//! The identity layer: between the node and a transport whose identities are addresses
//! (Mycelium's public keys), so that the rest of the node deals in node IDs
//! (`crate::identity`) only, whatever transport reaches a peer.
//!
//! A node proves who it is to an address with WHO (which asks back) or ME (the answer):
//! its public node key, and a signature over its own address and the recipient's
//! (`signed`). The transport authenticates the sender's address, so a proof is good for
//! that pair of addresses only: replayed from another address, or to another recipient,
//! it fails. Once a node proved itself at an address, messages from there are its.
//!
//! * A message from an address no node proved itself at is dropped, and the address is
//!   asked WHO (at most every `ASK_INTERVAL`): after a restart, the first message from
//!   a peer brings its proof back, and the peer's node retries what was lost.
//! * A seed (`resolve_seed`) is an address, or something the transport resolves to one;
//!   it is asked WHO, and its ME says which node is there.
//! * Where other nodes are comes from the member list and HELLO_REPLYs
//!   (`learn_addresses`): hints, not proofs. A message to a node goes to the address it
//!   proved itself at; with only hints, those are asked WHO, and the send is refused
//!   until one answers (the node retries refused sends).
//!
//! Addresses are `<scheme>:<transport identity>`, e.g. `mycelium:<public key>`.

use std::collections::HashMap;
use std::sync::{Arc, Mutex, MutexGuard};
use std::time::Duration;

use anyhow::{Result, anyhow, bail};
use async_trait::async_trait;
use tokio::sync::oneshot;
use tokio::time::Instant;
use tracing::{debug, info};

use crate::identity::{self, NodeKey};
use crate::locks::MutexExt;
use crate::transport::traffic::Traffic;
use crate::transport::{
    EncodedMessage, HandlerFn, IdentityProof, MessageHandler, MessageKind, ParsedMessage, Transport,
};

pub(crate) const WHO_LABEL: &[u8] = b"continuum/who/1";
pub(crate) const ME_LABEL: &[u8] = b"continuum/me/1";
/// An address is asked WHO at most this often, unless a seed is being resolved.
const ASK_INTERVAL: Duration = Duration::from_secs(5);
/// How long resolving a seed waits for its ME.
const RESOLVE_TIMEOUT: Duration = Duration::from_secs(10);
/// Addresses kept per node from hints: about one per transport.
const MAX_HINTS: usize = 8;
/// Addresses with a proven node, nodes with hints, and addresses asked, each at most:
/// anyone can send us proofs (keys are free), so the books are bounded.
const MAX_ENTRIES: usize = 4096;

/// What a node under the identity layer needs (`node::NodeConfig::identity`).
#[derive(Clone)]
pub(crate) struct IdentityConfig {
    pub(crate) key: Arc<NodeKey>,
    /// The transport's address scheme, e.g. `mycelium`.
    pub(crate) scheme: &'static str,
}

/// The bytes a WHO (`label`: `WHO_LABEL`) or ME proof signs: the label, then the
/// sender's and the recipient's address, each with its length.
pub(crate) fn signed(label: &[u8], from: &str, to: &str) -> Vec<u8> {
    let mut bytes = label.to_vec();
    for address in [from, to] {
        bytes.extend_from_slice(&(address.len() as u32).to_be_bytes());
        bytes.extend_from_slice(address.as_bytes());
    }
    bytes
}

pub(crate) struct IdentityTransport {
    shared: Arc<Shared>,
}

struct Shared {
    inner: Arc<dyn Transport>,
    scheme: &'static str,
    key: Arc<NodeKey>,
    node_id: String,
    /// WHO and ME are counted here, under the address (`traffic`).
    traffic: Arc<Traffic>,
    // A std mutex: every access is short and never spans an await.
    book: Mutex<Book>,
}

/// Who is where. Keys are transport identities (an address without its scheme).
#[derive(Default)]
struct Book {
    /// The node that proved itself at a transport identity, and when.
    at: HashMap<String, (String, Instant)>,
    /// The transport identity each node last proved itself at.
    proven: HashMap<String, String>,
    /// Where others say a node is.
    hints: HashMap<String, Vec<String>>,
    /// When each transport identity was last asked WHO.
    asked: HashMap<String, Instant>,
    /// Seeds being resolved: who waits for a ME from where.
    waiting: HashMap<String, Vec<oneshot::Sender<String>>>,
}

impl Book {
    /// `node` proved itself at `target`. Returns whether that is news.
    fn bind(&mut self, target: &str, node: &str, now: Instant) -> bool {
        let news = self.at.get(target).is_none_or(|(known, _)| known != node);
        // One address per node and one node per address: a node that moved, or an
        // address taken over by another node, leaves no stale entry behind.
        if let Some(previous) = self.proven.insert(node.to_owned(), target.to_owned())
            && previous != target
        {
            self.at.remove(&previous);
        }
        if let Some((previous, _)) = self.at.insert(target.to_owned(), (node.to_owned(), now))
            && previous != node
            && self.proven.get(&previous).is_some_and(|at| at == target)
        {
            self.proven.remove(&previous);
        }
        if self.at.len() > MAX_ENTRIES
            && let Some(oldest) = self
                .at
                .iter()
                .min_by_key(|(_, (_, at))| *at)
                .map(|(target, _)| target.clone())
            && let Some((node, _)) = self.at.remove(&oldest)
        {
            self.proven.remove(&node);
        }
        for waiter in self.waiting.remove(target).into_iter().flatten() {
            let _ = waiter.send(node.to_owned());
        }
        news
    }

    /// Whether to ask `target` WHO now (always, if `force`), noting that it was asked.
    fn ask(&mut self, target: &str, now: Instant, force: bool) -> bool {
        if !force
            && self
                .asked
                .get(target)
                .is_some_and(|at| now.duration_since(*at) < ASK_INTERVAL)
        {
            return false;
        }
        if self.asked.len() >= MAX_ENTRIES {
            self.asked
                .retain(|_, at| now.duration_since(*at) < ASK_INTERVAL);
        }
        self.asked.insert(target.to_owned(), now);
        true
    }
}

impl IdentityTransport {
    pub(crate) fn new(
        inner: Arc<dyn Transport>,
        config: IdentityConfig,
        traffic: Arc<Traffic>,
    ) -> Self {
        let node_id = config.key.id();
        Self {
            shared: Arc::new(Shared {
                inner,
                scheme: config.scheme,
                key: config.key,
                node_id,
                traffic,
                book: Mutex::default(),
            }),
        }
    }
}

impl Shared {
    fn lock(&self) -> MutexGuard<'_, Book> {
        self.book.lock_or_recover()
    }

    fn address(&self, target: &str) -> String {
        format!("{}:{target}", self.scheme)
    }

    /// The transport identity in `address`, if it is one of this transport's.
    fn target_of<'a>(&self, address: &'a str) -> Option<&'a str> {
        address
            .strip_prefix(self.scheme)
            .and_then(|rest| rest.strip_prefix(':'))
            .filter(|target| self.inner.is_peer_id(target))
    }

    fn proof(&self, label: &[u8], target: &str) -> IdentityProof {
        let message = signed(
            label,
            &self.address(self.inner.local_id()),
            &self.address(target),
        );
        IdentityProof {
            key: self.key.public(),
            signature: self.key.sign(&message),
        }
    }

    /// Notes the node `proof` proves to be at `sender`; None if it proves nothing.
    fn prove(&self, sender: &str, label: &[u8], proof: &IdentityProof) -> Option<String> {
        let message = signed(
            label,
            &self.address(sender),
            &self.address(self.inner.local_id()),
        );
        if !identity::verify(&proof.key, &message, &proof.signature) {
            debug!(from = %self.address(sender), "Ignoring an identity proof that doesn't hold");
            return None;
        }
        let node = identity::node_id_of(&proof.key);
        if node == self.node_id {
            debug!(from = %self.address(sender), "Ignoring a proof of our own node key");
            return None;
        }
        if self.lock().bind(sender, &node, Instant::now()) {
            debug!(node = %node, address = %self.address(sender), "Node proved its address");
        }
        Some(node)
    }

    /// Asks `target` WHO, unless it was asked recently (`force`: anyway).
    fn ask(self: &Arc<Self>, target: &str, force: bool) {
        if !self.lock().ask(target, Instant::now(), force) {
            return;
        }
        let shared = Arc::clone(self);
        let target = target.to_owned();
        tokio::spawn(async move { shared.send_proof(MessageKind::Who, &target).await });
    }

    /// Sends a WHO or ME (`kind`) to `target`.
    async fn send_proof(&self, kind: MessageKind, target: &str) {
        let message = if kind == MessageKind::Who {
            ParsedMessage::Who(self.proof(WHO_LABEL, target))
        } else {
            ParsedMessage::Me(self.proof(ME_LABEL, target))
        };
        let encoded = match self.inner.encode(target, &message) {
            Ok(encoded) => encoded,
            Err(error) => {
                debug!(to = %target, error = %format!("{error:#}"), "Couldn't encode an identity proof");
                return;
            }
        };
        let address = self.address(target);
        match self.inner.send_encoded(&encoded).await {
            Ok(()) => self
                .traffic
                .sent(&address, encoded.kind, encoded.payload_len),
            Err(error) => {
                self.traffic
                    .refused(&address, encoded.kind, encoded.payload_len);
                debug!(to = %address, error = %format!("{error:#}"), "Couldn't send an identity proof");
            }
        }
    }

    async fn receive(
        self: Arc<Self>,
        sender: String,
        message: ParsedMessage,
        payload_len: usize,
        handler: &HandlerFn,
    ) -> Result<()> {
        match message {
            ParsedMessage::Who(proof) => {
                self.traffic
                    .received(&self.address(&sender), MessageKind::Who, payload_len);
                if self.prove(&sender, WHO_LABEL, &proof).is_some() {
                    let shared = Arc::clone(&self);
                    tokio::spawn(async move { shared.send_proof(MessageKind::Me, &sender).await });
                }
                Ok(())
            }
            ParsedMessage::Me(proof) => {
                self.traffic
                    .received(&self.address(&sender), MessageKind::Me, payload_len);
                self.prove(&sender, ME_LABEL, &proof);
                Ok(())
            }
            message => {
                let node = self.lock().at.get(&sender).map(|(node, _)| node.clone());
                match node {
                    Some(node) => handler(node, message, payload_len).await,
                    None => {
                        debug!(
                            from = %self.address(&sender),
                            kind = ?message.kind(),
                            "Dropping a message from an address no node proved itself at; asking who is there"
                        );
                        self.ask(&sender, false);
                        Ok(())
                    }
                }
            }
        }
    }
}

#[async_trait]
impl Transport for IdentityTransport {
    fn local_id(&self) -> &str {
        &self.shared.node_id
    }

    fn is_peer_id(&self, id: &str) -> bool {
        identity::is_node_id(id)
    }

    fn delivery_limit(&self) -> Duration {
        self.shared.inner.delivery_limit()
    }

    fn local_addresses(&self) -> Vec<String> {
        vec![self.shared.address(self.shared.inner.local_id())]
    }

    fn addresses_of(&self, peer: &str) -> Vec<String> {
        let book = self.shared.lock();
        match book.proven.get(peer) {
            Some(target) => vec![self.shared.address(target)],
            None => book
                .hints
                .get(peer)
                .into_iter()
                .flatten()
                .map(|target| self.shared.address(target))
                .collect(),
        }
    }

    fn learn_addresses(&self, peer: &str, addresses: &[String]) {
        if peer == self.shared.node_id || !identity::is_node_id(peer) {
            return;
        }
        let local = self.shared.inner.local_id();
        let mut targets: Vec<String> = addresses
            .iter()
            .filter_map(|address| self.shared.target_of(address))
            .filter(|target| *target != local)
            .map(str::to_owned)
            .collect();
        targets.dedup();
        targets.truncate(MAX_HINTS);
        if targets.is_empty() {
            return;
        }
        let mut book = self.shared.lock();
        if book.hints.len() < MAX_ENTRIES || book.hints.contains_key(peer) {
            book.hints.insert(peer.to_owned(), targets);
        }
    }

    /// A node ID resolves to itself if this node knows where it is; anything else is
    /// an address (`<scheme>:...`) or what the transport resolves to one, asked WHO.
    async fn resolve_seed(&self, seed: &str) -> Result<String> {
        if identity::is_node_id(seed) {
            let book = self.shared.lock();
            if book.proven.contains_key(seed) || book.hints.contains_key(seed) {
                return Ok(seed.to_owned());
            }
            bail!("a node ID alone doesn't say where that node is; give its address");
        }
        let target = match self.shared.target_of(seed) {
            Some(target) => target.to_owned(),
            None => self.shared.inner.resolve_seed(seed).await?,
        };
        if target == self.shared.inner.local_id() {
            bail!("{seed} is this node");
        }
        let receiver = {
            let mut book = self.shared.lock();
            if let Some((node, _)) = book.at.get(&target) {
                return Ok(node.clone());
            }
            let (sender, receiver) = oneshot::channel();
            let waiting = book.waiting.entry(target.clone()).or_default();
            waiting.retain(|waiter| !waiter.is_closed());
            waiting.push(sender);
            receiver
        };
        self.shared.ask(&target, true);
        match tokio::time::timeout(RESOLVE_TIMEOUT, receiver).await {
            Ok(Ok(node)) => {
                info!(seed = %seed, node = %node, "Seed is node");
                Ok(node)
            }
            _ => Err(anyhow!(
                "no node answered at {} within {RESOLVE_TIMEOUT:?}",
                self.shared.address(&target)
            )),
        }
    }

    /// For the address `peer` proved itself at, or else one it is said to be at (a send
    /// there is refused until it proves itself, `send_encoded`).
    fn encode(&self, peer: &str, message: &ParsedMessage) -> Result<EncodedMessage> {
        let target = {
            let book = self.shared.lock();
            book.proven
                .get(peer)
                .or_else(|| book.hints.get(peer).and_then(|hints| hints.first()))
                .cloned()
        };
        let Some(target) = target else {
            bail!("no address known for {peer}");
        };
        let mut encoded = self.shared.inner.encode(&target, message)?;
        encoded.peer = peer.to_owned();
        Ok(encoded)
    }

    async fn send_encoded(&self, message: &EncodedMessage) -> Result<()> {
        let hints = {
            let book = self.shared.lock();
            if book.proven.get(&message.peer) == Some(&message.address) {
                None
            } else {
                Some(book.hints.get(&message.peer).cloned().unwrap_or_default())
            }
        };
        if let Some(hints) = hints {
            for target in hints {
                self.shared.ask(&target, false);
            }
            bail!(
                "{} hasn't proven where it is yet; asked where it is said to be",
                message.peer
            );
        }
        let mut inner = message.clone();
        inner.peer = message.address.clone();
        self.shared.inner.send_encoded(&inner).await
    }

    async fn start_consumer(&self, handler: MessageHandler) -> Result<()> {
        let handler: Arc<HandlerFn> = Arc::from(handler);
        let shared = Arc::clone(&self.shared);
        self.shared
            .inner
            .start_consumer(Box::new(move |sender, message, payload_len| {
                let shared = Arc::clone(&shared);
                let handler = Arc::clone(&handler);
                Box::pin(async move {
                    shared
                        .receive(sender, message, payload_len, &*handler)
                        .await
                })
            }))
            .await
    }

    async fn wait_for_exit(&self) -> std::io::Result<std::process::ExitStatus> {
        self.shared.inner.wait_for_exit().await
    }
}

#[cfg(test)]
mod tests {
    use super::*;

    #[test]
    fn signed_bytes_say_both_addresses() {
        // A length before each address: "a:bc" to "d" isn't "a:b" to "cd".
        assert_ne!(
            signed(WHO_LABEL, "a:bc", "d"),
            signed(WHO_LABEL, "a:b", "cd")
        );
        assert_ne!(signed(WHO_LABEL, "a", "b"), signed(ME_LABEL, "a", "b"));
        assert_ne!(signed(WHO_LABEL, "a", "b"), signed(WHO_LABEL, "b", "a"));
    }

    #[test]
    fn one_address_per_node_and_one_node_per_address() {
        let now = Instant::now();
        let mut book = Book::default();
        assert!(book.bind("x", "n1", now));
        assert!(!book.bind("x", "n1", now));
        // n1 moved to y: x is no longer n1's.
        assert!(book.bind("y", "n1", now));
        assert!(!book.at.contains_key("x"));
        assert_eq!(book.proven["n1"], "y");
        // n2 took over y: n1 is nowhere proven.
        assert!(book.bind("y", "n2", now));
        assert!(!book.proven.contains_key("n1"));
        assert_eq!(book.at["y"].0, "n2");
    }

    #[test]
    fn asking_is_rate_limited() {
        let now = Instant::now();
        let mut book = Book::default();
        assert!(book.ask("x", now, false));
        assert!(!book.ask("x", now, false));
        assert!(book.ask("x", now, true));
        assert!(book.ask("x", now + ASK_INTERVAL, false));
    }

    #[tokio::test]
    async fn a_waiting_seed_learns_the_node() {
        let mut book = Book::default();
        let (sender, receiver) = oneshot::channel();
        book.waiting.insert("x".into(), vec![sender]);
        book.bind("x", "n1", Instant::now());
        assert_eq!(receiver.await.unwrap(), "n1");
    }
}