//! Multi-hop mesh router using the distributed routing table.
//!
//! The [`MeshRouter`] delivers messages using the best available path:
//! direct transport -> multi-hop via intermediate nodes -> store-and-forward.
//!
//! # Routing Algorithm
//!
//! ```text
//! send(destination, payload):
//!   1. Look up destination in routing table
//!   2. If direct transport available -> send via transport
//!   3. If next-hop known -> wrap in MeshEnvelope, send to next-hop
//!   4. If no route -> queue in store-and-forward
//! ```

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

use anyhow::{bail, Result};

use crate::announce::compute_address;
use crate::envelope::MeshEnvelope;
use crate::identity::MeshIdentity;
use crate::routing_table::RoutingTable;
use crate::store::MeshStore;
use crate::transport::TransportAddr;
use crate::transport_manager::TransportManager;

/// How a message was delivered.
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
pub enum DeliveryResult {
    /// Sent directly to destination via a transport.
    Direct,
    /// Forwarded to next-hop node for relay.
    Forwarded,
    /// Queued in store-and-forward (destination unreachable).
    Stored,
    /// Delivered via server relay (legacy fallback).
    ServerRelay,
}

/// What to do with an incoming envelope.
#[derive(Debug)]
pub enum IncomingAction {
    /// Message is for us — deliver to application.
    Deliver(MeshEnvelope),
    /// Message is for someone else — forward it.
    Forward {
        envelope: MeshEnvelope,
        next_hop: TransportAddr,
    },
    /// Message should be stored for later forwarding.
    Store(MeshEnvelope),
    /// Message was dropped (expired, max hops, invalid).
    Dropped(String),
}

/// Per-destination delivery statistics.
#[derive(Debug, Clone, Default)]
pub struct DeliveryStats {
    pub direct_count: u64,
    pub forwarded_count: u64,
    pub stored_count: u64,
    pub relay_count: u64,
    pub last_delivery: Option<Instant>,
    pub avg_latency: Option<Duration>,
}

impl DeliveryStats {
    fn record(&mut self, method: DeliveryResult, latency: Duration) {
        match method {
            DeliveryResult::Direct => self.direct_count += 1,
            DeliveryResult::Forwarded => self.forwarded_count += 1,
            DeliveryResult::Stored => self.stored_count += 1,
            DeliveryResult::ServerRelay => self.relay_count += 1,
        }
        self.last_delivery = Some(Instant::now());
        self.avg_latency = Some(match self.avg_latency {
            Some(prev) => (prev + latency) / 2,
            None => latency,
        });
    }

    /// Total number of deliveries across all methods.
    pub fn total(&self) -> u64 {
        self.direct_count + self.forwarded_count + self.stored_count + self.relay_count
    }
}

/// Multi-hop mesh message router.
pub struct MeshRouter {
    /// This node's mesh identity.
    identity: MeshIdentity,
    /// This node's 16-byte truncated address.
    local_address: [u8; 16],
    /// Distributed routing table.
    routes: Arc<RwLock<RoutingTable>>,
    /// Transport manager for sending packets.
    transports: Arc<TransportManager>,
    /// Store-and-forward queue for unreachable destinations.
    store: Arc<Mutex<MeshStore>>,
    /// Per-destination delivery stats.
    stats: Mutex<HashMap<[u8; 16], DeliveryStats>>,
}

impl MeshRouter {
    /// Create a new mesh router.
    pub fn new(
        identity: MeshIdentity,
        routes: Arc<RwLock<RoutingTable>>,
        transports: Arc<TransportManager>,
        store: Arc<Mutex<MeshStore>>,
    ) -> Self {
        let local_address = compute_address(&identity.public_key());
        Self {
            identity,
            local_address,
            routes,
            transports,
            store,
            stats: Mutex::new(HashMap::new()),
        }
    }

    /// Send a payload to a destination identified by its 16-byte mesh address.
    ///
    /// Routing priority:
    /// 1. Route found in routing table -> wrap in envelope and send via transport
    /// 2. No route -> store for later forwarding
    pub async fn send(&self, dest_address: &[u8; 16], payload: &[u8]) -> Result<DeliveryResult> {
        let start = Instant::now();

        // Look up destination in routing table.
        let route_info = {
            let table = self
                .routes
                .read()
                .map_err(|e| anyhow::anyhow!("routing table lock poisoned: {e}"))?;
            table.lookup(dest_address).map(|entry| {
                (
                    entry.identity_key,
                    entry.next_hop_addr.clone(),
                    entry.hops,
                )
            })
        };

        if let Some((dest_key, next_hop_addr, hops)) = route_info {
            // Build an envelope addressed to the destination.
            let envelope =
                MeshEnvelope::new(&self.identity, &dest_key, payload.to_vec(), 300, 0);
            let wire = envelope.to_wire();

            self.transports.send(&next_hop_addr, &wire).await?;

            // Classify: if destination is directly reachable (hop count <= 1),
            // consider it Direct; otherwise it's Forwarded through intermediaries.
            let result = if hops <= 1 {
                DeliveryResult::Direct
            } else {
                DeliveryResult::Forwarded
            };

            let latency = start.elapsed();
            self.record_stats(dest_address, result, latency);
            Ok(result)
        } else {
            // No route — store for later forwarding.
            // We need a recipient key for the store. Since we only have the address
            // and no key, store with the address zero-padded to 32 bytes as a key
            // placeholder. The drain_store_for method matches on this convention.
            let mut recipient_key = [0u8; 32];
            recipient_key[..16].copy_from_slice(dest_address);

            let envelope = MeshEnvelope::new(
                &self.identity,
                &recipient_key,
                payload.to_vec(),
                300,
                0,
            );
            let stored = {
                let mut store = self
                    .store
                    .lock()
                    .map_err(|e| anyhow::anyhow!("store lock poisoned: {e}"))?;
                store.store(envelope)
            };
            if !stored {
                bail!("store rejected envelope (duplicate or at capacity)");
            }

            let latency = start.elapsed();
            let result = DeliveryResult::Stored;
            self.record_stats(dest_address, result, latency);
            Ok(result)
        }
    }

    /// Convenience: compute the 16-byte address from a 32-byte key, then send.
    pub async fn send_to_key(
        &self,
        dest_key: &[u8; 32],
        payload: &[u8],
    ) -> Result<DeliveryResult> {
        let addr = compute_address(dest_key);
        self.send(&addr, payload).await
    }

    /// Process a received envelope and decide what to do with it.
    pub fn handle_incoming(&self, envelope: MeshEnvelope) -> Result<IncomingAction> {
        // Verify envelope signature.
        if !envelope.verify() {
            return Ok(IncomingAction::Dropped(
                "invalid signature".to_string(),
            ));
        }

        // Check if it's for us (recipient_key matches our identity).
        let our_key = self.identity.public_key();
        if envelope.recipient_key.len() == 32 {
            let recipient: [u8; 32] = envelope
                .recipient_key
                .as_slice()
                .try_into()
                .map_err(|_| anyhow::anyhow!("invalid recipient key length"))?;
            if recipient == our_key {
                return Ok(IncomingAction::Deliver(envelope));
            }
        }

        // Broadcast (empty recipient) — always deliver locally.
        if envelope.recipient_key.is_empty() {
            return Ok(IncomingAction::Deliver(envelope));
        }

        // Not for us — check if we can forward.
        if !envelope.can_forward() {
            let reason = if envelope.is_expired() {
                "envelope expired"
            } else {
                "max hops reached"
            };
            return Ok(IncomingAction::Dropped(reason.to_string()));
        }

        // Look up the recipient in the routing table.
        let dest_address = compute_address(&envelope.recipient_key);
        let next_hop = {
            let table = self
                .routes
                .read()
                .map_err(|e| anyhow::anyhow!("routing table lock poisoned: {e}"))?;
            table
                .lookup(&dest_address)
                .map(|entry| entry.next_hop_addr.clone())
        };

        match next_hop {
            Some(addr) => {
                let forwarded = envelope.forwarded();
                Ok(IncomingAction::Forward {
                    envelope: forwarded,
                    next_hop: addr,
                })
            }
            None => Ok(IncomingAction::Store(envelope)),
        }
    }

    /// Forward an envelope to its next hop based on the routing table.
    ///
    /// The envelope is sent as-is (callers such as [`handle_incoming`](Self::handle_incoming)
    /// are expected to have already incremented the hop count via [`MeshEnvelope::forwarded`]).
    pub async fn forward(&self, envelope: MeshEnvelope) -> Result<DeliveryResult> {
        let start = Instant::now();
        let dest_address = compute_address(&envelope.recipient_key);

        let next_hop_addr = {
            let table = self
                .routes
                .read()
                .map_err(|e| anyhow::anyhow!("routing table lock poisoned: {e}"))?;
            table
                .lookup(&dest_address)
                .map(|entry| entry.next_hop_addr.clone())
                .ok_or_else(|| anyhow::anyhow!("no route for forwarding target"))?
        };

        let wire = envelope.to_wire();
        self.transports.send(&next_hop_addr, &wire).await?;

        let latency = start.elapsed();
        let result = DeliveryResult::Forwarded;
        self.record_stats(&dest_address, result, latency);
        Ok(result)
    }

    /// Drain stored messages for a destination and attempt to forward them.
    ///
    /// Call this when a new route appears (e.g., from an announce) to flush
    /// queued messages. Returns the count of successfully forwarded messages.
    pub async fn drain_store_for(&self, dest_address: &[u8; 16]) -> Result<usize> {
        // Look up the route to get identity key and next-hop.
        let (identity_key, next_hop_addr) = {
            let table = self
                .routes
                .read()
                .map_err(|e| anyhow::anyhow!("routing table lock poisoned: {e}"))?;
            match table.lookup(dest_address) {
                Some(entry) => (entry.identity_key, entry.next_hop_addr.clone()),
                None => return Ok(0),
            }
        };

        // Fetch stored envelopes keyed by the full identity key.
        let envelopes = {
            let mut store = self
                .store
                .lock()
                .map_err(|e| anyhow::anyhow!("store lock poisoned: {e}"))?;
            let mut result = store.fetch(&identity_key);
            // Also try the zero-padded address convention used by send().
            let mut padded_key = [0u8; 32];
            padded_key[..16].copy_from_slice(dest_address);
            result.extend(store.fetch(&padded_key));
            result
        };

        let mut forwarded_count = 0;
        for env in envelopes {
            if env.can_forward() {
                let fwd = env.forwarded();
                let wire = fwd.to_wire();
                if self.transports.send(&next_hop_addr, &wire).await.is_ok() {
                    forwarded_count += 1;
                }
            }
        }

        Ok(forwarded_count)
    }

    /// Get delivery statistics for a specific destination.
    pub fn stats(&self, address: &[u8; 16]) -> Option<DeliveryStats> {
        self.stats
            .lock()
            .ok()
            .and_then(|s| s.get(address).cloned())
    }

    /// Get delivery statistics for all known destinations.
    pub fn all_stats(&self) -> HashMap<[u8; 16], DeliveryStats> {
        self.stats
            .lock()
            .map(|s| s.clone())
            .unwrap_or_default()
    }

    /// This node's 16-byte truncated mesh address.
    pub fn local_address(&self) -> &[u8; 16] {
        &self.local_address
    }

    /// Record a delivery in the per-destination stats.
    fn record_stats(&self, address: &[u8; 16], method: DeliveryResult, latency: Duration) {
        if let Ok(mut stats) = self.stats.lock() {
            stats
                .entry(*address)
                .or_default()
                .record(method, latency);
        }
    }
}

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

    #[test]
    fn delivery_stats_tracking() {
        let mut stats = DeliveryStats::default();
        assert_eq!(stats.total(), 0);

        stats.record(DeliveryResult::Direct, Duration::from_millis(10));
        assert_eq!(stats.direct_count, 1);
        assert_eq!(stats.total(), 1);
        assert!(stats.last_delivery.is_some());
        assert!(stats.avg_latency.is_some());

        stats.record(DeliveryResult::Forwarded, Duration::from_millis(20));
        assert_eq!(stats.forwarded_count, 1);
        assert_eq!(stats.total(), 2);

        stats.record(DeliveryResult::Stored, Duration::from_millis(5));
        assert_eq!(stats.stored_count, 1);
        assert_eq!(stats.total(), 3);

        stats.record(DeliveryResult::ServerRelay, Duration::from_millis(50));
        assert_eq!(stats.relay_count, 1);
        assert_eq!(stats.total(), 4);

        // avg_latency should be present and reasonable.
        let avg = stats.avg_latency.unwrap();
        assert!(avg.as_millis() > 0);
    }

    #[test]
    fn incoming_action_deliver_to_self() {
        let identity = MeshIdentity::generate();
        let our_key = identity.public_key();
        let routes = Arc::new(RwLock::new(RoutingTable::new(Duration::from_secs(300))));
        let transports = Arc::new(TransportManager::new());
        let store = Arc::new(Mutex::new(MeshStore::new(100)));

        let router = MeshRouter::new(identity, routes, transports, store);

        // Create an envelope addressed to our key.
        let sender = MeshIdentity::generate();
        let envelope =
            MeshEnvelope::new(&sender, &our_key, b"hello self".to_vec(), 3600, 5);

        let action = router.handle_incoming(envelope).expect("handle_incoming");
        match action {
            IncomingAction::Deliver(env) => {
                assert_eq!(env.payload, b"hello self");
            }
            other => panic!("expected Deliver, got {:?}", std::mem::discriminant(&other)),
        }
    }

    #[test]
    fn incoming_action_broadcast_delivers() {
        let identity = MeshIdentity::generate();
        let routes = Arc::new(RwLock::new(RoutingTable::new(Duration::from_secs(300))));
        let transports = Arc::new(TransportManager::new());
        let store = Arc::new(Mutex::new(MeshStore::new(100)));

        let router = MeshRouter::new(identity, routes, transports, store);

        // Create a broadcast envelope (empty recipient key).
        let sender = MeshIdentity::generate();
        let envelope =
            MeshEnvelope::new(&sender, &[], b"broadcast msg".to_vec(), 3600, 5);

        let action = router.handle_incoming(envelope).expect("handle_incoming");
        match action {
            IncomingAction::Deliver(env) => {
                assert_eq!(env.payload, b"broadcast msg");
                assert!(env.recipient_key.is_empty());
            }
            other => panic!("expected Deliver, got {:?}", std::mem::discriminant(&other)),
        }
    }

    #[test]
    fn incoming_action_dropped_expired() {
        let identity = MeshIdentity::generate();
        let routes = Arc::new(RwLock::new(RoutingTable::new(Duration::from_secs(300))));
        let transports = Arc::new(TransportManager::new());
        let store = Arc::new(Mutex::new(MeshStore::new(100)));

        let router = MeshRouter::new(identity, routes, transports, store);

        // Create an envelope addressed to someone else with TTL=0.
        // is_expired() checks: now - timestamp > ttl_secs.
        // With ttl=0 and timestamp=now, we need to wait >0 seconds for expiry.
        let sender = MeshIdentity::generate();
        let other_key = [0xBB; 32];
        let envelope =
            MeshEnvelope::new(&sender, &other_key, b"expired".to_vec(), 0, 5);

        // Sleep briefly so that now - timestamp > 0 (the TTL).
        std::thread::sleep(Duration::from_millis(1100));

        let action = router.handle_incoming(envelope).expect("handle_incoming");
        match action {
            IncomingAction::Dropped(reason) => {
                assert!(
                    reason.contains("expired"),
                    "expected expired reason, got: {reason}"
                );
            }
            other => panic!("expected Dropped, got {:?}", std::mem::discriminant(&other)),
        }
    }

    #[test]
    fn incoming_action_dropped_invalid_sig() {
        let identity = MeshIdentity::generate();
        let routes = Arc::new(RwLock::new(RoutingTable::new(Duration::from_secs(300))));
        let transports = Arc::new(TransportManager::new());
        let store = Arc::new(Mutex::new(MeshStore::new(100)));

        let router = MeshRouter::new(identity, routes, transports, store);

        // Create a valid envelope then tamper with the payload.
        let sender = MeshIdentity::generate();
        let other_key = [0xCC; 32];
        let mut envelope =
            MeshEnvelope::new(&sender, &other_key, b"original".to_vec(), 3600, 5);
        envelope.payload = b"tampered".to_vec();

        let action = router.handle_incoming(envelope).expect("handle_incoming");
        match action {
            IncomingAction::Dropped(reason) => {
                assert!(
                    reason.contains("invalid signature"),
                    "expected invalid signature reason, got: {reason}"
                );
            }
            other => panic!("expected Dropped, got {:?}", std::mem::discriminant(&other)),
        }
    }
}