using System;
using System.Collections.Generic;
using System.Linq;
using System.Numerics;
using AcDream.Core.Meshing;
using AcDream.Core.Terrain;
using AcDream.Core.World;
using DatReaderWriter;
using DatReaderWriter.DBObjs;
using DatReaderWriter.Enums;
using Silk.NET.OpenGL;

namespace AcDream.App.Rendering.Sky;

/// <summary>
/// Port of <c>references/WorldBuilder/Chorizite.OpenGLSDLBackend/Lib/SkyboxRenderManager.cs</c>.
/// Draws the retail sky as a stack of independent celestial meshes (the
/// "it's not a dome" insight from r12 §2) rather than a cube/sphere
/// with a gradient texture. Each <see cref="SkyObjectData"/> is
/// visible in a window of day-fraction space, sweeps from
/// <c>BeginAngle</c> to <c>EndAngle</c> across the sky, and samples its
/// texture with a per-frame UV scroll driven by <c>TexVelocityX/Y</c>.
///
/// <para>
/// GL state delta per frame:
/// <list type="bullet">
///   <item><description>Depth mask OFF, depth test OFF, cull OFF — the sky
///     should never occlude scene geometry.</description></item>
///   <item><description>Separate projection matrix with a 0.1–1e6 near/far
///     so mesh vertices at large distance don't clip.</description></item>
///   <item><description>View matrix with translation zeroed — sky is
///     always camera-centred; moving doesn't get you closer to the
///     sun.</description></item>
/// </list>
/// </para>
///
/// <para>
/// Meshes are built lazily per GfxObj id on first reference. The
/// per-object arc transform matches WorldBuilder's composition:
/// <c>scale × RotZ(-heading) × RotY(-rotation)</c> — the negative signs
/// come from AC's Z-up right-handed convention where heading is
/// measured clockwise from north.
/// </para>
/// </summary>
public sealed unsafe class SkyRenderer : IDisposable
{
    private readonly GL _gl;
    private readonly DatCollection _dats;
    private readonly Shader _shader;
    private readonly TextureCache _textures;

    // Lazily-built GPU resources per sky-GfxObj.
    private readonly Dictionary<uint, List<SubMeshGpu>> _gpuByGfxObj = new();

    // When did we start running — used to accumulate TexVelocityX/Y over
    // real time (independent of the day-fraction clock).
    private readonly DateTime _startedAt = DateTime.UtcNow;

    // Configurable render distance — retail uses ~1e6; anything larger
    // than the scene far plane works.
    public float Near { get; set; } = 0.1f;
    public float Far  { get; set; } = 1_000_000f;

    public SkyRenderer(GL gl, DatCollection dats, Shader shader, TextureCache textures)
    {
        _gl = gl ?? throw new ArgumentNullException(nameof(gl));
        _dats = dats ?? throw new ArgumentNullException(nameof(dats));
        _shader = shader ?? throw new ArgumentNullException(nameof(shader));
        _textures = textures ?? throw new ArgumentNullException(nameof(textures));
    }

    /// <summary>
    /// Draw the sky for this frame. Called FIRST in the render loop —
    /// terrain / meshes / debug lines / overlay land on top.
    ///
    /// <para>
    /// Each submesh renders with retail's per-vertex lighting formula:
    /// <c>tint = clamp(emissive + ambient + max(dot(N, -sunDir), 0) * sunColor, 0, 1)</c>
    /// where <c>emissive</c> is the submesh's <c>Surface.Luminosity</c>
    /// float (1.0 for dome + sun + moon → texture passthrough via
    /// saturation; 0.0 for clouds → get the full time-of-day tint).
    /// <paramref name="keyframe"/> supplies the AmbientColor and SunColor
    /// already pre-multiplied by AmbBright / DirBright (loader-side).
    /// </para>
    /// <para>
    /// See <c>docs/research/2026-04-23-sky-retail-verbatim.md</c> §6 for
    /// the full decompile citation. The empirical Dereth dump (
    /// <c>ACDREAM_DUMP_SKY=1</c>, logged 2026-04-23) confirmed the
    /// <c>SurfaceType.Luminous</c> flag bit is NOT set on any Dereth sky
    /// mesh — the differentiator is the <c>Surface.Luminosity</c> FLOAT
    /// field.
    /// </para>
    /// </summary>
    public void Render(
        ICamera camera,
        Vector3 cameraWorldPos,
        float dayFraction,
        DayGroupData? group,
        SkyKeyframe keyframe)
    {
        if (group is null || group.SkyObjects.Count == 0) return;

        // Build a sky projection with a huge far plane so 1e6m-distant
        // celestial meshes don't clip. The FOV is cargo-culted from the
        // camera's projection — see WorldBuilder's implementation.
        float fovY = MathF.PI / 3f;  // 60° — matches FlyCamera/ChaseCamera
        float aspect = camera.Aspect;
        if (aspect <= 0f) aspect = 16f / 9f;
        var skyProj = Matrix4x4.CreatePerspectiveFieldOfView(fovY, aspect, Near, Far);

        // View with translation zeroed — keeps the sky at camera origin
        // regardless of camera position in the world.
        var skyView = camera.View;
        skyView.M41 = 0f;
        skyView.M42 = 0f;
        skyView.M43 = 0f;

        _shader.Use();
        _shader.SetMatrix4("uSkyView", skyView);
        _shader.SetMatrix4("uSkyProjection", skyProj);

        // Retail per-vertex lighting inputs (AdjustPlanes formula).
        // AmbColor/SunColor are already × AmbBright/DirBright from
        // SkyDescLoader. SunDir is the unit vector FROM surface TO sun
        // derived from the keyframe's DirHeading/DirPitch.
        _shader.SetVec3("uAmbientColor", keyframe.AmbientColor);
        _shader.SetVec3("uSunColor",     keyframe.SunColor);
        _shader.SetVec3("uSunDir",
            AcDream.Core.World.SkyStateProvider.SunDirectionFromKeyframe(keyframe));

        // Save + override GL state.
        _gl.DepthMask(false);
        _gl.Disable(EnableCap.DepthTest);
        _gl.Disable(EnableCap.CullFace);
        _gl.Enable(EnableCap.Blend);
        // Default blend — overridden per-submesh inside the inner loop.
        // Additive surfaces (sun/moon/stars via SurfaceType.Additive =
        // 0x10000) get GL_SRC_ALPHA / GL_ONE; alpha-blended (clouds, dome
        // with Alpha flag) get GL_SRC_ALPHA / GL_ONE_MINUS_SRC_ALPHA.
        _gl.BlendFunc(BlendingFactor.SrcAlpha, BlendingFactor.OneMinusSrcAlpha);

        // Look up the keyframe's override list so we can apply
        // SkyObjReplace (r12 §2.3): per-keyframe GfxObj swaps + rotation
        // override + transparency fade + luminosity cap.
        var replaces = PickReplaces(group, dayFraction);

        float secondsSinceStart = (float)(DateTime.UtcNow - _startedAt).TotalSeconds;

        for (int i = 0; i < group.SkyObjects.Count; i++)
        {
            var obj = group.SkyObjects[i];
            if (!obj.IsVisible(dayFraction)) continue;

            // Apply per-keyframe replace overrides.
            uint gfxObjId = obj.GfxObjId;
            float headingDeg = 0f;
            float transparent = 0f;
            float luminosity = 1f;
            if (replaces.TryGetValue((uint)i, out var rep))
            {
                if (rep.GfxObjId != 0) gfxObjId = rep.GfxObjId;
                if (rep.Rotate != 0f)  headingDeg = rep.Rotate;
                transparent = Math.Clamp(rep.Transparent, 0f, 1f);
                if (rep.Luminosity > 0f) luminosity = rep.Luminosity;
                if (rep.MaxBright > 0f) luminosity = MathF.Min(luminosity, rep.MaxBright);
            }
            if (gfxObjId == 0) continue;

            // Current arc angle across the sky.
            float rotationDeg = obj.CurrentAngle(dayFraction);
            float headingRad = headingDeg * (MathF.PI / 180f);
            float rotationRad = rotationDeg * (MathF.PI / 180f);

            // Matches WorldBuilder's composition for a Z-up right-handed
            // frame with heading measured clockwise from north.
            var model = Matrix4x4.CreateScale(1.0f)
                      * Matrix4x4.CreateRotationZ(-headingRad)
                      * Matrix4x4.CreateRotationY(-rotationRad);

            _shader.SetMatrix4("uModel", model);

            // UV scroll accumulates real-time × velocity. Wrap to [0, 1]
            // so long-running sessions don't accumulate float precision
            // loss in the fragment UV.
            float uOffset = (obj.TexVelocityX * secondsSinceStart) % 1f;
            float vOffset = (obj.TexVelocityY * secondsSinceStart) % 1f;
            _shader.SetVec2("uUvScroll", new Vector2(uOffset, vOffset));
            _shader.SetFloat("uTransparency", transparent);
            _shader.SetFloat("uLuminosity", luminosity);

            EnsureMeshUploaded(gfxObjId);
            if (!_gpuByGfxObj.TryGetValue(gfxObjId, out var subMeshes)) continue;

            foreach (var sub in subMeshes)
            {
                // Per-submesh blend mode: sun/moon/stars are Additive
                // (SurfaceType.Additive = 0x10000), clouds are AlphaBlend,
                // sky dome is Base1Image (Opaque, mapped to
                // SrcAlpha/InvSrcAlpha for a no-op blend at alpha=1).
                // See FUN_00508010 (chunk_00500000.c:7535) for the retail
                // pattern — retail routes sky meshes through the normal
                // mesh pipeline where Surface flags dictate state.
                if (sub.IsAdditive)
                    _gl.BlendFunc(BlendingFactor.SrcAlpha, BlendingFactor.One);
                else
                    _gl.BlendFunc(BlendingFactor.SrcAlpha, BlendingFactor.OneMinusSrcAlpha);

                // Emissive source: retail's FUN_0059da60 for non-luminous
                // surfaces writes rep.Luminosity into D3DMATERIAL9.Emissive
                // (via material cache +0x3c). This PROMOTES bright-keyframe
                // clouds into the self-lit term so the litColor saturates
                // and the texture renders at full brightness rather than
                // being dimmed by a per-fragment multiply.
                //
                // If no rep.Luminosity override: fall back to the Surface's
                // static Luminosity (1.0 for dome/sun/moon → saturates;
                // 0.0 for stars → stays ambient-lit, correct retail look).
                float effEmissive = (luminosity > 0f) ? luminosity : sub.SurfLuminosity;
                _shader.SetFloat("uEmissive", effEmissive);

                uint tex = _textures.GetOrUpload(sub.SurfaceId);
                _gl.ActiveTexture(TextureUnit.Texture0);
                _gl.BindTexture(TextureTarget.Texture2D, tex);
                _gl.BindVertexArray(sub.Vao);
                _gl.DrawElements(PrimitiveType.Triangles,
                    (uint)sub.IndexCount,
                    DrawElementsType.UnsignedInt,
                    (void*)0);
            }
        }

        // Restore GL state expected by the rest of the pipeline.
        _gl.Disable(EnableCap.Blend);
        _gl.DepthMask(true);
        _gl.Enable(EnableCap.DepthTest);
        _gl.BindVertexArray(0);
    }

    /// <summary>
    /// Find the <see cref="SkyObjectReplaceData"/> entries for the
    /// keyframe currently "active" at <paramref name="dayFraction"/>.
    /// Matches WorldBuilder's single-keyframe lookup (it picks <c>t1</c>
    /// and doesn't interpolate the replace fields).
    /// </summary>
    private static Dictionary<uint, SkyObjectReplaceData> PickReplaces(
        DayGroupData group, float dayFraction)
    {
        var result = new Dictionary<uint, SkyObjectReplaceData>();
        var times = group.SkyTimes;
        if (times.Count == 0) return result;

        // Pick k1 = last keyframe with Begin <= dayFraction.
        DatSkyKeyframeData k1 = times[^1];
        for (int i = 0; i < times.Count; i++)
        {
            if (times[i].Keyframe.Begin <= dayFraction)
                k1 = times[i];
            else
                break;
        }

        foreach (var r in k1.Replaces)
            result[r.ObjectIndex] = r;

        return result;
    }

    /// <summary>
    /// Lazy GfxObj build — reuses <see cref="GfxObjMesh"/> so the
    /// pos/neg polygon splitting logic stays consistent with the main
    /// static-mesh pipeline. Most sky meshes are single-surface.
    /// </summary>
    private void EnsureMeshUploaded(uint gfxObjId)
    {
        if (_gpuByGfxObj.ContainsKey(gfxObjId)) return;

        // DatCollection isn't thread-safe and the streaming loader can be
        // actively reading a shared DatBinReader buffer; sky meshes are
        // loaded on the render thread but GfxObj.Unpack can race with the
        // streamer. Cache a null entry on any read failure so we don't
        // retry every frame and crash the render loop. A future
        // refactor should move all dat access behind the _datLock.
        GfxObj? gfx = null;
        try { gfx = _dats.Get<GfxObj>(gfxObjId); }
        catch { gfx = null; }

        if (gfx is null)
        {
            _gpuByGfxObj[gfxObjId] = new List<SubMeshGpu>();
            return;
        }

        System.Collections.Generic.IReadOnlyList<GfxObjSubMesh>? subMeshes = null;
        try { subMeshes = GfxObjMesh.Build(gfx, _dats); }
        catch { subMeshes = null; }

        if (subMeshes is null)
        {
            _gpuByGfxObj[gfxObjId] = new List<SubMeshGpu>();
            return;
        }

        // Phase 1 diagnostic: dump Surface.Type flags on every sky GfxObj
        // once, so we can determine which submeshes carry Luminous (0x40)
        // vs plain-lit. This settles the retail "cloud tint = per-vertex
        // lighting on non-Luminous meshes" hypothesis — see
        // docs/research/2026-04-23-sky-retail-verbatim.md §6.
        if (System.Environment.GetEnvironmentVariable("ACDREAM_DUMP_SKY") == "1")
            DumpGfxObjSurfaces(gfxObjId, gfx, subMeshes);

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

    /// <summary>
    /// Log each surface's raw flag bits and the derived
    /// <see cref="TranslucencyKind"/>. Called once per GfxObj when
    /// <c>ACDREAM_DUMP_SKY=1</c>. Output format is grep-friendly so
    /// we can pipe the launch log through <c>| grep sky-dump</c> and
    /// recover a complete picture of the Dereth sky without re-running.
    /// </summary>
    private void DumpGfxObjSurfaces(
        uint gfxObjId,
        GfxObj gfx,
        System.Collections.Generic.IReadOnlyList<GfxObjSubMesh> subMeshes)
    {
        Console.WriteLine(
            $"[sky-dump] GfxObj 0x{gfxObjId:X8} Surfaces.Count={gfx.Surfaces.Count} Polygons.Count={gfx.Polygons.Count} SubMeshes.Count={subMeshes.Count}");

        for (int i = 0; i < gfx.Surfaces.Count; i++)
        {
            uint surfaceId = (uint)gfx.Surfaces[i];
            DatReaderWriter.DBObjs.Surface? surface = null;
            try { surface = _dats.Get<DatReaderWriter.DBObjs.Surface>(surfaceId); }
            catch { surface = null; }

            if (surface is null)
            {
                Console.WriteLine($"[sky-dump]   Surface[{i}] 0x{surfaceId:X8} -- (dat read failed)");
                continue;
            }

            // SurfaceType is a flag enum — `ToString()` gives the
            // comma-joined names (e.g. "Base1Image, Additive").
            uint rawType = (uint)surface.Type;
            string names = surface.Type.ToString();
            uint origTex = surface.OrigTextureId?.DataId ?? 0u;
            var trans = TranslucencyKindExtensions.FromSurfaceType(surface.Type);
            // Surface's own Luminosity (0..1 fraction per test fixture —
            // different from SkyObjectReplace.Luminosity which lives in the keyframe).
            Console.WriteLine(
                $"[sky-dump]   Surface[{i}] 0x{surfaceId:X8} Type=0x{rawType:X8} ({names}) " +
                $"OrigTexture=0x{origTex:X8} Translucency={trans} " +
                $"SurfLuminosity={surface.Luminosity:F4} SurfTranslucency={surface.Translucency:F4}");
        }
    }

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

        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 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);

        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)));

        _gl.BindVertexArray(0);

        // Classify blend mode from the Surface's flags. Sun/moon/stars with
        // `SurfaceType.Additive = 0x10000` get GL_ONE / GL_ONE (their texture
        // has a black background and a bright body; additive makes the
        // background contribute nothing and the body glow on top of the sky).
        //
        // NOTE: earlier revision also treated `SurfaceType.Luminous = 0x40`
        // as additive, but that flag is present on the sky DOME itself and
        // on cloud sheets — turning those additive blew the whole sky to
        // white. `Luminous` means "self-illuminated / unshaded" in retail's
        // render pipeline, not "additive blend". Only the Additive bit
        // toggles the blend mode.
        bool isAdditive = sm.Translucency == TranslucencyKind.Additive;

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

    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);
            }
        }
        _gpuByGfxObj.Clear();
    }

    private sealed class SubMeshGpu
    {
        public uint Vao;
        public uint Vbo;
        public uint Ebo;
        public int  IndexCount;
        public uint SurfaceId;
        /// <summary>
        /// True if the Surface's <c>SurfaceType.Additive</c> flag (0x10000)
        /// is set. Drives the blend func switch (GL_ONE vs GL_ONE_MINUS_SRC_ALPHA).
        /// Computed once at upload; avoids a per-frame dat lookup.
        /// </summary>
        public bool IsAdditive;
        /// <summary>
        /// <c>Surface.Luminosity</c> float (0..1 — NOT the SurfaceType.Luminous
        /// flag bit). Passed to the sky fragment shader as <c>uEmissive</c>;
        /// when 1.0 it saturates the lighting math so the mesh renders at
        /// full texture brightness (dome, sun). When 0.0 the mesh picks up
        /// the time-of-day ambient+diffuse tint (clouds). See
        /// <c>docs/research/2026-04-23-sky-retail-verbatim.md</c> §6.
        /// </summary>
        public float SurfLuminosity;
    }
}