minstrel/internal/recommendation/songs_like_limits_test.go

package recommendation

import "testing"

// Songs-like's pool must lean on arms that MEASURE distance from the seed.
// The default gave ~29% of candidates a sim_score of literally 0
// (taste_overlap and random_fill are both `0.0::float8` in
// recommendation.sql), which is what let "Songs like X" wander.
func TestSongsLikeLimits_FavourTheArmsThatMeasureTheSeed(t *testing.T) {
	d := DefaultCandidateSourceLimits()
	s := SongsLikeCandidateSourceLimits()

	if s.LBSimilar <= d.LBSimilar {
		t.Errorf("LBSimilar %d is not above the default %d — the only arm that "+
			"measures track-level distance from the seed should be favoured here",
			s.LBSimilar, d.LBSimilar)
	}
	// The two seed-INDEPENDENT arms, which is the whole complaint.
	zeroSimDefault := d.TasteOverlap + d.RandomFill
	zeroSimSongsLike := s.TasteOverlap + s.RandomFill
	if zeroSimSongsLike >= zeroSimDefault {
		t.Errorf("seed-independent arms total %d, not reduced from the default %d; "+
			"these carry sim_score 0 by construction", zeroSimSongsLike, zeroSimDefault)
	}
}

// RULE 131: a system playlist degrades, it never vanishes.
//
// Zeroing the seed-independent arms was the first instinct and is exactly the
// vanish-or-nothing shape that rule forbids: a seed whose artist has thin
// ListenBrainz coverage would yield a short mix or none at all. They are the
// tier-3 FLOOR — reduced hard, never removed — and the songs_like weights are
// what keep them at the bottom of the ranking rather than out of the pool.
//
// "A few tracks further from the seed than we would like" beats "no playlist".
func TestSongsLikeLimits_KeepATierThreeFloor(t *testing.T) {
	s := SongsLikeCandidateSourceLimits()
	if s.RandomFill <= 0 {
		t.Error("RandomFill is zero: a seed with thin similarity coverage now produces " +
			"a short or empty mix instead of degrading (rule 131)")
	}
	if s.TasteOverlap <= 0 {
		t.Error("TasteOverlap is zero: the graded floor is gone, leaving only random " +
			"fill between a sparse seed and an empty playlist (rule 131)")
	}
}

// The pool should stay roughly the size it was — this change is about
// COMPOSITION, not about starving the surface. A much smaller pool would also
// shrink what the per-artist cap has to work with.
func TestSongsLikeLimits_KeepThePoolRoughlyTheSameSize(t *testing.T) {
	total := func(l CandidateSourceLimits) int {
		return l.LBSimilar + l.SimilarArtist + l.TagOverlap + l.LikesOverlap +
			l.RandomFill + l.TasteOverlap + l.UserCoplay
	}
	d, s := total(DefaultCandidateSourceLimits()), total(SongsLikeCandidateSourceLimits())
	if s < d/2 {
		t.Errorf("songs_like pool is %d against the default %d — less than half; "+
			"this was meant to re-weight the pool, not starve it", s, d)
	}
}

// The constraint that is invisible in the numbers, and that this file exists
// to keep visible.
//
// `likes_overlap` and `random_fill` end in a bare `ORDER BY random()` with no
// daily seed (recommendation.sql:118, :161). Such an arm returns a stable set
// only while its LIMIT exceeds the eligible rows; below that it returns a
// random SUBSET that differs between two builds on the same day, and the
// daily-determinism promise quietly stops holding.
//
// This is not hypothetical — it is how this change first failed CI. Cutting
// RandomFill to 10 broke TestBuildSystemPlaylists_DailyNonceDeterminism,
// whose library is smaller than the default limit and whose determinism was
// therefore an accident of the limit exceeding the library.
//
// Growing these arms is always safe. Only shrinking is, and the fix that
// would make shrinking safe is a seeded ordering (#3889), not a smaller
// number here.
func TestSongsLikeLimits_DoNotShrinkTheUnseededRandomArms(t *testing.T) {
	d := DefaultCandidateSourceLimits()
	s := SongsLikeCandidateSourceLimits()

	for _, tc := range []struct {
		arm             string
		songsLike, dflt int
	}{
		{"RandomFill", s.RandomFill, d.RandomFill},
		{"LikesOverlap", s.LikesOverlap, d.LikesOverlap},
	} {
		if tc.songsLike < tc.dflt {
			t.Errorf("%s cut from %d to %d. That arm is ordered by unseeded random(), "+
				"so a smaller limit makes pool membership vary between same-day "+
				"rebuilds — it breaks daily determinism rather than merely narrowing "+
				"the mix. Fix the ordering (#3889) before trimming this.",
				tc.arm, tc.dflt, tc.songsLike)
		}
	}
}