248:    private static final byte[] keepAliveBytes = {
361:            0x01, 0x02, 0x00, 0x29,  0x00, 0x00, 0x00, 0x00,   0x00
390:    private void setTempoMaster(DeviceUpdate newMaster) {
421:    public void setTempoEpsilon(double epsilon) {
452:                metronome.setTempo(newTempo);
554:                setTempoMaster(update);
563:                    if (update.deviceNumber != masterYieldedFrom.get()) {
564:                        if (masterYieldedFrom.get() == 0) {
567:                            logger.warn("Expected player {} to yield master to us, but player {} did.", masterYieldedFrom.get(), update.deviceNumber);
571:                    masterYieldedFrom.set(0);
572:                    setTempoMaster(null);
582:                setTempoMaster(null);
1225:            setTempoMaster(null);
1254:                    setSendingStatus(false);
1273:    private void sendAnnouncement(InetAddress broadcastAddress) {
2006:                    if (number >= 0x29 && number < 0x30) { // We can see rekordbox mobile, this will work too.
2147:                    setSendingStatus(false);
2188:    public long sendBeat() {
2189:        return sendBeat(getPlaybackPosition());
2201:    public long sendBeat(Snapshot snapshot) {
2250:    public synchronized void setSendingStatus(boolean send) throws IOException {
2300:                sendStatus();
2424:    private static final byte[] MASTER_HANDOFF_REQUEST_PAYLOAD = { 0x01,
2441:    public synchronized void becomeTempoMaster() throws IOException {
2451:            byte[] payload = new byte[MASTER_HANDOFF_REQUEST_PAYLOAD.length];
2452:            System.arraycopy(MASTER_HANDOFF_REQUEST_PAYLOAD, 0, payload, 0, MASTER_HANDOFF_REQUEST_PAYLOAD.length);
2459:            assembleAndSendPacket(Util.PacketType.MASTER_HANDOFF_REQUEST, payload, currentMaster.address, BeatFinder.BEAT_PORT);
2491:                metronome.setTempo(tempo);
2527:                setTempo(getMasterTempo());
2582:    public void setTempo(double bpm) {
2588:        metronome.setTempo(bpm);
2671:    private final AtomicInteger masterYieldedFrom = new AtomicInteger(0);
2729:    private void sendStatus() {
2809:                            becomeTempoMaster();
2838:                                assembleAndSendPacket(Util.PacketType.MASTER_HANDOFF_RESPONSE, payload, lastStatusFromNewMaster.getAddress(), UPDATE_PORT);
2859:                            masterYieldedFrom.set(deviceNumber);
Shell cwd was reset to /Users/noah/conductor/workspaces/deadca7/san-diego


also do the load-track / load settings too


I'm looking at the key sections of the code that handle tempo synchronization—specifically the status packet template, the beat-sending logic, status updates, and the master handoff mechanism. I need to examine how these pieces work together to understand the tempo master transition flow.


Bash

cd /tmp && echo "=== sendBeat + setSendingStatus (2180-2320) ==="; sed -n '2180,2320p' vcdj.java

Read sendBeat and setSendingStatus


=== sendBeat + setSendingStatus (2180-2320) ===

    /**
     * Sends a beat packet. Generally this should only be invoked when our {@link BeatSender} has determined that it is
     * time to do so, but it is public to allow experimentation.
     *
     * @return the beat number that was sent, computed from the current (or stopped) playback position
     */
    @API(status = API.Status.STABLE)
    public long sendBeat() {
        return sendBeat(getPlaybackPosition());
    }

    /**
     * Sends a beat packet. Generally this should only be invoked when our {@link BeatSender} has determined that it is
     * time to do so, but it is public to allow experimentation.
     *
     * @param snapshot the time at which the beat to be sent occurred, for computation of its beat number
     *
     * @return the beat number that was sent
     */
    @API(status = API.Status.STABLE)
    public long sendBeat(Snapshot snapshot) {
        byte[] payload = new byte[BEAT_PAYLOAD.length];
        System.arraycopy(BEAT_PAYLOAD, 0, payload, 0, BEAT_PAYLOAD.length);
        payload[0x02] = getDeviceNumber();

        Util.numberToBytes((int)snapshot.getBeatInterval(), payload, 0x05, 4);
        Util.numberToBytes((int)(snapshot.getBeatInterval() * 2), payload, 0x09, 4);
        Util.numberToBytes((int)(snapshot.getBeatInterval() * 4), payload, 0x11, 4);
        Util.numberToBytes((int)(snapshot.getBeatInterval() * 8), payload, 0x19, 4);

        final int beatsLeft = 5 - snapshot.getBeatWithinBar();
        final int nextBar = (int)(snapshot.getBeatInterval() * beatsLeft);
        Util.numberToBytes(nextBar, payload, 0x0d, 4);
        Util.numberToBytes(nextBar + (int)snapshot.getBarInterval(), payload, 0x15, 4);

        Util.numberToBytes((int)Math.round(snapshot.getTempo() * 100), payload, 0x3b, 2);
        payload[0x3d] = (byte)snapshot.getBeatWithinBar();
        payload[0x40] = getDeviceNumber();

        try {
            assembleAndSendPacket(Util.PacketType.BEAT, payload, broadcastAddress.get(), BeatFinder.BEAT_PORT);
        } catch (IOException e) {
            logger.error("VirtualCdj Failed to send beat packet.", e);
        }

        return snapshot.getBeat();
    }

    /**
     * Will hold a non-null value when we are sending our own status packets, which can be used to stop the thread
     * doing so. Most uses of Beat Link will not require this level of activity. However, if you want to be able to
     * take over the tempo master role, and control the tempo and beat alignment of other players, you will need to
     * turn on this feature, which also requires that you are using one of the standard player numbers, 1-4.
     */
    private AtomicBoolean sendingStatus = null;

    /**
     * Control whether the Virtual CDJ sends status packets to the other players. Most uses of Beat Link will not
     * require this level of activity. However, if you want to be able to take over the tempo master role, and control
     * the tempo and beat alignment of other players, you will need to turn on this feature, which also requires that
     * you are using one of the standard player numbers, 1-4.
     *
     * @param send if {@code true} we will send status packets, and can participate in (and control) tempo and beat sync
     *
     * @throws IllegalStateException if the virtual CDJ is not running, or if it is not using a device number in the
     *                               range 1 through 4
     * @throws IOException if there is a problem starting the {@link BeatFinder}
     */
    @API(status = API.Status.STABLE)
    public synchronized void setSendingStatus(boolean send) throws IOException {
        if (isSendingStatus() == send) {
            return;
        }

        if (send) {  // Start sending status packets.
            ensureRunning();
            if (proxyingForVirtualRekordbox.get()) {
                throw new IllegalStateException("Cannot send status when in Opus Quad rekordbox lighting mode.");
            }
            if ((getDeviceNumber() < 1) || (getDeviceNumber() > 4)) {
                throw new IllegalStateException("Can only send status when using a standard player number, 1 through 4.");
            }

            BeatFinder.getInstance().start();
            BeatFinder.getInstance().addLifecycleListener(beatFinderLifecycleListener);

            final AtomicBoolean stillRunning = new AtomicBoolean(true);
            sendingStatus =  stillRunning;  // Allow other threads to stop us when necessary.
            startSenderThread(stillRunning);

            if (isSynced()) {  // If we are supposed to be synced, we need to respond to master beats and tempo changes.
                addMasterListener(ourSyncMasterListener);
            }

            if (isPlaying()) {  // Start the beat sender too, if we are supposed to be playing.
                beatSender.set(new BeatSender(metronome));
            }
        } else {  // Stop sending status packets, and responding to master beats and tempo changes if we were synced.
            BeatFinder.getInstance().removeLifecycleListener(beatFinderLifecycleListener);
            removeMasterListener(ourSyncMasterListener);

            sendingStatus.set(false);                          // Stop the status sending thread.
            sendingStatus = null;                              // Indicate that we are no longer sending status.
            final BeatSender activeSender = beatSender.get();  // And stop the beat sender if we have one.
            if (activeSender != null) {
                activeSender.shutDown();
                beatSender.set(null);
            }
        }
    }

    /**
     * Creates and starts the thread which sends our status packets as long as {@code stillRunning} remains {@code true}.
     *
     * @param stillRunning used to determine when to shut down
     */
    private void startSenderThread(AtomicBoolean stillRunning) {
        Thread sender = new Thread(null, () -> {
            while (stillRunning.get()) {
                sendStatus();
                try {
                    //noinspection BusyWait
                    Thread.sleep(getStatusInterval());
                } catch (InterruptedException e) {
                    logger.warn("beat-link VirtualCDJ status sender thread was interrupted; continuing");
                }
            }
        }, "beat-link VirtualCdj status sender");
        sender.setDaemon(true);
        sender.start();
    }

    /**
     * Check whether we are currently sending status packets.
     *
     * @return {@code true} if we are sending status packets, and can participate in (and control) tempo and beat sync
     */
    @API(status = API.Status.STABLE)
    public synchronized boolean isSendingStatus() {
        return (sendingStatus != null);
Shell cwd was reset to /Users/noah/conductor/workspaces/deadca7/san-diego


Prompt: also do the load-track / load settings too


yes we do want to drive tempo, and we also want to just close out packet dumps for now, so start it


 The user wants to expand the scope to include load-track and load-settings commands alongside the VCDJ status messaging. There's a critical constraint from beat-link: I can only send status messages using standard player numbers 1-4, so the virtual CDJ needs to claim one of those device slots rather than using slot 5. This is a substantial undertaking with that device numbering limitation as a key architectural decision.

I need to gather the remaining protocol templates: the beat payload structure, the status packet format with sendStatus, the master handoff request and response handling, and the load-track and load-settings payloads. I'm also looking for the status interval configuration. This is a large protocol implementation, so I'm being strategic about which templates to extract first—focusing on the core pieces needed for status emission, beat emission, and the load commands rather than trying to pull everything at once.


Noted — adding load-track and load-settings to the build. This is a big chunk of protocol; let me gather all the remaining templates (status, beat, master-handoff, load-track, load-settings) before writing code.


cd /tmp && echo "=== payload/template arrays ==="; grep -nE 'BEAT_PAYLOAD|LOAD_TRACK|LOAD_SETTINGS|MASTER_HANDOFF.*PAYLOAD|SYNC_CONTROL_PAYLOAD|playerStatusBytes|statusPayload|getStatusInterval' vcdj.java | head; echo; echo "=== status template + sendStatus (2675-2760) ==="; sed -n '2675,2760p' vcdj.java

Find payload templates and status sender


=== payload/template arrays ===
495:            case LOAD_TRACK_ACK:
1733:    private final static byte[] SYNC_CONTROL_PAYLOAD = { 0x01,
1747:        byte[] payload = new byte[SYNC_CONTROL_PAYLOAD.length];
1748:        System.arraycopy(SYNC_CONTROL_PAYLOAD, 0, payload, 0, SYNC_CONTROL_PAYLOAD.length);
1934:    private final static byte[] LOAD_TRACK_PAYLOAD = { 0x01,
1982:        byte[] payload = new byte[LOAD_TRACK_PAYLOAD.length];
1983:        System.arraycopy(LOAD_TRACK_PAYLOAD, 0, payload, 0, LOAD_TRACK_PAYLOAD.length);
2015:        assembleAndSendPacket(Util.PacketType.LOAD_TRACK_COMMAND, payload, target.getAddress(), UPDATE_PORT);
2028:    private final static byte[] LOAD_SETTINGS_PAYLOAD = { 0x02,
2068:        byte[] payload = new byte[LOAD_SETTINGS_PAYLOAD.length];

=== status template + sendStatus (2675-2760) ===
     */
    private final AtomicInteger packetCounter = new AtomicInteger(0);

    /**
     * The template used to assemble a status packet when we are sending them.
     */
    private final static byte[] STATUS_PAYLOAD = { 0x01,
            0x04, 0x00, 0x00, (byte)0xf8, 0x00, 0x00, 0x01, 0x00, 0x00,  0x03,  0x01,  0x00, 0x00, 0x00, 0x00, 0x01,  // 0x020
            0x00, 0x00, 0x00, 0x01, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, (byte)0xa0, 0x00, 0x00, 0x00, 0x00, 0x00,  // 0x030
            0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,  // 0x040
            0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,  // 0x050
            0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x01, 0x00, 0x04, 0x04, 0x00, 0x00, 0x00, 0x04,  // 0x060
            0x00, 0x00, 0x00, 0x04, 0x00, 0x01, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x31, 0x2e, 0x34, 0x33,  // 0x070
            0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, (byte)0xff, 0x00, 0x00, 0x10, 0x00, 0x00,  // 0x080
            (byte)0x80, 0x00, 0x00, 0x00, 0x7f, (byte)0xff, (byte)0xff, (byte)0xff, 0x00, 0x10, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,  // 0x090
            0x00, 0x00, 0x00, 0x00, 0x01, (byte)0xff, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,  // 0x0a0
            0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x01, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,  // 0x0b0
            0x00, 0x10, 0x00, 0x00, 0x00, 0x10, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x0f, 0x01, 0x00, 0x00,  // 0x0c0
            0x12, 0x34, 0x56, 0x78, 0x00, 0x00, 0x00, 0x01, 0x01, 0x01, 0x01, 0x01, 0x00, 0x00, 0x00, 0x00,  // 0x0d0
            0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,  // 0x0e0
            0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,  // 0x0f0
            0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,  // 0x100
            0x00, 0x00, 0x00, 0x15, 0x00, 0x00, 0x07, 0x61, 0x00, 0x00, 0x06, 0x2f  // 0x110
    };

    /**
     * Gets the current playback position, then checks if we are within {@link BeatSender#SLEEP_THRESHOLD} ms before
     * an upcoming beat, or {@link BeatSender#BEAT_THRESHOLD} ms after one, sleeping until that is no longer the case.
     *
     * @return the current playback position, potentially after having delayed a bit so that is not too near a beat
     */
    private Snapshot avoidBeatPacket() {
        Snapshot playState = getPlaybackPosition();
        double distance = playState.distanceFromBeat();
        while (playing.get() &&
                (((distance < 0.0) && (Math.abs(distance) <= BeatSender.SLEEP_THRESHOLD)) ||
                ((distance >= 0.0) && (distance <= (BeatSender.BEAT_THRESHOLD + 1))))) {
            try {
                //noinspection BusyWait
                Thread.sleep(2);
            } catch (InterruptedException e) {
                logger.warn("Interrupted while sleeping to avoid beat packet; ignoring.", e);
            }
            playState = getPlaybackPosition();
            distance = playState.distanceFromBeat();
        }
        return playState;
    }

    /**
     * Send a status packet to all devices on the network. Used when we are actively sending status, presumably so we
     * can be the tempo master. Avoids sending one within twice {@link BeatSender#BEAT_THRESHOLD} milliseconds of
     * a beat, to make sure the beat packet announces the new beat before an early status packet confuses matters.
     */
    private void sendStatus() {
        final Snapshot playState = avoidBeatPacket();
        final boolean playing = this.playing.get();
        byte[] payload = new byte[STATUS_PAYLOAD.length];
        System.arraycopy(STATUS_PAYLOAD, 0, payload, 0, STATUS_PAYLOAD.length);
        payload[0x02] = getDeviceNumber();
        payload[0x05] = payload[0x02];
        payload[0x08] = (byte)(playing ? 1 : 0);        // a, playing flag
        payload[0x09] = payload[0x02];                  // Dr, the player from which the track was loaded
        payload[0x5c] = (byte)(playing ? 3 : 5);        // P1, playing flag
        Util.numberToBytes(syncCounter.get(), payload, 0x65, 4);
        payload[0x6a] = (byte)(0x84 +                   // F, main status bit vector
                (playing ? 0x40 : 0) + (master.get() ? 0x20 : 0) + (synced.get() ? 0x10 : 0) + (onAir.get() ? 0x08 : 0));
        payload[0x6c] = (byte)(playing ? 0x7a : 0x7e);  // P2, playing flag
        Util.numberToBytes((int)Math.round(getTempo() * 100), payload, 0x73, 2);
        payload[0x7e] = (byte)(playing ? 9 : 1);        // P3, playing flag
        payload[0x7f] = (byte)(master.get() ? 1 : 0);   // Mm, tempo master flag
        payload[0x80] = (byte)nextMaster.get();         // Mh, tempo master handoff indicator
        Util.numberToBytes((int)playState.getBeat(), payload, 0x81, 4);
        payload[0x87] = (byte)(playState.getBeatWithinBar());
        Util.numberToBytes(packetCounter.incrementAndGet(), payload, 0xa9, 4);

        DatagramPacket packet = Util.buildPacket(Util.PacketType.CDJ_STATUS,
                ByteBuffer.wrap(keepAliveBytes, DEVICE_NAME_OFFSET, DEVICE_NAME_LENGTH).asReadOnlyBuffer(),
                ByteBuffer.wrap(payload));
        packet.setPort(UPDATE_PORT);
        for (DeviceAnnouncement device : DeviceFinder.getInstance().getCurrentDevices()) {
            packet.setAddress(device.getAddress());
            try {
                socket.get().send(packet);
            } catch (IOException e) {
                logger.warn("Unable to send status packet to {}", device, e);
Shell cwd was reset to /Users/noah/conductor/workspaces/deadca7/san-diego


 I'm working through the offset calculations for the status packet—the payload offsets versus the full packet offsets where the header adds 0x1f bytes. The STATUS_PAYLOAD template starts with 0x01 followed by data at what's marked as offset 0x020, but I need to clarify whether those comments refer to positions within the payload or the complete packet structure.

Mapping out the sendStatus field patches confirms the alignment: payload[0x02] for device number lands at full offset 0x21, payload[0x09] for source player at 0x28, payload[0x6a] for status flags at 0x89, payload[0x73] for tempo at 0x92—all matching the DecodeStatus structure offsets I've already identified. The pattern holds across multiple fields, which validates the offset scheme.

The status packet itself is roughly 284 bytes based on the template structure, which is much smaller than a CDJ-3000's full status but still acceptable to real CDJs. Given the scope of what needs to be implemented—status emission, beat tracking, master handoff, sync coordination, track loading, and settings—this is a substantial feature that'll likely run 400-600 lines. I need to gather the remaining payload templates and prioritize strategically. I need to gather the remaining protocol templates—BEAT_PAYLOAD, the load payloads, master handoff payload, and sync payload—then implement a focused subset: a metronome-driven status and beat emitter for tempo mastery, plus simple command senders for load-track and load-settings. I'll create a new master.go file for the tempo logic and add the load commands to the existing media handling.


The status template maps cleanly to our known offsets (`+0x1f`). Let me grab the remaining templates — beat, master-handoff, sync-control, load-track, load-settings — and the handoff-response logic in one pass.


cd /tmp && echo "=== BEAT_PAYLOAD ==="; grep -n 'BEAT_PAYLOAD =' vcdj.java; awk '/byte\[\] BEAT_PAYLOAD = /{f=1} f{print} f&&/};/{exit}' vcdj.java
echo "=== LOAD_TRACK_PAYLOAD + sender (1934-2016) ==="; sed -n '1934,2016p' vcdj.java

Read BEAT_PAYLOAD and load-track


=== BEAT_PAYLOAD ===
2175:    private static final byte[] BEAT_PAYLOAD = { 0x01,
    private static final byte[] BEAT_PAYLOAD = { 0x01,
            0x00, 0x0d, 0x00, 0x3c,  0x01, 0x01, 0x01, 0x01,   0x02, 0x02, 0x02, 0x02,  0x10, 0x10, 0x10, 0x10,
            0x04, 0x04, 0x04, 0x04,  0x20, 0x20, 0x20, 0x20,   0x08, 0x08, 0x08, 0x08,    -1,   -1,   -1,   -1,
            -1,   -1,   -1,   -1,    -1,   -1,   -1,   -1,     -1,   -1,   -1,   -1,    -1,   -1,   -1,   -1,
            -1,   -1,   -1,   -1,  0x00, 0x10, 0x00, 0x00,   0x00, 0x00, 0x00, 0x00,  0x0b, 0x00, 0x00, 0x0d};
=== LOAD_TRACK_PAYLOAD + sender (1934-2016) ===
    private final static byte[] LOAD_TRACK_PAYLOAD = { 0x01,
            0x00, 0x0d, 0x00, 0x34,  0x0d, 0x00, 0x00, 0x00,   0x00, 0x00, 0x00, 0x00,  0x00, 0x00, 0x00, 0x00,
            0x00, 0x00, 0x00, 0x32,  0x00, 0x00, 0x00, 0x00,   0x00, 0x00, 0x00, 0x00,  0x00, 0x00, 0x00, 0x00,
            0x00, 0x00, 0x00, 0x00,  0x00, 0x00, 0x00, 0x00,   0x00, 0x00, 0x00, 0x00,  0x00, 0x00, 0x00, 0x00,
            0x00, 0x00, 0x00, 0x00,  0x00, 0x00, 0x00, 0x00
    };

    /**
     * Send a packet to the target player telling it to load the specified track from the specified source player.
     *
     * @param targetPlayer the device number of the player that you want to have load a track
     * @param rekordboxId the identifier of a track within the source player's rekordbox database
     * @param sourcePlayer the device number of the player from which the track should be loaded
     * @param sourceSlot the media slot from which the track should be loaded
     * @param sourceType the type of track to be loaded
     *
     * @throws IOException if there is a problem sending the command
     * @throws IllegalStateException if the {@code VirtualCdj} is not active or the target device cannot be found
     */
    @API(status = API.Status.STABLE)
    public void sendLoadTrackCommand(int targetPlayer, int rekordboxId,
                                     int sourcePlayer, CdjStatus.TrackSourceSlot sourceSlot, CdjStatus.TrackType sourceType)
            throws IOException {
        final DeviceUpdate update = getLatestStatusFor(targetPlayer);
        if (update == null) {
            throw new IllegalArgumentException("Device " + targetPlayer + " not found on network.");
        }
        sendLoadTrackCommand(update, rekordboxId, sourcePlayer, sourceSlot, sourceType);
    }

    /**
     * Send a packet to the target device telling it to load the specified track from the specified source player.
     *
     * @param target an update from the player that you want to have load a track
     * @param rekordboxId the identifier of a track within the source player's rekordbox database
     * @param sourcePlayer the device number of the player from which the track should be loaded
     * @param sourceSlot the media slot from which the track should be loaded
     * @param sourceType the type of track to be loaded
     *
     * @throws IOException if there is a problem sending the command
     * @throws IllegalStateException if the {@code VirtualCdj} is not active
     */
    @API(status = API.Status.STABLE)
    public void sendLoadTrackCommand(DeviceUpdate target, int rekordboxId,
                                     int sourcePlayer, CdjStatus.TrackSourceSlot sourceSlot, CdjStatus.TrackType sourceType)
            throws IOException {
        ensureRunning();
        logger.info("Sending track load command, target player: {}, id: {}, sourcePlayer: {}, sourceSlot: {}, sourceType: {}", target.getDeviceNumber(), rekordboxId, sourcePlayer, sourceSlot, sourceType);
        byte[] payload = new byte[LOAD_TRACK_PAYLOAD.length];
        System.arraycopy(LOAD_TRACK_PAYLOAD, 0, payload, 0, LOAD_TRACK_PAYLOAD.length);
        payload[0x02] = getDeviceNumber();
        payload[0x05] = getDeviceNumber();
        payload[0x09] = (byte)sourcePlayer;
        payload[0x0a] = sourceSlot.protocolValue;
        payload[0x0b] = sourceType.protocolValue;
        payload[0x21] = (byte)(target.getDeviceNumber() - 1);
        if (target.deviceName.startsWith("XDJ-XZ")) {
            // Use the packet variant that seems to be necessary for the XDJ-XZ to accept it.
            payload[0x01] = 1;
            payload[0x2c] = 0x32;
            // See if we can find a rekordbox instance to pose as, which is also necessary to make this work.
            for (DeviceAnnouncement device : DeviceFinder.getInstance().getCurrentDevices()) {
                final byte number = (byte)device.getDeviceNumber();
                if (number >= 0x11 && number < 0x20) {  // Aha! We can see a regular rekordbox instance, this will work.
                    payload[0x02] = number;
                    payload[0x05] = number;
                    break;
                }
            }
            if (payload[0x02] == getDeviceNumber()) {  // We did not find a laptop rekordbox, see if there's a mobile one.
                for (DeviceAnnouncement device : DeviceFinder.getInstance().getCurrentDevices()) {
                    final byte number = (byte)device.getDeviceNumber();
                    if (number >= 0x29 && number < 0x30) { // We can see rekordbox mobile, this will work too.
                        payload[0x02] = number;
                        payload[0x05] = number;
                        break;
                    }
                }
            }
        }
        Util.numberToBytes(rekordboxId, payload, 0x0d, 4);
        assembleAndSendPacket(Util.PacketType.LOAD_TRACK_COMMAND, payload, target.getAddress(), UPDATE_PORT);
    }
Shell cwd was reset to /Users/noah/conductor/workspaces/deadca7/san-diego


 I've identified the load-track payload structure with specific byte offsets for device, source player, slot, type, and rekordbox ID, and now I need to locate the remaining payloads like LOAD_SETTINGS_PAYLOAD and MASTER_HANDOFF_REQUEST_PAYLOAD.

Now I'm looking at the master handoff response handling and sync control payload, and I'm realizing this is a massive feature scope. I'll focus on implementing the core pieces: a metronome with tempo tracking, VCDJ status emission with the STATUS_PAYLOAD, beat emission, the handoff protocol (sending 0x26 requests and handling 0x27 responses), and a main loop that broadcasts status and beats at the chosen tempo to each device's unicast address.

For the implementation, I'll need to handle a lot of state—master status, sync flags, playing state, on-air indicators, sync and packet counters—and build out LoadTrack and LoadSettings command builders, keeping the focus on driving tempo as the master device.

The real challenge is integrating this with the existing Online mode, which already owns port 50000 and runs keepalive. Status goes on 50002 and beats on 50001, so master mode needs to run alongside Online while pulling the device list from observed keepalives. Rather than trying to do everything at once, I'll build the core emitters as reusable templates in the link package, then wire them into a master.go with a capture-tool integration that can run in master mode with load-track and load-settings actions.


Let me get the last templates: load-settings, master-handoff, sync-control, and the handoff-response logic.


cd /tmp && echo "=== SYNC_CONTROL_PAYLOAD (1733) + sender ==="; sed -n '1733,1760p' vcdj.java
echo "=== LOAD_SETTINGS_PAYLOAD (2028-2075) ==="; sed -n '2028,2075p' vcdj.java
echo "=== MASTER_HANDOFF_REQUEST_PAYLOAD + becomeTempoMaster (2424-2470) ==="; sed -n '2424,2470p' vcdj.java
echo "=== handoff response handling (2800-2865) ==="; sed -n '2800,2865p' vcdj.java

Read remaining templates and handoff logic


=== SYNC_CONTROL_PAYLOAD (1733) + sender ===
    private final static byte[] SYNC_CONTROL_PAYLOAD = { 0x01,
            0x00, 0x0d, 0x00, 0x08, 0x00, 0x00, 0x00, 0x0d, 0x00, 0x00, 0x00, 0x0f };

    /**
     * Assemble and send a packet that performs sync control, turning a device's sync mode on or off, or telling it
     * to become the tempo master.
     *
     * @param target an update from the device whose sync state is to be set
     * @param command the byte identifying the specific sync command to be sent
     *
     * @throws IOException if there is a problem sending the command to the device
     */
    private void sendSyncControlCommand(DeviceUpdate target, byte command) throws IOException {
        ensureRunning();
        byte[] payload = new byte[SYNC_CONTROL_PAYLOAD.length];
        System.arraycopy(SYNC_CONTROL_PAYLOAD, 0, payload, 0, SYNC_CONTROL_PAYLOAD.length);
        payload[2] = getDeviceNumber();
        payload[8] = getDeviceNumber();
        payload[12] = command;
        assembleAndSendPacket(Util.PacketType.SYNC_CONTROL, payload, target.getAddress(), BeatFinder.BEAT_PORT);
    }

    /**
     * Tell a device to turn sync on or off.
     *
     * @param deviceNumber the device whose sync state is to be set
     * @param synced {@code} true if sync should be turned on, else it will be turned off
     *
=== LOAD_SETTINGS_PAYLOAD (2028-2075) ===
    private final static byte[] LOAD_SETTINGS_PAYLOAD = { 0x02,
            0x0d, 0x0d, 0x00, 0x50,  0x12, 0x34, 0x56, 0x78,   0x00, 0x00, 0x00, 0x03,  0x00, 0x00, 0x00, 0x00,
            0x00, 0x01, 0x00, 0x00,  0x00, 0x01, 0x01, 0x01,   0x00, 0x00, 0x00, 0x00,  0x00, 0x00, 0x00, 0x00,
            0x00, 0x00, 0x00, 0x00,  0x00, 0x00, 0x00, 0x00,   0x00, 0x00, 0x00, 0x00,  0x00, 0x00, 0x00, 0x00,
            0x00, 0x00, 0x00, 0x00,  0x00, 0x00, 0x00, 0x00,   0x00, 0x00, 0x00, 0x00,  0x00, 0x00, 0x00, 0x00,
            0x00, 0x00, 0x00, 0x00,  0x00, 0x00, 0x00, 0x00,   0x00, 0x00, 0x00, 0x00,  0x00, 0x00, 0x00, 0x00,
            0x00, 0x00, 0x00, 0x00
    };

    /**
     * Send a packet to the target player telling it to apply the supplied settings.
     *
     * @param targetPlayer the device number of the player that you want to configure
     * @param settings the device settings that should be loaded
     *
     * @throws IOException if there is a problem sending the command
     * @throws IllegalStateException if the {@code VirtualCdj} is not active or the target device cannot be found
     */
    @API(status = API.Status.STABLE)
    public void sendLoadSettingsCommand(int targetPlayer, PlayerSettings settings)
            throws IOException {
        final DeviceUpdate update = getLatestStatusFor(targetPlayer);
        if (update == null) {
            throw new IllegalArgumentException("Device " + targetPlayer + " not found on network.");
        }
        sendLoadSettingsCommand(update, settings);
    }

    /**
     * Send a packet to the target device telling it to apply the supplied settings.
     *
     * @param target an update from the player that you want to configure
     * @param settings the device settings that should be loaded
     *
     * @throws IOException if there is a problem sending the command
     * @throws IllegalStateException if the {@code VirtualCdj} is not active
     */
    @API(status = API.Status.STABLE)
    public void sendLoadSettingsCommand(DeviceUpdate target, PlayerSettings settings) throws IOException {
        ensureRunning();
        byte[] payload = new byte[LOAD_SETTINGS_PAYLOAD.length];
        System.arraycopy(LOAD_SETTINGS_PAYLOAD, 0, payload, 0, LOAD_SETTINGS_PAYLOAD.length);
        Util.setPayloadByte(payload, 0x20, getDeviceNumber());
        Util.setPayloadByte(payload, 0x21, (byte)target.getDeviceNumber());
        Util.setPayloadByte(payload, 0x2c, settings.onAirDisplay.protocolValue);
        Util.setPayloadByte(payload, 0x2d, settings.lcdBrightness.protocolValue);
        Util.setPayloadByte(payload, 0x2e, settings.quantize.protocolValue);
        Util.setPayloadByte(payload, 0x2f, settings.autoCueLevel.protocolValue);
=== MASTER_HANDOFF_REQUEST_PAYLOAD + becomeTempoMaster (2424-2470) ===
    private static final byte[] MASTER_HANDOFF_REQUEST_PAYLOAD = { 0x01,
            0x00, 0x0d, 0x00, 0x04, 0x00, 0x00, 0x00, 0x0d };

    /**
     * When have started the process of requesting the tempo master role from another player, this gets set to its
     * device number. Otherwise, it has the value {@code 0}.
     */
    private final AtomicInteger requestingMasterRoleFromPlayer = new AtomicInteger(0);

    /**
     * Arrange to become the tempo master. Starts a sequence of interactions with the other players that should end
     * up with us in charge of the group tempo and beat alignment.
     *
     * @throws IllegalStateException if we are not sending status updates
     * @throws IOException if there is a problem sending the master yield request
     */
    @API(status = API.Status.STABLE)
    public synchronized void becomeTempoMaster() throws IOException {
        logger.debug("Trying to become master.");
        if (!isSendingStatus()) {
            throw new IllegalStateException("Must be sending status updates to become the tempo master.");
        }

        // Is there someone we need to ask to yield to us?
        final DeviceUpdate currentMaster = getTempoMaster();
        if (currentMaster != null) {
            // Send the yield request; we will become master when we get a successful response.
            byte[] payload = new byte[MASTER_HANDOFF_REQUEST_PAYLOAD.length];
            System.arraycopy(MASTER_HANDOFF_REQUEST_PAYLOAD, 0, payload, 0, MASTER_HANDOFF_REQUEST_PAYLOAD.length);
            payload[2] = getDeviceNumber();
            payload[8] = getDeviceNumber();
            if (logger.isDebugEnabled()) {
                logger.debug("Sending master yield request to player {}", currentMaster);
            }
            requestingMasterRoleFromPlayer.set(currentMaster.deviceNumber);
            assembleAndSendPacket(Util.PacketType.MASTER_HANDOFF_REQUEST, payload, currentMaster.address, BeatFinder.BEAT_PORT);
        } else if (!master.get()) {
            // There is no other master, we can just become it immediately.
            requestingMasterRoleFromPlayer.set(0);
            setMasterTempo(getTempo());
            master.set(true);
        }
    }

    /**
     * Check whether we are currently in charge of the tempo and beat alignment.
     *
=== handoff response handling (2800-2865) ===
                setSynced(synced);
            }

            @Override
            public void becomeMaster() {
                logger.debug("Received packet telling us to become master.");
                if (isSendingStatus()) {
                    new Thread(() -> {
                        try {
                            becomeTempoMaster();
                        } catch (Throwable t) {
                            logger.error("Problem becoming tempo master in response to sync command packet", t);
                        }
                    }).start();
                } else {
                    logger.warn("Ignoring sync command to become tempo master, since we are not sending status packets.");
                }
            }
        });

        BeatFinder.getInstance().addMasterHandoffListener(new MasterHandoffListener() {
            @Override
            public void yieldMasterTo(int deviceNumber) {
                if (logger.isDebugEnabled()) {
                    logger.debug("Received instruction to yield master to device {}", deviceNumber);
                }
                if (isTempoMaster()) {
                    if (isSendingStatus() && getDeviceNumber() != deviceNumber) {
                        nextMaster.set(deviceNumber);
                        final DeviceUpdate lastStatusFromNewMaster = getLatestStatusFor(deviceNumber);
                        if (lastStatusFromNewMaster == null) {
                            logger.warn("Unable to send master yield response to device {}: no status updates have been received from it!", deviceNumber);
                        } else {
                            byte[] payload = new byte[YIELD_ACK_PAYLOAD.length];
                            System.arraycopy(YIELD_ACK_PAYLOAD, 0, payload, 0, YIELD_ACK_PAYLOAD.length);
                            payload[0x02] = getDeviceNumber();
                            payload[0x08] = getDeviceNumber();
                            try {
                                assembleAndSendPacket(Util.PacketType.MASTER_HANDOFF_RESPONSE, payload, lastStatusFromNewMaster.getAddress(), UPDATE_PORT);
                            } catch (Throwable t) {
                                logger.error("Problem sending master yield acknowledgment to player {}", deviceNumber, t);
                            }
                        }
                    }
                } else {
                    logger.warn("Ignoring instruction to yield master to device {}: we were not tempo master.", deviceNumber);

                }
            }

            @Override
            public void yieldResponse(int deviceNumber, boolean yielded) {
                if (logger.isDebugEnabled()) {
                    logger.debug("Received yield response of {} from device {}", yielded, deviceNumber);
                }
                if (yielded) {
                    if (isSendingStatus()) {
                        if (deviceNumber == requestingMasterRoleFromPlayer.get()) {
                            requestingMasterRoleFromPlayer.set(0);
                            masterYieldedFrom.set(deviceNumber);
                        } else {
                            if (requestingMasterRoleFromPlayer.get() == 0) {
                                logger.warn("Ignoring master yield response from player {} because we are not trying to become tempo master.", deviceNumber);
                            } else {
                                logger.warn("Ignoring master yield response from player {} because we asked player {}", deviceNumber, requestingMasterRoleFromPlayer.get());
                            }
Shell cwd was reset to /Users/noah/conductor/workspaces/deadca7/san-diego


 I'm reviewing the payload templates and protocol details—specifically noting that the MASTER_HANDOFF_RESPONSE packet gets sent over the UPDATE_PORT channel through the handoff handler's packet assembly function.

The request travels on BEAT_PORT while the response uses UPDATE_PORT, which is an interesting asymmetry. Given the scope here, I'm identifying the core building blocks: status emission with device state patches sent to all known devices, beat emission broadcast at tempo intervals, a metronome to calculate beat positions from BPM, the master handoff flow where a device requests mastership and waits for acknowledgment, handling incoming yield requests when we're the current master, sync control messaging, and track/settings loaders. This is a substantial feature—likely over 500 lines with careful concurrency handling—so I'll focus on building a working, focused implementation.

The real challenge is integration: the master needs to both emit packets and listen on 50001/50002 to react to incoming requests and track other devices. Rather than trying to bolt this onto the existing capture tool, I should create a dedicated `cmd/master` command that's self-contained. It would handle the device announcement handshake, bind both ports, run the status and beat senders at a configured tempo, read incoming packets to record them and respond to master-yield requests, optionally initiate a master takeover, and support load-track and load-settings commands. This duplicates some capture logic but keeps the master mode clean and independent. Given the context constraints, I need to identify the absolute minimum viable implementation that drives tempo, captures directed packets, and handles track/settings loading. I'm realizing the scope is getting unwieldy, so let me focus on what actually matters: the user needs to capture those directed packets (0x26/0x27/0x2a) when master changes, and I can do that by having our VCDJ announce as a device, become master, and listen for incoming requests. I'll keep it self-contained in cmd/master — announce online, track devices from keepalives, run status and beat senders on the right ports, and have a reader that captures and reacts to master requests when another deck tries to take over.

For the device number, I need to use 1 or 2 since the real decks are at 3 and 4, and the auto-assign logic starts at 5. I'll just hardcode device 1 with a flag override if needed.

On the status sending, I'm debating whether to broadcast or unicast to each device — beat-link unicasts to individual device IPs, but broadcasting on 50002 would be simpler and probably works fine for this test.

Now I'm setting up the metronome logic: given a start time and current time, I calculate the elapsed duration, divide by the interval (60000/tempo) to get the beat number, and use modulo 4 to get the beat-in-bar. For the actual packets, I need to define the STATUS_PAYLOAD and BEAT_PAYLOAD templates as hex byte arrays — I'll generate those from Python like I did before, though some bytes might be negative and need careful handling.

I'm planning to write a helper function that assembles the full packet by combining the magic header, type, device name (padded to 20 bytes), and payload. The payload templates are relative to offset 0x1f in the full packet, so I'll either patch the payload first then prepend the header, or build the full buffer and patch at absolute offsets. I'll generate all the payload hex strings in one Python call to stay within context limits, then embed them in the Go code.


I have the complete master/handoff/load protocol. This is a large feature — let me generate exact hex for all the payload templates in one shot, then build it.


Generate hex for status/beat/handoff/load templates


STATUS  len=253 "01040000f800000100000301000000000100000001000000000000a00000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000100040400000004000000040001000000000000312e343300000000000000000000ff0000100000800000007fffffff00100000000000000000000001ff00000000000000000000000000000000010000000000000000000010000000100000000000000f0100001234567800000001010101010000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000015000007610000062f"
BEAT    len= 65 "01000d003c010101010202020210101010040404042020202008080808ffffffffffffffffffffffffffffffffffffffffffffffff00100000000000000b00000d"
MREQ    len=  9 "01000d00040000000d"
SYNC    len= 13 "01000d00080000000d0000000f"
LOADT   len= 57 "01000d00340d000000000000000000000000000032000000000000000000000000000000000000000000000000000000000000000000000000"


 I'm organizing the code structure to fit the context budget—consolidating the master emission logic and builders into `master.go`, putting the load commands in `commands.go`, and keeping the entry point in `cmd/master/main.go`.

For the driver loop, I need to handle discovery and announcement through the Online mechanism, then manage three separate UDP channels: one for broadcast discovery on 50000, one for master negotiation on 50001, and one for responses and status on 50002. The key is tracking devices by their keepalives and status messages, maintaining shared state about which device is currently master, and implementing the master request/yield handshake—when a deck sends 0x26 asking us to yield, we respond with 0x27 if we're the master, and if we want to become master ourselves, we send 0x26 to the current master and wait for that acknowledgment. I'll run separate readers on 50001 and 50002 to record packets and react to these control messages, while also spinning up goroutines to emit status updates every ~200ms and beat signals at the appropriate intervals.

Given the scope, I'm going to keep this pragmatic and implement it directly in cmd/master rather than over-abstracting into the library. I'll use exported builder functions from the link package to construct packets cleanly, and define a simple StatusOut struct that captures the essential fields—beat, beat-in-bar, counter, master flag—so I can generate status packets without passing a dozen parameters.

Building out the packet constructors now: StatusPacket takes that StatusOut struct and patches the right byte offsets into a STATUS template (device ID at 0x02 and 0x05, playing flag at 0x08, sync counter at 0x65–0x68, the master/synced/onair flags packed into 0x6a, tempo at 0x73–0x74), BeatPacket for driving sync, MasterRequest and MasterResponse for the handshake, plus helpers for SyncControl and track/settings loading. These are all testable in isolation, then the driver loop just calls them in sequence. I need to fetch the YIELD_ACK_PAYLOAD structure to properly capture the master's response message, since getting the full handshake right is more important than approximating it.


All templates captured. Let me grab the one remaining payload (yield-ack / master-response) so the handoff response is byte-correct.


cd /tmp && grep -n 'YIELD_ACK_PAYLOAD =' vcdj.java; awk '/byte\[\] YIELD_ACK_PAYLOAD = /{f=1} f{print} f&&/};/{exit}' vcdj.java

Read yield-ack payload


2021:    private final static byte[] YIELD_ACK_PAYLOAD = { 0x01,
    private final static byte[] YIELD_ACK_PAYLOAD = { 0x01,
            0x00, 0x0d, 0x00, 0x08,  0x00, 0x00, 0x00, 0x0d,   0x00, 0x00, 0x00, 0x01
    };
Shell cwd was reset to /Users/noah/conductor/workspaces/deadca7/san-diego


 I've identified the yield acknowledgment payload structure with 13 bytes, where specific positions hold device identifiers and the flag at position 0x0c indicates the yielded state. Now I'm ready to write the Go code, starting with the packet builder and then implementing the various packet types like status, heartbeat, master request/response, and sync control handlers.

I need to generate the hex constants for all the packet templates from the Python output — status, beat, master request, yield acknowledgment, sync control, and load track payloads. For load settings, I'll compute the hex representation from the byte array I already have rather than making another round trip. I'm uncertain about how `setPayloadByte` maps offsets to the payload array, which makes implementing load-settings risky without seeing the actual function. Since load-track is well-understood and the user explicitly requested both commands, I should either quickly look up `setPayloadByte` to get it right, or implement load-track confidently and clearly mark load-settings as lower priority given the time constraints.


Last detail — `setPayloadByte`'s offset convention (load-settings uses it) and the load-settings hex:


Read setPayloadByte and load-settings hex


277:    public static void setPayloadByte(byte[] payload, int address, byte value) {
278-        payload[address - 0x1f] = value;
279-    }
280-
LOADS len=85 020d0d00501234567800000003000000000001000000010101000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000
Shell cwd was reset to /Users/noah/conductor/workspaces/deadca7/san-diego


 I'm working out the address mapping for the payload—the `setPayloadByte` function translates full addresses like 0x20, 0x21, 0x2c into payload indices by subtracting 0x1f, so I need to patch payload[0x01] when loading settings.

Now I'm setting up the code structure: I'll create a `buildPacket` helper that constructs the full packet with magic bytes, type field, name, and payload, then define the command builders for LoadTrack and LoadSettings with their respective type codes, and write tests to verify the packet structure.

Writing the Go code... Finishing the StatusPacket function by writing the counter at the second location, then moving on to implement BeatPacket which calculates beat intervals based on tempo and constructs a packet with timing information for the next beat and bar boundaries.

Now I'm setting up the master request, response, and sync control functions that all follow a similar pattern of copying a template, setting device identifiers, and building the appropriate packet type. Then I'm starting on the commands module with track loading functions that specify source and destination devices along with slot and track type information.

For the master driver, I need to build a focused tool that announces the device, runs the master emission loop to broadcast status and beats to all devices, and handles the handoff protocol while recording everything. I'll add command-line flags for device selection, tempo, playback state, track loading, and output options. The key challenge is discovering and maintaining the device list from keepalives and status packets, so I'm planning to use a mutex-guarded map that gets updated as packets arrive on the network.

The main flow is: open the three UDP channels and set broadcast on c0, discover the VCDJ device, then spawn reader goroutines on c1 and c2 that record packets and react to protocol messages—handling master requests by responding with our master status, tracking device IPs from status packets, and capturing master handoff signals. Once online, I'll start ticker goroutines for status emission every 200ms and beat emission if playing, building the status packet with our master flag, current tempo, and metronome beat calculated from elapsed time. For the metronome, I'm computing the beat number and position in the bar based on the interval between beats. After that, I can handle the load-track and load-settings commands by sending them to the target device.

Given the scope, I'll focus on writing the core builders and tests first—master.go and commands.go with their test coverage—then a working cmd/master driver to validate the packet emission, and optionally capture a directed packet in a brief session.

Writing master.go...


`setPayloadByte` = `payload[address-0x1f]`. I now have everything. Let me build it — `master.go` (status/beat/handoff/sync emitters), `commands.go` (load-track/settings), tests, and a `cmd/master` driver.


Write

package link

import "encoding/binary"

// Packet types a virtual CDJ sends to drive tempo and beat sync. A master role
// is handed off with a request (0x26) on the beat port answered by a response
// (0x27) on the update port; sync (0x2a) tells a device to sync or take master.
const (
	TypeMasterRequest  Type = 0x26
	TypeMasterResponse Type = 0x27
	TypeSyncControl    Type = 0x2a
)

// Sync control command bytes.
const (
	SyncOff    byte = 0x00
	SyncOn     byte = 0x10
	SyncMaster byte = 0x01
)

var (
	tmplBeat     = mustHex("01000d003c010101010202020210101010040404042020202008080808ffffffffffffffffffffffffffffffffffffffffffffffff00100000000000000b00000d")
	tmplMReq     = mustHex("01000d00040000000d")
	tmplStatus   = mustHex("01040000f800000100000301000000000100000001000000000000a00000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000100040400000004000000040001000000000000312e343300000000000000000000ff0000100000800000007fffffff00100000000000000000000001ff00000000000000000000000000000000010000000000000000000010000000100000000000000f0100001234567800000001010101010000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000015000007610000062f")
	tmplSync     = mustHex("01000d00080000000d0000000f")
	tmplYieldAck = mustHex("01000d00080000000d00000001")
)

// buildPacket frames a payload as a DJ Link packet: magic, type, 20-byte name.
func buildPacket(t Type, name string, payload []byte) []byte {
	b := make([]byte, 0x1f+len(payload))
	copy(b, Magic)
	b[0x0a] = byte(t)
	copy(b[0x0b:0x1f], name)
	copy(b[0x1f:], payload)
	return b
}

// StatusOut is the playback state a virtual CDJ broadcasts in a status packet.
type StatusOut struct {
	Beat      int
	BeatInBar byte
	Counter   uint32
	Master    bool
	OnAir     bool
	Playing   bool
	Synced    bool
	Tempo     float64
}

// StatusPacket builds a CDJ status (0x0a) packet announcing our playback state.
func (v VCDJ) StatusPacket(s StatusOut) []byte {
	p := append([]byte(nil), tmplStatus...)
	p[0x02] = v.Device
	p[0x05] = v.Device
	p[0x09] = v.Device
	flag := byte(0x84)
	if s.Playing {
		p[0x08] = 1
		p[0x5c] = 3
		p[0x6c] = 0x7a
		p[0x7e] = 9
		flag |= 0x40
	} else {
		p[0x5c] = 5
		p[0x6c] = 0x7e
		p[0x7e] = 1
	}
	if s.Master {
		flag |= 0x20
		p[0x7f] = 1
	}
	if s.Synced {
		flag |= 0x10
	}
	if s.OnAir {
		flag |= 0x08
	}
	p[0x6a] = flag
	binary.BigEndian.PutUint32(p[0x65:0x69], s.Counter)
	binary.BigEndian.PutUint16(p[0x73:0x75], uint16(s.Tempo*100))
	binary.BigEndian.PutUint32(p[0x81:0x85], uint32(s.Beat))
	p[0x87] = s.BeatInBar
	binary.BigEndian.PutUint32(p[0xa9:0xad], s.Counter)
	return buildPacket(TypeStatus, v.Name, p)
}

// BeatPacket builds a beat (0x28) packet for the given bar position and tempo.
func (v VCDJ) BeatPacket(beatInBar byte, tempo float64) []byte {
	p := append([]byte(nil), tmplBeat...)
	interval := uint32(60000 / tempo)
	p[0x02] = v.Device
	binary.BigEndian.PutUint32(p[0x05:0x09], interval)
	binary.BigEndian.PutUint32(p[0x09:0x0d], interval*2)
	binary.BigEndian.PutUint32(p[0x11:0x15], interval*4)
	binary.BigEndian.PutUint32(p[0x19:0x1d], interval*8)
	nextBar := interval * uint32(5-beatInBar)
	binary.BigEndian.PutUint32(p[0x0d:0x11], nextBar)
	binary.BigEndian.PutUint32(p[0x15:0x19], nextBar+interval*4)
	binary.BigEndian.PutUint16(p[0x3b:0x3d], uint16(tempo*100))
	p[0x3d] = beatInBar
	p[0x40] = v.Device
	return buildPacket(TypeBeat, v.Name, p)
}

// MasterRequest builds a master-handoff request (0x26) sent to the current master.
func (v VCDJ) MasterRequest() []byte {
	p := append([]byte(nil), tmplMReq...)
	p[0x02] = v.Device
	p[0x08] = v.Device
	return buildPacket(TypeMasterRequest, v.Name, p)
}

// MasterResponse builds a master-handoff yield acknowledgment (0x27).
func (v VCDJ) MasterResponse() []byte {
	p := append([]byte(nil), tmplYieldAck...)
	p[0x02] = v.Device
	p[0x08] = v.Device
	return buildPacket(TypeMasterResponse, v.Name, p)
}

// SyncControl builds a sync-control (0x2a) command for a target device.
func (v VCDJ) SyncControl(command byte) []byte {
	p := append([]byte(nil), tmplSync...)
	p[0x02] = v.Device
	p[0x08] = v.Device
	p[0x0c] = command
	return buildPacket(TypeSyncControl, v.Name, p)
}


File created successfully at: /Users/noah/conductor/workspaces/deadca7/san-diego/link/master.go (file state is current in your context — no need to Read it back)

package link

import "encoding/binary"

// Packet types a virtual CDJ sends to drive tempo and beat sync. A master role
// is handed off with a request (0x26) on the beat port answered by a response
// (0x27) on the update port; sync (0x2a) tells a device to sync or take master.
const (
	TypeMasterRequest  Type = 0x26
	TypeMasterResponse Type = 0x27
	TypeSyncControl    Type = 0x2a
)

// Sync control command bytes.
const (
	SyncOff    byte = 0x00
	SyncOn     byte = 0x10
	SyncMaster byte = 0x01
)

var (
	tmplBeat     = mustHex("01000d003c010101010202020210101010040404042020202008080808ffffffffffffffffffffffffffffffffffffffffffffffff00100000000000000b00000d")
	tmplMReq     = mustHex("01000d00040000000d")
	tmplStatus   = mustHex("01040000f800000100000301000000000100000001000000000000a00000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000100040400000004000000040001000000000000312e343300000000000000000000ff0000100000800000007fffffff00100000000000000000000001ff00000000000000000000000000000000010000000000000000000010000000100000000000000f0100001234567800000001010101010000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000015000007610000062f")
	tmplSync     = mustHex("01000d00080000000d0000000f")
	tmplYieldAck = mustHex("01000d00080000000d00000001")
)

// buildPacket frames a payload as a DJ Link packet: magic, type, 20-byte name.
func buildPacket(t Type, name string, payload []byte) []byte {
	b := make([]byte, 0x1f+len(payload))
	copy(b, Magic)
	b[0x0a] = byte(t)
	copy(b[0x0b:0x1f], name)
	copy(b[0x1f:], payload)
	return b
}

// StatusOut is the playback state a virtual CDJ broadcasts in a status packet.
type StatusOut struct {
	Beat      int
	BeatInBar byte
	Counter   uint32
	Master    bool
	OnAir     bool
	Playing   bool
	Synced    bool
	Tempo     float64
}

// StatusPacket builds a CDJ status (0x0a) packet announcing our playback state.
func (v VCDJ) StatusPacket(s StatusOut) []byte {
	p := append([]byte(nil), tmplStatus...)
	p[0x02] = v.Device
	p[0x05] = v.Device
	p[0x09] = v.Device
	flag := byte(0x84)
	if s.Playing {
		p[0x08] = 1
		p[0x5c] = 3
		p[0x6c] = 0x7a
		p[0x7e] = 9
		flag |= 0x40
	} else {
		p[0x5c] = 5
		p[0x6c] = 0x7e
		p[0x7e] = 1
	}
	if s.Master {
		flag |= 0x20
		p[0x7f] = 1
	}
	if s.Synced {
		flag |= 0x10
	}
	if s.OnAir {
		flag |= 0x08
	}
	p[0x6a] = flag
	binary.BigEndian.PutUint32(p[0x65:0x69], s.Counter)
	binary.BigEndian.PutUint16(p[0x73:0x75], uint16(s.Tempo*100))
	binary.BigEndian.PutUint32(p[0x81:0x85], uint32(s.Beat))
	p[0x87] = s.BeatInBar
	binary.BigEndian.PutUint32(p[0xa9:0xad], s.Counter)
	return buildPacket(TypeStatus, v.Name, p)
}

// BeatPacket builds a beat (0x28) packet for the given bar position and tempo.
func (v VCDJ) BeatPacket(beatInBar byte, tempo float64) []byte {
	p := append([]byte(nil), tmplBeat...)
	interval := uint32(60000 / tempo)
	p[0x02] = v.Device
	binary.BigEndian.PutUint32(p[0x05:0x09], interval)
	binary.BigEndian.PutUint32(p[0x09:0x0d], interval*2)
	binary.BigEndian.PutUint32(p[0x11:0x15], interval*4)
	binary.BigEndian.PutUint32(p[0x19:0x1d], interval*8)
	nextBar := interval * uint32(5-beatInBar)
	binary.BigEndian.PutUint32(p[0x0d:0x11], nextBar)
	binary.BigEndian.PutUint32(p[0x15:0x19], nextBar+interval*4)
	binary.BigEndian.PutUint16(p[0x3b:0x3d], uint16(tempo*100))
	p[0x3d] = beatInBar
	p[0x40] = v.Device
	return buildPacket(TypeBeat, v.Name, p)
}

// MasterRequest builds a master-handoff request (0x26) sent to the current master.
func (v VCDJ) MasterRequest() []byte {
	p := append([]byte(nil), tmplMReq...)
	p[0x02] = v.Device
	p[0x08] = v.Device
	return buildPacket(TypeMasterRequest, v.Name, p)
}

// MasterResponse builds a master-handoff yield acknowledgment (0x27).
func (v VCDJ) MasterResponse() []byte {
	p := append([]byte(nil), tmplYieldAck...)
	p[0x02] = v.Device
	p[0x08] = v.Device
	return buildPacket(TypeMasterResponse, v.Name, p)
}

// SyncControl builds a sync-control (0x2a) command for a target device.
func (v VCDJ) SyncControl(command byte) []byte {
	p := append([]byte(nil), tmplSync...)
	p[0x02] = v.Device
	p[0x08] = v.Device
	p[0x0c] = command
	return buildPacket(TypeSyncControl, v.Name, p)
}


package link

import "encoding/binary"

// Commands a virtual CDJ sends to a player on the update port: tell it to load a
// track (0x19, acknowledged with 0x1a) or apply player settings (0x34).
const (
	TypeLoadTrack    Type = 0x19
	TypeLoadTrackAck Type = 0x1a
	TypeLoadSettings Type = 0x34
)

var (
	tmplLoadSettings = mustHex("020d0d00501234567800000003000000000001000000010101000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000")
	tmplLoadTrack    = mustHex("01000d00340d000000000000000000000000000032000000000000000000000000000000000000000000000000000000000000000000000000")
)

// LoadTrack builds a load-track (0x19) command telling target to load the
// rekordbox track id from source's slot.
func (v VCDJ) LoadTrack(target, source byte, slot Slot, kind TrackType, id uint32) []byte {
	p := append([]byte(nil), tmplLoadTrack...)
	p[0x02] = v.Device
	p[0x05] = v.Device
	p[0x09] = source
	p[0x0a] = byte(slot)
	p[0x0b] = byte(kind)
	binary.BigEndian.PutUint32(p[0x0d:0x11], id)
	p[0x21] = target - 1
	return buildPacket(TypeLoadTrack, v.Name, p)
}

// LoadSettings builds a load-settings (0x34) command for target, applying the
// template's default player settings.
func (v VCDJ) LoadSettings(target byte) []byte {
	p := append([]byte(nil), tmplLoadSettings...)
	p[0x01] = v.Device
	p[0x02] = target
	return buildPacket(TypeLoadSettings, v.Name, p)
}


File created successfully at: /Users/noah/conductor/workspaces/deadca7/san-diego/link/commands.go (file state is current in your context — no need to Read it back)

package link

import "encoding/binary"

// Commands a virtual CDJ sends to a player on the update port: tell it to load a
// track (0x19, acknowledged with 0x1a) or apply player settings (0x34).
const (
	TypeLoadTrack    Type = 0x19
	TypeLoadTrackAck Type = 0x1a
	TypeLoadSettings Type = 0x34
)

var (
	tmplLoadSettings = mustHex("020d0d00501234567800000003000000000001000000010101000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000")
	tmplLoadTrack    = mustHex("01000d00340d000000000000000000000000000032000000000000000000000000000000000000000000000000000000000000000000000000")
)

// LoadTrack builds a load-track (0x19) command telling target to load the
// rekordbox track id from source's slot.
func (v VCDJ) LoadTrack(target, source byte, slot Slot, kind TrackType, id uint32) []byte {
	p := append([]byte(nil), tmplLoadTrack...)
	p[0x02] = v.Device
	p[0x05] = v.Device
	p[0x09] = source
	p[0x0a] = byte(slot)
	p[0x0b] = byte(kind)
	binary.BigEndian.PutUint32(p[0x0d:0x11], id)
	p[0x21] = target - 1
	return buildPacket(TypeLoadTrack, v.Name, p)
}

// LoadSettings builds a load-settings (0x34) command for target, applying the
// template's default player settings.
func (v VCDJ) LoadSettings(target byte) []byte {
	p := append([]byte(nil), tmplLoadSettings...)
	p[0x01] = v.Device
	p[0x02] = target
	return buildPacket(TypeLoadSettings, v.Name, p)
}


initialization failed: packages.Load error: err: exit status 1: stderr: go: go.work requires go >= 1.26.4 (running go 1.26.0)


Now builder tests, then the `cmd/master` driver:


package link

import (
	"net"
	"testing"

	"github.com/stretchr/testify/assert"
)

func TestMasterPackets(t *testing.T) {
	v := VCDJ{Device: 1, IP: net.ParseIP("192.168.0.160"), Name: "deadca7"}

	t.Run("status round-trips through DecodeStatus", func(t *testing.T) {
		a := assert.New(t)
		b := v.StatusPacket(StatusOut{Beat: 17, BeatInBar: 2, Master: true, Playing: true, Synced: true, Tempo: 128})
		a.Equal(TypeStatus, Type(b[0x0a]))
		s, err := DecodeStatus(b)
		a.NoError(err)
		a.Equal(byte(1), s.Device)
		a.Equal(128.0, s.BPM)
		a.EqualValues(17, s.Beat)
		a.Equal(byte(2), s.BeatInBar)
		a.True(s.Master)
		a.True(s.Playing)
		a.True(s.Synced)
	})

	t.Run("beat round-trips through DecodeBeat", func(t *testing.T) {
		a := assert.New(t)
		b := v.BeatPacket(3, 128)
		a.Equal(TypeBeat, Type(b[0x0a]))
		bt, err := DecodeBeat(b)
		a.NoError(err)
		a.Equal(byte(1), bt.Device)
		a.Equal(128.0, bt.BPM)
		a.Equal(byte(3), bt.BeatInBar)
	})

	t.Run("handoff and sync packet types and device", func(t *testing.T) {
		a := assert.New(t)
		for _, tc := range []struct {
			b   []byte
			typ Type
		}{
			{v.MasterRequest(), TypeMasterRequest},
			{v.MasterResponse(), TypeMasterResponse},
			{v.SyncControl(SyncMaster), TypeSyncControl},
			{v.LoadTrack(2, 3, SlotSD, TrackTypeRekordbox, 1), TypeLoadTrack},
			{v.LoadSettings(2), TypeLoadSettings},
		} {
			a.Equal(tc.typ, Type(tc.b[0x0a]))
			a.Equal("deadca7", string(tc.b[0x0b:0x0b+7]))
		}
	})

	t.Run("load track carries id and target", func(t *testing.T) {
		a := assert.New(t)
		b := v.LoadTrack(2, 3, SlotSD, TrackTypeRekordbox, 0x1234)
		a.EqualValues(0x1234, uint32(b[0x1f+0x0d])<<24|uint32(b[0x1f+0x0e])<<16|uint32(b[0x1f+0x0f])<<8|uint32(b[0x1f+0x10]))
		a.Equal(byte(1), b[0x1f+0x21]) // target 2 - 1
		a.Equal(byte(3), b[0x1f+0x09]) // source player
	})
}


File created successfully at: /Users/noah/conductor/workspaces/deadca7/san-diego/link/master_test.go (file state is current in your context — no need to Read it back)

package link

import (
	"net"
	"testing"

	"github.com/stretchr/testify/assert"
)

func TestMasterPackets(t *testing.T) {
	v := VCDJ{Device: 1, IP: net.ParseIP("192.168.0.160"), Name: "deadca7"}

	t.Run("status round-trips through DecodeStatus", func(t *testing.T) {
		a := assert.New(t)
		b := v.StatusPacket(StatusOut{Beat: 17, BeatInBar: 2, Master: true, Playing: true, Synced: true, Tempo: 128})
		a.Equal(TypeStatus, Type(b[0x0a]))
		s, err := DecodeStatus(b)
		a.NoError(err)
		a.Equal(byte(1), s.Device)
		a.Equal(128.0, s.BPM)
		a.EqualValues(17, s.Beat)
		a.Equal(byte(2), s.BeatInBar)
		a.True(s.Master)
		a.True(s.Playing)
		a.True(s.Synced)
	})

	t.Run("beat round-trips through DecodeBeat", func(t *testing.T) {
		a := assert.New(t)
		b := v.BeatPacket(3, 128)
		a.Equal(TypeBeat, Type(b[0x0a]))
		bt, err := DecodeBeat(b)
		a.NoError(err)
		a.Equal(byte(1), bt.Device)
		a.Equal(128.0, bt.BPM)
		a.Equal(byte(3), bt.BeatInBar)
	})

	t.Run("handoff and sync packet types and device", func(t *testing.T) {
		a := assert.New(t)
		for _, tc := range []struct {
			b   []byte
			typ Type
		}{
			{v.MasterRequest(), TypeMasterRequest},
			{v.MasterResponse(), TypeMasterResponse},
			{v.SyncControl(SyncMaster), TypeSyncControl},
			{v.LoadTrack(2, 3, SlotSD, TrackTypeRekordbox, 1), TypeLoadTrack},
			{v.LoadSettings(2), TypeLoadSettings},
		} {
			a.Equal(tc.typ, Type(tc.b[0x0a]))
			a.Equal("deadca7", string(tc.b[0x0b:0x0b+7]))
		}
	})

	t.Run("load track carries id and target", func(t *testing.T) {
		a := assert.New(t)
		b := v.LoadTrack(2, 3, SlotSD, TrackTypeRekordbox, 0x1234)
		a.EqualValues(0x1234, uint32(b[0x1f+0x0d])<<24|uint32(b[0x1f+0x0e])<<16|uint32(b[0x1f+0x0f])<<8|uint32(b[0x1f+0x10]))
		a.Equal(byte(1), b[0x1f+0x21]) // target 2 - 1
		a.Equal(byte(3), b[0x1f+0x09]) // source player
	})
}


Bash

cd /Users/noah/conductor/workspaces/deadca7/san-diego/link && go test . 2>&1 | tail -15

Run builder tests


ok  	github.com/nzoschke/deadca7/link	0.220s


 The builders are validating that status and beat data round-trip correctly through our decoders, confirming the emitters work as expected. Now I'm moving to the `cmd/master` driver, which needs to announce itself as a standard player device, broadcast status and beat information to all connected devices at the current tempo, and handle the master takeover protocol by sending the appropriate command to the current master.

I'm sketching out the structure: open the three UDP channels, set broadcast mode on c0, discover the network and assign a device number, then spawn goroutines to track connected devices from keepalives and status packets, record all traffic to a JSON lines file, and listen on c1 and c2 to react to incoming master requests and update device state. The master loop itself will either request mastership from the current holder or emit status and beat packets at regular intervals depending on whether we're taking over or just participating.

Writing the driver implementation... Writing the master service...


Builders pass — and critically, our emitted status/beat **round-trip through our own `DecodeStatus`/`DecodeBeat`** (BPM 128, beat 17, flags all correct). Now the driver:


Write

// Command master brings a virtual CDJ online as tempo master: it emits status
// and beat packets at a chosen tempo, takes the master role, answers handoff
// requests, and records all traffic so the directed master/sync packets can be
// captured. It can also push load-track and load-settings commands to a player.
//
//	go run ./cmd/master -device 1 -tempo 124 -playing -out master.jsonl -secs 60
package main

import (
	"context"
	"encoding/json"
	"flag"
	"fmt"
	"net"
	"os"
	"os/signal"
	"sync"
	"time"

	"github.com/cockroachdb/errors"
	"github.com/nzoschke/deadca7/link"
	"golang.org/x/sync/errgroup"
)

type Packet struct {
	Data string `json:"data"`
	Len  int    `json:"len"`
	Port int    `json:"port"`
	Src  string `json:"src"`
	TS   int64  `json:"ts"`
}

func main() {
	device := flag.Int("device", 1, "virtual CDJ device number (must be 1-4 to be master)")
	loadTrack := flag.String("load-track", "", "load a track: target,source,id (e.g. 3,3,1)")
	loadSettings := flag.Int("load-settings", 0, "send load-settings to this target player")
	name := flag.String("name", "deadca7", "virtual CDJ device name")
	out := flag.String("out", "master.jsonl", "output JSONL fixture path")
	playing := flag.Bool("playing", false, "emit beats and a playing status")
	secs := flag.Int("secs", 60, "run duration in seconds")
	tempo := flag.Float64("tempo", 120, "tempo in BPM to broadcast")
	flag.Parse()

	if *device < 1 || *device > 4 {
		fmt.Fprintln(os.Stderr, "device must be 1-4 to act as master")
		os.Exit(1)
	}

	cfg := config{
		device:       byte(*device),
		loadSettings: byte(*loadSettings),
		loadTrack:    *loadTrack,
		name:         *name,
		out:          *out,
		playing:      *playing,
		secs:         time.Duration(*secs) * time.Second,
		tempo:        *tempo,
	}
	if err := run(cfg); err != nil {
		fmt.Fprintf(os.Stderr, "%+v\n", err)
		os.Exit(1)
	}
}

type config struct {
	device       byte
	loadSettings byte
	loadTrack    string
	name         string
	out          string
	playing      bool
	secs         time.Duration
	tempo        float64
}

func run(cfg config) error {
	f, err := os.Create(cfg.out)
	if err != nil {
		return errors.WithStack(err)
	}
	defer f.Close()

	ctx, cancel := signal.NotifyContext(context.Background(), os.Interrupt)
	defer cancel()
	ctx, cancel = context.WithTimeout(ctx, cfg.secs)
	defer cancel()

	c0, err := listenUDP(50000)
	if err != nil {
		return err
	}
	defer c0.Close()
	if err := link.SetBroadcast(c0); err != nil {
		return err
	}
	c1, err := listenUDP(50001)
	if err != nil {
		return err
	}
	defer c1.Close()
	c2, err := listenUDP(50002)
	if err != nil {
		return err
	}
	defer c2.Close()

	vcdj, ipNet, err := online(c0, cfg.device, cfg.name)
	if err != nil {
		return err
	}

	enc := json.NewEncoder(f)
	var encMu sync.Mutex
	record := func(b []byte, port int, src *net.UDPAddr) {
		encMu.Lock()
		defer encMu.Unlock()
		enc.Encode(Packet{Data: fmt.Sprintf("%x", b), Len: len(b), Port: port, Src: src.String(), TS: time.Now().UnixMilli()})
	}

	d := &devices{ip: map[byte]net.IP{}}

	g, ctx := errgroup.WithContext(ctx)
	g.Go(func() error {
		return vcdj.Online(ctx, c0, ipNet, func(b []byte, src *net.UDPAddr) {
			if k, err := link.DecodeKeepAlive(b); err == nil {
				d.set(k.Device, k.IP)
			}
			record(b, 50000, src)
		})
	})
	g.Go(func() error { return reader(ctx, c1, 50001, d, vcdj, c2, record) })
	g.Go(func() error { return reader(ctx, c2, 50002, d, vcdj, c2, record) })

	vcdj.OnOnline = func() { drive(ctx, vcdj, c1, c2, d, cfg) }

	if err := g.Wait(); err != nil && !errors.Is(err, context.DeadlineExceeded) {
		return err
	}
	return nil
}

// drive emits status and beats as master and runs any one-shot commands.
func drive(ctx context.Context, v *link.VCDJ, c1, c2 *net.UDPConn, d *devices, cfg config) {
	if cfg.loadTrack != "" {
		var target, source, id int
		if _, err := fmt.Sscanf(cfg.loadTrack, "%d,%d,%d", &target, &source, &id); err == nil {
			if ip := d.get(byte(target)); ip != nil {
				c2.WriteToUDP(v.LoadTrack(byte(target), byte(source), link.SlotSD, link.TrackTypeRekordbox, uint32(id)), &net.UDPAddr{IP: ip, Port: 50002})
				fmt.Fprintf(os.Stderr, "sent load-track to player %d: track %d\n", target, id)
			}
		}
	}
	if cfg.loadSettings != 0 {
		if ip := d.get(cfg.loadSettings); ip != nil {
			c2.WriteToUDP(v.LoadSettings(cfg.loadSettings), &net.UDPAddr{IP: ip, Port: 50002})
			fmt.Fprintf(os.Stderr, "sent load-settings to player %d\n", cfg.loadSettings)
		}
	}

	// Ask the current master to yield, then assert ourselves as master.
	if m := d.master(); m != 0 && m != v.Device {
		if ip := d.get(m); ip != nil {
			c1.WriteToUDP(v.MasterRequest(), &net.UDPAddr{IP: ip, Port: 50001})
			fmt.Fprintf(os.Stderr, "requested master from player %d\n", m)
		}
	}

	interval := time.Duration(60000/cfg.tempo) * time.Millisecond
	start := time.Now()
	status := time.NewTicker(200 * time.Millisecond)
	defer status.Stop()
	beat := time.NewTicker(interval)
	defer beat.Stop()

	var counter uint32
	for {
		select {
		case <-ctx.Done():
			return
		case <-status.C:
			n := int(time.Since(start) / interval)
			counter++
			s := link.StatusOut{Beat: n + 1, BeatInBar: byte(n%4) + 1, Counter: counter, Master: true, OnAir: true, Playing: cfg.playing, Synced: true, Tempo: cfg.tempo}
			for _, ip := range d.all() {
				c2.WriteToUDP(v.StatusPacket(s), &net.UDPAddr{IP: ip, Port: 50002})
			}
		case <-beat.C:
			if cfg.playing {
				n := int(time.Since(start) / interval)
				c1.WriteToUDP(v.BeatPacket(byte(n%4)+1, cfg.tempo), &net.UDPAddr{IP: link.Broadcast(nil), Port: 50001})
			}
		}
	}
}

// reader records every datagram and answers master-handoff requests sent to us.
func reader(ctx context.Context, c *net.UDPConn, port int, d *devices, v *link.VCDJ, c2 *net.UDPConn, record func([]byte, int, *net.UDPAddr)) error {
	go func() { <-ctx.Done(); c.SetReadDeadline(time.Now()) }()
	buf := make([]byte, 1500)
	for {
		n, src, err := c.ReadFromUDP(buf)
		if err != nil {
			if ctx.Err() != nil {
				return nil
			}
			return errors.WithStack(err)
		}
		b := append([]byte(nil), buf[:n]...)
		if port == 50002 {
			if s, err := link.DecodeStatus(b); err == nil {
				d.set(s.Device, src.IP)
				if s.Master {
					d.setMaster(s.Device)
				}
			}
		}
		if port == 50001 && n > 0x0a && link.Type(b[0x0a]) == link.TypeMasterRequest {
			c2.WriteToUDP(v.MasterResponse(), &net.UDPAddr{IP: src.IP, Port: 50002})
			fmt.Fprintf(os.Stderr, "answered master-handoff request from %s\n", src.IP)
		}
		record(b, port, src)
	}
}

type devices struct {
	mu  sync.Mutex
	ip  map[byte]net.IP
	mas byte
}

func (d *devices) set(dev byte, ip net.IP) {
	d.mu.Lock()
	defer d.mu.Unlock()
	d.ip[dev] = append(net.IP(nil), ip...)
}

func (d *devices) setMaster(dev byte) { d.mu.Lock(); d.mas = dev; d.mu.Unlock() }

func (d *devices) master() byte { d.mu.Lock(); defer d.mu.Unlock(); return d.mas }

func (d *devices) get(dev byte) net.IP { d.mu.Lock(); defer d.mu.Unlock(); return d.ip[dev] }

func (d *devices) all() []net.IP {
	d.mu.Lock()
	defer d.mu.Unlock()
	var out []net.IP
	for _, ip := range d.ip {
		out = append(out, ip)
	}
	return out
}

func listenUDP(port int) (*net.UDPConn, error) {
	c, err := net.ListenUDP("udp", &net.UDPAddr{IP: net.IPv4zero, Port: port})
	return c, errors.WithStack(err)
}

func online(c0 *net.UDPConn, device byte, name string) (*link.VCDJ, *net.IPNet, error) {
	c0.SetReadDeadline(time.Now().Add(5 * time.Second))
	defer c0.SetReadDeadline(time.Time{})
	buf := make([]byte, 1500)
	for {
		n, _, err := c0.ReadFromUDP(buf)
		if err != nil {
			return nil, nil, errors.WithStack(err)
		}
		k, err := link.DecodeKeepAlive(buf[:n])
		if err != nil || k.Device == device {
			continue
		}
		iface, ipNet, err := link.Iface(k.IP)
		if err != nil {
			return nil, nil, err
		}
		return &link.VCDJ{Device: device, Log: func(s string) { fmt.Fprintln(os.Stderr, s) }, MAC: iface.HardwareAddr, Name: name}, ipNet, nil
	}
}


File created successfully at: /Users/noah/conductor/workspaces/deadca7/san-diego/link/cmd/master/main.go (file state is current in your context — no need to Read it back)

// Command master brings a virtual CDJ online as tempo master: it emits status
// and beat packets at a chosen tempo, takes the master role, answers handoff
// requests, and records all traffic so the directed master/sync packets can be
// captured. It can also push load-track and load-settings commands to a player.
//
//	go run ./cmd/master -device 1 -tempo 124 -playing -out master.jsonl -secs 60
package main

import (
	"context"
	"encoding/json"
	"flag"
	"fmt"
	"net"
	"os"
	"os/signal"
	"sync"
	"time"

	"github.com/cockroachdb/errors"
	"github.com/nzoschke/deadca7/link"
	"golang.org/x/sync/errgroup"
)

type Packet struct {
	Data string `json:"data"`
	Len  int    `json:"len"`
	Port int    `json:"port"`
	Src  string `json:"src"`
	TS   int64  `json:"ts"`
}

func main() {
	device := flag.Int("device", 1, "virtual CDJ device number (must be 1-4 to be master)")
	loadTrack := flag.String("load-track", "", "load a track: target,source,id (e.g. 3,3,1)")
	loadSettings := flag.Int("load-settings", 0, "send load-settings to this target player")
	name := flag.String("name", "deadca7", "virtual CDJ device name")
	out := flag.String("out", "master.jsonl", "output JSONL fixture path")
	playing := flag.Bool("playing", false, "emit beats and a playing status")
	secs := flag.Int("secs", 60, "run duration in seconds")
	tempo := flag.Float64("tempo", 120, "tempo in BPM to broadcast")
	flag.Parse()

	if *device < 1 || *device > 4 {
		fmt.Fprintln(os.Stderr, "device must be 1-4 to act as master")
		os.Exit(1)
	}

	cfg := config{
		device:       byte(*device),
		loadSettings: byte(*loadSettings),
		loadTrack:    *loadTrack,
		name:         *name,
		out:          *out,
		playing:      *playing,
		secs:         time.Duration(*secs) * time.Second,
		tempo:        *tempo,
	}
	if err := run(cfg); err != nil {
		fmt.Fprintf(os.Stderr, "%+v\n", err)
		os.Exit(1)
	}
}

type config struct {
	device       byte
	loadSettings byte
	loadTrack    string
	name         string
	out          string
	playing      bool
	secs         time.Duration
	tempo        float64
}

func run(cfg config) error {
	f, err := os.Create(cfg.out)
	if err != nil {
		return errors.WithStack(err)
	}
	defer f.Close()

	ctx, cancel := signal.NotifyContext(context.Background(), os.Interrupt)
	defer cancel()
	ctx, cancel = context.WithTimeout(ctx, cfg.secs)
	defer cancel()

	c0, err := listenUDP(50000)
	if err != nil {
		return err
	}
	defer c0.Close()
	if err := link.SetBroadcast(c0); err != nil {
		return err
	}
	c1, err := listenUDP(50001)
	if err != nil {
		return err
	}
	defer c1.Close()
	c2, err := listenUDP(50002)
	if err != nil {
		return err
	}
	defer c2.Close()

	vcdj, ipNet, err := online(c0, cfg.device, cfg.name)
	if err != nil {
		return err
	}

	enc := json.NewEncoder(f)
	var encMu sync.Mutex
	record := func(b []byte, port int, src *net.UDPAddr) {
		encMu.Lock()
		defer encMu.Unlock()
		enc.Encode(Packet{Data: fmt.Sprintf("%x", b), Len: len(b), Port: port, Src: src.String(), TS: time.Now().UnixMilli()})
	}

	d := &devices{ip: map[byte]net.IP{}}

	g, ctx := errgroup.WithContext(ctx)
	g.Go(func() error {
		return vcdj.Online(ctx, c0, ipNet, func(b []byte, src *net.UDPAddr) {
			if k, err := link.DecodeKeepAlive(b); err == nil {
				d.set(k.Device, k.IP)
			}
			record(b, 50000, src)
		})
	})
	g.Go(func() error { return reader(ctx, c1, 50001, d, vcdj, c2, record) })
	g.Go(func() error { return reader(ctx, c2, 50002, d, vcdj, c2, record) })

	vcdj.OnOnline = func() { drive(ctx, vcdj, c1, c2, d, cfg) }

	if err := g.Wait(); err != nil && !errors.Is(err, context.DeadlineExceeded) {
		return err
	}
	return nil
}

// drive emits status and beats as master and runs any one-shot commands.
func drive(ctx context.Context, v *link.VCDJ, c1, c2 *net.UDPConn, d *devices, cfg config) {
	if cfg.loadTrack != "" {
		var target, source, id int
		if _, err := fmt.Sscanf(cfg.loadTrack, "%d,%d,%d", &target, &source, &id); err == nil {
			if ip := d.get(byte(target)); ip != nil {
				c2.WriteToUDP(v.LoadTrack(byte(target), byte(source), link.SlotSD, link.TrackTypeRekordbox, uint32(id)), &net.UDPAddr{IP: ip, Port: 50002})
				fmt.Fprintf(os.Stderr, "sent load-track to player %d: track %d\n", target, id)
			}
		}
	}
	if cfg.loadSettings != 0 {
		if ip := d.get(cfg.loadSettings); ip != nil {
			c2.WriteToUDP(v.LoadSettings(cfg.loadSettings), &net.UDPAddr{IP: ip, Port: 50002})
			fmt.Fprintf(os.Stderr, "sent load-settings to player %d\n", cfg.loadSettings)
		}
	}

	// Ask the current master to yield, then assert ourselves as master.
	if m := d.master(); m != 0 && m != v.Device {
		if ip := d.get(m); ip != nil {
			c1.WriteToUDP(v.MasterRequest(), &net.UDPAddr{IP: ip, Port: 50001})
			fmt.Fprintf(os.Stderr, "requested master from player %d\n", m)
		}
	}

	interval := time.Duration(60000/cfg.tempo) * time.Millisecond
	start := time.Now()
	status := time.NewTicker(200 * time.Millisecond)
	defer status.Stop()
	beat := time.NewTicker(interval)
	defer beat.Stop()

	var counter uint32
	for {
		select {
		case <-ctx.Done():
			return
		case <-status.C:
			n := int(time.Since(start) / interval)
			counter++
			s := link.StatusOut{Beat: n + 1, BeatInBar: byte(n%4) + 1, Counter: counter, Master: true, OnAir: true, Playing: cfg.playing, Synced: true, Tempo: cfg.tempo}
			for _, ip := range d.all() {
				c2.WriteToUDP(v.StatusPacket(s), &net.UDPAddr{IP: ip, Port: 50002})
			}
		case <-beat.C:
			if cfg.playing {
				n := int(time.Since(start) / interval)
				c1.WriteToUDP(v.BeatPacket(byte(n%4)+1, cfg.tempo), &net.UDPAddr{IP: link.Broadcast(nil), Port: 50001})
			}
		}
	}
}

// reader records every datagram and answers master-handoff requests sent to us.
func reader(ctx context.Context, c *net.UDPConn, port int, d *devices, v *link.VCDJ, c2 *net.UDPConn, record func([]byte, int, *net.UDPAddr)) error {
	go func() { <-ctx.Done(); c.SetReadDeadline(time.Now()) }()
	buf := make([]byte, 1500)
	for {
		n, src, err := c.ReadFromUDP(buf)
		if err != nil {
			if ctx.Err() != nil {
				return nil
			}
			return errors.WithStack(err)
		}
		b := append([]byte(nil), buf[:n]...)
		if port == 50002 {
			if s, err := link.DecodeStatus(b); err == nil {
				d.set(s.Device, src.IP)
				if s.Master {
					d.setMaster(s.Device)
				}
			}
		}
		if port == 50001 && n > 0x0a && link.Type(b[0x0a]) == link.TypeMasterRequest {
			c2.WriteToUDP(v.MasterResponse(), &net.UDPAddr{IP: src.IP, Port: 50002})
			fmt.Fprintf(os.Stderr, "answered master-handoff request from %s\n", src.IP)
		}
		record(b, port, src)
	}
}

type devices struct {
	mu  sync.Mutex
	ip  map[byte]net.IP
	mas byte
}

func (d *devices) set(dev byte, ip net.IP) {
	d.mu.Lock()
	defer d.mu.Unlock()
	d.ip[dev] = append(net.IP(nil), ip...)
}

func (d *devices) setMaster(dev byte) { d.mu.Lock(); d.mas = dev; d.mu.Unlock() }

func (d *devices) master() byte { d.mu.Lock(); defer d.mu.Unlock(); return d.mas }

func (d *devices) get(dev byte) net.IP { d.mu.Lock(); defer d.mu.Unlock(); return d.ip[dev] }

func (d *devices) all() []net.IP {
	d.mu.Lock()
	defer d.mu.Unlock()
	var out []net.IP
	for _, ip := range d.ip {
		out = append(out, ip)
	}
	return out
}

func listenUDP(port int) (*net.UDPConn, error) {
	c, err := net.ListenUDP("udp", &net.UDPAddr{IP: net.IPv4zero, Port: port})
	return c, errors.WithStack(err)
}

func online(c0 *net.UDPConn, device byte, name string) (*link.VCDJ, *net.IPNet, error) {
	c0.SetReadDeadline(time.Now().Add(5 * time.Second))
	defer c0.SetReadDeadline(time.Time{})
	buf := make([]byte, 1500)
	for {
		n, _, err := c0.ReadFromUDP(buf)
		if err != nil {
			return nil, nil, errors.WithStack(err)
		}
		k, err := link.DecodeKeepAlive(buf[:n])
		if err != nil || k.Device == device {
			continue
		}
		iface, ipNet, err := link.Iface(k.IP)
		if err != nil {
			return nil, nil, err
		}
		return &link.VCDJ{Device: device, Log: func(s string) { fmt.Fprintln(os.Stderr, s) }, MAC: iface.HardwareAddr, Name: name}, ipNet, nil
	}
}


I'm realizing that passing `nil` to `Broadcast` won't work since it expects a `*net.IPNet` and would panic. I need to compute the actual broadcast address from the IP network and thread it through to the drive function so the beat packets can properly broadcast to the subnet's broadcast address on port 50001.


The beat broadcast uses `link.Broadcast(nil)` which would panic — let me thread the real broadcast address through:


Edit

vcdj.OnOnline = func() { drive(ctx, vcdj, c1, c2, d, cfg) }

	bcast := link.Broadcast(ipNet)
	vcdj.OnOnline = func() { drive(ctx, vcdj, c1, c2, d, cfg, bcast) }


The file /Users/noah/conductor/workspaces/deadca7/san-diego/link/cmd/master/main.go has been updated successfully. (file state is current in your context — no need to Read it back)

	bcast := link.Broadcast(ipNet)
	vcdj.OnOnline = func() { drive(ctx, vcdj, c1, c2, d, cfg, bcast) }

// Command master brings a virtual CDJ online as tempo master: it emits status
// and beat packets at a chosen tempo, takes the master role, answers handoff
// requests, and records all traffic so the directed master/sync packets can be
// captured. It can also push load-track and load-settings commands to a player.
//
//	go run ./cmd/master -device 1 -tempo 124 -playing -out master.jsonl -secs 60
package main

import (
	"context"
	"encoding/json"
	"flag"
	"fmt"
	"net"
	"os"
	"os/signal"
	"sync"
	"time"

	"github.com/cockroachdb/errors"
	"github.com/nzoschke/deadca7/link"
	"golang.org/x/sync/errgroup"
)

type Packet struct {
	Data string `json:"data"`
	Len  int    `json:"len"`
	Port int    `json:"port"`
	Src  string `json:"src"`
	TS   int64  `json:"ts"`
}

func main() {
	device := flag.Int("device", 1, "virtual CDJ device number (must be 1-4 to be master)")
	loadTrack := flag.String("load-track", "", "load a track: target,source,id (e.g. 3,3,1)")
	loadSettings := flag.Int("load-settings", 0, "send load-settings to this target player")
	name := flag.String("name", "deadca7", "virtual CDJ device name")
	out := flag.String("out", "master.jsonl", "output JSONL fixture path")
	playing := flag.Bool("playing", false, "emit beats and a playing status")
	secs := flag.Int("secs", 60, "run duration in seconds")
	tempo := flag.Float64("tempo", 120, "tempo in BPM to broadcast")
	flag.Parse()

	if *device < 1 || *device > 4 {
		fmt.Fprintln(os.Stderr, "device must be 1-4 to act as master")
		os.Exit(1)
	}

	cfg := config{
		device:       byte(*device),
		loadSettings: byte(*loadSettings),
		loadTrack:    *loadTrack,
		name:         *name,
		out:          *out,
		playing:      *playing,
		secs:         time.Duration(*secs) * time.Second,
		tempo:        *tempo,
	}
	if err := run(cfg); err != nil {
		fmt.Fprintf(os.Stderr, "%+v\n", err)
		os.Exit(1)
	}
}

type config struct {
	device       byte
	loadSettings byte
	loadTrack    string
	name         string
	out          string
	playing      bool
	secs         time.Duration
	tempo        float64
}

func run(cfg config) error {
	f, err := os.Create(cfg.out)
	if err != nil {
		return errors.WithStack(err)
	}
	defer f.Close()

	ctx, cancel := signal.NotifyContext(context.Background(), os.Interrupt)
	defer cancel()
	ctx, cancel = context.WithTimeout(ctx, cfg.secs)
	defer cancel()

	c0, err := listenUDP(50000)
	if err != nil {
		return err
	}
	defer c0.Close()
	if err := link.SetBroadcast(c0); err != nil {
		return err
	}
	c1, err := listenUDP(50001)
	if err != nil {
		return err
	}
	defer c1.Close()
	c2, err := listenUDP(50002)
	if err != nil {
		return err
	}
	defer c2.Close()

	vcdj, ipNet, err := online(c0, cfg.device, cfg.name)
	if err != nil {
		return err
	}

	enc := json.NewEncoder(f)
	var encMu sync.Mutex
	record := func(b []byte, port int, src *net.UDPAddr) {
		encMu.Lock()
		defer encMu.Unlock()
		enc.Encode(Packet{Data: fmt.Sprintf("%x", b), Len: len(b), Port: port, Src: src.String(), TS: time.Now().UnixMilli()})
	}

	d := &devices{ip: map[byte]net.IP{}}

	g, ctx := errgroup.WithContext(ctx)
	g.Go(func() error {
		return vcdj.Online(ctx, c0, ipNet, func(b []byte, src *net.UDPAddr) {
			if k, err := link.DecodeKeepAlive(b); err == nil {
				d.set(k.Device, k.IP)
			}
			record(b, 50000, src)
		})
	})
	g.Go(func() error { return reader(ctx, c1, 50001, d, vcdj, c2, record) })
	g.Go(func() error { return reader(ctx, c2, 50002, d, vcdj, c2, record) })

	vcdj.OnOnline = func() { drive(ctx, vcdj, c1, c2, d, cfg) }

	if err := g.Wait(); err != nil && !errors.Is(err, context.DeadlineExceeded) {
		return err
	}
	return nil
}

// drive emits status and beats as master and runs any one-shot commands.
func drive(ctx context.Context, v *link.VCDJ, c1, c2 *net.UDPConn, d *devices, cfg config) {
	if cfg.loadTrack != "" {
		var target, source, id int
		if _, err := fmt.Sscanf(cfg.loadTrack, "%d,%d,%d", &target, &source, &id); err == nil {
			if ip := d.get(byte(target)); ip != nil {
				c2.WriteToUDP(v.LoadTrack(byte(target), byte(source), link.SlotSD, link.TrackTypeRekordbox, uint32(id)), &net.UDPAddr{IP: ip, Port: 50002})
				fmt.Fprintf(os.Stderr, "sent load-track to player %d: track %d\n", target, id)
			}
		}
	}
	if cfg.loadSettings != 0 {
		if ip := d.get(cfg.loadSettings); ip != nil {
			c2.WriteToUDP(v.LoadSettings(cfg.loadSettings), &net.UDPAddr{IP: ip, Port: 50002})
			fmt.Fprintf(os.Stderr, "sent load-settings to player %d\n", cfg.loadSettings)
		}
	}

	// Ask the current master to yield, then assert ourselves as master.
	if m := d.master(); m != 0 && m != v.Device {
		if ip := d.get(m); ip != nil {
			c1.WriteToUDP(v.MasterRequest(), &net.UDPAddr{IP: ip, Port: 50001})
			fmt.Fprintf(os.Stderr, "requested master from player %d\n", m)
		}
	}

	interval := time.Duration(60000/cfg.tempo) * time.Millisecond
	start := time.Now()
	status := time.NewTicker(200 * time.Millisecond)
	defer status.Stop()
	beat := time.NewTicker(interval)
	defer beat.Stop()

	var counter uint32
	for {
		select {
		case <-ctx.Done():
			return
		case <-status.C:
			n := int(time.Since(start) / interval)
			counter++
			s := link.StatusOut{Beat: n + 1, BeatInBar: byte(n%4) + 1, Counter: counter, Master: true, OnAir: true, Playing: cfg.playing, Synced: true, Tempo: cfg.tempo}
			for _, ip := range d.all() {
				c2.WriteToUDP(v.StatusPacket(s), &net.UDPAddr{IP: ip, Port: 50002})
			}
		case <-beat.C:
			if cfg.playing {
				n := int(time.Since(start) / interval)
				c1.WriteToUDP(v.BeatPacket(byte(n%4)+1, cfg.tempo), &net.UDPAddr{IP: link.Broadcast(nil), Port: 50001})
			}
		}
	}
}

// reader records every datagram and answers master-handoff requests sent to us.
func reader(ctx context.Context, c *net.UDPConn, port int, d *devices, v *link.VCDJ, c2 *net.UDPConn, record func([]byte, int, *net.UDPAddr)) error {
	go func() { <-ctx.Done(); c.SetReadDeadline(time.Now()) }()
	buf := make([]byte, 1500)
	for {
		n, src, err := c.ReadFromUDP(buf)
		if err != nil {
			if ctx.Err() != nil {
				return nil
			}
			return errors.WithStack(err)
		}
		b := append([]byte(nil), buf[:n]...)
		if port == 50002 {
			if s, err := link.DecodeStatus(b); err == nil {
				d.set(s.Device, src.IP)
				if s.Master {
					d.setMaster(s.Device)
				}
			}
		}
		if port == 50001 && n > 0x0a && link.Type(b[0x0a]) == link.TypeMasterRequest {
			c2.WriteToUDP(v.MasterResponse(), &net.UDPAddr{IP: src.IP, Port: 50002})
			fmt.Fprintf(os.Stderr, "answered master-handoff request from %s\n", src.IP)
		}
		record(b, port, src)
	}
}

type devices struct {
	mu  sync.Mutex
	ip  map[byte]net.IP
	mas byte
}

func (d *devices) set(dev byte, ip net.IP) {
	d.mu.Lock()
	defer d.mu.Unlock()
	d.ip[dev] = append(net.IP(nil), ip...)
}

func (d *devices) setMaster(dev byte) { d.mu.Lock(); d.mas = dev; d.mu.Unlock() }

func (d *devices) master() byte { d.mu.Lock(); defer d.mu.Unlock(); return d.mas }

func (d *devices) get(dev byte) net.IP { d.mu.Lock(); defer d.mu.Unlock(); return d.ip[dev] }

func (d *devices) all() []net.IP {
	d.mu.Lock()
	defer d.mu.Unlock()
	var out []net.IP
	for _, ip := range d.ip {
		out = append(out, ip)
	}
	return out
}

func listenUDP(port int) (*net.UDPConn, error) {
	c, err := net.ListenUDP("udp", &net.UDPAddr{IP: net.IPv4zero, Port: port})
	return c, errors.WithStack(err)
}

func online(c0 *net.UDPConn, device byte, name string) (*link.VCDJ, *net.IPNet, error) {
	c0.SetReadDeadline(time.Now().Add(5 * time.Second))
	defer c0.SetReadDeadline(time.Time{})
	buf := make([]byte, 1500)
	for {
		n, _, err := c0.ReadFromUDP(buf)
		if err != nil {
			return nil, nil, errors.WithStack(err)
		}
		k, err := link.DecodeKeepAlive(buf[:n])
		if err != nil || k.Device == device {
			continue
		}
		iface, ipNet, err := link.Iface(k.IP)
		if err != nil {
			return nil, nil, err
		}
		return &link.VCDJ{Device: device, Log: func(s string) { fmt.Fprintln(os.Stderr, s) }, MAC: iface.HardwareAddr, Name: name}, ipNet, nil
	}
}

-  vcdj.OnOnline = func() { drive(ctx, vcdj, c1, c2, d, cfg) }

+  vcdj.OnOnline = func() { drive(ctx, vcdj, c1, c2, d, cfg, bcast) }


func drive(ctx context.Context, v *link.VCDJ, c1, c2 *net.UDPConn, d *devices, cfg config) {

func drive(ctx context.Context, v *link.VCDJ, c1, c2 *net.UDPConn, d *devices, cfg config, bcast net.IP) {