Five diversity mechanics — applied to both For-You and Songs-Like-X.
1. For-You seed rotates daily across the user's top-5 most-played
tracks. pickForYouSeedForDay uses userIDHash(user, day) mod
len(seeds) so today's mix uses an entirely different similarity
pool than tomorrow's. Within-day determinism preserved.
2. JitterMagnitude bumped 0.0 → 0.1. The scoring RNG is now seeded
by userIDHash(user, day) rather than the no-op, so near-tied
candidates shuffle daily without breaking within-day stability.
3. Head/tail split moves from 20+5 to 12+13. Roughly half the
playlist comes from the tail now (daily-deterministic via
tieBreakHash), giving the user substantially different content
while a 12-track anchor of strong similarity matches keeps the
mix recognizable.
4. Songs-Like-X seed artists shuffle daily across the user's top-5
played artists. pickSeedArtistsForDay applies a userIDHash-seeded
Fisher-Yates and takes 3.
5. scoreAndSortCandidates / pickTopN / pickHeadAndTail gain a userID
parameter so the RNG can be seeded per-user; existing call sites
updated; noopRNG removed.
Test fixtures widened similarity gaps (e.g. float64(50-i) instead of
(50-i)/50) so the new jitter (±0.1) doesn't perturb head ordering in
assertions about the head/tail mechanism. New seed_selection_test
coverage for userIDHash + pickForYouSeedForDay + pickSeedArtistsForDay
spans deterministic-within-day, varies-across-days, and graceful
degradation with small candidate pools.
PickTopPlayedTrackForUser replaced by PickTopPlayedTracksForUser
:many in the prior commit (b4801c2). The For-You seed lookup now
goes through pickForYouSeedForDay over the returned slice.
PickSeedArtists's LIMIT widened to 5 in the same prior commit.
For #392 Half A — system playlist content diversity. Half B
(per-user timezone scheduling) is a separate spec.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
bvandeusen
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