Disable Extrapolation (#803)

* Split GameState processing logic out of ClientGameStateManager into the new GameStateProcessor class.

* Adds a test fixture for GameStateProcessor with a basic use test.

* More unit tests and added Extrapolated property to GameStateProcessor.

Robust.Client/GameStates/ClientGameStateManager.cs

@@ -1,11 +1,9 @@
-using System.Collections.Generic;
-using Robust.Client.Interfaces;
+using Robust.Client.Interfaces;
 using Robust.Client.Interfaces.GameObjects;
 using Robust.Client.Interfaces.GameStates;
 using Robust.Shared.GameStates;
 using Robust.Shared.Interfaces.Network;
 using Robust.Shared.IoC;
-using Robust.Shared.Log;
 using Robust.Shared.Network.Messages;
 using Robust.Client.Player;
 using Robust.Shared.Configuration;
@@ -13,20 +11,14 @@ using Robust.Shared.Interfaces.Configuration;
 using Robust.Shared.Interfaces.Map;
 using Robust.Shared.Interfaces.Timing;
 using Robust.Shared.Timing;
-using Robust.Shared.Utility;
 
 namespace Robust.Client.GameStates
 {
     /// <inheritdoc />
     public class ClientGameStateManager : IClientGameStateManager
     {
-        private readonly List<GameState> _stateBuffer = new List<GameState>();
-        private GameState _lastFullState;
-        private bool _waitingForFull = true;
-        private bool _interpEnabled;
-        private int _interpRatio;
-        private bool _logging;
-
+        private GameStateProcessor _processor;
+        
         [Dependency] private readonly IClientEntityManager _entities;
         [Dependency] private readonly IPlayerManager _players;
         [Dependency] private readonly IClientNetManager _network;
@@ -36,41 +28,38 @@ namespace Robust.Client.GameStates
         [Dependency] private readonly IConfigurationManager _config;
 
         /// <inheritdoc />
-        public int MinBufferSize => _interpEnabled ? 3 : 2;
+        public int MinBufferSize => _processor.MinBufferSize;
 
         /// <inheritdoc />
-        public int TargetBufferSize => MinBufferSize + _interpRatio;
+        public int TargetBufferSize => _processor.TargetBufferSize;
 
         /// <inheritdoc />
         public void Initialize()
         {
+            _processor = new GameStateProcessor(_timing);
+
             _network.RegisterNetMessage<MsgState>(MsgState.NAME, HandleStateMessage);
             _network.RegisterNetMessage<MsgStateAck>(MsgStateAck.NAME);
             _client.RunLevelChanged += RunLevelChanged;
 
             if(!_config.IsCVarRegistered("net.interp"))
-                _config.RegisterCVar("net.interp", false, CVar.ARCHIVE, b => _interpEnabled = b);
+                _config.RegisterCVar("net.interp", false, CVar.ARCHIVE, b => _processor.Interpolation = b);
 
             if (!_config.IsCVarRegistered("net.interp_ratio"))
-                _config.RegisterCVar("net.interp_ratio", 0, CVar.ARCHIVE, i => _interpRatio = i < 0 ? 0 : i);
+                _config.RegisterCVar("net.interp_ratio", 0, CVar.ARCHIVE, i => _processor.InterpRatio = i);
 
             if (!_config.IsCVarRegistered("net.logging"))
-                _config.RegisterCVar("net.logging", false, CVar.ARCHIVE, b => _logging = b);
+                _config.RegisterCVar("net.logging", false, CVar.ARCHIVE, b => _processor.Logging = b);
 
-            _interpEnabled = _config.GetCVar<bool>("net.interp");
-
-            _interpRatio = _config.GetCVar<int>("net.interp_ratio");
-            _interpRatio = _interpRatio < 0 ? 0 : _interpRatio; // min bound, < 0 makes no sense
-
-            _logging = _config.GetCVar<bool>("net.logging");
+            _processor.Interpolation = _config.GetCVar<bool>("net.interp");
+            _processor.InterpRatio = _config.GetCVar<int>("net.interp_ratio");
+            _processor.Logging = _config.GetCVar<bool>("net.logging");
         }
 
         /// <inheritdoc />
         public void Reset()
         {
-            _stateBuffer.Clear();
-            _lastFullState = null;
-            _waitingForFull = true;
+            _processor.Reset();
         }
 
         private void RunLevelChanged(object sender, RunLevelChangedEventArgs args)
@@ -78,7 +67,7 @@ namespace Robust.Client.GameStates
             if (args.NewLevel == ClientRunLevel.Initialize)
             {
                 // We JUST left a server or the client started up, Reset everything.
-                _stateBuffer.Clear();
+                Reset();
             }
         }
 
@@ -86,207 +75,19 @@ namespace Robust.Client.GameStates
         {
             var state = message.State;
 
+            _processor.AddNewState(state, message.MsgSize);
+
             // we always ack everything we receive, even if it is late
             AckGameState(state.ToSequence);
-
-            // any state from tick 0 is a full state, and needs to be handled different
-            if (state.FromSequence == GameTick.Zero)
-            {
-                // this is a newer full state, so discard the older one.
-                if(_lastFullState == null || (_lastFullState != null && _lastFullState.ToSequence < state.ToSequence))
-                {
-                    _lastFullState = state;
-
-                    if(_logging)
-                        Logger.InfoS("net", $"Received Full GameState: to={state.ToSequence}, sz={message.MsgSize}");
-
-                    return;
-                }
-            }
-
-            // NOTE: DispatchTick will be modifying CurTick, this is NOT thread safe.
-            var lastTick = new GameTick(_timing.CurTick.Value - 1);
-
-            if (state.ToSequence <= lastTick && !_waitingForFull) // CurTick isn't set properly when WaitingForFull
-            {
-                if (_logging)
-                    Logger.DebugS("net.state", $"Received Old GameState: cTick={_timing.CurTick}, fSeq={state.FromSequence}, tSeq={state.ToSequence}, sz={message.MsgSize}, buf={_stateBuffer.Count}");
-
-                return;
-            }
-
-            // lets check for a duplicate state now.
-            for (var i = 0; i < _stateBuffer.Count; i++)
-            {
-                var iState = _stateBuffer[i];
-
-                if (state.ToSequence != iState.ToSequence)
-                    continue;
-
-                if (iState.Extrapolated)
-                {
-                    _stateBuffer.RemoveSwap(i); // remove the fake extrapolated state
-                    break; // break from the loop, add the new state normally
-                }
-
-                if (_logging)
-                    Logger.DebugS("net.state", $"Received Dupe GameState: cTick={_timing.CurTick}, fSeq={state.FromSequence}, tSeq={state.ToSequence}, sz={message.MsgSize}, buf={_stateBuffer.Count}");
-
-                return;
-            }
-
-            // this is a good state that we will be using.
-            _stateBuffer.Add(state);
-
-            if (_logging)
-                Logger.DebugS("net.state", $"Received New GameState: cTick={_timing.CurTick}, fSeq={state.FromSequence}, tSeq={state.ToSequence}, sz={message.MsgSize}, buf={_stateBuffer.Count}");
         }
-
+        
         /// <inheritdoc />
         public void ApplyGameState()
         {
-            if (CalculateNextStates(_timing.CurTick, out var curState, out var nextState, TargetBufferSize))
-            {
-                if (_logging)
-                    Logger.DebugS("net.state", $"Applying State:  ext={curState.Extrapolated}, cTick={_timing.CurTick}, fSeq={curState.FromSequence}, tSeq={curState.ToSequence}, buf={_stateBuffer.Count}");
+            if (!_processor.ProcessTickStates(_timing.CurTick, out var curState, out var nextState))
+                return;
 
-                ApplyGameState(curState, nextState);
-            }
-
-            if (!_waitingForFull)
-            {
-                // This will slightly speed up or slow down the client tickrate based on the contents of the buffer.
-                // CalcNextState should have just cleaned out any old states, so the buffer contains [t-1(last), t+0(cur), t+1(next), t+2, t+3, ..., t+n]
-                // we can use this info to properly time our tickrate so it does not run fast or slow compared to the server.
-                _timing.TickTimingAdjustment = (_stateBuffer.Count - (float) TargetBufferSize) * 0.10f;
-            }
-            else
-            {
-                _timing.TickTimingAdjustment = 0f;
-            }
-        }
-
-        private bool CalculateNextStates(GameTick curTick, out GameState curState, out GameState nextState, int targetBufferSize)
-        {
-            if (_waitingForFull)
-            {
-                return CalculateFullState(out curState, out nextState, targetBufferSize);
-            }
-            else // this will be how almost all states are calculated
-            {
-                return CalculateDeltaState(curTick, out curState, out nextState);
-            }
-        }
-
-        private bool CalculateFullState(out GameState curState, out GameState nextState, int targetBufferSize)
-        {
-            if (_lastFullState != null)
-            {
-                if (_logging)
-                    Logger.DebugS("net", $"Resync CurTick to: {_lastFullState.ToSequence}");
-
-                var curTick = _timing.CurTick = _lastFullState.ToSequence;
-
-                if (_interpEnabled)
-                {
-                    // look for the next state
-                    var lastTick = new GameTick(curTick.Value - 1);
-                    var nextTick = new GameTick(curTick.Value + 1);
-                    nextState = null;
-
-                    for (var i = 0; i < _stateBuffer.Count; i++)
-                    {
-                        var state = _stateBuffer[i];
-                        if (state.ToSequence == nextTick)
-                        {
-                            nextState = state;
-                        }
-                        else if (state.ToSequence < lastTick) // remove any old states we find to keep the buffer clean
-                        {
-                            _stateBuffer.RemoveSwap(i);
-                            i--;
-                        }
-                    }
-
-                    // we let the buffer fill up before starting to tick
-                    if (nextState != null && _stateBuffer.Count >= targetBufferSize)
-                    {
-                        curState = _lastFullState;
-                        _waitingForFull = false;
-                        return true;
-                    }
-                }
-                else if (_stateBuffer.Count >= targetBufferSize)
-                {
-                    curState = _lastFullState;
-                    nextState = default;
-                    _waitingForFull = false;
-                    return true;
-                }
-            }
-
-            if (_logging)
-                Logger.DebugS("net", $"Have FullState, filling buffer... ({_stateBuffer.Count}/{targetBufferSize})");
-
-            // waiting for full state or buffer to fill
-            curState = default;
-            nextState = default;
-            return false;
-        }
-
-        private bool CalculateDeltaState(GameTick curTick, out GameState curState, out GameState nextState)
-        {
-            var lastTick = new GameTick(curTick.Value - 1);
-            var nextTick = new GameTick(curTick.Value + 1);
-
-            curState = null;
-            nextState = null;
-
-            for (var i = 0; i < _stateBuffer.Count; i++)
-            {
-                var state = _stateBuffer[i];
-
-                // remember there are no duplicate ToSequence states in the list.
-                if (state.ToSequence == curTick)
-                {
-                    curState = state;
-                }
-                else if (_interpEnabled && state.ToSequence == nextTick)
-                {
-                    nextState = state;
-                }
-                else if (state.ToSequence < lastTick) // remove any old states we find to keep the buffer clean
-                {
-                    _stateBuffer.RemoveSwap(i);
-                    i--;
-                }
-            }
-
-            // we found both the states to interpolate between, this should almost always be true.
-            if ((_interpEnabled && curState != null) || (!_interpEnabled && curState != null && nextState != null))
-                return true;
-
-            if (curState == null)
-            {
-                curState = ExtrapolateState(lastTick, curTick);
-            }
-
-            if (nextState == null && _interpEnabled)
-            {
-                nextState = ExtrapolateState(curTick, nextTick);
-            }
-
-            return true;
-        }
-
-        /// <summary>
-        ///     Generates a completely fake GameState.
-        /// </summary>
-        private static GameState ExtrapolateState(GameTick fromSequence, GameTick toSequence)
-        {
-           var state = new GameState(fromSequence, toSequence, null, null, null, null);
-           state.Extrapolated = true;
-           return state;
+            ApplyGameState(curState, nextState);
         }
 
         private void AckGameState(GameTick sequence)

Robust.Client/GameStates/GameStateProcessor.cs

@@ -0,0 +1,268 @@
+using System.Collections.Generic;
+using Robust.Shared.GameStates;
+using Robust.Shared.Interfaces.Timing;
+using Robust.Shared.Log;
+using Robust.Shared.Timing;
+using Robust.Shared.Utility;
+
+namespace Robust.Client.GameStates
+{
+    /// <inheritdoc />
+    internal class GameStateProcessor : IGameStateProcessor
+    {
+        private readonly IGameTiming _timing;
+
+        private readonly List<GameState> _stateBuffer = new List<GameState>();
+        private GameState _lastFullState;
+        private bool _waitingForFull = true;
+        private int _interpRatio;
+        private GameTick _lastProcessedRealState;
+
+        /// <inheritdoc />
+        public int MinBufferSize => Interpolation ? 3 : 2;
+
+        /// <inheritdoc />
+        public int TargetBufferSize => MinBufferSize + InterpRatio;
+
+        /// <inheritdoc />
+        public int CurrentBufferSize => _stateBuffer.Count;
+
+        /// <inheritdoc />
+        public bool Interpolation { get; set; }
+
+        /// <inheritdoc />
+        public int InterpRatio
+        {
+            get => _interpRatio;
+            set => _interpRatio = value < 0 ? 0 : value;
+        }
+
+        /// <inheritdoc />
+        public bool Extrapolation { get; set; }
+
+        /// <inheritdoc />
+        public bool Logging { get; set; }
+
+        /// <summary>
+        ///     Constructs a new instance of <see cref="GameStateProcessor"/>.
+        /// </summary>
+        /// <param name="timing">Timing information of the current state.</param>
+        public GameStateProcessor(IGameTiming timing)
+        {
+            _timing = timing;
+        }
+
+        /// <inheritdoc />
+        public void AddNewState(GameState state, int stateSize)
+        {
+            // any state from tick 0 is a full state, and needs to be handled different
+            if (state.FromSequence == GameTick.Zero)
+            {
+                // this is a newer full state, so discard the older one.
+                if (_lastFullState == null || (_lastFullState != null && _lastFullState.ToSequence < state.ToSequence))
+                {
+                    _lastFullState = state;
+
+                    if (Logging)
+                        Logger.InfoS("net", $"Received Full GameState: to={state.ToSequence}, sz={stateSize}");
+
+                    return;
+                }
+            }
+
+            // NOTE: DispatchTick will be modifying CurTick, this is NOT thread safe.
+            var lastTick = new GameTick(_timing.CurTick.Value - 1);
+
+            if (state.ToSequence <= lastTick && !_waitingForFull) // CurTick isn't set properly when WaitingForFull
+            {
+                if (Logging)
+                    Logger.DebugS("net.state", $"Received Old GameState: cTick={_timing.CurTick}, fSeq={state.FromSequence}, tSeq={state.ToSequence}, sz={stateSize}, buf={_stateBuffer.Count}");
+
+                return;
+            }
+
+            // lets check for a duplicate state now.
+            for (var i = 0; i < _stateBuffer.Count; i++)
+            {
+                var iState = _stateBuffer[i];
+
+                if (state.ToSequence != iState.ToSequence)
+                    continue;
+
+                if (Logging)
+                    Logger.DebugS("net.state", $"Received Dupe GameState: cTick={_timing.CurTick}, fSeq={state.FromSequence}, tSeq={state.ToSequence}, sz={stateSize}, buf={_stateBuffer.Count}");
+
+                return;
+            }
+
+            // this is a good state that we will be using.
+            _stateBuffer.Add(state);
+
+            if (Logging)
+                Logger.DebugS("net.state", $"Received New GameState: cTick={_timing.CurTick}, fSeq={state.FromSequence}, tSeq={state.ToSequence}, sz={stateSize}, buf={_stateBuffer.Count}");
+        }
+
+        /// <inheritdoc />
+        public bool ProcessTickStates(GameTick curTick, out GameState curState, out GameState nextState)
+        {
+            bool applyNextState;
+            if (_waitingForFull)
+            {
+                applyNextState = CalculateFullState(out curState, out nextState, TargetBufferSize);
+            }
+            else // this will be how almost all states are calculated
+            {
+                applyNextState = CalculateDeltaState(curTick, out curState, out nextState);
+            }
+
+            if (applyNextState && !curState.Extrapolated)
+                _lastProcessedRealState = curState.ToSequence;
+            
+            if (!_waitingForFull)
+            {
+                if (!applyNextState)
+                    _timing.CurTick = _lastProcessedRealState;
+
+                // This will slightly speed up or slow down the client tickrate based on the contents of the buffer.
+                // CalcNextState should have just cleaned out any old states, so the buffer contains [t-1(last), t+0(cur), t+1(next), t+2, t+3, ..., t+n]
+                // we can use this info to properly time our tickrate so it does not run fast or slow compared to the server.
+                _timing.TickTimingAdjustment = (_stateBuffer.Count - (float)TargetBufferSize) * 0.10f;
+            }
+            else
+            {
+                _timing.TickTimingAdjustment = 0f;
+            }
+
+            if (Logging && applyNextState)
+                Logger.DebugS("net.state", $"Applying State:  ext={curState.Extrapolated}, cTick={_timing.CurTick}, fSeq={curState.FromSequence}, tSeq={curState.ToSequence}, buf={_stateBuffer.Count}");
+
+            return applyNextState;
+        }
+
+        private bool CalculateFullState(out GameState curState, out GameState nextState, int targetBufferSize)
+        {
+            if (_lastFullState != null)
+            {
+                if (Logging)
+                    Logger.DebugS("net", $"Resync CurTick to: {_lastFullState.ToSequence}");
+
+                var curTick = _timing.CurTick = _lastFullState.ToSequence;
+
+                if (Interpolation)
+                {
+                    // look for the next state
+                    var lastTick = new GameTick(curTick.Value - 1);
+                    var nextTick = new GameTick(curTick.Value + 1);
+                    nextState = null;
+
+                    for (var i = 0; i < _stateBuffer.Count; i++)
+                    {
+                        var state = _stateBuffer[i];
+                        if (state.ToSequence == nextTick)
+                        {
+                            nextState = state;
+                        }
+                        else if (state.ToSequence < lastTick) // remove any old states we find to keep the buffer clean
+                        {
+                            _stateBuffer.RemoveSwap(i);
+                            i--;
+                        }
+                    }
+
+                    // we let the buffer fill up before starting to tick
+                    if (nextState != null && _stateBuffer.Count >= targetBufferSize)
+                    {
+                        curState = _lastFullState;
+                        _waitingForFull = false;
+                        return true;
+                    }
+                }
+                else if (_stateBuffer.Count >= targetBufferSize)
+                {
+                    curState = _lastFullState;
+                    nextState = default;
+                    _waitingForFull = false;
+                    return true;
+                }
+            }
+
+            if (Logging)
+                Logger.DebugS("net", $"Have FullState, filling buffer... ({_stateBuffer.Count}/{targetBufferSize})");
+
+            // waiting for full state or buffer to fill
+            curState = default;
+            nextState = default;
+            return false;
+        }
+
+        private bool CalculateDeltaState(GameTick curTick, out GameState curState, out GameState nextState)
+        {
+            var lastTick = new GameTick(curTick.Value - 1);
+            var nextTick = new GameTick(curTick.Value + 1);
+
+            curState = null;
+            nextState = null;
+
+            for (var i = 0; i < _stateBuffer.Count; i++)
+            {
+                var state = _stateBuffer[i];
+
+                // remember there are no duplicate ToSequence states in the list.
+                if (state.ToSequence == curTick)
+                {
+                    curState = state;
+                }
+                else if (Interpolation && state.ToSequence == nextTick)
+                {
+                    nextState = state;
+                }
+                else if (state.ToSequence < lastTick) // remove any old states we find to keep the buffer clean
+                {
+                    _stateBuffer.RemoveSwap(i);
+                    i--;
+                }
+            }
+
+            // we won't extrapolate, and curState was not found.
+            if (!Extrapolation && curState == null)
+                return false;
+
+            // we found both the states to interpolate between, this should almost always be true.
+            if (Interpolation && curState != null)
+                return true;
+
+            if (!Interpolation && curState != null && nextState != null)
+                return true;
+
+            if (curState == null)
+            {
+                curState = ExtrapolateState(lastTick, curTick);
+            }
+
+            if (nextState == null && Interpolation)
+            {
+                nextState = ExtrapolateState(curTick, nextTick);
+            }
+
+            return true;
+        }
+
+        /// <summary>
+        ///     Generates a completely fake GameState.
+        /// </summary>
+        private static GameState ExtrapolateState(GameTick fromSequence, GameTick toSequence)
+        {
+            var state = new GameState(fromSequence, toSequence, null, null, null, null);
+            state.Extrapolated = true;
+            return state;
+        }
+
+        /// <inheritdoc />
+        public void Reset()
+        {
+            _stateBuffer.Clear();
+            _lastFullState = null;
+            _waitingForFull = true;
+        }
+    }
+}

Robust.Client/GameStates/IGameStateProcessor.cs

@@ -0,0 +1,76 @@
+using Robust.Shared.GameStates;
+using Robust.Shared.Timing;
+
+namespace Robust.Client.GameStates
+{
+    /// <summary>
+    ///     Holds a collection of game states and calculates which ones to apply at a given game tick.
+    /// </summary>
+    internal interface IGameStateProcessor
+    {
+        /// <summary>
+        ///     Minimum number of states needed in the buffer for everything to work.
+        /// </summary>
+        /// <remarks>
+        ///     With interpolation enabled minimum is 3 states in buffer for the system to work (last, cur, next).
+        ///     Without interpolation enabled minimum is 2 states in buffer for the system to work (last, cur).
+        /// </remarks>
+        int MinBufferSize { get; }
+
+        /// <summary>
+        ///     The number of states the system is trying to keep in the buffer. This will always
+        ///     be greater or equal to <see cref="MinBufferSize"/>.
+        /// </summary>
+        int TargetBufferSize { get; }
+
+        /// <summary>
+        ///     Number of game states currently in the state buffer.
+        /// </summary>
+        int CurrentBufferSize { get; }
+
+        /// <summary>
+        ///     Is frame interpolation turned on?
+        /// </summary>
+        bool Interpolation { get; set; }
+
+        /// <summary>
+        ///     The target number of states to keep in the buffer for network smoothing.
+        /// </summary>
+        /// <remarks>
+        ///     For Lan, set this to 0. For Excellent net conditions, set this to 1. For normal network conditions,
+        ///     set this to 2. For worse conditions, set it higher.
+        /// </remarks>
+        int InterpRatio { get; set; }
+
+        /// <summary>
+        ///     If the client clock runs ahead of the server and the buffer gets emptied, should fake extrapolated states be generated?
+        /// </summary>
+        bool Extrapolation { get; set; }
+
+        /// <summary>
+        ///     Is debug logging enabled? This will dump debug info about every state to the log.
+        /// </summary>
+        bool Logging { get; set; }
+
+        /// <summary>
+        ///     Adds a new state into the processor. These are usually from networking or replays.
+        /// </summary>
+        /// <param name="state">Newly received state.</param>
+        /// <param name="stateSize">Optionally provide the size in bytes of this new state. This is strictly for debug logging.</param>
+        void AddNewState(GameState state, int stateSize);
+
+        /// <summary>
+        ///     Calculates the current and next state to apply for a given game tick.
+        /// </summary>
+        /// <param name="curTick">Tick to get the states for.</param>
+        /// <param name="curState">Current state for the given tick. This can be null.</param>
+        /// <param name="nextState">Current state for tick + 1. This can be null.</param>
+        /// <returns>Was the function able to correctly calculate the states for the given tick?</returns>
+        bool ProcessTickStates(GameTick curTick, out GameState curState, out GameState nextState);
+
+        /// <summary>
+        ///     Resets the processor back to its initial state.
+        /// </summary>
+        void Reset();
+    }
+}

Robust.Client/Robust.Client.csproj

@@ -140,6 +140,8 @@
     <Compile Include="Console\Commands\QuitCommand.cs" />
     <Compile Include="Console\Commands\Debug.cs" />
     <Compile Include="GameStates\ClientGameStateManager.cs" />
+    <Compile Include="GameStates\GameStateProcessor.cs" />
+    <Compile Include="GameStates\IGameStateProcessor.cs" />
     <Compile Include="GameStates\NetGraphOverlay.cs" />
     <Compile Include="GameStates\NetInterpOverlay.cs" />
     <Compile Include="Graphics\CanvasLayers.cs" />

Robust.UnitTesting/Client/GameStates/GameStateProcessor_Tests.cs

@@ -0,0 +1,188 @@
+using Moq;
+using NUnit.Framework;
+using Robust.Client.GameStates;
+using Robust.Shared.GameStates;
+using Robust.Shared.Interfaces.Timing;
+using Robust.Shared.Timing;
+
+namespace Robust.UnitTesting.Client.GameStates
+{
+    [TestFixture, Parallelizable, TestOf(typeof(GameStateProcessor))]
+    class GameStateProcessor_Tests
+    {
+        [Test]
+        public void FillBufferBlocksProcessing()
+        {
+            var timingMock = new Mock<IGameTiming>();
+            timingMock.SetupProperty(p => p.CurTick);
+
+            var timing = timingMock.Object;
+            var processor = new GameStateProcessor(timing);
+
+            processor.AddNewState(GameStateFactory(0, 1), 0);
+            processor.AddNewState(GameStateFactory(1, 2), 0); // buffer is at 2/3, so processing should be blocked
+
+            // calculate states for first tick
+            timing.CurTick = new GameTick(3);
+            var result = processor.ProcessTickStates(new GameTick(1), out _, out _);
+
+            Assert.That(result, Is.False);
+            Assert.That(timing.CurTick.Value, Is.EqualTo(1));
+        }
+
+        [Test]
+        public void FillBufferAndCalculateFirstState()
+        {
+            var timingMock = new Mock<IGameTiming>();
+            timingMock.SetupProperty(p => p.CurTick);
+
+            var timing = timingMock.Object;
+            var processor = new GameStateProcessor(timing);
+
+            processor.AddNewState(GameStateFactory(0, 1), 0);
+            processor.AddNewState(GameStateFactory(1, 2), 0);
+            processor.AddNewState(GameStateFactory(2, 3), 0); // buffer is now full, otherwise cannot calculate states.
+
+            // calculate states for first tick
+            timing.CurTick = new GameTick(1);
+            var result = processor.ProcessTickStates(new GameTick(1), out var curState, out var nextState);
+
+            Assert.That(result, Is.True);
+            Assert.That(curState, Is.Not.Null);
+            Assert.That(curState.Extrapolated, Is.False);
+            Assert.That(curState.ToSequence.Value, Is.EqualTo(1));
+            Assert.That(nextState, Is.Null);
+        }
+
+        /// <summary>
+        ///     When a full state is in the queue (fromSequence = 0), it will modify CurTick to the states' toSequence,
+        ///     then return the state as curState.
+        /// </summary>
+        [Test]
+        public void FullStateResyncsCurTick()
+        {
+            var timingMock = new Mock<IGameTiming>();
+            timingMock.SetupProperty(p => p.CurTick);
+
+            var timing = timingMock.Object;
+            var processor = new GameStateProcessor(timing);
+
+            processor.AddNewState(GameStateFactory(0, 1), 0);
+            processor.AddNewState(GameStateFactory(1, 2), 0);
+            processor.AddNewState(GameStateFactory(2, 3), 0); // buffer is now full, otherwise cannot calculate states.
+
+            // calculate states for first tick
+            timing.CurTick = new GameTick(3);
+            processor.ProcessTickStates(timing.CurTick, out _, out _);
+
+            Assert.That(timing.CurTick.Value, Is.EqualTo(1));
+        }
+
+        [Test]
+        public void StatesReceivedPastCurTickAreDropped()
+        {
+            var (timing, processor) = SetupProcessorFactory();
+
+            processor.Extrapolation = false;
+
+            // a few moments later...
+            timing.CurTick = new GameTick(5); // current clock is ahead of server
+            processor.AddNewState(GameStateFactory(3, 4), 0); // received a late state
+            var result = processor.ProcessTickStates(timing.CurTick, out _, out _);
+
+            Assert.That(result, Is.False);
+        }
+
+        /// <summary>
+        ///     The server fell behind the client, so the client clock is now ahead of the incoming states.
+        ///     Without extrapolation, processing blocks.
+        /// </summary>
+        [Test]
+        public void ServerLagsWithoutExtrapolation()
+        {
+            var (timing, processor) = SetupProcessorFactory();
+
+            processor.Extrapolation = false;
+
+            // a few moments later...
+            timing.CurTick = new GameTick(5); // current clock is ahead of server
+            var result = processor.ProcessTickStates(timing.CurTick, out _, out _);
+
+            Assert.That(result, Is.False);
+        }
+
+        /// <summary>
+        ///     When processing is blocked because the client is ahead of the server, reset CurTick to the last
+        ///     received state.
+        /// </summary>
+        [Test]
+        public void ServerLagsWithoutExtrapolationSetsCurTick()
+        {
+            var (timing, processor) = SetupProcessorFactory();
+
+            processor.Extrapolation = false;
+
+            // a few moments later...
+            timing.CurTick = new GameTick(4); // current clock is ahead of server (server=1, client=5)
+            var result = processor.ProcessTickStates(timing.CurTick, out _, out _);
+
+            Assert.That(result, Is.False);
+            Assert.That(timing.CurTick.Value, Is.EqualTo(1));
+        }
+
+        /// <summary>
+        ///     The server fell behind the client, so the client clock is now ahead of the incoming states.
+        ///     With extrapolation, processing returns a fake extrapolated state for the current tick.
+        /// </summary>
+        [Test]
+        public void ServerLagsWithExtrapolation()
+        {
+            var (timing, processor) = SetupProcessorFactory();
+
+            processor.Extrapolation = true;
+
+            // a few moments later...
+            timing.CurTick = new GameTick(5); // current clock is ahead of server
+
+            var result = processor.ProcessTickStates(timing.CurTick, out var curState, out var nextState);
+
+            Assert.That(result, Is.True);
+            Assert.That(curState, Is.Not.Null);
+            Assert.That(curState.Extrapolated, Is.True);
+            Assert.That(curState.ToSequence.Value, Is.EqualTo(5));
+            Assert.That(nextState, Is.Null);
+        }
+
+        /// <summary>
+        ///     Creates a new empty GameState with the given to and from properties.
+        /// </summary>
+        private static GameState GameStateFactory(uint from, uint to)
+        {
+            return new GameState(new GameTick(from), new GameTick(to), null, null, null, null);
+        }
+
+        /// <summary>
+        ///     Creates a new GameTiming and GameStateProcessor, fills the processor with enough states, and calculate the first tick.
+        ///     CurTick = 1, states 1 - 3 are in the buffer.
+        /// </summary>
+        private static (IGameTiming timing, GameStateProcessor processor) SetupProcessorFactory()
+        {
+            var timingMock = new Mock<IGameTiming>();
+            timingMock.SetupProperty(p => p.CurTick);
+            timingMock.SetupProperty(p => p.TickTimingAdjustment);
+
+            var timing = timingMock.Object;
+            var processor = new GameStateProcessor(timing);
+
+            processor.AddNewState(GameStateFactory(0, 1), 0);
+            processor.AddNewState(GameStateFactory(1, 2), 0);
+            processor.AddNewState(GameStateFactory(2, 3), 0); // buffer is now full, otherwise cannot calculate states.
+
+            // calculate states for first tick
+            timing.CurTick = new GameTick(1);
+            processor.ProcessTickStates(timing.CurTick, out _, out _);
+            
+            return (timing, processor);
+        }
+    }
+}

Robust.UnitTesting/Robust.UnitTesting.csproj

@@ -92,6 +92,7 @@
     <Compile Include="ApproxEqualityConstraint.cs" />
     <Compile Include="Client\GameObjects\Components\Transform_Test.cs" />
     <Compile Include="Client\GameControllerProxyDummy.cs" />
+    <Compile Include="Client\GameStates\GameStateProcessor_Tests.cs" />
     <Compile Include="Client\Graphics\StyleBoxTest.cs" />
     <Compile Include="Client\Graphics\TextureLoadParametersTest.cs" />
     <Compile Include="Client\UserInterface\Controls\BoxContainerTest.cs" />