acdream/src/AcDream.App/Rendering/Wb/BuildingLoader.cs

using System.Collections.Generic;
using System.Numerics;
using AcDream.App.Rendering;
using DatReaderWriter.DBObjs;

namespace AcDream.App.Rendering.Wb;

/// <summary>
/// Phase A8 (2026-05-26): static factory that builds a per-landblock
/// <see cref="BuildingRegistry"/> from a <see cref="LandBlockInfo"/>'s
/// <c>Buildings</c> array.
///
/// <para>Algorithm (mirrors WB's <c>PortalService.GetPortalsByBuilding</c> at
/// <c>WorldBuilder.Shared/Services/PortalService.cs:43-97</c>):</para>
/// <list type="bullet">
///   <item>Step A — seed the cell set from <c>BuildingInfo.Portals</c> entry portals.</item>
///   <item>Step B — BFS through <see cref="LoadedCell.Portals"/> to discover all
///     interior cells reachable from the entry portals (interior portals only;
///     exit portals — <c>OtherCellId == 0xFFFF</c> — terminate each BFS branch).</item>
///   <item>Step C — collect exit portal polygons in world space for the stencil
///     pipeline (Phase A8 Steps 1+2, RR7 scope).</item>
/// </list>
///
/// <para>Cells whose <c>LoadedCell</c> entries are missing from
/// <paramref name="cellsByCellId"/> are silently skipped (BFS bails at the
/// unloaded cell). In production, streaming loads all cells for a landblock
/// before <see cref="Build"/> runs, so the dict is always complete.</para>
///
/// <para><c>LoadedCell.BuildingId</c> is stamped here in RR4 after <c>reg.Add(building)</c>.</para>
///
/// <para>Retail references:
/// <c>docs/research/named-retail/acclient.h:32035</c> (<c>BuildInfo</c>) and
/// <c>:32094</c> (<c>CBldPortal</c>).</para>
/// </summary>
public static class BuildingLoader
{
    /// <summary>
    /// Builds a <see cref="BuildingRegistry"/> from the supplied landblock data.
    /// Building IDs are allocated sequentially starting at 1 (0 is reserved for
    /// "no building" semantics used by <c>LoadedCell.BuildingId</c> in RR4).
    /// </summary>
    /// <param name="info">The dat-loaded <see cref="LandBlockInfo"/> for this landblock.</param>
    /// <param name="landblockId">The 32-bit landblock id (e.g. <c>0xA9B40000</c>).
    /// The high 16 bits are ORed with each 16-bit <c>OtherCellId</c> to produce
    /// the full cell id.</param>
    /// <param name="cellsByCellId">Pre-loaded cells keyed by full 32-bit cell id.
    /// Used for BFS extension (Step B) and exit-polygon collection (Step C).
    /// An empty dict is valid for unit tests; Step B and C are skipped per cell.</param>
    /// <returns>A fully populated <see cref="BuildingRegistry"/>; never null.</returns>
    public static BuildingRegistry Build(
        LandBlockInfo info,
        uint landblockId,
        IReadOnlyDictionary<uint, LoadedCell> cellsByCellId)
    {
        var reg = new BuildingRegistry();
        if (info.Buildings is null || info.Buildings.Count == 0)
            return reg;

        uint lbMask = landblockId & 0xFFFF0000u;
        uint nextId = 1;

        foreach (var bInfo in info.Buildings)
        {
            var envCellIds    = new HashSet<uint>();
            var exitPortalPolys = new List<Vector3[]>();

            // Step A: seed the cell set from BuildingInfo.Portals (entry portals).
            // Each BuildingPortal.OtherCellId is a 16-bit cell-local id; OR with
            // the landblock prefix for the full id.
            // Defensive: skip OtherCellId == 0xFFFF (exit-portal sentinel in
            // BuildingInfo — rare but WB guards against it too at PortalService.cs:58).
            if (bInfo.Portals is not null)
            {
                foreach (var portal in bInfo.Portals)
                {
                    if (portal.OtherCellId == 0xFFFF) continue;
                    envCellIds.Add(lbMask | portal.OtherCellId);
                }
            }

            // Step B: BFS through interior CellPortals to find the full cell set.
            // Uses pre-loaded LoadedCell.Portals (avoids a duplicate dat fetch per
            // BFS step). Mirrors WB PortalService.cs:67-79.
            // When cellsByCellId is empty (unit-test path), BFS immediately exits.
            var queue = new Queue<uint>(envCellIds);
            while (queue.Count > 0)
            {
                var current = queue.Dequeue();
                if (!cellsByCellId.TryGetValue(current, out var cell)) continue;
                foreach (var p in cell.Portals)
                {
                    if (p.OtherCellId == 0xFFFF) continue;   // exit portal — stop BFS here
                    uint neighbourId = lbMask | p.OtherCellId;
                    if (envCellIds.Add(neighbourId))
                        queue.Enqueue(neighbourId);
                }
            }

            // Step C: collect exit portal polygons in world space.
            // For each interior cell, iterate its portals; for each exit portal
            // (OtherCellId == 0xFFFF), transform the portal polygon vertices from
            // cell-local space to world space via WorldTransform.
            // Mirrors WB PortalService.cs:81-86 (GetPortalsForCell return path).
            foreach (var cellId in envCellIds)
            {
                if (!cellsByCellId.TryGetValue(cellId, out var cell)) continue;
                for (int pi = 0; pi < cell.Portals.Count; pi++)
                {
                    if (cell.Portals[pi].OtherCellId != 0xFFFF) continue;
                    if (pi >= cell.PortalPolygons.Count) continue;
                    var localPoly = cell.PortalPolygons[pi];
                    if (localPoly.Length < 3) continue;
                    var worldPoly = new Vector3[localPoly.Length];
                    for (int v = 0; v < localPoly.Length; v++)
                        worldPoly[v] = Vector3.Transform(localPoly[v], cell.WorldTransform);
                    exitPortalPolys.Add(worldPoly);
                }
            }

            // WB PortalService.cs:89: skip buildings with no interior cells.
            if (envCellIds.Count == 0) continue;

            var building = new Building
            {
                BuildingId         = nextId++,
                EnvCellIds         = envCellIds,
                ExitPortalPolygons = exitPortalPolys,
            };
            reg.Add(building);

            // Step 4: stamp BuildingId on each cell (Option C — both directions
            // O(1)). The internal setter on LoadedCell.BuildingId is accessible
            // because this class lives in the same assembly (AcDream.App).
            foreach (var cellId in envCellIds)
            {
                if (cellsByCellId.TryGetValue(cellId, out var cell))
                    cell.BuildingId = building.BuildingId;
            }
        }

        return reg;
    }
}