//! methods for 2D vector
pub fn sub_<T>(
a: &[T],
b: &[T]) -> [T; 2]
where T: std::ops::Sub<Output=T> + Copy
{
[a[0] - b[0], a[1] - b[1]]
}
pub fn to_na<T>(vtx2xyz: &[T], i_vtx: usize) -> nalgebra::Vector2::<T>
where T: Copy + nalgebra::RealField
{
nalgebra::Vector2::<T>::from_row_slice(&vtx2xyz[i_vtx *2..(i_vtx +1)*2])
}