fix(physics): make collision activation starvation-free

src/AcDream.Core/Physics/CollisionWorldState.cs

@@ -0,0 +1,86 @@
+using System.Collections.Concurrent;
+using AcDream.Core.World.Cells;
+
+namespace AcDream.Core.Physics;
+
+/// <summary>
+/// One exclusive-by-ownership collision-world root. A preparation mutates only
+/// its private root; activation transfers the complete root through one volatile
+/// reference publication shared by every collision facade.
+/// </summary>
+internal sealed class CollisionWorldState
+{
+    internal Dictionary<uint, PhysicsEngine.LandblockPhysics> Landblocks { get; } = new();
+    internal List<uint> LandblockSlots { get; } = new();
+    internal Dictionary<uint, int> LandblockIndices { get; } = new();
+    internal Stack<int> LandblockFreeSlots { get; } = new();
+    internal ConcurrentDictionary<uint, CellPhysics> CellStruct { get; } = new();
+    internal ConcurrentDictionary<uint, FlatCellStructureCollisionAsset>
+        FlatCellStruct { get; } = new();
+    internal ConcurrentDictionary<uint, FlatEnvCellTopology> FlatEnvCell { get; } = new();
+    internal ConcurrentDictionary<uint, BuildingPhysics> Buildings { get; } = new();
+    internal ConcurrentDictionary<uint, EnvCell> EnvCells { get; } = new();
+    internal ConcurrentDictionary<uint, CellGraphTerrain> Terrain { get; } = new();
+    internal ConcurrentDictionary<uint, ObjCell> OutdoorCells { get; } = new();
+    internal Dictionary<uint, List<ShadowEntry>> ShadowCells { get; } = new();
+    internal Dictionary<uint, List<uint>> ShadowEntityCells { get; } = new();
+    internal HashSet<uint> SuspendedShadowEntities { get; } = new();
+    internal Dictionary<uint, HashSet<uint>> WithdrawnPrefixesByOwner { get; } = new();
+    internal Dictionary<uint, IReadOnlyList<ShadowShape>> ShadowEntityShapes { get; } = new();
+    internal Dictionary<uint, ShadowObjectRegistry.RegistrationRecord>
+        ShadowEntityRegistrations { get; } = new();
+    internal Dictionary<uint, ulong> ShadowOwnerVersions { get; } = new();
+    internal Dictionary<uint, HashSet<uint>> ShadowOwnerPrefixes { get; } = new();
+    internal Dictionary<uint, List<uint>> ShadowPrefixOwnerSlots { get; } = new();
+    internal Dictionary<uint, Dictionary<uint, int>> ShadowPrefixOwnerIndices { get; } = new();
+    internal Dictionary<uint, Stack<int>> ShadowPrefixFreeSlots { get; } = new();
+    internal List<uint> ShadowOwnerSlots { get; } = new();
+    internal Dictionary<uint, int> ShadowOwnerIndices { get; } = new();
+    internal Stack<int> ShadowOwnerFreeSlots { get; } = new();
+}
+
+/// <summary>
+/// Stable indirection shared by PhysicsEngine, PhysicsDataCache, CellGraph,
+/// and ShadowObjectRegistry. Readers observe either complete root, never a
+/// mixture assembled by several facade assignments.
+/// </summary>
+internal sealed class CollisionWorldStateSlot
+{
+    private CollisionWorldState? _current = new();
+    private bool _revoked;
+
+    internal CollisionWorldStateSlot()
+    {
+    }
+
+    internal CollisionWorldStateSlot(CollisionWorldState current)
+    {
+        _current = current ?? throw new ArgumentNullException(nameof(current));
+    }
+
+    internal CollisionWorldState Current
+    {
+        get
+        {
+            if (_revoked)
+                throw new ObjectDisposedException("Transferred collision generation");
+            return Volatile.Read(ref _current)
+                ?? throw new ObjectDisposedException("Transferred collision generation");
+        }
+    }
+
+    internal CollisionWorldState TransferTo(CollisionWorldStateSlot destination)
+    {
+        ArgumentNullException.ThrowIfNull(destination);
+        if (_revoked)
+            throw new ObjectDisposedException("Transferred collision generation");
+        CollisionWorldState transferred = _current
+            ?? throw new ObjectDisposedException("Transferred collision generation");
+        _revoked = true;
+        Volatile.Write(ref destination._current, transferred);
+        _current = null;
+        return transferred;
+    }
+
+    internal CollisionWorldState Capture() => Current;
+}