using Silk.NET.Maths;
using Silk.NET.OpenGL;

namespace Nebulix.Rendering
{
	public sealed class Mesh
	{
		public Vector3D<float>[] Vertices { get => vertices; 
			set { vertices = value; regenerate = true; normals = new Vector3D<float>[vertices.Length];} }
		public uint[] Indices { get => indices; set { indices = value; regenerate = true; } }
		
		private uint vao = 0, vbo = 0, ebo = 0;
		private bool regenerate = true;

		private Vector3D<float>[] vertices = [];
		private uint[] indices = [];
		private Vector3D<float>[] normals = [];

		public void Clear()
		{
			vertices = [];
			indices = [];
		}

		public void CalculateNormals()
		{
			for (int i = 0; i < indices.Length; i += 3)
			{
				uint i0 = indices[i];
				uint i1 = indices[i+1];
				uint i2 = indices[i+2];
				Vector3D<float> v0 = vertices[i0];
				Vector3D<float> v1 = vertices[i1];
				Vector3D<float> v2 = vertices[i2];

				Vector3D<float> normal = Vector3D.Cross(v1-v0, v2-v0);
				// Commenting this out, will result in the normals being weighted based on the triangle area
				normal = Vector3D.Normalize(normal);
				normals[i0] += normal;
				normals[i1] += normal;
				normals[i2] += normal;
			}

			for (int i = 0; i < normals.Length; i++)
			{
				normals[i] = Vector3D.Normalize(normals[i]); // smoothing for shared vertices
			}
		}
		
		// getting called by "Engine" which currently is in another assembly, meaning I probably need to make this public
		// needs to be change for the real engine
		/// <summary>
		/// Binds the necessary buffers and draws the mesh. Does not use any Shaders 
		/// </summary>
		/// <param name="gl"></param>
		public unsafe void Render(GL gl)
		{
			if (regenerate) Generate(gl);

			gl.BindVertexArray(vao);
			gl.DrawElements(PrimitiveType.Triangles, (uint)indices.Length, DrawElementsType.UnsignedInt, null);
		}

		private unsafe void Generate(GL gl)
		{
			regenerate = false;

			if(vao == 0)
				vao = gl.CreateVertexArray();
			gl.BindVertexArray(vao);

			if(vbo == 0)
				vbo = gl.GenBuffer();
			if(ebo == 0) 
				ebo = gl.GenBuffer();

			float[] meshData = new float[vertices.Length * 3 + normals.Length * 3];
			for (int i = 0, insert = 0; i < vertices.Length; i++)
			{
				meshData[insert] = vertices[i].X;
				meshData[insert + 1] = vertices[i].Y;
				meshData[insert + 2] = vertices[i].Z;
				
				meshData[insert + 3] = normals[i].X;
				meshData[insert + 4] = normals[i].Y;
				meshData[insert + 5] = normals[i].Z;
				insert += 6;
			}

			gl.BindBuffer(BufferTargetARB.ArrayBuffer, vbo);
			fixed(void* data = &meshData[0])
			{
				gl.BufferData(BufferTargetARB.ArrayBuffer, (nuint)(meshData.Length * sizeof(float)), data, BufferUsageARB.StaticDraw);
			}

			gl.BindBuffer(BufferTargetARB.ElementArrayBuffer, ebo);
			fixed(void* indicesData = &indices[0])
			{
				gl.BufferData(BufferTargetARB.ElementArrayBuffer, (nuint)(indices.Length * sizeof(uint)), indicesData, BufferUsageARB.StaticDraw);
			}
			
			// vertices
			gl.EnableVertexAttribArray(0);
			gl.VertexAttribPointer(0, 3, VertexAttribPointerType.Float, false, 6 * sizeof(float), null);
			// normals
			gl.EnableVertexAttribArray(1);
			gl.VertexAttribPointer(1, 3, VertexAttribPointerType.Float, false, 6 * sizeof(float), (void*)(3 * sizeof(float)));
		}
	}
}