using System;
using System.Collections.Generic;
using System.IO;
using System.Linq;
using System.Numerics;
using AcDream.Core.Physics;
using DatReaderWriter.Enums;
using DatReaderWriter.Types;
using Xunit;

namespace AcDream.Core.Tests.Physics;

/// <summary>
/// A6.P3 issue #98 (2026-05-23) — deterministic TRAJECTORY replay
/// harness for the cottage cellar-ascent failure. Drives
/// <see cref="PhysicsEngine.ResolveWithTransition"/> through N physics
/// ticks against pre-loaded cell fixtures, capturing a per-tick
/// trajectory record.
///
/// <para>
/// Unlike <see cref="Issue98CellarUpReplayTests"/> (which tests a SINGLE
/// failing-frame's geometry against our walkable predicates), this
/// harness drives MANY ticks through the full engine to reproduce the
/// trajectory itself — once the fixtures support it (see below).
/// </para>
///
/// <h3>Status as of 2026-05-23 evening: harness mechanics WORK, fixtures
/// INCOMPLETE.</h3>
///
/// <para>
/// The harness compiles and runs the engine through N ticks in
/// &lt; 100 ms total. Two findings during commissioning:
/// </para>
///
/// <list type="number">
///   <item>The three issue-#98 cell fixtures
///         (<c>tests/AcDream.Core.Tests/Fixtures/issue98/0xA9B40*.json</c>)
///         contain ONLY axis-aligned polygons — cellar floor, cellar
///         ceiling, four cellar walls, cottage floor, cottage walls. The
///         live capture's CELLAR RAMP polygon
///         (normal ≈ <c>(0, ±0.719, 0.695)</c>) is NOT in any of the
///         fixtures. Without it the harness can't reproduce the climb
///         trajectory — the sphere walks across the cellar floor
///         horizontally and never encounters a slope.</item>
///   <item>Independently: at the sphere's initial position resting on
///         the cellar floor, the engine reports
///         <c>hit=yes n=(0,0,1) walkable=False</c> on tick 1 and rejects
///         the forward move. The grounded state flips off and subsequent
///         ticks proceed as airborne (no Z change). This may be a real
///         engine bug (touching the floor classified as non-walkable
///         collision) or a fixture issue (cellar floor poly's
///         containment test mis-firing). Either way, the harness
///         exposes it deterministically — that's the point.</item>
/// </list>
///
/// <para>
/// <b>Before this harness can drive issue-#98 trajectory fix attempts,
/// the fixtures need a re-capture</b> that includes:
/// </para>
///
/// <list type="bullet">
///   <item>The cellar ramp polygon (whichever cell it actually lives
///         in — the live capture said cellar cell <c>0xA9B40147</c>,
///         but our dump doesn't have it; investigate
///         <see cref="CellDumpSerializer"/> to see whether some
///         polygons are being skipped during capture).</item>
///   <item>Any neighboring cells the sphere may transit into during
///         the climb (the live capture's
///         <c>[cell-set-summary]</c> showed overlap with
///         <c>0xA9B40143</c> and <c>0xA9B40146</c>, both already in
///         the fixture set — but additional cells beyond these may
///         appear at tick boundaries we haven't observed).</item>
/// </list>
///
/// <para>
/// The current tests document the harness mechanics + the two
/// findings above. When fixtures are re-captured, flip
/// <see cref="CellarUp_FreezesAtRampTop_DocumentsBug"/>'s assertion
/// to require a successful climb and add additional tests for the
/// trajectory shape.
/// </para>
/// </summary>
public class CellarUpTrajectoryReplayTests
{
    // ── Cellar / cottage geometry constants ────────────────────────
    private const uint CellarId         = 0xA9B40147u;
    private const uint CottageNeighborA = 0xA9B40143u;
    private const uint CottageNeighborB = 0xA9B40146u;

    private const float CellarFloorZ  = 90.95f;
    private const float CottageFloorZ = 94.00f;

    private const float SphereRadius  = 0.48f;
    private const float SphereHeight  = 1.20f;
    private const float StepUpHeight   = 0.60f;
    private const float StepDownHeight = 0.04f;

    /// <summary>
    /// Sphere center starts exactly at its natural resting position on
    /// the cellar floor: bottom on floor, center at Z = floor + radius.
    /// Y=9.5 is ~0.75 m before the ramp foot at Y=8.75 (live-capture
    /// ramp plane: <c>0.719·y + 0.695·z = 69.5035</c> → y=8.75 at z=90.95).
    /// X=141.5 matches the live capture's X.
    /// </summary>
    private static readonly Vector3 InitialSphereWorld =
        new(141.5f, 9.5f, CellarFloorZ + SphereRadius);

    /// <summary>
    /// Per-tick forward offset (−Y direction toward the ramp).
    /// Magnitude (~0.10 m) matches the live capture's observed per-tick
    /// requested offset.
    /// </summary>
    private static readonly Vector3 PerTickOffset =
        new(0f, -0.10f, 0f);

    private const int SimulationTicks = 200;

    // ───────────────────────────────────────────────────────────────
    // Tests
    // ───────────────────────────────────────────────────────────────

    /// <summary>
    /// Confirms the harness compiles, the engine runs the simulation,
    /// and a trajectory comes back with the expected number of points.
    /// Does NOT assert on trajectory CONTENT — fixture limitations
    /// (see class summary) make content-level assertions premature.
    /// </summary>
    [Fact]
    public void Harness_CompilesAndRunsSimulation()
    {
        var (engine, _) = BuildEngineWithCellarFixtures();
        var body = BuildInitialBody();
        var trajectory = SimulateTicks(engine, body, CellarId, SimulationTicks);

        Assert.Equal(SimulationTicks + 1, trajectory.Count);
        Assert.Equal(0, trajectory[0].Tick);
        Assert.Equal(SimulationTicks, trajectory[^1].Tick);
    }

    /// <summary>
    /// Diagnostic dump: print the first 10 trajectory points + the
    /// engine's resolve-probe decisions. Useful when investigating
    /// what the harness is actually doing.
    /// </summary>
    [Fact]
    public void Harness_DiagnosticDump_FirstTenTicks()
    {
        PhysicsDiagnostics.ProbeResolveEnabled    = true;
        PhysicsDiagnostics.ProbeStepWalkEnabled   = true;
        PhysicsDiagnostics.ProbeIndoorBspEnabled  = true;
        PhysicsDiagnostics.ProbePolyDumpEnabled   = true;
        try
        {
            var (engine, _) = BuildEngineWithCellarFixtures();
            var body = BuildInitialBody();
            var trajectory = SimulateTicks(engine, body, CellarId, 2);

            var msg = "Trajectory (2 ticks):\n  " +
                string.Join("\n  ", trajectory.Select(p =>
                    $"tick={p.Tick} pos=({p.Position.X:F4},{p.Position.Y:F4},{p.Position.Z:F4}) " +
                    $"cell=0x{p.CellId:X8} onGround={p.IsOnGround} cpValid={p.CpValid}"));

            // Always pass — this is a diagnostic test; the probe
            // output appears in the test runner's captured stdout
            // and the trajectory in the assertion message on failure.
            Assert.True(true, msg);
        }
        finally
        {
            PhysicsDiagnostics.ProbeResolveEnabled    = false;
            PhysicsDiagnostics.ProbeStepWalkEnabled   = false;
            PhysicsDiagnostics.ProbeIndoorBspEnabled  = false;
            PhysicsDiagnostics.ProbePolyDumpEnabled   = false;
        }
    }

    /// <summary>
    /// Experiment: drive without a PhysicsBody (no CP seeding, no
    /// cross-tick state). Tests whether the airborne-at-tick-1 issue
    /// is caused by the seeded CP creating a false collision against
    /// the cellar floor.
    /// </summary>
    [Fact]
    public void Harness_DiagnosticDump_NoBodySeed()
    {
        PhysicsDiagnostics.ProbeResolveEnabled = true;
        try
        {
            var (engine, _) = BuildEngineWithCellarFixtures();

            uint cellId      = CellarId;
            bool isOnGround  = true;
            Vector3 pos      = InitialSphereWorld;

            var trajectory = new List<TrajectoryPoint>
            {
                new(0, pos, cellId, isOnGround, false),
            };

            for (int tick = 1; tick <= 10; tick++)
            {
                Vector3 target = pos + PerTickOffset;
                var result = engine.ResolveWithTransition(
                    pos, target, cellId,
                    SphereRadius, SphereHeight,
                    StepUpHeight, StepDownHeight,
                    isOnGround,
                    body:           null,            // ← no body, no CP seed
                    moverFlags:     ObjectInfoState.IsPlayer | ObjectInfoState.EdgeSlide,
                    movingEntityId: 0);

                pos = result.Position;
                cellId = result.CellId;
                isOnGround = result.IsOnGround;
                trajectory.Add(new(tick, pos, cellId, isOnGround, false));
            }

            var msg = "No-body trajectory (10 ticks):\n  " +
                string.Join("\n  ", trajectory.Select(p =>
                    $"tick={p.Tick} pos=({p.Position.X:F4},{p.Position.Y:F4},{p.Position.Z:F4}) " +
                    $"onGround={p.IsOnGround}"));

            Assert.True(true, msg);
        }
        finally
        {
            PhysicsDiagnostics.ProbeResolveEnabled = false;
        }
    }

    /// <summary>
    /// Documents the deep-investigation finding (2026-05-23 evening
    /// extension): the seeded grounded sphere still goes airborne at
    /// tick 1 with hit=(0,1,0) — a +Y wall normal that doesn't match
    /// any registered geometry. The hit is set by ValidateTransition
    /// after the inner TransitionalInsert returns Collided, but the
    /// source of the (0,1,0) inside TransitionalInsert is not yet
    /// isolated.
    ///
    /// <para>
    /// Investigation excluded:
    /// <list type="bullet">
    ///   <item>Stub landblock terrain (removed; same hit)</item>
    ///   <item>Synthetic stair GfxObj (removed; same hit)</item>
    ///   <item>Cell BSP=null on Hydrate (attached synthetic BSP; same hit)</item>
    ///   <item>WalkablePolygon NOT seeded vs seeded (seeded now: walkable=True survives, but (0,1,0) hit remains)</item>
    ///   <item>Initial sphere Z lift 0.0 vs 0.05 m (same hit)</item>
    ///   <item>PhysicsBody seeded vs body=null (same hit)</item>
    /// </list>
    /// </para>
    ///
    /// <para>
    /// Next session's investigation move: build a side-by-side
    /// instrumentation harness that calls the EXACT same
    /// ResolveWithTransition invocation as production's
    /// PlayerMovementController, with identical body state, and
    /// compare per-tick state divergence. The harness setup must be
    /// missing some piece of state that production carries from a
    /// prior live tick — find what piece.
    /// </para>
    /// </summary>
    [Fact]
    public void Harness_Finding_SphereGoesAirborneAtTick1()
    {
        var (engine, _) = BuildEngineWithCellarFixtures();
        var body = BuildInitialBody();
        var trajectory = SimulateTicks(engine, body, CellarId, 3);

        Assert.True(trajectory[0].IsOnGround,
            "Tick 0 is the seeded starting state and must report grounded.");
        Assert.False(trajectory[1].IsOnGround,
            "Open finding: at tick 1 the engine reports the sphere is NOT " +
            "grounded, even though it started seeded with ContactPlane + " +
            "WalkablePolygon on the cellar floor and the cell has a " +
            "synthetic BSP wrapping every polygon. Hit normal is (0,1,0) — " +
            "doesn't match any registered geometry. Source of (0,1,0) " +
            "inside TransitionalInsert is not yet isolated. See the class " +
            "doc for the exclusion list and next investigation move.");
    }

    /// <summary>
    /// Perf budget for the harness: 200 ticks must complete in well
    /// under 500 ms. If this ever fails, the inner loop has regressed
    /// and the whole point of the harness — fast iteration on physics
    /// fixes — is at risk.
    /// </summary>
    [Fact]
    public void Harness_SimulationRunsInUnder500ms()
    {
        var (engine, _) = BuildEngineWithCellarFixtures();
        var body = BuildInitialBody();

        var sw = System.Diagnostics.Stopwatch.StartNew();
        _ = SimulateTicks(engine, body, CellarId, SimulationTicks);
        sw.Stop();

        Assert.True(sw.ElapsedMilliseconds < 500,
            $"200-tick simulation should complete in under 500 ms. " +
            $"Took: {sw.ElapsedMilliseconds} ms.");
    }

    /// <summary>
    /// A6.P3 #98 (2026-05-23 evening apparatus extension) — smoke-tests
    /// the <see cref="PhysicsResolveCapture"/> probe. Drives 3 ticks with
    /// capture enabled, then reads the JSON-Lines file back and verifies:
    /// <list type="bullet">
    ///   <item>One record per call.</item>
    ///   <item>Inputs round-trip (currentPos, targetPos, cellId, flags).</item>
    ///   <item>Body-before and body-after snapshots are present.</item>
    /// </list>
    /// This proves the production probe is wire-correct before we ask the
    /// user to run a live capture in the cellar — if this test passes, the
    /// only variable left is what's different about the live run.
    /// </summary>
    [Fact]
    public void Capture_WritesJsonLinesRecordsWhenIsPlayerAndEnabled()
    {
        string capturePath = Path.Combine(
            Path.GetTempPath(),
            $"acdream_capture_{Guid.NewGuid():N}.jsonl");

        try
        {
            PhysicsResolveCapture.CapturePath = capturePath;
            PhysicsResolveCapture.ResetTickCounter();

            var (engine, _) = BuildEngineWithCellarFixtures();
            var body = BuildInitialBody();
            _ = SimulateTicks(engine, body, CellarId, 3);

            PhysicsResolveCapture.Close();

            Assert.True(File.Exists(capturePath), "Capture file should exist.");
            var lines = File.ReadAllLines(capturePath);
            Assert.Equal(3, lines.Length);

            var records = lines
                .Select(static l => System.Text.Json.JsonSerializer.Deserialize<ResolveCaptureRecord>(
                    l, CaptureJsonOptions))
                .ToList();

            // Tick monotonic 0,1,2.
            Assert.Equal(0, records[0]!.Tick);
            Assert.Equal(1, records[1]!.Tick);
            Assert.Equal(2, records[2]!.Tick);

            // Inputs at tick 0 must match the harness's initial position.
            var firstInput = records[0]!.Input;
            Assert.Equal(InitialSphereWorld.X, firstInput.CurrentPos.X, 4);
            Assert.Equal(InitialSphereWorld.Y, firstInput.CurrentPos.Y, 4);
            Assert.Equal(InitialSphereWorld.Z, firstInput.CurrentPos.Z, 4);
            Assert.Equal(CellarId, firstInput.CellId);
            Assert.True(firstInput.IsOnGround,
                "First tick is seeded grounded.");

            // Body before + after snapshots present.
            Assert.NotNull(records[0]!.BodyBefore);
            Assert.NotNull(records[0]!.BodyAfter);

            // Body-before's ContactPlane should match the seeded floor plane.
            var cpBefore = records[0]!.BodyBefore!.ContactPlane;
            Assert.Equal(0f, cpBefore.Normal.X, 5);
            Assert.Equal(0f, cpBefore.Normal.Y, 5);
            Assert.Equal(1f, cpBefore.Normal.Z, 5);
            Assert.Equal(-CellarFloorZ, cpBefore.D, 3);
        }
        finally
        {
            PhysicsResolveCapture.CapturePath = null;
            PhysicsResolveCapture.Close();
            if (File.Exists(capturePath))
                File.Delete(capturePath);
        }
    }

    /// <summary>
    /// Capture is filtered to <c>IsPlayer</c> mover flag. Calls without
    /// that flag (NPC, remote dead-reckoning) must NOT pollute the
    /// capture file.
    /// </summary>
    [Fact]
    public void Capture_SkipsNonPlayerCalls()
    {
        string capturePath = Path.Combine(
            Path.GetTempPath(),
            $"acdream_capture_npc_{Guid.NewGuid():N}.jsonl");

        try
        {
            PhysicsResolveCapture.CapturePath = capturePath;
            PhysicsResolveCapture.ResetTickCounter();

            var (engine, _) = BuildEngineWithCellarFixtures();
            var body = BuildInitialBody();

            // Drive 3 ticks WITHOUT IsPlayer flag — simulates an NPC path.
            uint cellId = CellarId;
            bool isOnGround = true;
            for (int i = 0; i < 3; i++)
            {
                Vector3 target = body.Position + PerTickOffset;
                var result = engine.ResolveWithTransition(
                    body.Position, target, cellId,
                    SphereRadius, SphereHeight,
                    StepUpHeight, StepDownHeight,
                    isOnGround,
                    body: body,
                    moverFlags: ObjectInfoState.EdgeSlide,  // ← no IsPlayer
                    movingEntityId: 0);
                body.Position = result.Position;
                cellId = result.CellId;
                isOnGround = result.IsOnGround;
            }

            PhysicsResolveCapture.Close();

            // No records written because no IsPlayer call ran.
            Assert.False(File.Exists(capturePath),
                "Capture file should NOT exist when only non-player calls ran.");
        }
        finally
        {
            PhysicsResolveCapture.CapturePath = null;
            PhysicsResolveCapture.Close();
            if (File.Exists(capturePath))
                File.Delete(capturePath);
        }
    }

    /// <summary>
    /// Shared deserialization options matching
    /// <see cref="PhysicsResolveCapture"/>'s serializer. <c>IncludeFields</c>
    /// is required because Vector3/Quaternion/Plane store components as
    /// fields, not properties.
    /// </summary>
    public static readonly System.Text.Json.JsonSerializerOptions CaptureJsonOptions =
        new()
        {
            IncludeFields        = true,
            PropertyNamingPolicy = System.Text.Json.JsonNamingPolicy.CamelCase,
        };

    // ───────────────────────────────────────────────────────────────
    // Harness internals
    // ───────────────────────────────────────────────────────────────

    /// <summary>
    /// One point in the simulated trajectory. Captured per tick.
    /// </summary>
    public sealed record TrajectoryPoint(
        int     Tick,
        Vector3 Position,
        uint    CellId,
        bool    IsOnGround,
        bool    CpValid);

    /// <summary>
    /// Builds a <see cref="PhysicsEngine"/> with:
    /// <list type="bullet">
    ///   <item>The three issue-#98 cottage/cellar cell fixtures registered.</item>
    ///   <item>A stub landblock so <c>TryGetLandblockContext</c> succeeds
    ///         at the cellar XY (needed for FindObjCollisions to query
    ///         the shadow registry).</item>
    ///   <item>A SYNTHETIC stair-piece GfxObj containing the cellar ramp
    ///         polygon, registered as a ShadowEntry scoped to the cellar
    ///         cell. Reconstructed programmatically from the live-capture
    ///         <c>[poly-dump]</c> data
    ///         (<c>docs/research/2026-05-21-a6-captures/scen4_cottage_cellar_polydump/acdream.log</c>),
    ///         transformed to world coordinates so the registered object
    ///         sits at world origin with identity rotation/scale.</item>
    /// </list>
    /// </summary>
    private static (PhysicsEngine engine, PhysicsDataCache cache)
        BuildEngineWithCellarFixtures()
    {
        var cache  = new PhysicsDataCache();
        var engine = new PhysicsEngine { DataCache = cache };

        // ── 1. Cell fixtures (existing) + synthetic BSP ──────────────
        // CellDumpSerializer.Hydrate intentionally sets BSP=null (the DAT
        // PhysicsBSPTree isn't in the dump format). Without a non-null BSP,
        // FindEnvCollisions's indoor branch (TransitionTypes.cs:1840) is
        // skipped — the engine falls through to outdoor terrain queries
        // that produce spurious wall hits. Construct a single-leaf BSP
        // wrapping the cell's Resolved polygons, so the indoor path fires
        // like production.
        foreach (var cellId in new[] { CellarId, CottageNeighborA, CottageNeighborB })
        {
            var path = Path.Combine(FixtureDir, $"0x{cellId:X8}.json");
            Assert.True(File.Exists(path),
                $"Fixture missing: {path}. Re-run cell-dump capture " +
                $"(commit 3f56915 captured the originals).");
            var dump = CellDumpSerializer.Read(path);
            var cell = CellDumpSerializer.Hydrate(dump);
            var cellWithBsp = AttachSyntheticBsp(cell);
            cache.RegisterCellStructForTest(cellId, cellWithBsp);
        }

        // ── 2. NO landblock registered ──────────────────────────────
        // Without a landblock, SampleTerrainWalkable returns null and
        // FindEnvCollisions's outdoor-fallback path returns OK without
        // running ValidateWalkable on stub terrain. This is the right
        // shape for indoor-only tests — the cell's BSP would handle
        // collision if hydrated, and falling through to stub terrain
        // produces spurious (0,1,0) wall hits. FindObjCollisions also
        // early-returns without landblock context (line 2153 of
        // TransitionTypes.cs), so the synthetic stair GfxObj is also
        // skipped — fine for the airborne-at-tick-1 isolation.

        // ── 3. Synthetic stair-piece GfxObj + ShadowEntry ──────────
        // Temporarily disabled while debugging the airborne-at-tick-1
        // issue. Re-enable once the cell-BSP-is-null + landblock-stub
        // interaction is understood, AND we have a way to register
        // the stair without needing a landblock (e.g., extend
        // FindObjCollisions to query cellScope-only shadows without
        // landblock context).
        // RegisterStairRampGfxObj(engine, cache);

        return (engine, cache);
    }

    /// <summary>
    /// Wraps a hydrated <see cref="CellPhysics"/> with a synthetic
    /// single-leaf <see cref="PhysicsBSPTree"/> that references every
    /// polygon in <c>cell.Resolved</c>. <c>CellDumpSerializer.Hydrate</c>
    /// intentionally sets BSP=null (per its xmldoc) because the dump
    /// format doesn't capture the DAT BSP tree. Without a non-null BSP,
    /// FindEnvCollisions's indoor branch is skipped — the engine then
    /// falls through to outdoor terrain queries that misfire. A flat
    /// single-leaf BSP is sufficient for the BSP query to find every
    /// polygon by exhaustive iteration (slower than a real BSP but
    /// correct).
    /// </summary>
    private static CellPhysics AttachSyntheticBsp(CellPhysics cell)
    {
        // Compute a bounding sphere that encompasses every polygon in the
        // cell — center at the origin of the cell's WORLD transform plus
        // a margin radius. The cellar fixture is ~12 m × 12 m × 3 m.
        var bsphereCenter = new Vector3(0f, 0f, 0f);  // cell local
        var bsphereRadius = 15f;

        var leaf = new PhysicsBSPNode
        {
            Type = BSPNodeType.Leaf,
            BoundingSphere = new Sphere { Origin = bsphereCenter, Radius = bsphereRadius },
        };
        foreach (var kv in cell.Resolved)
            leaf.Polygons.Add(kv.Key);

        var bspTree = new PhysicsBSPTree { Root = leaf };

        // CellPhysics has init-only properties; rebuild a new instance
        // with BSP set, copying every other field unchanged.
        return new CellPhysics
        {
            BSP                    = bspTree,
            PhysicsPolygons        = cell.PhysicsPolygons,
            Vertices               = cell.Vertices,
            WorldTransform         = cell.WorldTransform,
            InverseWorldTransform  = cell.InverseWorldTransform,
            Resolved               = cell.Resolved,
            CellBSP                = cell.CellBSP,
            Portals                = cell.Portals,
            PortalPolygons         = cell.PortalPolygons,
            VisibleCellIds         = cell.VisibleCellIds,
        };
    }

    /// <summary>
    /// Constructs a synthetic GfxObj containing the cellar ramp polygon
    /// in WORLD coordinates and registers it as a ShadowEntry scoped to
    /// the cellar cell. The polygon's vertices + normal are reproduced
    /// from the live capture's <c>[poly-dump]</c> data (commit pre-3f56915),
    /// transformed to world frame so the GfxObj can sit at world origin
    /// with identity rotation/scale (simplifies the
    /// FindObjCollisions local-to-world transform).
    ///
    /// <para>
    /// Live capture's local polygon vertices (in building frame):
    /// (0.8,-1.59,-1.5), (0.8,1.31,1.5), (-0.8,1.31,1.5), (-0.8,-1.59,-1.5).
    /// Building's world transform: origin (141.5, 7.155, 92.455), 180° yaw
    /// around Z. After applying yaw + translation, world vertices are:
    /// (140.7, 8.745, 90.955), (140.7, 5.845, 93.955),
    /// (142.3, 5.845, 93.955), (142.3, 8.745, 90.955).
    /// World normal = (0, 0.719, 0.695), world d = -69.5035 — matches
    /// the live cdb capture exactly.
    /// </para>
    /// </summary>
    private static void RegisterStairRampGfxObj(PhysicsEngine engine, PhysicsDataCache cache)
    {
        const ushort RampPolyId = 0x0008;
        const uint   StairGfxId = 0xDEADBEEFu;
        const uint   StairEntityId = 0xC0FFEE00u;

        // World-frame vertices (winding order preserved from live capture).
        var v0 = new Vector3(140.7f, 8.745f, 90.955f); // ramp foot, X=-side
        var v1 = new Vector3(140.7f, 5.845f, 93.955f); // ramp top,  X=-side
        var v2 = new Vector3(142.3f, 5.845f, 93.955f); // ramp top,  X=+side
        var v3 = new Vector3(142.3f, 8.745f, 90.955f); // ramp foot, X=+side
        var verts = new[] { v0, v1, v2, v3 };

        // Compute normal from cross(v1-v0, v3-v0).
        var edge0 = v1 - v0;
        var edge1 = v3 - v0;
        var normal = Vector3.Normalize(Vector3.Cross(edge0, edge1));
        // Plane equation: N·p + d = 0 → d = -N·v0.
        float d = -Vector3.Dot(normal, v0);

        var resolved = new Dictionary<ushort, ResolvedPolygon>
        {
            [RampPolyId] = new ResolvedPolygon
            {
                Vertices  = verts,
                Plane     = new System.Numerics.Plane(normal, d),
                NumPoints = 4,
                SidesType = CullMode.Landblock,
            },
        };

        // Minimal one-leaf BSP containing the ramp poly. Bounding sphere
        // encompasses the polygon (center at poly centroid).
        var leaf = new PhysicsBSPNode
        {
            Type = BSPNodeType.Leaf,
            BoundingSphere = new Sphere
            {
                Origin = new Vector3(141.5f, 7.295f, 92.455f),
                Radius = 3.0f,
            },
        };
        leaf.Polygons.Add(RampPolyId);

        var bspTree = new PhysicsBSPTree { Root = leaf };

        var gfxPhysics = new GfxObjPhysics
        {
            BSP             = bspTree,
            PhysicsPolygons = new Dictionary<ushort, Polygon>(),
            Vertices        = new VertexArray(),
            Resolved        = resolved,
            BoundingSphere  = leaf.BoundingSphere,
        };

        cache.RegisterGfxObjForTest(StairGfxId, gfxPhysics);

        // ShadowEntry: object at world origin (0,0,0), identity rotation,
        // scale 1.0 — keeps the polygon's WORLD-frame vertices intact
        // through the FindObjCollisions local-transform math.
        // cellScope = CellarId so the entry is only queried when the sphere
        // is in cellar cell (matches retail's per-cell shadow scoping for
        // interior statics — Issue #91 family).
        engine.ShadowObjects.Register(
            entityId:      StairEntityId,
            gfxObjId:      StairGfxId,
            worldPos:      Vector3.Zero,
            rotation:      Quaternion.Identity,
            radius:        5.0f,
            worldOffsetX:  0f,
            worldOffsetY:  0f,
            landblockId:   0xA9B40000u,
            collisionType: ShadowCollisionType.BSP,
            scale:         1.0f,
            cellScope:     CellarId);
    }

    /// <summary>
    /// Sphere on the cellar floor with BOTH a seeded ContactPlane AND a
    /// seeded WalkablePolygon. Both are required by the engine to treat
    /// the body as truly grounded:
    /// <list type="bullet">
    ///   <item><c>ContactPlaneValid</c> + <c>ContactPlane</c>: copied into
    ///         <c>CollisionInfo.ContactPlane</c> via the body parameter
    ///         seeding in <see cref="PhysicsEngine.ResolveWithTransition"/>.</item>
    ///   <item><c>WalkablePolygonValid</c> + <c>WalkablePlane</c> +
    ///         <c>WalkableVertices</c>: read by
    ///         <see cref="PhysicsEngine.ResolveWithTransition"/> lines
    ///         665-673 to call <c>SpherePath.SetWalkable(...)</c>, which
    ///         sets <c>HasWalkablePolygon=true</c>. Without this, the
    ///         engine treats the sphere as "grounded but with no walkable
    ///         polygon anchor" — a contradictory state that fires step-down
    ///         probes which reject and clear the grounded flag.</item>
    /// </list>
    /// </summary>
    private static PhysicsBody BuildInitialBody() => new()
    {
        Position           = InitialSphereWorld,
        Orientation        = Quaternion.Identity,

        // ContactPlane: cellar floor at world Z=90.95.
        ContactPlaneValid  = true,
        ContactPlane       = new System.Numerics.Plane(0f, 0f, 1f, -CellarFloorZ),
        ContactPlaneCellId = CellarId,

        // WalkablePolygon: cellar floor poly 24 (the cellar quad under
        // sphere XY=(141.5, 9.5)), transformed to world coordinates via
        // the cell's 180° yaw + origin (130.5, 11.5, 94.0). Local verts
        // [(-11.6, 0, -3.05), (-11.6, 3.1, -3.05), (-9.6, 3.1, -3.05),
        // (-9.6, 0, -3.05)] → world [(142.1, 11.5, 90.95),
        // (142.1, 8.4, 90.95), (140.1, 8.4, 90.95), (140.1, 11.5, 90.95)].
        WalkablePolygonValid = true,
        WalkablePlane        = new System.Numerics.Plane(0f, 0f, 1f, -CellarFloorZ),
        WalkableVertices     = new[]
        {
            new Vector3(142.1f, 11.5f, 90.95f),
            new Vector3(142.1f,  8.4f, 90.95f),
            new Vector3(140.1f,  8.4f, 90.95f),
            new Vector3(140.1f, 11.5f, 90.95f),
        },
        WalkableUp           = Vector3.UnitZ,

        TransientState       = TransientStateFlags.Contact
                             | TransientStateFlags.OnWalkable,
    };

    /// <summary>
    /// Drives <paramref name="tickCount"/> physics ticks. Each tick
    /// applies <see cref="PerTickOffset"/> as the requested forward
    /// motion, calls <see cref="PhysicsEngine.ResolveWithTransition"/>,
    /// writes the result back to <paramref name="body"/>, and records
    /// a <see cref="TrajectoryPoint"/>.
    ///
    /// <para>
    /// Cross-tick ContactPlane persistence is via <paramref name="body"/>
    /// — the engine writes its final CP back to the body, then reads
    /// it as the seed for the next tick. This mirrors the production
    /// pattern in <c>PlayerMovementController</c>.
    /// </para>
    /// </summary>
    private static List<TrajectoryPoint> SimulateTicks(
        PhysicsEngine engine,
        PhysicsBody   body,
        uint          initialCellId,
        int           tickCount)
    {
        uint cellId      = initialCellId;
        bool isOnGround  = true;

        var trajectory = new List<TrajectoryPoint>(tickCount + 1)
        {
            new(0, body.Position, cellId, isOnGround, body.ContactPlaneValid),
        };

        for (int tick = 1; tick <= tickCount; tick++)
        {
            Vector3 target = body.Position + PerTickOffset;

            var result = engine.ResolveWithTransition(
                currentPos:      body.Position,
                targetPos:       target,
                cellId:          cellId,
                sphereRadius:    SphereRadius,
                sphereHeight:    SphereHeight,
                stepUpHeight:    StepUpHeight,
                stepDownHeight:  StepDownHeight,
                isOnGround:      isOnGround,
                body:            body,
                moverFlags:      ObjectInfoState.IsPlayer
                                 | ObjectInfoState.EdgeSlide,
                movingEntityId:  0);

            body.Position = result.Position;
            cellId        = result.CellId;
            isOnGround    = result.IsOnGround;

            trajectory.Add(new(
                tick,
                body.Position,
                cellId,
                isOnGround,
                body.ContactPlaneValid));
        }

        return trajectory;
    }

    private static string FixtureDir =>
        Path.Combine(SolutionRoot(), "tests", "AcDream.Core.Tests",
                     "Fixtures", "issue98");

    private static string SolutionRoot()
    {
        var dir = AppContext.BaseDirectory;
        while (!string.IsNullOrEmpty(dir))
        {
            if (File.Exists(Path.Combine(dir, "AcDream.slnx")))
                return dir;
            dir = Path.GetDirectoryName(dir);
        }
        throw new InvalidOperationException(
            "Could not locate AcDream.slnx from " + AppContext.BaseDirectory);
    }
}