feat(scanner): add folder hash for smarter quick scan change detection (#4220)

* Simplify folder hash migration

* fix hashing lint

* refactor

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

* Update scanner/folder_entry.go

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

---------

Signed-off-by: Deluan <deluan@navidrome.org>
Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

scanner/folder_entry.go

@@ -0,0 +1,96 @@
+package scanner
+
+import (
+	"crypto/md5"
+	"encoding/hex"
+	"fmt"
+	"io"
+	"io/fs"
+	"maps"
+	"slices"
+	"strings"
+	"time"
+
+	"github.com/navidrome/navidrome/core"
+	"github.com/navidrome/navidrome/model"
+	"github.com/navidrome/navidrome/utils/chrono"
+)
+
+func newFolderEntry(job *scanJob, path string) *folderEntry {
+	id := model.FolderID(job.lib, path)
+	info := job.popLastUpdate(id)
+	f := &folderEntry{
+		id:         id,
+		job:        job,
+		path:       path,
+		audioFiles: make(map[string]fs.DirEntry),
+		imageFiles: make(map[string]fs.DirEntry),
+		albumIDMap: make(map[string]string),
+		updTime:    info.UpdatedAt,
+		prevHash:   info.Hash,
+	}
+	return f
+}
+
+type folderEntry struct {
+	job             *scanJob
+	elapsed         chrono.Meter
+	path            string    // Full path
+	id              string    // DB ID
+	modTime         time.Time // From FS
+	updTime         time.Time // from DB
+	audioFiles      map[string]fs.DirEntry
+	imageFiles      map[string]fs.DirEntry
+	numPlaylists    int
+	numSubFolders   int
+	imagesUpdatedAt time.Time
+	prevHash        string // Previous hash from DB
+	tracks          model.MediaFiles
+	albums          model.Albums
+	albumIDMap      map[string]string
+	artists         model.Artists
+	tags            model.TagList
+	missingTracks   []*model.MediaFile
+}
+
+func (f *folderEntry) hasNoFiles() bool {
+	return len(f.audioFiles) == 0 && len(f.imageFiles) == 0 && f.numPlaylists == 0 && f.numSubFolders == 0
+}
+
+func (f *folderEntry) isNew() bool {
+	return f.updTime.IsZero()
+}
+
+func (f *folderEntry) toFolder() *model.Folder {
+	folder := model.NewFolder(f.job.lib, f.path)
+	folder.NumAudioFiles = len(f.audioFiles)
+	if core.InPlaylistsPath(*folder) {
+		folder.NumPlaylists = f.numPlaylists
+	}
+	folder.ImageFiles = slices.Collect(maps.Keys(f.imageFiles))
+	folder.ImagesUpdatedAt = f.imagesUpdatedAt
+	folder.Hash = f.hash()
+	return folder
+}
+
+func (f *folderEntry) hash() string {
+	audioKeys := slices.Collect(maps.Keys(f.audioFiles))
+	slices.Sort(audioKeys)
+	imageKeys := slices.Collect(maps.Keys(f.imageFiles))
+	slices.Sort(imageKeys)
+
+	h := md5.New()
+	_, _ = io.WriteString(h, f.modTime.UTC().String())
+	_, _ = io.WriteString(h, strings.Join(audioKeys, ","))
+	_, _ = io.WriteString(h, strings.Join(imageKeys, ","))
+	fmt.Fprintf(h, "%d-%d", f.numPlaylists, f.numSubFolders)
+	_, _ = io.WriteString(h, f.imagesUpdatedAt.UTC().String())
+	return hex.EncodeToString(h.Sum(nil))
+}
+
+func (f *folderEntry) isOutdated() bool {
+	if f.job.lib.FullScanInProgress && f.updTime.Before(f.job.lib.LastScanStartedAt) {
+		return true
+	}
+	return f.prevHash != f.hash()
+}