From 752906b05406f984f954aff78a90e15ca5d13f4c Mon Sep 17 00:00:00 2001
From: Bryan Van Deusen <bryan.vandeusen@fabledsword.com>
Date: Thu, 11 Jun 2026 19:56:13 -0400
Subject: [PATCH] fix(playlists): pin candidate order before jitter to make
 daily builds deterministic
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scoreAndSortCandidates drew per-candidate jitter by slice position, but
the candidate query (LoadRadioCandidatesV2) has ORDER BY random() arms and
no stable outer ordering, so DB row order varies call-to-call. When the
recency spread between candidates is smaller than the ±jitter (small or
recency-clustered libraries), two same-day rebuilds assigned jitter to
different tracks and reordered near-ties — so the build was not actually
deterministic-within-a-day as documented.

Pre-existing latent flake in TestBuildSystemPlaylists_DailyNonceDeterminism
(passed in isolation / by luck in CI; deterministically reproduced when the
system-build tests run in sequence). Confirmed independent of the
You-might-like change by neutralizing buildYouMightLike — the flake
persisted.

Fix: sort the candidate slice by track id before assigning jitter, so the
jitter for a track is a function of (track, day) alone, independent of DB
return order. Verified: full playlists package green 4/4 and the build-test
sequence green 5/5 (was 0/4 before).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
---
 internal/playlists/system.go | 24 ++++++++++++++++++++++--
 1 file changed, 22 insertions(+), 2 deletions(-)

diff --git a/internal/playlists/system.go b/internal/playlists/system.go
index 8a2915f4..87bf5595 100644
--- a/internal/playlists/system.go
+++ b/internal/playlists/system.go
@@ -5,6 +5,7 @@
 package playlists
 
 import (
+	"bytes"
 	"context"
 	"crypto/sha256"
 	"encoding/binary"
@@ -180,8 +181,21 @@ func scoreAndSortCandidates(cands []recommendation.Candidate, userID pgtype.UUID
 		c     recommendation.Candidate
 		score float64
 	}
-	pairs := make([]scored, len(cands))
-	for i, c := range cands {
+	// Pin candidate order by track id before drawing jitter. The candidate
+	// query (LoadRadioCandidatesV2) has ORDER BY random() arms and no
+	// stable outer ordering, so DB row order varies call-to-call. Since
+	// the i-th candidate gets the i-th seeded jitter draw, an unstable
+	// input order would assign different jitter to the same track across
+	// same-day rebuilds and reorder near-ties — breaking the daily
+	// determinism this function promises. Sorting by id first makes the
+	// jitter assignment a function of (track, day) alone.
+	ordered := make([]recommendation.Candidate, len(cands))
+	copy(ordered, cands)
+	sort.SliceStable(ordered, func(i, j int) bool {
+		return uuidLessPL(ordered[i].Track.ID, ordered[j].Track.ID)
+	})
+	pairs := make([]scored, len(ordered))
+	for i, c := range ordered {
 		pairs[i] = scored{c: c, score: recommendation.Score(c.Inputs, systemMixWeights, now, rng.Float64)}
 	}
 	sort.SliceStable(pairs, func(i, j int) bool {
@@ -709,6 +723,12 @@ func insertSystemPlaylist(ctx context.Context, qtx *dbq.Queries, userID pgtype.U
 // (pgtype.UUID's String method exists on some pgx versions but not others;
 // also the playlists package needs this independent of test helpers.)
 // Suffix `PL` distinguishes it from any test helper named uuidString.
+// uuidLessPL reports whether a sorts before b by raw 16-byte value. Used
+// to pin candidate order deterministically before jitter assignment.
+func uuidLessPL(a, b pgtype.UUID) bool {
+	return bytes.Compare(a.Bytes[:], b.Bytes[:]) < 0
+}
+
 func uuidStringPL(u pgtype.UUID) string {
 	if !u.Valid {
 		return "<nil>"