pure playlist navigation

internal/playback/playlist_navigation.go

@@ -0,0 +1,93 @@
+package playback
+
+import "errors"
+
+type PlaylistCommandKind uint8
+
+const (
+	PlaylistSelect PlaylistCommandKind = iota + 1
+	PlaylistNext
+	PlaylistPrevious
+)
+
+type PlaylistCommand struct {
+	Kind  PlaylistCommandKind
+	Index int
+}
+
+type PlaylistState struct {
+	CurrentIndex int
+	HasSelection bool
+}
+
+var (
+	ErrPlaylistEmpty           = errors.New("playlist is empty")
+	ErrPlaylistIndexOutOfRange = errors.New("playlist index is out of range")
+	ErrUnknownPlaylistCommand  = errors.New("unknown playlist command")
+)
+
+func ApplyPlaylistCommand(
+	playlist Playlist,
+	current PlaylistState,
+	command PlaylistCommand,
+) (PlaylistState, error) {
+	if err := playlist.Validate(); err != nil {
+		return current, err
+	}
+	if len(playlist.Entries) == 0 {
+		return current, ErrPlaylistEmpty
+	}
+
+	lastIndex := len(playlist.Entries) - 1
+	switch command.Kind {
+	case PlaylistSelect:
+		if command.Index < 0 || command.Index > lastIndex {
+			return current, ErrPlaylistIndexOutOfRange
+		}
+		return PlaylistState{CurrentIndex: command.Index, HasSelection: true}, nil
+
+	case PlaylistNext:
+		if !current.HasSelection {
+			return PlaylistState{CurrentIndex: 0, HasSelection: true}, nil
+		}
+		if current.CurrentIndex < 0 || current.CurrentIndex > lastIndex {
+			return current, ErrPlaylistIndexOutOfRange
+		}
+		if current.CurrentIndex == lastIndex {
+			if playlist.Loop {
+				return PlaylistState{CurrentIndex: 0, HasSelection: true}, nil
+			}
+			return current, nil
+		}
+		return PlaylistState{CurrentIndex: current.CurrentIndex + 1, HasSelection: true}, nil
+
+	case PlaylistPrevious:
+		if !current.HasSelection {
+			index := 0
+			if playlist.Loop {
+				index = lastIndex
+			}
+			return PlaylistState{CurrentIndex: index, HasSelection: true}, nil
+		}
+		if current.CurrentIndex < 0 || current.CurrentIndex > lastIndex {
+			return current, ErrPlaylistIndexOutOfRange
+		}
+		if current.CurrentIndex == 0 {
+			if playlist.Loop {
+				return PlaylistState{CurrentIndex: lastIndex, HasSelection: true}, nil
+			}
+			return current, nil
+		}
+		return PlaylistState{CurrentIndex: current.CurrentIndex - 1, HasSelection: true}, nil
+
+	default:
+		return current, ErrUnknownPlaylistCommand
+	}
+}
+
+func (s PlaylistState) Entry(playlist Playlist) (PlaylistEntry, bool) {
+	if !s.HasSelection || s.CurrentIndex < 0 || s.CurrentIndex >= len(playlist.Entries) {
+		return PlaylistEntry{}, false
+	}
+	return playlist.Entries[s.CurrentIndex], true
+}