Gas overlay chunking (#1678)

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

Content.Shared/GameObjects/EntitySystems/Atmos/SharedGasTileOverlaySystem.cs

@@ -0,0 +1,98 @@
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using Robust.Shared.GameObjects;
+using Robust.Shared.GameObjects.Systems;
+using Robust.Shared.Map;
+using Robust.Shared.Serialization;
+using Robust.Shared.Utility;
+
+namespace Content.Shared.GameObjects.EntitySystems.Atmos
+{
+    public abstract class SharedGasTileOverlaySystem : EntitySystem
+    {
+        public const byte ChunkSize = 8;
+        protected float AccumulatedFrameTime;
+
+        public static MapIndices GetGasChunkIndices(MapIndices indices)
+        {
+            return new MapIndices((int) Math.Floor((float) indices.X / ChunkSize) * ChunkSize, (int) MathF.Floor((float) indices.Y / ChunkSize) * ChunkSize);
+        }
+        
+        [Serializable, NetSerializable]
+        public struct GasData
+        {
+            public byte Index { get; set; }
+            public byte Opacity { get; set; }
+
+            public GasData(byte gasId, byte opacity)
+            {
+                Index = gasId;
+                Opacity = opacity;
+            }
+        }
+
+        [Serializable, NetSerializable]
+        public readonly struct GasOverlayData : IEquatable<GasOverlayData>
+        {
+            public readonly byte FireState;
+            public readonly float FireTemperature;
+            public readonly GasData[] Gas;
+
+            public GasOverlayData(byte fireState, float fireTemperature, GasData[] gas)
+            {
+                FireState = fireState;
+                FireTemperature = fireTemperature;
+                Gas = gas;
+            }
+
+            public bool Equals(GasOverlayData other)
+            {
+                // TODO: Moony had a suggestion on how to do this faster with the hash
+                // https://discordapp.com/channels/310555209753690112/310555209753690112/744080145219846204
+                // Aside from that I can't really see any low-hanging fruit CPU perf wise.
+                if (Gas?.Length != other.Gas?.Length) return false;
+                if (FireState != other.FireState) return false;
+                if (FireTemperature != other.FireTemperature) return false;
+
+                if (Gas == null)
+                {
+                    return true;
+                }
+                
+                DebugTools.Assert(other.Gas != null);
+                
+                for (var i = 0; i < Gas.Length; i++)
+                {
+                    var thisGas = Gas[i];
+                    var otherGas = other.Gas[i];
+
+                    if (!thisGas.Equals(otherGas))
+                    {
+                        return false;
+                    }
+                }
+
+                return true;
+            }
+        }
+
+        /// <summary>
+        ///     Invalid tiles for the gas overlay.
+        ///     No point re-sending every tile if only a subset might have been updated.
+        /// </summary>
+        [Serializable, NetSerializable]
+        public sealed class GasOverlayMessage : EntitySystemMessage
+        {
+            public GridId GridId { get; }
+
+            public List<(MapIndices, GasOverlayData)> OverlayData { get; }
+
+            public GasOverlayMessage(GridId gridIndices, List<(MapIndices,GasOverlayData)> overlayData)
+            {
+                GridId = gridIndices;
+                OverlayData = overlayData;
+            }
+        }
+    }
+}