From 03f08f00c1a9c4a6c9980f189dc45bdc155a671a Mon Sep 17 00:00:00 2001
From: Erik <erik.nihlen@gmail.com>
Date: Tue, 2 Jun 2026 09:15:37 +0200
Subject: [PATCH] =?UTF-8?q?fix(core):=20UCG=20Stage=201=20=E2=80=94=20Reso?=
 =?UTF-8?q?lvePortalPolygon=20all-or-nothing=20(match=20BuildLoadedCell)?=
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Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
---
 src/AcDream.Core/World/Cells/EnvCell.cs       | 14 +++++---
 .../World/Cells/EnvCellFromDatTests.cs        | 32 +++++++++++++++++++
 2 files changed, 41 insertions(+), 5 deletions(-)

diff --git a/src/AcDream.Core/World/Cells/EnvCell.cs b/src/AcDream.Core/World/Cells/EnvCell.cs
index f4dc2e0..10a0407 100644
--- a/src/AcDream.Core/World/Cells/EnvCell.cs
+++ b/src/AcDream.Core/World/Cells/EnvCell.cs
@@ -76,11 +76,15 @@ public sealed class EnvCell : ObjCell
 
     private static IReadOnlyList<Vector3> ResolvePortalPolygon(CellStruct cellStruct, ushort polygonId)
     {
-        if (!cellStruct.Polygons.TryGetValue(polygonId, out var poly)) return Array.Empty<Vector3>();
-        var verts = new List<Vector3>(poly.VertexIds.Count);
-        foreach (var vid in poly.VertexIds)
-            if (cellStruct.VertexArray.Vertices.TryGetValue((ushort)vid, out var v))
-                verts.Add(new Vector3(v.Origin.X, v.Origin.Y, v.Origin.Z));
+        if (!cellStruct.Polygons.TryGetValue(polygonId, out var poly) || poly.VertexIds.Count < 3)
+            return Array.Empty<Vector3>();
+        var verts = new Vector3[poly.VertexIds.Count];
+        for (int i = 0; i < poly.VertexIds.Count; i++)
+        {
+            if (!cellStruct.VertexArray.Vertices.TryGetValue((ushort)poly.VertexIds[i], out var v))
+                return Array.Empty<Vector3>();   // any miss -> empty (matches BuildLoadedCell:5686-5691)
+            verts[i] = new Vector3(v.Origin.X, v.Origin.Y, v.Origin.Z);
+        }
         return verts;
     }
 }
diff --git a/tests/AcDream.Core.Tests/World/Cells/EnvCellFromDatTests.cs b/tests/AcDream.Core.Tests/World/Cells/EnvCellFromDatTests.cs
index b13d5c7..09d6741 100644
--- a/tests/AcDream.Core.Tests/World/Cells/EnvCellFromDatTests.cs
+++ b/tests/AcDream.Core.Tests/World/Cells/EnvCellFromDatTests.cs
@@ -40,4 +40,36 @@ public class EnvCellFromDatTests
         Assert.Equal(Vector3.Zero, env.LocalBoundsMax);
         Assert.Null(env.ContainmentBsp);
     }
+
+    [Fact]
+    public void FromDat_PortalPolygonMissingAVertex_YieldsEmptyPolygonLocal()
+    {
+        var cellStruct = new CellStruct
+        {
+            VertexArray = new VertexArray { Vertices = new Dictionary<ushort, SWVertex>
+            {
+                [0] = new SWVertex { Origin = new Vector3(0, 0, 0) },
+                [1] = new SWVertex { Origin = new Vector3(1, 0, 0) },
+                // vertex 2 intentionally MISSING
+            }},
+            Polygons = new Dictionary<ushort, Polygon>
+            {
+                [0] = new Polygon { VertexIds = new List<short> { 0, 1, 2 } },
+            },
+        };
+        var dat = new DatEnvCell
+        {
+            Flags = (EnvCellFlags)0,
+            CellPortals = new List<DatCellPortal>
+            {
+                new() { OtherCellId = 0x0105, PolygonId = 0, OtherPortalId = 0, Flags = (PortalFlags)0 },
+            },
+            VisibleCells = new List<ushort>(),
+        };
+
+        var env = EnvCell.FromDat(0xA9B40104u, dat, cellStruct, Matrix4x4.Identity);
+
+        Assert.Single(env.Portals);
+        Assert.Empty(env.Portals[0].PolygonLocal);
+    }
 }