fn main() {
    println!("Hello, world!");
}

pub fn longest_palindrome(s: String) -> String {
    let mut best = Vec::with_capacity(s.len());

    let bytes = s.as_bytes();

    for center in 0..bytes.len() {
        for (mut left_idx, mut right_idx) in [(center, center), (center, center + 1)] {
            expand(bytes, &mut left_idx, &mut right_idx);

            if right_idx - left_idx > best.len() {
                best = bytes[left_idx..right_idx].into();
            }
        }
    }
    String::from_utf8(best).unwrap()
}

fn expand(bytes: &[u8], left_idx: &mut usize, right_idx: &mut usize) {
    loop {
        if bytes.get(*right_idx).is_none_or(|&r| bytes[*left_idx] != r) {
            *left_idx += 1;
            return;
        }
        match left_idx.checked_sub(1) {
            Some(new_left) => *left_idx = new_left,
            None => {
                *right_idx += 1;
                return;
            }
        }
        *right_idx += 1;
    }
}

#[cfg(test)]
mod tests {
    use crate::longest_palindrome;

    #[test]
    fn test_1() {
        assert!(
            ["aba".to_string(), "bab".to_string()]
                .contains(&longest_palindrome("babad".to_string()))
        );
    }
    #[test]
    fn test_2() {
        assert_eq!(longest_palindrome("cbbd".to_string()), "bb".to_string());
    }

    #[test]
    fn test_3() {
        assert_eq!(longest_palindrome("whdqcudjpisufnrtsyupwtnnbsvfptrcgvobbjglmpynebblpigaflpbezjvjgbmofejyjssdhbgghgrhzuplbeptpaecfdanhlylgusptlgobkqnulxvnwuzwauewcplnvcwowmbxxnhsdmgxtvbfgnuqdpxennqglgmspbagvmjcmzmbsuacxlqfxjggrwsnbblnnwisvmpwwhomyjylbtedzrptejjsaiqzprnadkjxeqfdpkddmbzokkegtypxaafodjdwirynzurzkjzrkufsokhcdkajwmqvhcbzcnysrbsfxhfvtodqabvbuosxtonbpmgoemcgkudandrioncjigbyizekiakmrfjvezuzddjxqyevyenuebfwugqelxwpirsoyixowcmtgosuggrkdciehktojageynqkazsqxraimeopcsjxcdtzhlbvtlvzytgblwkmbfwmggrkpioeofkrmfdgfwknrbaimhefpzckrzwdvddhdqujffwvtvfyjlimkljrsnnhudyejcrtrwvtsbkxaplchgbikscfcbhovlepdojmqybzhbiionyjxqsmquehkhzdiawfxunguhqhkxqdiiwsbuhosebxrpcstpklukjcsnnzpbylzaoyrmyjatuovmaqiwfdfwyhugbeehdzeozdrvcvghekusiahfxhlzclhbegdnvkzeoafodnqbtanfwixjzirnoaiqamjgkcapeopbzbgtxsjhqurbpbuduqjziznblrhxbydxsmtjdfeepntijqpkuwmqezkhnkwbvwgnkxmkyhlbfuwaslmjzlhocsgtoujabbexvxweigplmlewumcone".to_string()), "wfdfw".to_string());
    }

    #[test]
    fn test_4() {
        assert_eq!(longest_palindrome("a".to_string()), "a".to_string());
    }
}