Lag compensation for melee (#11885)

Co-authored-by: metalgearsloth <metalgearsloth@gmail.com>

Content.Server/Movement/Systems/LagCompensationSystem.cs

@@ -0,0 +1,113 @@
+using Content.Server.Movement.Components;
+using Robust.Server.Player;
+using Robust.Shared.Map;
+using Robust.Shared.Timing;
+
+namespace Content.Server.Movement.Systems;
+
+/// <summary>
+/// Stores a buffer of previous positions of the relevant entity.
+/// Can be used to check the entity's position at a recent point in time.
+/// </summary>
+public sealed class LagCompensationSystem : EntitySystem
+{
+    [Dependency] private readonly IGameTiming _timing = default!;
+
+    // I figured 500 ping is max, so 1.5 is 750.
+    // Max ping I've had is 350ms from aus to spain.
+    public static readonly TimeSpan BufferTime = TimeSpan.FromMilliseconds(750);
+
+    private ISawmill _sawmill = Logger.GetSawmill("lagcomp");
+
+    public override void Initialize()
+    {
+        base.Initialize();
+        _sawmill.Level = LogLevel.Info;
+        SubscribeLocalEvent<LagCompensationComponent, MoveEvent>(OnLagMove);
+    }
+
+    public override void Update(float frameTime)
+    {
+        base.Update(frameTime);
+
+        var curTime = _timing.CurTime;
+        var earliestTime = curTime - BufferTime;
+
+        // Cull any old ones from active updates
+        // Probably fine to include ignored.
+        foreach (var (_, comp) in EntityQuery<ActiveLagCompensationComponent, LagCompensationComponent>(true))
+        {
+            while (comp.Positions.TryPeek(out var pos))
+            {
+                if (pos.Item1 < earliestTime)
+                {
+                    comp.Positions.Dequeue();
+                    continue;
+                }
+
+                break;
+            }
+
+            if (comp.Positions.Count == 0)
+            {
+                RemComp<ActiveLagCompensationComponent>(comp.Owner);
+            }
+        }
+    }
+
+    private void OnLagMove(EntityUid uid, LagCompensationComponent component, ref MoveEvent args)
+    {
+        EnsureComp<ActiveLagCompensationComponent>(uid);
+        component.Positions.Enqueue((_timing.CurTime, args.NewPosition, args.NewRotation));
+    }
+
+    public (EntityCoordinates Coordinates, Angle Angle) GetCoordinatesAngle(EntityUid uid, IPlayerSession pSession,
+        TransformComponent? xform = null)
+    {
+        if (!Resolve(uid, ref xform))
+            return (EntityCoordinates.Invalid, Angle.Zero);
+
+        if (!TryComp<LagCompensationComponent>(uid, out var lag) || lag.Positions.Count == 0)
+            return (xform.Coordinates, xform.LocalRotation);
+
+        var angle = Angle.Zero;
+        var coordinates = EntityCoordinates.Invalid;
+        var ping = pSession.Ping;
+        // Use 1.5 due to the trip buffer.
+        var sentTime = _timing.CurTime - TimeSpan.FromMilliseconds(ping * 1.5);
+
+        foreach (var pos in lag.Positions)
+        {
+            coordinates = pos.Item2;
+            angle = pos.Item3;
+
+            if (pos.Item1 >= sentTime)
+                break;
+        }
+
+        if (coordinates == default)
+        {
+            _sawmill.Debug($"No long comp coords found, using {xform.Coordinates}");
+            coordinates = xform.Coordinates;
+            angle = xform.LocalRotation;
+        }
+        else
+        {
+            _sawmill.Debug($"Actual coords is {xform.Coordinates} and got {coordinates}");
+        }
+
+        return (coordinates, angle);
+    }
+
+    public Angle GetAngle(EntityUid uid, IPlayerSession pSession, TransformComponent? xform = null)
+    {
+        var (_, angle) = GetCoordinatesAngle(uid, pSession, xform);
+        return angle;
+    }
+
+    public EntityCoordinates GetCoordinates(EntityUid uid, IPlayerSession pSession, TransformComponent? xform = null)
+    {
+        var (coordinates, _) = GetCoordinatesAngle(uid, pSession, xform);
+        return coordinates;
+    }
+}