pub fn letters_to_num(letters: &str) -> Option<u8> {
    let mut num: u8 = 0;
    for (i, letter) in letters.to_ascii_uppercase().chars().rev().enumerate() {
        let n = letter as u8;
        if !(65..=90).contains(&n) {
            return None;
        }
        num = num.checked_add((26_i32.pow(i as u32) * (n as i32 - 64)).try_into().ok()?)?;
    }
    Some(num)
}
pub fn num_to_letters(num: u8) -> Option<String> {
    if num == 0 {
        return None;
    } // Otherwise, we will not return none!
      // As another note, we can't represent higher than "IV" anyway;
      // thus there's no reason for a loop (26^n with n>1 will NOT occur).
    let mut text = "".to_string();
    let mut digit1 = num.div_euclid(26u8);
    let mut digit2 = num.rem_euclid(26u8);
    if digit1 > 0 && digit2 == 0u8 {
        digit1 -= 1;
        digit2 = 26;
    }
    if digit1 != 0 {
        text.push((64 + digit1) as char)
    }
    text.push((64 + digit2) as char);

    Some(text.to_string())
}

#[cfg(test)]
mod tests {
    use super::{letters_to_num, num_to_letters};

    #[test]
    fn test_letters_to_num() {
        assert_eq!(letters_to_num("D"), Some(4));
        assert_eq!(letters_to_num("d"), None);
        assert_eq!(letters_to_num("AD"), Some(26 + 4));
        assert_eq!(letters_to_num("CG"), Some(3 * 26 + 7));
    }

    #[test]
    fn test_num_to_letters() {
        println!("27 is {:?}", num_to_letters(27));
        assert_eq!(num_to_letters(1), Some("A".to_string()));
        assert_eq!(num_to_letters(26), Some("Z".to_string()));
        assert_eq!(num_to_letters(27), Some("AA".to_string()));
        assert_eq!(num_to_letters(111), Some("DG".to_string()));
    }

    #[test]
    fn test_l2n_and_n2l() {
        assert_eq!(
            num_to_letters(letters_to_num("A").unwrap()),
            Some("A".to_string())
        );
        assert_eq!(
            num_to_letters(letters_to_num("BJ").unwrap()),
            Some("BJ".to_string())
        );
        for i in 1..=255 {
            assert_eq!(letters_to_num(&num_to_letters(i).unwrap()), Some(i));
        }
    }

}