test(measure): VerticalShafts breaks the primordial law in 15 of 16 seeds

The margin is exonerated. Every earlier run excluded 8 voxels at each
strate boundary, and voxels 4-8 of those are ordinary interior, not seal
-- the same window mistake that produced "FlatPlain is 99.95% void" at
margin 32. So the query was re-run at margin 0, 2, 4 and 8. All four stay
negative; margin 0 includes the seal itself and still reports negative.
(The implementation had clamped explicit margins to >= 1, which made the
margin-0 check impossible; Codex found and fixed that.)

Diagnosis -- this is not a near-miss wall:
  arrival   LowerPoint (65.8,  95.8, -1047.25)
  departure UpperPoint (88.4, -104.6,  -991.58)
  XY separation 201.7 voxels (~50 m), Z delta 55.7
  arrival component   22,531 cells (22.0% of air)
  departure component 21,621 cells (21.1% of air)
  nearest approach between the two components: 135-179 voxels

Two large, healthy, entirely separate shaft networks half a strate apart.
Nothing to breach; the archetype has no lateral connection at all.

16-seed sweep: 1 pass, 15 fail. Not one unlucky world -- structural.

So: descend into a VerticalShafts strate and you probably cannot reach
its exit. This is the "live hole in the shipping game" COMPOSER-NOTES
Tier 1 predicted, now measured instead of assumed.

Stated precisely, because it names the fix: VF_SuggestLandingPoint makes
each mouth land in open space, but NOTHING makes the two mouths of one
strate land in the SAME open space.

New: VF_DiagnoseConnectivity returns component sizes and the spatial
separation between the two mouths' components, so a near-miss and a
never-meant-to-connect read differently instead of both being "false".

No generation change, no threshold tuning. The fix is a design decision.

Build clean, 19 succeeded / 0 warnings / 0 failed. Sweeps 3.4 s + 6.9 s.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Bp7FxoQcSoiuhxjuLr6aBM

CODEMAP.md

@@ -304,7 +304,7 @@ This is **where terrain shape lives.**
 
 ### 3.6b Strate measurement — `Public/VoxelStrateMeasure.h` + `Private/VoxelStrateMeasure.cpp`
 Headless, read-only Tier 2 measurement pass. `VF_MeasureStrate` samples one strate between a
-boundary-seal-derived (or explicitly overridden) interior margin into a bounded coarse grid,
+boundary-seal-derived (or explicitly overridden, including explicit zero) interior margin into a bounded coarse grid,
 performs one deterministic 6-connected air flood fill, and derives fractions, components,
 walkability, feature scale, and clearance from that grid. Metrics report the resolved margin and
 the inclusive/exclusive voxel Z window used, plus the largest component's deterministic lowest-cell
@@ -318,9 +318,10 @@ because a corridor thinner than `SampleStep` is invisible. No UObject state, cac
 or PIE is required.
 | Symbol | Role |
 |--------|------|
-| `FVoxelStrateMeasureSettings` / `FVoxelStrateMetrics` | Plain settings/result structs for bounded strate sampling and derived measurements; callers may override the interior margin, and results identify the resolved margin/Z window and deterministic component representatives. |
+| `FVoxelStrateMeasureSettings` / `FVoxelStrateMetrics` | Plain settings/result structs for bounded strate sampling and derived measurements; callers may override the interior margin (including zero to sample the seal), and results identify the resolved margin/Z window and deterministic component representatives. |
 | `VF_MeasureStrate` | One-grid/one-flood-fill strate metrics; refuses invalid bounds or a grid over `MaxCells`. |
 | `VF_AreConnected` | Coarse 6-connected BFS plus one full-resolution recheck of its recovered route, with explicit endpoint-solid, out-of-window, `CoarseLied`, `NotConnectedAtThisResolution`, and connected outcomes. |
+| `VF_DiagnoseConnectivity` | The same query plus each mouth's air-component size/share and exact geometric distance to the nearest cell of the other component; proximity is measured in coarse-cell coordinates and does not claim a route through solid. |
 
 ### 3.7 Cave morphology (SDF rooms/tunnels) — `Public/VoxelCaveMorphology.h` + `.cpp`
 Header is rich with inline docs. Two namespaces + a per-chunk cache system.
@@ -485,6 +486,7 @@ The plugin's first tests (`OPSTACK-PLAN.md` Phase 0.5). Run them from the editor
 | `VoxelForgeStrateParamCoverageTest.cpp` | `VoxelForge.Determinism.StrateParamBlendCoverage` | **The X-macro guard** (added 2026-08-17). `FStrateGenerationParams::Lerp` blends the hand-written `VF_STRATE_PARAM_FIELDS` list, **not** the struct — so a field added to one and not the other compiles, tests green, and silently takes its **default** inside every Gradient/Interleaved transition band. This expands the X-macro a **third** way (after LERP and SNAP), into a name list, and diffs it against the struct's UObject reflection. Pure shape test: no fixture, no world, instant. `GExemptFieldNames` is **empty** — every reflected field is covered today, and any exemption must be written down as a decision. Stakes rise with the world composer, which intends to invent parameter sets through this same `Lerp` (`COMPOSER-NOTES.md`). |
 | `VoxelForgeLayoutOrderIndependenceTest.cpp` | `VoxelForge.Determinism.LayoutOrderIndependence` | Builds a known transient soft-pointer pool, then rebuilds it in original, reversed, and swapped orders. Requires a non-empty layout and passage set, and compares every slot's definition/Z/height plus passage endpoints, radius, type, control geometry, and bounds bit-for-bit. |
 | `VoxelForgePassageOpenSpaceTest.cpp` | `VoxelForge.Determinism.PassageLandsInOpenSpace` | Uses the real fixture density path to check every generated inter-strate passage whose destination query answers: a 16-point ring outside the mouth's carve/blend band has at least half its samples in destination air, and the endpoint matches the pure open-point result within the mouth's float envelope. This is a connectivity proxy, not a flood-fill proof. Reports checked passages and false/unanswerable archetypes; fails if it inspects zero passages. |
+| `VoxelForgeStrateConnectivityTest.cpp` | `VoxelForge.Generation.StrateConnectivity` / `VoxelForge.Generation.StrateConnectivityRefinement` | Bounded strate metrics with density-polarity and solid-gap controls, deterministic route rechecks, and a VerticalShafts refinement sweep. The refinement test explicitly checks margins 0/2/4/8 at step 2/radius 192, then conditionally reports mouth components/proximity and a 16-seed arrival→departure result. |
 
 ## 4. The density pipeline (most-edited hot path)
 

Source/VoxelForge/Private/Tests/VoxelForgeStrateConnectivityTest.cpp

@@ -246,6 +246,7 @@ namespace
     {
         int32 SampleStep = 0;
         int32 RadiusInVoxels = 0;
+        int32 RequestedMarginVoxels = -1;
         FVoxelStrateMetrics Metrics;
         FConnectivityProbe Probe;
     };
@@ -1037,6 +1038,7 @@ bool FVoxelForgeStrateConnectivityRefinementTest::RunTest(const FString& Paramet
     {
         int32 SampleStep;
         int32 RadiusInVoxels;
+        int32 InteriorMarginVoxels = -1;
     };
     static constexpr FRefinementCase Cases[] = {
         {4, 256},
@@ -1056,10 +1058,12 @@ bool FVoxelForgeStrateConnectivityRefinementTest::RunTest(const FString& Paramet
         FRefinementSweepRow& Row = Rows.AddDefaulted_GetRef();
         Row.SampleStep = TestCase.SampleStep;
         Row.RadiusInVoxels = TestCase.RadiusInVoxels;
+        Row.RequestedMarginVoxels = TestCase.InteriorMarginVoxels;
 
         FVoxelStrateMeasureSettings RowSettings = BaseSettings;
         RowSettings.SampleStep = TestCase.SampleStep;
         RowSettings.RadiusInVoxels = TestCase.RadiusInVoxels;
+        RowSettings.InteriorMarginVoxels = TestCase.InteriorMarginVoxels;
 
         Row.Metrics = VF_MeasureStrate(
             *World.Generator,
@@ -1125,6 +1129,106 @@ bool FVoxelForgeStrateConnectivityRefinementTest::RunTest(const FString& Paramet
     }
     const double SweepSeconds = FPlatformTime::Seconds() - SweepStartSeconds;
 
+    static constexpr int32 MarginCases[] = {0, 2, 4, 8};
+    TArray<FRefinementSweepRow> MarginRows;
+    MarginRows.Reserve(UE_ARRAY_COUNT(MarginCases));
+
+    const double MarginSweepStartSeconds = FPlatformTime::Seconds();
+    for (const int32 RequestedMargin : MarginCases)
+    {
+        FRefinementSweepRow& Row = MarginRows.AddDefaulted_GetRef();
+        Row.SampleStep = 2;
+        Row.RadiusInVoxels = 192;
+        Row.RequestedMarginVoxels = RequestedMargin;
+
+        FVoxelStrateMeasureSettings RowSettings = BaseSettings;
+        RowSettings.SampleStep = Row.SampleStep;
+        RowSettings.RadiusInVoxels = Row.RadiusInVoxels;
+        RowSettings.InteriorMarginVoxels = RequestedMargin;
+
+        Row.Metrics = VF_MeasureStrate(
+            *World.Generator,
+            *World.StrateManager,
+            VerticalShaftsIndex,
+            RowSettings);
+
+        Row.Probe.bChecked = true;
+        Row.Probe.Result = VF_AreConnected(
+            *World.Generator,
+            *World.StrateManager,
+            VerticalShaftsIndex,
+            ArrivalPoint,
+            DeparturePoint,
+            RowSettings,
+            Row.Probe.bStartSnapped,
+            Row.Probe.bGoalSnapped);
+
+        if (!Row.Metrics.bValid
+            || Row.Metrics.NumSampled <= 0
+            || Row.Metrics.NumSampled > BaseSettings.MaxCells
+            || Row.Metrics.NumAir <= 0
+            || Row.Metrics.NumSolid <= 0)
+        {
+            AddError(FString::Printf(
+                TEXT("HARD FAILURE: VerticalShafts margin row margin=%d did not produce a "
+                     "bounded non-vacuous measurement (valid=%s, reason='%s', cells=%lld, "
+                     "air=%lld, solid=%lld)."),
+                RequestedMargin,
+                Row.Metrics.bValid ? TEXT("true") : TEXT("false"),
+                *Row.Metrics.RefusalReason,
+                Row.Metrics.NumSampled,
+                Row.Metrics.NumAir,
+                Row.Metrics.NumSolid));
+            bAllChecksPassed = false;
+        }
+
+        if (Row.Metrics.ResolvedMarginVoxels != RequestedMargin)
+        {
+            AddError(FString::Printf(
+                TEXT("HARD FAILURE: VerticalShafts margin row requested margin=%d but the "
+                     "measurement resolved margin=%d."),
+                RequestedMargin,
+                Row.Metrics.ResolvedMarginVoxels));
+            bAllChecksPassed = false;
+        }
+
+        if (Row.Probe.Result == EVoxelConnectivityResult::OutOfWindow
+            || Row.Probe.Result == EVoxelConnectivityResult::StartCellSolid
+            || Row.Probe.Result == EVoxelConnectivityResult::GoalCellSolid)
+        {
+            AddError(FString::Printf(
+                TEXT("HARD FAILURE: VerticalShafts margin row margin=%d could not query both "
+                     "in-window air mouths: %s."),
+                RequestedMargin,
+                *ConnectivityProbeText(Row.Probe)));
+            bAllChecksPassed = false;
+        }
+
+        if (Row.Probe.bStartSnapped || Row.Probe.bGoalSnapped)
+        {
+            AddError(FString::Printf(
+                TEXT("HARD FAILURE: VerticalShafts margin row margin=%d snapped an endpoint; "
+                     "the controlled comparison is not a mouth-to-mouth query."),
+                RequestedMargin));
+            bAllChecksPassed = false;
+        }
+    }
+    const double MarginSweepSeconds = FPlatformTime::Seconds() - MarginSweepStartSeconds;
+
+    bool bMarginArtifact = false;
+    bool bMarginSweepUsable = MarginRows.Num() == UE_ARRAY_COUNT(MarginCases);
+    for (const FRefinementSweepRow& Row : MarginRows)
+    {
+        bMarginArtifact |= Row.Probe.Result == EVoxelConnectivityResult::Connected;
+        bMarginSweepUsable &= Row.Metrics.bValid
+            && Row.Metrics.ResolvedMarginVoxels == Row.RequestedMarginVoxels
+            && Row.Probe.Result != EVoxelConnectivityResult::OutOfWindow
+            && Row.Probe.Result != EVoxelConnectivityResult::StartCellSolid
+            && Row.Probe.Result != EVoxelConnectivityResult::GoalCellSolid
+            && !Row.Probe.bStartSnapped
+            && !Row.Probe.bGoalSnapped;
+    }
+
     FString Summary = TEXT(
         "VerticalShafts arrival->departure refinement (seed 1337; source mouth pair; "
         "derived interior window):\n");
@@ -1158,6 +1262,199 @@ bool FVoxelForgeStrateConnectivityRefinementTest::RunTest(const FString& Paramet
     Summary += FString::Printf(
         TEXT("VerticalShafts refinement sweep wall-clock: %.3f seconds.\n"),
         SweepSeconds);
+
+    Summary += TEXT(
+        "Part A — margin artifact check (SampleStep 2, RadiusInVoxels 192; explicit margins):\n");
+    Summary += TEXT(
+        "  margin | verdict | components | largest share | sampled cells | Z window | endpoint snaps\n");
+    for (const FRefinementSweepRow& Row : MarginRows)
+    {
+        const FString VerdictText = ConnectivityProbeText(Row.Probe);
+        FString SnapText = TEXT("none");
+        if (Row.Probe.bStartSnapped || Row.Probe.bGoalSnapped)
+        {
+            SnapText = ConnectivityProbeText(Row.Probe);
+        }
+        Summary += FString::Printf(
+            TEXT("  %d | %s | %d | %.9g | %lld | [%d,%d) | %s\n"),
+            Row.RequestedMarginVoxels,
+            *VerdictText,
+            Row.Metrics.NumAirComponents,
+            Row.Metrics.LargestComponentShare,
+            Row.Metrics.NumSampled,
+            Row.Metrics.SampledMinZ,
+            Row.Metrics.SampledMaxZ,
+            *SnapText);
+    }
+    Summary += FString::Printf(
+        TEXT("Part A verdict: %s. Margin sweep wall-clock: %.3f seconds.\n"),
+        bMarginArtifact
+            ? TEXT("ARTIFACT — at least one margin-0/2/4 query is CONNECTED; the default margin excluded the connection")
+            : TEXT("FINDING SURVIVES — no margin-0/2/4 query is CONNECTED; the full window is exonerated"),
+        MarginSweepSeconds);
+
+    if (!bMarginArtifact && bMarginSweepUsable)
+    {
+        FVoxelStrateMeasureSettings DiagnosticSettings = BaseSettings;
+        DiagnosticSettings.SampleStep = 2;
+        DiagnosticSettings.RadiusInVoxels = 192;
+        DiagnosticSettings.InteriorMarginVoxels = 0;
+
+        const FVoxelConnectivityDiagnostics Diagnostics = VF_DiagnoseConnectivity(
+            *World.Generator,
+            *World.StrateManager,
+            VerticalShaftsIndex,
+            ArrivalPoint,
+            DeparturePoint,
+            DiagnosticSettings);
+        if (!Diagnostics.bValid
+            || Diagnostics.NumAirCells <= 0
+            || Diagnostics.StartComponentCells <= 0
+            || Diagnostics.GoalComponentCells <= 0
+            || Diagnostics.StartToGoalComponentDistanceCells < 0
+            || Diagnostics.GoalToStartComponentDistanceCells < 0)
+        {
+            AddError(TEXT("HARD FAILURE: the surviving VerticalShafts finding could not produce "
+                          "component and separation diagnostics."));
+            bAllChecksPassed = false;
+        }
+
+        const float MouthXYSeparation = FMath::Sqrt(
+            FMath::Square(ArrivalPoint.X - DeparturePoint.X)
+            + FMath::Square(ArrivalPoint.Y - DeparturePoint.Y));
+        const float MouthZDelta = DeparturePoint.Z - ArrivalPoint.Z;
+        const float MouthStraightLine = FVector::Dist(ArrivalPoint, DeparturePoint);
+        const float CoarseStraightLineCells = MouthStraightLine
+            / static_cast<float>(DiagnosticSettings.SampleStep);
+
+        Summary += TEXT("Part B — VerticalShafts diagnosis (seed 1337; margin 0; step 2; radius 192):\n");
+        Summary += FString::Printf(
+            TEXT("  arrival LowerPoint: (%.3f, %.3f, %.3f); departure UpperPoint: "
+                 "(%.3f, %.3f, %.3f); XY separation %.3f voxels; Z delta %.3f voxels.\n"),
+            ArrivalPoint.X,
+            ArrivalPoint.Y,
+            ArrivalPoint.Z,
+            DeparturePoint.X,
+            DeparturePoint.Y,
+            DeparturePoint.Z,
+            MouthXYSeparation,
+            MouthZDelta);
+        Summary += FString::Printf(
+            TEXT("  mouth components: air=%lld; arrival=%lld (share %.9g); departure=%lld "
+                 "(share %.9g).\n"),
+            Diagnostics.NumAirCells,
+            Diagnostics.StartComponentCells,
+            Diagnostics.StartComponentShare,
+            Diagnostics.GoalComponentCells,
+            Diagnostics.GoalComponentShare);
+        Summary += FString::Printf(
+            TEXT("  coarse-grid separation: straight-line %.3f cells (%.3f voxels); "
+                 "arrival to nearest departure-component cell %.3f cells (%.3f voxels); "
+                 "departure to nearest arrival-component cell %.3f cells (%.3f voxels).\n"),
+            CoarseStraightLineCells,
+            MouthStraightLine,
+            Diagnostics.StartToGoalComponentDistanceCells,
+            Diagnostics.StartToGoalComponentDistanceCells * static_cast<float>(DiagnosticSettings.SampleStep),
+            Diagnostics.GoalToStartComponentDistanceCells,
+            Diagnostics.GoalToStartComponentDistanceCells * static_cast<float>(DiagnosticSettings.SampleStep));
+        Summary += FString::Printf(
+            TEXT("  diagnostic route verdict: %s; endpoint snaps: start=%s, goal=%s.\n"),
+            ConnectivityResultName(Diagnostics.Result),
+            Diagnostics.bStartSnapped ? TEXT("yes") : TEXT("no"),
+            Diagnostics.bGoalSnapped ? TEXT("yes") : TEXT("no"));
+
+        static constexpr int32 SeedCases[] = {
+            1337, 1, 2, 3, 4, 5, 6, 7,
+            8, 9, 10, 11, 12, 13, 14, 15,
+        };
+        int32 SeedPasses = 0;
+        int32 SeedFailures = 0;
+        int32 SeedSnapEvents = 0;
+        int32 SeedResultCounts[6] = {};
+        const double SeedSweepStartSeconds = FPlatformTime::Seconds();
+        for (const int32 Seed : SeedCases)
+        {
+            FTestWorld SeedWorld;
+            SeedWorld.Build(Seed, /*InGapChunks=*/2);
+            if (!SeedWorld.IsValid())
+            {
+                AddError(FString::Printf(
+                    TEXT("HARD FAILURE: seed %d could not build the VerticalShafts fixture."),
+                    Seed));
+                ++SeedFailures;
+                continue;
+            }
+
+            FVector SeedArrivalPoint = FVector::ZeroVector;
+            FVector SeedDeparturePoint = FVector::ZeroVector;
+            if (!FindChainMouths(
+                    SeedWorld.StrateManager->GetPassages(),
+                    VerticalShaftsIndex,
+                    SeedArrivalPoint,
+                    SeedDeparturePoint))
+            {
+                AddError(FString::Printf(
+                    TEXT("HARD FAILURE: seed %d did not produce exactly one VerticalShafts "
+                         "arrival and departure mouth."),
+                    Seed));
+                ++SeedFailures;
+                continue;
+            }
+
+            bool bStartSnapped = false;
+            bool bGoalSnapped = false;
+            const EVoxelConnectivityResult SeedResult = VF_AreConnected(
+                *SeedWorld.Generator,
+                *SeedWorld.StrateManager,
+                VerticalShaftsIndex,
+                SeedArrivalPoint,
+                SeedDeparturePoint,
+                DiagnosticSettings,
+                bStartSnapped,
+                bGoalSnapped);
+            ++SeedResultCounts[static_cast<int32>(SeedResult)];
+            const int32 SnapEvents = (bStartSnapped ? 1 : 0) + (bGoalSnapped ? 1 : 0);
+            SeedSnapEvents += SnapEvents;
+            if (SeedResult == EVoxelConnectivityResult::Connected
+                && !bStartSnapped
+                && !bGoalSnapped)
+            {
+                ++SeedPasses;
+            }
+            else
+            {
+                ++SeedFailures;
+            }
+        }
+        const double SeedSweepSeconds = FPlatformTime::Seconds() - SeedSweepStartSeconds;
+
+        Summary += FString::Printf(
+            TEXT("  multi-seed arrival->departure sweep (%d seeds; margin 0, step 2, radius 192): "
+                 "%d pass, %d fail, %d snapped endpoint events; wall-clock %.3f seconds.\n"),
+            UE_ARRAY_COUNT(SeedCases),
+            SeedPasses,
+            SeedFailures,
+            SeedSnapEvents,
+            SeedSweepSeconds);
+        Summary += FString::Printf(
+            TEXT("  seed result breakdown: CONNECTED=%d, NOT_CONNECTED_AT_THIS_RESOLUTION=%d, "
+                 "START_CELL_SOLID=%d, GOAL_CELL_SOLID=%d, OUT_OF_WINDOW=%d, COARSE_LIED=%d.\n"),
+            SeedResultCounts[static_cast<int32>(EVoxelConnectivityResult::Connected)],
+            SeedResultCounts[static_cast<int32>(EVoxelConnectivityResult::NotConnectedAtThisResolution)],
+            SeedResultCounts[static_cast<int32>(EVoxelConnectivityResult::StartCellSolid)],
+            SeedResultCounts[static_cast<int32>(EVoxelConnectivityResult::GoalCellSolid)],
+            SeedResultCounts[static_cast<int32>(EVoxelConnectivityResult::OutOfWindow)],
+            SeedResultCounts[static_cast<int32>(EVoxelConnectivityResult::CoarseLied)]);
+    }
+    else
+    {
+        Summary += FString::Printf(
+            TEXT("Part B diagnosis and multi-seed sweep: SKIPPED because Part A %s.\n"),
+            bMarginArtifact
+                ? TEXT("found the connection in the previously excluded band")
+                : TEXT("did not produce a usable four-row measurement/query set"));
+    }
+
     AddInfo(Summary);
 
     return bAllChecksPassed;

Source/VoxelForge/Private/VoxelStrateMeasure.cpp

@@ -164,14 +164,14 @@ namespace VoxelStrateMeasurePrivate
             }
             else if (DoubleSealThickness <= static_cast<float>(INT32_MIN))
             {
-                RequestedMargin = 1;
+                RequestedMargin = 0;
             }
             else
             {
                 RequestedMargin = FMath::CeilToInt(DoubleSealThickness);
             }
         }
-        const int32 ResolvedMargin = FMath::Clamp(RequestedMargin, 1, MaxMargin);
+        const int32 ResolvedMargin = FMath::Clamp(RequestedMargin, 0, MaxMargin);
 
         const int64 InteriorMinZ = StrateBottomZ + static_cast<int64>(ResolvedMargin);
         const int64 InteriorMaxZ = StrateTopZ - static_cast<int64>(ResolvedMargin);
@@ -803,6 +803,94 @@ namespace VoxelStrateMeasurePrivate
         }
         return true;
     }
+
+    EVoxelConnectivityResult EvaluateConnectivityOnGrid(
+        const UVoxelGenerator& Generator,
+        const FSampleGrid& Grid,
+        const FVector& AVoxel,
+        const FVector& BVoxel,
+        int32& OutStart,
+        int32& OutGoal,
+        bool& bOutStartSnapped,
+        bool& bOutGoalSnapped)
+    {
+        OutStart = -1;
+        OutGoal = -1;
+        bOutStartSnapped = false;
+        bOutGoalSnapped = false;
+
+        const EEndpointCellResult StartCell = ResolveEndpointCell(
+            Grid, AVoxel, OutStart, bOutStartSnapped);
+        const EEndpointCellResult GoalCell = ResolveEndpointCell(
+            Grid, BVoxel, OutGoal, bOutGoalSnapped);
+        if (StartCell == EEndpointCellResult::OutOfWindow
+            || GoalCell == EEndpointCellResult::OutOfWindow)
+        {
+            return EVoxelConnectivityResult::OutOfWindow;
+        }
+        if (StartCell == EEndpointCellResult::Solid)
+        {
+            return EVoxelConnectivityResult::StartCellSolid;
+        }
+        if (GoalCell == EEndpointCellResult::Solid)
+        {
+            return EVoxelConnectivityResult::GoalCellSolid;
+        }
+
+        TArray<int32> Path;
+        if (!FindCoarsePath(Grid, OutStart, OutGoal, Path))
+        {
+            return EVoxelConnectivityResult::NotConnectedAtThisResolution;
+        }
+
+        // The coarse route is evidence, not a pass. Re-check this one BFS route at one-voxel
+        // spacing in the original voxel field. A single solid sample is the dangerous coarse
+        // false positive.
+        if (!FullResolutionPathIsAir(Generator, Grid, AVoxel, BVoxel, Path))
+        {
+            return EVoxelConnectivityResult::CoarseLied;
+        }
+        return EVoxelConnectivityResult::Connected;
+    }
+
+    float DistanceToComponent(
+        const FSampleGrid& Grid,
+        int32 StartCell,
+        int32 TargetComponent,
+        const TArray<int32>& Components)
+    {
+        if (Components[StartCell] == TargetComponent)
+        {
+            return 0.0f;
+        }
+
+        int32 StartX = 0;
+        int32 StartY = 0;
+        int32 StartZ = 0;
+        DecodeIndex(Grid, StartCell, StartX, StartY, StartZ);
+        int64 BestDistanceSquared = MAX_int64;
+        for (int32 Index = 0; Index < Grid.CellCount; ++Index)
+        {
+            if (Components[Index] != TargetComponent)
+            {
+                continue;
+            }
+
+            int32 X = 0;
+            int32 Y = 0;
+            int32 Z = 0;
+            DecodeIndex(Grid, Index, X, Y, Z);
+            const int64 DX = static_cast<int64>(X) - static_cast<int64>(StartX);
+            const int64 DY = static_cast<int64>(Y) - static_cast<int64>(StartY);
+            const int64 DZ = static_cast<int64>(Z) - static_cast<int64>(StartZ);
+            const int64 DistanceSquared = DX * DX + DY * DY + DZ * DZ;
+            BestDistanceSquared = FMath::Min(BestDistanceSquared, DistanceSquared);
+        }
+
+        return BestDistanceSquared == MAX_int64
+            ? -1.0f
+            : FMath::Sqrt(static_cast<float>(BestDistanceSquared));
+    }
 }
 
 FVoxelStrateMetrics VF_MeasureStrate(
@@ -859,38 +947,109 @@ EVoxelConnectivityResult VF_AreConnected(
 
     int32 Start = -1;
     int32 Goal = -1;
-    const VoxelStrateMeasurePrivate::EEndpointCellResult StartCell =
-        VoxelStrateMeasurePrivate::ResolveEndpointCell(
-            Grid, AVoxel, Start, bOutStartSnapped);
-    const VoxelStrateMeasurePrivate::EEndpointCellResult GoalCell =
-        VoxelStrateMeasurePrivate::ResolveEndpointCell(
-            Grid, BVoxel, Goal, bOutGoalSnapped);
-    if (StartCell == VoxelStrateMeasurePrivate::EEndpointCellResult::OutOfWindow
-        || GoalCell == VoxelStrateMeasurePrivate::EEndpointCellResult::OutOfWindow)
+    return VoxelStrateMeasurePrivate::EvaluateConnectivityOnGrid(
+        Generator,
+        Grid,
+        AVoxel,
+        BVoxel,
+        Start,
+        Goal,
+        bOutStartSnapped,
+        bOutGoalSnapped);
+}
+
+FVoxelConnectivityDiagnostics VF_DiagnoseConnectivity(
+    const UVoxelGenerator& Generator,
+    const UVoxelStrateManager& Manager,
+    int32 StrateIndex,
+    const FVector& AVoxel,
+    const FVector& BVoxel,
+    const FVoxelStrateMeasureSettings& Settings)
+{
+    FVoxelConnectivityDiagnostics Result;
+    if (!FMath::IsFinite(AVoxel.X) || !FMath::IsFinite(AVoxel.Y) || !FMath::IsFinite(AVoxel.Z)
+        || !FMath::IsFinite(BVoxel.X) || !FMath::IsFinite(BVoxel.Y) || !FMath::IsFinite(BVoxel.Z))
+    {
+        return Result;
+    }
+
+    VoxelStrateMeasurePrivate::FSampleGrid Grid;
+    FString RefusalReason;
+    if (!VoxelStrateMeasurePrivate::BuildSampleGrid(
+            Generator, Manager, StrateIndex, Settings, Grid, RefusalReason))
     {
-        return EVoxelConnectivityResult::OutOfWindow;
+        return Result;
     }
-    if (StartCell == VoxelStrateMeasurePrivate::EEndpointCellResult::Solid)
+
+    int32 Start = -1;
+    int32 Goal = -1;
+    Result.Result = VoxelStrateMeasurePrivate::EvaluateConnectivityOnGrid(
+        Generator,
+        Grid,
+        AVoxel,
+        BVoxel,
+        Start,
+        Goal,
+        Result.bStartSnapped,
+        Result.bGoalSnapped);
+    Result.bValid = Result.Result != EVoxelConnectivityResult::OutOfWindow
+        || (Start >= 0 && Goal >= 0);
+    if (Start < 0 || Goal < 0)
     {
-        return EVoxelConnectivityResult::StartCellSolid;
+        return Result;
     }
-    if (GoalCell == VoxelStrateMeasurePrivate::EEndpointCellResult::Solid)
+
+    TArray<int32> Components;
+    int32 NumComponents = 0;
+    int64 LargestComponentCells = 0;
+    int32 LargestComponentLowestCell = INDEX_NONE;
+    int32 NumComponentsAtLeast1Pct = 0;
+    int64 NumAir = 0;
+    for (const uint8 bAir : Grid.Air)
     {
-        return EVoxelConnectivityResult::GoalCellSolid;
+        NumAir += bAir != 0u ? 1 : 0;
     }
+    FloodFillAir(
+        Grid,
+        Components,
+        NumComponents,
+        NumAir,
+        LargestComponentCells,
+        LargestComponentLowestCell,
+        NumComponentsAtLeast1Pct);
 
-    TArray<int32> Path;
-    if (!VoxelStrateMeasurePrivate::FindCoarsePath(Grid, Start, Goal, Path))
+    const int32 StartComponent = Components[Start];
+    const int32 GoalComponent = Components[Goal];
+    if (StartComponent < 0 || GoalComponent < 0)
     {
-        return EVoxelConnectivityResult::NotConnectedAtThisResolution;
+        return Result;
+    }
+
+    TArray<int32> ComponentCells;
+    ComponentCells.Init(0, NumComponents);
+    for (const int32 Component : Components)
+    {
+        if (Component >= 0)
+        {
+            ++ComponentCells[Component];
+        }
     }
 
-    // The coarse route is evidence, not a pass. Re-check this one BFS route at one-voxel spacing
-    // in the original voxel field. A single solid sample is the dangerous coarse false positive.
-    if (!VoxelStrateMeasurePrivate::FullResolutionPathIsAir(
-            Generator, Grid, AVoxel, BVoxel, Path))
+    Result.NumAirCells = NumAir;
+    Result.StartComponentCells = ComponentCells[StartComponent];
+    Result.GoalComponentCells = ComponentCells[GoalComponent];
+    if (NumAir > 0)
     {
-        return EVoxelConnectivityResult::CoarseLied;
+        Result.StartComponentShare = static_cast<float>(
+            static_cast<double>(Result.StartComponentCells) / static_cast<double>(NumAir));
+        Result.GoalComponentShare = static_cast<float>(
+            static_cast<double>(Result.GoalComponentCells) / static_cast<double>(NumAir));
     }
-    return EVoxelConnectivityResult::Connected;
+    Result.StartToGoalComponentDistanceCells =
+        VoxelStrateMeasurePrivate::DistanceToComponent(
+            Grid, Start, GoalComponent, Components);
+    Result.GoalToStartComponentDistanceCells =
+        VoxelStrateMeasurePrivate::DistanceToComponent(
+            Grid, Goal, StartComponent, Components);
+    return Result;
 }

Source/VoxelForge/Public/VoxelStrateMeasure.h

@@ -26,7 +26,7 @@ struct VOXELFORGE_API FVoxelStrateMeasureSettings
     FVector2D CenterXY       = FVector2D::ZeroVector; // Actor-space voxel coordinates.
     int32     MaxCells       = 8000000; // Refuse a grid larger than this many cells.
     int32     HeadroomCells  = 2;    // Air cells above a floor cell required for walkable.
-    int32     InteriorMarginVoxels = -1; // <0 = derive from BoundarySealThickness (2x, clamped).
+    int32     InteriorMarginVoxels = -1; // <0 = derive from BoundarySealThickness (2x, clamped); explicit 0 includes the seal.
 };
 
 /** Metrics derived from one coarse grid and its single air flood fill. */
@@ -55,6 +55,33 @@ struct VOXELFORGE_API FVoxelStrateMetrics
     int32 SampledMaxZ = 0;
 };
 
+/**
+ * Component facts for one coarse mouth-to-mouth query.
+ *
+ * The component shares are fractions of the sampled air cells. The two component distances are
+ * exact spatial Euclidean distances in coarse-cell coordinates to the nearest cell of the other
+ * mouth's component. Solid cells do not participate in that proximity calculation because it
+ * measures geometric separation between air components, not an alternate route. A distance of
+ * zero means both mouths resolve to the same component. Negative distances mean that the
+ * requested grid or endpoint could not be analyzed.
+ */
+struct VOXELFORGE_API FVoxelConnectivityDiagnostics
+{
+    bool bValid = false;
+    EVoxelConnectivityResult Result = EVoxelConnectivityResult::OutOfWindow;
+    bool bStartSnapped = false;
+    bool bGoalSnapped = false;
+
+    int64 NumAirCells = 0;
+    int64 StartComponentCells = 0;
+    int64 GoalComponentCells = 0;
+    float StartComponentShare = 0.0f;
+    float GoalComponentShare = 0.0f;
+
+    float StartToGoalComponentDistanceCells = -1.0f;
+    float GoalToStartComponentDistanceCells = -1.0f;
+};
+
 /**
  * Measure one strate in actor-space voxel coordinates.
  *
@@ -87,3 +114,17 @@ VOXELFORGE_API EVoxelConnectivityResult VF_AreConnected(
     const FVoxelStrateMeasureSettings& Settings,
     bool& bOutStartSnapped,
     bool& bOutGoalSnapped);
+
+/**
+ * Return the component sizes and spatial separation facts for the same query performed by
+ * VF_AreConnected. The returned Result uses the same full-resolution route re-check. bValid means
+ * the sample grid was built and both endpoint coordinates were in its bounds; component fields
+ * remain zero/negative when an endpoint has no air cell to analyze.
+ */
+VOXELFORGE_API FVoxelConnectivityDiagnostics VF_DiagnoseConnectivity(
+    const UVoxelGenerator& Generator,
+    const UVoxelStrateManager& Manager,
+    int32 StrateIndex,
+    const FVector& AVoxel,
+    const FVector& BVoxel,
+    const FVoxelStrateMeasureSettings& Settings);