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

- [x] two-deck status captured (deck 3 track 1; deck 4 empty, bpm 655.35 sentinel)
- [x] dynamic scenarios captured & decoded: playing, on-air toggle, tempo change,
      track load (see Beat sync section)

## Not passively observable (need active participation, not bystander capture)

A bystander virtual CDJ has now captured every broadcast packet on the network.
The remaining enum types are directed device-to-device or request/response, so
they require our virtual CDJ to act, not just listen:

- 0x26/0x27 master-handoff, 0x2a sync-control (50001): directed between the
  master and the controlled deck. Evidence: a live capture where sync went true
  and master moved between decks produced zero of these packets. To capture,
  our VCDJ must be tempo master or the sync target (needs outgoing status/beat).
- 0x02 fader-start (50001): only when the DJM fader-start feature is enabled.
- 0x29 mixer-status (50002): DJM-V10 does NOT emit it — uses 0x03 + 0x04 instead.
- 0x05/0x06 media-query/response, 0x19/0x1a load-track, 0x34 load-settings:
  request/response we must initiate (the dbserver-style flow).
- 0x10 rekordbox-lighting-hello, 0x56 opus-metadata: other device classes, N/A.

# link TODO

Go port of Pioneer Pro DJ Link, built capture-first: document each packet,
capture a real fixture from the CDJ-3000, then decode it under test. No test
should require live hardware — `cmd/capture` records fixtures into `testdata/`.

## Setup

- CDJ-3000 (device 3) at `192.168.0.192`, DJM-V10 mixer (device 33) at
  `192.168.0.193`, this Mac at `192.168.0.160` (en0), one `/24`.
- Reference SD card loaded in the CDJ for the database (crate-digger) flows.
- Passive capture: `go run ./cmd/capture -out testdata/<name>.jsonl -secs 8`.
- Active flows need a virtual CDJ that announces first — see Announce below.
  RPC/NFS uses dynamic ports, so capture those with `tcpdump` alongside.

## beat-link — live network protocol (UDP)

Each packet starts with Magic `51 73 70 74 31 57 6d 4a 4f 4c` + a type byte
read together with the port.

### Discovery / startup — port 50000
- [x] keep-alive (type 0x06): name, device #, IP, MAC — `testdata/devices.jsonl`
      covers CDJ-3000 (device 3) and DJM-V10 (device 33); name at 0x0c
- [x] virtual CDJ four-stage startup handshake (announce.go): hello (0x0a) ×3,
      claim 0x00/0x02/0x04 ×3 at 300ms, then keep-alive (0x06) at 1500ms.
      Verified the CDJ-3000 then streams 0x0a status — `testdata/status.jsonl`.
- [x] device-number negotiation (announce.go Online): self-assign (device 0)
      after a watch period, accept mixer assignment (0x01/0x03/0x05), abort/retry
      on a defense (0x08), and defend our own number once online. Verified fixed
      and auto-assign both reach 0x0a status; auto picked 5 (CDJ=3, mixer=33 used).
- [x] defense-out verified live: held device 4 while a second CDJ (.194) fixed
      to 4 was powered on; it oscillated 4<->25 as we defended 28 times. Real
      claim-stage-2 packets in `testdata/claim.jsonl`, parsed by ClaimNumber.
- [ ] mixer/gateway packet seen on 50002 (type 0x40, "NXS-GW") — identify & decode

### Beat sync — port 50001
- [x] beat packet (type 0x28): BPM, beat-in-bar, pitch — `testdata/beats.jsonl`
      (name@0x0b; bpm@0x5a centi; beat@0x5c; pitch@0x55 3-byte, 0x100000=100%)
- [x] precise position (type 0x0b, DecodePosition): device@0x21, length(s)@0x24,
      playback(ms)@0x28, pitch@0x2c, bpm@0x38 — `testdata/position.jsonl`
- [x] channels-on-air (type 0x03, DecodeOnAir): ch1-4@0x24-0x27, ch5-6@0x2d-0x2e
      (DJM-V10 0x35-len), device@0x21 — `testdata/onair.jsonl` (toggle 3<->4)
- [x] live two-deck session validated all decoders dynamically: playing flag,
      advancing position (deck 4 1466->93023ms), tempo change (deck 3 105 & 114
      BPM), on-air toggle, master/sync, track loads (deck 3 tracks 1->2->3).
- [ ] next/prev beat-timing distances (six 4-byte BE values from ~0x24)
- [x] mixer type 0x04 (len 100, DJM-V10) — `testdata/mixer04.jsonl`: device 33,
      near-static, only byte 0x24 varies; sparse periodic housekeeping, low value

### Status — port 50002 (needs virtual CDJ online)
- [x] captured CDJ 0x0a status (len 1116) — `testdata/status.jsonl`
- [x] decode 0x0a (status.go DecodeStatus): play/master/sync/on-air flags @0x89,
      bpm @0x92, pitch @0x8d, beat @0xa0, beat-in-bar @0xa6
- [x] loaded-track fields: track ID @0x2c, source player @0x28, slot @0x29,
      type @0x2a. Verified: track 1, player 3, SD, rekordbox.
- [x] CDJ type 0x40 on 50002 identified: "NXS-GW" (Nexus Gateway) keepalive,
      device 25, low-rate, undocumented on djl-analysis — not player status, skip
- [x] two-deck status captured (deck 3 track 1; deck 4 empty, bpm 655.35 sentinel)
- [ ] capture scenarios: playing, on-air toggle, tempo change, track load

## crate-digger — rekordbox database (over NFS)

Reached only after the virtual CDJ announces and a deck reports a loaded track.

- [x] no portmap needed: CDJ serves mount + NFS on UDP 2049 (nfs/nfs.go)
- [x] MOUNT slot path (SD=/B/, USB=/C/) → root file handle
- [x] NFS LOOKUP + READ `PIONEER/rekordbox/export.pdb` (cmd/fetch) —
      pulled 184320 bytes off CDJ-3000 SD into `testdata/export.pdb`
- [x] PDB parse (pdb/): kaitai-generated rekordbox_pdb.go + wrapper resolving
      tracks->title/artist/album/artwork/anlz. Tested vs `testdata/export.pdb`.
      (go vet warns on self-assignment in the generated file; upstream artifact)
- [x] metadata-by-track-ID: track 1 -> "get it" by mobygratis, matches status
- [ ] playlists / playlist tree
- [ ] ANLZ parse: `.DAT`/`.EXT` (beat grid, cue points, waveforms)
- [ ] wire status track ID + slot -> NFS fetch -> PDB lookup as one Link call

## Reliability (port the vizlab code without its known bugs)

- [x] proper startup handshake instead of vizlab blasting only keep-alives —
      this was the main instability: real players need hello + claim first
- [x] SetBroadcast (SO_BROADCAST) so subnet-broadcast sends don't silently fail
- [ ] replace `time.Sleep(5s)` FIXME in `Start()` with discovery/mount readiness
- [ ] make the media query deterministic (vizlab's was "unreliable")
- [x] swap `github.com/pkg/errors` → `github.com/cockroachdb/errors`
- [ ] live integration suite gated behind `testing.Short()` / env, separate from
      the hermetic fixture-replay tests

-- [ ] capture scenarios: playing, on-air toggle, tempo change, track load

+- [x] dynamic scenarios captured & decoded: playing, on-air toggle, tempo change,

+      track load (see Beat sync section)

+## Not passively observable (need active participation, not bystander capture)

+A bystander virtual CDJ has now captured every broadcast packet on the network.

+The remaining enum types are directed device-to-device or request/response, so

+they require our virtual CDJ to act, not just listen:

+- 0x26/0x27 master-handoff, 0x2a sync-control (50001): directed between the

+  master and the controlled deck. Evidence: a live capture where sync went true

+  and master moved between decks produced zero of these packets. To capture,

+  our VCDJ must be tempo master or the sync target (needs outgoing status/beat).

+- 0x02 fader-start (50001): only when the DJM fader-start feature is enabled.

+- 0x29 mixer-status (50002): DJM-V10 does NOT emit it — uses 0x03 + 0x04 instead.

+- 0x05/0x06 media-query/response, 0x19/0x1a load-track, 0x34 load-settings:

+  request/response we must initiate (the dbserver-style flow).

+- 0x10 rekordbox-lighting-hello, 0x56 opus-metadata: other device classes, N/A.


Running the final test suite to verify everything passes, then I'll compile the results.


Bash

rm -f testdata/control.jsonl && go test ./... 2>&1 | tail -6 && echo "--- fixtures ---" && ls testdata

Clean up, run tests, list fixtures


ok  	github.com/nzoschke/deadca7/link	(cached)
?   	github.com/nzoschke/deadca7/link/cmd/capture	[no test files]
?   	github.com/nzoschke/deadca7/link/cmd/fetch	[no test files]
?   	github.com/nzoschke/deadca7/link/nfs	[no test files]
ok  	github.com/nzoschke/deadca7/link/pdb	(cached)
--- fixtures ---
beats.jsonl
claim.jsonl
devices.jsonl
export.pdb
mixer04.jsonl
onair.jsonl
position.jsonl
status.jsonl


We've now chased down everything observable. Here's the definitive conclusion.

## Network capture coverage is complete for a bystander virtual CDJ

I built the full packet-type checklist from beat-link's enum and verified each against our captures. **Every packet that's broadcast on the network has been captured and decoded.** The control capture this round produced no new types — and crucially, the earlier *live* capture where **sync went true and master moved between decks still produced zero `0x2a`/`0x26`/`0x27`** — which proves those aren't broadcast to bystanders.

The remaining enum types fall into three "not passively observable" buckets (now documented in `TODO.md`):

1. **Directed device-to-device** — master-handoff (`0x26`/`0x27`), sync-control (`0x2a`). Sent between the master and the controlled deck; a third-party VCDJ never sees them. Capturing them requires our VCDJ to *be* master or the sync target → needs outgoing status/beat (a feature, not a capture).
2. **Request/response we must initiate** — media-query/response (`0x05`/`0x06`), load-track (`0x19`/`0x1a`), load-settings (`0x34`).
3. **Not emitted by this gear / N/A** — `0x29` mixer-status (**finding: the DJM-V10 doesn't send it**, uses `0x03`+`0x04`); `0x02` fader-start (needs the DJM feature enabled); `0x10` rekordbox-lighting and `0x56` opus-metadata (other device classes).

Also resolved the last unknown: **mixer `0x04`** is a near-static DJM-V10 housekeeping packet (only byte `0x24` varies across samples, ~0.05/s) — no decodable dynamic fields, low value.

## Final state of the capture/decode work

Decoded + tested against real fixtures: **keep-alive, full startup negotiation, beat, precise position, channels-on-air, CDJ status** (all flags + loaded track), plus the **NFS fetch → PDB parse** database path. Fixtures kept small (`devices`, `beats`, `position`, `onair`, `status`, `claim`, `mixer04`, `export.pdb`); all raw multi-MB captures discarded. All tests green.

To go beyond passive capture from here, the two doors are: **(a)** make the VCDJ act as tempo master / send status (unlocks the directed control packets and lets it drive sync), or **(b)** implement the **media-query / load-track** initiator flow. Both are "active participant" features rather than capture work — say the word if you want to open either, or we can move to wiring this into the app.


can we trigger / capture the media query request response,
if its not too hard lets do the VCDJ master outgoing status / capture too
does my V10 have the fader start feature? how do I enable


 I've got three things to work through here. First, I need to capture the media query request and response on that endpoint—we'll initiate the query and grab what comes back. Second, if it's feasible, I want to monitor the VCDJ master's outgoing status and capture it, sending our own status as master with beat information to see the master handoff and sync control directed at us. Third, I'm checking whether the DJM-V10 supports Fader Start and how to turn it on—looks like it does have this feature with channel fader start capability.

For the DJM-V10 specifically, Fader Start is configured through the Utility settings or via a SHIFT shortcut on Pioneer mixers when connected over PRO DJ LINK. The exact button sequence varies by model, so I should verify the precise steps for the V10 rather than guess at the controls.

On the media query side, I'm looking at the protocol structure—we send a 0x05 packet to the target player's IP on port 50002 with our device number and the target slot, and the player responds with a 0x06 packet containing media details like name, track count, and color. I'm checking how beat-link constructs this media query packet to understand the exact format.

For the VCDJ outgoing status and master control, this is more substantial. Sending a basic status packet periodically is moderate work, but becoming tempo master with beat broadcasting and handling master handoffs is considerably more complex. I should prioritize the media query first since it's concrete and high-value, then assess whether the master status implementation is feasible or if it's too involved for this scope.

Let me run parallel research on the DJM-V10 fader start behavior, pull the beat-link media query packet structure, and check how VirtualCdj currently handles outgoing status packets.


Let me research all three in parallel: the DJM-V10 fader-start procedure, the media-query packet format, and how beat-link sends outgoing status.


WebSearch

Question: Pioneer AlphaTheta DJM-V10 fader start enable channel fader start setting utility


Bash

cd /tmp && echo "=== media query in VirtualCdj.java ==="; grep -nE 'MEDIA_QUERY|mediaQuery|requestMediaDetails|sendMediaDetail|0x05|MEDIA_RESPONSE|MediaDetails' vcdj.java | head -20; echo "=== search MediaDetailsListener / packet build ==="; gh api repos/Deep-Symmetry/beat-link/git/trees/main?recursive=1 --jq '.tree[].path' 2>/dev/null | grep -iE 'media' | head

Find media query code and DJM-V10 fader start


can we trigger / capture the media query request response, if its not too hard lets do the VCDJ master outgoing status / capture too does my V10 have the fader start feature? how do I enable


Web search results for query: "Pioneer AlphaTheta DJM-V10 fader start enable channel fader start setting utility"

Links: [{"title":"Does the DJM-V10 support the Fader Start feature? – AlphaTheta Help Center","url":"https://support.pioneerdj.com/hc/en-us/articles/4408479744409-Does-the-DJM-V10-support-the-Fader-Start-feature"},{"title":"Using my DJM-V10 – AlphaTheta Help Center","url":"https://support.pioneerdj.com/hc/en-us/sections/4408486280089-Using-my-DJM-V10"},{"title":"DJM-V10 - AlphaTheta Help Center","url":"https://support.pioneerdj.com/hc/en-us/sections/4416568754457-DJM-V10"},{"title":"DJM-V10 > FAQ | AlphaTheta Support","url":"https://faq.pioneerdj.com/product.php?c=6880&lang=en&p=DJM-V10&t=faq"},{"title":"DJM-V10-LF > FAQ | AlphaTheta Support","url":"https://faq.pioneerdj.com/product.php?c=13&lang=en&p=DJM-V10-LF&t=faq"},{"title":"How to use the Pioneer DJ DJM-V10 DJ Mixer (Instruction manual) - AlphaTheta Help Center","url":"https://support.pioneerdj.com/hc/en-us/articles/4404659749145-How-to-use-the-Pioneer-DJ-DJM-V10-DJ-Mixer-Instruction-manual"},{"title":"There are three controls to adjust the volume: TRIM, channel fader, and MASTER LEVEL. What's the difference? – AlphaTheta Help Center","url":"https://support.pioneerdj.com/hc/en-us/articles/4409816634009-There-are-three-controls-to-adjust-the-volume-TRIM-channel-fader-and-MASTER-LEVEL-What-s-the-difference"},{"title":"Pioneer DJM V10 User Manual | PDF | Disc Jockey | Headphones","url":"https://www.scribd.com/document/658531563/Pioneer-DJM-V10-User-Manual"},{"title":"Are the channel fader curves on the DJM-V10 and the DJM-V10-LF the same? – AlphaTheta Help Center","url":"https://support.pioneerdj.com/hc/en-us/articles/4406611970713-Are-the-channel-fader-curves-on-the-DJM-V10-and-the-DJM-V10-LF-the-same"}]

Based on the search results, the DJM-V10 does not support the Fader Start feature. This means there is no fader start enable setting or channel fader start utility available for this mixer model.

The search results show that Pioneer/AlphaTheta support documentation explicitly addresses this limitation, indicating that fader start functionality is not part of the DJM-V10's capabilities.

REMINDER: You MUST include the sources above in your response to the user using markdown hyperlinks.

Question: Pioneer AlphaTheta DJM-V10 fader start enable channel fader start setting utility

DJM-V10 - AlphaTheta Help Center

DJM-V10 > FAQ | AlphaTheta Support

DJM-V10-LF > FAQ | AlphaTheta Support


=== media query in VirtualCdj.java ===
499:            case MEDIA_QUERY:
503:            case MEDIA_RESPONSE:
504:                deliverMediaDetailsUpdate(new MediaDetails(packet));
1540:    private final Set<MediaDetailsListener> detailsListeners =
1560:    public void addMediaDetailsListener(MediaDetailsListener listener) {
1574:    public void removeMediaDetailsListener(MediaDetailsListener listener) {
1586:    public Set<MediaDetailsListener> getMediaDetailsListeners() {
1597:    public void deliverMediaDetailsUpdate(final MediaDetails details) {
1598:        for (MediaDetailsListener listener : getMediaDetailsListeners()) {
1703:    private final static byte[] MEDIA_QUERY_PAYLOAD = { 0x01,
1721:        byte[] payload = new byte[MEDIA_QUERY_PAYLOAD.length];
1722:        System.arraycopy(MEDIA_QUERY_PAYLOAD, 0, payload, 0, MEDIA_QUERY_PAYLOAD.length);
1727:        assembleAndSendPacket(Util.PacketType.MEDIA_QUERY, payload, announcement.getAddress(), UPDATE_PORT);
1985:        payload[0x05] = getDeviceNumber();
1999:                    payload[0x05] = number;
2008:                        payload[0x05] = number;
2206:        Util.numberToBytes((int)snapshot.getBeatInterval(), payload, 0x05, 4);
2685:            0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,  // 0x050
2735:        payload[0x05] = payload[0x02];
=== search MediaDetailsListener / packet build ===
(eval):1: no matches found: repos/Deep-Symmetry/beat-link/git/trees/main?recursive=1
Shell cwd was reset to /Users/noah/conductor/workspaces/deadca7/san-diego


The DJM-V10 doesn't support fader start, which explains why we never saw that 0x02 message — the mixer simply can't emit it. Now I need to examine the media query implementation in vcdj.java and how it parses the 0x06 response, along with the MediaDetails class and the packet assembly logic.


**Question 3 answered definitively: the DJM-V10 does *not* support Fader Start** — per [AlphaTheta's own support article](https://support.pioneerdj.com/hc/en-us/articles/4408479744409-Does-the-DJM-V10-support-the-Fader-Start-feature). That's exactly why we never saw `0x02` — your mixer can't emit it. One mystery closed for free.

Now let me get the media-query packet format and the response parser.


cd /tmp && echo "=== MEDIA_QUERY_PAYLOAD + requestMediaDetails ==="; sed -n '1700,1735p' vcdj.java; echo "=== assembleAndSendPacket ==="; awk '/private (static )?DatagramPacket assembleAndSendPacket|void assembleAndSendPacket|assembleAndSendPacket\(/{f=1} f{print} /^    }$/{if(f)c++; if(c>=1 && f && /^    }$/){exit}}' vcdj.java | head -40

Read media query payload and packet assembly


=== MEDIA_QUERY_PAYLOAD + requestMediaDetails ===
    /**
     * The bytes after the device name in a media query packet.
     */
    private final static byte[] MEDIA_QUERY_PAYLOAD = { 0x01,
    0x00, 0x0d, 0x00, 0x0c, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00};

    /**
     * Ask a device for information about the media mounted in a particular slot. Will update the
     * {@link MetadataFinder} when a response is received.
     *
     * @param slot the slot holding media we want to know about.
     *
     * @throws IOException if there is a problem sending the request.
     */
    @API(status = API.Status.STABLE)
    public void sendMediaQuery(SlotReference slot) throws IOException {
        final DeviceAnnouncement announcement = DeviceFinder.getInstance().getLatestAnnouncementFrom(slot.player);
        if (announcement == null) {
            throw new IllegalArgumentException("Device for " + slot + " not found on network.");
        }
        ensureRunning();
        byte[] payload = new byte[MEDIA_QUERY_PAYLOAD.length];
        System.arraycopy(MEDIA_QUERY_PAYLOAD, 0, payload, 0, MEDIA_QUERY_PAYLOAD.length);
        payload[2] = getDeviceNumber();
        System.arraycopy(keepAliveBytes, 44, payload, 5, 4);  // Copy in our IP address.
        payload[12] = (byte)slot.player;
        payload[16] = slot.slot.protocolValue;
        assembleAndSendPacket(Util.PacketType.MEDIA_QUERY, payload, announcement.getAddress(), UPDATE_PORT);
    }

    /**
     * The bytes after the device name in a sync control command packet.
     */
    private final static byte[] SYNC_CONTROL_PAYLOAD = { 0x01,
            0x00, 0x0d, 0x00, 0x08, 0x00, 0x00, 0x00, 0x0d, 0x00, 0x00, 0x00, 0x0f };

=== assembleAndSendPacket ===
    private void assembleAndSendPacket(Util.PacketType kind, byte[] payload, InetAddress destination, int port) throws IOException {
        DatagramPacket packet = Util.buildPacket(kind,
                ByteBuffer.wrap(keepAliveBytes, DEVICE_NAME_OFFSET, DEVICE_NAME_LENGTH).asReadOnlyBuffer(),
                ByteBuffer.wrap(payload));
        packet.setAddress(destination);
        packet.setPort(port);
        try {
            socket.get().send(packet);
        } catch (java.net.NoRouteToHostException e) {
            logger.warn("Unable to route packet to player at address {}", destination);
            listNetworkInterfaces();
            throw e;
        }
    }
Shell cwd was reset to /Users/noah/conductor/workspaces/deadca7/san-diego


I'm working through the packet structure for media queries—the header contains magic bytes plus a type byte, followed by the device name at offset 0x0b for 20 bytes, then the payload. From the ByteBuffer allocation I saw earlier, the total packet size is 0x1f (31 bytes) plus the payload length, so I need to figure out exactly how the header is laid out before the device name field. I need to trace through the exact packet structure in buildPacket to understand where the device name sits relative to the type byte, since this affects how the media query payload is constructed. The payload itself starts with specific bytes that get partially overwritten—device number replaces a byte at index 2, the IP address goes into indices 5-8, and the target player and slot protocol value go into later positions. I need to check the exact structure of buildPacket to confirm where the name field starts and whether there's padding before it.


The media query is clear. Let me get the exact packet framing (`Util.buildPacket`) and the response parser (`MediaDetails`) so I build it byte-correct.


cd /tmp && gh api repos/Deep-Symmetry/beat-link/contents/src/main/java/org/deepsymmetry/beatlink/Util.java --jq '.content' 2>/dev/null | base64 -d | awk '/static DatagramPacket buildPacket/,/return new DatagramPacket/' | head -30
echo "=== MediaDetails parsing (0x06 response) ==="
gh api repos/Deep-Symmetry/beat-link/contents/src/main/java/org/deepsymmetry/beatlink/MediaDetails.java --jq '.content' 2>/dev/null | base64 -d | grep -nE 'bytesToNumber|getText|0x[0-9a-f]+|name =|trackCount|playlistCount|hasMySettings|color|mediaType|protocolValue|slot' | head -30

Read buildPacket and MediaDetails parser


    public static DatagramPacket buildPacket(PacketType type, ByteBuffer deviceName, ByteBuffer payload) {
        ByteBuffer content = ByteBuffer.allocate(0x1f + payload.remaining());
        content.put(getMagicHeader());
        content.put(type.protocolValue);
        content.put(deviceName);
        content.put(payload);
        return new DatagramPacket(content.array(), content.capacity());
=== MediaDetails parsing (0x06 response) ===
17: * Represents information about the media mounted in a player's slot; returned in response to a media query packet.
25:     * The player and slot in which this media resides.
28:    public final SlotReference slotReference;
34:    public final CdjStatus.TrackType mediaType;
49:     * The number of rekordbox tracks in the media database. Will be zero if {@link #mediaType} is not rekordbox.
52:    public final int trackCount;
55:     * The number of rekordbox playlists in the media database. Will be zero if {@link #mediaType} is not rekordbox.
58:    public final int playlistCount;
61:     * The color tint that should be applied to the user interface when a player mounts this media.
64:    public final Color color;
71:    public final boolean hasMySettings;
105:    private static final Set<Integer> expectedMediaPacketSizes = new HashSet<>(Collections.singletonList(0xc0));
112:    public static final int MINIMUM_PACKET_SIZE = 0xc0;
128:            if (source[i] == 0x00 && source[i + 1] == 0x00) {
140:     * @param slotReference Slot Reference to which these media details belong
141:     * @param mediaType     Media Type to Emulate
143:     * @param trackCount    How many tracks are present in the media
144:     * @param playlistCount How many playlists are present in the media
148:    public MediaDetails(SlotReference slotReference, CdjStatus.TrackType mediaType, String name,
149:                 int trackCount, int playlistCount, long lastModified) {
153:        this.slotReference = slotReference;
154:        this.mediaType = mediaType;
155:        this.name = name;
157:        this.trackCount = trackCount;
159:        this.playlistCount = playlistCount;
161:        this.color = new Color(0);
163:        this.hasMySettings = false;
192:        final int payloadLength = (int) Util.bytesToNumber(packetCopy, 0x22, 2);
193:        if (packetCopy.length != payloadLength + 0x24) {
195:                    payloadLength, packetCopy.length - 0x24);
Shell cwd was reset to /Users/noah/conductor/workspaces/deadca7/san-diego


 I'm mapping out the packet structure for the media query: it starts with a 10-byte magic header, followed by a type byte (0x05 for MEDIA_QUERY), then a 20-byte device name field, and finally the payload data starting at offset 0x1f.

The payload itself is 17 bytes where I'm setting specific fields: the device number at offset 0x21, our IP address across bytes 0x24-0x27, the target player at offset 0x2b, and the slot protocol value (2 for SD, 3 for USB) at offset 0x2f, with the remaining bytes filled with zeros or fixed values like 0x01 and 0x0c.

For the MediaDetails response, I need to understand the packet structure — the payload length sits at offset 0x22 and the total packet size is calculated from that, with a minimum of 192 bytes. The response contains fields like media type, name (UTF-16 terminated with 0x0000), track count, playlist count, and other metadata. Rather than fully decode the response right now, I should focus on sending the media query correctly and capturing the response bytes, then analyze them afterward.

To actually get a response from the target player, I need to be announced as a participating device — the player will respond back to my IP on port 50002. My capture tool already binds that port, so I can extend it with a media query function. The approach is to add a `-media <player>:<slot>` flag that leverages the existing Online announcement (which claims a device number and IP), then sends the media query and captures the 0x06 response in the normal output. The challenge is that Online runs a keepalive loop indefinitely, so I need to figure out how to send the query while keeping the online status active.

I'm thinking of adding an `OnOnline` callback to the VCDJ struct that fires once the device is announced, allowing the capture tool to trigger the media query send from a separate goroutine. Then I'll add a `MediaQuery` method that constructs and sends the 0x05 packet to the target's port 50002. The response comes back on our 50002 socket and gets recorded with the rest of the capture data.

For the capture tool, I'll add flags for `-query-ip`, `-query-player`, and `-query-slot` so we can specify which device to query without duplicating the announce logic in a separate command.

The media query needs to be sent a few times with retries since it can be unreliable. Once we're online, we'll send the query to the target IP and capture the 0x06 response in the jsonl output.

For the master status feature, that's more involved — it requires building a full status packet and managing a master-handoff state machine. I'll deliver the media query first since it's concrete and high-value, then assess the master status work as a follow-up.

Now I need to pull the MediaDetails constructor offsets so I can decode the response properly.


Everything I need for the media query is clear. The packet is: magic + `0x05` + 20-byte name + payload (our device# at `0x21`, our IP at `0x24`, target player at `0x2b`, slot at `0x2f`). Let me get the response (`0x06`) field offsets, then implement.


cd /tmp && gh api repos/Deep-Symmetry/beat-link/contents/src/main/java/org/deepsymmetry/beatlink/MediaDetails.java --jq '.content' 2>/dev/null | base64 -d | sed -n '165,230p'

Read MediaDetails response field offsets


    /**
     * Constructor sets all the immutable interpreted fields based on the packet content.
     *
     * @param packet the media response packet that was received
     */
    MediaDetails(DatagramPacket packet) {
        this(packet.getData(), packet.getLength());
    }

    /**
     * Constructor sets all the immutable interpreted fields based on the packet content.
     *
     * @param packet       the media response packet that was received or cached
     * @param packetLength the number of bytes within the packet which were actually received
     */
    @API(status = API.Status.STABLE)
    public MediaDetails(byte[] packet, int packetLength) {
        byte[] packetCopy = new byte[packetLength];  // Make a defensive copy
        System.arraycopy(packet, 0, packetCopy, 0, packetLength);
        rawBytes = ByteBuffer.wrap(packetCopy).asReadOnlyBuffer();  // Save for safe sharing.

        if (packetCopy.length < MINIMUM_PACKET_SIZE) {
            throw new IllegalArgumentException("Unable to create a MediaDetails object, packet too short: we need " + MINIMUM_PACKET_SIZE +
                    " bytes and were given only " + packetCopy.length);
        }

        final int payloadLength = (int) Util.bytesToNumber(packetCopy, 0x22, 2);
        if (packetCopy.length != payloadLength + 0x24) {
            logger.warn("Received Media response packet with reported payload length of {} and actual payload length of {}",
                    payloadLength, packetCopy.length - 0x24);
        }

        if (!expectedMediaPacketSizes.contains(packetCopy.length)) {
            logger.warn("Processing Media response packets with unexpected lengths {}.", packetCopy.length);
            expectedMediaPacketSizes.add(packetCopy.length);
        }

        final int hostPlayer = packetCopy[0x27];
        final CdjStatus.TrackSourceSlot hostSLot = CdjStatus.TRACK_SOURCE_SLOT_MAP.get(packetCopy[0x2b]);
        if (hostSLot == null) {
            throw new IllegalArgumentException("Unrecognized slot for media response:" + packetCopy[0x2b]);
        }
        slotReference = SlotReference.getSlotReference(hostPlayer, hostSLot);

        final CdjStatus.TrackType type = CdjStatus.TRACK_TYPE_MAP.get(packetCopy[0xaa]);
        if (type == null) {
            throw new IllegalArgumentException("Unrecognized media type for media response:" + packetCopy[0xaa]);
        }
        mediaType = type;

        if (hostPlayer >= 40) {  // Rekordbox mobile does not send media name or creation date.
            name = "rekordbox mobile";
            creationDate = "";
        } else {
            int mediaNameLength = getUTF16StringLength(packetCopy, 0x2c, 0x40);
            int creationDateLength = getUTF16StringLength(packetCopy, 0x6c, 0x18);

            name = new String(packetCopy, 0x2c, mediaNameLength, StandardCharsets.UTF_16BE).trim();
            creationDate = new String(packetCopy, 0x6c, creationDateLength, StandardCharsets.UTF_16BE).trim();
        }

        trackCount = (int) Util.bytesToNumber(packetCopy, 0xa6, 2);
        color = ColorItem.colorForId(packetCopy[0xa8]);
        playlistCount = (int) Util.bytesToNumber(packetCopy, 0xae, 2);
        hasMySettings = packetCopy[0xab] != 0;
Shell cwd was reset to /Users/noah/conductor/workspaces/deadca7/san-diego


 I'm mapping out the MediaDetails response structure, which includes the host player and slot identifiers, media type classification, a UTF-16BE encoded name field, creation date, track count, and color identifier spread across specific byte offsets.

Now I need to add the remaining fields: a settings flag at 0xab, playlist count at 0xae, and ensure the minimum packet size is 0xc0 bytes. I'm also planning to implement a MediaQuery builder function that constructs the query payload with the magic header, device information, target player, and slot number, then add an OnOnline callback to the VCDJ struct that triggers queries when the device comes online. For the capture tool, I'll add command-line flags to specify the target IP, player, and slot, then send the query through the c2 port. I'll also create a decoder for the 0x06 response and test it against captured data, using the slot protocol values where SD maps to 2 and USB to 3.

Building the MediaQuery function directly... The device name field goes at 0x0b per the buildPacket convention, so I'm setting it there along with the other MediaQuery fields like the player slot and IP address. I'm also adding the TypeMediaQuery and TypeMediaResponse constants (both happen to collide with existing values on different ports, which is fine in Go), and I'll organize the media-related code into a new media.go file with the query builder, response decoder, and MediaDetails struct. Now moving on to handle the OnOnline logic.

I need to add an OnOnline callback field to VCDJ that fires after the device comes online, but since it runs in the keepalive goroutine, I should make sure it doesn't block — the capture tool will spawn a separate goroutine to send the media query a few times with retries via the c2 socket. I'll add the OnOnline field and invocation to announce.go, flesh out media.go with the query builder and details decoder, then wire it all up in the capture tool with command-line flags for the query parameters.

Defining the MediaDetails struct... Writing media.go...


Complete picture now. The response (`0x06`) carries: host player `0x27`, slot `0x2b`, **media name (UTF-16BE) `0x2c`**, **track count `0xa6`**, media type `0xaa`, **playlist count `0xae`**. Let me implement: a `MediaQuery` builder + `DecodeMediaDetails`, an `OnOnline` hook so the capture tool fires the query once we're online, and wire it up.


Write

package link

import (
	"bytes"
	"encoding/binary"
	"unicode/utf16"

	"github.com/cockroachdb/errors"
)

// Media query and response on port 50002. A query asks a player what media is
// mounted in a slot; the player replies with its name, track and playlist count.
const (
	TypeMediaQuery    Type = 0x05
	TypeMediaResponse Type = 0x06
)

// Media is a player's description of the media mounted in one slot.
type Media struct {
	Name          string
	Player        byte
	PlaylistCount int
	Slot          Slot
	TrackCount    int
	Type          TrackType
}

// MediaQuery builds a media-query packet asking player about slot. Slot uses the
// protocol values SlotCD=1, SlotSD=2, SlotUSB=3.
func (v VCDJ) MediaQuery(player byte, slot Slot) []byte {
	b := make([]byte, 0x30)
	copy(b, Magic)
	b[0x0a] = byte(TypeMediaQuery)
	copy(b[0x0b:0x1f], v.Name)
	b[0x1f] = 0x01
	b[0x21] = v.Device
	b[0x23] = 0x0c
	copy(b[0x24:0x28], v.IP.To4())
	b[0x2b] = player
	b[0x2f] = byte(slot)
	return b
}

// DecodeMediaDetails parses a media-response (type 0x06) packet.
func DecodeMediaDetails(b []byte) (Media, error) {
	if len(b) < 0xc0 || !bytes.Equal(b[:len(Magic)], Magic) {
		return Media{}, ErrNotDJLink
	}
	if t := Type(b[0x0a]); t != TypeMediaResponse {
		return Media{}, errors.Errorf("want media response 0x06, got 0x%02x", byte(t))
	}

	return Media{
		Name:          utf16beString(b[0x2c : 0x2c+0x40]),
		Player:        b[0x27],
		PlaylistCount: int(binary.BigEndian.Uint16(b[0xae:0xb0])),
		Slot:          Slot(b[0x2b]),
		TrackCount:    int(binary.BigEndian.Uint16(b[0xa6:0xa8])),
		Type:          TrackType(b[0xaa]),
	}, nil
}

// utf16beString decodes a UTF-16BE string, stopping at the first NUL unit.
func utf16beString(b []byte) string {
	var u []uint16
	for i := 0; i+1 < len(b); i += 2 {
		c := binary.BigEndian.Uint16(b[i : i+2])
		if c == 0 {
			break
		}
		u = append(u, c)
	}
	return string(utf16.Decode(u))
}


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

package link

import (
	"bytes"
	"encoding/binary"
	"unicode/utf16"

	"github.com/cockroachdb/errors"
)

// Media query and response on port 50002. A query asks a player what media is
// mounted in a slot; the player replies with its name, track and playlist count.
const (
	TypeMediaQuery    Type = 0x05
	TypeMediaResponse Type = 0x06
)

// Media is a player's description of the media mounted in one slot.
type Media struct {
	Name          string
	Player        byte
	PlaylistCount int
	Slot          Slot
	TrackCount    int
	Type          TrackType
}

// MediaQuery builds a media-query packet asking player about slot. Slot uses the
// protocol values SlotCD=1, SlotSD=2, SlotUSB=3.
func (v VCDJ) MediaQuery(player byte, slot Slot) []byte {
	b := make([]byte, 0x30)
	copy(b, Magic)
	b[0x0a] = byte(TypeMediaQuery)
	copy(b[0x0b:0x1f], v.Name)
	b[0x1f] = 0x01
	b[0x21] = v.Device
	b[0x23] = 0x0c
	copy(b[0x24:0x28], v.IP.To4())
	b[0x2b] = player
	b[0x2f] = byte(slot)
	return b
}

// DecodeMediaDetails parses a media-response (type 0x06) packet.
func DecodeMediaDetails(b []byte) (Media, error) {
	if len(b) < 0xc0 || !bytes.Equal(b[:len(Magic)], Magic) {
		return Media{}, ErrNotDJLink
	}
	if t := Type(b[0x0a]); t != TypeMediaResponse {
		return Media{}, errors.Errorf("want media response 0x06, got 0x%02x", byte(t))
	}

	return Media{
		Name:          utf16beString(b[0x2c : 0x2c+0x40]),
		Player:        b[0x27],
		PlaylistCount: int(binary.BigEndian.Uint16(b[0xae:0xb0])),
		Slot:          Slot(b[0x2b]),
		TrackCount:    int(binary.BigEndian.Uint16(b[0xa6:0xa8])),
		Type:          TrackType(b[0xaa]),
	}, nil
}

// utf16beString decodes a UTF-16BE string, stopping at the first NUL unit.
func utf16beString(b []byte) string {
	var u []uint16
	for i := 0; i+1 < len(b); i += 2 {
		c := binary.BigEndian.Uint16(b[i : i+2])
		if c == 0 {
			break
		}
		u = append(u, c)
	}
	return string(utf16.Decode(u))
}


Now the `OnOnline` hook in `announce.go`:


Edit

type VCDJ struct {
	Device byte
	IP     net.IP
	Log    func(string)
	MAC    net.HardwareAddr
	Name   string
}

type VCDJ struct {
	Device   byte
	IP       net.IP
	Log      func(string)
	MAC      net.HardwareAddr
	Name     string
	OnOnline func()
}


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

type VCDJ struct {
	Device   byte
	IP       net.IP
	Log      func(string)
	MAC      net.HardwareAddr
	Name     string
	OnOnline func()
}

package link

import (
	"bytes"
	"context"
	"encoding/hex"
	"fmt"
	"net"
	"sync"
	"sync/atomic"
	"time"

	"github.com/cockroachdb/errors"
)

// Packet types exchanged while a virtual CDJ comes online, interpreted on port
// 50000. A player sends hello, then three device-number claim stages, before
// keep-alives; a mixer may interrupt to assign a number, and any device
// defends a number it already owns. See
// https://djl-analysis.deepsymmetry.org/djl-analysis/startup.html#cdj-startup
const (
	TypeClaim1         Type = 0x00
	TypeWillAssign     Type = 0x01
	TypeClaim2         Type = 0x02
	TypeAssign         Type = 0x03
	TypeClaim3         Type = 0x04
	TypeAssignFinished Type = 0x05
	TypeDefense        Type = 0x08
	TypeHello          Type = 0x0a
)

// ClaimInterval is the gap between packets within a startup stage.
const ClaimInterval = 300 * time.Millisecond

// KeepAliveInterval is the gap between keep-alive packets once online.
const KeepAliveInterval = 1500 * time.Millisecond

// WatchPeriod is how long to observe keep-alives before self-assigning, so we
// avoid claiming a number already in use.
const WatchPeriod = 2 * time.Second

// Packet templates copied verbatim from beat-link; per-device fields (name,
// number, MAC, IP, packet counter) are patched into a copy before sending.
var (
	tmplAssign    = mustHex("5173707431576d4a4f4c0201626561742d6c696e6b0000000000000000000000010200320000000000000000000000000100")
	tmplClaim1    = mustHex("5173707431576d4a4f4c0000626561742d6c696e6b00000000000000000000000103002c0d01000000000000")
	tmplClaim2    = mustHex("5173707431576d4a4f4c0200626561742d6c696e6b000000000000000000000001030032000000000000000000000d000100")
	tmplClaim3    = mustHex("5173707431576d4a4f4c0400626561742d6c696e6b0000000000000000000000010300260d00")
	tmplDefense   = mustHex("5173707431576d4a4f4c0800626561742d6c696e6b0000000000000000000000010200290000000000")
	tmplHello     = mustHex("5173707431576d4a4f4c0a00626561742d6c696e6b0000000000000000000000010400260140")
	tmplKeepAlive = mustHex("5173707431576d4a4f4c0600626561742d6c696e6b000000000000000000000001020036000100000000000000000000020000000164")
)

func mustHex(s string) []byte {
	b, err := hex.DecodeString(s)
	if err != nil {
		panic(err)
	}
	return b
}

// ClaimNumber returns the device number a claim packet is trying to take, found
// at a different offset for claim stage 2 (0x02) than stage 3 (0x04).
func ClaimNumber(b []byte) (byte, bool) {
	if len(b) < 0x0b || !bytes.Equal(b[:len(Magic)], Magic) {
		return 0, false
	}
	switch Type(b[0x0a]) {
	case TypeClaim2:
		if len(b) > 0x2e {
			return b[0x2e], true
		}
	case TypeClaim3:
		if len(b) > 0x24 {
			return b[0x24], true
		}
	}
	return 0, false
}

// SelfAssign returns the lowest player number not in used that is greater than
// after, starting no lower than 5 (the first number a CDJ-3000 may take) and
// stopping below the 33+ range mixers use.
func SelfAssign(used map[byte]bool, after byte) (byte, bool) {
	n := after + 1
	if n < 5 {
		n = 5
	}
	for ; n <= 15; n++ {
		if !used[n] {
			return n, true
		}
	}
	return 0, false
}

// VCDJ is a virtual CDJ that participates in the network so real players send
// it status, metadata, and database access. Device 0 means self-assign.
type VCDJ struct {
	Device byte
	IP     net.IP
	Log    func(string)
	MAC    net.HardwareAddr
	Name   string
}

func (v VCDJ) logf(format string, args ...any) {
	if v.Log != nil {
		v.Log(fmt.Sprintf(format, args...))
	}
}

// Online brings the virtual CDJ online and holds its device number until ctx is
// done. It owns conn (bound to port 50000): it runs the claim handshake while
// reacting to defenses and mixer assignments, then sends keep-alives and
// defends its number. Every received datagram is passed to onPacket so callers
// can record it.
func (v *VCDJ) Online(ctx context.Context, conn *net.UDPConn, ipNet *net.IPNet, onPacket func([]byte, *net.UDPAddr)) error {
	v.IP = ipNet.IP
	dst := &net.UDPAddr{IP: Broadcast(ipNet), Port: 50000}

	var (
		assigned atomic.Int32
		claiming atomic.Int32
		online   atomic.Bool
		rejected atomic.Bool
		seen     sync.Map
	)

	send := func(b []byte, to *net.UDPAddr) {
		conn.WriteToUDP(b, to)
	}

	handle := func(b []byte, src *net.UDPAddr) {
		if len(b) < 0x25 || !bytes.Equal(b[:len(Magic)], Magic) {
			return
		}
		peer := &net.UDPAddr{IP: src.IP, Port: 50000}
		switch Type(b[0x0a]) {
		case TypeKeepAlive:
			seen.Store(b[0x24], true)
		case TypeWillAssign:
			if claiming.Load() != 0 {
				send(v.assignRequest(), peer)
			}
		case TypeAssign:
			assigned.Store(int32(b[0x24]))
		case TypeAssignFinished:
			assigned.Store(claiming.Load())
		case TypeDefense:
			if b[0x24] == byte(claiming.Load()) {
				rejected.Store(true)
			}
		case TypeClaim2, TypeClaim3:
			if n, ok := ClaimNumber(b); ok && online.Load() && n == v.Device {
				v.logf("defending device %d against %s", v.Device, src.IP)
				send(v.defense(), peer)
			}
		}
	}

	errc := make(chan error, 1)
	go func() {
		go func() { <-ctx.Done(); conn.SetReadDeadline(time.Now()) }()
		buf := make([]byte, 1500)
		for {
			n, src, err := conn.ReadFromUDP(buf)
			if err != nil {
				if ctx.Err() != nil {
					errc <- nil
				} else {
					errc <- errors.WithStack(err)
				}
				return
			}
			b := append([]byte(nil), buf[:n]...)
			handle(b, src)
			if onPacket != nil {
				onPacket(b, src)
			}
		}
	}()

	wait := func(d time.Duration) error {
		select {
		case <-ctx.Done():
			return errors.WithStack(ctx.Err())
		case err := <-errc:
			if err == nil {
				err = errors.New("listener stopped")
			}
			return err
		case <-time.After(d):
			return nil
		}
	}

	for i := 0; i < 3; i++ {
		send(v.hello(), dst)
		if err := wait(ClaimInterval); err != nil {
			return err
		}
	}

	auto := v.Device == 0
	if auto {
		if err := wait(WatchPeriod); err != nil {
			return err
		}
	}

	claim := func(num byte) (bool, error) {
		v.Device = num
		claiming.Store(int32(num))
		rejected.Store(false)
		assigned.Store(0)

		stages := [][]func(byte) []byte{
			{v.claim1}, {v.claim2}, {v.claim3},
		}
		for s, stage := range stages {
			if s == 2 {
				if n := assigned.Swap(0); n > 0 {
					v.logf("mixer assigned device %d", n)
					v.Device = byte(n)
					claiming.Store(n)
				}
			}
			for i := byte(1); i <= 3 && assigned.Load() == 0; i++ {
				send(stage[0](i), dst)
				if err := wait(ClaimInterval); err != nil {
					return false, err
				}
				if rejected.Load() {
					return false, nil
				}
			}
		}
		return true, nil
	}

	var after byte
	for {
		num := v.Device
		if auto {
			n, ok := SelfAssign(usedNumbers(&seen), after)
			if !ok {
				return errors.New("no available device number")
			}
			num, after = n, n
		}

		ok, err := claim(num)
		if err != nil {
			return err
		}
		if ok {
			break
		}
		if !auto {
			return errors.Errorf("device number %d defended by another device", num)
		}
		v.logf("device %d defended, trying another", num)
	}

	online.Store(true)
	claiming.Store(0)
	v.logf("online as device %d", v.Device)

	t := time.NewTicker(KeepAliveInterval)
	defer t.Stop()
	for {
		send(v.keepAlive(), dst)
		select {
		case <-ctx.Done():
			return nil
		case err := <-errc:
			return err
		case <-t.C:
		}
	}
}

func usedNumbers(seen *sync.Map) map[byte]bool {
	used := map[byte]bool{}
	seen.Range(func(k, _ any) bool {
		used[k.(byte)] = true
		return true
	})
	return used
}

func (v VCDJ) name(b []byte) {
	for i := 0; i < 0x14; i++ {
		b[0x0c+i] = 0
	}
	copy(b[0x0c:0x0c+0x14], v.Name)
}

func (v VCDJ) hello() []byte {
	b := append([]byte(nil), tmplHello...)
	v.name(b)
	return b
}

func (v VCDJ) claim1(counter byte) []byte {
	b := append([]byte(nil), tmplClaim1...)
	v.name(b)
	b[0x24] = counter
	copy(b[0x26:0x2c], v.MAC)
	return b
}

func (v VCDJ) claim2(counter byte) []byte {
	b := append([]byte(nil), tmplClaim2...)
	v.name(b)
	copy(b[0x24:0x28], v.IP.To4())
	copy(b[0x28:0x2e], v.MAC)
	b[0x2e] = v.Device
	b[0x2f] = counter
	b[0x31] = v.assignFlag()
	return b
}

func (v VCDJ) claim3(counter byte) []byte {
	b := append([]byte(nil), tmplClaim3...)
	v.name(b)
	b[0x24] = v.Device
	b[0x25] = counter
	return b
}

func (v VCDJ) assignRequest() []byte {
	b := append([]byte(nil), tmplAssign...)
	v.name(b)
	copy(b[0x24:0x28], v.IP.To4())
	copy(b[0x28:0x2e], v.MAC)
	b[0x2f] = 0x01
	b[0x31] = v.assignFlag()
	return b
}

func (v VCDJ) defense() []byte {
	b := append([]byte(nil), tmplDefense...)
	v.name(b)
	b[0x24] = v.Device
	copy(b[0x25:0x29], v.IP.To4())
	return b
}

func (v VCDJ) keepAlive() []byte {
	b := append([]byte(nil), tmplKeepAlive...)
	v.name(b)
	b[0x24] = v.Device
	copy(b[0x26:0x2c], v.MAC)
	copy(b[0x2c:0x30], v.IP.To4())
	return b
}

func (v VCDJ) assignFlag() byte {
	if v.Device == 0 {
		return 0x01
	}
	return 0x02
}


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


	online.Store(true)
	claiming.Store(0)
	v.logf("online as device %d", v.Device)

	online.Store(true)
	claiming.Store(0)
	v.logf("online as device %d", v.Device)
	if v.OnOnline != nil {
		go v.OnOnline()
	}


	online.Store(true)
	claiming.Store(0)
	v.logf("online as device %d", v.Device)
	if v.OnOnline != nil {
		go v.OnOnline()
	}

package link

import (
	"bytes"
	"context"
	"encoding/hex"
	"fmt"
	"net"
	"sync"
	"sync/atomic"
	"time"

	"github.com/cockroachdb/errors"
)

// Packet types exchanged while a virtual CDJ comes online, interpreted on port
// 50000. A player sends hello, then three device-number claim stages, before
// keep-alives; a mixer may interrupt to assign a number, and any device
// defends a number it already owns. See
// https://djl-analysis.deepsymmetry.org/djl-analysis/startup.html#cdj-startup
const (
	TypeClaim1         Type = 0x00
	TypeWillAssign     Type = 0x01
	TypeClaim2         Type = 0x02
	TypeAssign         Type = 0x03
	TypeClaim3         Type = 0x04
	TypeAssignFinished Type = 0x05
	TypeDefense        Type = 0x08
	TypeHello          Type = 0x0a
)

// ClaimInterval is the gap between packets within a startup stage.
const ClaimInterval = 300 * time.Millisecond

// KeepAliveInterval is the gap between keep-alive packets once online.
const KeepAliveInterval = 1500 * time.Millisecond

// WatchPeriod is how long to observe keep-alives before self-assigning, so we
// avoid claiming a number already in use.
const WatchPeriod = 2 * time.Second

// Packet templates copied verbatim from beat-link; per-device fields (name,
// number, MAC, IP, packet counter) are patched into a copy before sending.
var (
	tmplAssign    = mustHex("5173707431576d4a4f4c0201626561742d6c696e6b0000000000000000000000010200320000000000000000000000000100")
	tmplClaim1    = mustHex("5173707431576d4a4f4c0000626561742d6c696e6b00000000000000000000000103002c0d01000000000000")
	tmplClaim2    = mustHex("5173707431576d4a4f4c0200626561742d6c696e6b000000000000000000000001030032000000000000000000000d000100")
	tmplClaim3    = mustHex("5173707431576d4a4f4c0400626561742d6c696e6b0000000000000000000000010300260d00")
	tmplDefense   = mustHex("5173707431576d4a4f4c0800626561742d6c696e6b0000000000000000000000010200290000000000")
	tmplHello     = mustHex("5173707431576d4a4f4c0a00626561742d6c696e6b0000000000000000000000010400260140")
	tmplKeepAlive = mustHex("5173707431576d4a4f4c0600626561742d6c696e6b000000000000000000000001020036000100000000000000000000020000000164")
)

func mustHex(s string) []byte {
	b, err := hex.DecodeString(s)
	if err != nil {
		panic(err)
	}
	return b
}

// ClaimNumber returns the device number a claim packet is trying to take, found
// at a different offset for claim stage 2 (0x02) than stage 3 (0x04).
func ClaimNumber(b []byte) (byte, bool) {
	if len(b) < 0x0b || !bytes.Equal(b[:len(Magic)], Magic) {
		return 0, false
	}
	switch Type(b[0x0a]) {
	case TypeClaim2:
		if len(b) > 0x2e {
			return b[0x2e], true
		}
	case TypeClaim3:
		if len(b) > 0x24 {
			return b[0x24], true
		}
	}
	return 0, false
}

// SelfAssign returns the lowest player number not in used that is greater than
// after, starting no lower than 5 (the first number a CDJ-3000 may take) and
// stopping below the 33+ range mixers use.
func SelfAssign(used map[byte]bool, after byte) (byte, bool) {
	n := after + 1
	if n < 5 {
		n = 5
	}
	for ; n <= 15; n++ {
		if !used[n] {
			return n, true
		}
	}
	return 0, false
}

// VCDJ is a virtual CDJ that participates in the network so real players send
// it status, metadata, and database access. Device 0 means self-assign.
type VCDJ struct {
	Device   byte
	IP       net.IP
	Log      func(string)
	MAC      net.HardwareAddr
	Name     string
	OnOnline func()
}

func (v VCDJ) logf(format string, args ...any) {
	if v.Log != nil {
		v.Log(fmt.Sprintf(format, args...))
	}
}

// Online brings the virtual CDJ online and holds its device number until ctx is
// done. It owns conn (bound to port 50000): it runs the claim handshake while
// reacting to defenses and mixer assignments, then sends keep-alives and
// defends its number. Every received datagram is passed to onPacket so callers
// can record it.
func (v *VCDJ) Online(ctx context.Context, conn *net.UDPConn, ipNet *net.IPNet, onPacket func([]byte, *net.UDPAddr)) error {
	v.IP = ipNet.IP
	dst := &net.UDPAddr{IP: Broadcast(ipNet), Port: 50000}

	var (
		assigned atomic.Int32
		claiming atomic.Int32
		online   atomic.Bool
		rejected atomic.Bool
		seen     sync.Map
	)

	send := func(b []byte, to *net.UDPAddr) {
		conn.WriteToUDP(b, to)
	}

	handle := func(b []byte, src *net.UDPAddr) {
		if len(b) < 0x25 || !bytes.Equal(b[:len(Magic)], Magic) {
			return
		}
		peer := &net.UDPAddr{IP: src.IP, Port: 50000}
		switch Type(b[0x0a]) {
		case TypeKeepAlive:
			seen.Store(b[0x24], true)
		case TypeWillAssign:
			if claiming.Load() != 0 {
				send(v.assignRequest(), peer)
			}
		case TypeAssign:
			assigned.Store(int32(b[0x24]))
		case TypeAssignFinished:
			assigned.Store(claiming.Load())
		case TypeDefense:
			if b[0x24] == byte(claiming.Load()) {
				rejected.Store(true)
			}
		case TypeClaim2, TypeClaim3:
			if n, ok := ClaimNumber(b); ok && online.Load() && n == v.Device {
				v.logf("defending device %d against %s", v.Device, src.IP)
				send(v.defense(), peer)
			}
		}
	}

	errc := make(chan error, 1)
	go func() {
		go func() { <-ctx.Done(); conn.SetReadDeadline(time.Now()) }()
		buf := make([]byte, 1500)
		for {
			n, src, err := conn.ReadFromUDP(buf)
			if err != nil {
				if ctx.Err() != nil {
					errc <- nil
				} else {
					errc <- errors.WithStack(err)
				}
				return
			}
			b := append([]byte(nil), buf[:n]...)
			handle(b, src)
			if onPacket != nil {
				onPacket(b, src)
			}
		}
	}()

	wait := func(d time.Duration) error {
		select {
		case <-ctx.Done():
			return errors.WithStack(ctx.Err())
		case err := <-errc:
			if err == nil {
				err = errors.New("listener stopped")
			}
			return err
		case <-time.After(d):
			return nil
		}
	}

	for i := 0; i < 3; i++ {
		send(v.hello(), dst)
		if err := wait(ClaimInterval); err != nil {
			return err
		}
	}

	auto := v.Device == 0
	if auto {
		if err := wait(WatchPeriod); err != nil {
			return err
		}
	}

	claim := func(num byte) (bool, error) {
		v.Device = num
		claiming.Store(int32(num))
		rejected.Store(false)
		assigned.Store(0)

		stages := [][]func(byte) []byte{
			{v.claim1}, {v.claim2}, {v.claim3},
		}
		for s, stage := range stages {
			if s == 2 {
				if n := assigned.Swap(0); n > 0 {
					v.logf("mixer assigned device %d", n)
					v.Device = byte(n)
					claiming.Store(n)
				}
			}
			for i := byte(1); i <= 3 && assigned.Load() == 0; i++ {
				send(stage[0](i), dst)
				if err := wait(ClaimInterval); err != nil {
					return false, err
				}
				if rejected.Load() {
					return false, nil
				}
			}
		}
		return true, nil
	}

	var after byte
	for {
		num := v.Device
		if auto {
			n, ok := SelfAssign(usedNumbers(&seen), after)
			if !ok {
				return errors.New("no available device number")
			}
			num, after = n, n
		}

		ok, err := claim(num)
		if err != nil {
			return err
		}
		if ok {
			break
		}
		if !auto {
			return errors.Errorf("device number %d defended by another device", num)
		}
		v.logf("device %d defended, trying another", num)
	}

	online.Store(true)
	claiming.Store(0)
	v.logf("online as device %d", v.Device)

	t := time.NewTicker(KeepAliveInterval)
	defer t.Stop()
	for {
		send(v.keepAlive(), dst)
		select {
		case <-ctx.Done():
			return nil
		case err := <-errc:
			return err
		case <-t.C:
		}
	}
}

func usedNumbers(seen *sync.Map) map[byte]bool {
	used := map[byte]bool{}
	seen.Range(func(k, _ any) bool {
		used[k.(byte)] = true
		return true
	})
	return used
}

func (v VCDJ) name(b []byte) {
	for i := 0; i < 0x14; i++ {
		b[0x0c+i] = 0
	}
	copy(b[0x0c:0x0c+0x14], v.Name)
}

func (v VCDJ) hello() []byte {
	b := append([]byte(nil), tmplHello...)
	v.name(b)
	return b
}

func (v VCDJ) claim1(counter byte) []byte {
	b := append([]byte(nil), tmplClaim1...)
	v.name(b)
	b[0x24] = counter
	copy(b[0x26:0x2c], v.MAC)
	return b
}

func (v VCDJ) claim2(counter byte) []byte {
	b := append([]byte(nil), tmplClaim2...)
	v.name(b)
	copy(b[0x24:0x28], v.IP.To4())
	copy(b[0x28:0x2e], v.MAC)
	b[0x2e] = v.Device
	b[0x2f] = counter
	b[0x31] = v.assignFlag()
	return b
}

func (v VCDJ) claim3(counter byte) []byte {
	b := append([]byte(nil), tmplClaim3...)
	v.name(b)
	b[0x24] = v.Device
	b[0x25] = counter
	return b
}

func (v VCDJ) assignRequest() []byte {
	b := append([]byte(nil), tmplAssign...)
	v.name(b)
	copy(b[0x24:0x28], v.IP.To4())
	copy(b[0x28:0x2e], v.MAC)
	b[0x2f] = 0x01
	b[0x31] = v.assignFlag()
	return b
}

func (v VCDJ) defense() []byte {
	b := append([]byte(nil), tmplDefense...)
	v.name(b)
	b[0x24] = v.Device
	copy(b[0x25:0x29], v.IP.To4())
	return b
}

func (v VCDJ) keepAlive() []byte {
	b := append([]byte(nil), tmplKeepAlive...)
	v.name(b)
	b[0x24] = v.Device
	copy(b[0x26:0x2c], v.MAC)
	copy(b[0x2c:0x30], v.IP.To4())
	return b
}

func (v VCDJ) assignFlag() byte {
	if v.Device == 0 {
		return 0x01
	}
	return 0x02
}


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

	announce := flag.Bool("announce", false, "bring a virtual CDJ online while capturing")
	device := flag.Int("device", 5, "virtual CDJ device number")
	name := flag.String("name", "deadca7", "virtual CDJ device name")
	out := flag.String("out", "capture.jsonl", "output JSONL fixture path")
	queryIP := flag.String("query-ip", "", "once online, send a media query to this player IP")
	queryPlayer := flag.Int("query-player", 0, "target player number for the media query")
	querySlot := flag.Int("query-slot", 2, "target slot for the media query (CD=1 SD=2 USB=3)")
	secs := flag.Int("secs", 8, "capture duration in seconds")
	flag.Parse()

	q := query{ip: *queryIP, player: byte(*queryPlayer), slot: byte(*querySlot)}
	if err := run(*out, time.Duration(*secs)*time.Second, *announce, byte(*device), *name, q); err != nil {

// Command capture records raw Pro DJ Link UDP datagrams to a JSONL fixture.
//
// It listens on the DJ Link discovery, beat, and status ports and writes one
// Packet per line so tests can replay real traffic without hardware. With
// -announce it first brings a virtual CDJ online (the full startup handshake)
// so real players send status, metadata, and database traffic to it.
package main

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

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

// Packet is one recorded UDP datagram.
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() {
	announce := flag.Bool("announce", false, "bring a virtual CDJ online while capturing")
	device := flag.Int("device", 5, "virtual CDJ device number")
	name := flag.String("name", "deadca7", "virtual CDJ device name")
	out := flag.String("out", "capture.jsonl", "output JSONL fixture path")
	secs := flag.Int("secs", 8, "capture duration in seconds")
	flag.Parse()

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

func run(out string, d time.Duration, announce bool, device byte, name string) error {
	f, err := os.Create(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, d)
	defer cancel()

	c0, err := net.ListenUDP("udp", &net.UDPAddr{IP: net.IPv4zero, Port: 50000})
	if err != nil {
		return errors.WithStack(err)
	}
	defer c0.Close()
	if err := link.SetBroadcast(c0); err != nil {
		return err
	}

	c1, err := net.ListenUDP("udp", &net.UDPAddr{IP: net.IPv4zero, Port: 50001})
	if err != nil {
		return errors.WithStack(err)
	}
	defer c1.Close()

	c2, err := net.ListenUDP("udp", &net.UDPAddr{IP: net.IPv4zero, Port: 50002})
	if err != nil {
		return errors.WithStack(err)
	}
	defer c2.Close()

	var vcdj *link.VCDJ
	var ipNet *net.IPNet
	if announce {
		vcdj, ipNet, err = online(c0, device, name)
		if err != nil {
			return err
		}
	}

	enc := json.NewEncoder(f)
	pkts := make(chan Packet)
	record := func(b []byte, src *net.UDPAddr) {
		select {
		case pkts <- packet(b, 50000, src):
		case <-ctx.Done():
		}
	}

	g, ctx := errgroup.WithContext(ctx)
	g.Go(func() error { return listen(ctx, c1, 50001, pkts) })
	g.Go(func() error { return listen(ctx, c2, 50002, pkts) })

	if announce {
		g.Go(func() error {
			err := vcdj.Online(ctx, c0, ipNet, record)
			fmt.Fprintf(os.Stderr, "virtual CDJ %q ran as device %d\n", vcdj.Name, vcdj.Device)
			return err
		})
	} else {
		g.Go(func() error { return listen(ctx, c0, 50000, pkts) })
	}

	g.Go(func() error {
		n := 0
		for {
			select {
			case <-ctx.Done():
				fmt.Fprintf(os.Stderr, "captured %d packets to %s\n", n, out)
				return nil
			case p := <-pkts:
				if err := enc.Encode(p); err != nil {
					return errors.WithStack(err)
				}
				n++
			}
		}
	})

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

// online discovers a real device to learn the local interface, then returns a
// virtual CDJ and its IPv4 network for announcing.
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
	}
}

func packet(b []byte, port int, src *net.UDPAddr) Packet {
	return Packet{
		Data: fmt.Sprintf("%x", b),
		Len:  len(b),
		Port: port,
		Src:  src.String(),
		TS:   time.Now().UnixMilli(),
	}
}

func listen(ctx context.Context, c *net.UDPConn, port int, pkts chan<- Packet) error {
	go func() {
		<-ctx.Done()
		c.SetReadDeadline(time.Now())
	}()

	buf := make([]byte, 1500)
	for {
		n, addr, err := c.ReadFromUDP(buf)
		if err != nil {
			if ctx.Err() != nil {
				return nil
			}
			return errors.WithStack(err)
		}
		select {
		case pkts <- packet(buf[:n], port, addr):
		case <-ctx.Done():
			return nil
		}
	}
}

+  queryIP := flag.String("query-ip", "", "once online, send a media query to this player IP")

+  queryPlayer := flag.Int("query-player", 0, "target player number for the media query")

+  querySlot := flag.Int("query-slot", 2, "target slot for the media query (CD=1 SD=2 USB=3)")

-  if err := run(*out, time.Duration(*secs)*time.Second, *announce, byte(*device), *name); err != nil {

+  q := query{ip: *queryIP, player: byte(*queryPlayer), slot: byte(*querySlot)}

+  if err := run(*out, time.Duration(*secs)*time.Second, *announce, byte(*device), *name, q); err != nil {


Now wire the query flags into the capture tool:


	announce := flag.Bool("announce", false, "bring a virtual CDJ online while capturing")
	device := flag.Int("device", 5, "virtual CDJ device number")
	name := flag.String("name", "deadca7", "virtual CDJ device name")
	out := flag.String("out", "capture.jsonl", "output JSONL fixture path")
	secs := flag.Int("secs", 8, "capture duration in seconds")
	flag.Parse()

	if err := run(*out, time.Duration(*secs)*time.Second, *announce, byte(*device), *name); err != nil {

	announce := flag.Bool("announce", false, "bring a virtual CDJ online while capturing")
	device := flag.Int("device", 5, "virtual CDJ device number")
	name := flag.String("name", "deadca7", "virtual CDJ device name")
	out := flag.String("out", "capture.jsonl", "output JSONL fixture path")
	queryIP := flag.String("query-ip", "", "once online, send a media query to this player IP")
	queryPlayer := flag.Int("query-player", 0, "target player number for the media query")
	querySlot := flag.Int("query-slot", 2, "target slot for the media query (CD=1 SD=2 USB=3)")
	secs := flag.Int("secs", 8, "capture duration in seconds")
	flag.Parse()

	q := query{ip: *queryIP, player: byte(*queryPlayer), slot: byte(*querySlot)}
	if err := run(*out, time.Duration(*secs)*time.Second, *announce, byte(*device), *name, q); err != nil {


func run(out string, d time.Duration, announce bool, device byte, name string) error {

type query struct {
	ip     string
	player byte
	slot   byte
}

func run(out string, d time.Duration, announce bool, device byte, name string, q query) error {


type query struct {
	ip     string
	player byte
	slot   byte
}

func run(out string, d time.Duration, announce bool, device byte, name string, q query) error {

// Command capture records raw Pro DJ Link UDP datagrams to a JSONL fixture.
//
// It listens on the DJ Link discovery, beat, and status ports and writes one
// Packet per line so tests can replay real traffic without hardware. With
// -announce it first brings a virtual CDJ online (the full startup handshake)
// so real players send status, metadata, and database traffic to it.
package main

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

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

// Packet is one recorded UDP datagram.
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() {
	announce := flag.Bool("announce", false, "bring a virtual CDJ online while capturing")
	device := flag.Int("device", 5, "virtual CDJ device number")
	name := flag.String("name", "deadca7", "virtual CDJ device name")
	out := flag.String("out", "capture.jsonl", "output JSONL fixture path")
	queryIP := flag.String("query-ip", "", "once online, send a media query to this player IP")
	queryPlayer := flag.Int("query-player", 0, "target player number for the media query")
	querySlot := flag.Int("query-slot", 2, "target slot for the media query (CD=1 SD=2 USB=3)")
	secs := flag.Int("secs", 8, "capture duration in seconds")
	flag.Parse()

	q := query{ip: *queryIP, player: byte(*queryPlayer), slot: byte(*querySlot)}
	if err := run(*out, time.Duration(*secs)*time.Second, *announce, byte(*device), *name, q); err != nil {
		fmt.Fprintf(os.Stderr, "%+v\n", err)
		os.Exit(1)
	}
}

func run(out string, d time.Duration, announce bool, device byte, name string) error {
	f, err := os.Create(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, d)
	defer cancel()

	c0, err := net.ListenUDP("udp", &net.UDPAddr{IP: net.IPv4zero, Port: 50000})
	if err != nil {
		return errors.WithStack(err)
	}
	defer c0.Close()
	if err := link.SetBroadcast(c0); err != nil {
		return err
	}

	c1, err := net.ListenUDP("udp", &net.UDPAddr{IP: net.IPv4zero, Port: 50001})
	if err != nil {
		return errors.WithStack(err)
	}
	defer c1.Close()

	c2, err := net.ListenUDP("udp", &net.UDPAddr{IP: net.IPv4zero, Port: 50002})
	if err != nil {
		return errors.WithStack(err)
	}
	defer c2.Close()

	var vcdj *link.VCDJ
	var ipNet *net.IPNet
	if announce {
		vcdj, ipNet, err = online(c0, device, name)
		if err != nil {
			return err
		}
	}

	enc := json.NewEncoder(f)
	pkts := make(chan Packet)
	record := func(b []byte, src *net.UDPAddr) {
		select {
		case pkts <- packet(b, 50000, src):
		case <-ctx.Done():
		}
	}

	g, ctx := errgroup.WithContext(ctx)
	g.Go(func() error { return listen(ctx, c1, 50001, pkts) })
	g.Go(func() error { return listen(ctx, c2, 50002, pkts) })

	if announce {
		g.Go(func() error {
			err := vcdj.Online(ctx, c0, ipNet, record)
			fmt.Fprintf(os.Stderr, "virtual CDJ %q ran as device %d\n", vcdj.Name, vcdj.Device)
			return err
		})
	} else {
		g.Go(func() error { return listen(ctx, c0, 50000, pkts) })
	}

	g.Go(func() error {
		n := 0
		for {
			select {
			case <-ctx.Done():
				fmt.Fprintf(os.Stderr, "captured %d packets to %s\n", n, out)
				return nil
			case p := <-pkts:
				if err := enc.Encode(p); err != nil {
					return errors.WithStack(err)
				}
				n++
			}
		}
	})

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

// online discovers a real device to learn the local interface, then returns a
// virtual CDJ and its IPv4 network for announcing.
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
	}
}

func packet(b []byte, port int, src *net.UDPAddr) Packet {
	return Packet{
		Data: fmt.Sprintf("%x", b),
		Len:  len(b),
		Port: port,
		Src:  src.String(),
		TS:   time.Now().UnixMilli(),
	}
}

func listen(ctx context.Context, c *net.UDPConn, port int, pkts chan<- Packet) error {
	go func() {
		<-ctx.Done()
		c.SetReadDeadline(time.Now())
	}()

	buf := make([]byte, 1500)
	for {
		n, addr, err := c.ReadFromUDP(buf)
		if err != nil {
			if ctx.Err() != nil {
				return nil
			}
			return errors.WithStack(err)
		}
		select {
		case pkts <- packet(buf[:n], port, addr):
		case <-ctx.Done():
			return nil
		}
	}
}

-func run(out string, d time.Duration, announce bool, device byte, name string) error {

+func run(out string, d time.Duration, announce bool, device byte, name string, q query) error {


Now set `OnOnline` to send the query via the 50002 socket. Let me look at the current online/setup section:


78		c1, err := net.ListenUDP("udp", &net.UDPAddr{IP: net.IPv4zero, Port: 50001})
79		if err != nil {
80			return errors.WithStack(err)
81		}
82		defer c1.Close()
83
84		c2, err := net.ListenUDP("udp", &net.UDPAddr{IP: net.IPv4zero, Port: 50002})
85		if err != nil {
86			return errors.WithStack(err)
87		}
88		defer c2.Close()
89
90		var vcdj *link.VCDJ
91		var ipNet *net.IPNet
92		if announce {
93			vcdj, ipNet, err = online(c0, device, name)
94			if err != nil {
95				return err
96			}
97		}
98
99		enc := json.NewEncoder(f)
100		pkts := make(chan Packet)
101		record := func(b []byte, src *net.UDPAddr) {
102			select {
103			case pkts <- packet(b, 50000, src):
104			case <-ctx.Done():
105			}
106		}
107
108		g, ctx := errgroup.WithContext(ctx)
109		g.Go(func() error { return listen(ctx, c1, 50001, pkts) })
110		g.Go(func() error { return listen(ctx, c2, 50002, pkts) })
111
112		if announce {
113			g.Go(func() error {
114				err := vcdj.Online(ctx, c0, ipNet, record)
115				fmt.Fprintf(os.Stderr, "virtual CDJ %q ran as device %d\n", vcdj.Name, vcdj.Device)
116				return err
117			})

	c1, err := net.ListenUDP("udp", &net.UDPAddr{IP: net.IPv4zero, Port: 50001})
	if err != nil {
		return errors.WithStack(err)
	}
	defer c1.Close()

	c2, err := net.ListenUDP("udp", &net.UDPAddr{IP: net.IPv4zero, Port: 50002})
	if err != nil {
		return errors.WithStack(err)
	}
	defer c2.Close()

	var vcdj *link.VCDJ
	var ipNet *net.IPNet
	if announce {
		vcdj, ipNet, err = online(c0, device, name)
		if err != nil {
			return err
		}
	}

	enc := json.NewEncoder(f)
	pkts := make(chan Packet)
	record := func(b []byte, src *net.UDPAddr) {
		select {
		case pkts <- packet(b, 50000, src):
		case <-ctx.Done():
		}
	}

	g, ctx := errgroup.WithContext(ctx)
	g.Go(func() error { return listen(ctx, c1, 50001, pkts) })
	g.Go(func() error { return listen(ctx, c2, 50002, pkts) })

	if announce {
		g.Go(func() error {
			err := vcdj.Online(ctx, c0, ipNet, record)
			fmt.Fprintf(os.Stderr, "virtual CDJ %q ran as device %d\n", vcdj.Name, vcdj.Device)
			return err
		})


	var vcdj *link.VCDJ
	var ipNet *net.IPNet
	if announce {
		vcdj, ipNet, err = online(c0, device, name)
		if err != nil {
			return err
		}
	}

	var vcdj *link.VCDJ
	var ipNet *net.IPNet
	if announce {
		vcdj, ipNet, err = online(c0, device, name)
		if err != nil {
			return err
		}
		if q.ip != "" {
			dst := &net.UDPAddr{IP: net.ParseIP(q.ip), Port: 50002}
			vcdj.OnOnline = func() {
				for range 5 {
					c2.WriteToUDP(vcdj.MediaQuery(q.player, link.Slot(q.slot)), dst)
					time.Sleep(500 * time.Millisecond)
				}
				fmt.Fprintf(os.Stderr, "sent media query to %s player %d slot %d\n", q.ip, q.player, q.slot)
			}
		}
	}


	var vcdj *link.VCDJ
	var ipNet *net.IPNet
	if announce {
		vcdj, ipNet, err = online(c0, device, name)
		if err != nil {
			return err
		}
		if q.ip != "" {
			dst := &net.UDPAddr{IP: net.ParseIP(q.ip), Port: 50002}
			vcdj.OnOnline = func() {
				for range 5 {
					c2.WriteToUDP(vcdj.MediaQuery(q.player, link.Slot(q.slot)), dst)
					time.Sleep(500 * time.Millisecond)
				}
				fmt.Fprintf(os.Stderr, "sent media query to %s player %d slot %d\n", q.ip, q.player, q.slot)
			}
		}
	}

// Command capture records raw Pro DJ Link UDP datagrams to a JSONL fixture.
//
// It listens on the DJ Link discovery, beat, and status ports and writes one
// Packet per line so tests can replay real traffic without hardware. With
// -announce it first brings a virtual CDJ online (the full startup handshake)
// so real players send status, metadata, and database traffic to it.
package main

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

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

// Packet is one recorded UDP datagram.
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() {
	announce := flag.Bool("announce", false, "bring a virtual CDJ online while capturing")
	device := flag.Int("device", 5, "virtual CDJ device number")
	name := flag.String("name", "deadca7", "virtual CDJ device name")
	out := flag.String("out", "capture.jsonl", "output JSONL fixture path")
	queryIP := flag.String("query-ip", "", "once online, send a media query to this player IP")
	queryPlayer := flag.Int("query-player", 0, "target player number for the media query")
	querySlot := flag.Int("query-slot", 2, "target slot for the media query (CD=1 SD=2 USB=3)")
	secs := flag.Int("secs", 8, "capture duration in seconds")
	flag.Parse()

	q := query{ip: *queryIP, player: byte(*queryPlayer), slot: byte(*querySlot)}
	if err := run(*out, time.Duration(*secs)*time.Second, *announce, byte(*device), *name, q); err != nil {
		fmt.Fprintf(os.Stderr, "%+v\n", err)
		os.Exit(1)
	}
}

type query struct {
	ip     string
	player byte
	slot   byte
}

func run(out string, d time.Duration, announce bool, device byte, name string, q query) error {
	f, err := os.Create(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, d)
	defer cancel()

	c0, err := net.ListenUDP("udp", &net.UDPAddr{IP: net.IPv4zero, Port: 50000})
	if err != nil {
		return errors.WithStack(err)
	}
	defer c0.Close()
	if err := link.SetBroadcast(c0); err != nil {
		return err
	}

	c1, err := net.ListenUDP("udp", &net.UDPAddr{IP: net.IPv4zero, Port: 50001})
	if err != nil {
		return errors.WithStack(err)
	}
	defer c1.Close()

	c2, err := net.ListenUDP("udp", &net.UDPAddr{IP: net.IPv4zero, Port: 50002})
	if err != nil {
		return errors.WithStack(err)
	}
	defer c2.Close()

	var vcdj *link.VCDJ
	var ipNet *net.IPNet
	if announce {
		vcdj, ipNet, err = online(c0, device, name)
		if err != nil {
			return err
		}
	}

	enc := json.NewEncoder(f)
	pkts := make(chan Packet)
	record := func(b []byte, src *net.UDPAddr) {
		select {
		case pkts <- packet(b, 50000, src):
		case <-ctx.Done():
		}
	}

	g, ctx := errgroup.WithContext(ctx)
	g.Go(func() error { return listen(ctx, c1, 50001, pkts) })
	g.Go(func() error { return listen(ctx, c2, 50002, pkts) })

	if announce {
		g.Go(func() error {
			err := vcdj.Online(ctx, c0, ipNet, record)
			fmt.Fprintf(os.Stderr, "virtual CDJ %q ran as device %d\n", vcdj.Name, vcdj.Device)
			return err
		})
	} else {
		g.Go(func() error { return listen(ctx, c0, 50000, pkts) })
	}

	g.Go(func() error {
		n := 0
		for {
			select {
			case <-ctx.Done():
				fmt.Fprintf(os.Stderr, "captured %d packets to %s\n", n, out)
				return nil
			case p := <-pkts:
				if err := enc.Encode(p); err != nil {
					return errors.WithStack(err)
				}
				n++
			}
		}
	})

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

// online discovers a real device to learn the local interface, then returns a
// virtual CDJ and its IPv4 network for announcing.
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
	}
}

func packet(b []byte, port int, src *net.UDPAddr) Packet {
	return Packet{
		Data: fmt.Sprintf("%x", b),
		Len:  len(b),
		Port: port,
		Src:  src.String(),
		TS:   time.Now().UnixMilli(),
	}
}

func listen(ctx context.Context, c *net.UDPConn, port int, pkts chan<- Packet) error {
	go func() {
		<-ctx.Done()
		c.SetReadDeadline(time.Now())
	}()

	buf := make([]byte, 1500)
	for {
		n, addr, err := c.ReadFromUDP(buf)
		if err != nil {
			if ctx.Err() != nil {
				return nil
			}
			return errors.WithStack(err)
		}
		select {
		case pkts <- packet(buf[:n], port, addr):
		case <-ctx.Done():
			return nil
		}
	}
}

+      dst := &net.UDPAddr{IP: net.ParseIP(q.ip), Port: 50002}

+          c2.WriteToUDP(vcdj.MediaQuery(q.player, link.Slot(q.slot)), dst)

+          time.Sleep(500 * time.Millisecond)

+        fmt.Fprintf(os.Stderr, "sent media query to %s player %d slot %d\n", q.ip, q.player, q.slot)


Bash

cd /Users/noah/conductor/workspaces/deadca7/san-diego/link && go build ./... 2>&1 | tail && echo "BUILD OK" && echo "--- trigger media query to .192 (player 3) SD slot 2 ---" && go run ./cmd/capture -announce -device 5 -query-ip 192.168.0.192 -query-player 3 -query-slot 2 -out testdata/media.jsonl -secs 15 2>&1 | grep -vE '^\{'

Build and trigger media query against CDJ-3


BUILD OK
--- trigger media query to .192 (player 3) SD slot 2 ---
online as device 5
sent media query to 192.168.0.192 player 3 slot 2
captured 996 packets to testdata/media.jsonl
virtual CDJ "deadca7" ran as device 5


Checking if we got a 0x06 media response from that address on port 50002, and looking through the media.jsonl file to find entries of that type.


Query sent. Let me check whether the CDJ replied with a `0x06` media response.


cd /Users/noah/conductor/workspaces/deadca7/san-diego/link
python3 -c '
import json
resp=[(p,bytes.fromhex(p["data"])) for p in map(json.loads,open("testdata/media.jsonl"))
      if p["port"]==50002 and bytes.fromhex(p["data"])[0x0a]==0x06]
qs=[p for p in map(json.loads,open("testdata/media.jsonl")) if p["port"]==50002 and bytes.fromhex(p["data"])[0x0a]==0x05]
print("media query 0x05 sent (captured as our own):",len(qs))
print("media response 0x06 received:",len(resp))
if resp:
    p,d=resp[0]
    print("  from",p["src"],"len",len(d))
    print("  hex",d.hex())
'

Check for media response


media query 0x05 sent (captured as our own): 0
media response 0x06 received: 5
  from 192.168.0.192:39331 len 192
  hex 5173707431576d4a4f4c0643444a2d33303030000000000000000000000000010003009c0000000300000002004400690067004c00610062000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000032003000320036002d00300033002d003200390000000000310030003000300000000000000000000000000000000000000000000000000000000408000101000000030000001dc4600000000000154eec0000


 We got the media response back five times from the player, one for each retry attempt. The response is 192 bytes, which matches the minimum expected size. Now I'm decoding it using DecodeMediaDetails—looking at the hex structure, I can see the magic bytes followed by 0x06, the "CDJ-3000" device name starting at offset 0x0b, and another name field around 0x2c that I need to parse more carefully from the hex data.

Decoding the bytes at that offset as UTF-16BE gives me "DigLab"—that's the SD card volume name. The trackCount and playlistCount fields are at 0xa6 and 0xae respectively. Rather than manually tracing through the hex, I should just run DecodeMediaDetails on the actual captured response and see what it produces. I'll trim the media fixture down to just the 0x06 responses (the full file is huge), then write a quick test to decode and verify the structure.


**Media response received** — 5 of them (CDJ replied to each retry), 192 bytes. I can already see "DigLab" in the UTF-16BE name field. Let me decode it with our parser and trim the fixture.


cd /Users/noah/conductor/workspaces/deadca7/san-diego/link
# trim fixture to the 0x06 responses
python3 -c '
import json
out=[json.dumps(p,separators=(",",":")) for p in map(json.loads,open("testdata/media.jsonl"))
     if p["port"]==50002 and bytes.fromhex(p["data"])[0x0a]==0x06]
open("testdata/media.jsonl","w").write("\n".join(out[:2])+"\n")
print("media.jsonl trimmed to %d response(s)"%min(2,len(out)))
'
# decode with our parser
cat > pdb_media_explore_test.go <<'EOF'
package link
import ("encoding/hex";"encoding/json";"os";"bufio";"fmt";"testing")
func TestMediaExplore(t *testing.T){
  f,_:=os.Open("testdata/media.jsonl"); defer f.Close()
  s:=bufio.NewScanner(f)
  for s.Scan(){
    var p struct{Data string `json:"data"`}
    json.Unmarshal(s.Bytes(),&p)
    b,_:=hex.DecodeString(p.Data)
    m,err:=DecodeMediaDetails(b)
    if err!=nil{t.Fatal(err)}
    fmt.Printf("Media: name=%q player=%d slot=%s type=%v tracks=%d playlists=%d\n",m.Name,m.Player,m.Slot,m.Type,m.TrackCount,m.PlaylistCount)
    break
  }
}
EOF
go test -run TestMediaExplore -v 2>&1 | grep -E 'Media:|FAIL|ok|PASS' | head; rm pdb_media_explore_test.go

Decode media response with our parser


media.jsonl trimmed to 2 response(s)
Media: name="DigLab" player=3 slot=sd type=1 tracks=4 playlists=3
--- PASS: TestMediaExplore (0.00s)
PASS
ok  	github.com/nzoschke/deadca7/link	0.222s


 The DecodeMediaDetails validation is working perfectly—the track count of 4 matches what we parsed from the PDB, and we're also getting a playlist count of 3 that we hadn't extracted yet. This confirms the media query is both functional and consistent with our PDB parsing. Now I'm ready to write out the proper implementation.

I'll create a test in media_test.go that validates the DecodeMediaDetails against our fixture, asserting all the key fields match what we expect. Once that's solid, I can mark the media query/response feature as complete.

For the VCDJ master status feature, I need to be realistic about the scope—it's a substantial undertaking that involves building a full status packet, broadcasting it periodically, and implementing the master handoff state machine to capture the relevant packets. Given how much ground we've covered already with the media query, I think the right move is to deliver this feature as a complete, tested piece, report back on the V10 fader-start finding, and then offer an honest assessment of the master status work as a next step rather than trying to half-build it now. The status packet is really the crux here — it's a complex feature that's risky to implement without real hardware validation, and the actual value for a catalog/visualizer is limited since we already capture master and sync state from the deck status flags. Since they made it conditional ("if it's not too hard"), the honest move is to report that it *is* hard and ask them to confirm before I invest time in it. I'll wrap up the media query test, document the findings, and present all three answers with a clear recommendation on the master-status feature.

Writing media_test.go...


Decoded cleanly: **name "DigLab" (the SD volume), player 3, slot SD, rekordbox, 4 tracks, 3 playlists** — and **tracks=4 matches our PDB parse exactly**, cross-validating the two paths. Let me lock it in with a test and update `TODO.md`.


Write

package link

import (
	"encoding/hex"
	"testing"

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

func TestDecodeMediaDetails(t *testing.T) {
	a := assert.New(t)
	r := require.New(t)

	ps := fixture(t, "testdata/media.jsonl")
	r.NotEmpty(ps)

	b, err := hex.DecodeString(ps[0].Data)
	r.NoError(err)

	m, err := DecodeMediaDetails(b)
	r.NoError(err)
	a.Equal("DigLab", m.Name)
	a.Equal(byte(3), m.Player)
	a.Equal(SlotSD, m.Slot)
	a.Equal(TrackTypeRekordbox, m.Type)
	a.Equal(4, m.TrackCount)
	a.Equal(3, m.PlaylistCount)
}


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

package link

import (
	"encoding/hex"
	"testing"

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

func TestDecodeMediaDetails(t *testing.T) {
	a := assert.New(t)
	r := require.New(t)

	ps := fixture(t, "testdata/media.jsonl")
	r.NotEmpty(ps)

	b, err := hex.DecodeString(ps[0].Data)
	r.NoError(err)

	m, err := DecodeMediaDetails(b)
	r.NoError(err)
	a.Equal("DigLab", m.Name)
	a.Equal(byte(3), m.Player)
	a.Equal(SlotSD, m.Slot)
	a.Equal(TrackTypeRekordbox, m.Type)
	a.Equal(4, m.TrackCount)
	a.Equal(3, m.PlaylistCount)
}


Bash

go test ./... 2>&1 | tail -3 && wc -c testdata/media.jsonl

Run tests; check fixture size


?   	github.com/nzoschke/deadca7/link/cmd/fetch	[no test files]
?   	github.com/nzoschke/deadca7/link/nfs	[no test files]
ok  	github.com/nzoschke/deadca7/link/pdb	(cached)
     932 testdata/media.jsonl


Edit

- 0x05/0x06 media-query/response, 0x19/0x1a load-track, 0x34 load-settings:
  request/response we must initiate (the dbserver-style flow).

- [x] 0x05/0x06 media-query/response: VCDJ.MediaQuery + DecodeMediaDetails, fired
      via cmd/capture -query-ip/-query-player/-query-slot once online. Verified
      against CDJ-3 SD: name "DigLab", 4 tracks, 3 playlists (tracks match the PDB).
      `testdata/media.jsonl`.
- 0x19/0x1a load-track, 0x34 load-settings: request/response we must initiate.
- 0x02 fader-start: the DJM-V10 does NOT support fader start (per AlphaTheta),
  so this packet cannot be produced on this rig.
- VCDJ outgoing status (0x0a) + master/sync emission: needed to capture the
  directed 0x26/0x27/0x2a packets (a deck only sends them to the master/target).
  This is an "act as master" controller feature, not passive capture; real CDJs
  are picky about accepting a virtual status packet, so it is a sizable build.


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

- [x] 0x05/0x06 media-query/response: VCDJ.MediaQuery + DecodeMediaDetails, fired
      via cmd/capture -query-ip/-query-player/-query-slot once online. Verified
      against CDJ-3 SD: name "DigLab", 4 tracks, 3 playlists (tracks match the PDB).
      `testdata/media.jsonl`.
- 0x19/0x1a load-track, 0x34 load-settings: request/response we must initiate.
- 0x02 fader-start: the DJM-V10 does NOT support fader start (per AlphaTheta),
  so this packet cannot be produced on this rig.
- VCDJ outgoing status (0x0a) + master/sync emission: needed to capture the
  directed 0x26/0x27/0x2a packets (a deck only sends them to the master/target).
  This is an "act as master" controller feature, not passive capture; real CDJs
  are picky about accepting a virtual status packet, so it is a sizable build.

# link TODO

Go port of Pioneer Pro DJ Link, built capture-first: document each packet,
capture a real fixture from the CDJ-3000, then decode it under test. No test
should require live hardware — `cmd/capture` records fixtures into `testdata/`.

## Setup

- CDJ-3000 (device 3) at `192.168.0.192`, DJM-V10 mixer (device 33) at
  `192.168.0.193`, this Mac at `192.168.0.160` (en0), one `/24`.
- Reference SD card loaded in the CDJ for the database (crate-digger) flows.
- Passive capture: `go run ./cmd/capture -out testdata/<name>.jsonl -secs 8`.
- Active flows need a virtual CDJ that announces first — see Announce below.
  RPC/NFS uses dynamic ports, so capture those with `tcpdump` alongside.

## beat-link — live network protocol (UDP)

Each packet starts with Magic `51 73 70 74 31 57 6d 4a 4f 4c` + a type byte
read together with the port.

### Discovery / startup — port 50000
- [x] keep-alive (type 0x06): name, device #, IP, MAC — `testdata/devices.jsonl`
      covers CDJ-3000 (device 3) and DJM-V10 (device 33); name at 0x0c
- [x] virtual CDJ four-stage startup handshake (announce.go): hello (0x0a) ×3,
      claim 0x00/0x02/0x04 ×3 at 300ms, then keep-alive (0x06) at 1500ms.
      Verified the CDJ-3000 then streams 0x0a status — `testdata/status.jsonl`.
- [x] device-number negotiation (announce.go Online): self-assign (device 0)
      after a watch period, accept mixer assignment (0x01/0x03/0x05), abort/retry
      on a defense (0x08), and defend our own number once online. Verified fixed
      and auto-assign both reach 0x0a status; auto picked 5 (CDJ=3, mixer=33 used).
- [x] defense-out verified live: held device 4 while a second CDJ (.194) fixed
      to 4 was powered on; it oscillated 4<->25 as we defended 28 times. Real
      claim-stage-2 packets in `testdata/claim.jsonl`, parsed by ClaimNumber.
- [ ] mixer/gateway packet seen on 50002 (type 0x40, "NXS-GW") — identify & decode

### Beat sync — port 50001
- [x] beat packet (type 0x28): BPM, beat-in-bar, pitch — `testdata/beats.jsonl`
      (name@0x0b; bpm@0x5a centi; beat@0x5c; pitch@0x55 3-byte, 0x100000=100%)
- [x] precise position (type 0x0b, DecodePosition): device@0x21, length(s)@0x24,
      playback(ms)@0x28, pitch@0x2c, bpm@0x38 — `testdata/position.jsonl`
- [x] channels-on-air (type 0x03, DecodeOnAir): ch1-4@0x24-0x27, ch5-6@0x2d-0x2e
      (DJM-V10 0x35-len), device@0x21 — `testdata/onair.jsonl` (toggle 3<->4)
- [x] live two-deck session validated all decoders dynamically: playing flag,
      advancing position (deck 4 1466->93023ms), tempo change (deck 3 105 & 114
      BPM), on-air toggle, master/sync, track loads (deck 3 tracks 1->2->3).
- [ ] next/prev beat-timing distances (six 4-byte BE values from ~0x24)
- [x] mixer type 0x04 (len 100, DJM-V10) — `testdata/mixer04.jsonl`: device 33,
      near-static, only byte 0x24 varies; sparse periodic housekeeping, low value

### Status — port 50002 (needs virtual CDJ online)
- [x] captured CDJ 0x0a status (len 1116) — `testdata/status.jsonl`
- [x] decode 0x0a (status.go DecodeStatus): play/master/sync/on-air flags @0x89,
      bpm @0x92, pitch @0x8d, beat @0xa0, beat-in-bar @0xa6
- [x] loaded-track fields: track ID @0x2c, source player @0x28, slot @0x29,
      type @0x2a. Verified: track 1, player 3, SD, rekordbox.
- [x] CDJ type 0x40 on 50002 identified: "NXS-GW" (Nexus Gateway) keepalive,
      device 25, low-rate, undocumented on djl-analysis — not player status, skip
- [x] two-deck status captured (deck 3 track 1; deck 4 empty, bpm 655.35 sentinel)
- [x] dynamic scenarios captured & decoded: playing, on-air toggle, tempo change,
      track load (see Beat sync section)

## Not passively observable (need active participation, not bystander capture)

A bystander virtual CDJ has now captured every broadcast packet on the network.
The remaining enum types are directed device-to-device or request/response, so
they require our virtual CDJ to act, not just listen:

- 0x26/0x27 master-handoff, 0x2a sync-control (50001): directed between the
  master and the controlled deck. Evidence: a live capture where sync went true
  and master moved between decks produced zero of these packets. To capture,
  our VCDJ must be tempo master or the sync target (needs outgoing status/beat).
- 0x02 fader-start (50001): only when the DJM fader-start feature is enabled.
- 0x29 mixer-status (50002): DJM-V10 does NOT emit it — uses 0x03 + 0x04 instead.
- 0x05/0x06 media-query/response, 0x19/0x1a load-track, 0x34 load-settings:
  request/response we must initiate (the dbserver-style flow).
- 0x10 rekordbox-lighting-hello, 0x56 opus-metadata: other device classes, N/A.

## crate-digger — rekordbox database (over NFS)

Reached only after the virtual CDJ announces and a deck reports a loaded track.

- [x] no portmap needed: CDJ serves mount + NFS on UDP 2049 (nfs/nfs.go)
- [x] MOUNT slot path (SD=/B/, USB=/C/) → root file handle
- [x] NFS LOOKUP + READ `PIONEER/rekordbox/export.pdb` (cmd/fetch) —
      pulled 184320 bytes off CDJ-3000 SD into `testdata/export.pdb`
- [x] PDB parse (pdb/): kaitai-generated rekordbox_pdb.go + wrapper resolving
      tracks->title/artist/album/artwork/anlz. Tested vs `testdata/export.pdb`.
      (go vet warns on self-assignment in the generated file; upstream artifact)
- [x] metadata-by-track-ID: track 1 -> "get it" by mobygratis, matches status
- [ ] playlists / playlist tree
- [ ] ANLZ parse: `.DAT`/`.EXT` (beat grid, cue points, waveforms)
- [ ] wire status track ID + slot -> NFS fetch -> PDB lookup as one Link call

## Reliability (port the vizlab code without its known bugs)

- [x] proper startup handshake instead of vizlab blasting only keep-alives —
      this was the main instability: real players need hello + claim first
- [x] SetBroadcast (SO_BROADCAST) so subnet-broadcast sends don't silently fail
- [ ] replace `time.Sleep(5s)` FIXME in `Start()` with discovery/mount readiness
- [ ] make the media query deterministic (vizlab's was "unreliable")
- [x] swap `github.com/pkg/errors` → `github.com/cockroachdb/errors`
- [ ] live integration suite gated behind `testing.Short()` / env, separate from
      the hermetic fixture-replay tests

-- 0x05/0x06 media-query/response, 0x19/0x1a load-track, 0x34 load-settings:

-  request/response we must initiate (the dbserver-style flow).

+- [x] 0x05/0x06 media-query/response: VCDJ.MediaQuery + DecodeMediaDetails, fired

+      via cmd/capture -query-ip/-query-player/-query-slot once online. Verified

+      against CDJ-3 SD: name "DigLab", 4 tracks, 3 playlists (tracks match the PDB).

+- 0x19/0x1a load-track, 0x34 load-settings: request/response we must initiate.

+- 0x02 fader-start: the DJM-V10 does NOT support fader start (per AlphaTheta),

+  so this packet cannot be produced on this rig.

+- VCDJ outgoing status (0x0a) + master/sync emission: needed to capture the

+  directed 0x26/0x27/0x2a packets (a deck only sends them to the master/target).

+  This is an "act as master" controller feature, not passive capture; real CDJs

+  are picky about accepting a virtual status packet, so it is a sizable build.


All tests green. Here are your three answers.

## 1. Media query/response — done ✅
Built `VCDJ.MediaQuery` + `DecodeMediaDetails`, fired via the capture tool once online (`-query-ip/-query-player/-query-slot`). Triggered it against CDJ-3's SD and the player replied:

> **name "DigLab", player 3, SD, rekordbox, 4 tracks, 3 playlists**

The **4 tracks matches our PDB parse exactly** — the two independent paths cross-validate. Captured as `testdata/media.jsonl`, decoded and asserted by `TestDecodeMediaDetails`. (This is the flow vizlab found "unreliable"; sending 5x with our proper handshake, the CDJ answered every time.) It also gives **playlist count (3)** — something we haven't pulled from the PDB yet.

## 2. DJM-V10 fader start — it doesn't have one
Per [AlphaTheta's own support article](https://support.pioneerdj.com/hc/en-us/articles/4408479744409-Does-the-DJM-V10-support-the-Fader-Start-feature), **the DJM-V10 does not support Fader Start.** So the `0x02` packet simply can't be produced on your rig — that's why we never saw it. Nothing to enable.

## 3. VCDJ master outgoing status — real feature, not a quick add
This is the honest part. To capture the directed `0x26`/`0x27` (master handoff) and `0x2a` (sync control) packets, our VCDJ has to **be the tempo master or sync target** — a deck only sends those to the master/target, never broadcast. That means **emitting our own `0x0a` CDJ status** (with master flag + tempo + beat) so the network treats us as master, then handling the handoff exchange.

That's a sizable build, and the tricky part is that real CDJs are picky about accepting a synthesized status packet (beat-link has a lot of code making theirs acceptable). It's an "act as master / controller" feature rather than packet capture — and notably, we **already get master/sync state from the real decks' status flags**, so for a catalog/visualizer it adds little.

My recommendation: it's worth doing only if you want deadca7 to *drive* sync/tempo (be the master), not just observe. If that's a goal, I'll take it on as a focused next step — port beat-link's virtual status emitter, get a real CDJ to accept us as master, then capture the handoff. Want me to start that, or is observing master/sync via the status flags enough?

Capture-wise, we've now got everything: all broadcast packets decoded, plus the media-query initiator. The only remaining initiated flows are load-track/load-settings (commands to push to a deck) — say the word if you want those too.


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


 I need to implement the VCDJ outgoing status feature so I can act as the master and capture those remaining directed packets—the master handoff and sync control messages. This means building out the CDJ status packet emission logic using beat-link's virtual CDJ template.

Let me pull the key components from VirtualCdj.java: the status packet template bytes, the sendStatus method and its sender loop, the beat packet emission logic, and the master handoff handling including the 0x26/0x27 exchange and the becomeTempoMaster flow. I'll search for those method names and the template definition to gather everything I need.


On it — we'll build the VCDJ master/tempo emitter to drive sync and capture the last directed packets. Let me pull beat-link's outgoing status + beat + master-handoff implementation.


Bash

cd /tmp && grep -nE 'private.*byte\[\] (keepAlive|beat|status)|new byte\[\]|sendStatus|sendBeat|playerStatusBytes|beatBytes|void sendAnnounce|becomeTempoMaster|deliverMasterYield|handoffMasterTo|masterYield|0x29|MASTER_HANDOFF|setTempo|setSendingStatus|startSendingStatus' vcdj.java | head -50

Find status/beat emission and master handoff code