acdream/src/AcDream.Core/World/LandblockLoader.cs

using DatReaderWriter;
using DatReaderWriter.DBObjs;

namespace AcDream.Core.World;

public static class LandblockLoader
{
    private const uint GfxObjMask = 0x01000000u;
    private const uint SetupMask  = 0x02000000u;
    private const uint TypeMask   = 0xFF000000u;

    /// <summary>
    /// Load a single landblock (heightmap + static objects) from the dats.
    /// </summary>
    /// <returns>Null if the landblock is missing from the cell dat.</returns>
    public static LoadedLandblock? Load(DatCollection dats, uint landblockId)
    {
        var block = dats.Get<LandBlock>(landblockId);
        if (block is null)
            return null;

        var info = dats.Get<LandBlockInfo>((landblockId & 0xFFFF0000u) | 0xFFFEu);
        var entities = info is null
            ? Array.Empty<WorldEntity>()
            : BuildEntitiesFromInfo(info, landblockId);

        return new LoadedLandblock(landblockId, block, entities);
    }

    /// <summary>
    /// Pure mapping from a parsed LandBlockInfo to a list of WorldEntity.
    /// Each Stab and BuildingInfo becomes one entity. Unsupported id types
    /// (neither GfxObj 0x01xxxxxx nor Setup 0x02xxxxxx) are silently skipped.
    /// MeshRefs is left empty at this stage — Task 5 populates it.
    /// </summary>
    public static IReadOnlyList<WorldEntity> BuildEntitiesFromInfo(LandBlockInfo info, uint landblockId = 0)
    {
        var result = new List<WorldEntity>(info.Objects.Count + info.Buildings.Count);

        // When landblockId is non-zero, namespace stab Ids globally:
        //   0xC0XXYY00 + n, where XX = lbX byte, YY = lbY byte
        // matching the scenery (0x80XXYY00) and interior (0x40XXYY00) patterns
        // in GameWindow.cs. The 0xC0 top byte distinguishes stabs from those.
        //
        // Pre-Tier-1 callers (existing tests) pass landblockId=0 and get the
        // legacy starting-from-1 monotonic Ids — compatible with their assertions
        // which check uniqueness within a single landblock.
        //
        // Latent: if a landblock has >256 stabs (rare), nextId overflows the
        // low byte and bleeds into the lbY byte → cross-LB collision. Same
        // pattern + same limitation as scenery/interior. Document but don't
        // fix in this commit — out of scope for the Tier 1 cache bug fix.
        uint stabIdBase = landblockId == 0
            ? 0u
            : 0xC0000000u | ((landblockId >> 24) & 0xFFu) << 16 | ((landblockId >> 16) & 0xFFu) << 8;
        uint nextId = stabIdBase == 0 ? 1u : stabIdBase + 1u;

        foreach (var stab in info.Objects)
        {
            if (!IsSupported(stab.Id))
                continue;
            var stabEntity = new WorldEntity
            {
                Id = nextId++,
                SourceGfxObjOrSetupId = stab.Id,
                Position = stab.Frame.Origin,
                Rotation = stab.Frame.Orientation,
                MeshRefs = Array.Empty<MeshRef>(),
            };
            stabEntity.RefreshAabb(); // A.5 T18: populate cached AABB at construction
            result.Add(stabEntity);
        }

        foreach (var building in info.Buildings)
        {
            if (!IsSupported(building.ModelId))
                continue;
            var buildingEntity = new WorldEntity
            {
                Id = nextId++,
                SourceGfxObjOrSetupId = building.ModelId,
                Position = building.Frame.Origin,
                Rotation = building.Frame.Orientation,
                MeshRefs = Array.Empty<MeshRef>(),
                IsBuildingShell = true,  // Phase A8: tag at source array boundary
            };
            buildingEntity.RefreshAabb(); // A.5 T18: populate cached AABB at construction
            result.Add(buildingEntity);
        }

        return result;
    }

    private static bool IsSupported(uint id)
    {
        var type = id & TypeMask;
        return type == GfxObjMask || type == SetupMask;
    }
}