Discover playlists could surface the same track twice with the
duplicates landing back-to-back — a "first song plays, then plays
again, skip works" symptom user reported on v2026.05.13.0. Root
cause: interleaveBuckets rotates one track per pass per bucket but
never tracks which IDs it has already emitted, so a track that's
both a dormant-artist pick AND a random-unheard pick comes out
once from each bucket.
On a single-user server the crossUser bucket is empty, so the
redistribute step rolls its slots into dormant + random. Their
output then interleaves d0, r0, d1, r1, … — and when d0 == r0
(common: a dormant-artist track is also valid for random-unheard)
the result is [X, X, …] with adjacent duplicates.
Fix: track seen track IDs across all buckets while interleaving;
skip already-taken IDs and advance to the next index in that
bucket. Dedup priority is bucket order, so a track in both
dormant and random comes from dormant.
Regression test covers the single-user case directly. The existing
round-robin test still passes — no shared IDs in that fixture.
Note: stale duplicates already written to drift / served as cached
playlists will clear naturally on the next playlist rebuild (the
03:00-local refresh, or any manual /api/me/playlists/refresh).
Adds the third system playlist variant: 'discover'. Surfaces 100
tracks the operator has not played and not liked, biased toward
artists they rarely play (< 10 plays) and complemented by tracks
liked by other users plus a random unheard sample. Cold start
collapses to all-random; single-user servers redistribute the
cross-user-likes allocation equally across the other two buckets.
The build runs as a fourth phase inside BuildSystemPlaylists
alongside For You and the seed-artist mixes. Same daily-deterministic
md5(track_id || dateStr) ordering as the other variants. Per-album
(<=2) and per-artist (<=3) caps prevent collapse onto a single
prolific artist. Buckets interleave round-robin so the playlist
order doesn't front-load one bucket's flavour.
Post-commit collage generation (already wired) gives Discover the
same 4-cell cover treatment as the other system playlists — no
new collage code needed.
Tests cover the slot-redistribution table (all-available,
cold-start, single-user, partial-dormant, fully-empty), the
per-album/per-artist caps, the round-robin interleave, and a
DB-backed cold-start integration test that asserts a Discover
playlist lands with non-zero tracks.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
bvandeusen