Function idx4d 

fn idx4d(
    dim: usize,
    i: usize,
    j: usize,
    k: usize,
    nx: usize,
    ny: usize,
    nz: usize,
) -> usize

Convert 4D index (for weights array) to flat index (Fortran order) weights layout: [dim][i][j][k] where dim is 0,1,2 for x,y,z For array shape (3, nx, nx, ny), index [dim,i,j,k] maps to: i + jnx + knxny + dimnxnynz