Implement AMCI (#681)

Main.cs

@@ -931,6 +931,7 @@ public class Main : BasePlugin
         BanManager.Init();
         TemplateManager.Init();
         SpamManager.Init();
+        AmciRegistration.Apply();
 
         IRandom.SetInstance(new NetRandomWrapper());
 

Modules/AmciRegistration.cs

@@ -0,0 +1,69 @@
+using System;
+using AmongUs.GameOptions;
+using HarmonyLib;
+using Hazel;
+using InnerNet;
+
+namespace EHR.Modules;
+
+/// <summary>
+/// Handles AU MCI (Among Us Modded Client Identification) registration for EHR.
+/// See: https://github.com/Innersloth-LLC/AmongUsModdingInformation
+///
+/// On startup, EHR's GUID is written into the game's mod registration slot.
+/// The matchmaker picks this up automatically and injects a mod filter into
+/// every lobby search, so Find Game exclusively returns EHR lobbies to EHR clients.
+///
+/// The hosting side is handled by <see cref="HostGamePatch"/>, which appends
+/// EHR's GUID to the host packet so Innersloth's servers register the lobby
+/// as modded and remove it from the vanilla matchmaking pool.
+/// </summary>
+public static class AmciRegistration
+{
+    /// <summary>
+    /// EHR's AMCI mod GUID. Do not change this after it ships to players or they won't be able
+    /// to find EHR lobbies until they update.
+    /// </summary>
+    private const string ModGuidString = "446420cc-ce35-4c2f-8f5d-b14f84354421";
+
+    /// <summary>
+    /// Writes EHR's GUID into the game's mod registration slot so the matchmaker
+    /// can filter lobby searches to EHR lobbies only. 
+    /// Call once from <see cref="Main.Load"/>
+    /// </summary>
+    public static void Apply()
+    {
+        CurrentModRegistration.ModRegistrationGuidString = ModGuidString;
+        Logger.Info($"AMCI mod GUID registered: {ModGuidString}", nameof(AmciRegistration));
+    }
+
+    /// <summary>
+    /// Replaces the standard HostGame packet with the modded variant and appends
+    /// EHR's GUID, so Innersloth's servers register this lobby as a modded EHR
+    /// lobby and remove it from vanilla matchmaking.
+    /// </summary>
+    [HarmonyPatch(typeof(InnerNetClient), nameof(InnerNetClient.HostGame))]
+    public static class HostGamePatch
+    {
+        public static bool Prefix(InnerNetClient __instance, IGameOptions settings, GameFilterOptions filterOpts)
+        {
+            if (!Guid.TryParse(ModGuidString, out Guid guid))
+            {
+                Logger.Warn("Failed to parse AMCI mod GUID, falling back to standard HostGame", nameof(AmciRegistration));
+                return true;
+            }
+
+            MessageWriter msg = MessageWriter.Get(SendOption.Reliable);
+            msg.StartMessage(Tags.HostModdedGame);
+            msg.WriteBytesAndSize(__instance.gameOptionsFactory.ToBytes(settings, AprilFoolsMode.IsAprilFoolsModeToggledOn));
+            msg.Write(CrossplayMode.GetCrossplayFlags());
+            filterOpts.Serialize(msg);
+            msg.Write(guid.ToByteArray());
+            msg.EndMessage();
+            __instance.SendOrDisconnect(msg);
+            msg.Recycle();
+
+            return false;
+        }
+    }
+}