pub fn discard_exact(avail: usize, len: usize) -> Result<usize, u8> {
    let mut avail = avail;
    let mut len = len;
    let mut rounds = 0usize;
    while len > 0 {
        let consume_len = if avail < len { avail } else { len };
        avail -= consume_len;
        len -= consume_len;
        rounds += 1;
    }
    Ok(rounds)
}