//! The identity layer: between the node and its transports, 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 may have several
//! transports; a peer is reached on one both have, so nodes on several transports
//! carry the traffic between the others.
//!
//! 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, to another recipient or
//! on another transport, 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 one of the transports 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 addresses
//!   it proved itself at, best first (`Paths::rank`, `Paths::send_first`: the next
//!   when a send fails); with only hints, those are asked WHO, and the send is refused
//!   until one answers (the node retries refused sends). A node proven on some
//!   transports is asked WHO at its hints on the others too, now and then
//!   (`OTHER_PATHS_ASK_INTERVAL`), so that each transport both have becomes a path.
//!
//! Addresses and the transports behind them are the paths layer's
//! (`transport::paths`): the books keep addresses as paths keep them.

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::firewall::Firewall;
use crate::transport::paths::{Gate, PathHandler, Paths, scheme_of};
use crate::transport::stats::{IdentityCounts, IdentityStats, TransportStats};
use crate::transport::traffic::Traffic;
use crate::transport::{
    Destination, 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);
/// A node's addresses on transports it hasn't proven itself on are asked WHO at most
/// this often (`Book::unproven_hints`): over Tor, a WHO is a dial that takes seconds,
/// and an address that doesn't answer shouldn't be pressed.
const OTHER_PATHS_ASK_INTERVAL: Duration = Duration::from_secs(60);
/// 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;

/// A send refused because the peer hasn't proven where it is yet (`send_encoded`):
/// expected when a peer known only from hints is first contacted, and retried.
#[derive(Debug, thiserror::Error)]
#[error("{0} hasn't proven where it is yet; asked where it is said to be")]
pub(crate) struct NotProven(String);

/// Whether `error` is (or was caused by) a [`NotProven`].
pub(crate) fn is_not_proven(error: &anyhow::Error) -> bool {
    error.chain().any(|cause| cause.is::<NotProven>())
}

/// What a node under the identity layer needs (`node::NodeConfig::identity`).
#[derive(Clone)]
pub(crate) struct IdentityConfig {
    pub(crate) key: Arc<NodeKey>,
    /// The address scheme of the node's transport (`NodeConfig::transport`), e.g.
    /// `mycelium`.
    pub(crate) scheme: &'static str,
    /// Further transports, by scheme, after the first in preference.
    pub(crate) more: Vec<(&'static str, Arc<dyn Transport>)>,
}

/// 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 {
    paths: Arc<Paths>,
    /// The debug firewall, by node ID; paths applies it (`Gate`).
    firewall: Option<Arc<Firewall>>,
    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, by address.
#[derive(Default)]
struct Book {
    /// The node that proved itself at an address, and when.
    at: HashMap<String, (String, Instant)>,
    /// The addresses each node last proved itself at, one per scheme.
    proven: HashMap<String, Vec<String>>,
    /// Where others say a node is.
    hints: HashMap<String, Vec<String>>,
    /// When each address 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>>>,
    /// Since start (`stats`).
    counts: IdentityCounts,
}

impl Book {
    /// `node` proved itself at `address`. Returns whether that is news.
    fn bind(&mut self, address: &str, node: &str, now: Instant) -> bool {
        let news = self.at.get(address).is_none_or(|(known, _)| known != node);
        // One address per node and scheme, and one node per address: a node that
        // moved, or an address taken over by another node, leaves no stale entry.
        let scheme = scheme_of(address);
        let addresses = self.proven.entry(node.to_owned()).or_default();
        if let Some(slot) = addresses.iter_mut().find(|at| scheme_of(at) == scheme) {
            if *slot != address {
                let previous = std::mem::replace(slot, address.to_owned());
                self.at.remove(&previous);
                self.counts.address_changes += 1;
            }
        } else {
            addresses.push(address.to_owned());
        }
        if let Some((previous, _)) = self.at.insert(address.to_owned(), (node.to_owned(), now))
            && previous != node
        {
            self.forget_proof(&previous, address);
        }
        if self.at.len() > MAX_ENTRIES
            && let Some(oldest) = self
                .at
                .iter()
                .min_by_key(|(_, (_, at))| *at)
                .map(|(address, _)| address.clone())
            && let Some((node, _)) = self.at.remove(&oldest)
        {
            self.forget_proof(&node, &oldest);
        }
        for waiter in self.waiting.remove(address).into_iter().flatten() {
            let _ = waiter.send(node.to_owned());
        }
        news
    }

    fn forget_proof(&mut self, node: &str, address: &str) {
        if let Some(addresses) = self.proven.get_mut(node) {
            addresses.retain(|at| at != address);
            if addresses.is_empty() {
                self.proven.remove(node);
            }
        }
    }

    /// What the books hold now, and the counts.
    fn stats(&self) -> IdentityStats {
        let mut proven = std::collections::BTreeMap::new();
        for addresses in self.proven.values() {
            for address in addresses {
                *proven.entry(scheme_of(address).to_owned()).or_insert(0) += 1;
            }
        }
        IdentityStats {
            proven,
            hinted: self
                .hints
                .keys()
                .filter(|node| !self.proven.contains_key(*node))
                .count(),
            counts: self.counts,
        }
    }

    /// Where `node` is said to be on transports it hasn't proven itself on yet.
    fn unproven_hints(&self, node: &str) -> Vec<String> {
        let proven = self.proven.get(node);
        self.hints
            .get(node)
            .into_iter()
            .flatten()
            .filter(|hint| {
                proven.is_none_or(|proven| {
                    !proven
                        .iter()
                        .any(|address| scheme_of(address) == scheme_of(hint))
                })
            })
            .cloned()
            .collect()
    }

    /// Whether to ask `address` WHO now (always, if `force`), noting that it was asked.
    fn ask(&mut self, address: &str, now: Instant, force: bool) -> bool {
        let interval = if force { Duration::ZERO } else { ASK_INTERVAL };
        self.ask_every(address, now, interval)
    }

    /// Whether to ask `address` WHO now, if it wasn't within `interval`, noting that it
    /// was asked.
    fn ask_every(&mut self, address: &str, now: Instant, interval: Duration) -> bool {
        if self
            .asked
            .get(address)
            .is_some_and(|at| now.duration_since(*at) < interval)
        {
            return false;
        }
        if self.asked.len() >= MAX_ENTRIES {
            // The longest interval: other paths' asks are kept until they are due again.
            self.asked
                .retain(|_, at| now.duration_since(*at) < OTHER_PATHS_ASK_INTERVAL);
        }
        self.asked.insert(address.to_owned(), now);
        true
    }
}

impl IdentityTransport {
    /// Over `transport`, whose addresses have `config.scheme`, and `config.more`.
    /// `firewall`: the debug firewall, if any, which paths applies by address.
    pub(crate) fn new(
        transport: Arc<dyn Transport>,
        config: IdentityConfig,
        traffic: Arc<Traffic>,
        firewall: Option<Arc<Firewall>>,
    ) -> Self {
        let node_id = config.key.id();
        let shared = Arc::new_cyclic(|shared: &std::sync::Weak<Shared>| {
            let gate: std::sync::Weak<dyn Gate> = shared.clone();
            Shared {
                paths: Arc::new(Paths::new(
                    std::iter::once((config.scheme, transport))
                        .chain(config.more)
                        .collect(),
                    Some(Arc::clone(&traffic)),
                    firewall.is_some().then_some(gate),
                )),
                firewall,
                key: config.key,
                node_id,
                traffic,
                book: Mutex::default(),
            }
        });
        Self { shared }
    }
}

/// The firewall's rules are by node ID: an address is blocked if the node proven at it
/// is. Addresses no node proved itself at aren't (nothing could name them).
impl Gate for Shared {
    fn blocks_inbound(&self, address: &str) -> bool {
        self.blocks(address, Firewall::blocks_inbound)
    }

    fn blocks_outbound(&self, address: &str) -> bool {
        self.blocks(address, Firewall::blocks_outbound)
    }
}

impl Shared {
    fn blocks(&self, address: &str, rule: fn(&Firewall, &str) -> bool) -> bool {
        let Some(firewall) = &self.firewall else {
            return false;
        };
        let node = self.lock().at.get(address).map(|(node, _)| node.clone());
        node.is_some_and(|node| rule(firewall, &node))
    }
}

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

    /// Our proof for `address` (as paths keep it), from our address on its transport.
    fn proof(&self, label: &[u8], address: &str) -> Option<IdentityProof> {
        let message = signed(label, &self.paths.own_address(address)?, address);
        Some(IdentityProof {
            key: self.key.public(),
            signature: self.key.sign(&message),
        })
    }

    /// Notes the node `proof` proves to be at `address` (as paths keep it); None if it
    /// proves nothing.
    fn prove(&self, address: &str, label: &[u8], proof: &IdentityProof) -> Option<String> {
        let message = signed(label, address, &self.paths.own_address(address)?);
        if !identity::verify(&proof.key, &message, &proof.signature) {
            self.lock().counts.proofs_rejected += 1;
            debug!(from = %address, "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 = %address, "Ignoring a proof of our own node key");
            return None;
        }
        if self.lock().bind(address, &node, Instant::now()) {
            debug!(node = %node, address = %address, "Node proved its address");
        }
        Some(node)
    }

    /// Asks `address` WHO, unless it was asked recently (`force`: anyway).
    fn ask(self: &Arc<Self>, address: &str, force: bool) {
        if !self.lock().ask(address, Instant::now(), force) {
            return;
        }
        self.send_who(address);
    }

    /// Asks `node`'s addresses on transports it hasn't proven itself on WHO, now and then
    /// (`OTHER_PATHS_ASK_INTERVAL`): its ME there proves another path to it, which the
    /// paths layer then measures, and fails over to.
    fn ask_other_paths(self: &Arc<Self>, node: &str) {
        let due: Vec<String> = {
            let mut book = self.lock();
            let now = Instant::now();
            book.unproven_hints(node)
                .into_iter()
                .filter(|address| book.ask_every(address, now, OTHER_PATHS_ASK_INTERVAL))
                .collect()
        };
        for address in due {
            self.send_who(&address);
        }
    }

    fn send_who(self: &Arc<Self>, address: &str) {
        let shared = Arc::clone(self);
        let address = address.to_owned();
        tokio::spawn(async move { shared.send_proof(MessageKind::Who, &address).await });
    }

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

    /// A message from `address`.
    async fn receive(
        self: Arc<Self>,
        address: String,
        message: ParsedMessage,
        payload_len: usize,
        handler: &HandlerFn,
    ) -> Result<()> {
        match message {
            ParsedMessage::Who(proof) => {
                self.traffic
                    .received(&address, MessageKind::Who, payload_len);
                if self.prove(&address, WHO_LABEL, &proof).is_some() {
                    let shared = Arc::clone(&self);
                    tokio::spawn(async move { shared.send_proof(MessageKind::Me, &address).await });
                }
                Ok(())
            }
            ParsedMessage::Me(proof) => {
                self.traffic
                    .received(&address, MessageKind::Me, payload_len);
                self.prove(&address, ME_LABEL, &proof);
                Ok(())
            }
            message => {
                let node = self.lock().at.get(&address).map(|(node, _)| node.clone());
                match node {
                    Some(node) => handler(node, message, payload_len).await,
                    None => {
                        self.lock().counts.dropped_unproven += 1;
                        debug!(
                            from = %address,
                            kind = ?message.kind(),
                            "Dropping a message from an address no node proved itself at; asking who is there"
                        );
                        self.ask(&address, false);
                        Ok(())
                    }
                }
            }
        }
    }

    /// Where to send to `peer`: an address it proved itself at, else one it is said to
    /// be at; the most preferred transport's first.
    fn destination(&self, peer: &str) -> Option<String> {
        self.proven_at(peer)
            .into_iter()
            .next()
            .or_else(|| self.hinted_at(peer).into_iter().next())
    }

    /// The addresses `peer` proved itself at, best first (`Paths::rank`).
    fn proven_at(&self, peer: &str) -> Vec<String> {
        let proven = self.lock().proven.get(peer).cloned().unwrap_or_default();
        self.paths.rank(&proven)
    }

    /// Where `peer` is said to be, best first.
    fn hinted_at(&self, peer: &str) -> Vec<String> {
        let hints = self.lock().hints.get(peer).cloned().unwrap_or_default();
        self.paths.rank(&hints)
    }
}

#[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.paths.delivery_limit()
    }

    fn local_addresses(&self) -> Vec<String> {
        self.shared.paths.local_addresses()
    }

    /// Where `peer` proved itself, else where it is said to be, as others can use it.
    fn addresses_of(&self, peer: &str) -> Vec<String> {
        let addresses = {
            let book = self.shared.lock();
            book.proven
                .get(peer)
                .or_else(|| book.hints.get(peer))
                .cloned()
                .unwrap_or_default()
        };
        let mut locations = Vec::new();
        for address in addresses {
            for location in self.shared.paths.locations(&address) {
                if !locations.contains(&location) {
                    locations.push(location);
                }
            }
        }
        locations
    }

    fn learn_addresses(&self, peer: &str, addresses: &[String]) {
        if peer == self.shared.node_id || !identity::is_node_id(peer) {
            return;
        }
        let mut known: Vec<String> = Vec::new();
        for address in addresses {
            if let Some(address) = self.shared.paths.canonical(address)
                && !self.shared.paths.is_own(&address)
                && !known.contains(&address)
            {
                known.push(address);
            }
        }
        known.truncate(MAX_HINTS);
        if known.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(), known);
        }
    }

    fn can_reach(&self, peer: &str) -> bool {
        self.shared.destination(peer).is_some()
    }

    fn transports(&self) -> Vec<String> {
        self.shared.paths.transports()
    }

    /// The address `encode` picks, the transport's connection there, and every path
    /// to `peer` as paths measures it, best first.
    fn destination(&self, peer: &str) -> Option<Destination> {
        let proven = self.shared.proven_at(peer);
        let (address, is_proven) = match proven.first() {
            Some(address) => (address.clone(), true),
            None => (self.shared.hinted_at(peer).into_iter().next()?, false),
        };
        let connection = self.shared.paths.connection(&address);
        Some(Destination {
            address: self.shared.paths.location(&address),
            proven: is_proven,
            connection,
            paths: self.shared.paths.info(&proven),
        })
    }

    /// The paths layer answers them (`Paths::receive`).
    fn answers_pings(&self) -> bool {
        true
    }

    /// The best path's, where messages to `peer` go.
    fn rtt(&self, peer: &str) -> Option<Duration> {
        let address = self.shared.proven_at(peer).into_iter().next()?;
        self.shared.paths.rtt(&address)
    }

    /// The identity layer's, and each transport's.
    fn stats(&self) -> TransportStats {
        let mut stats = self.shared.paths.stats();
        stats.merge(TransportStats {
            identity: Some(self.shared.lock().stats()),
            ..TransportStats::default()
        });
        stats
    }

    fn discovered_seeds(&self) -> Vec<String> {
        self.shared.paths.discovered_seeds()
    }

    /// A node ID resolves to itself if this node knows where it is; anything else is
    /// an address (`<scheme>:...`) or what one of the transports resolves to one (the
    /// first that does), asked WHO.
    async fn resolve_seed(&self, seed: &str) -> Result<String> {
        if identity::is_node_id(seed) {
            if self.shared.destination(seed).is_some() {
                return Ok(seed.to_owned());
            }
            bail!("a node ID alone doesn't say where that node is; give its address");
        }
        let address = self.shared.paths.resolve(seed).await?;
        if self.shared.paths.is_own(&address) {
            bail!("{seed} is this node");
        }
        let receiver = {
            let mut book = self.shared.lock();
            if let Some((node, _)) = book.at.get(&address) {
                return Ok(node.clone());
            }
            let (sender, receiver) = oneshot::channel();
            let waiting = book.waiting.entry(address.clone()).or_default();
            waiting.retain(|waiter| !waiter.is_closed());
            waiting.push(sender);
            receiver
        };
        self.shared.ask(&address, true);
        match tokio::time::timeout(RESOLVE_TIMEOUT, receiver).await {
            Ok(Ok(node)) => {
                info!(seed = %seed, node = %node, "Seed is node");
                Ok(node)
            }
            _ => {
                self.shared.lock().counts.seeds_unanswered += 1;
                Err(anyhow!(
                    "no node answered at {address} within {RESOLVE_TIMEOUT:?}"
                ))
            }
        }
    }

    /// For the address `destination` picks (a send there is refused until `peer`
    /// proved itself there, `send_encoded`).
    fn encode(&self, peer: &str, message: &ParsedMessage) -> Result<EncodedMessage> {
        let Some(address) = self.shared.destination(peer) else {
            bail!("no address known for {peer} on this node's transports");
        };
        let mut encoded = self.shared.paths.encode(&address, message)?;
        encoded.peer = peer.to_owned();
        Ok(encoded)
    }

    /// To the addresses `peer` proved itself at, best first, the next when a send
    /// fails (`Paths::send_first`); refused if it proved itself nowhere yet.
    async fn send_encoded(&self, message: &EncodedMessage) -> Result<()> {
        let proven = self.shared.proven_at(&message.peer);
        if !proven.is_empty() {
            self.shared.ask_other_paths(&message.peer);
        }
        if proven.is_empty() {
            let hints = {
                let mut book = self.shared.lock();
                book.counts.refused_unproven += 1;
                book.hints.get(&message.peer).cloned().unwrap_or_default()
            };
            for address in hints {
                self.shared.ask(&address, false);
            }
            return Err(NotProven(message.peer.clone()).into());
        }
        self.shared
            .paths
            .send_first(message, &proven)
            .await
            .map(|_| ())
    }

    /// Every transport's consumer (`Paths::start_consumer`); returns when the first
    /// one does.
    async fn start_consumer(&self, handler: MessageHandler) -> Result<()> {
        let handler: Arc<HandlerFn> = Arc::from(handler);
        let shared = Arc::clone(&self.shared);
        let receive: Arc<PathHandler> = Arc::new(move |address, message, payload_len| {
            let shared = Arc::clone(&shared);
            let handler = Arc::clone(&handler);
            Box::pin(async move {
                shared
                    .receive(address, message, payload_len, &*handler)
                    .await
            })
        });
        self.shared.paths.start_consumer(receive).await
    }

    /// When the first transport exits.
    async fn wait_for_exit(&self) -> std::io::Result<std::process::ExitStatus> {
        self.shared.paths.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_scheme_and_one_node_per_address() {
        let now = Instant::now();
        let mut book = Book::default();
        assert!(book.bind("s:x", "n1", now));
        assert!(!book.bind("s:x", "n1", now));
        // n1 moved to s:y: s:x is no longer n1's.
        assert!(book.bind("s:y", "n1", now));
        assert!(!book.at.contains_key("s:x"));
        assert_eq!(book.proven["n1"], ["s:y"]);
        // On another transport too.
        assert!(book.bind("t:x", "n1", now));
        assert_eq!(book.proven["n1"], ["s:y", "t:x"]);
        // n2 took over s:y: n1 is proven on t only.
        assert!(book.bind("s:y", "n2", now));
        assert_eq!(book.proven["n1"], ["t:x"]);
        assert_eq!(book.at["s:y"].0, "n2");
        // Only n1's move from s:x to s:y changed an address.
        assert_eq!(book.counts.address_changes, 1);
        let stats = book.stats();
        assert_eq!(stats.proven["s"], 1);
        assert_eq!(stats.proven["t"], 1);
    }

    #[test]
    fn other_paths_are_the_hints_on_transports_not_proven() {
        let now = Instant::now();
        let mut book = Book::default();
        book.hints
            .insert("n1".into(), vec!["s:x".into(), "t:x".into(), "u:x".into()]);
        // Proven nowhere: every hint.
        assert_eq!(book.unproven_hints("n1"), ["s:x", "t:x", "u:x"]);
        // Proven on s (even at another address): the others.
        book.bind("s:y", "n1", now);
        assert_eq!(book.unproven_hints("n1"), ["t:x", "u:x"]);
        assert!(book.ask_every("t:x", now, OTHER_PATHS_ASK_INTERVAL));
        assert!(!book.ask_every("t:x", now + ASK_INTERVAL, OTHER_PATHS_ASK_INTERVAL));
        assert!(book.ask_every(
            "t:x",
            now + OTHER_PATHS_ASK_INTERVAL,
            OTHER_PATHS_ASK_INTERVAL
        ));
        assert!(book.unproven_hints("n2").is_empty());
    }

    #[test]
    fn asking_is_rate_limited() {
        let now = Instant::now();
        let mut book = Book::default();
        assert!(book.ask("s:x", now, false));
        assert!(!book.ask("s:x", now, false));
        assert!(book.ask("s:x", now, true));
        assert!(book.ask("s: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("s:x".into(), vec![sender]);
        book.bind("s:x", "n1", Instant::now());
        assert_eq!(receiver.await.unwrap(), "n1");
    }
}