String rot13(String text) {
    var result = new StringBuilder();

    for (var character : text.toCharArray()) {
        if (character >= 'a' && character <= 'z')
            character = (char) ('a' + (character - 'a' + 13) % 26);
        else if (character >= 'A' && character <= 'Z')
            character = (char) ('A' + (character - 'A' + 13) % 26);

        result.append(character);
    }

    return result.toString();
}

void main() throws Exception {
    try (var subscriptions = new FileSubscriptions()) {
        var controlMailbox = "rot13/inbox";
        String textMailbox = null;
        subscriptions.subscribe(controlMailbox);

        while (true) {
            var incoming = subscriptions.take();
            var message = incoming.message();

            if (incoming.mailbox().equals(controlMailbox)
                    && message.type().equals("TOPIC")) {
                if (textMailbox != null)
                    subscriptions.unsubscribe(textMailbox);

                textMailbox = message.value();
                subscriptions.subscribe(textMailbox);
                IO.println("ROT13 now listens to " + textMailbox);
            } else if (message.type().equals("TEXT")) {
                FileMessageBoard.publish(
                    "rot13/outbox",
                    new Message("TEXT", rot13(message.value()))
                );
            }
        }
    }
}