enum ValidationError: Error {
    case tooShort
    case tooLong
    case containsInvalidCharacters
}

func validate(username: String) throws -> String {
    if username.count < 3 {
        throw ValidationError.tooShort
    }
    if username.count > 20 {
        throw ValidationError.tooLong
    }
    guard username.allSatisfy({ $0.isLetter || $0.isNumber }) else {
        throw ValidationError.containsInvalidCharacters
    }
    return username
}