#[derive(Debug, Default)]
struct Checksum(Option<(u8, u8)>);

impl Checksum {
    fn push(&mut self, data: u8) {
        let (ck_a, ck_b): &mut (u8, u8) = self.0.get_or_insert((0, 0));
        *ck_a = ck_a.wrapping_add(data);
        *ck_b = ck_b.wrapping_add(*ck_a);
    }

    fn take(&mut self) -> (u8, u8) {
        self.0.take().unwrap_or((0, 0))
    }
}

pub fn calculate_checksum(buf: &[u8], offset: usize) -> (u8, u8) {
    let mut ck_a: u8 = 0;
    let mut ck_b: u8 = 0;

    let len = buf.len() - 2;

    for i in buf.iter().take(len).skip(offset) {
        ck_a = ck_a.wrapping_add(*i);
        ck_b = ck_b.wrapping_add(ck_a);
    }
    (ck_a, ck_b)
}

fn main() {
    let vals: Vec<u8> = (0..100).collect();
    let mut ckobj = Checksum::default();
    let ck0 = {
        for v in &vals[..vals.len()-2] {
            ckobj.push(*v);
        }
    ckobj.take()
    };
    let ck1 = calculate_checksum(&vals, 0);
    assert_eq!(ck0, ck1);
}