//! KeyPackage cache for mesh-based MLS group setup.
//!
//! The [`KeyPackageCache`] stores MLS KeyPackages received from other nodes,
//! enabling group creation without a central server. KeyPackages are:
//!
//! - Indexed by the node's 16-byte mesh address
//! - Hashed (8 bytes) for announce inclusion
//! - TTL-managed for expiry (MLS KeyPackages are single-use but we cache N of them)
//! - Bounded by capacity to prevent memory exhaustion
//!
//! # Protocol Flow
//!
//! 1. Bob generates KeyPackage, computes hash, includes hash in MeshAnnounce
//! 2. Bob broadcasts full KeyPackage periodically (or on request)
//! 3. Alice receives Bob's KeyPackage, stores in cache
//! 4. Alice wants to create group with Bob: fetches from cache, creates Welcome
//! 5. Alice sends Welcome to Bob via mesh routing

use std::collections::HashMap;
use std::time::{Duration, Instant};

use crate::address::MeshAddress;
use crate::announce::compute_keypackage_hash;

/// Default TTL for cached KeyPackages (24 hours).
const DEFAULT_TTL: Duration = Duration::from_secs(24 * 60 * 60);

/// Default maximum KeyPackages per address (allow rotation).
const DEFAULT_MAX_PER_ADDRESS: usize = 3;

/// A cached KeyPackage entry.
#[derive(Clone, Debug)]
pub struct CachedKeyPackage {
    /// The serialized MLS KeyPackage bytes.
    pub bytes: Vec<u8>,
    /// 8-byte truncated hash for matching against announces.
    pub hash: [u8; 8],
    /// When this entry was stored.
    pub stored_at: Instant,
    /// When this entry expires.
    pub expires_at: Instant,
}

impl CachedKeyPackage {
    /// Create a new cached entry with default TTL.
    pub fn new(bytes: Vec<u8>) -> Self {
        Self::with_ttl(bytes, DEFAULT_TTL)
    }

    /// Create with custom TTL.
    pub fn with_ttl(bytes: Vec<u8>, ttl: Duration) -> Self {
        let hash = compute_keypackage_hash(&bytes);
        let now = Instant::now();
        Self {
            bytes,
            hash,
            stored_at: now,
            expires_at: now + ttl,
        }
    }

    /// Check if this entry has expired.
    pub fn is_expired(&self) -> bool {
        Instant::now() > self.expires_at
    }
}

/// Cache for KeyPackages received from mesh peers.
pub struct KeyPackageCache {
    /// Address -> list of cached KeyPackages (multiple for rotation).
    entries: HashMap<MeshAddress, Vec<CachedKeyPackage>>,
    /// Maximum KeyPackages stored per address.
    max_per_address: usize,
    /// Total capacity (max addresses).
    max_addresses: usize,
}

impl KeyPackageCache {
    /// Create a new cache with default settings.
    pub fn new() -> Self {
        Self::with_capacity(1000, DEFAULT_MAX_PER_ADDRESS)
    }

    /// Create with custom capacity.
    pub fn with_capacity(max_addresses: usize, max_per_address: usize) -> Self {
        Self {
            entries: HashMap::new(),
            max_per_address,
            max_addresses,
        }
    }

    /// Store a KeyPackage for a given address.
    ///
    /// Returns `true` if stored, `false` if rejected (at capacity or duplicate hash).
    pub fn store(&mut self, address: MeshAddress, keypackage_bytes: Vec<u8>) -> bool {
        let entry = CachedKeyPackage::new(keypackage_bytes);
        self.store_entry(address, entry)
    }

    /// Store a KeyPackage entry.
    fn store_entry(&mut self, address: MeshAddress, entry: CachedKeyPackage) -> bool {
        // Check if we already have this exact KeyPackage
        if let Some(existing) = self.entries.get(&address) {
            if existing.iter().any(|e| e.hash == entry.hash) {
                return false; // Duplicate
            }
        }

        // Check total capacity
        if !self.entries.contains_key(&address) && self.entries.len() >= self.max_addresses {
            // Evict oldest entry
            self.evict_oldest();
        }

        let list = self.entries.entry(address).or_default();

        // Enforce per-address limit
        while list.len() >= self.max_per_address {
            list.remove(0); // Remove oldest
        }

        list.push(entry);
        true
    }

    /// Get the newest KeyPackage for an address.
    pub fn get(&self, address: &MeshAddress) -> Option<&CachedKeyPackage> {
        self.entries
            .get(address)
            .and_then(|list| list.iter().rev().find(|e| !e.is_expired()))
    }

    /// Get a KeyPackage by its hash.
    pub fn get_by_hash(&self, address: &MeshAddress, hash: &[u8; 8]) -> Option<&CachedKeyPackage> {
        self.entries.get(address).and_then(|list| {
            list.iter()
                .rev()
                .find(|e| &e.hash == hash && !e.is_expired())
        })
    }

    /// Get the newest KeyPackage bytes for an address.
    pub fn get_bytes(&self, address: &MeshAddress) -> Option<Vec<u8>> {
        self.get(address).map(|e| e.bytes.clone())
    }

    /// Check if we have a KeyPackage matching a given hash.
    pub fn has_hash(&self, address: &MeshAddress, hash: &[u8; 8]) -> bool {
        self.get_by_hash(address, hash).is_some()
    }

    /// Remove all expired entries. Returns count removed.
    pub fn gc_expired(&mut self) -> usize {
        let mut removed = 0;
        self.entries.retain(|_, list| {
            let before = list.len();
            list.retain(|e| !e.is_expired());
            removed += before - list.len();
            !list.is_empty()
        });
        removed
    }

    /// Evict the oldest entry across all addresses.
    fn evict_oldest(&mut self) {
        let oldest_addr = self
            .entries
            .iter()
            .filter_map(|(addr, list)| {
                list.first().map(|e| (addr.clone(), e.stored_at))
            })
            .min_by_key(|(_, stored)| *stored)
            .map(|(addr, _)| addr);

        if let Some(addr) = oldest_addr {
            if let Some(list) = self.entries.get_mut(&addr) {
                list.remove(0);
                if list.is_empty() {
                    self.entries.remove(&addr);
                }
            }
        }
    }

    /// Number of addresses with cached KeyPackages.
    pub fn len(&self) -> usize {
        self.entries.len()
    }

    /// Whether the cache is empty.
    pub fn is_empty(&self) -> bool {
        self.entries.is_empty()
    }

    /// Total number of cached KeyPackages.
    pub fn total_keypackages(&self) -> usize {
        self.entries.values().map(|v| v.len()).sum()
    }

    /// Consume a KeyPackage (remove after use, as MLS KeyPackages are single-use).
    ///
    /// Returns the KeyPackage bytes if found.
    pub fn consume(&mut self, address: &MeshAddress, hash: &[u8; 8]) -> Option<Vec<u8>> {
        let list = self.entries.get_mut(address)?;
        let idx = list.iter().position(|e| &e.hash == hash)?;
        let entry = list.remove(idx);
        if list.is_empty() {
            self.entries.remove(address);
        }
        Some(entry.bytes)
    }
}

impl Default for KeyPackageCache {
    fn default() -> Self {
        Self::new()
    }
}

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

    fn make_keypackage(seed: u8) -> Vec<u8> {
        vec![seed; 100 + seed as usize]
    }

    fn make_address(seed: u8) -> MeshAddress {
        MeshAddress::from_bytes([seed; 16])
    }

    #[test]
    fn store_and_retrieve() {
        let mut cache = KeyPackageCache::new();
        let addr = make_address(1);
        let kp = make_keypackage(1);
        let hash = compute_keypackage_hash(&kp);

        assert!(cache.store(addr, kp.clone()));
        assert_eq!(cache.len(), 1);

        let retrieved = cache.get(&addr).expect("should exist");
        assert_eq!(retrieved.bytes, kp);
        assert_eq!(retrieved.hash, hash);
    }

    #[test]
    fn reject_duplicate() {
        let mut cache = KeyPackageCache::new();
        let addr = make_address(2);
        let kp = make_keypackage(2);

        assert!(cache.store(addr, kp.clone()));
        assert!(!cache.store(addr, kp), "duplicate should be rejected");
        assert_eq!(cache.total_keypackages(), 1);
    }

    #[test]
    fn multiple_per_address() {
        let mut cache = KeyPackageCache::with_capacity(100, 3);
        let addr = make_address(3);

        assert!(cache.store(addr, make_keypackage(1)));
        assert!(cache.store(addr, make_keypackage(2)));
        assert!(cache.store(addr, make_keypackage(3)));
        assert_eq!(cache.total_keypackages(), 3);

        // Fourth should evict first
        assert!(cache.store(addr, make_keypackage(4)));
        assert_eq!(cache.total_keypackages(), 3);

        // First should be gone
        let hash1 = compute_keypackage_hash(&make_keypackage(1));
        assert!(!cache.has_hash(&addr, &hash1));

        // Fourth should be present
        let hash4 = compute_keypackage_hash(&make_keypackage(4));
        assert!(cache.has_hash(&addr, &hash4));
    }

    #[test]
    fn consume_removes_keypackage() {
        let mut cache = KeyPackageCache::new();
        let addr = make_address(4);
        let kp = make_keypackage(4);
        let hash = compute_keypackage_hash(&kp);

        cache.store(addr, kp.clone());
        assert!(cache.has_hash(&addr, &hash));

        let consumed = cache.consume(&addr, &hash).expect("should consume");
        assert_eq!(consumed, kp);
        assert!(!cache.has_hash(&addr, &hash));
        assert!(cache.is_empty());
    }

    #[test]
    fn get_by_hash() {
        let mut cache = KeyPackageCache::new();
        let addr = make_address(5);
        let kp1 = make_keypackage(51);
        let kp2 = make_keypackage(52);
        let hash1 = compute_keypackage_hash(&kp1);
        let hash2 = compute_keypackage_hash(&kp2);

        cache.store(addr, kp1.clone());
        cache.store(addr, kp2.clone());

        let found1 = cache.get_by_hash(&addr, &hash1).expect("hash1");
        assert_eq!(found1.bytes, kp1);

        let found2 = cache.get_by_hash(&addr, &hash2).expect("hash2");
        assert_eq!(found2.bytes, kp2);

        let wrong_hash = [0xFFu8; 8];
        assert!(cache.get_by_hash(&addr, &wrong_hash).is_none());
    }

    #[test]
    fn capacity_eviction() {
        let mut cache = KeyPackageCache::with_capacity(2, 1);

        let addr1 = make_address(1);
        let addr2 = make_address(2);
        let addr3 = make_address(3);

        cache.store(addr1, make_keypackage(1));
        cache.store(addr2, make_keypackage(2));
        assert_eq!(cache.len(), 2);

        // Third should evict oldest (addr1)
        cache.store(addr3, make_keypackage(3));
        assert_eq!(cache.len(), 2);
        assert!(cache.get(&addr1).is_none());
        assert!(cache.get(&addr2).is_some());
        assert!(cache.get(&addr3).is_some());
    }

    #[test]
    fn expiry() {
        let mut cache = KeyPackageCache::new();
        let addr = make_address(6);

        // Create entry with very short TTL
        let kp = make_keypackage(6);
        let entry = CachedKeyPackage::with_ttl(kp, Duration::from_millis(1));
        cache.store_entry(addr, entry);

        assert_eq!(cache.total_keypackages(), 1);

        // Wait for expiry
        std::thread::sleep(Duration::from_millis(10));

        // GC should remove it
        let removed = cache.gc_expired();
        assert_eq!(removed, 1);
        assert!(cache.is_empty());
    }
}