Add OrderBuffer and time synchronisation.

OpenRA.Game/Server/OrderBuffer.cs

@@ -0,0 +1,136 @@
+#region Copyright & License Information
+/*
+ * Copyright 2007-2022 The OpenRA Developers (see AUTHORS)
+ * This file is part of OpenRA, which is free software. It is made
+ * available to you under the terms of the GNU General Public License
+ * as published by the Free Software Foundation, either version 3 of
+ * the License, or (at your option) any later version. For more
+ * information, see COPYING.
+ */
+#endregion
+
+using System;
+using System.Collections.Concurrent;
+using System.Collections.Generic;
+using System.Diagnostics;
+using System.Linq;
+using System.Threading;
+
+namespace OpenRA.Server
+{
+	public class OrderBuffer
+	{
+		const int NumberOfFrames = 20;
+		const int Interval = 1000;
+
+		// Limit the TickScale to maximum of 10%
+		const float MaxTickScale = 1.1f;
+
+		const int EmptyValue = -1;
+
+		Stopwatch gameTimer;
+		long nextUpdate = 0;
+
+		readonly ConcurrentDictionary<int, long> timestamps = new ConcurrentDictionary<int, long>();
+		readonly ConcurrentDictionary<int, Queue<long>> deltas = new ConcurrentDictionary<int, Queue<long>>();
+
+		int timestep;
+		int ticksPerInterval;
+		int baselinePlayer;
+		List<int> players;
+
+		public void AddOrderTimestamp(int playerIndex)
+		{
+			timestamps[playerIndex] = gameTimer.ElapsedMilliseconds;
+
+			if (timestamps.Values.All(t => t != EmptyValue))
+			{
+				var baseline = timestamps[baselinePlayer];
+
+				foreach (var (p, q) in timestamps)
+				{
+					var dt = baseline - q;
+
+					var playerDeltas = deltas[p];
+					playerDeltas.Enqueue(dt);
+
+					if (playerDeltas.Count > NumberOfFrames)
+						playerDeltas.Dequeue();
+
+					timestamps[p] = EmptyValue;
+				}
+			}
+		}
+
+		public void Start(GameSpeed gameSpeed, IEnumerable<int> players)
+		{
+			timestep = gameSpeed.Timestep;
+			ticksPerInterval = Interval / timestep;
+
+			this.players = players.ToList();
+			baselinePlayer = this.players.First();
+
+			foreach (var player in this.players)
+			{
+				timestamps.TryAdd(player, EmptyValue);
+				deltas.TryAdd(player, new Queue<long>());
+			}
+
+			gameTimer = Stopwatch.StartNew();
+			nextUpdate = gameTimer.ElapsedMilliseconds + Interval;
+		}
+
+		public IEnumerable<(int PlayerIndex, float TickScale)> GetTickScales()
+		{
+			var now = gameTimer.ElapsedMilliseconds;
+			if (now < nextUpdate)
+				yield break;
+
+			nextUpdate = now + Interval;
+
+			if (deltas.Values.Any(q => q.Count != NumberOfFrames))
+				yield break;
+
+			var medians = deltas.Select(d => (PlayerIndex: d.Key, Delta: Median(d.Value.ToArray()))).ToList();
+
+			// We need to check if we have a connection slower than our baseline and then use that as our offset.
+			var minValue = medians.MinBy(p => p.Delta).Delta;
+			var offset = minValue < 0 ? Math.Abs(minValue) : 0;
+
+			foreach (var (playerIndex, delta) in medians)
+			{
+				var deltaPerTick = (delta + offset) / (float)ticksPerInterval;
+
+				var tickScale = (timestep + deltaPerTick) / timestep;
+
+				var adjustedTickScale = tickScale.Clamp(1f, MaxTickScale);
+
+				yield return (playerIndex, adjustedTickScale);
+			}
+		}
+
+		long Median(long[] a)
+		{
+			Array.Sort(a);
+			var n = a.Length;
+
+			if (n % 2 != 0)
+				return a[n / 2];
+
+			return (a[(n - 1) / 2] + a[n / 2]) / 2;
+		}
+
+		public void RemovePlayer(int player)
+		{
+			players.Remove(player);
+			if (player == baselinePlayer && players.Count > 0)
+			{
+				var newBaseline = players.First();
+				Interlocked.Exchange(ref baselinePlayer, newBaseline);
+			}
+
+			timestamps.TryRemove(player, out _);
+			deltas.TryRemove(player, out _);
+		}
+	}
+}

OpenRA.Game/Server/Server.cs

@@ -69,6 +69,8 @@ namespace OpenRA.Server
 		readonly TypeDictionary serverTraits = new TypeDictionary();
 		readonly PlayerDatabase playerDatabase;
 
+		OrderBuffer orderBuffer;
+
 		volatile ServerState internalState = ServerState.WaitingPlayers;
 
 		readonly BlockingCollection<IServerEvent> events = new BlockingCollection<IServerEvent>();
@@ -361,6 +363,18 @@ namespace OpenRA.Server
 
 						foreach (var t in serverTraits.WithInterface<ITick>())
 							t.Tick(this);
+
+						if (State == ServerState.GameStarted)
+						{
+							foreach (var (playerIndex, scale) in orderBuffer.GetTickScales())
+							{
+								var frame = CreateTickScaleFrame(scale);
+								var con = Conns.SingleOrDefault(c => c.PlayerIndex == playerIndex);
+
+								if (con != null && con.Validated)
+									DispatchFrameToClient(con, playerIndex, frame);
+							}
+						}
 					}
 
 					if (State == ServerState.ShuttingDown)
@@ -890,6 +904,8 @@ namespace OpenRA.Server
 					frame += OrderLatency;
 					DispatchFrameToClient(conn, conn.PlayerIndex, CreateAckFrame(frame, 1));
 
+					orderBuffer.AddOrderTimestamp(conn.PlayerIndex);
+
 					// Track the last frame for each client so the disconnect handling can write
 					// an EndOfOrders marker with the correct frame number.
 					// TODO: This should be handled by the order buffering system too
@@ -1155,6 +1171,7 @@ namespace OpenRA.Server
 		{
 			lock (LobbyInfo)
 			{
+				orderBuffer?.RemovePlayer(toDrop.PlayerIndex);
 				Conns.Remove(toDrop);
 
 				var dropClient = LobbyInfo.Clients.FirstOrDefault(c => c.Index == toDrop.PlayerIndex);
@@ -1330,12 +1347,16 @@ namespace OpenRA.Server
 				SyncLobbyInfo();
 				State = ServerState.GameStarted;
 
+				var gameSpeeds = Game.ModData.Manifest.Get<GameSpeeds>();
+				var gameSpeedName = LobbyInfo.GlobalSettings.OptionOrDefault("gamespeed", gameSpeeds.DefaultSpeed);
+
+				var gameSpeed = gameSpeeds.Speeds[gameSpeedName];
+
+				orderBuffer = new OrderBuffer();
+				orderBuffer.Start(gameSpeed, Conns.Where(c => c.Validated).Select(c => c.PlayerIndex));
+
 				if (Type != ServerType.Local)
-				{
-					var gameSpeeds = Game.ModData.Manifest.Get<GameSpeeds>();
-					var gameSpeedName = LobbyInfo.GlobalSettings.OptionOrDefault("gamespeed", gameSpeeds.DefaultSpeed);
-					OrderLatency = gameSpeeds.Speeds[gameSpeedName].OrderLatency;
-				}
+					OrderLatency = gameSpeed.OrderLatency;
 
 				if (GameSave == null && LobbyInfo.GlobalSettings.GameSavesEnabled)
 					GameSave = new GameSave();