.handle_peer_list(&resp.list[..]);
			}
		}
	}

	fn handle_peer_list(&self, list: &[(NodeID, SocketAddr)]) {
		let mut known_hosts = self.known_hosts.write().unwrap();

		let mut changed = false;
		for (id, addr) in list.iter() {
			if !known_hosts.list.contains_key(id) {
				known_hosts.list.insert(*id, self.new_peer(id, *addr));
				changed = true;
			}
		}

		if changed {
			known_hosts.update_hash();
			self.update_public_peer_list(&known_hosts);
		}
	}

	async fn try_connect(self: Arc<Self>, id: NodeID, addr: SocketAddr) {
		let conn_result = self.netapp.clone().try_connect(addr, id).await;
		if let Err(e) = conn_result {
			warn!("Error connecting to {}: {}", hex::encode(&id[..8]), e);
			let mut known_hosts = self.known_hosts.write().unwrap();
			if let Some(host) = known_hosts.list.get_mut(&id) {
				host.state = match host.state {
					PeerConnState::Trying(i) => {
						if i >= CONN_MAX_RETRIES {
							PeerConnState::Abandonned
						} else {
							PeerConnState::Waiting(i + 1, Instant::now() + CONN_RETRY_INTERVAL)
						}
					}
					_ => PeerConnState::Waiting(0, Instant::now() + CONN_RETRY_INTERVAL),
				};
				self.update_public_peer_list(&known_hosts);
			}
		} else {
			self.on_connected(id, addr, false);
		}
	}

	fn on_connected(self: Arc<Self>, id: NodeID, addr: SocketAddr, is_incoming: bool) {
		if is_incoming {
			let mut known_hosts = self.known_hosts.write().unwrap();
			if let Vacant(entry) = known_hosts.list.entry(id) {
				entry.insert(self.new_peer(&id, addr));
				known_hosts.update_hash();
				self.update_public_peer_list(&known_hosts);
			}
		} else {
			info!(
				"Successfully connected to {} at {}",
				hex::encode(&id[..8]),
				addr
			);
			let mut known_hosts = self.known_hosts.write().unwrap();
			if let Some(host) = known_hosts.list.get_mut(&id) {
				host.state = PeerConnState::Connected;
				host.addr = addr;
			} else {
				known_hosts.list.insert(
					id,
					PeerInfoInternal {
						state: PeerConnState::Connected,
						addr,
						last_seen: None,
						ping: VecDeque::new(),
					},
				);
			}
			known_hosts.update_hash();
			self.update_public_peer_list(&known_hosts);
		}
	}

	fn on_disconnected(self: Arc<Self>, id: NodeID, is_incoming: bool) {
		if !is_incoming {
			info!("Connection to {} was closed", hex::encode(&id[..8]));
			let mut known_hosts = self.known_hosts.write().unwrap();
			if let Some(host) = known_hosts.list.get_mut(&id) {
				host.state = PeerConnState::Waiting(0, Instant::now());
				known_hosts.update_hash();
				self.update_public_peer_list(&known_hosts);
			}
		}
	}

	fn new_peer(&self, id: &NodeID, addr: SocketAddr) -> PeerInfoInternal {
		let state = if *id == self.netapp.id {
			PeerConnState::Ourself
		} else {
			PeerConnState::Waiting(0, Instant::now())
		};
		PeerInfoInternal {
			addr,
			state,
			last_seen: None,
			ping: VecDeque::new(),
		}
	}
}

#[async_trait]
impl EndpointHandler<PingMessage> for FullMeshPeeringStrategy {
	async fn handle(self: &Arc<Self>, ping: &PingMessage, from: NodeID) -> PingMessage {
		let ping_resp = PingMessage {
			id: ping.id,
			peer_list_hash: self.known_hosts.read().unwrap().hash,
		};
		debug!("Ping from {}", hex::encode(&from[..8]));
		ping_resp
	}
}

#[async_trait]
impl EndpointHandler<PeerListMessage> for FullMeshPeeringStrategy {
	async fn handle(
		self: &Arc<Self>,
		peer_list: &PeerListMessage,
		_from: NodeID,
	) -> PeerListMessage {
		self.handle_peer_list(&peer_list.list[..]);
		let peer_list = KnownHosts::map_into_vec(&self.known_hosts.read().unwrap().list);
		PeerListMessage { list: peer_list }
	}
}