record DynamicRoute(String from, String to) {}

DynamicRoute readRoute(Message message, String controlMailbox) {
    var mailboxes = message.value().lines().toList();

    if (mailboxes.size() != 2
            || mailboxes.get(0).isBlank()
            || mailboxes.get(1).isBlank()) {
        IO.println("ROUTE needs exactly two lines: <from> and <to>");
        return null;
    }

    var route = new DynamicRoute(mailboxes.get(0), mailboxes.get(1));

    try {
        FileMessageBoard.directory(route.from());
        FileMessageBoard.directory(route.to());
    } catch (IllegalArgumentException exception) {
        IO.println("Rejected ROUTE: " + exception.getMessage());
        return null;
    }

    if (route.from().equals(route.to())) {
        IO.println("Rejected ROUTE: from and to must differ");
        return null;
    }

    if (route.from().equals(controlMailbox)
            || route.to().equals(controlMailbox)) {
        IO.println("Rejected ROUTE: the control mailbox is not a data mailbox");
        return null;
    }

    return route;
}

void main(String[] arguments) throws Exception {
    if (arguments.length != 1
            || !arguments[0].matches("[A-Za-z0-9_-]+")) {
        IO.println("Usage: java DynamicMediatorService.java <mediator-name>");
        return;
    }

    var name = arguments[0];
    var controlMailbox = "mediators/" + name + "/inbox";

    try (var subscriptions = new FileSubscriptions()) {
        DynamicRoute route = null;
        subscriptions.subscribe(controlMailbox);

        IO.println(name + " waits for ROUTE on " + controlMailbox);

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

            if (incoming.mailbox().equals(controlMailbox)) {
                if (!message.type().equals("ROUTE")) {
                    IO.println("Ignored " + message.type() + "; expected ROUTE");
                    continue;
                }

                var requestedRoute = readRoute(message, controlMailbox);

                if (requestedRoute == null)
                    continue;

                if (route == null
                        || !route.from().equals(requestedRoute.from())) {
                    subscriptions.subscribe(requestedRoute.from());

                    if (route != null)
                        subscriptions.unsubscribe(route.from());
                }

                route = requestedRoute;

                IO.println(
                    name + " now mediates: "
                    + route.from() + " -> " + route.to()
                );
            } else if (route != null
                    && incoming.mailbox().equals(route.from())) {
                FileMessageBoard.publish(route.to(), message);
            }
        }
    }
}