# music — recommendation / vibe engine spec

Layers on top of core (spec 03). Handles enrichment (MusicBrainz + Last.fm), the vibe-endless-queue, candidate lifecycle, and the dislike → delayed-delete flow. Acquisition is a pluggable hook — the engine never fetches copyrighted audio itself.

## External providers
- **MusicBrainz** (no key, rate-limited 1 req/s, set a proper User-Agent): canonical recording/artist/release MBIDs, genres/tags. Used during enrichment to stamp `mbid` and genre data.
- **Last.fm** (free API key required): `track.getSimilar`, `artist.getSimilar`, `tag.getTopTracks`. Drives recommendations + vibe-queue.
- Store the **recommendation source** per candidate (`rec_source` on tracks: `lastfm` | `musicbrainz`) so feedback can be attributed.
- Respect rate limits: queue external calls, cache responses in sqlite.

```sql
CREATE TABLE IF NOT EXISTS mb_cache (
    key TEXT PRIMARY KEY,          -- e.g. "recording:<artist>:<title>"
    payload TEXT NOT NULL,         -- JSON
    fetched_at TEXT NOT NULL
);

CREATE TABLE IF NOT EXISTS lastfm_cache (
    key TEXT PRIMARY KEY,          -- e.g. "similar:<mbid|artist-title>"
    payload TEXT NOT NULL,
    fetched_at TEXT NOT NULL
);

-- candidate dislike lifecycle
CREATE TABLE IF NOT EXISTS dislikes (
    track_id INTEGER PRIMARY KEY REFERENCES tracks(id) ON DELETE CASCADE,
    disliked_at TEXT NOT NULL,
    warn_after TEXT NOT NULL,      -- disliked_at + grace period ("a couple days")
    warned_at TEXT,                -- set when reminder fired
    delete_after TEXT,             -- warned_at + 24h
    state TEXT NOT NULL            -- 'hidden' | 'warned' | 'deleted'
);

CREATE TABLE IF NOT EXISTS feedback (
    id INTEGER PRIMARY KEY AUTOINCREMENT,
    track_id INTEGER,
    rec_source TEXT,               -- which provider suggested it
    action TEXT NOT NULL,          -- 'promoted' | 'disliked' | 'skipped'
    at TEXT NOT NULL
);
```

## Enrichment
- After core indexing, a background enrichment pass fills missing `mbid`, `genre`, artist `genres`, artist `image_path`:
  - MusicBrainz lookup by artist+title → recording MBID + tags.
  - Last.fm `artist.getInfo` / `track.getInfo` for tags + similar seeds; cache.
- Throttled, cached, resumable. Never blocks playback.

## Recommendation / vibe queue
```
GET /api/vibe?seed_track_id=&limit=          -> ordered vibe queue (track ids)
GET /api/vibe/from-genre?genre=&limit=
```
Algorithm:
1. Seed = current track / chosen track / genre.
2. Pull similar from Last.fm (`track.getSimilar`, `artist.getSimilar`, `tag.getTopTracks`), cached.
3. Score each candidate against the **owned library**:
   - +score if artist/album already owned, genre overlap, similar to favorites, low recent-play (freshness), not disliked.
4. **Mix**: vibe queue interleaves owned tracks with promoted-from-recommendation tracks — recommendations are NOT front-loaded; they're shuffled into the stream so it feels organic.
5. Candidates **not yet owned** → enter the candidate pipeline (below) rather than playing immediately.

## Candidate lifecycle (acquisition)
- Hard cap: **max 5 candidates in probation at once** (`tracks.probation=1`). Engine won't request new acquisitions beyond the cap.
- When the engine wants to surface an unowned suggestion, it calls the acquisition hook:

```kotlin
// or python equivalent — defined seam, implemented by the operator
interface AcquisitionProvider {
    /** Fetch audio for a candidate. Return the local file path on success, null to skip.
     *  The engine does NOT care where the bytes come from. */
    suspend fun acquire(candidate: TrackCandidate): AcquiredFile?
}

data class TrackCandidate(
    val title: String,
    val artist: String,
    val album: String?,
    val mbid: String?,
    val recSource: String,        // lastfm | musicbrainz
)
data class AcquiredFile(val path: String)
```
  - Default shipped impl: `NoopAcquisitionProvider` (returns null → candidate stays metadata-only, never plays). Operator wires a real one (bandcamp purchase dl, FMA/Jamendo CC, internet archive, personal rips, etc.).
  - On successful acquire: file lands in music root, indexed as a track with `probation=1`, `source='recommendation'`, `rec_source` set, hidden from normal library lists (filtered by `probation=0`) but eligible to appear mixed into the **vibe queue**.

### Promotion
- A probation track that gets played and **not disliked** (passes the play-completion threshold) → `probation=0`, `source` stays `recommendation` for analytics but it's now full library, eligible as a future recommendation seed. Log `feedback(action='promoted')`.

## Dislike → delayed delete flow
Exact lifecycle:
1. **User dislikes a track** → insert/[update] `dislikes` row: `state='hidden'`, `disliked_at=now`, `warn_after=now + GRACE` (GRACE = "a couple days", e.g. 48h). Track is **hidden** from library + queues immediately. **File stays on disk.** Also `feedback(action='disliked')`.
2. **Reminder**: a periodic sweep finds `state='hidden'` rows past `warn_after` → fire notification (ntfy) / surface a toast in UI: *"You disliked '<track>'. It will be deleted in 24h — are you sure?"* Set `warned_at=now`, `delete_after=now + 24h`, `state='warned'`.
3. **Final**: sweep finds `state='warned'` rows past `delete_after` where the dislike still stands → **delete file from fs + all db rows** (calls core `DELETE /api/track`). Set `state='deleted'` (or just remove the dislike row since the track row is gone).
4. **Un-dislike** at any point before final deletion → remove `dislikes` row, un-hide the track. It's spared.

```
POST   /api/dislike/{track_id}      -> start lifecycle (hide + schedule)
DELETE /api/dislike/{track_id}      -> un-dislike (spare it), un-hide
GET    /api/dislikes                -> pending dislikes + their states/timers
POST   /api/dislikes/sweep          -> manual trigger of the sweep (also runs on a timer)
```

### Sweep scheduler
- A background timer (e.g. hourly) runs the sweep: hidden→warned (fires ntfy), warned→deleted. Hourly granularity is fine for day-scale timers.
- ntfy integration: POST to the ntfy topic (reuse the server's ntfy from the parked list) for the reminder.

## Feedback loop (future-facing, stub now)
- `feedback` table records promoted/disliked/skipped with `rec_source`.
- Later: weight providers/genres by acceptance rate (promoted vs disliked) to bias future recommendations. v1 just records; scoring tweak is a later iteration.

## Endpoints summary (this layer)
```
GET    /api/vibe
GET    /api/vibe/from-genre
POST   /api/dislike/{id}
DELETE /api/dislike/{id}
GET    /api/dislikes
POST   /api/dislikes/sweep
POST   /api/enrich            -> manual enrichment pass
GET    /api/recommendations   -> current probation candidates + why (rec_source, seed)
```

## Config / prefs
- `lastfm_api_key` (prefs or env)
- `mb_user_agent` (required by MusicBrainz)
- `dislike_grace_hours` (default 48)
- `dislike_final_hours` (default 24)
- `max_candidates` (default 5)
- `ntfy_topic_url` for reminders
