minstrel/internal/recommendation/library_scale_test.go

package recommendation

import (
	"context"
	"errors"
	"testing"
	"time"
)

// Small libraries must behave exactly as before. Scaling is meant to help a
// growing collection, not to change what every existing install already does.
func TestScaleForLibrary_SmallLibrariesAreUntouched(t *testing.T) {
	base := DefaultCandidateSourceLimits()
	for _, size := range []int64{0, 1, 500, libraryScaleReference} {
		if got := ScaleForLibrary(base, size); got != base {
			t.Errorf("library of %d changed the limits: %+v, want %+v", size, got, base)
		}
	}
}

// The complaint, restated as a property: a bigger library must reach further
// into itself. Operator, 2026-09-10 — "is the pool that we draw from somehow
// scaled to the amount of music in the library".
func TestScaleForLibrary_BigLibrariesGetABiggerPool(t *testing.T) {
	base := DefaultCandidateSourceLimits()
	small := ScaleForLibrary(base, libraryScaleReference)
	big := ScaleForLibrary(base, libraryScaleReference*16)

	if big.RandomFill <= small.RandomFill {
		t.Errorf("RandomFill %d did not grow for a 16x library (was %d); that arm "+
			"IS the fraction-of-library problem", big.RandomFill, small.RandomFill)
	}
	if big.LBSimilar <= small.LBSimilar {
		t.Errorf("LBSimilar %d did not grow for a 16x library (was %d)",
			big.LBSimilar, small.LBSimilar)
	}
}

// THE SUBSTANCE: scaling is per-arm, decided by what each arm is BOUNDED BY.
// Raising a limit only helps if there are rows for it to cut off, so arms
// bounded by the user's likes, the instance's co-play graph or the taste
// profile gain nothing from a bigger library — raising them would sample more
// of a set that did not grow.
//
// A uniform scale would look right and be wrong, which is why this is pinned
// separately from "the pool got bigger".
func TestScaleForLibrary_OnlyLibraryBoundedArmsGrow(t *testing.T) {
	base := DefaultCandidateSourceLimits()
	big := ScaleForLibrary(base, libraryScaleReference*16)

	for _, tc := range []struct {
		arm       string
		got, want int
		why       string
	}{
		{"LikesOverlap", big.LikesOverlap, base.LikesOverlap, "bounded by the user's likes"},
		{"UserCoplay", big.UserCoplay, base.UserCoplay, "bounded by the instance's co-play graph"},
		{"TasteOverlap", big.TasteOverlap, base.TasteOverlap, "bounded by the taste profile's artists"},
	} {
		if tc.got != tc.want {
			t.Errorf("%s scaled to %d (base %d) but is %s — a bigger library gives it "+
				"nothing more to return", tc.arm, tc.got, tc.want, tc.why)
		}
	}
}

// Growth is bounded, or a huge library turns every recommendation query into
// a huge scan. sqrt keeps it proportional; the ceiling keeps it affordable.
func TestScaleForLibrary_GrowthIsBounded(t *testing.T) {
	base := DefaultCandidateSourceLimits()
	huge := ScaleForLibrary(base, 100_000_000)

	if huge.RandomFill > int(float64(base.RandomFill)*maxLibraryScale) {
		t.Errorf("RandomFill %d exceeds the %.0fx ceiling on base %d",
			huge.RandomFill, maxLibraryScale, base.RandomFill)
	}
	// sqrt, not linear — and the test point matters. At SIXTEEN times the
	// reference both curves land on 4x, because linear has already been
	// clamped by the ceiling; asserting there proves nothing. Four times the
	// reference is below the ceiling for both, so the curves separate: sqrt
	// gives 2x, linear would give 4x.
	four := ScaleForLibrary(base, libraryScaleReference*4)
	if four.RandomFill != base.RandomFill*2 {
		t.Errorf("4x library gave RandomFill %d, want %d — sqrt growth (linear "+
			"would give %d)", four.RandomFill, base.RandomFill*2, base.RandomFill*4)
	}
}

// Never below the base. The base limits are a floor, not a midpoint — and
// #3889 makes this load-bearing rather than tidy: shrinking an arm ordered by
// unseeded random() changes pool membership between same-day rebuilds.
func TestScaleForLibrary_NeverShrinksAnArm(t *testing.T) {
	base := DefaultCandidateSourceLimits()
	for _, size := range []int64{0, 1, 100, 4999, 5001, 1_000_000} {
		got := ScaleForLibrary(base, size)
		for _, tc := range []struct {
			arm       string
			got, want int
		}{
			{"LBSimilar", got.LBSimilar, base.LBSimilar},
			{"SimilarArtist", got.SimilarArtist, base.SimilarArtist},
			{"TagOverlap", got.TagOverlap, base.TagOverlap},
			{"RandomFill", got.RandomFill, base.RandomFill},
			{"LikesOverlap", got.LikesOverlap, base.LikesOverlap},
			{"UserCoplay", got.UserCoplay, base.UserCoplay},
			{"TasteOverlap", got.TasteOverlap, base.TasteOverlap},
		} {
			if tc.got < tc.want {
				t.Errorf("library %d shrank %s to %d (base %d)", size, tc.arm, tc.got, tc.want)
			}
		}
	}
}

// A pool-sizing hint must never fail the request it is sizing. A count that
// errors leaves the previous value in place, and a never-counted cache
// returns 0 — which scales to the base limits, i.e. exactly today's
// behaviour.
func TestLibrarySize_DegradesOnFailure(t *testing.T) {
	clock := time.Now()
	c := NewLibrarySize(func() time.Time { return clock })

	boom := func(context.Context) (int64, error) { return 0, errors.New("db is down") }
	if got := c.Get(context.Background(), boom); got != 0 {
		t.Errorf("first failure returned %d, want 0 (which scales to the base limits)", got)
	}
	if got := ScaleForLibrary(DefaultCandidateSourceLimits(), 0); got != DefaultCandidateSourceLimits() {
		t.Error("a zero library size did not scale to the base limits")
	}

	// A good count, then a failure: the last known value survives.
	if got := c.Get(context.Background(), func(context.Context) (int64, error) { return 40_000, nil }); got != 40_000 {
		t.Fatalf("got %d, want 40000", got)
	}
	clock = clock.Add(librarySizeTTL + time.Second)
	if got := c.Get(context.Background(), boom); got != 40_000 {
		t.Errorf("a failed refresh returned %d, discarding the last known 40000", got)
	}
}

// The count is a full table scan, so it must not run per request.
func TestLibrarySize_CachesWithinTheTTL(t *testing.T) {
	clock := time.Now()
	c := NewLibrarySize(func() time.Time { return clock })
	calls := 0
	load := func(context.Context) (int64, error) { calls++; return 1234, nil }

	for i := 0; i < 5; i++ {
		c.Get(context.Background(), load)
	}
	if calls != 1 {
		t.Errorf("counted %d times within the TTL, want 1 — this is a full table scan", calls)
	}

	clock = clock.Add(librarySizeTTL + time.Second)
	c.Get(context.Background(), load)
	if calls != 2 {
		t.Errorf("counted %d times after the TTL expired, want 2 — the size never refreshes", calls)
	}
}

// A transient failure must not pin a stale value for the whole TTL: the
// failed refresh does not stamp the clock, so the next call retries.
func TestLibrarySize_RetriesAfterAFailedRefresh(t *testing.T) {
	clock := time.Now()
	c := NewLibrarySize(func() time.Time { return clock })

	c.Get(context.Background(), func(context.Context) (int64, error) { return 100, nil })
	clock = clock.Add(librarySizeTTL + time.Second)
	c.Get(context.Background(), func(context.Context) (int64, error) { return 0, errors.New("blip") })

	// Immediately after, with no clock movement — a stamped failure would
	// serve the stale 100 until the TTL expired again.
	if got := c.Get(context.Background(), func(context.Context) (int64, error) { return 900, nil }); got != 900 {
		t.Errorf("got %d after a failed refresh, want 900 — the failure pinned a stale value", got)
	}
}