//! Locally originated routes (the "Loc-RIB" of routes this router announces
//! on its own behalf, as opposed to routes learned from peers).
//!
//! The actor stores one route per prefix together with the attributes chosen
//! by whoever announced it. It does not know about peers or sessions: the
//! server subscribes to [`LocalRibEvent`]s and turns them into UPDATEs.

use std::collections::HashMap;
use std::time::SystemTime;

use bgp_packet::message::{LargeCommunity, OriginPathAttribute};
use bgp_primitives::ip_prefix::IpPrefix;
use serde::{Deserialize, Serialize};
use tokio::sync::{broadcast, mpsc, oneshot};
use tracing::warn;

const EVENT_CHANNEL_CAPACITY: usize = 1024;
const CMD_CHANNEL_CAPACITY: usize = 256;

/// User-selectable attributes for a locally originated route.
#[derive(Debug, Clone, PartialEq, Eq, Serialize, Deserialize)]
pub struct LocalRouteAttrs {
    pub origin: OriginPathAttribute,
    /// Number of *additional* copies of the local ASN to prepend to the
    /// AS_PATH. `0` yields the plain `[local_asn]` path.
    pub prepend: u8,
    pub med: Option<u32>,
    /// RFC 1997 standard communities as `(high, low)` 16-bit halves.
    pub communities: Vec<(u16, u16)>,
    /// RFC 8092 large communities.
    pub large_communities: Vec<LargeCommunity>,
}

impl Default for LocalRouteAttrs {
    fn default() -> Self {
        Self {
            origin: OriginPathAttribute::IGP,
            prepend: 0,
            med: None,
            communities: vec![],
            large_communities: vec![],
        }
    }
}

/// A route originated by this router.
#[derive(Debug, Clone, PartialEq, Eq, Serialize, Deserialize)]
pub struct LocalRoute {
    pub prefix: IpPrefix,
    /// Free-form identity of whoever announced the route (e.g. a student
    /// name). Used to stop one owner replacing or withdrawing another's route.
    pub owner: String,
    pub attrs: LocalRouteAttrs,
    pub announced_at: SystemTime,
}

impl LocalRoute {
    pub fn new(prefix: IpPrefix, owner: impl Into<String>, attrs: LocalRouteAttrs) -> Self {
        Self { prefix, owner: owner.into(), attrs, announced_at: SystemTime::now() }
    }
}

/// Limits on what may be announced. Enforced inside the actor so every caller
/// (HTTP API, tests, future CLI) goes through the same checks.
#[derive(Debug, Clone, Default, PartialEq, Eq)]
pub struct LocalRibPolicy {
    /// A route is accepted only if it is covered by at least one of these
    /// prefixes. Empty means no restriction.
    pub allowed_prefixes: Vec<IpPrefix>,
    /// Maximum number of routes a single owner may hold. `0` means unlimited.
    pub max_routes_per_owner: usize,
}

impl LocalRibPolicy {
    fn allows(&self, prefix: &IpPrefix) -> bool {
        self.allowed_prefixes.is_empty() || self.allowed_prefixes.iter().any(|p| p.contains(prefix))
    }
}

#[derive(Debug, Clone, PartialEq, Eq, thiserror::Error)]
pub enum LocalRibError {
    #[error("prefix {0} is not covered by the allowed prefixes")]
    NotAllowed(IpPrefix),
    #[error("owner {owner:?} already has {limit} routes")]
    OwnerLimit { owner: String, limit: usize },
    #[error("prefix {prefix} is owned by {owner:?}")]
    OwnedByOther { prefix: IpPrefix, owner: String },
    #[error("local RIB actor has stopped")]
    Closed,
}

#[derive(Debug, Clone)]
pub enum LocalRibEvent {
    /// A route was added or its attributes replaced.
    Announced(LocalRoute),
    Withdrawn { prefix: IpPrefix },
}

enum Command {
    Announce { route: LocalRoute, reply: oneshot::Sender<Result<(), LocalRibError>> },
    Withdraw {
        prefix: IpPrefix,
        requester: Option<String>,
        reply: oneshot::Sender<Result<Option<LocalRoute>, LocalRibError>>,
    },
    Snapshot { reply: oneshot::Sender<Vec<LocalRoute>> },
}

/// Handle to the local RIB actor. Cheap to clone; all clones share one table.
#[derive(Clone)]
pub struct LocalRib {
    tx: mpsc::Sender<Command>,
    events: broadcast::Sender<LocalRibEvent>,
}

impl LocalRib {
    pub fn spawn(policy: LocalRibPolicy) -> Self {
        let (tx, rx) = mpsc::channel(CMD_CHANNEL_CAPACITY);
        let (events, _) = broadcast::channel(EVENT_CHANNEL_CAPACITY);
        tokio::spawn(run_actor(rx, events.clone(), policy));
        Self { tx, events }
    }

    /// Adds a route, or replaces the attributes of an existing route with the
    /// same prefix and owner.
    pub async fn announce(&self, route: LocalRoute) -> Result<(), LocalRibError> {
        let (reply, rx) = oneshot::channel();
        self.tx
            .send(Command::Announce { route, reply })
            .await
            .map_err(|_| LocalRibError::Closed)?;
        rx.await.map_err(|_| LocalRibError::Closed)?
    }

    /// Removes a route. `requester` must match the route's owner; pass `None`
    /// to withdraw regardless of owner (administrative override). Returns the
    /// removed route, or `None` if the prefix was not present.
    pub async fn withdraw(
        &self,
        prefix: IpPrefix,
        requester: Option<&str>,
    ) -> Result<Option<LocalRoute>, LocalRibError> {
        let (reply, rx) = oneshot::channel();
        self.tx
            .send(Command::Withdraw { prefix, requester: requester.map(str::to_owned), reply })
            .await
            .map_err(|_| LocalRibError::Closed)?;
        rx.await.map_err(|_| LocalRibError::Closed)?
    }

    /// Point-in-time copy of every route, in no particular order.
    pub async fn snapshot(&self) -> Vec<LocalRoute> {
        let (reply, rx) = oneshot::channel();
        let _ = self.tx.send(Command::Snapshot { reply }).await;
        rx.await.unwrap_or_default()
    }

    /// Subscribe to mutations. See [`crate::AdjRibIn::subscribe`] for lag
    /// semantics.
    pub fn subscribe(&self) -> broadcast::Receiver<LocalRibEvent> {
        self.events.subscribe()
    }
}

async fn run_actor(
    mut rx: mpsc::Receiver<Command>,
    events: broadcast::Sender<LocalRibEvent>,
    policy: LocalRibPolicy,
) {
    let mut table: HashMap<IpPrefix, LocalRoute> = HashMap::new();

    while let Some(cmd) = rx.recv().await {
        match cmd {
            Command::Announce { route, reply } => {
                let result = check_announce(&table, &policy, &route).map(|()| {
                    table.insert(route.prefix.clone(), route.clone());
                    let _ = events.send(LocalRibEvent::Announced(route));
                });
                let _ = reply.send(result);
            }
            Command::Withdraw { prefix, requester, reply } => {
                let result = match (table.get(&prefix), requester) {
                    (Some(existing), Some(req)) if existing.owner != req => {
                        Err(LocalRibError::OwnedByOther {
                            prefix: prefix.clone(),
                            owner: existing.owner.clone(),
                        })
                    }
                    _ => {
                        let removed = table.remove(&prefix);
                        if removed.is_some() {
                            let _ = events.send(LocalRibEvent::Withdrawn { prefix });
                        }
                        Ok(removed)
                    }
                };
                let _ = reply.send(result);
            }
            Command::Snapshot { reply } => {
                if reply.send(table.values().cloned().collect()).is_err() {
                    warn!("Snapshot reply channel closed before send");
                }
            }
        }
    }
}

fn check_announce(
    table: &HashMap<IpPrefix, LocalRoute>,
    policy: &LocalRibPolicy,
    route: &LocalRoute,
) -> Result<(), LocalRibError> {
    if !policy.allows(&route.prefix) {
        return Err(LocalRibError::NotAllowed(route.prefix.clone()));
    }
    if let Some(existing) = table.get(&route.prefix) {
        if existing.owner != route.owner {
            return Err(LocalRibError::OwnedByOther {
                prefix: route.prefix.clone(),
                owner: existing.owner.clone(),
            });
        }
        // Replacing our own route never changes the per-owner count.
        return Ok(());
    }
    if policy.max_routes_per_owner > 0 {
        let held = table.values().filter(|r| r.owner == route.owner).count();
        if held >= policy.max_routes_per_owner {
            return Err(LocalRibError::OwnerLimit {
                owner: route.owner.clone(),
                limit: policy.max_routes_per_owner,
            });
        }
    }
    Ok(())
}

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

    fn prefix(s: &str) -> IpPrefix {
        IpPrefix::try_from(s).unwrap()
    }

    fn route(p: &str, owner: &str) -> LocalRoute {
        LocalRoute::new(prefix(p), owner, LocalRouteAttrs::default())
    }

    #[tokio::test]
    async fn announce_and_withdraw_emit_events() {
        let rib = LocalRib::spawn(LocalRibPolicy::default());
        let mut events = rib.subscribe();

        rib.announce(route("10.0.0.0/24", "alice")).await.unwrap();
        assert!(matches!(events.recv().await.unwrap(), LocalRibEvent::Announced(r) if r.prefix == prefix("10.0.0.0/24")));
        assert_eq!(rib.snapshot().await.len(), 1);

        let removed = rib.withdraw(prefix("10.0.0.0/24"), Some("alice")).await.unwrap();
        assert_eq!(removed.unwrap().owner, "alice");
        assert!(matches!(events.recv().await.unwrap(), LocalRibEvent::Withdrawn { prefix: p } if p == prefix("10.0.0.0/24")));
        assert!(rib.snapshot().await.is_empty());

        // Withdrawing an absent prefix is a no-op, not an error.
        assert_eq!(rib.withdraw(prefix("10.0.0.0/24"), None).await.unwrap(), None);
    }

    #[tokio::test]
    async fn policy_allowlist_is_enforced() {
        let rib = LocalRib::spawn(LocalRibPolicy {
            allowed_prefixes: vec![prefix("10.100.0.0/16")],
            max_routes_per_owner: 0,
        });
        rib.announce(route("10.100.3.0/24", "alice")).await.unwrap();
        let err = rib.announce(route("192.0.2.0/24", "alice")).await.unwrap_err();
        assert_eq!(err, LocalRibError::NotAllowed(prefix("192.0.2.0/24")));
    }

    #[tokio::test]
    async fn owner_limit_ignores_replacements() {
        let rib = LocalRib::spawn(LocalRibPolicy { allowed_prefixes: vec![], max_routes_per_owner: 1 });
        rib.announce(route("10.0.0.0/24", "alice")).await.unwrap();
        // Replacing the same prefix is fine.
        rib.announce(route("10.0.0.0/24", "alice")).await.unwrap();
        let err = rib.announce(route("10.0.1.0/24", "alice")).await.unwrap_err();
        assert!(matches!(err, LocalRibError::OwnerLimit { limit: 1, .. }));
        // Other owners are unaffected.
        rib.announce(route("10.0.1.0/24", "bob")).await.unwrap();
    }

    #[tokio::test]
    async fn other_owner_cannot_replace_or_withdraw() {
        let rib = LocalRib::spawn(LocalRibPolicy::default());
        rib.announce(route("10.0.0.0/24", "alice")).await.unwrap();

        let err = rib.announce(route("10.0.0.0/24", "bob")).await.unwrap_err();
        assert!(matches!(err, LocalRibError::OwnedByOther { .. }));

        let err = rib.withdraw(prefix("10.0.0.0/24"), Some("bob")).await.unwrap_err();
        assert!(matches!(err, LocalRibError::OwnedByOther { .. }));

        // Administrative override works.
        assert!(rib.withdraw(prefix("10.0.0.0/24"), None).await.unwrap().is_some());
    }
}