pub fn decode_hex(s: &str) -> Result<Vec<u8>, String> {

    if !s.len().is_multiple_of(2) {

        Err("Hex string must have an even length".to_string())

        (0..s.len())

            .step_by(2)

            .map(|i| u8::from_str_radix(&s[i..i + 2], 16).map_err(|e| format!("{e}")))

            .collect()

}

    fn test_decode_hex() {

        assert_eq!(decode_hex("48656c6c6f").unwrap(), b"Hello");

    }

    fn test_decode_hex_empty() {

        assert_eq!(decode_hex("").unwrap(), Vec::<u8>::new());

    }

    fn test_decode_hex_odd_length() {

        let result = decode_hex("48656c6c6");

        assert!(result.is_err());

        assert_eq!(result.unwrap_err(), "Hex string must have an even length");

    }

    fn test_decode_hex_invalid_char() {

        let result = decode_hex("48656c6g6f");

        assert!(result.is_err());

        assert!(

            result

                .unwrap_err()

                .contains("invalid digit found in string")

    }

    fn test_decode_hex_uppercase() {

        assert_eq!(decode_hex("48656C6C6F").unwrap(), b"Hello");

    }

    fn test_decode_hex_zero_bytes() {

        assert_eq!(decode_hex("0000").unwrap(), vec![0, 0]);

    }

    fn test_decode_hex_all_ff() {

        assert_eq!(decode_hex("ffff").unwrap(), vec![255, 255]);

    }