#[derive(Default)]
struct RB<T> {
    v: Vec<T>,
    oldest: usize,
}

impl RB<T> {
    fn add(&mut self, e: T) {
        if self.v.len() < 30 {
            self.v.push(e);
            return;
        }
        v[oldest] = e;
        oldest += 1;
        oldest %= 30;
    }
}

let rb = RB::default();

rb.add(1);
rb.add(2);