refactor: split recommendation engine into subpackage

internal/anime/recommendations/store.go

@@ -0,0 +1,72 @@
+package recommendations
+
+import (
+	"mal/internal/db"
+	"sort"
+	"sync"
+)
+
+type candidateStore struct {
+	watchlistAnimeIDs map[int]struct{}
+	byID              map[int]rankedCandidate
+	mu                sync.Mutex
+}
+
+func newCandidateStore(watchlist []db.GetUserWatchListRow) *candidateStore {
+	watchlistAnimeIDs := make(map[int]struct{}, len(watchlist))
+	for _, entry := range watchlist {
+		if entry.AnimeID <= 0 {
+			continue
+		}
+		watchlistAnimeIDs[int(entry.AnimeID)] = struct{}{}
+	}
+
+	return &candidateStore{
+		watchlistAnimeIDs: watchlistAnimeIDs,
+		byID:              map[int]rankedCandidate{},
+	}
+}
+
+func (s *candidateStore) upsert(candidate rankedCandidate) {
+	if candidate.id <= 0 {
+		return
+	}
+	if _, exists := s.watchlistAnimeIDs[candidate.id]; exists {
+		return
+	}
+
+	s.mu.Lock()
+	defer s.mu.Unlock()
+
+	current, ok := s.byID[candidate.id]
+	if !ok {
+		s.byID[candidate.id] = candidate
+		return
+	}
+
+	current.collaborativeScore += candidate.collaborativeScore
+	current.profileSearchScore += candidate.profileSearchScore
+	if candidate.hasAnime {
+		current.anime = candidate.anime
+		current.hasAnime = true
+	}
+	s.byID[candidate.id] = current
+}
+
+func (s *candidateStore) ranked() []rankedCandidate {
+	ranked := make([]rankedCandidate, 0, len(s.byID))
+	for _, item := range s.byID {
+		ranked = append(ranked, item)
+	}
+
+	sort.Slice(ranked, func(i, j int) bool {
+		left := rankedCandidateRetrievalScore(ranked[i].collaborativeScore, ranked[i].profileSearchScore)
+		right := rankedCandidateRetrievalScore(ranked[j].collaborativeScore, ranked[j].profileSearchScore)
+		if left == right {
+			return ranked[i].id < ranked[j].id
+		}
+		return left > right
+	})
+
+	return ranked
+}