Java

1// illustrative java model for this page: not Nexus source.2// the lobby on the right walks these exact states; a test checks the transitions match.3import java.time.Duration;4import java.time.Instant;5import java.util.EnumSet;6import java.util.Optional;7import java.util.Set;8 9enum SessionState {10    IDLE, HOSTING, INVITED, JOINING, VALIDATING, AUTHENTICATING, CONNECTED, REJECTED, CLOSED;11 12    Set<SessionState> next() {13        return switch (this) {14            case IDLE -> EnumSet.of(HOSTING);15            case HOSTING -> EnumSet.of(INVITED, CLOSED);16            case INVITED -> EnumSet.of(JOINING, CLOSED);17            case JOINING -> EnumSet.of(VALIDATING, REJECTED, CLOSED);18            case VALIDATING -> EnumSet.of(AUTHENTICATING, REJECTED, CLOSED);19            case AUTHENTICATING -> EnumSet.of(CONNECTED, CLOSED);20            case CONNECTED -> EnumSet.of(CLOSED);21            case REJECTED -> EnumSet.noneOf(SessionState.class);22            case CLOSED -> EnumSet.noneOf(SessionState.class);23        };24    }25}26 27enum Rejection { INVITE_EXPIRED, INVITE_ALREADY_USED, AUTH_FAILED }28 29/** a short token someone can type: valid once, and only for a little while. */30record Invite(String token, Instant expiresAt, boolean used) {31    static Invite issue(String token, Duration ttl) {32        return new Invite(token, Instant.now().plus(ttl), false);33    }34 35    /** empty means the invite is fine. expiry is checked before reuse. */36    Optional<Rejection> check(Instant now) {37        if (!now.isBefore(expiresAt)) return Optional.of(Rejection.INVITE_EXPIRED);38        if (used) return Optional.of(Rejection.INVITE_ALREADY_USED);39        return Optional.empty();40    }41 42    Invite consume() {43        return new Invite(token, expiresAt, true);44    }45}46 47final class Session {48    private SessionState state = SessionState.IDLE;49    private Invite invite;50 51    SessionState state() {52        return state;53    }54 55    void moveTo(SessionState target) {56        if (!state.next().contains(target)) {57            throw new IllegalStateException(state + " cannot go to " + target);58        }59        state = target;60    }61 62    void host() {63        moveTo(SessionState.HOSTING);64    }65 66    void createInvite(String token) {67        moveTo(SessionState.INVITED);68        invite = Invite.issue(token, Duration.ofSeconds(90));69    }70 71    void playerJoins() {72        moveTo(SessionState.JOINING);73    }74 75    /** the invite is checked and spent here, before anyone is trusted. */76    Optional<Rejection> validate(Instant now) {77        Optional<Rejection> problem = invite.check(now);78        if (problem.isPresent()) {79            moveTo(SessionState.REJECTED);80            return problem;81        }82        invite = invite.consume();83        moveTo(SessionState.VALIDATING);84        return Optional.empty();85    }86 87    Optional<Rejection> authenticate(boolean credentialsOk) {88        if (!credentialsOk) {89            moveTo(SessionState.REJECTED);90            return Optional.of(Rejection.AUTH_FAILED);91        }92        moveTo(SessionState.AUTHENTICATING);93        return Optional.empty();94    }95 96    void connect() {97        moveTo(SessionState.CONNECTED);98    }99 100    void hostOffline() {101        moveTo(SessionState.CLOSED);102    }103}