using System;
using System.Collections.Generic;
using System.Numerics;
using System.Runtime.InteropServices;
using AcDream.Core.Meshing;
using AcDream.Core.Terrain;
using AcDream.Core.World;
using Chorizite.OpenGLSDLBackend.Lib;
using Silk.NET.OpenGL;

namespace AcDream.App.Rendering.Wb;

/// <summary>
/// Draws entities using WB's <see cref="ObjectRenderData"/> (a single global
/// VAO/VBO/IBO under modern rendering) with acdream's <see cref="TextureCache"/>
/// for bindless texture resolution and <see cref="AcSurfaceMetadataTable"/> for
/// translucency classification.
///
/// <para>
/// <b>Atlas-tier</b> entities (<c>ServerGuid == 0</c>): mesh data comes from WB's
/// <see cref="ObjectMeshManager"/> via <see cref="WbMeshAdapter.TryGetRenderData"/>.
/// Textures resolve through the bindless-suffixed
/// <see cref="TextureCache.GetOrUploadBindless"/> variants, returning 64-bit
/// resident handles stored in the per-group SSBO.
/// </para>
///
/// <para>
/// <b>Per-instance-tier</b> entities (<c>ServerGuid != 0</c>): mesh data also from
/// WB, but textures resolve through
/// <see cref="TextureCache.GetOrUploadWithPaletteOverrideBindless"/> with palette
/// and surface overrides applied. <see cref="AnimatedEntityState"/> is currently
/// unused at draw time — GameWindow's spawn path already bakes AnimPartChanges +
/// GfxObjDegradeResolver (Issue #47 close-detail mesh) into <c>MeshRefs</c>.
/// </para>
///
/// <para>
/// <b>GL strategy (N.5 — mandatory):</b> <c>glMultiDrawElementsIndirect</c> with SSBOs
/// and <c>GL_ARB_bindless_texture</c> + <c>GL_ARB_shader_draw_parameters</c>.
/// All visible (entity, batch) pairs are bucketed by <see cref="GroupKey"/>;
/// each group becomes one <c>DrawElementsIndirectCommand</c>. Three GPU buffers
/// are uploaded per frame: instance matrices (SSBO binding 0), per-group batch
/// metadata/texture handles (SSBO binding 1), and the indirect draw commands.
/// Two <c>glMultiDrawElementsIndirect</c> calls cover the opaque and transparent
/// passes respectively — one GL call per pass regardless of group count.
/// </para>
///
/// <para>
/// <b>Shader:</b> <c>mesh_modern</c> (bindless + <c>gl_DrawIDARB</c> /
/// <c>gl_BaseInstanceARB</c>). Missing bindless/draw-parameters throws
/// <see cref="NotSupportedException"/> at startup — there is no legacy fallback.
/// </para>
///
/// <para>
/// <b>Modern rendering assumption:</b> WB's <c>_useModernRendering</c> path (GL
/// 4.3 + bindless) puts every mesh in a single shared VAO/VBO/IBO and uses
/// <c>FirstIndex</c> + <c>BaseVertex</c> per batch. The dispatcher honors those
/// offsets inside each <c>DrawElementsIndirectCommand</c> via
/// <c>glMultiDrawElementsIndirect</c>.
/// </para>
/// </summary>
public sealed unsafe class WbDrawDispatcher : IDisposable
{
    private readonly GL _gl;
    private readonly Shader _shader;
    private readonly TextureCache _textures;
    private readonly WbMeshAdapter _meshAdapter;
    private readonly EntitySpawnAdapter _entitySpawnAdapter;

    private readonly BindlessSupport _bindless;

    // SSBO buffer ids
    private uint _instanceSsbo;
    private uint _batchSsbo;
    private uint _indirectBuffer;

    // Per-frame scratch arrays — Tasks 9-10 fully wire these.
    private float[] _instanceData = new float[256 * 16];      // mat4 floats per instance
    private BatchData[] _batchData = new BatchData[256];
    private DrawElementsIndirectCommand[] _indirectCommands = new DrawElementsIndirectCommand[256];

    private int _opaqueDrawCount;
    private int _transparentDrawCount;
    private int _transparentByteOffset;

    // std430 layout: ulong TextureHandle (uvec2) at offset 0, uint TextureLayer
    // at offset 8, uint Flags at offset 12. Total 16 bytes.
    // Pack=8 (not 4) because std430's uvec2 requires 8-byte alignment — Pack=4
    // works today by accident (TextureHandle is the first field, so offset 0 is
    // always 8-byte aligned), but adding a 4-byte field before TextureHandle
    // without bumping Pack would silently misalign the GPU struct.
    [StructLayout(LayoutKind.Sequential, Pack = 8)]
    private struct BatchData
    {
        public ulong TextureHandle;   // bindless handle (uvec2 in GLSL)
        public uint  TextureLayer;
        public uint  Flags;
    }

    // Per-frame scratch — reused across frames to avoid per-frame allocation.
    private readonly Dictionary<GroupKey, InstanceGroup> _groups = new();
    private readonly List<InstanceGroup> _opaqueDraws = new();
    private readonly List<InstanceGroup> _translucentDraws = new();

    // Per-entity-cull AABB radius. Conservative — covers most entities; large
    // outliers (long banners, tall columns) are still landblock-culled.
    private const float PerEntityCullRadius = 5.0f;

    private bool _disposed;

    // Diagnostic counters logged once per ~5s under ACDREAM_WB_DIAG=1.
    private int _entitiesSeen;
    private int _entitiesDrawn;
    private int _meshesMissing;
    private int _drawsIssued;
    private int _instancesIssued;
    private long _lastLogTick;

    // CPU + GPU timing for [WB-DIAG] under ACDREAM_WB_DIAG=1.
    private readonly System.Diagnostics.Stopwatch _cpuStopwatch = new();
    private readonly long[] _cpuSamples = new long[256];   // microseconds
    private int _cpuSampleCursor;
    private uint _gpuQueryOpaque;
    private uint _gpuQueryTransparent;
    private readonly long[] _gpuSamples = new long[256];   // microseconds
    private int _gpuSampleCursor;
    private bool _gpuQueriesInitialized;

    public WbDrawDispatcher(
        GL gl,
        Shader shader,
        TextureCache textures,
        WbMeshAdapter meshAdapter,
        EntitySpawnAdapter entitySpawnAdapter,
        BindlessSupport bindless)
    {
        ArgumentNullException.ThrowIfNull(gl);
        ArgumentNullException.ThrowIfNull(shader);
        ArgumentNullException.ThrowIfNull(textures);
        ArgumentNullException.ThrowIfNull(meshAdapter);
        ArgumentNullException.ThrowIfNull(entitySpawnAdapter);

        _gl = gl;
        _shader = shader;
        _textures = textures;
        _meshAdapter = meshAdapter;
        _entitySpawnAdapter = entitySpawnAdapter;

        _bindless = bindless ?? throw new ArgumentNullException(nameof(bindless));
        _instanceSsbo   = _gl.GenBuffer();
        _batchSsbo      = _gl.GenBuffer();
        _indirectBuffer = _gl.GenBuffer();
    }

    public static Matrix4x4 ComposePartWorldMatrix(
        Matrix4x4 entityWorld,
        Matrix4x4 animOverride,
        Matrix4x4 restPose)
        => restPose * animOverride * entityWorld;

    public void Draw(
        ICamera camera,
        IEnumerable<(uint LandblockId, Vector3 AabbMin, Vector3 AabbMax, IReadOnlyList<WorldEntity> Entities)> landblockEntries,
        FrustumPlanes? frustum = null,
        uint? neverCullLandblockId = null,
        HashSet<uint>? visibleCellIds = null,
        HashSet<uint>? animatedEntityIds = null)
    {
        _shader.Use();
        var vp = camera.View * camera.Projection;
        _shader.SetMatrix4("uViewProjection", vp);

        bool diag = string.Equals(Environment.GetEnvironmentVariable("ACDREAM_WB_DIAG"), "1", StringComparison.Ordinal);

        if (diag && !_gpuQueriesInitialized)
        {
            _gpuQueryOpaque      = _gl.GenQuery();
            _gpuQueryTransparent = _gl.GenQuery();
            _gpuQueriesInitialized = true;
        }

        // Always run the CPU stopwatch — cheap; only logged under diag.
        _cpuStopwatch.Restart();

        // Camera world-space position for front-to-back sort (perf #2). The view
        // matrix is the inverse of the camera's world transform, so the world
        // translation lives in the inverse's translation row.
        Vector3 camPos = Vector3.Zero;
        if (Matrix4x4.Invert(camera.View, out var invView))
            camPos = invView.Translation;

        // ── Phase 1: clear groups, walk entities, build groups ──────────────
        foreach (var grp in _groups.Values) grp.Matrices.Clear();

        var metaTable = _meshAdapter.MetadataTable;
        uint anyVao = 0;

        foreach (var entry in landblockEntries)
        {
            bool landblockVisible = frustum is null
                || entry.LandblockId == neverCullLandblockId
                || FrustumCuller.IsAabbVisible(frustum.Value, entry.AabbMin, entry.AabbMax);

            if (!landblockVisible && (animatedEntityIds is null || animatedEntityIds.Count == 0))
                continue;

            foreach (var entity in entry.Entities)
            {
                if (entity.MeshRefs.Count == 0) continue;

                bool isAnimated = animatedEntityIds?.Contains(entity.Id) == true;
                if (!landblockVisible && !isAnimated) continue;

                if (entity.ParentCellId.HasValue && visibleCellIds is not null
                    && !visibleCellIds.Contains(entity.ParentCellId.Value))
                    continue;

                // Per-entity AABB frustum cull (perf #3). Skips work for distant
                // entities even when their landblock is visible. Animated
                // entities bypass — they're tracked at landblock level + need
                // per-frame work for animation regardless. Conservative 5m
                // radius covers typical entity bounds.
                if (frustum is not null && !isAnimated && entry.LandblockId != neverCullLandblockId)
                {
                    var p = entity.Position;
                    var aMin = new Vector3(p.X - PerEntityCullRadius, p.Y - PerEntityCullRadius, p.Z - PerEntityCullRadius);
                    var aMax = new Vector3(p.X + PerEntityCullRadius, p.Y + PerEntityCullRadius, p.Z + PerEntityCullRadius);
                    if (!FrustumCuller.IsAabbVisible(frustum.Value, aMin, aMax))
                        continue;
                }

                if (diag) _entitiesSeen++;

                var entityWorld =
                    Matrix4x4.CreateFromQuaternion(entity.Rotation) *
                    Matrix4x4.CreateTranslation(entity.Position);

                // Compute palette-override hash ONCE per entity (perf #4).
                // Reused across every (part, batch) lookup so the FNV-1a fold
                // over SubPalettes runs once instead of N times. Zero when the
                // entity has no palette override (trees, scenery).
                ulong palHash = 0;
                if (entity.PaletteOverride is not null)
                    palHash = TextureCache.HashPaletteOverride(entity.PaletteOverride);

                bool drewAny = false;
                for (int partIdx = 0; partIdx < entity.MeshRefs.Count; partIdx++)
                {
                    // Note: GameWindow's spawn path already applies
                    // AnimPartChanges + GfxObjDegradeResolver (Issue #47 fix —
                    // close-detail mesh swap for humanoids) to MeshRefs. We
                    // trust MeshRefs as the source of truth here. AnimatedEntityState's
                    // overrides become relevant only for hot-swap (0xF625
                    // ObjDescEvent) which today rebuilds MeshRefs anyway.
                    var meshRef = entity.MeshRefs[partIdx];
                    ulong gfxObjId = meshRef.GfxObjId;

                    var renderData = _meshAdapter.TryGetRenderData(gfxObjId);
                    if (renderData is null)
                    {
                        if (diag) _meshesMissing++;
                        continue;
                    }
                    drewAny = true;
                    if (anyVao == 0) anyVao = renderData.VAO;

                    if (renderData.IsSetup && renderData.SetupParts.Count > 0)
                    {
                        foreach (var (partGfxObjId, partTransform) in renderData.SetupParts)
                        {
                            var partData = _meshAdapter.TryGetRenderData(partGfxObjId);
                            if (partData is null) continue;

                            var model = ComposePartWorldMatrix(
                                entityWorld, meshRef.PartTransform, partTransform);

                            ClassifyBatches(partData, partGfxObjId, model, entity, meshRef, palHash, metaTable);
                        }
                    }
                    else
                    {
                        var model = meshRef.PartTransform * entityWorld;
                        ClassifyBatches(renderData, gfxObjId, model, entity, meshRef, palHash, metaTable);
                    }
                }

                if (diag && drewAny) _entitiesDrawn++;
            }
        }

        // Nothing visible — skip the GL pass entirely.
        if (anyVao == 0)
        {
            _cpuStopwatch.Stop();
            if (diag) MaybeFlushDiag();
            return;
        }

        // ── Phase 3: assign FirstInstance per group, lay matrices contiguously, sort opaque ──
        int totalInstances = 0;
        foreach (var grp in _groups.Values) totalInstances += grp.Matrices.Count;
        if (totalInstances == 0)
        {
            _cpuStopwatch.Stop();
            if (diag) MaybeFlushDiag();
            return;
        }

        int needed = totalInstances * 16;
        if (_instanceData.Length < needed)
            _instanceData = new float[needed + 256 * 16];

        _opaqueDraws.Clear();
        _translucentDraws.Clear();

        int cursor = 0;
        foreach (var grp in _groups.Values)
        {
            if (grp.Matrices.Count == 0) continue;

            grp.FirstInstance = cursor;
            grp.InstanceCount = grp.Matrices.Count;

            // Use the first instance's translation as the group's representative
            // position for front-to-back sort (perf #2). Cheap heuristic; works
            // well when instances of one group are spatially coherent
            // (typical for trees in one landblock area, NPCs at one spawn).
            var first = grp.Matrices[0];
            var grpPos = new Vector3(first.M41, first.M42, first.M43);
            grp.SortDistance = Vector3.DistanceSquared(camPos, grpPos);

            for (int i = 0; i < grp.Matrices.Count; i++)
            {
                WriteMatrix(_instanceData, cursor * 16, grp.Matrices[i]);
                cursor++;
            }

            if (IsOpaque(grp.Translucency))
                _opaqueDraws.Add(grp);
            else
                _translucentDraws.Add(grp);
        }

        // Front-to-back sort for opaque pass: nearer groups draw first so the
        // depth test rejects fragments hidden behind them, reducing fragment
        // shader cost from overdraw on dense scenes (Holtburg courtyard,
        // Foundry interior).
        _opaqueDraws.Sort(static (a, b) => a.SortDistance.CompareTo(b.SortDistance));

        // ── Phase 4: build IndirectGroupInput list (opaque sorted, then translucent),
        //            fill via BuildIndirectArrays ──────────────────────────────────
        int totalDraws = _opaqueDraws.Count + _translucentDraws.Count;
        if (_batchData.Length < totalDraws)
            _batchData = new BatchData[totalDraws + 64];
        if (_indirectCommands.Length < totalDraws)
            _indirectCommands = new DrawElementsIndirectCommand[totalDraws + 64];

        var groupInputs = new List<IndirectGroupInput>(totalDraws);
        foreach (var g in _opaqueDraws) groupInputs.Add(ToInput(g));
        foreach (var g in _translucentDraws) groupInputs.Add(ToInput(g));

        // Cast _batchData (private BatchData) to public-mirror BatchDataPublic for BuildIndirectArrays.
        // Layout is asserted at test time (BatchDataPublic_LayoutMatchesPrivateBatchData test).
        var batchPublic = new BatchDataPublic[totalDraws];
        var layout = BuildIndirectArrays(groupInputs, _indirectCommands, batchPublic);

        // Copy back into _batchData
        for (int i = 0; i < totalDraws; i++)
        {
            _batchData[i] = new BatchData
            {
                TextureHandle = batchPublic[i].TextureHandle,
                TextureLayer  = batchPublic[i].TextureLayer,
                Flags         = batchPublic[i].Flags,
            };
        }
        _opaqueDrawCount       = layout.OpaqueCount;
        _transparentDrawCount  = layout.TransparentCount;
        _transparentByteOffset = layout.TransparentByteOffset;

        // ── Phase 5: upload three buffers ───────────────────────────────────
        fixed (float* ip = _instanceData)
            UploadSsbo(_instanceSsbo, 0, ip, totalInstances * 16 * sizeof(float));

        fixed (BatchData* bp = _batchData)
            UploadSsbo(_batchSsbo, 1, bp, totalDraws * sizeof(BatchData));

        fixed (DrawElementsIndirectCommand* cp = _indirectCommands)
        {
            _gl.BindBuffer(BufferTargetARB.DrawIndirectBuffer, _indirectBuffer);
            _gl.BufferData(BufferTargetARB.DrawIndirectBuffer,
                (nuint)(totalDraws * sizeof(DrawElementsIndirectCommand)), cp, BufferUsageARB.DynamicDraw);
        }

        // ── Phase 6: bind global VAO once ───────────────────────────────────
        _gl.BindVertexArray(anyVao);

        if (string.Equals(Environment.GetEnvironmentVariable("ACDREAM_NO_CULL"), "1", StringComparison.Ordinal))
            _gl.Disable(EnableCap.CullFace);

        // ── Phase 7: opaque pass ─────────────────────────────────────────────
        if (_opaqueDrawCount > 0)
        {
            _gl.Disable(EnableCap.Blend);
            _gl.DepthMask(true);
            _shader.SetInt("uRenderPass", 0);
            _gl.BindBuffer(BufferTargetARB.DrawIndirectBuffer, _indirectBuffer);
            if (diag && _gpuQueriesInitialized) _gl.BeginQuery(QueryTarget.TimeElapsed, _gpuQueryOpaque);
            _gl.MultiDrawElementsIndirect(
                PrimitiveType.Triangles,
                DrawElementsType.UnsignedShort,
                (void*)0,
                (uint)_opaqueDrawCount,
                (uint)DrawCommandStride);
            if (diag && _gpuQueriesInitialized) _gl.EndQuery(QueryTarget.TimeElapsed);
        }

        // ── Phase 8: transparent pass ────────────────────────────────────────
        if (_transparentDrawCount > 0)
        {
            _gl.Enable(EnableCap.Blend);
            _gl.BlendFunc(BlendingFactor.SrcAlpha, BlendingFactor.OneMinusSrcAlpha);
            _gl.DepthMask(false);
            _shader.SetInt("uRenderPass", 1);
            if (diag && _gpuQueriesInitialized) _gl.BeginQuery(QueryTarget.TimeElapsed, _gpuQueryTransparent);
            _gl.MultiDrawElementsIndirect(
                PrimitiveType.Triangles,
                DrawElementsType.UnsignedShort,
                (void*)_transparentByteOffset,
                (uint)_transparentDrawCount,
                (uint)DrawCommandStride);
            if (diag && _gpuQueriesInitialized) _gl.EndQuery(QueryTarget.TimeElapsed);
            _gl.DepthMask(true);
            _gl.Disable(EnableCap.Blend);
        }

        _gl.Disable(EnableCap.CullFace);
        _gl.BindVertexArray(0);

        _cpuStopwatch.Stop();

        if (diag)
        {
            long cpuUs = _cpuStopwatch.ElapsedTicks * 1_000_000L / System.Diagnostics.Stopwatch.Frequency;
            _cpuSamples[_cpuSampleCursor] = cpuUs;
            _cpuSampleCursor = (_cpuSampleCursor + 1) % _cpuSamples.Length;

            // Read GPU samples non-blocking; the result for the previous frame's
            // queries should be ready by now. If not, drop the sample (don't stall
            // the CPU waiting for the GPU).
            if (_gpuQueriesInitialized)
            {
                _gl.GetQueryObject(_gpuQueryOpaque, QueryObjectParameterName.ResultAvailable, out int avail);
                if (avail != 0)
                {
                    _gl.GetQueryObject(_gpuQueryOpaque, QueryObjectParameterName.Result, out ulong opaqueNs);
                    _gl.GetQueryObject(_gpuQueryTransparent, QueryObjectParameterName.Result, out ulong transNs);
                    long gpuUs = (long)((opaqueNs + transNs) / 1000UL);
                    _gpuSamples[_gpuSampleCursor] = gpuUs;
                    _gpuSampleCursor = (_gpuSampleCursor + 1) % _gpuSamples.Length;
                }
            }

            _drawsIssued     += _opaqueDrawCount + _transparentDrawCount;
            _instancesIssued += totalInstances;
            MaybeFlushDiag();
        }
    }

    private static IndirectGroupInput ToInput(InstanceGroup g) => new(
        IndexCount:    g.IndexCount,
        FirstIndex:    g.FirstIndex,
        BaseVertex:    g.BaseVertex,
        InstanceCount: g.InstanceCount,
        FirstInstance: g.FirstInstance,
        TextureHandle: g.BindlessTextureHandle,
        TextureLayer:  g.TextureLayer,
        Translucency:  g.Translucency);

    private unsafe void UploadSsbo(uint ssbo, uint binding, void* data, int byteCount)
    {
        _gl.BindBuffer(BufferTargetARB.ShaderStorageBuffer, ssbo);
        _gl.BufferData(BufferTargetARB.ShaderStorageBuffer, (nuint)byteCount, data, BufferUsageARB.DynamicDraw);
        _gl.BindBufferBase(BufferTargetARB.ShaderStorageBuffer, binding, ssbo);
    }

    private void MaybeFlushDiag()
    {
        long now = Environment.TickCount64;
        if (now - _lastLogTick > 5000)
        {
            long cpuMed = MedianMicros(_cpuSamples);
            long cpuP95 = Percentile95Micros(_cpuSamples);
            long gpuMed = MedianMicros(_gpuSamples);
            long gpuP95 = Percentile95Micros(_gpuSamples);
            Console.WriteLine(
                $"[WB-DIAG] entSeen={_entitiesSeen} entDrawn={_entitiesDrawn} meshMissing={_meshesMissing} drawsIssued={_drawsIssued} instances={_instancesIssued} groups={_groups.Count} " +
                $"cpu_us={cpuMed}m/{cpuP95}p95 gpu_us={gpuMed}m/{gpuP95}p95");
            _entitiesSeen = _entitiesDrawn = _meshesMissing = _drawsIssued = _instancesIssued = 0;
            _lastLogTick = now;
            // Don't reset the sample buffers — they're a moving window of the
            // last 256 frames; clearing per 5s flush would lose recent history.
        }
    }

    private static long MedianMicros(long[] samples)
    {
        var copy = (long[])samples.Clone();
        Array.Sort(copy);
        int nz = 0;
        foreach (var v in copy) if (v > 0) nz++;
        if (nz == 0) return 0;
        return copy[copy.Length - nz / 2];
    }

    private static long Percentile95Micros(long[] samples)
    {
        var copy = (long[])samples.Clone();
        Array.Sort(copy);
        int nz = 0;
        foreach (var v in copy) if (v > 0) nz++;
        if (nz == 0) return 0;
        int idx = copy.Length - 1 - (int)(nz * 0.05);
        return copy[idx];
    }

    private void ClassifyBatches(
        ObjectRenderData renderData,
        ulong gfxObjId,
        Matrix4x4 model,
        WorldEntity entity,
        MeshRef meshRef,
        ulong palHash,
        AcSurfaceMetadataTable metaTable)
    {
        for (int batchIdx = 0; batchIdx < renderData.Batches.Count; batchIdx++)
        {
            var batch = renderData.Batches[batchIdx];

            TranslucencyKind translucency;
            if (metaTable.TryLookup(gfxObjId, batchIdx, out var meta))
            {
                translucency = meta.Translucency;
            }
            else
            {
                translucency = batch.IsAdditive ? TranslucencyKind.Additive
                    : batch.IsTransparent ? TranslucencyKind.AlphaBlend
                    : TranslucencyKind.Opaque;
            }

            ulong texHandle = ResolveTexture(entity, meshRef, batch, palHash);
            if (texHandle == 0) continue;

            // TextureLayer is always 0 for per-instance composites; non-zero when
            // WB atlas is adopted in N.6+ and batches reference a shared atlas layer.
            uint texLayer = 0;

            var key = new GroupKey(
                batch.IBO, batch.FirstIndex, (int)batch.BaseVertex,
                batch.IndexCount, texHandle, texLayer, translucency);

            if (!_groups.TryGetValue(key, out var grp))
            {
                grp = new InstanceGroup
                {
                    Ibo = batch.IBO,
                    FirstIndex = batch.FirstIndex,
                    BaseVertex = (int)batch.BaseVertex,
                    IndexCount = batch.IndexCount,
                    BindlessTextureHandle = texHandle,
                    TextureLayer = texLayer,
                    Translucency = translucency,
                };
                _groups[key] = grp;
            }
            grp.Matrices.Add(model);
        }
    }

    private ulong ResolveTexture(WorldEntity entity, MeshRef meshRef, ObjectRenderBatch batch, ulong palHash)
    {
        uint surfaceId = batch.Key.SurfaceId;
        if (surfaceId == 0 || surfaceId == 0xFFFFFFFF) return 0;

        uint overrideOrigTex = 0;
        bool hasOrigTexOverride = meshRef.SurfaceOverrides is not null
            && meshRef.SurfaceOverrides.TryGetValue(surfaceId, out overrideOrigTex);
        uint? origTexOverride = hasOrigTexOverride ? overrideOrigTex : (uint?)null;

        if (entity.PaletteOverride is not null)
        {
            return _textures.GetOrUploadWithPaletteOverrideBindless(
                surfaceId, origTexOverride, entity.PaletteOverride, palHash);
        }
        else if (hasOrigTexOverride)
        {
            return _textures.GetOrUploadWithOrigTextureOverrideBindless(surfaceId, overrideOrigTex);
        }
        else
        {
            return _textures.GetOrUploadBindless(surfaceId);
        }
    }

    private static void WriteMatrix(float[] buf, int offset, in Matrix4x4 m)
    {
        buf[offset + 0]  = m.M11; buf[offset + 1]  = m.M12; buf[offset + 2]  = m.M13; buf[offset + 3]  = m.M14;
        buf[offset + 4]  = m.M21; buf[offset + 5]  = m.M22; buf[offset + 6]  = m.M23; buf[offset + 7]  = m.M24;
        buf[offset + 8]  = m.M31; buf[offset + 9]  = m.M32; buf[offset + 10] = m.M33; buf[offset + 11] = m.M34;
        buf[offset + 12] = m.M41; buf[offset + 13] = m.M42; buf[offset + 14] = m.M43; buf[offset + 15] = m.M44;
    }

    public void Dispose()
    {
        if (_disposed) return;
        _disposed = true;
        _gl.DeleteBuffer(_instanceSsbo);
        _gl.DeleteBuffer(_batchSsbo);
        _gl.DeleteBuffer(_indirectBuffer);
        if (_gpuQueriesInitialized)
        {
            _gl.DeleteQuery(_gpuQueryOpaque);
            _gl.DeleteQuery(_gpuQueryTransparent);
        }
    }

    // ── Public types + helpers for BuildIndirectArrays (Task 9) ─────────────
    //
    // These are public so the pure-CPU unit tests in AcDream.Core.Tests can
    // exercise BuildIndirectArrays without needing a GL context.

    /// <summary>
    /// Stride in bytes of <c>DrawElementsIndirectCommand</c> in the indirect buffer.
    /// 5 × <c>uint</c> = 20 bytes. Tests and callers reference this symbolically
    /// rather than hard-coding <c>20</c> so a layout change produces a compile error.
    /// </summary>
    public const int DrawCommandStride = 20; // sizeof(DrawElementsIndirectCommand): 5 × uint

    /// <summary>
    /// Public view of the per-group inputs to <see cref="BuildIndirectArrays"/> — used in tests.
    /// </summary>
    public readonly record struct IndirectGroupInput(
        int IndexCount,
        uint FirstIndex,
        int BaseVertex,
        int InstanceCount,
        int FirstInstance,
        ulong TextureHandle,
        uint TextureLayer,
        TranslucencyKind Translucency);

    /// <summary>
    /// Public mirror of the per-group <see cref="BatchData"/> uploaded to the SSBO.
    /// Tests verify the layout. Same field shape as the private BatchData.
    /// </summary>
    [StructLayout(LayoutKind.Sequential, Pack = 8)]
    public struct BatchDataPublic
    {
        public ulong TextureHandle;
        public uint  TextureLayer;
        public uint  Flags;
    }

    /// <summary>Result of <see cref="BuildIndirectArrays"/>.</summary>
    public readonly record struct IndirectLayoutResult(
        int OpaqueCount,
        int TransparentCount,
        int TransparentByteOffset);

    /// <summary>
    /// Lays out the indirect commands + parallel BatchData array contiguously:
    /// opaque section first (caller sorts before calling), transparent section second.
    /// Pure CPU, no GL state. Caller passes pre-sized scratch arrays.
    /// </summary>
    /// <remarks>
    /// Classification: Opaque + ClipMap → opaque pass (ClipMap uses discard, not
    /// blending). Everything else (AlphaBlend, Additive, InvAlpha) → transparent pass.
    /// </remarks>
    public static IndirectLayoutResult BuildIndirectArrays(
        IReadOnlyList<IndirectGroupInput> groups,
        DrawElementsIndirectCommand[] indirectScratch,
        BatchDataPublic[] batchScratch)
    {
        int opaqueCount = 0;
        int transparentCount = 0;

        foreach (var g in groups)
        {
            if (IsOpaque(g.Translucency)) opaqueCount++;
            else transparentCount++;
        }

        int oi = 0;        // opaque write cursor (fills [0..opaqueCount))
        int ti = opaqueCount; // transparent write cursor (fills [opaqueCount..end))

        foreach (var g in groups)
        {
            var dec = new DrawElementsIndirectCommand
            {
                Count         = (uint)g.IndexCount,
                InstanceCount = (uint)g.InstanceCount,
                FirstIndex    = g.FirstIndex,
                BaseVertex    = g.BaseVertex,
                BaseInstance  = (uint)g.FirstInstance,
            };
            var bd = new BatchDataPublic
            {
                TextureHandle = g.TextureHandle,
                TextureLayer  = g.TextureLayer,
                Flags         = 0,
            };

            if (IsOpaque(g.Translucency))
            {
                indirectScratch[oi] = dec;
                batchScratch[oi]    = bd;
                oi++;
            }
            else
            {
                indirectScratch[ti] = dec;
                batchScratch[ti]    = bd;
                ti++;
            }
        }

        return new IndirectLayoutResult(opaqueCount, transparentCount, opaqueCount * DrawCommandStride);
    }

    /// <summary>
    /// Public test shim for <see cref="IsOpaque"/>. Locks in the N.5 Decision 2
    /// translucency partition: Opaque + ClipMap → opaque indirect; AlphaBlend +
    /// Additive + InvAlpha → transparent indirect.
    /// </summary>
    public static bool IsOpaquePublic(TranslucencyKind t) => IsOpaque(t);

    private static bool IsOpaque(TranslucencyKind t)
        => t == TranslucencyKind.Opaque || t == TranslucencyKind.ClipMap;

    // ────────────────────────────────────────────────────────────────────────

    private readonly record struct GroupKey(
        uint Ibo,
        uint FirstIndex,
        int BaseVertex,
        int IndexCount,
        ulong BindlessTextureHandle,
        uint TextureLayer,
        TranslucencyKind Translucency);

    private sealed class InstanceGroup
    {
        public uint Ibo;
        public uint FirstIndex;
        public int BaseVertex;
        public int IndexCount;
        public ulong BindlessTextureHandle;   // 64-bit (was uint TextureHandle in N.4)
        public uint TextureLayer;             // 0 for per-instance composites; non-zero when WB atlas is adopted in N.6+
        public TranslucencyKind Translucency;
        public int FirstInstance;   // offset into the shared instance VBO (in instances, not bytes)
        public int InstanceCount;
        public float SortDistance;  // squared distance from camera to first instance, for opaque sort
        public readonly List<Matrix4x4> Matrices = new();
    }
}