From 7910d51e7a163d57b8ea65448e004b21e5f58896 Mon Sep 17 00:00:00 2001
From: Erik <erik.nihlen@gmail.com>
Date: Mon, 25 May 2026 12:40:58 +0200
Subject: [PATCH] =?UTF-8?q?diag(phys):=20A6.P5=20[cellset-build]=20probe?=
 =?UTF-8?q?=20=E2=80=94=20log=20BuildCellSetAndPickContaining=20output?=
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One [cellset-build] line per call when ACDREAM_PROBE_CELLSET=1: seed cell,
sphere world XY, candidate count, full candidate id list. Used to prove
the cellSet for the player's start cell doesn't include the door's outdoor
cell across the over-penetration tick.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
---
 src/AcDream.Core/Physics/CellTransit.cs       |  5 ++++
 .../Physics/PhysicsDiagnostics.cs             | 29 +++++++++++++++++++
 2 files changed, 34 insertions(+)

diff --git a/src/AcDream.Core/Physics/CellTransit.cs b/src/AcDream.Core/Physics/CellTransit.cs
index 900157b..bdacceb 100644
--- a/src/AcDream.Core/Physics/CellTransit.cs
+++ b/src/AcDream.Core/Physics/CellTransit.cs
@@ -494,6 +494,11 @@ public static class CellTransit
             }
         }
 
+        if (PhysicsDiagnostics.ProbeCellSetEnabled)
+        {
+            PhysicsDiagnostics.LogCellSetBuild(currentCellId, worldSphereCenter, candidates);
+        }
+
         // Containment test: for each candidate, transform worldSphereCenter to
         // local and test PointInsideCellBsp.
         foreach (uint candId in candidates)
diff --git a/src/AcDream.Core/Physics/PhysicsDiagnostics.cs b/src/AcDream.Core/Physics/PhysicsDiagnostics.cs
index bb9008d..260ed07 100644
--- a/src/AcDream.Core/Physics/PhysicsDiagnostics.cs
+++ b/src/AcDream.Core/Physics/PhysicsDiagnostics.cs
@@ -69,6 +69,35 @@ public static class PhysicsDiagnostics
     public static bool ProbeBuildingEnabled { get; set; } =
         Environment.GetEnvironmentVariable("ACDREAM_PROBE_BUILDING") == "1";
 
+    /// <summary>
+    /// A6.P5 (2026-05-25) — dump the cellSet that
+    /// <c>BuildCellSetAndPickContaining</c> produces. One line per call:
+    /// seed cell, sphere world XY, candidate count, and the full candidate
+    /// list (hex). Pair with <c>[bsp-test]</c> / <c>[resolve]</c> to see
+    /// whether the door's outdoor cell is reachable from the player's
+    /// current indoor cell via the portal-walk.
+    /// </summary>
+    public static bool ProbeCellSetEnabled { get; set; }
+        = Environment.GetEnvironmentVariable("ACDREAM_PROBE_CELLSET") == "1";
+
+    public static void LogCellSetBuild(
+        uint seedCellId,
+        System.Numerics.Vector3 sphereCenter,
+        System.Collections.Generic.IReadOnlyCollection<uint> cellSet)
+    {
+        if (!ProbeCellSetEnabled) return;
+        var ids = new System.Text.StringBuilder();
+        bool first = true;
+        foreach (uint id in cellSet)
+        {
+            if (!first) ids.Append(',');
+            ids.Append(System.FormattableString.Invariant($"0x{id:X8}"));
+            first = false;
+        }
+        Console.WriteLine(System.FormattableString.Invariant(
+            $"[cellset-build] seed=0x{seedCellId:X8} sphere=({sphereCenter.X:F3},{sphereCenter.Y:F3},{sphereCenter.Z:F3}) count={cellSet.Count} ids={ids}"));
+    }
+
     /// <summary>
     /// L.2d slice 1 (2026-05-13). Diagnostic side-channel: the
     /// <see cref="ResolvedPolygon"/> that <see cref="BSPQuery"/>