pub(crate) struct IPv4ChecksumAccumulator {
    acc: u32,
}

impl IPv4ChecksumAccumulator {
    pub fn new() -> Self {
        return Self { acc: 0 };
    }

    pub fn add_first_byte(&mut self, b: u8) {
        self.acc += (b as u32) << 8;
    }

    pub  fn add_second_byte(&mut self, b: u8) {
        self.acc += b as u32;
    }

    pub  fn add_last_byte(&mut self, b: u8) {
        self.acc += b as u32;
    }

    pub  fn add_word(&mut self, b: u16) {
        self.acc += b as u32;
    }

    pub  fn add_trailing_data(&mut self, data: &[u8]) {
        for i in (0..(data.len() / 2)).into_iter().map(|i| i * 2) {
            self.add_first_byte(data[i]);
            self.add_second_byte(data[i + 1]);
        }
        if data.len() % 2 != 0 {
            self.add_last_byte(data[data.len() - 1]);
        }
    }

    pub  fn snapshot(&self) -> u16 {
        let mut out = self.acc;
        while out > 0x0000FFFFu32 {
            out = (out & 0x0000FFFFu32) + (out >> 16);
        }
        return !(out as u16);
    }
}