acdream/src/AcDream.App/Rendering/PortalVisibilityBuilder.cs

// PortalVisibilityBuilder.cs
//
// Phase A8.F: recursive portal-clip visibility (the builder). Port of retail
// PView::ConstructView (decomp:433750) -> ClipPortals (433572) -> AddViewToPortals
// (433446). Walks the portal graph from the camera cell, accumulating a per-cell
// screen-space CellView; exit portals union their clipped region into OutsideView.
// GL-free; unit-tested without a GPU context.
using System;
using System.Collections.Generic;
using System.Numerics;

namespace AcDream.App.Rendering;

/// <summary>Per-frame output of the portal-frame BFS.</summary>
public sealed class PortalVisibilityFrame
{
    /// <summary>Screen region (NDC) where outdoor terrain/scenery may draw — exit portals
    /// recursively clipped to their portal chain. The cellar-flap fix.</summary>
    public CellView OutsideView { get; } = new();

    /// <summary>Per-cell accumulated clip region, keyed by full cell id (wire-in #2).</summary>
    public Dictionary<uint, CellView> CellViews { get; } = new();

    /// <summary>Visible interior cells in the exact order they were first dequeued from the
    /// distance-priority work list — closest-first (Phase U.2a). Mirrors retail's
    /// PView::cell_draw_list, appended in PView::ConstructView (decomp:433783) as each cell pops
    /// off the nearest-vertex-sorted cell_todo_list (InsCellTodoList 433183). Deduplicated: a cell
    /// appears exactly once, on its first dequeue. The camera cell is always first.</summary>
    public List<uint> OrderedVisibleCells { get; } = new();

    /// <summary>Entry clip regions for other buildings reached through our portals, keyed by the
    /// neighbour cell id that left the camera building's cell set (wire-in #3 / Step 5).</summary>
    public Dictionary<uint, CellView> CrossBuildingViews { get; } = new();
}

public static class PortalVisibilityBuilder
{
    private const float PortalSideEpsilon = 0.01f; // matches CellVisibility.PointInCellEpsilon

    // TEMP diagnostic (Phase A8.F visual-gate triage; strip after): ACDREAM_A8_DUMP_PV=1 dumps the
    // local→NDC→clipped portal geometry for the first 2 Build calls per distinct camera cell.
    private static readonly bool s_pvDump =
        Environment.GetEnvironmentVariable("ACDREAM_A8_DUMP_PV") == "1";
    private static readonly Dictionary<uint, int> s_pvDumpCount = new();

    /// <param name="lookup">Resolve a full cell id to its LoadedCell, or null if not loaded.</param>
    /// <param name="buildingMembership">Optional: true if a cell id is in the camera building's cell
    /// set. When provided, a neighbour OUTSIDE the set routes to CrossBuildingViews instead of
    /// continuing the in-building BFS. Pass null to treat all reachable cells as in-building.</param>
    public static PortalVisibilityFrame Build(
        LoadedCell cameraCell,
        Vector3 cameraPos,
        Func<uint, LoadedCell?> lookup,
        Matrix4x4 viewProj,
        Func<uint, bool>? buildingMembership = null)
    {
        var frame = new PortalVisibilityFrame();
        if (cameraCell == null) return frame;

        // Interior portals never cross landblocks (same invariant as CellVisibility.GetVisibleCells);
        // building-boundary crossings are handled separately via the buildingMembership escape hatch.
        uint lbMask = cameraCell.CellId & 0xFFFF0000u;

        frame.CellViews[cameraCell.CellId] = CellView.FullScreen();

        // Distance-priority work list (retail PView::cell_todo_list). Cells pop closest-first;
        // each cell carries the camera→nearest-portal-vertex distance that put it on the list
        // (retail keys on InitCell's per-portal min-vertex distance, decomp 432988-433004). The
        // camera cell seeds at distance 0 (retail InsCellTodoList(this, arg2, 0f) at 433758) so it
        // always pops first.
        var todo = new CellTodoList();
        todo.Insert(cameraCell, 0f);

        // Fixpoint termination replacing the old MaxReprocessPerCell hard cap. This mirrors the
        // retail portal_view slice offset 0x44 (last-incorporated view-poly watermark) vs 0x38
        // (current view_count) decision in AddViewToPortals (433446): a cell is INSERTED into the
        // todo list exactly once — on first discovery (retail's ecx_5==0 branch calls
        // InsCellTodoList; the ecx_5!=eax_2 growth branch calls AddToCell IN PLACE and never
        // re-enqueues). Later growth into an already-discovered cell is unioned into its CellView but
        // does NOT re-enqueue it — the `cell_view_done` guarantee (ConstructView sets it at 433784
        // the instant a cell is popped). Enqueue-once across the cell set is the hard termination
        // guarantee for cyclic / hub / diamond graphs: at most N cells are ever processed. The
        // camera cell is pre-marked so a portal looping back to it can never re-enqueue it.
        var seen = new HashSet<uint> { cameraCell.CellId };

        bool pvDump = false;
        if (s_pvDump)
        {
            lock (s_pvDumpCount)
            {
                s_pvDumpCount.TryGetValue(cameraCell.CellId, out int dc);
                if (dc < 2) { s_pvDumpCount[cameraCell.CellId] = dc + 1; pvDump = true; }
            }
            if (pvDump)
            {
                Console.WriteLine($"[pv-dump] camCell=0x{cameraCell.CellId:X8} portals={cameraCell.Portals.Count} polyLists={cameraCell.PortalPolygons.Count} vp[M11={viewProj.M11:F3} M22={viewProj.M22:F3} M33={viewProj.M33:F3} M34={viewProj.M34:F3} M43={viewProj.M43:F3} M44={viewProj.M44:F3}]");
                // Camera-cell portal census (A8.F triage 2026-05-29): report, for EVERY
                // portal, the exact inputs the BFS guards read — BEFORE the guards run, so
                // a portal the loop silently `continue`s past is still visible here. An
                // empty OUTSIDEVIEW can then be traced to the precise gate: polyLen<3 (empty
                // polygon from BuildLoadedCell), interiorSide=false (camera back-facing the
                // portal — a legitimately-empty result, not a bug), or (if both OK) a
                // downstream projection/clip failure shown by the EXIT-PROJ/EXIT-CLIP lines.
                for (int ci = 0; ci < cameraCell.Portals.Count; ci++)
                {
                    int plen = ci < cameraCell.PortalPolygons.Count
                        ? (cameraCell.PortalPolygons[ci]?.Length ?? -1) : -2;
                    bool hasPlane = ci < cameraCell.ClipPlanes.Count;
                    bool interiorSide = !hasPlane || CameraOnInteriorSide(cameraCell, ci, cameraPos);
                    var n = hasPlane ? cameraCell.ClipPlanes[ci].Normal : Vector3.Zero;
                    Console.WriteLine($"[pv-dump]   CAMPORTAL[{ci}] other=0x{cameraCell.Portals[ci].OtherCellId:X4} polyLen={plen} hasPlane={hasPlane} interiorSide={interiorSide} planeN=({n.X:F3},{n.Y:F3},{n.Z:F3})");
                }
            }
        }

        while (todo.Count > 0)
        {
            var cell = todo.PopNearest();
            if (!frame.CellViews.TryGetValue(cell.CellId, out var currentView) || currentView.IsEmpty)
                continue;

            // `seen` guarantees each cell is inserted into the todo list exactly once, so this single
            // pop IS the cell's closest-first draw position (retail appends to cell_draw_list once per
            // pop, 433783) — no per-pop dedup needed, OrderedVisibleCells stays distinct by construction.
            frame.OrderedVisibleCells.Add(cell.CellId);

            for (int i = 0; i < cell.Portals.Count; i++)
            {
                if (i >= cell.PortalPolygons.Count) continue;
                var poly = cell.PortalPolygons[i];
                if (poly == null || poly.Length < 3) continue;

                bool dx = pvDump && cell.Portals[i].OtherCellId == 0xFFFF;

                // Portal-side test: only traverse a portal the camera is on the interior side of
                // (mirrors CellVisibility.GetVisibleCells + retail's 'seen' flag). Culls back-facing
                // portals so we never feed a degenerate/wrong-facing projection downstream.
                if (i < cell.ClipPlanes.Count && !CameraOnInteriorSide(cell, i, cameraPos))
                {
                    if (dx) Console.WriteLine($"[pv-dump]   EXIT-CULLED(side) cell=0x{cell.CellId:X8} p{i} localN={poly.Length} hasClipPlane={(i < cell.ClipPlanes.Count)}");
                    continue;
                }

                // Project to NDC, then normalize to CCW for the CCW-only ScreenPolygonClip
                // (ProjectToNdc preserves input winding; portal dat polygons may be CW).
                Vector2[] portalNdc = PortalProjection.ProjectToNdc(poly, cell.WorldTransform, viewProj);
                if (dx) Console.WriteLine($"[pv-dump]   EXIT-PROJ cell=0x{cell.CellId:X8} p{i} localN={poly.Length} ndcN={portalNdc.Length} local0=({poly[0].X:F2},{poly[0].Y:F2},{poly[0].Z:F2}) ndc=[{string.Join(" ", System.Array.ConvertAll(portalNdc, v => $"({v.X:F2},{v.Y:F2})"))}]");
                if (portalNdc.Length < 3) continue;
                EnsureCcw(portalNdc);

                // Intersect the portal opening with every polygon of the current cell's view.
                var clippedRegion = new List<ViewPolygon>();
                foreach (var vp in currentView.Polygons)
                {
                    var clipped = ScreenPolygonClip.Intersect(portalNdc, vp.Vertices);
                    if (clipped.Length >= 3) clippedRegion.Add(new ViewPolygon(clipped));
                }
                if (dx) Console.WriteLine($"[pv-dump]   EXIT-CLIP cell=0x{cell.CellId:X8} p{i} currentViewPolys={currentView.Polygons.Count} clipResult={clippedRegion.Count}");
                if (clippedRegion.Count == 0) continue; // portal not visible through this chain

                var portal = cell.Portals[i];

                if (portal.OtherCellId == 0xFFFF)
                {
                    if (pvDump)
                    {
                        Console.WriteLine($"[pv-dump]   EXIT cell=0x{cell.CellId:X8} p{i} localN={poly.Length} ndcN={portalNdc.Length} clipPolys={clippedRegion.Count}");
                        Console.WriteLine($"[pv-dump]     local=[{string.Join(" ", System.Array.ConvertAll(poly, v => $"({v.X:F2},{v.Y:F2},{v.Z:F2})"))}]");
                        Console.WriteLine($"[pv-dump]     ndc=[{string.Join(" ", System.Array.ConvertAll(portalNdc, v => $"({v.X:F3},{v.Y:F3})"))}]");
                        foreach (var cp in clippedRegion)
                            Console.WriteLine($"[pv-dump]     clipped({cp.Vertices.Length})=[{string.Join(" ", System.Array.ConvertAll((Vector2[])cp.Vertices, v => $"({v.X:F3},{v.Y:F3})"))}]");
                    }
                    // Exit portal -> outdoors visible through this (clipped) opening.
                    foreach (var cp in clippedRegion) frame.OutsideView.Add(cp);
                    continue;
                }

                uint neighbourId = lbMask | portal.OtherCellId;

                // Cross-building boundary: route to CrossBuildingViews, don't continue in-building BFS.
                // (Cross-building entry is retail's CBldPortal/AddToCell channel, not OtherPortalClip;
                // the reciprocal clip below is interior-cell-to-cell only, matching the OtherPortalClip
                // call inside ConstructView at decomp:433692.)
                if (buildingMembership != null && !buildingMembership(neighbourId))
                {
                    var xview = GetOrCreate(frame.CrossBuildingViews, neighbourId);
                    foreach (var cp in clippedRegion) xview.Add(cp);
                    continue;
                }

                var neighbour = lookup(neighbourId);
                if (neighbour == null) continue;

                // Phase U.2b — neighbour-side OtherPortalClip (retail PView::OtherPortalClip
                // decomp:433524). The portal opening seen from THIS cell may be wider than the
                // SAME opening seen from the neighbour (skewed/oblique apertures), so retail
                // re-clips the already-near-side-clipped region against the neighbour's matching
                // (reciprocal) portal polygon — the propagated region is the intersection of the
                // opening "seen from A" AND "seen from B". This can only TIGHTEN, never widen, and
                // degrades to the prior near-side-only region when the reciprocal is unresolvable
                // (over-include is the safe default). The reciprocal is the portal at index
                // `portal.OtherPortalId` in the NEIGHBOUR's portal list — retail's direct back-link
                // (arg2->other_portal_id, 433557), NOT a scan for the first OtherCellId match. The
                // direct index is what lets a cell with TWO portals to the same neighbour clip each
                // opening against its OWN reciprocal instead of the first one. Mutates clippedRegion
                // in place before the union below.
                ApplyReciprocalClip(clippedRegion, portal.OtherPortalId, neighbour, viewProj);
                if (clippedRegion.Count == 0) continue; // reciprocal opening doesn't overlap → not visible

                // Union the clipped region into the neighbour's accumulated view.
                var nview = GetOrCreate(frame.CellViews, neighbourId);
                foreach (var cp in clippedRegion) nview.Add(cp);

                // Insert the neighbour into the distance-priority list — but ONLY on first discovery
                // (retail enqueues via InsCellTodoList solely in the ecx_5==0 branch; growth into an
                // already-seen cell is handled in place, never by re-enqueue). `seen` is the
                // enqueue-once / `cell_view_done` gate: a neighbour already discovered is never
                // re-enqueued, which is what bounds cyclic & hub graphs. Distance = camera→nearest
                // portal-opening vertex in world space (retail InitCell min-vertex distance,
                // 432988-433004); derived from the portal geometry, so it works even when the cell's
                // WorldPosition was never populated.
                if (seen.Add(neighbourId))
                {
                    float dist = NearestPortalVertexDistance(poly, cell.WorldTransform, cameraPos);
                    todo.Insert(neighbour, dist);
                }
            }
        }

        if (pvDump)
            Console.WriteLine($"[pv-dump] OUTSIDEVIEW polys={frame.OutsideView.Polygons.Count} bfsCellViews={frame.CellViews.Count} crossBldg={frame.CrossBuildingViews.Count}");

        // Phase U.4c flap probe (ACDREAM_PROBE_FLAP) — read-only per-frame snapshot of the
        // root cell's per-portal side-test + projection + the frame's exit/visible counts.
        if (AcDream.Core.Rendering.RenderingDiagnostics.ProbeFlapEnabled)
            EmitFlapProbe(cameraCell, cameraPos, viewProj, frame);

        return frame;
    }

    // The NDC [-1,1] viewport quad (CCW), reused by the flap probe's clip recompute.
    private static readonly Vector2[] FullScreenQuad =
        { new Vector2(-1f, -1f), new Vector2(1f, -1f), new Vector2(1f, 1f), new Vector2(-1f, 1f) };

    // Phase U.4c flap probe. One [flap] line per Build: the root cell's per-portal
    // signed distance D (eye→portal plane), traverse/cull decision, and NDC projection
    // vertex count, plus the frame's OutsideView polygon count + visible-cell count.
    // `localEye` is the eye in root-local space — its component along an interior portal
    // plane reveals when the eye has crossed past that plane (the stale-root region that
    // makes the side test cull a still-needed portal). Read-only recompute; no effect on
    // the returned frame. Throwaway apparatus — strip with the probe.
    private static void EmitFlapProbe(
        LoadedCell cameraCell, Vector3 cameraPos, Matrix4x4 viewProj, PortalVisibilityFrame frame)
    {
        var localEye = Vector3.Transform(cameraPos, cameraCell.InverseWorldTransform);
        var sb = new System.Text.StringBuilder(220);
        sb.Append("[flap] root=0x").Append(cameraCell.CellId.ToString("X8"));
        sb.Append(" eye=(").Append(cameraPos.X.ToString("F2")).Append(',')
          .Append(cameraPos.Y.ToString("F2")).Append(',').Append(cameraPos.Z.ToString("F2")).Append(')');
        sb.Append(" localEye=(").Append(localEye.X.ToString("F2")).Append(',')
          .Append(localEye.Y.ToString("F2")).Append(',').Append(localEye.Z.ToString("F2")).Append(')');
        for (int i = 0; i < cameraCell.Portals.Count; i++)
        {
            var portal = cameraCell.Portals[i];
            float d = float.NaN;
            bool side = true;
            if (i < cameraCell.ClipPlanes.Count && cameraCell.ClipPlanes[i].Normal.LengthSquared() >= 1e-8f)
            {
                var pl = cameraCell.ClipPlanes[i];
                d = Vector3.Dot(pl.Normal, localEye) + pl.D;
                side = CameraOnInteriorSide(cameraCell, i, cameraPos);
            }
            // Replicate the walk's project → EnsureCcw → Intersect(FullScreen) exactly, so a
            // portal that PROJECTS (proj>=3) but still fails to ADD its neighbour shows WHY:
            // clip=0 with ndc inside [-1,1] ⇒ winding/self-intersection degeneracy; clip=0 with
            // ndc outside [-1,1] ⇒ genuinely off-screen. The ndc coords expose a near-plane bowtie.
            int projN = -1, clipN = -1;
            string ndcText = "";
            if (i < cameraCell.PortalPolygons.Count)
            {
                var poly = cameraCell.PortalPolygons[i];
                if (poly != null && poly.Length >= 3)
                {
                    var ndc = PortalProjection.ProjectToNdc(poly, cameraCell.WorldTransform, viewProj);
                    projN = ndc.Length;
                    if (ndc.Length >= 3)
                    {
                        EnsureCcw(ndc);
                        clipN = ScreenPolygonClip.Intersect(ndc, FullScreenQuad).Length;
                        var ns = new System.Text.StringBuilder(48);
                        foreach (var v in ndc) ns.Append('(').Append(v.X.ToString("F1")).Append(',').Append(v.Y.ToString("F1")).Append(')');
                        ndcText = ns.ToString();
                    }
                }
            }
            sb.Append(" | p").Append(i).Append("->0x").Append(portal.OtherCellId.ToString("X4"));
            sb.Append(" D=").Append(float.IsNaN(d) ? "na" : d.ToString("F2"));
            sb.Append(side ? " TRV" : " CULL");
            sb.Append(" proj=").Append(projN).Append(" clip=").Append(clipN);
            if (ndcText.Length > 0) sb.Append(" ndc=").Append(ndcText);
        }
        sb.Append(" || outPolys=").Append(frame.OutsideView.Polygons.Count);
        sb.Append(" vis=").Append(frame.OrderedVisibleCells.Count);
        Console.WriteLine(sb.ToString());
    }

    // Mirrors CellVisibility's portal-side test (InsideSide convention).
    private static bool CameraOnInteriorSide(LoadedCell cell, int portalIndex, Vector3 cameraPos)
    {
        var plane = cell.ClipPlanes[portalIndex];
        if (plane.Normal.LengthSquared() < 1e-8f) return true; // no usable plane → allow
        var localCam = Vector3.Transform(cameraPos, cell.InverseWorldTransform);
        float dot = Vector3.Dot(plane.Normal, localCam) + plane.D;
        return plane.InsideSide == 0 ? dot >= -PortalSideEpsilon : dot <= PortalSideEpsilon;
    }

    // Reverse vertex order in place if the polygon is wound clockwise (signed area < 0).
    private static void EnsureCcw(Vector2[] poly)
    {
        float area2 = 0f;
        for (int i = 0; i < poly.Length; i++)
        {
            var p = poly[i]; var q = poly[(i + 1) % poly.Length];
            area2 += p.X * q.Y - q.X * p.Y;
        }
        if (area2 < 0f) Array.Reverse(poly);
    }

    // Phase U.2b — reciprocal OtherPortalClip (retail PView::OtherPortalClip decomp:433524).
    // Resolves the neighbour's reciprocal back-portal by DIRECT INDEX (`otherPortalId`), projects
    // that reciprocal polygon through the NEIGHBOUR's world transform to NDC, and intersects it into
    // every polygon of `clippedRegion` (already clipped against the near-side opening + current
    // view). The net region is "opening seen from the near cell" ∩ "opening seen from the
    // neighbour" — a strict tightening that prevents over-inclusion through skewed apertures.
    //
    // `otherPortalId` is the near-side portal's reciprocal back-link, straight from the dat's
    // CellPortal.OtherPortalId. Retail indexes the neighbour's portal array with it directly —
    // `portals->portal[arg2->other_portal_id ...]` at 005a54b2/005a54f6 — rather than scanning for
    // the first OtherCellId match. A scan picks the FIRST back-portal for EVERY near-side portal to
    // the same neighbour, so a cell with two openings into one neighbour clips both against the same
    // (first) reciprocal — hiding the second opening when the apertures are disjoint (under-inclusion
    // bug #102 M-4). The direct index gives each opening its own reciprocal.
    //
    // GUARDS — degrade to over-include (leave `clippedRegion` untouched), NEVER clip against a
    // guessed polygon: the index is out of range, OR the indexed polygon is missing/degenerate
    // (< 3 verts), OR it projects entirely behind the camera. Over-inclusion is the safe default;
    // mis-resolution is the bug this method exists to remove. PortalPolygons is in lockstep with
    // Portals, so index `otherPortalId` selects the reciprocal polygon. NEVER throws.
    private static void ApplyReciprocalClip(
        List<ViewPolygon> clippedRegion, ushort otherPortalId, LoadedCell neighbour, Matrix4x4 viewProj)
    {
        if (clippedRegion.Count == 0) return;

        // Direct back-link index (retail arg2->other_portal_id). Out-of-range → over-include.
        if (otherPortalId >= neighbour.PortalPolygons.Count) return;
        Vector3[]? reciprocalPoly = neighbour.PortalPolygons[otherPortalId];
        if (reciprocalPoly == null || reciprocalPoly.Length < 3) return; // missing/degenerate → over-include

        // Project the reciprocal opening through the NEIGHBOUR's transform (retail positionPush(3,
        // &other_cell_ptr->pos) at 005a54d2), then normalize winding for the CCW-only clipper.
        Vector2[] reciprocalNdc = PortalProjection.ProjectToNdc(reciprocalPoly, neighbour.WorldTransform, viewProj);
        if (reciprocalNdc.Length < 3) return; // reciprocal entirely behind camera / degenerate → no-op
        EnsureCcw(reciprocalNdc);

        // Intersect the reciprocal opening into each near-side polygon; drop any that fall away.
        for (int k = clippedRegion.Count - 1; k >= 0; k--)
        {
            var tightened = ScreenPolygonClip.Intersect(reciprocalNdc, clippedRegion[k].Vertices);
            if (tightened.Length >= 3) clippedRegion[k] = new ViewPolygon(tightened);
            else clippedRegion.RemoveAt(k);
        }
    }

    private static CellView GetOrCreate(Dictionary<uint, CellView> map, uint key)
    {
        if (!map.TryGetValue(key, out var v)) { v = new CellView(); map[key] = v; }
        return v;
    }

    // Camera→nearest-vertex distance for a portal polygon, in world space. Mirrors the per-portal
    // min-distance loop retail runs in PView::InitCell (decomp:432988-433004) to key the todo list:
    // it walks the portal's vertices, transforms each to world space, and keeps the smallest
    // straight-line distance to the camera viewpoint. Keying on the portal opening (not the cell
    // origin) is both retail-faithful and robust to cells whose WorldPosition was never populated.
    private static float NearestPortalVertexDistance(Vector3[] localPoly, Matrix4x4 worldTransform, Vector3 cameraPos)
    {
        float best = float.MaxValue;
        for (int i = 0; i < localPoly.Length; i++)
        {
            var world = Vector3.Transform(localPoly[i], worldTransform);
            float d2 = Vector3.DistanceSquared(world, cameraPos);
            if (d2 < best) best = d2;
        }
        return best == float.MaxValue ? 0f : MathF.Sqrt(best);
    }

    /// <summary>
    /// Distance-sorted work list for the portal BFS, ported from retail PView::cell_todo_list +
    /// InsCellTodoList (decomp:433183). Insertion keeps the list ordered so the NEAREST cell sits at
    /// the tail; <see cref="PopNearest"/> removes the tail — giving closest-first traversal exactly
    /// as ConstructView's pop-from-(cell_todo_num-1) does (433767-433769). The insertion only shifts
    /// entries strictly farther than the newcomer (retail's flag test breaks on the first
    /// not-greater entry), so an equal-distance newcomer lands at the tail and pops FIRST —
    /// LIFO on ties, matching retail's break-on-first-not-greater + pop-from-tail.
    /// </summary>
    private sealed class CellTodoList
    {
        private readonly List<(LoadedCell Cell, float Distance)> _items = new();

        public int Count => _items.Count;

        public void Insert(LoadedCell cell, float distance)
        {
            // Find the slot: scan from the tail (nearest) toward the head while existing entries are
            // strictly nearer than `distance`, so the newcomer lands just ABOVE every entry that is
            // farther-or-equal — i.e. nearest-at-tail order, LIFO on ties (an equal-distance
            // newcomer inserts at the tail and pops first).
            int idx = _items.Count;
            while (idx > 0 && _items[idx - 1].Distance < distance)
                idx--;
            _items.Insert(idx, (cell, distance));
        }

        public LoadedCell PopNearest()
        {
            int last = _items.Count - 1;
            var cell = _items[last].Cell;
            _items.RemoveAt(last);
            return cell;
        }
    }
}