Makes grids that pass a certain size airtight. (#6934)

Co-authored-by: metalgearsloth <comedian_vs_clown@hotmail.com>

Content.Server/Atmos/EntitySystems/AutomaticAtmosSystem.cs

@@ -0,0 +1,52 @@
+using Content.Server.Atmos.Components;
+using Content.Server.Shuttles.EntitySystems;
+using Content.Shared.Maps;
+using Robust.Shared.Map;
+
+namespace Content.Server.Atmos.EntitySystems;
+
+/// <summary>
+/// Handles automatically adding a grid atmosphere to grids that become large enough, allowing players to build shuttles
+/// with a sealed atmosphere from scratch.
+/// </summary>
+public sealed class AutomaticAtmosSystem : EntitySystem
+{
+    [Dependency] private readonly IMapManager _mapManager = default!;
+    [Dependency] private readonly ITileDefinitionManager _tileDefinitionManager = default!;
+
+    public override void Initialize()
+    {
+        base.Initialize();
+        _mapManager.TileChanged += MapManagerOnTileChanged;
+    }
+
+    public override void Shutdown()
+    {
+        base.Shutdown();
+        _mapManager.TileChanged -= MapManagerOnTileChanged;
+    }
+
+    private void MapManagerOnTileChanged(object? sender, TileChangedEventArgs e)
+    {
+        // Only if a atmos-holding tile has been added or removed.
+        // Also, these calls are surprisingly slow.
+        // TODO: Make tiledefmanager cache the IsSpace property, and turn this lookup-through-two-interfaces into
+        // TODO: a simple array lookup, as tile IDs are likely contiguous, and there's at most 2^16 possibilities anyway.
+        if (!((e.OldTile.IsSpace(_tileDefinitionManager) && !e.NewTile.IsSpace(_tileDefinitionManager)) ||
+            (!e.OldTile.IsSpace(_tileDefinitionManager) && e.NewTile.IsSpace(_tileDefinitionManager))))
+            return;
+
+        var uid = _mapManager.GetGridEuid(e.NewTile.GridIndex);
+        if (!TryComp<PhysicsComponent>(uid, out var physics))
+            return;
+
+        // We can't actually count how many tiles there are efficiently, so instead estimate with the mass.
+        if (physics.Mass / ShuttleSystem.TileMassMultiplier >= 7.0f && !HasComp<IAtmosphereComponent>(uid))
+        {
+            AddComp<GridAtmosphereComponent>(uid);
+            Logger.InfoS("atmos", $"Giving grid {uid} GridAtmosphereComponent.");
+        }
+        // It's not super important to remove it should the grid become too small again.
+        // If explosions ever gain the ability to outright shatter grids, do rethink this.
+    }
+}