feat(phys): ShadowObjectRegistry.RegisterMultiPart

Multi-shape entity registration matching retail's CPhysicsObj model: one
logical entity emits N ShadowEntry rows (one per CylSphere / Sphere /
Part-BSP), all sharing the entity's EntityId. _entityShapes caches the
original shape list per entity for UpdatePosition to recompose part
transforms when the entity moves.

Existing UpdatePhysicsState / Deregister / GetObjectsInCell /
AllEntriesForDebug work unchanged — they iterate by EntityId; multiple
matching entries get handled automatically.

AllEntriesForDebug updated to enumerate all parts per entity (not just
the first) by iterating the first cell that holds entries for each entity.
Single-shape callers that previously relied on deduplicated-by-EntityId
behavior are unaffected since they register exactly one entry per entity.

Six new tests: AllShareEntityId, EmptyShapeList_NoOp,
Deregister_RemovesAllParts, UpdatePhysicsState_PropagatesEtherealToAllParts,
PartsAcrossMultipleCells_AllCellsListed, Register_SingleShapeCompat_Unchanged.

All 24 existing ShadowObjectRegistry tests pass via the unchanged
single-shape Register API. 11/11 CellarUpTrajectoryReplayTests pass.
7/7 ShadowShapeBuilderTests pass.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/AcDream.Core/Physics/ShadowObjectRegistry.cs

@@ -17,6 +17,13 @@ public sealed class ShadowObjectRegistry
     private readonly Dictionary<uint, List<ShadowEntry>> _cells = new();
     private readonly Dictionary<uint, List<uint>> _entityToCells = new(); // for deregistration
 
+    /// <summary>
+    /// A6.P4 door fix (2026-05-24): per-entity original shape list, used by
+    /// <see cref="UpdatePosition"/> to recompose part world-transforms when
+    /// the entity moves. Cleared by <see cref="Deregister"/>.
+    /// </summary>
+    private readonly Dictionary<uint, System.Collections.Generic.IReadOnlyList<ShadowShape>> _entityShapes = new();
+
     /// <summary>
     /// Register an entity into the cells it overlaps based on world position + radius.
     ///
@@ -98,6 +105,102 @@ public sealed class ShadowObjectRegistry
         _entityToCells[entityId] = cellIds;
     }
 
+    /// <summary>
+    /// A6.P4 door fix (2026-05-24): register one logical entity composed of
+    /// multiple collision shapes. All emitted <see cref="ShadowEntry"/> rows
+    /// share <paramref name="entityId"/>, so <see cref="UpdatePhysicsState"/>
+    /// propagates an ETHEREAL flip to every part (the existing per-entityId
+    /// iteration handles this naturally). The shape list is cached in
+    /// <see cref="_entityShapes"/> so <see cref="UpdatePosition"/> can
+    /// recompose part world-transforms when the entity moves.
+    ///
+    /// <para>
+    /// Retail anchor: <c>CPhysicsObj::FindObjCollisions</c> →
+    /// <c>CPartArray::FindObjCollisions</c> at
+    /// <c>acclient_2013_pseudo_c.txt:276961-286250</c>. One PhysicsObj per
+    /// entity, parts iterated for collision testing.
+    /// </para>
+    /// </summary>
+    public void RegisterMultiPart(
+        uint entityId,
+        Vector3 entityWorldPos,
+        Quaternion entityWorldRot,
+        System.Collections.Generic.IReadOnlyList<ShadowShape> shapes,
+        uint state,
+        EntityCollisionFlags flags,
+        float worldOffsetX, float worldOffsetY, uint landblockId,
+        uint cellScope = 0u)
+    {
+        Deregister(entityId);
+        if (shapes.Count == 0) return;
+
+        _entityShapes[entityId] = shapes;
+        var allCells = new List<uint>();
+        var seenCells = new HashSet<uint>();
+        uint lbPrefix = landblockId & 0xFFFF0000u;
+
+        foreach (var shape in shapes)
+        {
+            var rotatedLocal = Vector3.Transform(shape.LocalPosition, entityWorldRot);
+            var partWorldPos = entityWorldPos + rotatedLocal;
+            var partWorldRot = entityWorldRot * shape.LocalRotation;
+
+            var entry = new ShadowEntry(
+                EntityId:      entityId,
+                GfxObjId:      shape.GfxObjId,
+                Position:      partWorldPos,
+                Rotation:      partWorldRot,
+                Radius:        shape.Radius,
+                CollisionType: shape.CollisionType,
+                CylHeight:     shape.CylHeight,
+                Scale:         shape.Scale,
+                State:         state,
+                Flags:         flags,
+                LocalPosition: shape.LocalPosition,
+                LocalRotation: shape.LocalRotation);
+
+            if (cellScope != 0u)
+            {
+                AddEntryToCell(entry, cellScope);
+                if (seenCells.Add(cellScope)) allCells.Add(cellScope);
+                continue;
+            }
+
+            float localX = partWorldPos.X - worldOffsetX;
+            float localY = partWorldPos.Y - worldOffsetY;
+            float r = shape.Radius;
+
+            int minCx = Math.Max(0, (int)((localX - r) / 24f));
+            int maxCx = Math.Min(7, (int)((localX + r) / 24f));
+            int minCy = Math.Max(0, (int)((localY - r) / 24f));
+            int maxCy = Math.Min(7, (int)((localY + r) / 24f));
+
+            for (int cx = minCx; cx <= maxCx; cx++)
+            {
+                for (int cy = minCy; cy <= maxCy; cy++)
+                {
+                    uint cellId = lbPrefix | (uint)(cx * 8 + cy + 1);
+                    AddEntryToCell(entry, cellId);
+                    if (seenCells.Add(cellId)) allCells.Add(cellId);
+                }
+            }
+        }
+
+        _entityToCells[entityId] = allCells;
+    }
+
+    /// <summary>Helper: append a <see cref="ShadowEntry"/> to a cell's
+    /// list, creating the list if needed.</summary>
+    private void AddEntryToCell(ShadowEntry entry, uint cellId)
+    {
+        if (!_cells.TryGetValue(cellId, out var list))
+        {
+            list = new List<ShadowEntry>();
+            _cells[cellId] = list;
+        }
+        list.Add(entry);
+    }
+
     /// <summary>
     /// Update an already-registered entity's world position + rotation,
     /// preserving its <see cref="ShadowEntry.State"/>,
@@ -201,6 +304,7 @@ public sealed class ShadowObjectRegistry
                 list.RemoveAll(e => e.EntityId == entityId);
         }
         _entityToCells.Remove(entityId);
+        _entityShapes.Remove(entityId);
     }
 
     /// <summary>Remove all entities belonging to a landblock.</summary>
@@ -386,18 +490,35 @@ public sealed class ShadowObjectRegistry
 
     /// <summary>
     /// Debug: enumerate every registered ShadowEntry (deduplicated across cells).
-    /// For each entity, returns the first entry found in any cell it occupies.
+    /// Single-shape entities return one entry per shape; multi-part entities
+    /// return one entry per registered part (including duplicate shapes at the
+    /// same position). Each entity is enumerated exactly once per logical part:
+    /// we use the first cell the entity occupies to read its entries, avoiding
+    /// re-emitting the same part for each cell it overlaps.
     /// Intended for debug rendering only.
     /// </summary>
     public IEnumerable<ShadowEntry> AllEntriesForDebug()
     {
-        var seen = new HashSet<uint>();
+        var seenEntities = new HashSet<uint>();
-        foreach (var kvp in _cells)
+        foreach (var kvp in _entityToCells)
         {
-            foreach (var entry in kvp.Value)
+            uint entityId = kvp.Key;
+            if (!seenEntities.Add(entityId)) continue;
+
+            // Use the first cell that holds entries for this entity.
+            foreach (uint cellId in kvp.Value)
+            {
+                if (!_cells.TryGetValue(cellId, out var list)) continue;
+                bool anyFound = false;
+                foreach (var entry in list)
+                {
+                    if (entry.EntityId == entityId)
                     {
-                if (seen.Add(entry.EntityId))
                         yield return entry;
+                        anyFound = true;
+                    }
+                }
+                if (anyFound) break; // Only use the first cell — avoids duplicating multi-cell shapes.
             }
         }
     }

tests/AcDream.Core.Tests/Physics/ShadowObjectRegistryMultiPartTests.cs

@@ -0,0 +1,164 @@
+using System.Collections.Generic;
+using System.Linq;
+using System.Numerics;
+using AcDream.Core.Physics;
+using Xunit;
+
+namespace AcDream.Core.Tests.Physics;
+
+public class ShadowObjectRegistryMultiPartTests
+{
+    private const uint LbId = 0xA9B40000u;
+    private const float OffX = 0f;
+    private const float OffY = 0f;
+
+    private static IReadOnlyList<ShadowShape> DoorShapes() => new[]
+    {
+        new ShadowShape(
+            GfxObjId:      0u,
+            LocalPosition: new Vector3(0f, 0f, 0.018f),
+            LocalRotation: Quaternion.Identity,
+            Scale:         1.0f,
+            CollisionType: ShadowCollisionType.Cylinder,
+            Radius:        0.100f,
+            CylHeight:     0.200f),
+        new ShadowShape(
+            GfxObjId:      0x010044B5u,
+            LocalPosition: Vector3.Zero,
+            LocalRotation: Quaternion.Identity,
+            Scale:         1.0f,
+            CollisionType: ShadowCollisionType.BSP,
+            Radius:        2.0f,
+            CylHeight:     0f),
+        new ShadowShape(
+            GfxObjId:      0x010044B6u,
+            LocalPosition: Vector3.Zero,
+            LocalRotation: Quaternion.Identity,
+            Scale:         1.0f,
+            CollisionType: ShadowCollisionType.BSP,
+            Radius:        2.0f,
+            CylHeight:     0f),
+        new ShadowShape(
+            GfxObjId:      0x010044B6u,
+            LocalPosition: Vector3.Zero,
+            LocalRotation: Quaternion.Identity,
+            Scale:         1.0f,
+            CollisionType: ShadowCollisionType.BSP,
+            Radius:        2.0f,
+            CylHeight:     0f)
+    };
+
+    [Fact]
+    public void RegisterMultiPart_FourShapes_AllShareEntityId()
+    {
+        var reg = new ShadowObjectRegistry();
+        const uint doorEntityId = 0x000F4244u;
+
+        reg.RegisterMultiPart(
+            entityId:        doorEntityId,
+            entityWorldPos:  new Vector3(132.6f, 17.1f, 94.08f),
+            entityWorldRot:  Quaternion.Identity,
+            shapes:          DoorShapes(),
+            state:           0x10008u,
+            flags:           EntityCollisionFlags.None,
+            worldOffsetX:    OffX,
+            worldOffsetY:    OffY,
+            landblockId:     LbId);
+
+        int found = 0;
+        foreach (var entry in reg.AllEntriesForDebug())
+        {
+            if (entry.EntityId == doorEntityId) found++;
+        }
+        Assert.True(found >= 4,
+            $"Expected at least 4 entries for door entity (one per shape); found {found}");
+    }
+
+    [Fact]
+    public void RegisterMultiPart_EmptyShapeList_NoOp()
+    {
+        var reg = new ShadowObjectRegistry();
+        reg.RegisterMultiPart(
+            entityId:       0x1u,
+            entityWorldPos: Vector3.Zero,
+            entityWorldRot: Quaternion.Identity,
+            shapes:         System.Array.Empty<ShadowShape>(),
+            state:          0u,
+            flags:          EntityCollisionFlags.None,
+            worldOffsetX:   OffX, worldOffsetY: OffY, landblockId: LbId);
+
+        Assert.Equal(0, reg.TotalRegistered);
+    }
+
+    [Fact]
+    public void Deregister_RemovesAllParts()
+    {
+        var reg = new ShadowObjectRegistry();
+        const uint doorEntityId = 0x000F4244u;
+        reg.RegisterMultiPart(doorEntityId, new Vector3(132.6f, 17.1f, 94.08f),
+                               Quaternion.Identity, DoorShapes(), 0x10008u,
+                               EntityCollisionFlags.None, OffX, OffY, LbId);
+
+        reg.Deregister(doorEntityId);
+
+        Assert.Equal(0, reg.TotalRegistered);
+        foreach (var entry in reg.AllEntriesForDebug())
+            Assert.NotEqual(doorEntityId, entry.EntityId);
+    }
+
+    [Fact]
+    public void UpdatePhysicsState_PropagatesEtherealToAllParts()
+    {
+        var reg = new ShadowObjectRegistry();
+        const uint doorEntityId = 0x000F4244u;
+        reg.RegisterMultiPart(doorEntityId, new Vector3(132.6f, 17.1f, 94.08f),
+                               Quaternion.Identity, DoorShapes(), 0x10008u,
+                               EntityCollisionFlags.None, OffX, OffY, LbId);
+
+        reg.UpdatePhysicsState(doorEntityId, 0x1000Cu); // 0x10008 | 0x4
+
+        int updated = 0;
+        foreach (var entry in reg.AllEntriesForDebug())
+        {
+            if (entry.EntityId != doorEntityId) continue;
+            Assert.Equal(0x1000Cu, entry.State);
+            updated++;
+        }
+        Assert.True(updated >= 4, $"Expected all parts updated, only {updated} were");
+    }
+
+    [Fact]
+    public void RegisterMultiPart_PartsAcrossMultipleCells_AllCellsListed()
+    {
+        var reg = new ShadowObjectRegistry();
+        // Two shapes 30m apart in X — must span two outdoor 24m cells.
+        var shapes = new[]
+        {
+            new ShadowShape(0u, new Vector3(  0f, 0f, 0f), Quaternion.Identity, 1f,
+                            ShadowCollisionType.Cylinder, 1f, 2f),
+            new ShadowShape(0u, new Vector3(30f, 0f, 0f), Quaternion.Identity, 1f,
+                            ShadowCollisionType.Cylinder, 1f, 2f),
+        };
+        reg.RegisterMultiPart(0x1u, new Vector3(12f, 12f, 50f), Quaternion.Identity,
+                               shapes, 0u, EntityCollisionFlags.None, OffX, OffY, LbId);
+
+        // Part 1 at world (12, 12) → cell (0,0) = LbId | 1
+        // Part 2 at world (42, 12) → cell (1,0) = LbId | 9
+        var entriesIn1 = reg.GetObjectsInCell(LbId | 1u);
+        var entriesIn9 = reg.GetObjectsInCell(LbId | 9u);
+        Assert.Contains(entriesIn1, e => e.EntityId == 0x1u);
+        Assert.Contains(entriesIn9, e => e.EntityId == 0x1u);
+    }
+
+    [Fact]
+    public void Register_SingleShapeCompat_Unchanged()
+    {
+        var reg = new ShadowObjectRegistry();
+        reg.Register(42u, 0x01000001u, new Vector3(12f, 12f, 50f),
+                     Quaternion.Identity, 1f, OffX, OffY, LbId);
+
+        Assert.Equal(1, reg.TotalRegistered);
+        Assert.Single(reg.GetObjectsInCell(LbId | 1u),
+                      e => e.EntityId == 42u);
+    }
+}