Plumtree sync (#393)

* wip plumtree

* sync works

* fix logging

* ttl to 0

* fix send packet to one peer

* spec

* wip GCS instead of bloom

* remove bloom filter remainders

* clean

* prune old announcements

* remove announcements from sync after LEAVE

* sync after 1 second

* pruning

* track own announcement and prune messages without announcements

* fix pruning

* getGcsMaxFilterBytes default value 400 bytes

* parameters

app/src/main/java/com/bitchat/android/sync/PacketIdUtil.kt

@@ -0,0 +1,31 @@
+package com.bitchat.android.sync
+
+import com.bitchat.android.protocol.BitchatPacket
+import java.security.MessageDigest
+
+/**
+ * Deterministic packet ID helper for sync purposes.
+ * Uses SHA-256 over a canonical subset of packet fields:
+ * [type | senderID | timestamp | payload] to generate a stable ID.
+ * Returns a 16-byte (128-bit) truncated hash for compactness.
+ */
+object PacketIdUtil {
+    fun computeIdBytes(packet: BitchatPacket): ByteArray {
+        val md = MessageDigest.getInstance("SHA-256")
+        md.update(packet.type.toByte())
+        md.update(packet.senderID)
+        // Timestamp as 8 bytes big-endian
+        val ts = packet.timestamp.toLong()
+        for (i in 7 downTo 0) {
+            md.update(((ts ushr (i * 8)) and 0xFF).toByte())
+        }
+        md.update(packet.payload)
+        val digest = md.digest()
+        return digest.copyOf(16) // 128-bit ID
+    }
+
+    fun computeIdHex(packet: BitchatPacket): String {
+        return computeIdBytes(packet).joinToString("") { b -> "%02x".format(b) }
+    }
+}
+