// src/AcDream.App/Rendering/InstancedMeshRenderer.cs
//
// True instanced rendering for static-object meshes.
//   Groups entities by GfxObjId. All instance model matrices are written into
//   a single shared instance VBO once per frame. Each sub-mesh is drawn with
//   DrawElementsInstanced — one GL draw call per (GfxObj × sub-mesh) instead
//   of one per entity. For a scene with N unique GfxObjs and M total entities
//   this reduces draw calls from M*subMeshes to N*subMeshes.
//
// Matrix layout:
//   System.Numerics.Matrix4x4 is row-major. Written to the float[] buffer in
//   natural memory order (M11..M44). The GLSL shader reads 4 vec4 attributes
//   (aInstanceRow0-3) and constructs mat4(row0, row1, row2, row3). Because
//   GLSL mat4() takes column vectors, the rows of the C# matrix become the
//   columns of the GLSL mat4 — which is the same transpose that UniformMatrix4
//   with transpose=false produces. Visual result is identical to the old
//   SetMatrix4("uModel", ...) path.
//
// Architecture note: public API matches StaticMeshRenderer so GameWindow only
// needs to update the shader and uniform setup at the call sites.
using System.Numerics;
using System.Runtime.InteropServices;
using AcDream.Core.Meshing;
using AcDream.Core.Terrain;
using AcDream.Core.World;
using Silk.NET.OpenGL;

namespace AcDream.App.Rendering;

public sealed unsafe class InstancedMeshRenderer : IDisposable
{
    private readonly GL _gl;
    private readonly Shader _shader;
    private readonly TextureCache _textures;

    // One GPU bundle per unique GfxObj id. Each GfxObj can have multiple sub-meshes.
    private readonly Dictionary<uint, List<SubMeshGpu>> _gpuByGfxObj = new();

    // Shared instance VBO — filled every frame with all instance model matrices.
    private readonly uint _instanceVbo;

    // Per-frame scratch: reused float buffer for instance matrix data.
    // 16 floats per mat4.  Grown on demand; never shrunk.
    private float[] _instanceBuffer = new float[256 * 16]; // start at 256 instances

    // ── Instance grouping scratch ─────────────────────────────────────────────
    //
    // Reused every frame to avoid per-frame allocation.
    //
    // **Group key = (GfxObjId, PaletteOverrideHash, SurfaceOverridesHash).**
    //
    // An earlier implementation grouped on <c>GfxObjId</c> alone and resolved
    // the per-sub-mesh texture from the first instance in the group — which
    // is fine for scenery where every tree shares the same palette, but
    // utterly broken for NPCs: every humanoid uses the same base body
    // GfxObjs and they all piled into one group, so the first NPC's palette
    // was used for every NPC in the frame. Frustum culling + iteration
    // order meant that "first NPC" changed as the camera turned — producing
    // the "NPC clothing changes when I turn" symptom.
    //
    // Now we also key by the entity's PaletteOverride + per-MeshRef
    // SurfaceOverrides signature so only entities that decode to the
    // SAME texture for every sub-mesh can share a batch. Entities with
    // unique appearance fall to single-instance groups (still correct,
    // marginally slower than true instancing).
    private readonly Dictionary<GroupKey, InstanceGroup> _groups = new();

    private readonly record struct GroupKey(uint GfxObjId, ulong TextureSignature);

    public InstancedMeshRenderer(GL gl, Shader shader, TextureCache textures)
    {
        _gl = gl;
        _shader = shader;
        _textures = textures;

        _instanceVbo = _gl.GenBuffer();
    }

    // ── Upload ────────────────────────────────────────────────────────────────

    public void EnsureUploaded(uint gfxObjId, IReadOnlyList<GfxObjSubMesh> subMeshes)
    {
        if (_gpuByGfxObj.ContainsKey(gfxObjId))
            return;

        var list = new List<SubMeshGpu>(subMeshes.Count);
        foreach (var sm in subMeshes)
            list.Add(UploadSubMesh(sm));
        _gpuByGfxObj[gfxObjId] = list;
    }

    private SubMeshGpu UploadSubMesh(GfxObjSubMesh sm)
    {
        uint vao = _gl.GenVertexArray();
        _gl.BindVertexArray(vao);

        // ── Vertex buffer (positions, normals, UVs) ───────────────────────────
        uint vbo = _gl.GenBuffer();
        _gl.BindBuffer(BufferTargetARB.ArrayBuffer, vbo);
        fixed (void* p = sm.Vertices)
            _gl.BufferData(BufferTargetARB.ArrayBuffer,
                (nuint)(sm.Vertices.Length * sizeof(Vertex)), p, BufferUsageARB.StaticDraw);

        uint stride = (uint)sizeof(Vertex);
        _gl.EnableVertexAttribArray(0);
        _gl.VertexAttribPointer(0, 3, VertexAttribPointerType.Float, false, stride, (void*)0);
        _gl.EnableVertexAttribArray(1);
        _gl.VertexAttribPointer(1, 3, VertexAttribPointerType.Float, false, stride, (void*)(3 * sizeof(float)));
        _gl.EnableVertexAttribArray(2);
        _gl.VertexAttribPointer(2, 2, VertexAttribPointerType.Float, false, stride, (void*)(6 * sizeof(float)));
        // Note: location 3 (uint TerrainLayer) is NOT used by mesh_instanced.vert;
        // that slot is reserved for per-instance mat4 row 0 from the instance VBO.

        // ── Index buffer ──────────────────────────────────────────────────────
        uint ebo = _gl.GenBuffer();
        _gl.BindBuffer(BufferTargetARB.ElementArrayBuffer, ebo);
        fixed (void* p = sm.Indices)
            _gl.BufferData(BufferTargetARB.ElementArrayBuffer,
                (nuint)(sm.Indices.Length * sizeof(uint)), p, BufferUsageARB.StaticDraw);

        // ── Per-instance model matrix (locations 3-6) ─────────────────────────
        // Bind the shared instance VBO. The VAO captures this binding at each
        // attribute location. At draw time we re-call VertexAttribPointer with
        // the per-group byte offset (to address different groups in the VBO
        // without DrawElementsInstancedBaseInstance).
        _gl.BindBuffer(BufferTargetARB.ArrayBuffer, _instanceVbo);
        // mat4 = 4 × vec4, stride = 64 bytes, divisor = 1 (advance once per instance)
        for (uint row = 0; row < 4; row++)
        {
            uint loc = 3 + row;
            _gl.EnableVertexAttribArray(loc);
            _gl.VertexAttribPointer(loc, 4, VertexAttribPointerType.Float, false, 64, (void*)(row * 16));
            _gl.VertexAttribDivisor(loc, 1);
        }

        _gl.BindVertexArray(0);

        return new SubMeshGpu
        {
            Vao = vao,
            Vbo = vbo,
            Ebo = ebo,
            IndexCount = sm.Indices.Length,
            SurfaceId = sm.SurfaceId,
            Translucency = sm.Translucency,
        };
    }

    // ── Draw ──────────────────────────────────────────────────────────────────

    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,
        // L-fix1 (2026-04-28): set of entity ids that should bypass the
        // landblock-level frustum cull. Animated entities (other
        // players, NPCs, monsters) are always rendered if their
        // landblock is loaded — without this they vanish whenever the
        // camera rotates away from their landblock, even though
        // they're within visible distance of the player. Pass null /
        // empty to keep the previous "cull everything by landblock"
        // behavior.
        HashSet<uint>? animatedEntityIds = null)
    {
        _shader.Use();

        var vp = camera.View * camera.Projection;
        _shader.SetMatrix4("uViewProjection", vp);

        // Phase G: lighting + ambient + fog are owned by the
        // SceneLighting UBO (binding=1) uploaded once per frame by
        // GameWindow. The instanced mesh fragment shader reads it
        // directly — no per-draw uniform uploads needed.

        // ── Collect and group instances ───────────────────────────────────────
        CollectGroups(landblockEntries, frustum, neverCullLandblockId, visibleCellIds, animatedEntityIds);

        // ── Build and upload the instance buffer ──────────────────────────────
        // Count total instances.
        int totalInstances = 0;
        foreach (var grp in _groups.Values)
            totalInstances += grp.Count;

        // Grow the scratch buffer if needed.
        int needed = totalInstances * 16;
        if (_instanceBuffer.Length < needed)
            _instanceBuffer = new float[needed + 256 * 16]; // extra headroom

        // Write all groups contiguously. Record each group's starting offset
        // (in units of instances, not bytes) so we can address them at draw time.
        int instanceOffset = 0;
        foreach (var grp in _groups.Values)
        {
            grp.BufferOffset = instanceOffset;
            foreach (ref readonly var inst in CollectionsMarshal.AsSpan(grp.Entries))
                WriteMatrix(_instanceBuffer, instanceOffset++ * 16, inst.Model);
        }

        // Upload all instance data in a single DynamicDraw call.
        if (totalInstances > 0)
        {
            _gl.BindBuffer(BufferTargetARB.ArrayBuffer, _instanceVbo);
            fixed (void* p = _instanceBuffer)
                _gl.BufferData(BufferTargetARB.ArrayBuffer,
                    (nuint)(totalInstances * 16 * sizeof(float)), p, BufferUsageARB.DynamicDraw);
        }

        // ── Pass 1: Opaque + ClipMap ──────────────────────────────────────────
        // Diagnostic: ACDREAM_NO_CULL=1 disables backface culling entirely.
        if (string.Equals(Environment.GetEnvironmentVariable("ACDREAM_NO_CULL"), "1", StringComparison.Ordinal))
        {
            _gl.Disable(EnableCap.CullFace);
        }
        foreach (var (key, grp) in _groups)
        {
            if (!_gpuByGfxObj.TryGetValue(key.GfxObjId, out var subMeshes))
                continue;

            bool hasOpaqueSubMesh = false;
            foreach (var sub in subMeshes)
            {
                if (sub.Translucency == TranslucencyKind.Opaque ||
                    sub.Translucency == TranslucencyKind.ClipMap)
                {
                    hasOpaqueSubMesh = true;
                    break;
                }
            }
            if (!hasOpaqueSubMesh) continue;

            // For this group, instance data starts at grp.BufferOffset in the VBO.
            // We need to tell the VAO to read from that offset.
            uint byteOffset = (uint)(grp.BufferOffset * 64); // 64 bytes per mat4

            foreach (var sub in subMeshes)
            {
                if (sub.Translucency != TranslucencyKind.Opaque &&
                    sub.Translucency != TranslucencyKind.ClipMap)
                    continue;

                _shader.SetInt("uTranslucencyKind", (int)sub.Translucency);

                // Bind VAO + re-point instance attributes to the group's slice
                // in the shared VBO. This updates the VAO's stored offset for
                // locations 3-6 without touching the vertex or index bindings.
                _gl.BindVertexArray(sub.Vao);
                _gl.BindBuffer(BufferTargetARB.ArrayBuffer, _instanceVbo);
                for (uint row = 0; row < 4; row++)
                {
                    _gl.VertexAttribPointer(3 + row, 4, VertexAttribPointerType.Float,
                        false, 64, (void*)(byteOffset + row * 16));
                }

                // Resolve texture from the first instance (all instances in this
                // group share the same GfxObj so they have compatible overrides
                // only in the degenerate case of mixed-palette entities using the
                // same GfxObj — rare enough to accept the approximation here).
                if (grp.Count == 0) continue;
                var firstEntry = grp.Entries[0];
                uint tex = ResolveTex(firstEntry.Entity, firstEntry.MeshRef, sub);
                _gl.ActiveTexture(TextureUnit.Texture0);
                _gl.BindTexture(TextureTarget.Texture2D, tex);

                _gl.DrawElementsInstanced(PrimitiveType.Triangles,
                    (uint)sub.IndexCount,
                    DrawElementsType.UnsignedInt,
                    (void*)0,
                    (uint)grp.Count);
            }
        }

        // ── Pass 2: Translucent (AlphaBlend, Additive, InvAlpha) ─────────────
        _gl.Enable(EnableCap.Blend);
        _gl.DepthMask(false);
        // Diagnostic: ACDREAM_NO_CULL=1 disables backface culling (used 2026-05-01
        // to test if our mesh winding (0,i,i+1) vs ACME's (i+1,i,0) is causing
        // visible polygons to be culled, especially around the neck/coat seam).
        if (string.Equals(Environment.GetEnvironmentVariable("ACDREAM_NO_CULL"), "1", StringComparison.Ordinal))
        {
            _gl.Disable(EnableCap.CullFace);
        }
        else
        {
            _gl.Enable(EnableCap.CullFace);
            _gl.CullFace(TriangleFace.Back);
            _gl.FrontFace(FrontFaceDirection.Ccw);
        }

        foreach (var (key, grp) in _groups)
        {
            if (!_gpuByGfxObj.TryGetValue(key.GfxObjId, out var subMeshes))
                continue;

            bool hasTranslucentSubMesh = false;
            foreach (var sub in subMeshes)
            {
                if (sub.Translucency != TranslucencyKind.Opaque &&
                    sub.Translucency != TranslucencyKind.ClipMap)
                {
                    hasTranslucentSubMesh = true;
                    break;
                }
            }
            if (!hasTranslucentSubMesh) continue;

            uint byteOffset = (uint)(grp.BufferOffset * 64);

            foreach (var sub in subMeshes)
            {
                if (sub.Translucency == TranslucencyKind.Opaque ||
                    sub.Translucency == TranslucencyKind.ClipMap)
                    continue;

                switch (sub.Translucency)
                {
                    case TranslucencyKind.Additive:
                        _gl.BlendFunc(BlendingFactor.SrcAlpha, BlendingFactor.One);
                        break;
                    case TranslucencyKind.InvAlpha:
                        _gl.BlendFunc(BlendingFactor.OneMinusSrcAlpha, BlendingFactor.SrcAlpha);
                        break;
                    default: // AlphaBlend
                        _gl.BlendFunc(BlendingFactor.SrcAlpha, BlendingFactor.OneMinusSrcAlpha);
                        break;
                }

                _shader.SetInt("uTranslucencyKind", (int)sub.Translucency);

                _gl.BindVertexArray(sub.Vao);
                _gl.BindBuffer(BufferTargetARB.ArrayBuffer, _instanceVbo);
                for (uint row = 0; row < 4; row++)
                {
                    _gl.VertexAttribPointer(3 + row, 4, VertexAttribPointerType.Float,
                        false, 64, (void*)(byteOffset + row * 16));
                }

                if (grp.Count == 0) continue;
                var firstEntry = grp.Entries[0];
                uint tex = ResolveTex(firstEntry.Entity, firstEntry.MeshRef, sub);
                _gl.ActiveTexture(TextureUnit.Texture0);
                _gl.BindTexture(TextureTarget.Texture2D, tex);

                _gl.DrawElementsInstanced(PrimitiveType.Triangles,
                    (uint)sub.IndexCount,
                    DrawElementsType.UnsignedInt,
                    (void*)0,
                    (uint)grp.Count);
            }
        }

        // Restore default GL state.
        _gl.DepthMask(true);
        _gl.Disable(EnableCap.Blend);
        _gl.Disable(EnableCap.CullFace);
        _gl.BindVertexArray(0);
    }

    // ── Grouping ──────────────────────────────────────────────────────────────

    /// <summary>
    /// Iterates all visible landblock entries and groups every (entity, meshRef)
    /// pair by GfxObjId. Clears previous frame's groups before filling.
    /// </summary>
    private void CollectGroups(
        IEnumerable<(uint LandblockId, Vector3 AabbMin, Vector3 AabbMax, IReadOnlyList<WorldEntity> Entities)> landblockEntries,
        FrustumPlanes? frustum,
        uint? neverCullLandblockId,
        HashSet<uint>? visibleCellIds,
        HashSet<uint>? animatedEntityIds)
    {
        foreach (var grp in _groups.Values)
            grp.Entries.Clear();

        foreach (var entry in landblockEntries)
        {
            // L-fix1 (2026-04-28): the landblock cull decision is now
            // PER-LANDBLOCK boolean, not a continue. We still need to
            // walk the entity list because animated entities (in
            // animatedEntityIds) bypass the cull and render anyway.
            bool landblockVisible = frustum is null
                                  || entry.LandblockId == neverCullLandblockId
                                  || FrustumCuller.IsAabbVisible(frustum.Value, entry.AabbMin, entry.AabbMax);

            // Fast path: no animated entities globally → if landblock is
            // culled, skip the whole entity list (preserves the original
            // O(visible-landblocks) cost when the caller doesn't care
            // about animated bypass).
            if (!landblockVisible && (animatedEntityIds is null || animatedEntityIds.Count == 0))
                continue;

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

                // L-fix1: when the landblock is frustum-culled, only
                // render entities flagged as animated. This keeps
                // remote players / NPCs / monsters visible even when
                // their landblock rotates out of the view frustum.
                bool isAnimated = animatedEntityIds?.Contains(entity.Id) == true;
                if (!landblockVisible && !isAnimated)
                    continue;

                // Step 4: portal visibility filter. If we have a visible cell set,
                // skip interior entities whose parent cell isn't visible.
                // visibleCellIds == null means camera is outdoors → show all interiors.
                if (entity.ParentCellId.HasValue && visibleCellIds is not null
                    && !visibleCellIds.Contains(entity.ParentCellId.Value))
                    continue;

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

                // Hash the entity's PaletteOverride once — shared by every
                // MeshRef on this entity, so we compute it outside the loop.
                ulong palHash = HashPaletteOverride(entity.PaletteOverride);

                foreach (var meshRef in entity.MeshRefs)
                {
                    if (!_gpuByGfxObj.ContainsKey(meshRef.GfxObjId))
                        continue;

                    var model = meshRef.PartTransform * entityRoot;

                    // Texture signature = palette hash ^ surface-overrides hash.
                    // Two instances can share a batch only when their ResolveTex
                    // would return identical handles for every sub-mesh — that
                    // means identical palette AND identical surface overrides.
                    ulong surfHash = HashSurfaceOverrides(meshRef.SurfaceOverrides);
                    ulong texSig = palHash ^ surfHash;
                    var key = new GroupKey(meshRef.GfxObjId, texSig);

                    if (!_groups.TryGetValue(key, out var group))
                    {
                        group = new InstanceGroup();
                        _groups[key] = group;
                    }

                    group.Entries.Add(new InstanceEntry(model, entity, meshRef));
                }
            }
        }
    }

    private static ulong HashPaletteOverride(AcDream.Core.World.PaletteOverride? p)
    {
        if (p is null) return 0UL;
        ulong h = 0xCBF29CE484222325UL;
        const ulong prime = 0x100000001B3UL;
        h = (h ^ p.BasePaletteId) * prime;
        foreach (var sp in p.SubPalettes)
        {
            h = (h ^ sp.SubPaletteId) * prime;
            h = (h ^ sp.Offset) * prime;
            h = (h ^ sp.Length) * prime;
        }
        return h;
    }

    /// <summary>
    /// Order-independent hash of a SurfaceOverrides dictionary. XOR of each
    /// (key, value) pair keeps the result stable regardless of Dictionary
    /// iteration order, so two instances whose override maps contain the
    /// same pairs will hash identically.
    /// </summary>
    private static ulong HashSurfaceOverrides(IReadOnlyDictionary<uint, uint>? overrides)
    {
        if (overrides is null || overrides.Count == 0) return 0UL;
        ulong acc = 0UL;
        foreach (var kvp in overrides)
        {
            ulong pair = ((ulong)kvp.Key << 32) | kvp.Value;
            acc ^= pair;
        }
        // Fold with a prime so the zero case doesn't collide with "empty".
        return (acc ^ 0xCBF29CE484222325UL) * 0x100000001B3UL;
    }

    // ── Matrix write ──────────────────────────────────────────────────────────

    /// <summary>
    /// Writes a System.Numerics Matrix4x4 into <paramref name="buf"/> starting
    /// at <paramref name="offset"/> as 16 consecutive floats in row-major order
    /// (the C# natural memory layout).  The GLSL shader reads each 4-float row
    /// as a column of the mat4 — identical to what UniformMatrix4(transpose=false)
    /// produces for the uniform path.
    /// </summary>
    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;
    }

    // ── Texture resolution ────────────────────────────────────────────────────

    private uint ResolveTex(WorldEntity entity, MeshRef meshRef, SubMeshGpu sub)
    {
        uint overrideOrigTex = 0;
        bool hasOrigTexOverride = meshRef.SurfaceOverrides is not null
            && meshRef.SurfaceOverrides.TryGetValue(sub.SurfaceId, out overrideOrigTex);
        uint? origTexOverride = hasOrigTexOverride ? overrideOrigTex : (uint?)null;

        if (entity.PaletteOverride is not null)
        {
            return _textures.GetOrUploadWithPaletteOverride(
                sub.SurfaceId, origTexOverride, entity.PaletteOverride);
        }
        else if (hasOrigTexOverride)
        {
            return _textures.GetOrUploadWithOrigTextureOverride(sub.SurfaceId, overrideOrigTex);
        }
        else
        {
            return _textures.GetOrUpload(sub.SurfaceId);
        }
    }

    // ── Disposal ──────────────────────────────────────────────────────────────

    public void Dispose()
    {
        foreach (var subs in _gpuByGfxObj.Values)
        {
            foreach (var sub in subs)
            {
                _gl.DeleteBuffer(sub.Vbo);
                _gl.DeleteBuffer(sub.Ebo);
                _gl.DeleteVertexArray(sub.Vao);
            }
        }
        _gl.DeleteBuffer(_instanceVbo);
        _gpuByGfxObj.Clear();
        _groups.Clear();
    }

    // ── Private types ─────────────────────────────────────────────────────────

    private sealed class SubMeshGpu
    {
        public uint Vao;
        public uint Vbo;
        public uint Ebo;
        public int IndexCount;
        public uint SurfaceId;
        public TranslucencyKind Translucency;
    }

    /// <summary>
    /// All instances of one GfxObj for this frame, plus their starting offset
    /// in the shared instance VBO (in units of instances, not bytes).
    /// </summary>
    private sealed class InstanceGroup
    {
        public readonly List<InstanceEntry> Entries = new();
        public int BufferOffset;

        public int Count => Entries.Count;
    }

    private readonly struct InstanceEntry
    {
        public readonly Matrix4x4 Model;
        public readonly WorldEntity Entity;
        public readonly MeshRef MeshRef;

        public InstanceEntry(Matrix4x4 model, WorldEntity entity, MeshRef meshRef)
        {
            Model = model;
            Entity = entity;
            MeshRef = meshRef;
        }
    }
}