using OpenTK.Mathematics;

namespace TheLabs;

public static class Helpers
{
    public static void CalculateTangent(ref Vertex v1, ref Vertex v2, ref Vertex v3)
    {
        // 1. Calculate Edge Vectors (Geometry)
        Vector3 edge1 = v2.Position - v1.Position;
        Vector3 edge2 = v3.Position - v1.Position;

        // 2. Calculate Edge Vectors (Texture Space)
        Vector2 deltaUV1 = v2.TexCoords - v1.TexCoords;
        Vector2 deltaUV2 = v3.TexCoords - v1.TexCoords;

        // 3. The Math (Solving linear equations)
        float f = 1.0f / (deltaUV1.X * deltaUV2.Y - deltaUV2.X * deltaUV1.Y);

        Vector3 tangent;
        tangent.X = f * (deltaUV2.Y * edge1.X - deltaUV1.Y * edge2.X);
        tangent.Y = f * (deltaUV2.Y * edge1.Y - deltaUV1.Y * edge2.Y);
        tangent.Z = f * (deltaUV2.Y * edge1.Z - deltaUV1.Y * edge2.Z);

        // Normalize it
        tangent = Vector3.Normalize(tangent);
        Vector3 bitangent = Vector3.Normalize(Vector3.Cross(edge1, edge2));

        // 4. Assign the same tangent to all 3 vertices of the triangle
        // (For smooth objects, you would average them, but for a cube, this is fine)
        v1.Tangent = tangent; v1.Bitangent = bitangent;
        v2.Tangent = tangent; v2.Bitangent = bitangent;
        v3.Tangent = tangent; v3.Bitangent = bitangent;
    }

    public static void AddFace(List<Vertex> vertices, List<uint> indices,
        Vertex v0, Vertex v1, Vertex v2, Vertex v3)
    {
        // 1. Calculate Tangents
        // We calculate it twice to ensure the tangent is consistent across the whole face
        // Triangle 1 (Bottom-Left, Bottom-Right, Top-Right)
        CalculateTangent(ref v0, ref v1, ref v2);
        // Triangle 2 (Bottom-Left, Top-Right, Top-Left)
        CalculateTangent(ref v0, ref v2, ref v3);

        // 2. Track where these new vertices start in the main list
        // (If the list has 4 items, the new ones start at index 4)
        uint offset = (uint)vertices.Count;

        // 3. Add Vertices to the list
        vertices.Add(v0); // Index: offset + 0
        vertices.Add(v1); // Index: offset + 1
        vertices.Add(v2); // Index: offset + 2
        vertices.Add(v3); // Index: offset + 3

        // 4. Add Indices for the two triangles (Standard Quad)
        // Triangle 1
        indices.Add(offset + 0);
        indices.Add(offset + 1);
        indices.Add(offset + 2);

        // Triangle 2
        indices.Add(offset + 0);
        indices.Add(offset + 2);
        indices.Add(offset + 3);
    }
}