using System.Collections.Generic;
using System.Numerics;
using DatReaderWriter.Types;

namespace AcDream.Core.Physics;

/// <summary>
/// Indoor walking Phase 2 (2026-05-19). Portal-graph cell traversal,
/// ported from retail's <c>CObjCell::find_cell_list</c> family
/// (sphere variant for the player's path spheres).
///
/// <para>
/// Replaces Phase D's AABB containment. Uses the cell BSP for retail-
/// faithful point-in-cell tests via
/// <see cref="BSPQuery.PointInsideCellBsp"/>. Walks the portal graph
/// starting from a given current cell to find which cells a moving
/// sphere overlaps.
/// </para>
///
/// <para>
/// Reference pseudocode:
/// <c>docs/research/acclient_indoor_transitions_pseudocode.md</c>
/// (2026-04-13). Retail decomp: <c>CEnvCell::find_transit_cells</c>
/// (sphere variant) at <c>acclient_2013_pseudo_c.txt</c>.
/// </para>
/// </summary>
public static class CellTransit
{
    /// <summary>
    /// Small radius padding matching retail's <c>EPSILON</c> usage in the
    /// sphere-plane distance test (research doc §"EnvCell.find_transit_cells").
    /// </summary>
    private const float EPSILON = 0.02f;

    /// <summary>
    /// Retail <c>F_EPSILON</c> (acclient.exe data @ 007c8c70, 0.000199999995f) —
    /// the pad added to the sphere radius in the exterior-portal straddle test
    /// (<c>fadd [ecx+4]</c> at 0052c8eb, #112 rider live-binary read 2026-06-10).
    /// </summary>
    private const float FEpsilon = 0.000199999995f;

    /// <summary>
    /// Indoor portal-neighbour expansion. For each portal of
    /// <paramref name="currentCell"/>, test whether the sphere overlaps
    /// the portal polygon's plane in cell-local space. If so, add the
    /// neighbour cell to <paramref name="candidates"/>.
    ///
    /// <para>
    /// Ported from <c>CEnvCell::find_transit_cells</c> (sphere variant)
    /// per the pseudocode doc §"EnvCell.find_transit_cells (sphere variant)".
    /// </para>
    /// </summary>
    public static void FindTransitCellsSphere(
        PhysicsDataCache cache,
        CellPhysics currentCell,
        uint currentCellId,
        Vector3 worldSphereCenter,
        float sphereRadius,
        ICollection<uint> candidates,
        out bool exitOutside)
        => FindTransitCellsSphere(
            cache, currentCell, currentCellId, worldSphereCenter, sphereRadius,
            candidates, out exitOutside, out _);

    /// <inheritdoc cref="FindTransitCellsSphere(PhysicsDataCache, CellPhysics, uint, IReadOnlyList{Sphere}, int, ICollection{uint}, out bool, out bool)"/>
    public static void FindTransitCellsSphere(
        PhysicsDataCache cache,
        CellPhysics currentCell,
        uint currentCellId,
        Vector3 worldSphereCenter,
        float sphereRadius,
        ICollection<uint> candidates,
        out bool exitOutside,
        out bool hasExitPortal)
    {
        var spheres = new[]
        {
            new Sphere
            {
                Origin = worldSphereCenter,
                Radius = sphereRadius,
            },
        };

        FindTransitCellsSphere(
            cache, currentCell, currentCellId,
            spheres, spheres.Length, candidates, out exitOutside, out hasExitPortal);
    }

    /// <summary>
    /// Multi-sphere form used by retail's <c>CObjCell::find_cell_list</c>:
    /// pass <c>sphere_path.num_sphere</c> and <c>sphere_path.global_sphere</c>.
    /// Any sphere can trigger a portal neighbor or outdoor exit.
    /// </summary>
    /// <param name="exitOutside">RETAIL semantics (live-binary verified
    /// 2026-06-10, #112 rider): true iff some path sphere STRADDLES one of this
    /// cell's exterior portal polygon planes — <c>|dist| &lt; radius + EPSILON</c>.
    /// This is the only condition under which retail's
    /// <c>CEnvCell::find_transit_cells</c> calls <c>add_all_outside_cells</c>
    /// (acclient.exe 0052c8e5-0052c92d straddle test, 0052c9d2-0052c9f0 gate;
    /// pseudo-C :310070-310120). Drives the membership pick's outdoor branch.</param>
    /// <param name="hasExitPortal">Topology-only: this cell has at least one
    /// exterior (0xFFFF) portal, regardless of sphere position. NOT retail —
    /// kept for the A6.P5 collision cell-set widening (outdoor-registered door
    /// entities must stay findable from indoor cells until the A6.P4 per-cell
    /// shadow architecture ships; see the exterior-portal branch comment).</param>
    public static void FindTransitCellsSphere(
        PhysicsDataCache cache,
        CellPhysics currentCell,
        uint currentCellId,
        IReadOnlyList<Sphere> worldSpheres,
        int numSpheres,
        ICollection<uint> candidates,
        out bool exitOutside,
        out bool hasExitPortal)
    {
        exitOutside = false;
        hasExitPortal = false;

        uint lbPrefix = currentCellId & 0xFFFF0000u;
        int sphereCount = EffectiveSphereCount(worldSpheres, numSpheres);

        if (currentCell.PortalPolygons is null || sphereCount == 0) return;

        foreach (var portal in currentCell.Portals)
        {
            if (!currentCell.PortalPolygons.TryGetValue(portal.PolygonId, out var poly))
                continue;

            if (portal.OtherCellId == 0xFFFF)
            {
                hasExitPortal = true;

                // #112 rider (2026-06-10): retail straddle gate, RESTORED and
                // verified against the LIVE 2013 binary (cdb attach, function
                // 0052c820; x87 decode at 0052c8e5-0052c92d):
                //
                //   pad  = sphere.radius + F_EPSILON            (fadd [ecx+4])
                //   dist = dot(localCenter, portalPlane.n) + d  (cell-local)
                //   flag |= (dist > -pad) && (dist < +pad)      (fcompp/test ah,41h
                //                                                + fcomp/test ah,5/jp)
                //
                // add_all_outside_cells fires IFF flag (0052c9d6 je → skip). No
                // portal_side / exact_match in this branch — BN's pseudo-C
                // invented those (feedback_bn_decomp_field_names).
                //
                // History: this gate existed pre-A6.P5 and was removed
                // 2026-05-25 citing the CALLER (find_cell_list :308775-:308785
                // walks every array cell unconditionally — true, but each
                // callee still applies its own straddle gate). The A6.P5
                // symptom it fixed (outdoor-registered cottage DOORS invisible
                // to collision from indoor cells) is really the missing
                // per-cell shadow_object_list (#99/A6.P4): retail finds those
                // doors via shadow lists, not via outdoor transit cells. Until
                // A6.P4 ships, BuildCellSetAndPickContaining keeps widening the
                // COLLISION cell set on hasExitPortal — but the membership PICK
                // gates its outdoor branch on this retail flag, which is what
                // keeps deep-interior containment gaps on curr_cell (retail
                // keep-curr) instead of demoting to outdoor (#112).
                if (!exitOutside)
                {
                    for (int i = 0; i < sphereCount; i++)
                    {
                        var sphere = worldSpheres[i];
                        float pad = sphere.Radius + FEpsilon;
                        var localCenter = Vector3.Transform(
                            sphere.Origin, currentCell.InverseWorldTransform);
                        float dist = Vector3.Dot(localCenter, poly.Plane.Normal) + poly.Plane.D;
                        if (dist > -pad && dist < pad)
                        {
                            exitOutside = true;
                            break;
                        }
                    }
                }
                continue;
            }

            uint otherId = lbPrefix | portal.OtherCellId;

            // Retail CEnvCell::find_transit_cells first asks the loaded
            // neighbour cell whether the sphere intersects its CellBSP.
            // The portal-plane side test is only the unloaded-cell load hint.
            var otherCell = cache.GetCellStruct(otherId);
            if (otherCell?.CellBSP?.Root is not null)
            {
                for (int i = 0; i < sphereCount; i++)
                {
                    var sphere = worldSpheres[i];
                    var otherLocalCenter = Vector3.Transform(
                        sphere.Origin, otherCell.InverseWorldTransform);
                    bool hit = BSPQuery.SphereIntersectsCellBsp(
                        otherCell.CellBSP.Root, otherLocalCenter, sphere.Radius);
                    if (hit)
                    {
                        candidates.Add(otherId);
                        break;
                    }
                }

                continue;
            }

            // Conservative unloaded-cell hint: the sphere is near the portal
            // plane and on the outward side (per PortalSide).
            for (int i = 0; i < sphereCount; i++)
            {
                var sphere = worldSpheres[i];
                float rad = sphere.Radius + EPSILON;
                var localCenter = Vector3.Transform(
                    sphere.Origin, currentCell.InverseWorldTransform);
                float dist = Vector3.Dot(localCenter, poly.Plane.Normal) + poly.Plane.D;
                bool hit = portal.PortalSide ? dist > -rad : dist < rad;
                if (hit)
                {
                    candidates.Add(otherId);
                    break;
                }
            }
        }
    }

    /// <summary>
    /// Outdoor neighbour expansion. Ported from
    /// <c>CLandCell::add_all_outside_cells</c> (sphere variant,
    /// pc:317499 @0x00533630) per
    /// <c>docs/research/2026-06-09-landdefs-outside-cells-pseudocode.md</c>.
    ///
    /// <para>
    /// Retail runs this in the GLOBAL landcell grid (<see cref="LandDefs"/>
    /// lcoords, 0..2039 across the whole map): <c>adjust_to_outside</c> re-seats
    /// the (cell, position) pair onto the landcell actually under the sphere —
    /// crossing landblock boundaries when <c>floor(local/24)</c> leaves the
    /// current block's 8×8 grid — and <c>check_add_cell_boundary</c> adds up to
    /// 3 neighbour cells (strict &gt;/&lt; against the sphere radius), each id
    /// re-derived from its own global lcoord. Issue #106: the pre-fix port
    /// clamped everything to the current landblock's grid, so the candidate set
    /// emptied the moment the player stepped over a boundary and membership
    /// froze on the last in-block cell.
    /// </para>
    ///
    /// <para>
    /// <paramref name="worldSphereCenter"/> is in the floating world frame
    /// (anchor landblock at origin — the convention every physics caller uses);
    /// <paramref name="currentBlockOrigin"/> is the current cell's landblock
    /// world origin (SW corner; <see cref="World.Cells.CellGraph.TryGetTerrainOrigin"/>),
    /// which converts it to retail's block-local frame. Pass
    /// <see cref="Vector3.Zero"/> when the current block IS the anchor — the
    /// pre-#106 behavior, and what the A6.P4 (2026-05-24) "landblock-local
    /// coords" convention actually meant.
    /// </para>
    /// </summary>
    /// <returns>
    /// False when <c>adjust_to_outside</c> rejects the position (map edge /
    /// invalid cell id) — retail breaks out of the sphere loop on that.
    /// </returns>
    public static bool AddAllOutsideCells(
        Vector3 worldSphereCenter,
        float sphereRadius,
        uint currentCellId,
        Vector3 currentBlockOrigin,
        ICollection<uint> candidates)
    {
        // Retail's position is block-local to the current cell's landblock.
        var center = worldSphereCenter - currentBlockOrigin;

        uint cellId = currentCellId;
        if (!LandDefs.AdjustToOutside(ref cellId, ref center))
            return false;
        if (!LandDefs.GidToLcoord(cellId, out int lx, out int ly))
            return false;

        AddOutsideCell(candidates, lx, ly);

        // check_add_cell_boundary (pc:317229 @0x00533260): the point within the
        // 24 m cell, from the adjust_to_outside-normalized block-local center
        // (always [0, 192) post-adjust; floor-mod for safety). Strict >/< —
        // a sphere exactly tangent to a boundary does NOT add the neighbour.
        float pointX = center.X - MathF.Floor(center.X / LandDefs.CellLength) * LandDefs.CellLength;
        float pointY = center.Y - MathF.Floor(center.Y / LandDefs.CellLength) * LandDefs.CellLength;
        float minRad = sphereRadius;
        float maxRad = LandDefs.CellLength - sphereRadius;

        if (pointX > maxRad)
        {
            AddOutsideCell(candidates, lx + 1, ly);
            if (pointY > maxRad) AddOutsideCell(candidates, lx + 1, ly + 1);
            if (pointY < minRad) AddOutsideCell(candidates, lx + 1, ly - 1);
        }
        if (pointX < minRad)
        {
            AddOutsideCell(candidates, lx - 1, ly);
            if (pointY > maxRad) AddOutsideCell(candidates, lx - 1, ly + 1);
            if (pointY < minRad) AddOutsideCell(candidates, lx - 1, ly - 1);
        }
        if (pointY > maxRad) AddOutsideCell(candidates, lx, ly + 1);
        if (pointY < minRad) AddOutsideCell(candidates, lx, ly - 1);
        return true;
    }

    /// <summary>
    /// Multi-sphere outdoor expansion. Retail's sphere variant loops every
    /// path sphere and adds the outdoor landcells touched by any of them;
    /// an <c>adjust_to_outside</c> failure BREAKS the loop (pc:533699).
    /// </summary>
    public static void AddAllOutsideCells(
        IReadOnlyList<Sphere> worldSpheres,
        int numSpheres,
        uint currentCellId,
        Vector3 currentBlockOrigin,
        ICollection<uint> candidates)
    {
        int sphereCount = EffectiveSphereCount(worldSpheres, numSpheres);
        for (int i = 0; i < sphereCount; i++)
        {
            var sphere = worldSpheres[i];
            if (!AddAllOutsideCells(sphere.Origin, sphere.Radius, currentCellId, currentBlockOrigin, candidates))
                break;
        }
    }

    private static void AddOutsideCell(ICollection<uint> candidates, int lx, int ly)
    {
        // CLandCell::add_outside_cell (pc:317056 @0x00532ec0): map-bounds check,
        // then lcoord_to_gid — NO same-block filter (ACE's add_cell_block
        // "FIXME!" guard is an ACE divergence, not retail). The block id is
        // re-derived from the global lcoord, so neighbour-landblock cells come
        // out with the neighbour's prefix.
        uint gid = LandDefs.LcoordToGid(lx, ly);
        if (gid != 0u) candidates.Add(gid);
    }

    /// <summary>
    /// Outdoor→indoor entry path. Ported from retail's
    /// <c>BuildingObj::find_building_transit_cells</c> +
    /// <c>EnvCell::check_building_transit</c>. For each portal of the
    /// outdoor building, look up the destination interior cell and test
    /// whether the sphere overlaps it via
    /// <see cref="BSPQuery.SphereIntersectsCellBsp"/>. If so, add the
    /// interior cell to <paramref name="candidates"/>.
    ///
    /// <para>
    /// Issue #89 closed (2026-05-20): uses retail's radius-aware
    /// <c>CCellStruct::sphere_intersects_cell</c>
    /// (<c>acclient_2013_pseudo_c.txt:317666</c>) ported as
    /// <see cref="BSPQuery.SphereIntersectsCellBsp"/>. Promotes CellId to
    /// the interior cell the moment ANY part of the foot-sphere crosses
    /// the cell boundary — matches retail entry timing exactly and
    /// closes the login-inside-inn classification race where the player
    /// would briefly be classified outdoor and walk through walls.
    /// </para>
    /// </summary>
    public static void CheckBuildingTransit(
        PhysicsDataCache cache,
        BuildingPhysics building,
        Vector3 worldSphereCenter,
        float sphereRadius,
        ICollection<uint> candidates)
    {
        foreach (var portal in building.Portals)
        {
            var otherCell = cache.GetCellStruct(portal.OtherCellId);
            if (otherCell?.CellBSP?.Root is null)
            {
                if (PhysicsDiagnostics.ProbeIndoorBspEnabled)
                {
                    string reason = otherCell is null ? "cell not cached" : "CellBSP null";
                    Console.WriteLine(System.FormattableString.Invariant(
                        $"[check-bldg] portal->0x{portal.OtherCellId:X8} skipped: {reason}"));
                }
                continue;
            }

            // Sphere center in the OTHER cell's local space.
            // Issue #89 closed (2026-05-20): use radius-aware sphere-overlap
            // (matches retail's CCellStruct::sphere_intersects_cell at
            // acclient_2013_pseudo_c.txt:317666) instead of point-only. This
            // promotes the player's CellId to the interior cell the moment
            // ANY part of the foot-sphere crosses the cell boundary — the
            // entry-side counterpart to issue #90's sticky-stay fix. Without
            // it, login-inside-the-inn keeps the player classified outdoor
            // until they walk further in (sphere center crosses), letting
            // them run through exterior walls on the way out.
            var localCenter = Vector3.Transform(worldSphereCenter, otherCell.InverseWorldTransform);
            bool inside = BSPQuery.SphereIntersectsCellBsp(otherCell.CellBSP.Root, localCenter, sphereRadius);

            if (PhysicsDiagnostics.ProbeIndoorBspEnabled)
            {
                Console.WriteLine(System.FormattableString.Invariant(
                    $"[check-bldg] portal->0x{portal.OtherCellId:X8} wpos=({worldSphereCenter.X:F3},{worldSphereCenter.Y:F3},{worldSphereCenter.Z:F3}) lpos=({localCenter.X:F3},{localCenter.Y:F3},{localCenter.Z:F3}) r={sphereRadius:F3} inside={inside}"));
            }

            if (inside)
            {
                candidates.Add(portal.OtherCellId);
            }
        }
    }

    /// <summary>
    /// Verbatim port of <c>CEnvCell::find_visible_child_cell</c>
    /// (<c>acclient_2013_pseudo_c.txt:311397</c>). Returns the cell whose cell-BSP
    /// <c>point_in_cell</c> contains <paramref name="worldPoint"/>, checking the
    /// start cell first (:311402), then — when <paramref name="useStabList"/> is
    /// true (retail <c>arg3 != 0</c>, :311444) — the start's <c>stab_list</c>
    /// (<see cref="CellPhysics.VisibleCellIds"/>), else (<c>arg3 == 0</c>, :311411)
    /// its direct portal neighbours. Returns 0 when no cell contains the point
    /// (retail <c>return 0</c> at :311469).
    ///
    /// <para>
    /// Sibling of <see cref="FindCellList"/> (retail <c>find_cell_list</c>) — both
    /// resolve membership from the cell graph via <see cref="BSPQuery.PointInsideCellBsp"/>.
    /// Used by <c>CPhysicsObj::AdjustPosition</c> (pc:280028, <c>arg5 = 1</c> →
    /// stab-list mode) to seat the camera sweep's start cell at the head-pivot.
    /// </para>
    ///
    /// <para>
    /// acdream adaptation (matches <see cref="FindCellList"/> at line 518): a cell
    /// with no hydrated <see cref="CellPhysics.CellBSP"/> cannot run
    /// <c>point_in_cell</c>, so it is treated as NOT containing the point (skipped),
    /// rather than letting <see cref="BSPQuery.PointInsideCellBsp"/>'s null-node
    /// "inside" default make it spuriously claim every point.
    /// </para>
    /// </summary>
    public static uint FindVisibleChildCell(
        PhysicsDataCache cache, uint startCellId, Vector3 worldPoint, bool useStabList)
    {
        var start = cache.GetCellStruct(startCellId);
        if (start is null) return 0u;

        // this->point_in_cell(point) → return this  (:311402-311405)
        if (PointInCell(start, worldPoint)) return startCellId;

        if (useStabList)
        {
            // arg3 != 0 → iterate stab_list, GetVisible + point_in_cell (:311444-311465)
            foreach (uint id in start.VisibleCellIds)
                if (PointInCell(cache.GetCellStruct(id), worldPoint)) return id;
        }
        else
        {
            // arg3 == 0 → iterate direct portals, GetOtherCell + point_in_cell (:311411-311434)
            foreach (var portal in start.Portals)
                if (PointInCell(cache.GetCellStruct(portal.OtherCellId), worldPoint)) return portal.OtherCellId;
        }

        return 0u;
    }

    /// <summary>
    /// <c>CEnvCell::point_in_cell</c> (cell-BSP vtable[0x84]) against a world point:
    /// transform to the cell's local frame, then <see cref="BSPQuery.PointInsideCellBsp"/>.
    /// A cell with no hydrated <see cref="CellPhysics.CellBSP"/> returns false (see
    /// <see cref="FindVisibleChildCell"/>'s adaptation note).
    /// </summary>
    private static bool PointInCell(CellPhysics? cell, Vector3 worldPoint)
    {
        if (cell?.CellBSP?.Root is null) return false;
        var local = Vector3.Transform(worldPoint, cell.InverseWorldTransform);
        return BSPQuery.PointInsideCellBsp(cell.CellBSP.Root, local);
    }

    /// <summary>
    /// Top-level cell-tracking driver, ported from retail's
    /// <c>CObjCell::find_cell_list</c> (sphere variant).
    ///
    /// <para>
    /// Walks the portal graph from <paramref name="currentCellId"/>,
    /// finds the cell whose <see cref="CellPhysics.CellBSP"/> contains
    /// the sphere center, and returns its full id (landblock-prefixed).
    /// Falls back to <paramref name="currentCellId"/> when no candidate
    /// matches. The candidate set built internally is discarded; use
    /// <see cref="FindCellSet"/> to recover it.
    /// </para>
    ///
    /// <para>
    /// Pseudocode reference:
    /// <c>docs/research/acclient_indoor_transitions_pseudocode.md</c>
    /// §"Overall Driver: find_cell_list".
    /// </para>
    /// </summary>
    public static uint FindCellList(
        PhysicsDataCache cache,
        Vector3 worldSphereCenter,
        float sphereRadius,
        uint currentCellId)
    {
        return FindCellSet(cache, worldSphereCenter, sphereRadius, currentCellId, out _);
    }

    /// <summary>
    /// Phase A4 (2026-05-20). Same portal-graph traversal as
    /// <see cref="FindCellList"/> but additionally returns the full
    /// candidate set built during traversal. Used by
    /// <see cref="Transition.CheckOtherCells"/> to iterate every cell
    /// the sphere overlaps for per-cell BSP collision.
    ///
    /// <para>
    /// Retail oracle: <c>CTransition::check_other_cells</c> at
    /// <c>acclient_2013_pseudo_c.txt:272717-272798</c> calls
    /// <c>CObjCell::find_cell_list(&amp;this-&gt;cell_array, &amp;var_4c, ...)</c>
    /// which fills both the cell_array (set) and var_4c (containing cell).
    /// </para>
    /// </summary>
    public static uint FindCellSet(
        PhysicsDataCache cache,
        Vector3 worldSphereCenter,
        float sphereRadius,
        uint currentCellId,
        out IReadOnlyCollection<uint> cellSet)
    {
        var spheres = new[]
        {
            new Sphere
            {
                Origin = worldSphereCenter,
                Radius = sphereRadius,
            },
        };

        return FindCellSet(cache, spheres, spheres.Length, currentCellId, out cellSet);
    }

    /// <summary>
    /// Multi-sphere form of <see cref="FindCellSet(PhysicsDataCache, Vector3, float, uint, out IReadOnlyCollection{uint})"/>.
    /// Containment still uses sphere 0's center, matching retail's
    /// <c>CObjCell::find_cell_list</c> loop after the transit set is built.
    /// </summary>
    public static uint FindCellSet(
        PhysicsDataCache cache,
        IReadOnlyList<Sphere> worldSpheres,
        int numSpheres,
        uint currentCellId,
        out IReadOnlyCollection<uint> cellSet)
    {
        var containing = BuildCellSetAndPickContaining(
            cache, worldSpheres, numSpheres, currentCellId,
            out var candidates);
        cellSet = candidates;
        return containing;
    }

    private static uint BuildCellSetAndPickContaining(
        PhysicsDataCache cache,
        IReadOnlyList<Sphere> worldSpheres,
        int numSpheres,
        uint currentCellId,
        out CellArray candidates)
    {
        // Ordered, deduped candidate array — retail CELLARRAY (add_cell @701036).
        // The ORDER is load-bearing: the current cell is added at index 0 and the
        // pick iterates in order with interior-wins-break, so the current cell wins
        // a boundary straddle and the membership does not ping-pong (the R1 flap).
        candidates = new CellArray();
        int sphereCount = EffectiveSphereCount(worldSpheres, numSpheres);
        if (sphereCount == 0) return currentCellId;

        Vector3 worldSphereCenter = worldSpheres[0].Origin;
        float sphereRadius = worldSpheres[0].Radius;
        uint currentLow = currentCellId & 0xFFFFu;

        // #106: the current block's world origin converts the world-frame sphere
        // coords into retail's block-local frame for the LandDefs lcoord math.
        // Unregistered terrain (tests; pre-stream) falls back to Vector3.Zero —
        // the legacy anchor-frame assumption (world frame == block-local frame).
        cache.CellGraph.TryGetTerrainOrigin(currentCellId, out var blockOrigin);

        // #112 rider: outdoor candidates may win the pick only when retail would
        // have admitted them — outdoor seeds always; indoor seeds only when a
        // sphere straddled an exterior portal plane during the BFS (set below).
        bool outdoorPickAllowed = currentLow < 0x0100u;

        if (currentLow >= 0x0100u)
        {
            // Indoor seed: the CURRENT cell is added at INDEX 0 (retail
            // CObjCell::find_cell_list add_cell @ pseudo_c:308766). Index 0 is what
            // makes the pick current-cell-first — the hysteresis that stops the flap.
            var currentCell = cache.GetCellStruct(currentCellId);
            if (currentCell is null) return currentCellId;
            candidates.Add(currentCellId);

            // EXPAND — a single forward walk over the GROWING array, mirroring
            // retail's `for (i=0; i<num_cells; i++) cells[i].find_transit_cells(...)`
            // loop (pseudo_c:308775-308785). FindTransitCellsSphere APPENDS portal
            // neighbours (and, on an exit portal, the outdoor landcells) to the same
            // array; CellArray.Add dedups, so the walk terminates when no new cell is
            // appended. Read OrderedIds[i] by index because the list grows under us.
            bool outdoorAdded = false;
            for (int i = 0; i < candidates.Count; i++)
            {
                uint cellId = candidates.OrderedIds[i];
                var cell = cache.GetCellStruct(cellId);
                if (cell is null) continue;

                FindTransitCellsSphere(
                    cache, cell, cellId, worldSpheres, sphereCount,
                    candidates, out bool exitOutsideStraddle, out bool hasExitPortal);

                // #112 rider (2026-06-10): the retail straddle flag (live-binary
                // verified — see FindTransitCellsSphere) gates the PICK's outdoor
                // branch below. Retail only ever has outdoor cells in this array
                // when a path sphere straddles an exterior portal plane.
                outdoorPickAllowed |= exitOutsideStraddle;

                // A6.P5 (kept, NARROWED to the collision cell SET): the first
                // exit-portal cell triggers the outdoor neighbourhood add once, by
                // TOPOLOGY — wider than retail. This keeps outdoor-registered door
                // entities findable from indoor cells (the 2026-05-25 door capture)
                // until #99/A6.P4 ships per-cell shadow lists; the pick no longer
                // consumes these cells unless the retail flag fired, so membership
                // matches retail in both regimes. Appended AFTER the interior cells,
                // matching retail order (add_all_outside_cells at the end,
                // pseudo_c:310120) — interior-wins is preserved.
                if (hasExitPortal && !outdoorAdded)
                {
                    AddAllOutsideCells(worldSpheres, sphereCount, currentCellId, blockOrigin, candidates);
                    outdoorAdded = true;
                }
            }
        }
        else
        {
            // Outdoor seed: expand neighbour landcells (added first), then check each
            // for a building stab whose portals cross into an interior EnvCell.
            // (Stage 2 will make building entry intrinsic and remove CheckBuildingTransit.)
            AddAllOutsideCells(worldSpheres, sphereCount, currentCellId, blockOrigin, candidates);

            var landcellSnapshot = new List<uint>(candidates.OrderedIds);
            foreach (uint landcellId in landcellSnapshot)
            {
                var building = cache.GetBuilding(landcellId);
                if (building is null) continue;
                CheckBuildingTransit(cache, building, worldSphereCenter, sphereRadius, candidates);
            }
        }

        if (PhysicsDiagnostics.ProbeCellSetEnabled)
            PhysicsDiagnostics.LogCellSetBuild(currentCellId, worldSphereCenter, candidates);

        // THE PICK — verbatim CObjCell::find_cell_list containing-cell pick
        // (pseudo_c:308788-308825): iterate the array IN ORDER from index 0; for each
        // cell, point_in_cell; set the running result on ANY containing cell;
        // INTERIOR-WINS-BREAK. The current cell is at index 0, so if the sphere centre
        // is still inside it, it wins and the search stops — the retail hysteresis.
        // (Replaces the 5ca2f44 current-first pre-check, which approximated this for
        // the indoor-current case only; the ordered array now delivers it for every
        // seed by construction.)
        //
        // #106: the outdoor containing cell is the GLOBAL XY-column under the sphere
        // centre (LandDefs.AdjustToOutside from the current block's frame — retail
        // subtracts get_block_offset per candidate before point_in_cell, pc:308804;
        // landcells are disjoint columns so identity-compare is equivalent). The
        // pre-fix [0,8)-clamped, current-prefix-only computation could never match a
        // neighbour-block cell, freezing membership at landblock boundaries.
        uint containingOutdoorId = 0u;
        {
            var pickPos = worldSphereCenter - blockOrigin;
            uint pickCell = currentCellId;
            if (LandDefs.AdjustToOutside(ref pickCell, ref pickPos))
                containingOutdoorId = pickCell;
        }

        uint outdoorResult = 0u;
        foreach (uint candId in candidates.OrderedIds)
        {
            if ((candId & 0xFFFFu) >= 0x0100u)
            {
                // Interior candidate — point_in_cell via the cell BSP (vtable[0x84]).
                var cand = cache.GetCellStruct(candId);
                if (cand?.CellBSP?.Root is null) continue;
                var local = Vector3.Transform(worldSphereCenter, cand.InverseWorldTransform);
                if (BSPQuery.PointInsideCellBsp(cand.CellBSP.Root, local))
                    return candId;                       // interior-wins, stop (pseudo_c:308819)
            }
            else if (outdoorResult == 0u && containingOutdoorId != 0u && outdoorPickAllowed)
            {
                // Outdoor candidate — CLandCell::point_in_cell is the XY-column the
                // sphere is over (acdream landcells have no BSP point_in_cell; the
                // documented adaptation). Record as the running result but DO NOT
                // break — an interior cell later in the array can still win.
                // #112 rider: gated on outdoorPickAllowed — retail's array only
                // contains outdoor cells when a sphere straddled an exterior portal
                // plane (live-binary verified); ours may also contain them via the
                // A6.P5 collision widening, which the pick must ignore.
                if (candId == containingOutdoorId)
                    outdoorResult = candId;
            }
        }

        // No interior cell contained the centre. Return the outdoor XY-column cell if
        // it was a candidate, else stay on the current cell (retail leaves *result
        // null → caller keeps curr_cell).
        if (outdoorResult != 0u) return outdoorResult;

        // ── No containing cell: lateral recovery, then retail keep-curr ────
        // Retail find_cell_list leaves *result null here and the CALLER KEEPS
        // curr_cell (pc:308788-308825) — including when the centre sits in a
        // containment GAP between a house's cell volumes. #112 (2026-06-10):
        // the A9B3 hill cottage has a real gap inside the house; the 6dbbf95
        // escape hatch that used to live here demoted such gaps to the
        // outdoor column, stranding the player outdoor-classified deep inside
        // the house (outdoor→indoor promotion is portal-adjacent-only, retail-
        // identical) → the outdoor flood rendered the interior transparent.
        // The hatch's actual target — poisoned (cell, position) SAVES — is
        // handled at the SNAP by PhysicsEngine.Resolve's AdjustPosition
        // validation since #107/#111; mid-session farness cannot arise (the
        // sphere moves continuously, and real building exits flow through
        // exterior portals → outside cells enter the candidate array → the
        // normal outdoorResult path above demotes there, retail-faithfully).
        //
        // Before keeping a claim whose volume the sphere no longer overlaps,
        // try the claim's VISIBLE GRAPH for a containing cell (retail
        // CEnvCell::find_visible_child_cell in stab-list mode :311444 — the
        // same recovery AdjustPosition uses at :280028): a near-miss claim
        // one room off self-heals laterally instead of waiting for a doorway.
        if (currentLow >= 0x0100u)
        {
            var cur = cache.GetCellStruct(currentCellId);
            if (cur?.CellBSP?.Root is not null)
            {
                var curLocal = Vector3.Transform(worldSphereCenter, cur.InverseWorldTransform);
                if (!BSPQuery.SphereIntersectsCellBsp(cur.CellBSP.Root, curLocal, sphereRadius))
                {
                    uint recovered = FindVisibleChildCell(
                        cache, currentCellId, worldSphereCenter, useStabList: true);
                    if (recovered != 0u && recovered != currentCellId)
                        return recovered;
                }
            }
        }

        return currentCellId;
    }

    private static int EffectiveSphereCount(IReadOnlyList<Sphere> worldSpheres, int numSpheres)
    {
        if (numSpheres <= 0 || worldSpheres.Count == 0) return 0;
        return numSpheres < worldSpheres.Count ? numSpheres : worldSpheres.Count;
    }
}