pub fn normalize_uuid(u: &str) -> String {
    let s = u.trim().to_ascii_lowercase();
    const BASE_SUFFIX: &str = "-0000-1000-8000-00805f9b34fb";
    if s.ends_with(BASE_SUFFIX) && s.len() >= BASE_SUFFIX.len() + 4 {
        let start = s.len() - BASE_SUFFIX.len() - 4;
        return s[start..start + 4].to_string();
    }
    s
}

#[cfg(test)]
mod tests {
    use super::*;

    #[test]
    fn test_strips_standard_base_suffix() {
        assert_eq!(normalize_uuid("00001816-0000-1000-8000-00805f9b34fb"), "1816");
    }

    #[test]
    fn test_short_uuid_is_unchanged() {
        assert_eq!(normalize_uuid("1816"), "1816");
    }

    #[test]
    fn test_trims_whitespace() {
        assert_eq!(normalize_uuid(" 1816 "), "1816");
    }

    #[test]
    fn test_lowercases_input() {
        assert_eq!(normalize_uuid("181D"), "181d");
    }

    #[test]
    fn test_custom_128bit_uuid_is_left_untouched() {
        assert_eq!(
            normalize_uuid("f0001816-0451-4000-b000-000000000000"),
            "f0001816-0451-4000-b000-000000000000"
        );
    }
}