fn cmp_zorder(a, b):
    let mut j = 0;
    let mut x = 0;
    for k in 0..dim {
        let y = a[k] ^ b[k]
        if less_msb(x, y) {
            j = k
            x = y
        }
    }
    a[j] - b[j]
}

fn less_msb(x, y) {
    x < y && x < (x ^ y)
}