Add MPD client implementation

internal/client/client.go

@@ -0,0 +1,947 @@
+package client
+
+import (
+	"fmt"
+	"io"
+	"log"
+	"net/textproto"
+	"strconv"
+	"strings"
+	"sync"
+	"time"
+
+	"suruatoel.xyz/mcg/internal/common"
+)
+
+type QueueItem struct {
+	Command func(*Client) error
+}
+
+type Client struct {
+	Host            string
+	con             *textproto.Conn
+	runGroup        sync.WaitGroup
+	queue           chan QueueItem
+	idling          bool
+	version         string
+	state           string
+	playlist        []*common.PlaylistAlbum
+	albums          map[string]*common.Album
+	errorCB         ErrorCB
+	connectionCB    ConnectionCB
+	playlistCB      PlaylistCB
+	albumsCB        AlbumsCB
+	statusCB        StatusCB
+	statisticsCB    StatisticsCB
+	outputDevicesCB OutputDevicesCB
+	albumartCB      AlbumartCB
+	albumartChan    chan []byte
+}
+
+type ErrorCB func(string)
+
+type ConnectionCB func(bool)
+
+type PlaylistCB func([]*common.PlaylistAlbum)
+
+type AlbumsCB func(map[string]*common.Album)
+
+type StatusCB func(string, *common.PlaylistAlbum, uint64, uint64, int64, string, string, string, string)
+
+type StatisticsCB func(string, string, string, string, string, string)
+
+type OutputDevicesCB func(devices []*common.OutputDevice)
+
+type AlbumartCB func(*common.Album, []byte)
+
+var (
+	clientInstance *Client
+	clientInit     sync.Once
+)
+
+func Instance() *Client {
+	clientInit.Do(func() {
+		clientInstance = &Client{}
+	})
+
+	return clientInstance
+}
+
+func (c *Client) IsConnected() bool {
+	return (c.con != nil)
+}
+
+func (c *Client) Connect(host string, port int, password string) {
+	c.Host = host
+	c.queue = make(chan QueueItem, 300)
+	c.albumartChan = make(chan []byte)
+	// Add connect action
+	c.addAction(
+		func(client *Client) error {
+			return client.connect(host, port, password)
+		},
+	)
+
+	go c.run()
+}
+
+func (c *Client) connect(host string, port int, password string) error {
+	// Notify listeners
+	defer func() {
+		if c.connectionCB != nil {
+			c.connectionCB(c.con != nil)
+		}
+	}()
+
+	// Create socket
+	con, err := textproto.Dial("tcp", fmt.Sprintf("%s:%d", host, port))
+	c.con = con
+	if err != nil {
+		return err
+	}
+
+	// Greeting
+	greeting, err := c.readLine()
+	if err != nil {
+		return err
+	}
+	if greeting[0:6] != "OK MPD" {
+		return textproto.ProtocolError(fmt.Sprintf("Unexpected greeting: %s", greeting))
+	}
+
+	// Parse and store server version
+	c.version = greeting[7:]
+
+	// Add post-connect actions
+	c.addPostConnectActions(password)
+
+	return nil
+}
+
+func (c *Client) addPostConnectActions(password string) {
+	c.addAction(
+		func(client *Client) error {
+			return client.password(password)
+		},
+		func(client *Client) error {
+			return client.loadAlbums()
+		},
+		func(client *Client) error {
+			return client.loadPlaylist()
+		},
+		func(client *Client) error {
+			return client.getStatus()
+		},
+		func(client *Client) error {
+			return client.getStatistics()
+		},
+		func(client *Client) error {
+			return client.getOutputDevices()
+		},
+	)
+}
+
+func (c *Client) password(password string) error {
+	if password != "" {
+		reader := &emptyReader{}
+		return c.cmd(reader, "password", password)
+	}
+
+	return nil
+}
+
+func (c *Client) Disconnect() {
+	c.addDisconnectActions()
+
+	close(c.queue)
+	close(c.albumartChan)
+	c.runGroup.Wait()
+}
+
+func (c *Client) addDisconnectActions() {
+	c.addAction(
+		func(client *Client) error {
+			return client.disconnect()
+		},
+	)
+}
+
+func (c *Client) disconnect() error {
+	// Stop current idle process
+	c.noIdle()
+
+	// Close connection
+	err := c.con.Close()
+	if err != nil {
+		return err
+	}
+	c.con = nil
+
+	// Notify callback
+	if c.connectionCB != nil {
+		c.connectionCB(false)
+	}
+
+	return nil
+}
+
+func (c *Client) run() {
+	c.runGroup.Add(1)
+	defer func() {
+		c.runGroup.Done()
+	}()
+
+	for action := range c.queue {
+		err := action.Command(c)
+		if err != nil && c.errorCB != nil {
+			c.errorCB(fmt.Sprintf("Error running action: %s\n", err))
+		}
+	}
+}
+
+func (c *Client) Version() string {
+	return c.version
+}
+
+func (c *Client) ReceiveError(callback ErrorCB) {
+	c.errorCB = callback
+}
+
+func (c *Client) RegisterConnection(callback ConnectionCB) {
+	c.connectionCB = callback
+}
+
+func (c *Client) ReceivePlaylist(callback PlaylistCB) {
+	c.playlistCB = callback
+}
+
+func (c *Client) ReceiveAlbums(callback AlbumsCB) {
+	c.albumsCB = callback
+}
+
+func (c *Client) ReceiveStatus(callback StatusCB) {
+	c.statusCB = callback
+}
+
+func (c *Client) ReceiveStatistics(callback StatisticsCB) {
+	c.statisticsCB = callback
+}
+
+func (c *Client) ReceiveOutputDevices(callback OutputDevicesCB) {
+	c.outputDevicesCB = callback
+}
+
+func (c *Client) ReceiveAlbumart(callback AlbumartCB) {
+	c.albumartCB = callback
+}
+
+func (c *Client) loadPlaylist() error {
+	c.playlist = []*common.PlaylistAlbum{}
+
+	playlistReader := &listMapReader{Delimiter: "file"}
+	playlistErr := c.cmd(playlistReader, "playlistinfo")
+	if playlistErr != nil {
+		return playlistErr
+	}
+
+	for _, songData := range playlistReader.DictList() {
+		song := c.extractPlaylistSong(songData)
+		album := c.extractPlaylistAlbum(songData)
+		if len(c.playlist) == 0 || c.playlist[len(c.playlist)-1].ID() != album.ID() {
+			c.playlist = append(c.playlist, album)
+		} else {
+			album = c.playlist[len(c.playlist)-1]
+		}
+		if song != nil {
+			album.AddSong(song)
+		}
+	}
+
+	if c.playlistCB != nil {
+		c.playlistCB(c.playlist)
+	}
+
+	return nil
+}
+
+func (c *Client) loadAlbums() error {
+	// self._callback(Client.SIGNAL_INIT_ALBUMS)
+	c.albums = make(map[string]*common.Album)
+
+	albumReader := &listMapReader{Delimiter: "Album"}
+	albumErr := c.cmd(albumReader, "list album")
+	if albumErr != nil {
+		return albumErr
+	}
+	for _, entry := range albumReader.DictList() {
+		// self._callback(Client.SIGNAL_PULSE_ALBUMS)
+
+		// Album
+		album := c.extractAlbum(entry)
+		if album != nil {
+			// Songs
+			songReader := &listMapReader{Delimiter: "file"}
+			songErr := c.cmd(songReader, "find album", album.Title())
+			if songErr != nil {
+				return songErr
+			}
+			for _, songData := range songReader.DictList() {
+				song := c.extractSong(songData)
+				if song != nil {
+					album.AddSong(song)
+				}
+			}
+
+			c.albums[album.ID()] = album
+		}
+	}
+
+	if c.albumsCB != nil {
+		c.albumsCB(c.albums)
+	}
+
+	return nil
+}
+
+func (c *Client) getStatus() error {
+	statusReader := &mapReader{}
+	statusErr := c.cmd(statusReader, "status")
+	if statusErr != nil {
+		return statusErr
+	}
+
+	status := statusReader.Dict()
+	// State
+	c.state = status["state"]
+	// Time
+	var parseErr error
+	time := uint64(0)
+	if timeStr, timeSet := status["time"]; timeSet {
+		time, parseErr = strconv.ParseUint(strings.Split(timeStr, ":")[0], 10, 64)
+		if parseErr != nil {
+			return newValueParseError("time", timeStr)
+		}
+	}
+	// Volume
+	volume := int64(-1)
+	if volumeStr, volumeSet := status["volume"]; volumeSet {
+		volume, parseErr = strconv.ParseInt(volumeStr, 10, 64)
+		if parseErr != nil {
+			return newValueParseError("volume", volumeStr)
+		}
+	}
+	// Error
+	errorMessage := status["error"]
+
+	// Album information
+	file := ""
+	var album *common.PlaylistAlbum
+	pos := uint64(0)
+	songReader := &mapReader{}
+	songErr := c.cmd(songReader, "currentsong")
+	if songErr != nil {
+		return songErr
+	}
+	songData := songReader.Dict()
+	if len(songData) > 0 {
+		// File
+		file = songData["file"]
+		// Song
+		song := c.extractPlaylistSong(songData)
+		if song != nil {
+			// Album
+			album = c.extractPlaylistAlbum(songData)
+			pos = song.Pos()
+			for _, palbum := range c.playlist {
+				if palbum.ID() == album.ID() && uint64(len(palbum.Songs())) >= pos {
+					album = palbum
+					break
+				}
+				pos -= uint64(len(palbum.Songs()))
+			}
+		}
+	}
+	// Audio
+	audio := status["audio"]
+	// Bitrate
+	bitrate := status["bitrate"]
+
+	if c.statusCB != nil {
+		c.statusCB(c.state, album, pos, time, volume, file, audio, bitrate, errorMessage)
+	}
+
+	return nil
+}
+
+func (c *Client) getStatistics() error {
+	statisticsReader := &mapReader{}
+	statisticsErr := c.cmd(statisticsReader, "stats")
+	if statisticsErr != nil {
+		return statisticsErr
+	}
+
+	stats := statisticsReader.Dict()
+	artists := stats["artists"]
+	albums := stats["albums"]
+	songs := stats["songs"]
+	dbPlaytime := stats["db_playtime"]
+	playtime := stats["playtime"]
+	uptime := stats["uptime"]
+
+	if c.statisticsCB != nil {
+		c.statisticsCB(artists, albums, songs, dbPlaytime, playtime, uptime)
+	}
+
+	return nil
+}
+
+func (c *Client) getOutputDevices() error {
+	devicesReader := &listMapReader{Delimiter: "outputid"}
+	devicesErr := c.cmd(devicesReader, "outputs")
+	if devicesErr != nil {
+		return devicesErr
+	}
+
+	devices := []*common.OutputDevice{}
+	for _, deviceMap := range devicesReader.DictList() {
+		device := common.NewOutputDevice(
+			deviceMap["outputid"],
+			deviceMap["outputname"],
+		)
+
+		enabledString := deviceMap["outputenabled"]
+		if enabledString != "" {
+			enabled, enabledErr := strconv.ParseBool(enabledString)
+			if enabledErr == nil {
+				device.Enabled = enabled
+			} else {
+				log.Println("failed to parse status of output device", device.ID, device.Name)
+			}
+		}
+
+		devices = append(devices, device)
+	}
+
+	if c.outputDevicesCB != nil {
+		c.outputDevicesCB(devices)
+	}
+
+	return nil
+}
+
+func (c *Client) SetOutputDeviceState(device *common.OutputDevice, enable bool) {
+	c.addAction(func(client *Client) error {
+		return client.setOutputDeviceState(device, enable)
+	})
+}
+
+func (c *Client) setOutputDeviceState(device *common.OutputDevice, enable bool) error {
+	reader := &emptyReader{}
+	if enable {
+		return c.cmd(reader, "enableoutput", device.ID)
+	}
+	return c.cmd(reader, "disableoutput", device.ID)
+}
+
+func (c *Client) PlayPause() {
+	c.addAction(func(client *Client) error {
+		return client.playPause()
+	})
+}
+
+func (c *Client) playPause() error {
+	reader := &emptyReader{}
+	switch c.state {
+	case "play":
+		return c.cmd(reader, "pause")
+	default:
+		return c.cmd(reader, "play")
+	}
+}
+
+func (c *Client) PlayAlbum(album *common.Album) {
+	c.addAction(func(client *Client) error {
+		return client.playAlbum(album)
+	})
+}
+
+func (c *Client) playAlbum(album *common.Album) error {
+	songIDs, err := c.queueAlbum(album)
+	if err != nil {
+		return err
+	}
+
+	if len(songIDs) > 0 {
+		reader := &emptyReader{}
+		return c.cmd(reader, "playid", songIDs[0])
+	}
+
+	return nil
+}
+
+func (c *Client) QueueAlbum(album *common.Album) {
+	c.addAction(func(client *Client) error {
+		_, err := client.queueAlbum(album)
+		return err
+	})
+}
+
+func (c *Client) queueAlbum(album *common.Album) ([]string, error) {
+	songIDs := []string{}
+	for _, song := range album.Songs() {
+		reader := &mapReader{}
+		addErr := c.cmd(reader, "addid", song.File())
+		if addErr != nil {
+			return songIDs, addErr
+		}
+
+		addData := reader.Dict()
+		songID, found := addData["Id"]
+		if found {
+			songIDs = append(songIDs, songID)
+		}
+	}
+
+	return songIDs, nil
+}
+
+func (c *Client) QueueAlbums(albums []*common.Album) {
+	c.addAction(func(client *Client) error {
+		return client.queueAlbums(albums)
+	})
+}
+
+func (c *Client) queueAlbums(albums []*common.Album) error {
+	for _, album := range albums {
+		_, queueErr := c.queueAlbum(album)
+		if queueErr != nil {
+			return queueErr
+		}
+	}
+
+	return nil
+}
+
+func (c *Client) PlayQueuedAlbum(album *common.PlaylistAlbum) {
+	c.addAction(func(client *Client) error {
+		return client.playQueuedAlbum(album)
+	})
+}
+
+func (c *Client) playQueuedAlbum(album *common.PlaylistAlbum) error {
+	if len(album.Songs()) <= 0 {
+		return nil
+	}
+
+	reader := &emptyReader{}
+	return c.cmd(reader, "playid", album.Songs()[0].ID())
+}
+
+func (c *Client) UnqueueAlbum(album *common.PlaylistAlbum) {
+	c.addAction(func(client *Client) error {
+		return client.unqueueAlbum(album)
+	})
+}
+
+func (c *Client) unqueueAlbum(album *common.PlaylistAlbum) error {
+	commands := []listCommand{}
+	for _, song := range album.Songs() {
+		command := newListCommand("deleteid", song.ID())
+		commands = append(commands, command)
+	}
+
+	return c.cmdList(commands)
+}
+
+func (c *Client) UnqueueAlbums(albums []*common.PlaylistAlbum) {
+	c.addAction(func(client *Client) error {
+		return client.unqueueAlbums(albums)
+	})
+}
+
+func (c *Client) unqueueAlbums(albums []*common.PlaylistAlbum) error {
+	commands := []listCommand{}
+	for _, album := range albums {
+		for _, song := range album.Songs() {
+			command := newListCommand("deleteid", song.ID())
+			commands = append(commands, command)
+		}
+	}
+
+	return c.cmdList(commands)
+}
+
+func (c *Client) Seek(pos int, time uint64) {
+	c.addAction(func(client *Client) error {
+		return client.seek(pos, time)
+	})
+}
+
+func (c *Client) seek(pos int, time uint64) error {
+	reader := &emptyReader{}
+	return c.cmd(reader, "seek", strconv.Itoa(pos), strconv.FormatUint(time, 10))
+}
+
+func (c *Client) SetVolume(volume int) {
+	c.addAction(func(client *Client) error {
+		return client.setVolume(volume)
+	})
+}
+
+func (c *Client) setVolume(volume int) error {
+	reader := &emptyReader{}
+	return c.cmd(reader, "setvol", strconv.Itoa(volume))
+}
+
+func (c *Client) LoadAlbumart(album *common.Album) {
+	c.addAction(func(client *Client) error {
+		return client.loadAlbumart(album)
+	})
+}
+
+func (c *Client) LoadAlbumartNow(album *common.Album) []byte {
+	c.addAction(func(client *Client) error {
+		return client.loadAlbumartNow(album)
+	})
+
+	return <-c.albumartChan
+}
+
+func (c *Client) loadAlbumart(album *common.Album) error {
+	reader := &binaryReader{}
+	reader.size = 1
+	reader.offset = 0
+
+	if c.albumartCB != nil {
+		// We need to wrap this in an anonymous function to make sure the call to reader.Binary() is deferred as well
+		defer func() {
+			c.albumartCB(album, reader.Binary())
+		}()
+	}
+
+	if len(album.Songs()) > 0 {
+		return c.cmd(reader, "albumart", album.Songs()[0].File(), "0")
+	}
+
+	return nil
+}
+
+func (c *Client) loadAlbumartNow(album *common.Album) error {
+	reader := &binaryReader{}
+	reader.size = 1
+	reader.offset = 0
+
+	defer func() {
+		c.albumartChan <- reader.Binary()
+	}()
+
+	if len(album.Songs()) > 0 {
+		return c.cmd(reader, "albumart", album.Songs()[0].File(), "0")
+	}
+
+	return nil
+}
+
+func (c *Client) ClearPlaylist() {
+	c.addAction(func(client *Client) error {
+		return client.clearPlaylist()
+	})
+}
+
+func (c *Client) clearPlaylist() error {
+	reader := &emptyReader{}
+	return c.cmd(reader, "clear")
+}
+
+func (c *Client) idle() error {
+	reader := &listReader{
+		key: "changed",
+	}
+
+	c.idling = true
+	err := c.cmd(reader, "idle")
+	c.idling = false
+	if err != nil {
+		return err
+	}
+
+	for _, subsystem := range reader.List() {
+		switch subsystem {
+		case "player":
+			c.addAction(
+				func(client *Client) error {
+					return client.loadPlaylist()
+				},
+				func(client *Client) error {
+					return client.getStatus()
+				},
+			)
+		case "mixer":
+			fallthrough
+		case "output":
+			c.addAction(func(client *Client) error {
+				return client.getStatus()
+			})
+		case "playlist":
+			c.addAction(func(client *Client) error {
+				return client.loadPlaylist()
+			})
+		case "database":
+			fallthrough
+		case "update":
+			c.addAction(
+				func(client *Client) error {
+					return client.loadAlbums()
+				},
+				func(client *Client) error {
+					return client.loadPlaylist()
+				},
+				func(client *Client) error {
+					return client.getStatus()
+				},
+			)
+		}
+	}
+
+	return nil
+}
+
+func (c *Client) noIdle() error {
+	id, err := c.write("noidle")
+	if err != nil {
+		return err
+	}
+
+	c.con.StartResponse(id)
+	defer c.con.EndResponse(id)
+
+	return nil
+}
+
+func (c *Client) addAction(commands ...func(*Client) error) {
+	for _, command := range commands {
+		c.queue <- QueueItem{
+			command,
+		}
+	}
+	c.queue <- QueueItem{
+		func(client *Client) error {
+			if client.con != nil && len(client.queue) == 0 {
+				return client.idle()
+			}
+			return nil
+		},
+	}
+	if c.idling {
+		c.noIdle()
+	}
+}
+
+func (c *Client) cmd(reader reader, action string, args ...string) error {
+	readingComplete := false
+	for !readingComplete {
+		command := c.constructCommand(action, args...)
+		id, err := c.write(command)
+		if err != nil {
+			return textproto.ProtocolError(fmt.Sprintf("failed to write command \"%s\": %s", action, err))
+		}
+
+		complete, newArgs, err := c.readData(id, reader, args)
+		if err != nil {
+			return textproto.ProtocolError(fmt.Sprintf("failed to read response for command \"%s\": %s", action, err))
+		}
+		readingComplete = complete
+		args = newArgs
+	}
+
+	return nil
+}
+
+func (c *Client) cmdList(commands []listCommand) error {
+	beginReader := &noopReader{}
+	beginErr := c.cmd(beginReader, "command_list_begin")
+	if beginErr != nil {
+		return beginErr
+	}
+
+	commandReader := &noopReader{}
+	for _, command := range commands {
+		commandErr := c.cmd(commandReader, command.Action, command.Args...)
+		if commandErr != nil {
+			return commandErr
+		}
+	}
+
+	endReader := &emptyReader{}
+	return c.cmd(endReader, "command_list_end")
+}
+
+func (c *Client) constructCommand(command string, args ...string) string {
+	if len(args) == 0 {
+		return command
+	}
+
+	cleanArgs := c.escapeCommandArguments(args...)
+	joinedArgs := strings.Join(cleanArgs, "\" \"")
+
+	return fmt.Sprintf("%s \"%s\"", command, joinedArgs)
+}
+
+func (c *Client) escapeCommandArguments(args ...string) []string {
+	for i, arg := range args {
+		args[i] = strings.ReplaceAll(arg, "\"", "\\\"")
+	}
+
+	return args
+}
+
+func (c *Client) write(command string) (uint, error) {
+	id := c.con.Next()
+	c.con.StartRequest(id)
+	defer c.con.EndRequest(id)
+
+	fmt.Fprintf(c.con.W, "%s", command)
+	c.con.W.WriteByte('\n')
+	err := c.con.W.Flush()
+	if err != nil {
+		return id, err
+	}
+
+	return id, nil
+}
+
+func (c *Client) readData(id uint, reader reader, args []string) (bool, []string, error) {
+	c.con.StartResponse(id)
+	defer c.con.EndResponse(id)
+
+	return reader.ReadData(c, args)
+}
+
+func (c *Client) read() ([]string, error) {
+	response := []string{}
+
+	for {
+		line, err := c.readLine()
+		if err != nil {
+			return nil, err
+		}
+		if line == "OK" {
+			break
+		}
+		response = append(response, line)
+	}
+
+	return response, nil
+}
+
+func (c *Client) readLine() (string, error) {
+	line, err := c.con.ReadLine()
+	if err != nil {
+		return "", err
+	}
+	if strings.HasPrefix(line, "ACK ") {
+		return "", textproto.ProtocolError(fmt.Sprintf("Unexpected line: %s", line[4:]))
+	}
+
+	return line, nil
+}
+
+func (c *Client) readBytes(bytes []byte) error {
+	n, err := io.ReadFull(c.con.R, bytes)
+	if err != nil {
+		return err
+	}
+	if n != len(bytes) {
+		return textproto.ProtocolError(fmt.Sprintf("Wrong byte count: %d instead of %d", n, len(bytes)))
+	}
+
+	return nil
+}
+
+func (c *Client) extractAlbum(song map[string]string) *common.Album {
+	albumTitle := "Various"
+	if title, titleSet := song["Album"]; titleSet {
+		albumTitle = title
+	}
+	album := common.NewAlbum(albumTitle)
+
+	if albumLookup, albumExists := c.albums[album.ID()]; albumExists {
+		return albumLookup
+	}
+
+	return album
+}
+
+func (c *Client) extractPlaylistAlbum(song map[string]string) *common.PlaylistAlbum {
+	albumTitle := "Various"
+	if title, titleSet := song["Album"]; titleSet {
+		albumTitle = title
+	}
+	album := common.NewAlbum(albumTitle)
+
+	if albumLookup, albumExists := c.albums[album.ID()]; albumExists {
+		album = albumLookup
+	}
+
+	playlistAlbum := common.NewPlaylistAlbum(album)
+
+	return playlistAlbum
+}
+
+func (c *Client) extractSong(data map[string]string) *common.Song {
+	if artist, artistSet := data["Artist"]; artistSet {
+		if title, titleSet := data["Title"]; titleSet {
+			if file, fileSet := data["file"]; fileSet {
+				song := common.NewSong(artist, title, file)
+
+				if track, isSet := data["Track"]; isSet {
+					song.SetTrack(track)
+				}
+				if timeString, isSet := data["Time"]; isSet {
+					time, timeErr := strconv.ParseUint(timeString, 10, 64)
+					if timeErr == nil {
+						song.SetLength(time)
+					} else {
+						log.Println("failed to parse \"Time\" field of song file", file)
+					}
+				}
+				if date, isSet := data["Date"]; isSet {
+					song.SetDate(date)
+				}
+				if albumartist, isSet := data["Albumartist"]; isSet {
+					song.AddAlbumartist(albumartist)
+				}
+				if lastModifiedString, isSet := data["last-modified"]; isSet {
+					lastModified, dateErr := time.Parse("2006-01-02T15:04:05-0700", lastModifiedString)
+					if dateErr == nil {
+						song.SetLastModified(lastModified)
+					} else {
+						log.Println("failed to parse \"last-modified\" field of song file", file)
+					}
+				}
+
+				return song
+			}
+		}
+	}
+
+	return nil
+}
+
+func (c *Client) extractPlaylistSong(data map[string]string) *common.PlaylistSong {
+	song := c.extractSong(data)
+	if song != nil {
+		if id, idSet := data["Id"]; idSet {
+			if posString, posSet := data["Pos"]; posSet {
+				pos, posErr := strconv.ParseUint(posString, 10, 64)
+				if posErr != nil {
+					log.Println("failed to parse \"Pos\" field of playlist song")
+				}
+
+				return common.NewPlaylistSong(song, id, pos)
+			}
+		}
+	}
+
+	return nil
+}