refactor: reduce GC pressure by pre-allocating slices

Signed-off-by: Deluan <deluan@navidrome.org>

model/album.go

@@ -84,7 +84,7 @@ type Albums []Album
 // It assumes all albums have the same AlbumArtist, or else results are unpredictable.
 func (als Albums) ToAlbumArtist() Artist {
 	a := Artist{AlbumCount: len(als)}
-	var mbzArtistIds []string
+	mbzArtistIds := make([]string, 0, len(als))
 	for _, al := range als {
 		a.ID = al.AlbumArtistID
 		a.Name = al.AlbumArtist

model/lyrics.go

@@ -39,11 +39,11 @@ func ToLyrics(language, text string) (*Lyrics, error) {
 	text = str.SanitizeText(text)
 
 	lines := strings.Split(text, "\n")
+	structuredLines := make([]Line, 0, len(lines)*2)
 
 	artist := ""
 	title := ""
 	var offset *int64 = nil
-	structuredLines := []Line{}
 
 	synced := syncRegex.MatchString(text)
 	priorLine := ""
@@ -105,7 +105,7 @@ func ToLyrics(language, text string) (*Lyrics, error) {
 						Value: strings.TrimSpace(priorLine),
 					})
 				}
-				timestamps = []int64{}
+				timestamps = nil
 			}
 
 			end := 0

model/mediafile.go

@@ -108,11 +108,9 @@ type MediaFiles []MediaFile
 
 // Dirs returns a deduped list of all directories from the MediaFiles' paths
 func (mfs MediaFiles) Dirs() []string {
-	var dirs []string
+	dirs := slice.Map(mfs, func(m MediaFile) string {
-	for _, mf := range mfs {
+		return filepath.Dir(m.Path)
-		dir, _ := filepath.Split(mf.Path)
+	})
-		dirs = append(dirs, filepath.Clean(dir))
-	}
 	slices.Sort(dirs)
 	return slices.Compact(dirs)
 }
@@ -121,16 +119,16 @@ func (mfs MediaFiles) Dirs() []string {
 // It assumes all mediafiles have the same Album, or else results are unpredictable.
 func (mfs MediaFiles) ToAlbum() Album {
 	a := Album{SongCount: len(mfs)}
-	var fullText []string
+	fullText := make([]string, 0, len(mfs))
-	var albumArtistIds []string
+	albumArtistIds := make([]string, 0, len(mfs))
-	var songArtistIds []string
+	songArtistIds := make([]string, 0, len(mfs))
-	var mbzAlbumIds []string
+	mbzAlbumIds := make([]string, 0, len(mfs))
-	var comments []string
+	comments := make([]string, 0, len(mfs))
-	var years []int
+	years := make([]int, 0, len(mfs))
-	var dates []string
+	dates := make([]string, 0, len(mfs))
-	var originalYears []int
+	originalYears := make([]int, 0, len(mfs))
-	var originalDates []string
+	originalDates := make([]string, 0, len(mfs))
-	var releaseDates []string
+	releaseDates := make([]string, 0, len(mfs))
 	for _, m := range mfs {
 		// We assume these attributes are all the same for all songs on an album
 		a.ID = m.AlbumID

model/mediafile_test.go

@@ -1,6 +1,7 @@
 package model_test
 
 import (
+	"path/filepath"
 	"time"
 
 	"github.com/navidrome/navidrome/conf"
@@ -13,7 +14,7 @@ import (
 
 var _ = Describe("MediaFiles", func() {
 	var mfs MediaFiles
-
+	Describe("ToAlbum", func() {
 		Context("Simple attributes", func() {
 			BeforeEach(func() {
 				mfs = MediaFiles{
@@ -270,6 +271,67 @@ var _ = Describe("MediaFiles", func() {
 		})
 	})
 
+	Describe("Dirs", func() {
+		var mfs MediaFiles
+
+		When("there are no media files", func() {
+			BeforeEach(func() {
+				mfs = MediaFiles{}
+			})
+			It("returns an empty list", func() {
+				Expect(mfs.Dirs()).To(BeEmpty())
+			})
+		})
+
+		When("there is one media file", func() {
+			BeforeEach(func() {
+				mfs = MediaFiles{
+					{Path: "/music/artist/album/song.mp3"},
+				}
+			})
+			It("returns the directory of the media file", func() {
+				Expect(mfs.Dirs()).To(Equal([]string{filepath.Clean("/music/artist/album")}))
+			})
+		})
+
+		When("there are multiple media files in the same directory", func() {
+			BeforeEach(func() {
+				mfs = MediaFiles{
+					{Path: "/music/artist/album/song1.mp3"},
+					{Path: "/music/artist/album/song2.mp3"},
+				}
+			})
+			It("returns a single directory", func() {
+				Expect(mfs.Dirs()).To(Equal([]string{filepath.Clean("/music/artist/album")}))
+			})
+		})
+
+		When("there are multiple media files in different directories", func() {
+			BeforeEach(func() {
+				mfs = MediaFiles{
+					{Path: "/music/artist2/album/song2.mp3"},
+					{Path: "/music/artist1/album/song1.mp3"},
+				}
+			})
+			It("returns all directories", func() {
+				Expect(mfs.Dirs()).To(Equal([]string{filepath.Clean("/music/artist1/album"), filepath.Clean("/music/artist2/album")}))
+			})
+		})
+
+		When("there are media files with empty paths", func() {
+			BeforeEach(func() {
+				mfs = MediaFiles{
+					{Path: ""},
+					{Path: "/music/artist/album/song.mp3"},
+				}
+			})
+			It("ignores the empty paths", func() {
+				Expect(mfs.Dirs()).To(Equal([]string{".", filepath.Clean("/music/artist/album")}))
+			})
+		})
+	})
+})
+
 var _ = Describe("MediaFile", func() {
 	BeforeEach(func() {
 		DeferCleanup(configtest.SetupConfig())

persistence/sql_base_repository.go

@@ -140,11 +140,12 @@ func (r sqlRepository) buildSortOrder(sort, order string) string {
 		reverseOrder = "desc"
 	}
 
-	var newSort []string
 	parts := strings.FieldsFunc(sort, splitFunc(','))
+	newSort := make([]string, 0, len(parts))
 	for _, p := range parts {
 		f := strings.FieldsFunc(p, splitFunc(' '))
-		newField := []string{f[0]}
+		newField := make([]string, 1, len(f))
+		newField[0] = f[0]
 		if len(f) == 1 {
 			newField = append(newField, order)
 		} else {

scanner/mapping.go

@@ -143,7 +143,7 @@ func (s MediaFileMapper) albumArtistID(md metadata.Tags) string {
 func (s MediaFileMapper) mapGenres(genres []string) (string, model.Genres) {
 	var result model.Genres
 	unique := map[string]struct{}{}
-	var all []string
+	all := make([]string, 0, len(genres)*2)
 	for i := range genres {
 		gs := strings.FieldsFunc(genres[i], func(r rune) bool {
 			return strings.ContainsRune(conf.Server.Scanner.GenreSeparators, r)

scanner/metadata/metadata.go

@@ -84,7 +84,7 @@ func NewTag(filePath string, fileInfo os.FileInfo, tags ParsedTags) Tags {
 func removeDuplicatesAndEmpty(values []string) []string {
 	encountered := map[string]struct{}{}
 	empty := true
-	var result []string
+	result := make([]string, 0, len(values))
 	for _, v := range values {
 		if _, ok := encountered[v]; ok {
 			continue
@@ -300,7 +300,7 @@ func (t Tags) getFirstTagValue(tagNames ...string) string {
 }
 
 func (t Tags) getAllTagValues(tagNames ...string) []string {
-	var values []string
+	values := make([]string, 0, len(tagNames)*2)
 	for _, tag := range tagNames {
 		if v, ok := t.Tags[tag]; ok {
 			values = append(values, v...)
@@ -311,7 +311,8 @@ func (t Tags) getAllTagValues(tagNames ...string) []string {
 
 func (t Tags) getSortTag(originalTag string, tagNames ...string) string {
 	formats := []string{"sort%s", "sort_%s", "sort-%s", "%ssort", "%s_sort", "%s-sort"}
-	all := []string{originalTag}
+	all := make([]string, 1, len(tagNames)*len(formats)+1)
+	all[0] = originalTag
 	for _, tag := range tagNames {
 		for _, format := range formats {
 			name := fmt.Sprintf(format, tag)

scanner/tag_scanner.go

@@ -294,7 +294,7 @@ func (s *TagScanner) processChangedDir(ctx context.Context, refresher *refresher
 
 	// If track from folder is newer than the one in DB, select for update/insert in DB
 	log.Trace(ctx, "Processing changed folder", "dir", dir, "tracksInDB", len(currentTracks), "tracksInFolder", len(files))
-	var filesToUpdate []string
+	filesToUpdate := make([]string, 0, len(files))
 	for filePath, entry := range files {
 		c, inDB := currentTracks[filePath]
 		if !inDB || fullScan {

scanner/walk_dir_tree.go

@@ -64,7 +64,6 @@ func walkFolder(ctx context.Context, rootPath string, currentFolder string, resu
 }
 
 func loadDir(ctx context.Context, dirPath string) ([]string, *dirStats, error) {
-	var children []string
 	stats := &dirStats{}
 
 	dirInfo, err := os.Stat(dirPath)
@@ -77,11 +76,13 @@ func loadDir(ctx context.Context, dirPath string) ([]string, *dirStats, error) {
 	dir, err := os.Open(dirPath)
 	if err != nil {
 		log.Error(ctx, "Error in Opening directory", "path", dirPath, err)
-		return children, stats, err
+		return nil, stats, err
 	}
 	defer dir.Close()
 
-	for _, entry := range fullReadDir(ctx, dir) {
+	entries := fullReadDir(ctx, dir)
+	children := make([]string, 0, len(entries))
+	for _, entry := range entries {
 		isDir, err := isDirOrSymlinkToDir(dirPath, entry)
 		// Skip invalid symlinks
 		if err != nil {

utils/cache/simple_cache.go

@@ -100,7 +100,7 @@ func (c *simpleCache[K, V]) evictExpired() {
 }
 
 func (c *simpleCache[K, V]) Keys() []K {
-	var res []K
+	res := make([]K, 0, c.data.Len())
 	c.data.Range(func(item *ttlcache.Item[K, V]) bool {
 		if !item.IsExpired() {
 			res = append(res, item.Key())
@@ -111,7 +111,7 @@ func (c *simpleCache[K, V]) Keys() []K {
 }
 
 func (c *simpleCache[K, V]) Values() []V {
-	var res []V
+	res := make([]V, 0, c.data.Len())
 	c.data.Range(func(item *ttlcache.Item[K, V]) bool {
 		if !item.IsExpired() {
 			res = append(res, item.Value())