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

using System.Collections.Generic;
using System.Numerics;
using AcDream.Core.World;

namespace AcDream.App.Rendering;

/// <summary>
/// Splits a frame's landblock entities into the draw buckets used by the
/// retail-style DrawInside flood.
///
/// <para>T1 (fused BR-2/3, 2026-06-11) — retail draw-order contract: the
/// frame draws STATIC world first (terrain, building shells, scenery, then
/// flooded interior cells + their static object lists), and every DYNAMIC
/// (server-spawned: player, NPCs, doors, items) draws LAST, depth-tested,
/// never hard-clipped. This is what makes the aperture depth punch safe —
/// when the punch erases depth inside a doorway, no dynamic has been drawn
/// yet, so nothing visible is destroyed (retail: objects draw per cell AFTER
/// cells, PView::DrawCells epilogue Ghidra 0x005a4840; the first BR-2 attempt
/// punched after dynamics and erased the player, reverted 88be519).</para>
///
/// <list type="bullet">
///   <item><see cref="Result.ByCell"/> — indoor STATICS (dat-baked, ServerGuid==0)
///   per visible cell, drawn with their cell.</item>
///   <item><see cref="Result.OutdoorStatic"/> — outdoor statics (building
///   shells, scenery stabs), drawn with the world/landscape pass.</item>
///   <item><see cref="Result.Dynamics"/> — ALL server-spawned entities
///   (ServerGuid != 0) regardless of cell, plus unresolved-cell live entities;
///   drawn in the frame's single LAST entity pass.</item>
/// </list>
/// </summary>
public static class InteriorEntityPartition
{
    public sealed class Result
    {
        public Dictionary<uint, List<WorldEntity>> ByCell { get; } = new();
        public List<WorldEntity> OutdoorStatic { get; } = new();
        public List<WorldEntity> Dynamics { get; } = new();
    }

    public static Result Partition(
        HashSet<uint> visibleCells,
        IEnumerable<(uint LandblockId, Vector3 AabbMin, Vector3 AabbMax,
                     IReadOnlyList<WorldEntity> Entities,
                     IReadOnlyDictionary<uint, WorldEntity>? AnimatedById)> landblockEntries)
    {
        var result = new Result();
        foreach (var entry in landblockEntries)
        {
            foreach (var e in entry.Entities)
            {
                if (e.MeshRefs.Count == 0) continue;

                // Retail contract: every server-spawned entity is a DYNAMIC
                // and draws in the last pass — indoor, outdoor, or unresolved.
                if (e.ServerGuid != 0)
                {
                    result.Dynamics.Add(e);
                }
                else if (e.ParentCellId is uint cell && IsIndoorCellId(cell))
                {
                    if (!visibleCells.Contains(cell))
                        continue;
                    if (!result.ByCell.TryGetValue(cell, out var list))
                        result.ByCell[cell] = list = new List<WorldEntity>();
                    list.Add(e);
                }
                else
                {
                    result.OutdoorStatic.Add(e);
                }
            }
        }
        return result;
    }

    /// <summary>Shared indoor classification — keep DrawDynamicsLast, the
    /// outside-stage assignment (#118), and the partition in lockstep.</summary>
    public static bool IsIndoorCellId(uint cellId)
    {
        uint low = cellId & 0xFFFFu;
        return low >= 0x0100u && low != 0xFFFFu;
    }

    /// <inheritdoc cref="IsIndoorCellId(uint)"/>
    public static bool IsIndoorCellId(uint? cellId) => cellId is uint c && IsIndoorCellId(c);
}