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();

        // MP-Alloc: scratch for PruneEmptyCellBuckets — reused across frames
        // so pruning itself doesn't allocate.
        private readonly List<uint> _emptyCellScratch = new();

        /// <summary>
        /// MP-Alloc (2026-07-05): clear every collection in place for reuse
        /// by <see cref="Partition(Result, HashSet{uint}, IEnumerable{ValueTuple})"/>.
        /// The per-cell lists inside <see cref="ByCell"/> are cleared and
        /// KEPT (not removed) so a steady-state frame with the same visible
        /// cell set reuses the same List&lt;WorldEntity&gt; instances instead
        /// of reallocating one per cell every frame.
        /// </summary>
        internal void ClearForReuse()
        {
            foreach (var list in ByCell.Values)
                list.Clear();
            OutdoorStatic.Clear();
            Dynamics.Clear();
        }

        /// <summary>
        /// MP-Alloc: drop any cell bucket that ended this frame with zero
        /// entries (either newly emptied, or a leftover key from a previous
        /// frame's visible-cell set that this frame never touched). Keeps
        /// ByCell.Count / .Keys bit-identical to the old always-fresh-
        /// Dictionary behavior — callers that inspect key presence/count
        /// directly (not just TryGetValue) must see exactly the cells that
        /// actually received at least one static this frame.
        /// </summary>
        internal void PruneEmptyCellBuckets()
        {
            _emptyCellScratch.Clear();
            foreach (var (cellId, list) in ByCell)
            {
                if (list.Count == 0)
                    _emptyCellScratch.Add(cellId);
            }
            foreach (var cellId in _emptyCellScratch)
                ByCell.Remove(cellId);
        }
    }

    /// <summary>
    /// Allocating overload — always returns a brand-new <see cref="Result"/>.
    /// Kept for tests and any one-shot caller; the per-frame render path
    /// uses the <see cref="Partition(Result, HashSet{uint}, IEnumerable{ValueTuple})"/>
    /// reuse overload instead (see <see cref="RetailPViewRenderer"/>).
    /// </summary>
    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();
        Partition(result, visibleCells, landblockEntries);
        return result;
    }

    /// <summary>
    /// MP-Alloc (2026-07-05): reuse overload. Clears <paramref name="result"/>
    /// in place (see <see cref="Result.ClearForReuse"/>) and refills it,
    /// reusing each cell's existing <c>List&lt;WorldEntity&gt;</c> when the
    /// cell key survives from the previous frame instead of allocating a new
    /// one — the per-cell dictionary entries persist across frames (cleared,
    /// never removed) since the visible-cell set is usually stable frame to
    /// frame. Identical partitioning output to the allocating overload; only
    /// the backing storage is reused.
    /// </summary>
    public static void Partition(
        Result result,
        HashSet<uint> visibleCells,
        IEnumerable<(uint LandblockId, Vector3 AabbMin, Vector3 AabbMax,
                     IReadOnlyList<WorldEntity> Entities,
                     IReadOnlyDictionary<uint, WorldEntity>? AnimatedById)> landblockEntries)
    {
        result.ClearForReuse();
        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);
                }
            }
        }

        result.PruneEmptyCellBuckets();
    }

    /// <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);
}