fix: limit recommendation scoring

internal/anime/recommendations/constants.go

@@ -6,6 +6,7 @@ const (
 	maxSeeds               = 8
 	maxRecommendations     = 10
 	candidateFetchLimit    = 60
+	candidateFetchBuffer   = 6
 	TopPickLimit           = 18
 	TopPicksLimit          = 60
 	profileSearchLimit     = 8

internal/anime/recommendations/engine.go

@@ -64,7 +64,7 @@ func (e engine) getTopPicksForYou(ctx context.Context, userID string, resultLimi
 		return domain.CatalogSectionData{Animes: []domain.Anime{}}, nil
 	}
 
-	candidates, err := e.scoreRankedCandidates(ctx, now, profile, ranked)
+	candidates, err := e.scoreRankedCandidates(ctx, now, profile, ranked, resultLimit)
 	if err != nil {
 		return domain.CatalogSectionData{}, err
 	}
@@ -191,8 +191,9 @@ func (e engine) scoreRankedCandidates(
 	now time.Time,
 	profile userTasteProfile,
 	ranked []rankedCandidate,
+	resultLimit int,
 ) ([]recommendationCandidate, error) {
-	limit := min(len(ranked), candidateFetchLimit)
+	limit := min(len(ranked), candidateScoreLimit(resultLimit))
 	candidates := make([]recommendationCandidate, 0, limit)
 	var candidatesMu sync.Mutex
 	var g errgroup.Group
@@ -244,3 +245,11 @@ func (e engine) scoreRankedCandidates(
 
 	return candidates, nil
 }
+
+func candidateScoreLimit(resultLimit int) int {
+	if resultLimit <= 0 {
+		return 0
+	}
+
+	return min(candidateFetchLimit, resultLimit+candidateFetchBuffer)
+}

internal/anime/recommendations/recommendations_test.go

@@ -174,6 +174,18 @@ func TestRerankRecommendationCandidatesSpreadsRepeatedGenres(t *testing.T) {
 	}
 }
 
+func TestCandidateScoreLimitTracksRequestedResultSize(t *testing.T) {
+	if got := candidateScoreLimit(TopPickLimit); got != TopPickLimit+candidateFetchBuffer {
+		t.Fatalf("expected top-pick scoring to fetch a small oversample, got %d", got)
+	}
+	if got := candidateScoreLimit(TopPicksLimit); got != candidateFetchLimit {
+		t.Fatalf("expected full top-picks scoring to keep existing cap, got %d", got)
+	}
+	if got := candidateScoreLimit(0); got != 0 {
+		t.Fatalf("expected zero result limit to skip scoring, got %d", got)
+	}
+}
+
 func testRecommendationAnime(id int, genreID int) jikan.Anime {
 	return jikan.Anime{
 		MalID:  id,