SYNC_REQUEST 2 (#616)

* wip

* woohooo

* Plumtree gossip: don't subset REQUEST_SYNC fanout; make RequestSyncPacket.encode use const

* bloom -> gcs [wip]

* fix build

* fix broadcast

* prune old messages too

* faster sync

* prune better

* adjust parameters

* fix(sync): make cap a constant in GCSFilter.buildFilter to silence 'never mutated' warning

* fix(mesh): surface self-origin public messages recovered via sync; only ignore self when TTL != 0 in handleMessage

* sync: allow self messages via GCS restore and relax TTL==0 acceptance\n- Bypass dedup for self TTL==0 packets in handleReceivedPacket\n- Accept self TTL==0 in handleMessage and set nickname\n- Accept unknown senders for TTL==0 with anon# prefix to restore history

---------

Co-authored-by: jack <jackjackbits@users.noreply.github.com>

bitchat/Models/RequestSyncPacket.swift

@@ -0,0 +1,63 @@
+import Foundation
+
+// REQUEST_SYNC payload TLV (type, length16, value)
+//  - 0x01: P (uint8) — Golomb-Rice parameter
+//  - 0x02: M (uint32, big-endian) — hash range (N * 2^P)
+//  - 0x03: data (opaque) — GR bitstream bytes (MSB-first)
+struct RequestSyncPacket {
+    let p: Int
+    let m: UInt32
+    let data: Data
+
+    func encode() -> Data {
+        var out = Data()
+        func putTLV(_ t: UInt8, _ v: Data) {
+            out.append(t)
+            let len = UInt16(v.count)
+            out.append(UInt8((len >> 8) & 0xFF))
+            out.append(UInt8(len & 0xFF))
+            out.append(v)
+        }
+        // P
+        putTLV(0x01, Data([UInt8(p & 0xFF)]))
+        // M (uint32)
+        var mBE = m.bigEndian
+        putTLV(0x02, withUnsafeBytes(of: &mBE) { Data($0) })
+        // data
+        putTLV(0x03, data)
+        return out
+    }
+
+    static func decode(from data: Data, maxAcceptBytes: Int = 1024) -> RequestSyncPacket? {
+        var off = 0
+        var p: Int? = nil
+        var m: UInt32? = nil
+        var payload: Data? = nil
+
+        while off + 3 <= data.count {
+            let t = Int(data[off]); off += 1
+            guard off + 2 <= data.count else { return nil }
+            let len = (Int(data[off]) << 8) | Int(data[off+1]); off += 2
+            guard off + len <= data.count else { return nil }
+            let v = data.subdata(in: off..<(off+len)); off += len
+            switch t {
+            case 0x01:
+                if v.count == 1 { p = Int(v[0]) }
+            case 0x02:
+                if v.count == 4 {
+                    var mm: UInt32 = 0
+                    for b in v { mm = (mm << 8) | UInt32(b) }
+                    m = mm
+                }
+            case 0x03:
+                if v.count > maxAcceptBytes { return nil }
+                payload = v
+            default:
+                break // forward compatible; ignore unknown TLVs
+            }
+        }
+
+        guard let pp = p, let mm = m, let dd = payload, pp >= 1, mm > 0 else { return nil }
+        return RequestSyncPacket(p: pp, m: mm, data: dd)
+    }
+}

bitchat/Protocols/BitchatProtocol.swift

@@ -125,6 +125,7 @@ enum MessageType: UInt8 {
     case announce = 0x01        // "I'm here" with nickname
     case message = 0x02         // Public chat message  
     case leave = 0x03           // "I'm leaving"
+    case requestSync = 0x21     // GCS filter-based sync request (local-only)
     
     // Noise encryption
     case noiseHandshake = 0x10  // Handshake (init or response determined by payload)
@@ -138,6 +139,7 @@ enum MessageType: UInt8 {
         case .announce: return "announce"
         case .message: return "message"
         case .leave: return "leave"
+        case .requestSync: return "requestSync"
         case .noiseHandshake: return "noiseHandshake"
         case .noiseEncrypted: return "noiseEncrypted"
         case .fragment: return "fragment"

bitchat/Services/BLEService.swift

@@ -138,6 +138,9 @@ final class BLEService: NSObject {
     private var pendingDirectedRelays: [String: [String: (packet: BitchatPacket, enqueuedAt: Date)]] = [:]
     // Debounce for 'reconnected' logs
     private var lastReconnectLogAt: [String: Date] = [:]
+
+    // MARK: - Gossip Sync
+    private var gossipSyncManager: GossipSyncManager?
     
     // MARK: - Maintenance Timer
     
@@ -398,9 +401,15 @@ final class BLEService: NSObject {
         }
         timer.resume()
         maintenanceTimer = timer
-        
+
         // Publish initial empty state
         requestPeerDataPublish()
+
+        // Initialize gossip sync manager
+        let sync = GossipSyncManager(myPeerID: myPeerID)
+        sync.delegate = self
+        sync.start()
+        self.gossipSyncManager = sync
     }
     
     func setNickname(_ nickname: String) {
@@ -775,6 +784,8 @@ final class BLEService: NSObject {
                 self.messageDeduplicator.markProcessed(dedupID)
                 // Call synchronously since we're already on background queue
                 self.broadcastPacket(signedPacket)
+                // Track our own broadcast for sync
+                self.gossipSyncManager?.onPublicPacketSeen(signedPacket)
             }
         }
     }
@@ -1099,10 +1110,13 @@ final class BLEService: NSObject {
         }
 
         // For broadcast (no directed peer) and non-fragment, choose a subset deterministically
-        // Special-case announces: do NOT subset to maximize reach for presence
+        // Special-case control/presence messages: do NOT subset to maximize immediate coverage
         var selectedPeripheralIDs = Set(allowedPeripheralIDs)
         var selectedCentralIDs = Set(allowedCentralIDs)
-        if directedOnlyPeer == nil && packet.type != MessageType.fragment.rawValue && packet.type != MessageType.announce.rawValue {
+        if directedOnlyPeer == nil
+            && packet.type != MessageType.fragment.rawValue
+            && packet.type != MessageType.announce.rawValue
+            && packet.type != MessageType.requestSync.rawValue {
             let kp = subsetSizeForFanout(allowedPeripheralIDs.count)
             let kc = subsetSizeForFanout(allowedCentralIDs.count)
             selectedPeripheralIDs = selectDeterministicSubset(ids: allowedPeripheralIDs, k: kp, seed: messageID)
@@ -1133,6 +1147,12 @@ final class BLEService: NSObject {
         }
     }
 
+    // Directed send helper (unicast to a specific peerID) without altering packet contents
+    private func sendPacketDirected(_ packet: BitchatPacket, to peerID: String) {
+        guard let data = packet.toBinaryData(padding: false) else { return }
+        sendOnAllLinks(packet: packet, data: data, pad: false, directedOnlyPeer: peerID)
+    }
+
     // MARK: - Directed store-and-forward
     private func spoolDirectedPacket(_ packet: BitchatPacket, recipientPeerID: String) {
         let msgID = makeMessageID(for: packet)
@@ -1339,7 +1359,10 @@ final class BLEService: NSObject {
         
         // Efficient deduplication
         // Important: do not dedup fragment packets globally (each piece must pass)
-        if packet.type != MessageType.fragment.rawValue && messageDeduplicator.isDuplicate(messageID) {
+        // Special case: allow our own packets recovered via sync (TTL==0) to pass
+        // through even if we've marked them as seen at send time.
+        let allowSelfSyncReplay = (packet.ttl == 0) && (senderID == myPeerID)
+        if packet.type != MessageType.fragment.rawValue && !allowSelfSyncReplay && messageDeduplicator.isDuplicate(messageID) {
             // Announce packets (type 1) are sent every 10 seconds for peer discovery
             // It's normal to see these as duplicates - don't log them to reduce noise
             if packet.type != MessageType.announce.rawValue {
@@ -1383,6 +1406,9 @@ final class BLEService: NSObject {
         case .message:
             handleMessage(packet, from: senderID)
             
+        case .requestSync:
+            handleRequestSync(packet, from: senderID)
+            
         case .noiseHandshake:
             handleNoiseHandshake(packet, from: senderID)
             
@@ -1583,12 +1609,17 @@ final class BLEService: NSObject {
             // Only notify of connection for new or reconnected peers when it is a direct announce
             if (packet.ttl == self.messageTTL) && (isNewPeer || isReconnectedPeer) {
                 self.delegate?.didConnectToPeer(peerID)
+                // Schedule initial unicast sync to this peer
+                self.gossipSyncManager?.scheduleInitialSyncToPeer(peerID, delaySeconds: 1.0)
             }
             
             self.requestPeerDataPublish()
             self.delegate?.didUpdatePeerList(currentPeerIDs)
         }
         
+        // Track for sync (include our own and others' announces)
+        gossipSyncManager?.onPublicPacketSeen(packet)
+
         // Send announce back for bidirectional discovery (only once per peer)
         let announceBackID = "announce-back-\(peerID)"
         let shouldSendBack = !messageDeduplicator.contains(announceBackID)
@@ -1610,17 +1641,33 @@ final class BLEService: NSObject {
             }
         }
     }
+
+    // Handle REQUEST_SYNC: decode payload and respond with missing packets via sync manager
+    private func handleRequestSync(_ packet: BitchatPacket, from peerID: String) {
+        guard let req = RequestSyncPacket.decode(from: packet.payload) else {
+            SecureLogger.warning("⚠️ Malformed REQUEST_SYNC from \(peerID)", category: .session)
+            return
+        }
+        gossipSyncManager?.handleRequestSync(fromPeerID: peerID, request: req)
+    }
     
     // Mention parsing moved to ChatViewModel
     
     private func handleMessage(_ packet: BitchatPacket, from peerID: String) {
-        // Ignore self-origin public messages that may be seen again via relay
+        // Ignore self-origin public messages except when returned via sync (TTL==0).
-        if peerID == myPeerID { return }
+        // This allows our own messages to be surfaced when they come back via
+        // the sync path without re-processing regular relayed copies.
+        if peerID == myPeerID && packet.ttl != 0 { return }
 
         var accepted = false
         var senderNickname: String = ""
 
-        if let info = peers[peerID], info.isVerifiedNickname {
+        // If the packet is from ourselves (e.g., recovered via sync TTL==0), accept immediately
+        if peerID == myPeerID {
+            accepted = true
+            senderNickname = myNickname
+        }
+        else if let info = peers[peerID], info.isVerifiedNickname {
             // Known verified peer path
             accepted = true
             senderNickname = info.nickname
@@ -1648,6 +1695,22 @@ final class BLEService: NSObject {
                     }
                 }
             }
+            // If still not accepted and this is a sync-returned packet (TTL==0),
+            // accept with a generic nickname so history can be restored even for
+            // peers we haven't verified yet.
+            if !accepted && packet.ttl == 0 {
+                accepted = true
+                senderNickname = "anon" + String(peerID.prefix(4))
+            }
+        }
+
+        // Track broadcast messages for sync (treat nil or 0xFF..0xFF as broadcast)
+        let isBroadcastRecipient: Bool = {
+            guard let r = packet.recipientID else { return true }
+            return r.count == 8 && r.allSatisfy { $0 == 0xFF }
+        }()
+        if isBroadcastRecipient && packet.type == MessageType.message.rawValue {
+            gossipSyncManager?.onPublicPacketSeen(packet)
         }
 
         guard accepted else {
@@ -1780,6 +1843,8 @@ final class BLEService: NSObject {
             // Remove the peer when they leave
             peers.removeValue(forKey: peerID)
         }
+        // Remove any stored announcement for sync purposes
+        gossipSyncManager?.removeAnnouncementForPeer(peerID)
         // Send on main thread
         notifyUI { [weak self] in
             guard let self = self else { return }
@@ -1861,6 +1926,8 @@ final class BLEService: NSObject {
                 self?.broadcastPacket(signedPacket)
             }
         }
+        // Ensure our own announce is included in sync state
+        gossipSyncManager?.onPublicPacketSeen(signedPacket)
     }
     
     func sendDeliveryAck(for messageID: String, to peerID: String) {
@@ -2088,6 +2155,8 @@ final class BLEService: NSObject {
                 if !peer.isConnected {
                     if age > retention {
                         SecureLogger.debug("🗑️ Removing stale peer after reachability window: \(peerID) (\(peer.nickname))", category: .session)
+                        // Also remove any stored announcement from sync candidates
+                        gossipSyncManager?.removeAnnouncementForPeer(peerID)
                         peers.removeValue(forKey: peerID)
                         removedOfflineCount += 1
                     }
@@ -2249,6 +2318,29 @@ final class BLEService: NSObject {
     }
 }
 
+// MARK: - GossipSyncManager Delegate
+extension BLEService: GossipSyncManager.Delegate {
+    func sendPacket(_ packet: BitchatPacket) {
+        if DispatchQueue.getSpecific(key: messageQueueKey) != nil {
+            broadcastPacket(packet)
+        } else {
+            messageQueue.async { [weak self] in self?.broadcastPacket(packet) }
+        }
+    }
+
+    func sendPacket(to peerID: String, packet: BitchatPacket) {
+        if DispatchQueue.getSpecific(key: messageQueueKey) != nil {
+            sendPacketDirected(packet, to: peerID)
+        } else {
+            messageQueue.async { [weak self] in self?.sendPacketDirected(packet, to: peerID) }
+        }
+    }
+
+    func signPacketForBroadcast(_ packet: BitchatPacket) -> BitchatPacket {
+        return noiseService.signPacket(packet) ?? packet
+    }
+}
+
 // MARK: - CBCentralManagerDelegate
 
 extension BLEService: CBCentralManagerDelegate {

bitchat/Sync/GCSFilter.swift

@@ -0,0 +1,183 @@
+import Foundation
+import CryptoKit
+
+// Golomb-Coded Set (GCS) filter utilities for sync.
+// Hashing:
+//  - Packet ID is 16 bytes (see PacketIdUtil). For GCS mapping, use h64 = first 8 bytes of SHA-256 over the 16-byte ID.
+//  - Map to [0, M) via (h64 % M).
+// Encoding (v1):
+//  - Sort mapped values ascending; encode deltas (first is v0, then vi - v{i-1}) as positive integers x >= 1.
+//  - Golomb-Rice with parameter P: q = (x - 1) >> P encoded as unary (q ones then a zero), then write P-bit remainder r = (x - 1) & ((1<<P)-1).
+//  - Bitstream is MSB-first within each byte.
+enum GCSFilter {
+    struct Params { let p: Int; let m: UInt32; let data: Data }
+
+    // Derive P from FPR (~ 1 / 2^P)
+    static func deriveP(targetFpr: Double) -> Int {
+        let f = max(0.000001, min(0.25, targetFpr))
+        // ceil(log2(1/f))
+        let p = Int(ceil(log2(1.0 / f)))
+        return max(1, p)
+    }
+
+    // Estimate max elements that fit in size bytes: bits per element ~= P + 2 (approx)
+    static func estimateMaxElements(sizeBytes: Int, p: Int) -> Int {
+        let bits = max(8, sizeBytes * 8)
+        let per = max(3, p + 2)
+        return max(1, bits / per)
+    }
+
+    static func buildFilter(ids: [Data], maxBytes: Int, targetFpr: Double) -> Params {
+        let p = deriveP(targetFpr: targetFpr)
+        let cap = estimateMaxElements(sizeBytes: maxBytes, p: p)
+        let n = min(ids.count, cap)
+        let selected = Array(ids.prefix(n))
+        // Map to [0, M)
+        let mInit = UInt32(n << p)
+        var mapped = selected.map { id16 -> UInt64 in
+            let h = h64(id16)
+            return UInt64(h % UInt64(max(1, mInit)))
+        }.sorted()
+        var encoded = encode(sorted: mapped, p: p)
+        var trimmedN = n
+        // Trim if over budget
+        while encoded.count > maxBytes && trimmedN > 0 {
+            trimmedN = (trimmedN * 9) / 10 // drop ~10%
+            mapped = Array(mapped.prefix(trimmedN))
+            encoded = encode(sorted: mapped, p: p)
+        }
+        let finalM = UInt32(max(1, trimmedN << p))
+        return Params(p: p, m: finalM, data: encoded)
+    }
+
+    static func decodeToSortedSet(p: Int, m: UInt32, data: Data) -> [UInt64] {
+        var values: [UInt64] = []
+        let reader = BitReader(data)
+        var acc: UInt64 = 0
+        while true {
+            guard let q = reader.readUnary() else { break }
+            guard let r = reader.readBits(count: p) else { break }
+            let x = (UInt64(q) << UInt64(p)) + UInt64(r) + 1
+            acc &+= x
+            if acc >= UInt64(m) { break }
+            values.append(acc)
+        }
+        return values
+    }
+
+    static func contains(sortedValues: [UInt64], candidate: UInt64) -> Bool {
+        var lo = 0
+        var hi = sortedValues.count - 1
+        while lo <= hi {
+            let mid = (lo + hi) >> 1
+            let v = sortedValues[mid]
+            if v == candidate { return true }
+            if v < candidate { lo = mid + 1 } else { hi = mid - 1 }
+        }
+        return false
+    }
+
+    private static func h64(_ id16: Data) -> UInt64 {
+        var hasher = SHA256()
+        hasher.update(data: id16)
+        let d = hasher.finalize()
+        let db = Data(d)
+        var x: UInt64 = 0
+        let take = min(8, db.count)
+        for i in 0..<take { x = (x << 8) | UInt64(db[i]) }
+        return x & 0x7fff_ffff_ffff_ffff
+    }
+
+    private static func encode(sorted: [UInt64], p: Int) -> Data {
+        let writer = BitWriter()
+        var prev: UInt64 = 0
+        let mask: UInt64 = (p >= 64) ? ~0 : ((1 << UInt64(p)) - 1)
+        for v in sorted {
+            let delta = v &- prev
+            prev = v
+            let x = delta
+            let q = (x &- 1) >> UInt64(p)
+            let r = (x &- 1) & mask
+            // unary q ones then zero
+            if q > 0 { writer.writeOnes(count: Int(q)) }
+            writer.writeBit(0)
+            writer.writeBits(value: r, count: p)
+        }
+        return writer.toData()
+    }
+
+    // MARK: - Bit helpers (MSB-first)
+    private final class BitWriter {
+        private var buf = Data()
+        private var cur: UInt8 = 0
+        private var nbits: Int = 0
+        func writeBit(_ bit: Int) { // 0 or 1
+            cur = UInt8((Int(cur) << 1) | (bit & 1))
+            nbits += 1
+            if nbits == 8 {
+                buf.append(cur)
+                cur = 0; nbits = 0
+            }
+        }
+        func writeOnes(count: Int) {
+            guard count > 0 else { return }
+            for _ in 0..<count { writeBit(1) }
+        }
+        func writeBits(value: UInt64, count: Int) {
+            guard count > 0 else { return }
+            for i in stride(from: count - 1, through: 0, by: -1) {
+                let bit = Int((value >> UInt64(i)) & 1)
+                writeBit(bit)
+            }
+        }
+        func toData() -> Data {
+            if nbits > 0 {
+                let rem = UInt8(Int(cur) << (8 - nbits))
+                buf.append(rem)
+                cur = 0; nbits = 0
+            }
+            return buf
+        }
+    }
+
+    private final class BitReader {
+        private let data: Data
+        private var idx: Int = 0
+        private var cur: UInt8 = 0
+        private var left: Int = 0
+        init(_ data: Data) {
+            self.data = data
+            if !data.isEmpty {
+                cur = data[0]
+                left = 8
+            }
+        }
+        func readBit() -> Int? {
+            if idx >= data.count { return nil }
+            let bit = (Int(cur) >> 7) & 1
+            cur = UInt8((Int(cur) << 1) & 0xFF)
+            left -= 1
+            if left == 0 {
+                idx += 1
+                if idx < data.count { cur = data[idx]; left = 8 }
+            }
+            return bit
+        }
+        func readUnary() -> Int? {
+            var q = 0
+            while true {
+                guard let b = readBit() else { return nil }
+                if b == 1 { q += 1 } else { break }
+            }
+            return q
+        }
+        func readBits(count: Int) -> UInt64? {
+            var v: UInt64 = 0
+            for _ in 0..<count {
+                guard let b = readBit() else { return nil }
+                v = (v << 1) | UInt64(b)
+            }
+            return v
+        }
+    }
+}

bitchat/Sync/GossipSyncManager.swift

@@ -0,0 +1,200 @@
+import Foundation
+import CryptoKit
+
+// Gossip-based sync manager using on-demand GCS filters
+final class GossipSyncManager {
+    protocol Delegate: AnyObject {
+        func sendPacket(_ packet: BitchatPacket)
+        func sendPacket(to peerID: String, packet: BitchatPacket)
+        func signPacketForBroadcast(_ packet: BitchatPacket) -> BitchatPacket
+    }
+
+    struct Config {
+        var seenCapacity: Int = 1000          // max packets per sync (cap across types)
+        var gcsMaxBytes: Int = 400           // filter size budget (128..1024)
+        var gcsTargetFpr: Double = 0.01      // 1%
+    }
+
+    private let myPeerID: String
+    private let config: Config
+    weak var delegate: Delegate?
+
+    // Storage: broadcast messages (ordered by insert), and latest announce per sender
+    private var messages: [String: BitchatPacket] = [:] // idHex -> packet
+    private var messageOrder: [String] = []
+    private var latestAnnouncementByPeer: [String: (id: String, packet: BitchatPacket)] = [:]
+
+    // Timer
+    private var periodicTimer: DispatchSourceTimer?
+    private let queue = DispatchQueue(label: "mesh.sync", qos: .utility)
+
+    init(myPeerID: String, config: Config = Config()) {
+        self.myPeerID = myPeerID
+        self.config = config
+    }
+
+    func start() {
+        stop()
+        let timer = DispatchSource.makeTimerSource(queue: queue)
+        timer.schedule(deadline: .now() + 30.0, repeating: 30.0, leeway: .seconds(1))
+        timer.setEventHandler { [weak self] in self?.sendRequestSync() }
+        timer.resume()
+        periodicTimer = timer
+    }
+
+    func stop() {
+        periodicTimer?.cancel(); periodicTimer = nil
+    }
+
+    func scheduleInitialSyncToPeer(_ peerID: String, delaySeconds: TimeInterval = 5.0) {
+        queue.asyncAfter(deadline: .now() + delaySeconds) { [weak self] in
+            self?.sendRequestSync(to: peerID)
+        }
+    }
+
+    func onPublicPacketSeen(_ packet: BitchatPacket) {
+        let mt = MessageType(rawValue: packet.type)
+        let isBroadcastRecipient: Bool = {
+            guard let r = packet.recipientID else { return true }
+            return r.count == 8 && r.allSatisfy { $0 == 0xFF }
+        }()
+        let isBroadcastMessage = (mt == .message && isBroadcastRecipient)
+        let isAnnounce = (mt == .announce)
+        guard isBroadcastMessage || isAnnounce else { return }
+
+        let idHex = PacketIdUtil.computeId(packet).hexEncodedString()
+
+        if isBroadcastMessage {
+            if messages[idHex] == nil {
+                messages[idHex] = packet
+                messageOrder.append(idHex)
+                // Enforce capacity
+                let cap = max(1, config.seenCapacity)
+                while messageOrder.count > cap {
+                    let victim = messageOrder.removeFirst()
+                    messages.removeValue(forKey: victim)
+                }
+            }
+        } else if isAnnounce {
+            let sender = packet.senderID.hexEncodedString()
+            latestAnnouncementByPeer[sender] = (id: idHex, packet: packet)
+        }
+    }
+
+    private func sendRequestSync() {
+        let payload = buildGcsPayload()
+        let pkt = BitchatPacket(
+            type: MessageType.requestSync.rawValue,
+            senderID: Data(hexString: myPeerID) ?? Data(),
+            recipientID: nil, // broadcast
+            timestamp: UInt64(Date().timeIntervalSince1970 * 1000),
+            payload: payload,
+            signature: nil,
+            ttl: 0 // local-only
+        )
+        let signed = delegate?.signPacketForBroadcast(pkt) ?? pkt
+        delegate?.sendPacket(signed)
+    }
+
+    private func sendRequestSync(to peerID: String) {
+        let payload = buildGcsPayload()
+        var recipient = Data()
+        var temp = peerID
+        while temp.count >= 2 && recipient.count < 8 {
+            let hexByte = String(temp.prefix(2))
+            if let b = UInt8(hexByte, radix: 16) { recipient.append(b) }
+            temp = String(temp.dropFirst(2))
+        }
+        let pkt = BitchatPacket(
+            type: MessageType.requestSync.rawValue,
+            senderID: Data(hexString: myPeerID) ?? Data(),
+            recipientID: recipient,
+            timestamp: UInt64(Date().timeIntervalSince1970 * 1000),
+            payload: payload,
+            signature: nil,
+            ttl: 0 // local-only
+        )
+        let signed = delegate?.signPacketForBroadcast(pkt) ?? pkt
+        delegate?.sendPacket(to: peerID, packet: signed)
+    }
+
+    func handleRequestSync(fromPeerID: String, request: RequestSyncPacket) {
+        // Decode GCS into sorted set and prepare membership checker
+        let sorted = GCSFilter.decodeToSortedSet(p: request.p, m: request.m, data: request.data)
+        func mightContain(_ id: Data) -> Bool {
+            var hasher = SHA256()
+            hasher.update(data: id) // 16-byte PacketId
+            let digest = hasher.finalize()
+            let db = Data(digest)
+            var x: UInt64 = 0
+            let take = min(8, db.count)
+            for i in 0..<take { x = (x << 8) | UInt64(db[i]) }
+            let v = (x & 0x7fff_ffff_ffff_ffff) % UInt64(request.m)
+            return GCSFilter.contains(sortedValues: sorted, candidate: v)
+        }
+
+        // 1) Announcements: send latest per peer if requester lacks them
+        for (_, pair) in latestAnnouncementByPeer {
+            let (idHex, pkt) = pair
+            let idBytes = Data(hexString: idHex) ?? Data()
+            if !mightContain(idBytes) {
+                var toSend = pkt
+                toSend.ttl = 0
+                delegate?.sendPacket(to: fromPeerID, packet: toSend)
+            }
+        }
+
+        // 2) Broadcast messages: send all missing
+        let toSendMsgs = messageOrder.compactMap { messages[$0] }
+        for pkt in toSendMsgs {
+            let idBytes = PacketIdUtil.computeId(pkt)
+            if !mightContain(idBytes) {
+                var toSend = pkt
+                toSend.ttl = 0
+                delegate?.sendPacket(to: fromPeerID, packet: toSend)
+            }
+        }
+    }
+
+    // Build REQUEST_SYNC payload using current candidates and GCS params
+    private func buildGcsPayload() -> Data {
+        // Collect candidates: latest announce per peer + broadcast messages
+        var candidates: [BitchatPacket] = []
+        candidates.reserveCapacity(latestAnnouncementByPeer.count + messageOrder.count)
+        for (_, pair) in latestAnnouncementByPeer { candidates.append(pair.packet) }
+        for id in messageOrder { if let p = messages[id] { candidates.append(p) } }
+        // Sort by timestamp desc
+        candidates.sort { $0.timestamp > $1.timestamp }
+
+        let p = GCSFilter.deriveP(targetFpr: config.gcsTargetFpr)
+        let nMax = GCSFilter.estimateMaxElements(sizeBytes: config.gcsMaxBytes, p: p)
+        let cap = max(1, config.seenCapacity)
+        let takeN = min(candidates.count, min(nMax, cap))
+        if takeN <= 0 {
+            let req = RequestSyncPacket(p: p, m: 1, data: Data())
+            return req.encode()
+        }
+        let ids: [Data] = candidates.prefix(takeN).map { PacketIdUtil.computeId($0) }
+        let params = GCSFilter.buildFilter(ids: ids, maxBytes: config.gcsMaxBytes, targetFpr: config.gcsTargetFpr)
+        let req = RequestSyncPacket(p: params.p, m: params.m, data: params.data)
+        return req.encode()
+    }
+
+    // Explicit removal hook for LEAVE/stale peer
+    func removeAnnouncementForPeer(_ peerID: String) {
+        let normalizedPeerID = peerID.lowercased()
+        _ = latestAnnouncementByPeer.removeValue(forKey: normalizedPeerID)
+        
+        // Remove messages from this peer
+        // Collect IDs to remove first to avoid concurrent modification
+        let messageIdsToRemove = messages.compactMap { (id, message) -> String? in
+            message.senderID.hexEncodedString().lowercased() == normalizedPeerID ? id : nil
+        }
+        
+        // Remove messages and update messageOrder
+        for id in messageIdsToRemove {
+            messages.removeValue(forKey: id)
+            messageOrder.removeAll { $0 == id }
+        }
+    }
+}

bitchat/Sync/PacketIdUtil.swift

@@ -0,0 +1,21 @@
+import Foundation
+import CryptoKit
+
+// Deterministic packet ID used for gossip sync membership
+// ID = first 16 bytes of SHA-256 over: [type | senderID | timestamp | payload]
+enum PacketIdUtil {
+    static func computeId(_ packet: BitchatPacket) -> Data {
+        var hasher = SHA256()
+        hasher.update(data: Data([packet.type]))
+        hasher.update(data: packet.senderID)
+        var tsBE = packet.timestamp.bigEndian
+        withUnsafeBytes(of: &tsBE) { raw in hasher.update(data: Data(raw)) }
+        hasher.update(data: packet.payload)
+        let digest = hasher.finalize()
+        return Data(digest.prefix(16))
+    }
+
+    static func computeIdHex(_ packet: BitchatPacket) -> String {
+        return computeId(packet).hexEncodedString()
+    }
+}