diff --git a/src/AcDream.Core/Physics/TransitionTypes.cs b/src/AcDream.Core/Physics/TransitionTypes.cs
index 11dbd7c..e5f1e86 100644
--- a/src/AcDream.Core/Physics/TransitionTypes.cs
+++ b/src/AcDream.Core/Physics/TransitionTypes.cs
@@ -1502,7 +1502,7 @@ public sealed class Transition
}
}
- private TransitionState FindEnvCollisions(PhysicsEngine engine)
+ internal TransitionState FindEnvCollisions(PhysicsEngine engine)
{
var sp = SpherePath;
var ci = CollisionInfo;
@@ -1618,6 +1618,21 @@ public sealed class Transition
return cellState;
}
+ // ── Phase A4 (2026-05-20): query every other cell ──────────
+ // Retail oracle: CTransition::check_other_cells at
+ // acclient_2013_pseudo_c.txt:272717-272798. The vestibule
+ // walls bug (cell 0xA9B40164 has only 4 polys; adjacent
+ // 0xA9B40157 has the actual walls) closes here.
+ //
+ // Discard the containing-cell return — sp.CheckCellId is
+ // already authoritative for the primary cell we just queried.
+ _ = CellTransit.FindCellSet(engine.DataCache, footCenter, sphereRadius,
+ sp.CheckCellId, out var cellSet);
+ var otherCellsState = CheckOtherCells(engine, footCenter, sphereRadius, cellSet);
+ if (otherCellsState != TransitionState.OK)
+ return otherCellsState;
+ // ──────────────────────────────────────────────────────────
+
// ── Synthesize indoor walkable contact plane ──────────────
// Indoor walking Phase 2 follow-up (2026-05-19). When the BSP
// returns OK (no wall collision), the player is standing on a
diff --git a/tests/AcDream.Core.Tests/Physics/FindEnvCollisionsMultiCellTests.cs b/tests/AcDream.Core.Tests/Physics/FindEnvCollisionsMultiCellTests.cs
new file mode 100644
index 0000000..d973305
--- /dev/null
+++ b/tests/AcDream.Core.Tests/Physics/FindEnvCollisionsMultiCellTests.cs
@@ -0,0 +1,148 @@
+using System;
+using System.Collections.Generic;
+using System.Numerics;
+using DatReaderWriter.Enums;
+using DatReaderWriter.Types;
+using AcDream.Core.Physics;
+using Xunit;
+
+namespace AcDream.Core.Tests.Physics;
+
+///
+/// End-to-end test that the indoor branch of
+/// queries the cells the
+/// sphere overlaps, not just the cell whose CellBSP contains the
+/// sphere center. This is the core Phase A4 behaviour test — the
+/// Holtburg inn vestibule (cell 0xA9B40164) bug reduced to a minimal
+/// synthetic fixture.
+///
+public class FindEnvCollisionsMultiCellTests
+{
+ // Indoor cell IDs — both have low-byte ≥ 0x100 to trigger the
+ // indoor branch of FindEnvCollisions. Vestibule has the lower id so
+ // CellTransit.FindCellSet's sorted iteration encounters it first.
+ private const uint VestibuleCellId = 0xA9B40157u;
+ private const uint InteriorCellId = 0xA9B40164u;
+
+ private static CellBSPTree LeafCellBsp() => new CellBSPTree
+ {
+ Root = new CellBSPNode { Type = BSPNodeType.Leaf },
+ };
+
+ private static PhysicsBSPTree EmptyLeafBsp() => new PhysicsBSPTree
+ {
+ Root = new PhysicsBSPNode
+ {
+ Type = BSPNodeType.Leaf,
+ BoundingSphere = new Sphere { Origin = Vector3.Zero, Radius = 10f },
+ }
+ };
+
+ [Fact]
+ public void IndoorSphereOverlappingAdjacentCellWithWall_HaltsTransition()
+ {
+ // ── Secondary cell (interior) ─────────────────────────────────────
+ // Reuse BSPStepUp's TallWall fixture — proven to halt a grounded mover
+ // that can't scale it (test B2_GroundedMover_TallWall_BlockedOrSlides).
+ // Wall is at interior-local x=0.5. Translate the interior cell by
+ // +0.3 in world X so the wall ends up at world x=0.8, within reach
+ // of a sphere walking from x=0.1 toward x=0.6 (sphere radius 0.2).
+ var (wallRoot, wallResolved) = BSPStepUpFixtures.TallWall();
+ var interiorWT = Matrix4x4.CreateTranslation(new Vector3(0.3f, 0f, 0f));
+ Matrix4x4.Invert(interiorWT, out var interiorInv);
+
+ var interior = new CellPhysics
+ {
+ BSP = new PhysicsBSPTree { Root = wallRoot },
+ WorldTransform = interiorWT,
+ InverseWorldTransform = interiorInv,
+ Resolved = wallResolved,
+ CellBSP = LeafCellBsp(),
+ };
+
+ // ── Primary cell (vestibule) ──────────────────────────────────────
+ // Empty PhysicsBSP — no walls of its own. CellBSP contains a portal
+ // at world x=0.5 (vestibule-local x=0.5 since vestibule WorldTransform
+ // = Identity) leading to the interior cell. The sphere's foot reaches
+ // the portal at world x=0.5 during the sweep — that triggers
+ // CellTransit.FindCellSet to add the interior to the candidate set.
+ var portalPoly = new ResolvedPolygon
+ {
+ Vertices = new[]
+ {
+ new Vector3(0.5f, -2.5f, 0f),
+ new Vector3(0.5f, 2.5f, 0f),
+ new Vector3(0.5f, 2.5f, 5f),
+ new Vector3(0.5f, -2.5f, 5f),
+ },
+ Plane = new Plane(new Vector3(1f, 0f, 0f), -0.5f),
+ NumPoints = 4,
+ SidesType = CullMode.None,
+ };
+
+ var vestibule = new CellPhysics
+ {
+ BSP = EmptyLeafBsp(),
+ WorldTransform = Matrix4x4.Identity,
+ InverseWorldTransform = Matrix4x4.Identity,
+ Resolved = new Dictionary(),
+ CellBSP = LeafCellBsp(),
+ PortalPolygons = new Dictionary { [10] = portalPoly },
+ Portals = new[]
+ {
+ new PortalInfo(otherCellId: (ushort)(InteriorCellId & 0xFFFFu),
+ polygonId: 10, flags: 0),
+ },
+ };
+
+ // ── Engine + cache ────────────────────────────────────────────────
+ var engine = new PhysicsEngine();
+ engine.DataCache = new PhysicsDataCache();
+
+ // Provide a flat terrain strip at z=0 so FindEnvCollisions's outdoor
+ // fall-through has something to sample if it ever fires.
+ var heights = new byte[81];
+ Array.Fill(heights, (byte)0);
+ var ht = new float[256];
+ for (int i = 0; i < 256; i++) ht[i] = i * 1.0f;
+ engine.AddLandblock(0xA9B4FFFFu, new TerrainSurface(heights, ht),
+ Array.Empty(), Array.Empty(),
+ worldOffsetX: 0f, worldOffsetY: 0f);
+
+ engine.DataCache.RegisterCellStructForTest(VestibuleCellId, vestibule);
+ engine.DataCache.RegisterCellStructForTest(InteriorCellId, interior);
+
+ // ── Transition ────────────────────────────────────────────────────
+ // Grounded mover, foot at world x=0.1 walking to x=0.7. The sphere
+ // (radius 0.2, center at foot + 0.2 in Z) ends with its center at
+ // world x=0.7 = interior-local x=0.4 (since interior translation is
+ // +0.3). The TallWall sits at interior-local x=0.5 with normal -X —
+ // the sphere reach (0.4 + 0.2 = 0.6) penetrates the wall by 0.1.
+ // StepUpHeight 0.04 means the mover can't scale the 5m TallWall.
+ // MakeGroundedTransition seeds Contact + OnWalkable +
+ // LastKnownContactPlane so Path 5 fires for any wall the BSP query
+ // encounters.
+ var from = new Vector3(0.1f, 0f, 0f);
+ var to = new Vector3(0.7f, 0f, 0f);
+ var t = BSPStepUpFixtures.MakeGroundedTransition(from, to,
+ stepUpHeight: 0.04f,
+ cellId: VestibuleCellId);
+
+ // SetCheckPos sets the candidate position FindEnvCollisions evaluates.
+ t.SpherePath.SetCheckPos(to, VestibuleCellId);
+
+ // ── Act ───────────────────────────────────────────────────────────
+ // Call FindEnvCollisions directly (now internal). Bypasses
+ // FindTransitionalPosition's sub-step iteration so we can assert on
+ // the single result.
+ var result = t.FindEnvCollisions(engine);
+
+ // ── Assert ────────────────────────────────────────────────────────
+ // Pre-A4: empty vestibule BSP returns OK, interior is never queried,
+ // result == OK (sphere walks through the wall).
+ // Post-A4: CheckOtherCells iterates the interior cell, BSPQuery on
+ // TallWall returns Slid (the wall-slide path matching B2), and
+ // FindEnvCollisions returns Slid via ApplyOtherCellResult.
+ Assert.NotEqual(TransitionState.OK, result);
+ }
+}