using System;
using System.Diagnostics;
using System.Numerics;
using System.Runtime.InteropServices;
using gaemstone.ECS;
using gaemstone.Flecs;
using gaemstone.Utility;
using Silk.NET.OpenGL;
using Silk.NET.Windowing;
using static gaemstone.Client.Components.CameraComponents;
using static gaemstone.Client.Components.RenderingComponents;
using static gaemstone.Client.Systems.Windowing;
using static gaemstone.Components.TransformComponents;

namespace gaemstone.Client.Systems;

[Module]
[DependsOn<gaemstone.Client.Components.CameraComponents>]
[DependsOn<gaemstone.Client.Components.RenderingComponents>]
[DependsOn<gaemstone.Client.Systems.Windowing>]
[DependsOn<gaemstone.Components.TransformComponents>]
public class Renderer
{
	private uint _program;
	private int _cameraMatrixUniform;
	private int _modelMatrixUniform;
	private Rule? _renderEntityRule;

	[Observer<ObserverEvent.OnSet>]
	public void OnCanvasSet(Canvas canvas)
	{
		var GL = canvas.GL;

		GL.Enable(EnableCap.DebugOutputSynchronous);
		GL.DebugMessageCallback(DebugCallback, 0);

		GL.Enable(EnableCap.CullFace);
		GL.CullFace(CullFaceMode.Back);

		GL.Enable(EnableCap.DepthTest);
		GL.DepthFunc(DepthFunction.Less);

		GL.Enable(EnableCap.Blend);
		GL.BlendFunc(BlendingFactor.SrcAlpha, BlendingFactor.OneMinusSrcAlpha);

		var vertexShaderSource   = Resources.GetString("/gaemstone.Client/Resources/default.vs.glsl");
		var fragmentShaderSource = Resources.GetString("/gaemstone.Client/Resources/default.fs.glsl");

		var vertexShader   = GL.CreateAndCompileShader(ShaderType.VertexShader  , "vertex"  , vertexShaderSource);
		var fragmentShader = GL.CreateAndCompileShader(ShaderType.FragmentShader, "fragment", fragmentShaderSource);

		_program = GL.CreateAndLinkProgram("program", vertexShader, fragmentShader);
		_cameraMatrixUniform = GL.GetUniformLocation(_program, "cameraMatrix");
		_modelMatrixUniform  = GL.GetUniformLocation(_program, "modelMatrix");
	}

	[System<SystemPhase.PreStore>]
	public void Clear(Canvas canvas)
	{
		var GL = canvas.GL;
		GL.UseProgram(_program);
		GL.Viewport(canvas.Size);
		GL.ClearColor(canvas.BackgroundColor);
		GL.Clear(ClearBufferMask.ColorBufferBit | ClearBufferMask.DepthBufferBit);
	}

	[System<SystemPhase.OnStore>]
	public void Render(Universe universe, [Game] Canvas canvas,
		in GlobalTransform cameraTransform, in Camera camera, CameraViewport? viewport)
	{
		var color  = viewport?.ClearColor ?? Color.FromRGB(0.3f, 0.0f, 0.5f);
		var bounds = viewport?.Viewport   ?? new(default, canvas.Size);

		var GL = canvas.GL;
		GL.Enable(EnableCap.ScissorTest);
		GL.Viewport(bounds); GL.Scissor(bounds.Left, bounds.Top, (uint)bounds.Width, (uint)bounds.Height);
		GL.ClearColor(color); GL.Clear(ClearBufferMask.ColorBufferBit | ClearBufferMask.DepthBufferBit);
		GL.Disable(EnableCap.ScissorTest);

		// Create the camera's projection matrix.
		var cameraProjection = camera.IsOrthographic
			? Matrix4x4.CreateOrthographic(
				bounds.Width, -bounds.Height,
				camera.NearPlane, camera.FarPlane)
			: Matrix4x4.CreatePerspectiveFieldOfView(
				camera.FieldOfView * MathF.PI / 180,  // Degrees => Radians
				(float)bounds.Width / bounds.Height, // Aspect Ratio
				camera.NearPlane, camera.FarPlane);

		// Get the camera's transform matrix and invert it.
		Matrix4x4.Invert(cameraTransform, out var invertedTransform);
		// Set the uniform to the combined transform and projection.
		var cameraMatrix = invertedTransform * cameraProjection;
		GL.UniformMatrix4(_cameraMatrixUniform, 1, false, in cameraMatrix.M11);

		_renderEntityRule ??= new(universe, new("""
			GlobalTransform,
			(Mesh, $mesh), MeshHandle($mesh),
			?(Texture, $tex), ?TextureHandle($tex)
			"""));
		foreach (var iter in _renderEntityRule.Iter()) {
			var transforms = iter.Field<GlobalTransform>(1);
			var meshes     = iter.Field<MeshHandle>(3);
			// var texPairs   = iter.FieldOrEmpty<Identifier>(4);
			var textures   = iter.FieldOrEmpty<TextureHandle>(5);

			for (var i = 0; i < iter.Count; i++) {
				var rTransform = transforms[i];
				var mesh       = meshes[i];
				// var hasTexture = (texPairs.Length > 0);
				var texture    = textures.GetOrNull(i);

				// If entity has Texture, bind it now.
				if (texture.HasValue) GL.BindTexture(texture.Value.Target, texture.Value.Handle);

				// Draw the mesh.
				GL.UniformMatrix4(_modelMatrixUniform, 1, false, in rTransform.Value.M11);
				GL.BindVertexArray(mesh.Handle);
				if (!mesh.IsIndexed) GL.DrawArrays(PrimitiveType.Triangles, 0, (uint)mesh.Count);
				else unsafe { GL.DrawElements(PrimitiveType.Triangles, (uint)mesh.Count, DrawElementsType.UnsignedShort, null); }

				// If entity has Texture, unbind it after it has been rendered.
				if (texture.HasValue) GL.BindTexture(texture.Value.Target, 0);
			}
		}
	}

	[System<SystemPhase.PostFrame>]
	public static void SwapBuffers(GameWindow window)
		=> window.Handle.SwapBuffers();

	[DebuggerStepThrough]
	private static void DebugCallback(GLEnum source, GLEnum _type, int id, GLEnum _severity,
	                                  int length, nint _message, nint userParam)
	{
		var type     = (DebugType)_type;
		var severity = (DebugSeverity)_severity;
		var message  = Marshal.PtrToStringAnsi(_message, length);
		Console.WriteLine($"[GLDebug] [{severity}] {type}/{id}: {message}");
		if (type == DebugType.DebugTypeError) throw new Exception(message);
	}
}