Terrible mesh flattener

private static Vector3 GetSurfaceNormal(FLVER.Vertex[] face)
{
    Vector3 v1 = face[2].Position - face[1].Position;
    Vector3 v2 = face[1].Position - face[0].Position;
    return Vector3.Normalize(Vector3.Cross(v1, v2));
}

private static Vector4 MakeTangent(Vector3 normal)
{
    Vector3 t1 = Vector3.Cross(normal, Vector3.UnitX);
    Vector3 t2 = Vector3.Cross(normal, Vector3.UnitY);
    if (t1.Length() > t2.Length())
        return new Vector4(t1, 1);
    else
        return new Vector4(t2, 1);
}

private static void FlattenMesh(FLVER2.Mesh mesh)
{
    var faceGroups = new List<List<FLVER.Vertex[]>>();
    foreach (var face in mesh.GetFaces())
    {
        Vector3 normal = GetSurfaceNormal(face);
        var group = faceGroups.Find(fg => (normal - GetSurfaceNormal(fg[0])).Length() < 0.0001f); // Heuristic: my ass
        if (group == null)
        {
            group = new List<FLVER.Vertex[]>();
            faceGroups.Add(group);
        }
        group.Add(face);
    }

    var newVertices = new List<FLVER.Vertex>();
    var newIndices = new List<int>();
    foreach (var group in faceGroups)
    {
        var indices = new Dictionary<FLVER.Vertex, int>();
        int addVertex(FLVER.Vertex vert, Vector3 norm, Vector4 tang)
        {
            vert.Normal = norm;
            for (int i = 0; i < vert.Tangents.Count; i++)
                vert.Tangents[i] = tang;

            if (!indices.ContainsKey(vert))
            {
                indices[vert] = newVertices.Count;
                newVertices.Add(new FLVER.Vertex(vert));
            }
            return indices[vert];
        }

        Vector3 normal = GetSurfaceNormal(group[0]);
        Vector4 tangent = MakeTangent(normal);
        foreach (var face in group)
        {
            newIndices.Add(addVertex(face[0], normal, tangent));
            newIndices.Add(addVertex(face[1], normal, tangent));
            newIndices.Add(addVertex(face[2], normal, tangent));
        }
    }

    mesh.Vertices = newVertices;
    mesh.FaceSets.ForEach(fs => fs.Indices = newIndices);
}