using System;
using System.Collections.Generic;
using System.IO;
using System.Numerics;
using System.Runtime.InteropServices;
using Silk.NET.OpenGL;

namespace AcDream.App.Rendering;

/// <summary>
/// 2D batched quad renderer for text + solid rectangles. Coordinates are in
/// screen pixels with origin top-left, +X right, +Y down. Call
/// <see cref="Begin"/> at the start of a HUD pass, queue geometry via
/// <see cref="DrawString"/> / <see cref="DrawRect"/>, then <see cref="Flush"/>.
///
/// Uses two internal vertex buffers (text and rect) flushed in two draw calls
/// to avoid a per-vertex "use texture" flag. Rects are drawn first so text
/// sits on top of background panels.
/// </summary>
public sealed unsafe class TextRenderer : IDisposable
{
    private const int FloatsPerVertex = 8;   // pos(2) + uv(2) + color(4)

    private readonly GL _gl;
    private readonly Shader _shader;
    private readonly uint _vao;
    private readonly uint _vbo;
    private readonly uint _whiteTex;   // 1×1 white, for solid fills routed through the sprite bucket
    private int _vboCapacityBytes;

    private readonly List<float> _textBuf = new(8192);
    private readonly List<float> _rectBuf = new(1024);
    // Submission-ordered sprite segments: consecutive DrawSprite calls with the
    // SAME texture batch into one segment; a texture change starts a new segment.
    // Drawing segments in submission order preserves painter z-order for
    // sprite-on-sprite UI. (The old per-texture dictionary drew a REUSED texture
    // at its FIRST-insertion point, so later bar sprites covered glyphs emitted
    // earlier via the shared dat-font atlas — the stamina/mana numbers vanished.)
    private sealed class SpriteSeg { public uint Texture; public readonly List<float> Verts = new(256); }
    private readonly List<SpriteSeg> _spriteSegs = new();
    private int _segUsed;
    private int _textVerts;
    private int _rectVerts;
    private Vector2 _screenSize;

    // Overlay layer — a parallel set of buckets drawn AFTER the normal sprite/rect/text
    // buckets, so open popups/menus composite on top of EVERYTHING, including translucent
    // rect panel backgrounds (which otherwise always win because rects flush after
    // sprites). Routed by OverlayMode; the UI root sets it for the popup traversal.
    private readonly List<float> _overlayTextBuf = new(1024);
    private readonly List<float> _overlayRectBuf = new(256);
    private readonly List<SpriteSeg> _overlaySpriteSegs = new();
    private int _overlaySegUsed;
    private int _overlayTextVerts;
    private int _overlayRectVerts;

    /// <summary>When true, Draw* calls route to the overlay layer (flushed last, on top
    /// of all normal-layer geometry). Set by the UI root around the popup/overlay pass.</summary>
    public bool OverlayMode { get; set; }

    public TextRenderer(GL gl, string shaderDir)
    {
        _gl = gl;
        _shader = new Shader(gl,
            Path.Combine(shaderDir, "ui_text.vert"),
            Path.Combine(shaderDir, "ui_text.frag"));

        _vao = _gl.GenVertexArray();
        _vbo = _gl.GenBuffer();

        _gl.BindVertexArray(_vao);
        _gl.BindBuffer(BufferTargetARB.ArrayBuffer, _vbo);

        uint stride = FloatsPerVertex * sizeof(float);
        _gl.EnableVertexAttribArray(0);
        _gl.VertexAttribPointer(0, 2, VertexAttribPointerType.Float, false, stride, (void*)0);
        _gl.EnableVertexAttribArray(1);
        _gl.VertexAttribPointer(1, 2, VertexAttribPointerType.Float, false, stride, (void*)(2 * sizeof(float)));
        _gl.EnableVertexAttribArray(2);
        _gl.VertexAttribPointer(2, 4, VertexAttribPointerType.Float, false, stride, (void*)(4 * sizeof(float)));

        _gl.BindBuffer(BufferTargetARB.ArrayBuffer, 0);
        _gl.BindVertexArray(0);

        // 1×1 white texture so DrawFill can route solid-colour quads through the SPRITE
        // bucket (the shader multiplies texel×color → white×color = color). Lets a panel
        // background draw UNDER its text in painter order, which DrawRect's separate
        // bucket cannot (it always composites after all sprites).
        _whiteTex = _gl.GenTexture();
        _gl.BindTexture(TextureTarget.Texture2D, _whiteTex);
        Span<byte> whitePixel = stackalloc byte[] { 255, 255, 255, 255 };
        fixed (byte* wp = whitePixel)
            _gl.TexImage2D(TextureTarget.Texture2D, 0, (int)InternalFormat.Rgba8, 1, 1, 0,
                PixelFormat.Rgba, PixelType.UnsignedByte, wp);
        _gl.TexParameter(TextureTarget.Texture2D, TextureParameterName.TextureMinFilter, (int)TextureMinFilter.Nearest);
        _gl.TexParameter(TextureTarget.Texture2D, TextureParameterName.TextureMagFilter, (int)TextureMinFilter.Nearest);
        _gl.BindTexture(TextureTarget.Texture2D, 0);
    }

    /// <summary>Begin a HUD pass. Call once per frame before any Draw* calls.</summary>
    public void Begin(Vector2 screenSize)
    {
        _screenSize = screenSize;
        _textBuf.Clear();
        _rectBuf.Clear();
        _segUsed = 0; // pool the SpriteSeg objects across frames
        _textVerts = 0;
        _rectVerts = 0;
        _overlayTextBuf.Clear();
        _overlayRectBuf.Clear();
        _overlaySegUsed = 0;
        _overlayTextVerts = 0;
        _overlayRectVerts = 0;
        OverlayMode = false;
    }

    /// <summary>Draw a filled rectangle in screen pixel space.</summary>
    public void DrawRect(float x, float y, float w, float h, Vector4 color)
    {
        if (OverlayMode) { AppendQuad(_overlayRectBuf, x, y, w, h, 0, 0, 0, 0, color); _overlayRectVerts += 6; }
        else             { AppendQuad(_rectBuf,        x, y, w, h, 0, 0, 0, 0, color); _rectVerts += 6; }
    }

    /// <summary>Draw a solid-colour quad through the SPRITE bucket (and the overlay layer
    /// when active), so it composites in painter order with sprites + dat-font text. Use
    /// this — not <see cref="DrawRect"/> — for a panel BACKGROUND that text draws on top of:
    /// DrawRect's bucket always flushes after all sprites, so a rect background would cover
    /// the text instead.</summary>
    public void DrawFill(float x, float y, float w, float h, Vector4 color)
        => DrawSprite(_whiteTex, x, y, w, h, 0f, 0f, 1f, 1f, color);

    /// <summary>Draw a 1-pixel-thick outline rect.</summary>
    public void DrawRectOutline(float x, float y, float w, float h, Vector4 color, float thickness = 1f)
    {
        // top, bottom, left, right
        DrawRect(x, y, w, thickness, color);
        DrawRect(x, y + h - thickness, w, thickness, color);
        DrawRect(x, y, thickness, h, color);
        DrawRect(x + w - thickness, y, thickness, h, color);
    }

    /// <summary>
    /// Draw a single line of text at (x,y) where (x,y) is the top-left of the
    /// typographic block. Handles '\n' as a line break.
    /// </summary>
    public void DrawString(BitmapFont font, string text, float x, float y, Vector4 color)
    {
        float cursorX = x;
        // The caller provides top-y; shift to baseline for glyph offset math.
        float baseline = y + font.Ascent;

        for (int i = 0; i < text.Length; i++)
        {
            char c = text[i];
            if (c == '\n')
            {
                cursorX = x;
                baseline += font.LineHeight;
                continue;
            }
            if (!font.TryGetGlyph(c, out var g))
            {
                // Unknown glyph — skip its advance width if '?' exists.
                if (font.TryGetGlyph('?', out var q))
                    cursorX += q.Advance;
                continue;
            }

            float gx = cursorX + g.OffsetX;
            float gy = baseline + g.OffsetY;
            float gw = g.Width;
            float gh = g.Height;

            if (gw > 0 && gh > 0)
            {
                if (OverlayMode) { AppendQuad(_overlayTextBuf, gx, gy, gw, gh, g.UvMinX, g.UvMinY, g.UvMaxX, g.UvMaxY, color); _overlayTextVerts += 6; }
                else             { AppendQuad(_textBuf,        gx, gy, gw, gh, g.UvMinX, g.UvMinY, g.UvMaxX, g.UvMaxY, color); _textVerts += 6; }
            }
            cursorX += g.Advance;
        }
    }

    /// <summary>
    /// Draw a textured sprite quad in screen pixel space with an explicit
    /// source-UV rectangle (for 9-slice / atlas sub-regions). Batched per
    /// GL texture handle; flushed with uUseTexture=2 (RGBA modulate).
    /// </summary>
    public void DrawSprite(uint texture, float x, float y, float w, float h,
        float u0, float v0, float u1, float v1, Vector4 tint)
    {
        SpriteSeg seg = OverlayMode
            ? NextSpriteSeg(_overlaySpriteSegs, ref _overlaySegUsed, texture)
            : NextSpriteSeg(_spriteSegs,        ref _segUsed,        texture);
        AppendQuad(seg.Verts, x, y, w, h, u0, v0, u1, v1, tint);
    }

    /// <summary>Pick the sprite segment for <paramref name="texture"/>: extend the current
    /// same-texture run, else reuse a pooled segment, else allocate. Submission order is
    /// preserved (painter z-order for sprite-on-sprite UI).</summary>
    private static SpriteSeg NextSpriteSeg(List<SpriteSeg> segs, ref int used, uint texture)
    {
        if (used > 0 && segs[used - 1].Texture == texture)
            return segs[used - 1];
        if (used < segs.Count)
        {
            var s = segs[used++];
            s.Texture = texture;
            s.Verts.Clear();
            return s;
        }
        var ns = new SpriteSeg { Texture = texture };
        segs.Add(ns);
        used++;
        return ns;
    }

    private static void AppendQuad(List<float> buf,
        float x, float y, float w, float h,
        float u0, float v0, float u1, float v1, Vector4 color)
    {
        // Two triangles (6 verts). CCW in pixel space is clockwise in NDC
        // because the vertex shader flips Y, so OpenGL's default front-face
        // is GL_CCW — we rely on cull-face being disabled during HUD pass.
        //   (x, y) ─ (x+w, y)
        //     │         │
        //   (x, y+h) ─ (x+w, y+h)
        //
        // Triangle 1: (x,y) (x+w,y+h) (x+w,y)
        // Triangle 2: (x,y) (x,y+h)   (x+w,y+h)
        void V(float px, float py, float pu, float pv)
        {
            buf.Add(px); buf.Add(py);
            buf.Add(pu); buf.Add(pv);
            buf.Add(color.X); buf.Add(color.Y); buf.Add(color.Z); buf.Add(color.W);
        }
        V(x,     y,     u0, v0);
        V(x + w, y + h, u1, v1);
        V(x + w, y,     u1, v0);
        V(x,     y,     u0, v0);
        V(x,     y + h, u0, v1);
        V(x + w, y + h, u1, v1);
    }

    /// <summary>Upload + draw accumulated rects + text. font may be null if only DrawRect was used.</summary>
    public void Flush(BitmapFont? font)
    {
        bool anyNormal  = _segUsed > 0 || _textVerts > 0 || _rectVerts > 0;
        bool anyOverlay = _overlaySegUsed > 0 || _overlayTextVerts > 0 || _overlayRectVerts > 0;
        if (!anyNormal && !anyOverlay) return;

        _shader.Use();
        _shader.SetVec2("uScreenSize", _screenSize);

        _gl.BindVertexArray(_vao);
        _gl.BindBuffer(BufferTargetARB.ArrayBuffer, _vbo);

        // Save GL state.
        bool wasDepth = _gl.IsEnabled(EnableCap.DepthTest);
        bool wasBlend = _gl.IsEnabled(EnableCap.Blend);
        bool wasCull  = _gl.IsEnabled(EnableCap.CullFace);
        // The world pass leaves alpha-to-coverage + multisample enabled (WbDrawDispatcher,
        // QualitySettings MSAA). If they bleed into the UI pass, each glyph's soft alpha
        // EDGE is converted to dithered MSAA coverage instead of a clean alpha blend —
        // the "text not sharp / fuzzy" artifact. The UI composites with straight alpha
        // blending and must own this state (feedback_render_self_contained_gl_state).
        bool wasA2C  = _gl.IsEnabled(EnableCap.SampleAlphaToCoverage);
        bool wasMsaa = _gl.IsEnabled(EnableCap.Multisample);
        _gl.Disable(EnableCap.SampleAlphaToCoverage);
        _gl.Disable(EnableCap.Multisample);
        _gl.Disable(EnableCap.DepthTest);
        _gl.Disable(EnableCap.CullFace);
        _gl.DepthMask(false);
        _gl.Enable(EnableCap.Blend);
        _gl.BlendFunc(BlendingFactor.SrcAlpha, BlendingFactor.OneMinusSrcAlpha);

        // LAYERED compositing for the UI (background → fill → text):
        //   1. RGBA dat sprites  — window chrome / panel backgrounds (behind)
        //   2. Untextured rects  — widget fills (e.g. vital bars) on the chrome
        //   3. Text glyphs       — on top
        // Bucket 1 (sprites) draws in SUBMISSION (painter) order via _spriteSegs,
        // so sprite-on-sprite z is preserved. Buckets 2 (rects) + 3 (debug text)
        // composite on top, in that order. The OVERLAY layer repeats all three
        // AFTER the normal layer, so open popups beat even the rect backgrounds.
        DrawLayer(_spriteSegs, _segUsed, _rectBuf, _rectVerts, _textBuf, _textVerts, font);
        DrawLayer(_overlaySpriteSegs, _overlaySegUsed, _overlayRectBuf, _overlayRectVerts, _overlayTextBuf, _overlayTextVerts, font);

        // Restore GL state.
        _gl.DepthMask(true);
        if (!wasBlend) _gl.Disable(EnableCap.Blend);
        if (wasCull)   _gl.Enable(EnableCap.CullFace);
        if (wasDepth)  _gl.Enable(EnableCap.DepthTest);
        if (wasA2C)    _gl.Enable(EnableCap.SampleAlphaToCoverage);
        if (wasMsaa)   _gl.Enable(EnableCap.Multisample);

        _gl.BindVertexArray(0);
    }

    /// <summary>Draw one compositing layer: sprites (submission order, one call per
    /// texture) → untextured rects → debug-font text. Shared by the normal and overlay
    /// layers; GL state + shader are set up by <see cref="Flush"/>.</summary>
    private void DrawLayer(
        List<SpriteSeg> spriteSegs, int segUsed,
        List<float> rectBuf, int rectVerts,
        List<float> textBuf, int textVerts, BitmapFont? font)
    {
        // 1. RGBA dat sprites — one draw call per distinct GL texture.
        if (segUsed > 0)
        {
            _shader.SetInt("uUseTexture", 2);
            _gl.ActiveTexture(TextureUnit.Texture0);
            _shader.SetInt("uTex", 0);
            for (int i = 0; i < segUsed; i++)
            {
                var seg = spriteSegs[i];
                if (seg.Verts.Count == 0) continue;
                _gl.BindTexture(TextureTarget.Texture2D, seg.Texture);
                UploadBuffer(seg.Verts);
                _gl.DrawArrays(PrimitiveType.Triangles, 0, (uint)(seg.Verts.Count / FloatsPerVertex));
            }
        }

        // 2. Untextured rects — widget fills on top of the chrome.
        if (rectVerts > 0)
        {
            _shader.SetInt("uUseTexture", 0);
            UploadBuffer(rectBuf);
            _gl.DrawArrays(PrimitiveType.Triangles, 0, (uint)rectVerts);
        }

        // 3. Textured debug-font text glyphs on top.
        if (textVerts > 0 && font is not null)
        {
            _shader.SetInt("uUseTexture", 1);
            _gl.ActiveTexture(TextureUnit.Texture0);
            _gl.BindTexture(TextureTarget.Texture2D, font.TextureId);
            _shader.SetInt("uTex", 0);
            UploadBuffer(textBuf);
            _gl.DrawArrays(PrimitiveType.Triangles, 0, (uint)textVerts);
        }
    }

    private void UploadBuffer(List<float> buf)
    {
        int bytes = buf.Count * sizeof(float);
        if (bytes == 0) return;

        if (bytes > _vboCapacityBytes)
        {
            fixed (float* p = CollectionsMarshal.AsSpan(buf))
                _gl.BufferData(BufferTargetARB.ArrayBuffer, (nuint)bytes, p, BufferUsageARB.DynamicDraw);
            _vboCapacityBytes = bytes;
        }
        else
        {
            fixed (float* p = CollectionsMarshal.AsSpan(buf))
                _gl.BufferSubData(BufferTargetARB.ArrayBuffer, 0, (nuint)bytes, p);
        }
    }

    public void Dispose()
    {
        _gl.DeleteTexture(_whiteTex);
        _gl.DeleteBuffer(_vbo);
        _gl.DeleteVertexArray(_vao);
        _shader.Dispose();
    }
}