misc: Implement Iterator approach for artists

First step towards improving sorting for Artists. Use the same iterator
approach as albums and tracks.

backend/mediaprovider/subsonic/artistiterator.go

@@ -0,0 +1,79 @@
+package subsonic
+
+import (
+	"cmp"
+	"log"
+	"slices"
+	"strings"
+
+	"github.com/dweymouth/go-subsonic/subsonic"
+	"github.com/dweymouth/supersonic/backend/mediaprovider"
+	"github.com/dweymouth/supersonic/backend/mediaprovider/helpers"
+	"github.com/dweymouth/supersonic/sharedutil"
+)
+
+const (
+	ArtistSortNameAZ string = "Name (A-Z)"
+)
+
+func (s *subsonicMediaProvider) ArtistSortOrders() []string {
+	return []string{
+		ArtistSortNameAZ,
+	}
+}
+
+func (s *subsonicMediaProvider) IterateArtists(sortOrder string) mediaprovider.ArtistIterator {
+	if sortOrder == "" {
+		sortOrder = ArtistSortNameAZ // default
+	}
+	switch sortOrder {
+	case ArtistSortNameAZ:
+		return s.baseArtistIterFromSimpleSortOrder(
+			func(artists []*subsonic.ArtistID3) []*subsonic.ArtistID3 {
+				slices.SortFunc(artists, func(a, b *subsonic.ArtistID3) int {
+					return cmp.Compare(strings.ToLower(a.Name), strings.ToLower(b.Name))
+				})
+				return artists
+			},
+		)
+	default:
+		log.Printf("Undefined artist sort order: %s", sortOrder)
+		return nil
+	}
+}
+
+func (s *subsonicMediaProvider) baseArtistIterFromSimpleSortOrder(sortFn func([]*subsonic.ArtistID3) []*subsonic.ArtistID3) mediaprovider.ArtistIterator {
+	return helpers.NewArtistIterator(s.artistFetchFnFromStandardSort(sortFn))
+}
+
+func (s *subsonicMediaProvider) artistFetchFnFromStandardSort(sortFn func([]*subsonic.ArtistID3) []*subsonic.ArtistID3) helpers.ArtistFetchFn {
+	return makeArtistFetchFn(func(offset, limit int) ([]*subsonic.ArtistID3, error) {
+		// When the iterator asks for a second page of results, return nil, as Subsonic does not support pagination for artists.
+		if offset > 0 {
+			return nil, nil
+		}
+
+		idxs, err := s.client.GetArtists(map[string]string{})
+		if err != nil {
+			return nil, err
+		}
+		var artists []*subsonic.ArtistID3
+		for _, idx := range idxs.Index {
+			for _, ar := range idx.Artist {
+				artists = append(artists, ar)
+			}
+		}
+		artists = sortFn(artists)
+		return artists, nil
+	})
+}
+
+func makeArtistFetchFn(subsonicFetchFn func(offset, limit int) ([]*subsonic.ArtistID3, error)) helpers.ArtistFetchFn {
+	return func(offset, limit int) ([]*mediaprovider.Artist, error) {
+		ar, err := subsonicFetchFn(offset, limit)
+		if err != nil {
+			return nil, err
+		}
+		return sharedutil.MapSlice(ar, toArtistFromID3), nil
+	}
+}

backend/mediaprovider/subsonic/subsonicmediaprovider.go

@@ -130,20 +130,6 @@ func (s *subsonicMediaProvider) GetArtistInfo(artistID string) (*mediaprovider.A
 	}, nil
 }
 
-func (s *subsonicMediaProvider) GetArtists() ([]*mediaprovider.Artist, error) {
-	idxs, err := s.client.GetArtists(map[string]string{})
-	if err != nil {
-		return nil, err
-	}
-	var artists []*mediaprovider.Artist
-	for _, idx := range idxs.Index {
-		for _, ar := range idx.Artist {
-			artists = append(artists, toArtistFromID3(ar))
-		}
-	}
-	return artists, nil
-}
-
 func (s *subsonicMediaProvider) GetCoverArt(id string, size int) (image.Image, error) {
 	params := map[string]string{}
 	if size > 0 {