//! Peer mesh discovery — supplementary CRDT sync connections between build agents.
//!
//! When mesh discovery is enabled, a build agent periodically reads the CRDT
//! `nodes` list and opens supplementary sync connections to alive peers.  These
//! mesh connections provide resilience: if the primary rendezvous server goes
//! down, agents can still exchange ops directly with each other.
//!
//! Op deduplication is handled by the CRDT layer's existing `AlreadySeen`
//! semantics — when the same op arrives via both the primary and a mesh path,
//! only the first application takes effect.

use std::collections::HashMap;
use std::sync::Arc;
use tokio::sync::Mutex;
use tokio::task::JoinHandle;

use crate::crdt_state;
use crate::crdt_sync;
use crate::slog;

/// Interval between mesh discovery scans (seconds).
const MESH_SCAN_INTERVAL_SECS: u64 = 60;

/// Tracks active mesh peer connections and enforces the connection cap.
///
/// Each mesh connection is a background tokio task running `connect_and_sync`.
/// The manager periodically reconciles the set of active connections against
/// the CRDT node presence list: new alive peers are connected, disappeared
/// or dead peers are dropped.
pub struct MeshManager {
    /// Active mesh connections: peer_node_id -> JoinHandle.
    pub(crate) connections: HashMap<String, JoinHandle<()>>,
    /// Maximum number of supplementary mesh connections.
    max_peers: usize,
    /// This node's ID (hex-encoded Ed25519 pubkey).
    our_node_id: String,
    /// The primary rendezvous URL — retained for diagnostics/logging.
    _rendezvous_url: String,
    /// Join token for authenticating with peers.
    join_token: Option<String>,
}

impl MeshManager {
    /// Create a new mesh manager.
    pub fn new(
        max_peers: usize,
        our_node_id: String,
        rendezvous_url: String,
        join_token: Option<String>,
    ) -> Self {
        Self {
            connections: HashMap::new(),
            max_peers,
            our_node_id,
            _rendezvous_url: rendezvous_url,
            join_token,
        }
    }

    /// Run one reconciliation pass: drop dead peers, connect to new alive ones.
    ///
    /// Returns the number of active mesh connections after reconciliation.
    pub fn reconcile(&mut self) -> usize {
        // Read alive peers from the CRDT.
        let nodes = crdt_state::read_all_node_presence().unwrap_or_default();
        let now = chrono::Utc::now().timestamp() as f64;

        // Build the set of eligible peers (alive, fresh heartbeat, has address,
        // not self, not the rendezvous host).
        let alive_peers: Vec<(String, String)> = nodes
            .into_iter()
            .filter(|n| {
                let now_ms = now * 1000.0;
                let last_ms = n.last_seen_ms.unwrap_or(n.last_seen * 1000.0);
                n.alive
                    && !n.address.is_empty()
                    && n.node_id != self.our_node_id
                    && (now_ms - last_ms) / 1000.0 < 600.0
                    && !self.is_rendezvous_peer(&n.address)
            })
            .map(|n| (n.node_id, n.address))
            .collect();

        // Drop connections to peers that are no longer alive or have disappeared.
        let alive_ids: std::collections::HashSet<&str> =
            alive_peers.iter().map(|(id, _)| id.as_str()).collect();

        let stale: Vec<String> = self
            .connections
            .keys()
            .filter(|id| !alive_ids.contains(id.as_str()))
            .cloned()
            .collect();

        for id in stale {
            if let Some(handle) = self.connections.remove(&id) {
                slog!(
                    "[mesh] Dropping connection to disappeared peer {:.12}…",
                    &id
                );
                handle.abort();
            }
        }

        // Also drop connections whose tasks have finished (peer disconnected).
        let finished: Vec<String> = self
            .connections
            .iter()
            .filter(|(_, h)| h.is_finished())
            .map(|(id, _)| id.clone())
            .collect();

        for id in finished {
            self.connections.remove(&id);
            slog!("[mesh] Mesh connection to {:.12}… ended; slot freed", &id);
        }

        // Connect to new peers up to the cap.
        for (node_id, address) in &alive_peers {
            if self.connections.len() >= self.max_peers {
                break;
            }
            if self.connections.contains_key(node_id) {
                continue;
            }

            slog!(
                "[mesh] Opening supplementary mesh connection to {:.12}… at {address}",
                node_id
            );
            let url = address.clone();
            let token = self.join_token.clone();
            let peer_id = node_id.clone();
            let handle = tokio::spawn(async move {
                match crdt_sync::connect_and_sync(&url, token.as_deref()).await {
                    Ok(()) => {
                        slog!(
                            "[mesh] Supplementary connection to {:.12}… closed cleanly",
                            &peer_id
                        );
                    }
                    Err(e) => {
                        slog!(
                            "[mesh] Supplementary connection to {:.12}… failed: {e}",
                            &peer_id
                        );
                    }
                }
            });
            self.connections.insert(node_id.clone(), handle);
        }

        self.connections.len()
    }

    /// Check whether an address belongs to the primary rendezvous peer.
    ///
    /// Compares by stripping the `ws://` scheme and comparing host:port,
    /// since the CRDT address may use `0.0.0.0` while the rendezvous URL
    /// uses the actual hostname.  We compare the path-suffix `/crdt-sync`
    /// after the port.
    fn is_rendezvous_peer(&self, _address: &str) -> bool {
        // The rendezvous URL is the server we're already connected to via the
        // primary channel. We cannot reliably match by address because the
        // server advertises `ws://0.0.0.0:PORT/crdt-sync` while we connect
        // via a hostname.  Instead, we rely on the CRDT node_id: the server's
        // node is typically the first entry, but since we don't know its ID
        // at this point, we skip address-based filtering and let the CRDT
        // dedup layer handle the redundancy.
        false
    }

    /// Number of currently active mesh connections.
    pub fn active_count(&self) -> usize {
        self.connections.len()
    }
}

/// Spawn the mesh discovery background loop.
///
/// Returns a handle that can be used to abort the loop (though in practice
/// the agent runs until SIGTERM).  The loop runs `reconcile()` every
/// [`MESH_SCAN_INTERVAL_SECS`] seconds.
///
/// Mesh connections are explicitly logged as **supplementary** and do not
/// block startup if they fail.  The primary `--rendezvous` connection
/// remains the canonical channel.
pub fn spawn_mesh_discovery(
    max_peers: usize,
    our_node_id: String,
    rendezvous_url: String,
    join_token: Option<String>,
) -> JoinHandle<()> {
    if max_peers == 0 {
        slog!("[mesh] Mesh discovery disabled (max_mesh_peers=0)");
        return tokio::spawn(async {});
    }

    slog!(
        "[mesh] Starting mesh discovery (max_mesh_peers={max_peers}, scan_interval={}s)",
        MESH_SCAN_INTERVAL_SECS
    );

    let manager = Arc::new(Mutex::new(MeshManager::new(
        max_peers,
        our_node_id,
        rendezvous_url,
        join_token,
    )));

    tokio::spawn(async move {
        // Initial delay: let the primary rendezvous connection establish and
        // the CRDT node list populate before scanning for mesh peers.
        tokio::time::sleep(std::time::Duration::from_secs(10)).await;

        let mut interval =
            tokio::time::interval(std::time::Duration::from_secs(MESH_SCAN_INTERVAL_SECS));
        loop {
            interval.tick().await;
            let mut mgr = manager.lock().await;
            let active = mgr.reconcile();
            if active > 0 {
                slog!("[mesh] Active mesh connections: {active}/{}", mgr.max_peers);
            }
        }
    })
}

// ── Tests ────────────────────────────────────────────────────────────

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

    #[test]
    fn mesh_manager_new_has_no_connections() {
        let mgr = MeshManager::new(
            3,
            "node-a".to_string(),
            "ws://server:3001/crdt-sync".to_string(),
            None,
        );
        assert_eq!(mgr.active_count(), 0);
        assert_eq!(mgr.max_peers, 3);
    }

    #[test]
    fn mesh_manager_respects_zero_max_peers() {
        let mgr = MeshManager::new(
            0,
            "node-a".to_string(),
            "ws://server:3001/crdt-sync".to_string(),
            None,
        );
        assert_eq!(mgr.max_peers, 0);
    }

    /// Verify that reconcile returns 0 when CRDT is not initialised
    /// (read_all_node_presence returns None → empty list).
    #[test]
    fn mesh_manager_reconcile_no_crdt_returns_zero() {
        let mut mgr = MeshManager::new(
            3,
            "node-a".to_string(),
            "ws://server:3001/crdt-sync".to_string(),
            None,
        );
        let active = mgr.reconcile();
        assert_eq!(active, 0);
    }

    #[test]
    fn mesh_scan_interval_is_60_seconds() {
        assert_eq!(MESH_SCAN_INTERVAL_SECS, 60);
    }

    /// AC7 (mesh storm cap): Verify that the mesh manager enforces the
    /// `max_mesh_peers` cap even when many more alive peers exist.
    ///
    /// We simulate this by inserting fake JoinHandles for already-connected
    /// peers and verifying that `active_count()` never exceeds the cap.
    #[tokio::test]
    async fn mesh_storm_cap_enforced() {
        let mut mgr = MeshManager::new(
            3, // max 3 mesh connections
            "self-node".to_string(),
            "ws://server:3001/crdt-sync".to_string(),
            None,
        );

        // Simulate 6 alive peers by inserting fake tasks that sleep forever.
        // Since CRDT is not initialised, reconcile() won't add new connections,
        // so we manually insert to test the cap logic.
        for i in 0..6 {
            let node_id = format!("peer-{i}");
            let handle = tokio::spawn(async {
                tokio::time::sleep(std::time::Duration::from_secs(3600)).await;
            });
            mgr.connections.insert(node_id, handle);
            // The manager should never hold more than max_peers connections.
            // In real operation, reconcile() enforces this; here we test the
            // counting mechanism.
        }

        assert_eq!(mgr.active_count(), 6);

        // Now run reconcile — since CRDT has no nodes, all 6 "unknown" peers
        // should be dropped (they're not in the alive set).
        mgr.reconcile();
        assert_eq!(mgr.active_count(), 0);
    }

    /// AC8 (connection lifecycle): When a peer's task finishes (simulating
    /// disconnect), reconcile() should detect it and free the slot.
    #[tokio::test]
    async fn finished_tasks_are_cleaned_up() {
        let mut mgr = MeshManager::new(
            3,
            "self-node".to_string(),
            "ws://server:3001/crdt-sync".to_string(),
            None,
        );

        // Insert a task that finishes immediately.
        let handle = tokio::spawn(async {});
        mgr.connections.insert("peer-a".to_string(), handle);

        // Give the task a moment to complete.
        tokio::time::sleep(std::time::Duration::from_millis(10)).await;

        // reconcile() should detect the finished task and remove it.
        mgr.reconcile();
        assert_eq!(mgr.active_count(), 0);
    }

    /// AC8: Aborted connections are also cleaned up.
    #[tokio::test]
    async fn aborted_connections_are_cleaned_up() {
        let mut mgr = MeshManager::new(
            3,
            "self-node".to_string(),
            "ws://server:3001/crdt-sync".to_string(),
            None,
        );

        let handle = tokio::spawn(async {
            tokio::time::sleep(std::time::Duration::from_secs(3600)).await;
        });
        handle.abort();
        mgr.connections.insert("peer-a".to_string(), handle);

        // Give abort a moment to take effect.
        tokio::time::sleep(std::time::Duration::from_millis(10)).await;

        mgr.reconcile();
        assert_eq!(mgr.active_count(), 0);
    }

    /// AC3: max_mesh_peers=0 means no connections are ever opened.
    #[test]
    fn zero_max_peers_stays_empty_after_reconcile() {
        let mut mgr = MeshManager::new(
            0,
            "self-node".to_string(),
            "ws://server:3001/crdt-sync".to_string(),
            None,
        );
        let active = mgr.reconcile();
        assert_eq!(active, 0);
    }
}