From d2fae31d1aa283577e8cfbf84797225e5c320efb Mon Sep 17 00:00:00 2001
From: r4 <darwin@Arch.localdomain>
Date: Wed, 28 Jul 2021 17:30:01 +0200
Subject: [PATCH] add code

---
 TODO.md           |  13 ++
 args.go           |  16 ++
 commands.go       | 512 ++++++++++++++++++++++++++++++++++++++++++++++
 config.go         |  45 ++++
 dca0/dca0.go      | 261 +++++++++++++++++++++++
 dca0/pcm.go       |  73 +++++++
 go.mod            |   8 +
 go.sum            |   8 +
 main.go           | 359 ++++++++++++++++++++++++++++++++
 util/util.go      |  15 ++
 ytdl/error.go     |  26 +++
 ytdl/extractor.go | 165 +++++++++++++++
 12 files changed, 1501 insertions(+)
 create mode 100644 TODO.md
 create mode 100644 args.go
 create mode 100644 commands.go
 create mode 100644 config.go
 create mode 100644 dca0/dca0.go
 create mode 100644 dca0/pcm.go
 create mode 100644 go.mod
 create mode 100644 go.sum
 create mode 100644 main.go
 create mode 100644 util/util.go
 create mode 100644 ytdl/error.go
 create mode 100644 ytdl/extractor.go

diff --git a/TODO.md b/TODO.md
new file mode 100644
index 0000000..bbdf430
--- /dev/null
+++ b/TODO.md
@@ -0,0 +1,13 @@
+# TODO
+
+- [ ] Improve code documentation.
+
+- [ ] Reduce the number of long, thin, cylindrical Italian pasta contained within the code.
+
+- [x] Implement the queue system.
+
+- [ ] Fix seek for larger durations; inform the user if the desired position hasn't been loaded yet.
+
+- [x] Add playlist support.
+
+- [ ] Receive playlist items gradually, making it so you don't have to wait until youtube-dl has retrieved the entire playlist.
diff --git a/args.go b/args.go
new file mode 100644
index 0000000..e1ca4f2
--- /dev/null
+++ b/args.go
@@ -0,0 +1,16 @@
+// Parses the commands sent through Discord. This program does not use any
+// traditional command line options, as it uses config.json for configuration.
+package main
+
+import (
+	"strings"
+)
+
+// Returns a false and a nil slice if cmd was not intended for the bot.
+func CmdGetArgs(cmd string) (args []string, ok bool) {
+	if !strings.HasPrefix(cmd, cfg.Prefix) {
+		return nil, false
+	}
+	cmd = strings.TrimPrefix(cmd, cfg.Prefix)
+	return strings.Fields(cmd), true
+}
diff --git a/commands.go b/commands.go
new file mode 100644
index 0000000..da9d2f3
--- /dev/null
+++ b/commands.go
@@ -0,0 +1,512 @@
+package main
+
+import (
+	"fmt"
+	"math/rand"
+	"path"
+	"strconv"
+	"strings"
+	"time"
+
+	"dcbot/dca0"
+	"dcbot/ytdl"
+
+	"github.com/bwmarrin/discordgo"
+)
+
+////////////////////////////////
+// Helper functions.
+////////////////////////////////
+func generateHelpMsg() string {
+	// Align all commands nicely so that the descriptions are in the same
+	// column.
+	longestCmd := 0
+	var cmds, descs []string
+	addCmd := func(cmd, desc string) {
+		if len(cmd) > longestCmd {
+			longestCmd = len(cmd)
+		}
+		cmds = append(cmds, cmd)
+		descs = append(descs, desc)
+	}
+	addCmd("help", "show this page")
+	addCmd("play <URL|query>", "play audio from a URL or a youtube search query")
+	addCmd("play", "start playing queue/resume playback")
+	addCmd("seek <time>", "seek to the specified time (format: mm:ss or seconds)")
+	addCmd("pos", "get the current playback time")
+	addCmd("loop", "start/stop looping the current track")
+	addCmd("add <URL|query>", "add a URL or a youtube search query to the queue; note: be patient when adding large playlists")
+	addCmd("queue", "print the current queue; used to obtain track IDs for some other commands")
+	addCmd("pause", "pause playback")
+	addCmd("stop", "clear playlist and stop playback")
+	addCmd("skip", "skip the current track")
+	addCmd("delete <ID|ID-ID>...", "delete one or multiple tracks from the queue")
+	addCmd("swap <ID> <ID>", "swap the position of two tracks in the queue")
+	addCmd("shuffle", "shuffle all items in the current queue")
+
+	var msg strings.Builder
+	msg.WriteString("Commands:\n")
+	for i := range cmds {
+		msg.WriteString("\u2022 `" + cfg.Prefix + cmds[i])
+		msg.WriteString(strings.Repeat(" ", longestCmd-len(cmds[i])))
+		msg.WriteString(" - " + descs[i] + "`\n")
+	}
+	return msg.String()
+}
+
+// Converts seconds to string in format mm:ss.
+func secsToMinsSecs(secs int) string {
+	return fmt.Sprintf("%02d:%02d", secs/60, secs%60)
+}
+
+// Removes/replaces some characters with special formatting meaning in discord
+// messages (for example *).
+func dcSanitize(in string) string {
+	in = strings.ReplaceAll(in, "*", "\u2217")
+	in = strings.ReplaceAll(in, "~~", "\u02dc\u02dc")
+	in = strings.ReplaceAll(in, "`", "'")
+	return strings.ReplaceAll(in, "||", "\u2758\u2758")
+}
+
+////////////////////////////////
+// Global variables.
+////////////////////////////////
+var helpMsg string
+
+////////////////////////////////
+// The actual commands.
+////////////////////////////////
+func commandHelp(c *Client) {
+	// We're not generating the help message in the var declaration because
+	// generateHelpMsg() relies on the config which hasn't been read at that
+	// point.
+	if helpMsg == "" {
+		helpMsg = generateHelpMsg()
+	}
+	c.Messagef("%s", helpMsg)
+}
+
+func commandPlay(s *discordgo.Session, g *discordgo.Guild, c *Client, args []string) {
+	var playbackActive bool
+	{
+		var playback Playback
+		if playback, playbackActive = c.GetPlaybackInfo(); playbackActive {
+			if playback.Paused {
+				c.Messagef("Resuming playback.")
+				playback.CmdCh <- dca0.CommandResume{}
+				c.Lock()
+				c.Playback.Paused = false
+				c.Unlock()
+				return
+			}
+		}
+	}
+
+	if c.QueueLen() == 0 && len(args) == 0 {
+		c.Messagef("Nothing in queue. Please add an item to the queue or specify a URL or a youtube search query.")
+		return
+	}
+
+	if len(args) > 0 {
+		// Add the current track/playlist in place.
+		commandAdd(c, args, true)
+		// We only want one player active at once.
+		if playbackActive {
+			return
+		}
+	}
+
+	if c.VoiceChannelID == "" {
+		c.Messagef("I don't know which voice channel to join.")
+		return
+	}
+
+	vc, err := s.ChannelVoiceJoin(g.ID, c.VoiceChannelID, false, true)
+	if err != nil {
+		c.Messagef("Error joining voice channel: %s.", err)
+		return
+	}
+	defer vc.Disconnect()
+
+	// Set playback to nothing once we're done or if an error occurs.
+	defer func() {
+		c.Lock()
+		c.Playback = nil
+		c.Unlock()
+	}()
+
+	// Play the queue.
+	for c.QueueLen() > 0 {
+		track, _ := c.QueuePopFront()
+		mediaUrl := track.MediaUrl
+		c.Messagef("Playing: %s.\n", dcSanitize(track.Title))
+
+		// Set up dca0 encoder.
+		dcaOpts := dca0.GetDefaultOptions(cfg.FfmpegPath)
+		enc, err := dca0.NewEncoder(dcaOpts)
+		if err != nil {
+			c.Messagef("Error: %s.", err)
+			return
+		}
+
+		// Set up audio playback.
+		c.Lock()
+		c.Playback = &Playback{
+			CmdCh:  make(chan dca0.Command),
+			RespCh: make(chan dca0.Response),
+			Track: track,
+		}
+		c.Unlock()
+		// We just set the playback info so we don't have to check if it's there.
+		playback, _ := c.GetPlaybackInfo()
+		errCh := make(chan error)
+		// Start downloading and sending audio data.
+		vc.Speaking(true)
+		go func() {
+			enc.GetOpusFrames(mediaUrl, dcaOpts, vc.OpusSend, errCh, playback.CmdCh, playback.RespCh)
+			close(errCh)
+		}()
+		// Process errors: Get and print the first error put out by the extractor.
+		err = nil
+		for e := range errCh {
+			if e != nil && err == nil {
+				err = e
+			}
+		}
+		if err != nil {
+			c.Messagef("Playback error: %s.", err)
+			return
+		}
+		// Done with this song.
+		playback, _ = c.GetPlaybackInfo()
+	}
+	c.Messagef("Done playing queue.")
+}
+
+// If inPlace is set to true, the track will be added to the front and replace
+// the currently playing one. If inPlace is set to true when dealing with a
+// playlist, the entire queue is replaced with that playlist.
+func commandAdd(c *Client, args []string, inPlace bool) {
+	if len(args) < 1 {
+		c.Messagef("Plase specify a URL or a youtube search query.")
+		return
+	}
+
+	// URL or search query.
+	input := strings.Join(args, " ")
+
+	// TODO: This is some very shitty detection for if we're dealing with a
+	// playlist.
+	if strings.HasPrefix(path.Base(input), "playlist") {
+		c.Messagef("Long playlists may take a while to add, please be patient.")
+	}
+
+	ytdlEx := ytdl.NewExtractor(cfg.YtdlPath)
+
+	meta, err := ytdlEx.GetMetadata(input)
+	if err != nil {
+		c.Messagef("Error getting audio metadata: %s.", err)
+		return
+	}
+
+	var plural string
+	if len(meta) != 1 {
+		plural = "s"
+	}
+	c.Messagef("Adding %d track%s to queue.", len(meta), plural)
+
+	isPlaylist := len(meta) > 1
+	if inPlace && isPlaylist {
+		c.QueueClear()
+	}
+
+	for _, m := range meta {
+		title, titleOk := m["title"]
+		webpageUrl, webpageUrlOk := m["webpage_url"]
+		if !(titleOk && webpageUrlOk) {
+			c.Messagef("Error getting video metadata: title=%t, url=%t.", titleOk, webpageUrlOk)
+			return
+		}
+
+		mediaUrl, err := ytdl.GetAudioURL(m)
+		if err != nil {
+			c.Messagef("Error getting URL: %s.", err)
+			return
+		}
+
+		track := &Track{
+			Title:    title.(string),
+			Url:      webpageUrl.(string),
+			MediaUrl: mediaUrl,
+		}
+		if inPlace && !isPlaylist {
+			if playback, ok := c.GetPlaybackInfo(); ok {
+				// To replace the currently playing track, insert the new one
+				// at Queue[0] and skip the current one.
+				if c.QueueLen() == 0 {
+					c.QueuePushBack(track)
+				} else {
+					c.QueuePushFront(track)
+				}
+				// Skip the current track.
+				playback.CmdCh <- dca0.CommandStop{}
+			} else {
+				c.QueuePushFront(track)
+			}
+		} else {
+			c.QueuePushBack(track)
+		}
+	}
+}
+
+func commandQueue(c *Client) {
+	playback, playbackOk := c.GetPlaybackInfo()
+	ql := c.QueueLen()
+	if ql == 0 && !playbackOk {
+		c.Messagef("Queue is empty.")
+		return
+	}
+
+	const maxLines = 15
+	var msg strings.Builder
+	// flush() writes the string buffer into a new Discord message, then clears
+	// the buffer.
+	flush := func() {
+		c.Messagef("%s", msg.String())
+		msg.Reset()
+		msg.WriteString("\u2800\n")
+	}
+	msg.WriteString("\u2800\n")
+	if playbackOk {
+		var loop string
+		if playback.Loop {
+			loop = " [LOOP]"
+		}
+		msg.WriteString(fmt.Sprintf("PLAYING: " + dcSanitize(playback.Track.Title) + loop + "\n"))
+	}
+	for i := 0; i < ql; i++ {
+		t, _ := c.QueueAt(i)
+		msg.WriteString(fmt.Sprintf("%02d. %s\n", i+1, dcSanitize(t.Title)))
+		// Only send a maximum of 15 lines at a time.
+		if (i+1)%maxLines == 0 && i != ql-1 {
+			flush()
+		}
+	}
+	flush()
+}
+
+func commandSeek(c *Client, args []string) {
+	playback, ok := c.GetPlaybackInfo()
+	if !ok {
+		c.Messagef("Not playing anything.")
+		return
+	}
+	const invalidFormat = "Please specify where to seek, either in seconds or in the format of mm:ss."
+	if len(args) == 0 {
+		c.Messagef(invalidFormat)
+		return
+	}
+	splits := strings.Split(args[0], ":")
+	var sMins, sSecs string
+	if len(splits) == 2 {
+		sMins, sSecs = splits[0], splits[1]
+	} else if len(splits) == 1 {
+		sMins, sSecs = "", splits[0]
+	} else {
+		c.Messagef(invalidFormat)
+		return
+	}
+	var mins, secs int64
+	var err error
+	if sMins != "" {
+		mins, err = strconv.ParseInt(sMins, 10, 32)
+		if err != nil {
+			c.Messagef(invalidFormat)
+			return
+		}
+	}
+	secs, err = strconv.ParseInt(sSecs, 10, 32)
+	if err != nil {
+		c.Messagef(invalidFormat)
+		return
+	}
+	secs = 60*mins + secs
+	c.Messagef("Seeking to %s.", secsToMinsSecs(int(secs)))
+	playback.CmdCh <- dca0.CommandSeek(secs)
+}
+
+func commandPos(c *Client) {
+	playback, ok := c.GetPlaybackInfo()
+	if !ok {
+		c.Messagef("Not playing anything.")
+		return
+	}
+	var sTime, sDur string
+	// Get current playback time.
+	playback.CmdCh <- dca0.CommandGetPlaybackTime{}
+	respTime := <-playback.RespCh
+	if t, ok := respTime.(dca0.ResponsePlaybackTime); ok {
+		sTime = secsToMinsSecs(int(t))
+	} else {
+		c.Messagef("Error receiving response: invalid type.")
+		return
+	}
+	// Attempt to get duration.
+	playback.CmdCh <- dca0.CommandGetDuration{}
+	respDur := <-playback.RespCh
+	switch d := respDur.(type) {
+	case dca0.ResponseDurationUnknown:
+		sDur = "??:??"
+	case dca0.ResponseDuration:
+		sDur = secsToMinsSecs(int(d))
+	default:
+		c.Messagef("Error receiving response: invalid type.")
+		return
+	}
+
+	c.Messagef("Current playback position: %s / %s.", sTime, sDur)
+}
+
+func commandLoop(c *Client) {
+	playback, ok := c.GetPlaybackInfo()
+	if !ok {
+		c.Messagef("Not playing anything.")
+		return
+	}
+
+	if playback.Loop {
+		playback.CmdCh <- dca0.CommandStopLooping{}
+		c.Messagef("Looping disabled.")
+	} else {
+		playback.CmdCh <- dca0.CommandStartLooping{}
+		c.Messagef("Looping enabled.")
+	}
+	c.Lock()
+	c.Playback.Loop = !playback.Loop
+	c.Unlock()
+}
+
+func commandStop(c *Client) {
+	playback, ok := c.GetPlaybackInfo()
+	if !ok {
+		c.Messagef("Not playing anything.")
+		return
+	}
+	c.Messagef("Stopping playback.")
+	c.QueueClear()
+	playback.CmdCh <- dca0.CommandStop{}
+}
+
+func commandSkip(c *Client) {
+	playback, ok := c.GetPlaybackInfo()
+	if !ok {
+		c.Messagef("Not playing anything.")
+		return
+	}
+	c.Messagef("Skipping current track.")
+	playback.CmdCh <- dca0.CommandStop{}
+}
+
+func commandPause(c *Client) {
+	playback, ok := c.GetPlaybackInfo()
+	if !ok {
+		c.Messagef("Not playing anything.")
+		return
+	}
+	if playback.Paused {
+		c.Messagef("Already paused.")
+	} else {
+		c.Messagef("Pausing playback.")
+		playback.CmdCh <- dca0.CommandPause{}
+		c.Lock()
+		c.Playback.Paused = true
+		c.Unlock()
+	}
+}
+
+func commandDelete(c *Client, args []string) {
+	if len(args) < 1 {
+		c.Messagef("Please specify which item(s) to delete from the queue. IDs can be obtained with %squeue.", cfg.Prefix)
+		return
+	}
+
+	toDel := make(map[int]struct{})
+	for _, arg := range args {
+		splits := strings.Split(arg, "-")
+		switch len(splits) {
+		case 1, 2:
+			ids := [2]int{-1, -1}
+			for i, s := range splits {
+				id, err := strconv.ParseInt(s, 10, 32)
+				if err != nil {
+					c.Messagef("Invalid format: %s.", arg)
+					return
+				}
+				id--
+				if id < 0 || int(id) >= c.QueueLen() {
+					c.Messagef("Index out of bounds: %s.", arg)
+					return
+				}
+				ids[i] = int(id)
+			}
+			if ids[1] == -1 {
+				toDel[ids[0]] = struct{}{}
+			} else {
+				if ids[0] > ids[1] {
+					c.Messagef("The first id of the range must be not be larger: %s.", arg)
+					return
+				}
+				for i := ids[0]; i <= ids[1]; i++ {
+					toDel[i] = struct{}{}
+				}
+			}
+		default:
+			c.Messagef("Invalid format: %s.", arg)
+			return
+		}
+	}
+
+	var newQueue []*Track
+	queueLen := c.QueueLen()
+	for i := 0; i < queueLen; i++ {
+		if _, del := toDel[i]; !del {
+			c.RLock()
+			newQueue = append(newQueue, c.Queue[i])
+			c.RUnlock()
+		}
+	}
+	c.Lock()
+	c.Queue = newQueue
+	c.Unlock()
+	c.Messagef("Successfully deleted %d items.", len(toDel))
+}
+
+func commandSwap(c *Client, args []string) {
+	if len(args) != 2 {
+		c.Messagef("Please specify 2 items to swap with one another. IDs can be obtained with %squeue.", cfg.Prefix)
+		return
+	}
+	var ids [2]int
+	for i, arg := range args {
+		id, err := strconv.ParseInt(arg, 10, 32)
+		if err != nil {
+			c.Messagef("Invalid format: %s.", arg)
+			return
+		}
+		id--
+		ids[i] = int(id)
+	}
+	c.Messagef("Swapping: %d and %d.", ids[0]+1, ids[1]+1)
+	c.QueueSwap(ids[0], ids[1])
+}
+
+func commandShuffle(c *Client) {
+	rand.Seed(time.Now().Unix())
+	queueLen := c.QueueLen()
+	c.Lock()
+	rand.Shuffle(queueLen, func(a, b int) {
+		c.Queue[a], c.Queue[b] = c.Queue[b], c.Queue[a]
+	})
+	c.Unlock()
+	c.Messagef("Successfully shuffled %d items.", queueLen)
+}
diff --git a/config.go b/config.go
new file mode 100644
index 0000000..dc017e3
--- /dev/null
+++ b/config.go
@@ -0,0 +1,45 @@
+package main
+
+import (
+	"encoding/json"
+	"errors"
+	"os"
+)
+
+type Config struct {
+	Prefix     string `json:"prefix"`
+	Token      string `json:"token"`
+	YtdlPath   string `json:"youtube-dl_path"`
+	FfmpegPath string `json:"ffmpeg_path"`
+}
+
+const configFile = "config.json"
+
+const tokenDefaultString = "insert your discord bot token here"
+
+func ReadConfig(cfg *Config) error {
+	configData, err := os.ReadFile(configFile)
+	if err != nil {
+		return errors.New("unable to read config file: " + err.Error())
+	}
+
+	json.Unmarshal(configData, cfg)
+	if err != nil {
+		return errors.New("unable to decode config file: " + err.Error())
+	}
+	return nil
+}
+
+func WriteDefaultConfig() error {
+	data, err := json.MarshalIndent(Config{
+		Prefix:     "!",
+		Token:      tokenDefaultString,
+		YtdlPath:   "youtube-dl",
+		FfmpegPath: "ffmpeg",
+	}, "", "\t")
+	if err != nil {
+		return err
+	}
+
+	return os.WriteFile(configFile, data, 0666)
+}
diff --git a/dca0/dca0.go b/dca0/dca0.go
new file mode 100644
index 0000000..5fac9f5
--- /dev/null
+++ b/dca0/dca0.go
@@ -0,0 +1,261 @@
+package dca0
+
+import (
+	"encoding/binary"
+	"io"
+	"os"
+	"os/exec"
+	"strconv"
+	"time"
+
+	"layeh.com/gopus"
+)
+
+type CacheOverflowError struct {
+	MaxCacheBytes int
+}
+
+func (e *CacheOverflowError) Error() string {
+	return "audio too large: the maximum cache limit of " + strconv.Itoa(e.MaxCacheBytes) + " bytes has been exceeded"
+}
+
+type Command interface{}
+
+type CommandStop struct{}
+type CommandPause struct{}
+type CommandResume struct{}
+type CommandStartLooping struct{}
+type CommandStopLooping struct{}
+type CommandSeek float32             // In seconds.
+type CommandGetPlaybackTime struct{} // Gets the playback time.
+type CommandGetDuration struct{}     // Attempts to get the duration. Only succeeds if the encoder is already done.
+
+type Response interface{}
+
+type ResponsePlaybackTime float32     // Playback time in seconds.
+type ResponseDuration float32         // Duration in seconds.
+type ResponseDurationUnknown struct{} // Returned if the duration is unknown.
+
+type Dca0Options struct {
+	PcmOptions
+	// 960 (20ms at 48kHz) is currently the only one supported by discordgo.
+	// If it is changed nonetheless, it changes the playback speed (without
+	// changing the pitch because of how discord and opus work).
+	FrameSize int
+	Bitrate   int
+	// Maximum number of bytes that may be cached. A 3-minute song usually uses
+	// about 2.7MB. If the capacity is full, an error will be sent and the
+	// function will exit.
+	MaxCacheBytes int
+}
+
+func GetDefaultOptions(ffmpegPath string) Dca0Options {
+	return Dca0Options{
+		PcmOptions: getDefaultPcmOptions(ffmpegPath),
+		FrameSize:  960,
+		// 64000 is Discord's default.
+		Bitrate: 64000,
+		// Max cache size of 20MB.
+		MaxCacheBytes: 20000000,
+	}
+}
+
+type Dca0Encoder struct {
+	opusEnc *gopus.Encoder
+}
+
+func NewEncoder(opts Dca0Options) (*Dca0Encoder, error) {
+	opusEnc, err := gopus.NewEncoder(opts.SampleRate, opts.Channels, gopus.Audio)
+	if err != nil {
+		return nil, err
+	}
+	return &Dca0Encoder{
+		opusEnc: opusEnc,
+	}, nil
+}
+
+// Sends the individual opus frames as byte arrays through the specified
+// channel.
+// Input can be either a local file or an http(s) address. It can be of any
+// format supported by ffmpeg.
+// Caches the entire opus data due to some problems when reading from ffmpeg
+// too slowly.
+func (e *Dca0Encoder) GetOpusFrames(input string, opts Dca0Options, ch chan<- []byte, errCh chan<- error, cmdCh <-chan Command, respCh chan<- Response) {
+	pcm, cmd, err := getPcm(input, opts.PcmOptions)
+	if err != nil {
+		errCh <- err
+		return
+	}
+
+	// How many opus frames are played per second.
+	framesPerSecond := float32(opts.SampleRate) / float32(opts.FrameSize)
+	// Potential maximum samples an audio frame can have.
+	maxSamples := opts.FrameSize * opts.Channels
+	// One pcm sample equals two bytes.
+	maxBytes := maxSamples * 2
+
+	// Size of the opus frame cache.
+	cacheSize := 0
+	// We're storing all opus frames in this array as a cache.
+	opusFrames := make([][]byte, 0, 512)
+	// Opus frame read position.
+	rp := 0
+	// We're sending frames through this before appending them to opusFrames.
+	frameCh := make(chan []byte, 8)
+
+	sampleBytes := make([]byte, maxBytes)
+	samples := make([]int16, maxSamples)
+
+	// Used by the encoder to tell the main process when it's done encoding.
+	encoderDone := make(chan struct{})
+	// Used by the main process to tell the encoder to stop.
+	encoderStop := make(chan struct{})
+	// Launch the encoder.
+	// Encode opus data and send it through frameCh.
+	go func() {
+		var killedFfmpeg bool
+	encoderLoop:
+		for {
+			// Stop encoding if the main process tells us to.
+			select {
+			case <-encoderStop:
+				// Kill ffmpeg using SIGINT.
+				cmd.Process.Signal(os.Interrupt)
+				pcm.Close()
+				killedFfmpeg = true
+				// Exit the loop.
+				break encoderLoop
+			default:
+			}
+
+			// Efficiently read the sample bytes outputted by ffmpeg into the
+			// int16 sample slice.
+			_, err := io.ReadFull(pcm, sampleBytes)
+			if err != nil {
+				// We also want to stop on ErrUnexpectedEOF because a frame can
+				// currently ONLY be 960 * 2 samples large. If a frame were
+				// smaller, Discord would just slow the audio down. Since a
+				// frame is just 20ms long, we can discard the last one without
+				// any issues.
+				if err == io.EOF || err == io.ErrUnexpectedEOF {
+					break
+				} else {
+					errCh <- err
+				}
+			}
+
+			// Read samples from binary, similarly to how the go package binary does
+			// it but more efficiently since we're not allocating anything.
+			for i := range samples {
+				samples[i] = int16(binary.LittleEndian.Uint16(sampleBytes[2*i:]))
+			}
+
+			// Encode samples as opus.
+			frame, err := e.opusEnc.Encode(samples, opts.FrameSize, maxBytes)
+			if err != nil {
+				errCh <- err
+			}
+
+			if cacheSize > opts.MaxCacheBytes {
+				errCh <- &CacheOverflowError{
+					MaxCacheBytes: opts.MaxCacheBytes,
+				}
+				break
+			}
+
+			cacheSize += len(frame)
+			frameCh <- frame
+		}
+		// Wait for ffmpeg to close.
+		err = cmd.Wait()
+		if err != nil {
+			// Ffmpeg returns 255 if it was killed using SIGINT. Therefore that
+			// wouldn't be an error.
+			switch e := err.(type) {
+			case *exec.ExitError:
+				if e.ExitCode() == 255 {
+					if !killedFfmpeg {
+						errCh <- err
+					}
+				}
+			default:
+				errCh <- err
+			}
+		}
+		// Tell the main process that the encoder is done.
+		encoderDone <- struct{}{}
+	}()
+
+	encoderRunning := true
+	paused := false
+	loop := false
+
+loop:
+	for {
+		select {
+		case v := <-frameCh:
+			opusFrames = append(opusFrames, v)
+		case <-encoderDone:
+			encoderRunning = false
+		case receivedCmd := <-cmdCh:
+			switch v := receivedCmd.(type) {
+			case CommandStop:
+				if encoderRunning {
+					encoderStop <- struct{}{}
+				}
+				break loop
+			case CommandPause:
+				paused = true
+			case CommandResume:
+				paused = false
+			case CommandStartLooping:
+				loop = true
+			case CommandStopLooping:
+				loop = false
+			case CommandSeek:
+				rp = int(float32(v) * framesPerSecond)
+			case CommandGetPlaybackTime:
+				respCh <- ResponsePlaybackTime(float32(rp) / framesPerSecond)
+			case CommandGetDuration:
+				if encoderRunning {
+					respCh <- ResponseDurationUnknown{}
+				} else {
+					respCh <- ResponseDuration(float32(len(opusFrames)) / framesPerSecond)
+				}
+			}
+		default:
+			time.Sleep(2 * time.Millisecond)
+		}
+
+		if !paused && rp < len(opusFrames) {
+			if encoderRunning {
+				select {
+				case ch <- opusFrames[rp]:
+					rp++
+				default:
+				}
+			} else {
+				ch <- opusFrames[rp]
+				rp++
+			}
+		}
+
+		if !encoderRunning && rp >= len(opusFrames) {
+			if loop {
+				rp = 0
+			} else {
+				// We're done sending opus data.
+				break
+			}
+		}
+	}
+
+	// Wait for the encoder to finish if it's still running.
+	if encoderRunning {
+		<-encoderDone
+		encoderRunning = false
+		// TODO: I want to make this unnecessary. I have just noticed that
+		// this channel often get stuck so a panic is more helpful than that.
+		close(respCh)
+	}
+}
diff --git a/dca0/pcm.go b/dca0/pcm.go
new file mode 100644
index 0000000..f09a7dd
--- /dev/null
+++ b/dca0/pcm.go
@@ -0,0 +1,73 @@
+// Helper functions for extracting pcm audio from web and local sources.
+package dca0
+
+import (
+	"errors"
+	"io"
+	"os/exec"
+	"strconv"
+)
+
+type PcmOptions struct {
+	FfmpegPath string
+	Channels   int
+	SampleRate int
+	// Seek means where to start in seconds.
+	Seek float32
+	// Duration means where to stop (in seconds after the time specified by Seek).
+	// If Duration is set to 0, the stream will ignore it and encode all the way
+	// to the end of the input.
+	Duration float32
+}
+
+func getDefaultPcmOptions(ffmpegPath string) PcmOptions {
+	return PcmOptions{
+		FfmpegPath: ffmpegPath,
+		Channels:   2,
+		// 48000 is the only one used by discord currently.
+		SampleRate: 48000,
+		Seek:       0,
+		Duration:   0,
+	}
+}
+
+// Input can be either a local file or an http(s) address. It can be of any
+// audio format supported by ffmpeg.
+// Wait must be called on the returned command to free its resources after
+// everything has been read.
+func getPcm(input string, opts PcmOptions) (io.ReadCloser, *exec.Cmd, error) {
+	if input == "" {
+		return nil, nil, errors.New("dca0.getPcm() called with empty input")
+	}
+	var cmdOpts []string
+	cmdOpts = append(cmdOpts, []string{
+		"-vn", // No video.
+		"-sn", // No subtitle.
+		"-dn", // No data encoding.
+	}...)
+	if opts.Seek != 0.0 {
+		cmdOpts = append(cmdOpts,
+			"-accurate_seek",
+			"-ss", strconv.FormatFloat(float64(opts.Seek), 'f', 5, 32))
+	}
+	if opts.Duration != 0.0 {
+		cmdOpts = append(cmdOpts,
+			"-t", strconv.FormatFloat(float64(opts.Duration), 'f', 5, 32))
+	}
+	cmdOpts = append(cmdOpts, []string{
+		"-i", input,
+		"-f", "s16le", // Signed int16 samples.
+		"-ar", strconv.Itoa(opts.SampleRate),
+		"-ac", strconv.Itoa(opts.Channels), // Number of audio channels.
+		"pipe:1", // Output to stdout.
+	}...)
+	cmd := exec.Command(opts.FfmpegPath, cmdOpts...)
+	stdout, err := cmd.StdoutPipe()
+	if err != nil {
+		return nil, nil, err
+	}
+	if err := cmd.Start(); err != nil {
+		return nil, nil, err
+	}
+	return stdout, cmd, nil
+}
diff --git a/go.mod b/go.mod
new file mode 100644
index 0000000..f8dc011
--- /dev/null
+++ b/go.mod
@@ -0,0 +1,8 @@
+module dcbot
+
+go 1.16
+
+require (
+	github.com/bwmarrin/discordgo v0.23.2
+	layeh.com/gopus v0.0.0-20210501142526-1ee02d434e32
+)
diff --git a/go.sum b/go.sum
new file mode 100644
index 0000000..80317b5
--- /dev/null
+++ b/go.sum
@@ -0,0 +1,8 @@
+github.com/bwmarrin/discordgo v0.23.2 h1:BzrtTktixGHIu9Tt7dEE6diysEF9HWnXeHuoJEt2fH4=
+github.com/bwmarrin/discordgo v0.23.2/go.mod h1:c1WtWUGN6nREDmzIpyTp/iD3VYt4Fpx+bVyfBG7JE+M=
+github.com/gorilla/websocket v1.4.0 h1:WDFjx/TMzVgy9VdMMQi2K2Emtwi2QcUQsztZ/zLaH/Q=
+github.com/gorilla/websocket v1.4.0/go.mod h1:E7qHFY5m1UJ88s3WnNqhKjPHQ0heANvMoAMk2YaljkQ=
+golang.org/x/crypto v0.0.0-20181030102418-4d3f4d9ffa16 h1:y6ce7gCWtnH+m3dCjzQ1PCuwl28DDIc3VNnvY29DlIA=
+golang.org/x/crypto v0.0.0-20181030102418-4d3f4d9ffa16/go.mod h1:6SG95UA2DQfeDnfUPMdvaQW0Q7yPrPDi9nlGo2tz2b4=
+layeh.com/gopus v0.0.0-20210501142526-1ee02d434e32 h1:/S1gOotFo2sADAIdSGk1sDq1VxetoCWr6f5nxOG0dpY=
+layeh.com/gopus v0.0.0-20210501142526-1ee02d434e32/go.mod h1:yDtyzWZDFCVnva8NGtg38eH2Ns4J0D/6hD+MMeUGdF0=
diff --git a/main.go b/main.go
new file mode 100644
index 0000000..a7981ae
--- /dev/null
+++ b/main.go
@@ -0,0 +1,359 @@
+package main
+
+import (
+	"fmt"
+	"os"
+	"os/signal"
+	"sync"
+	"syscall"
+
+	"dcbot/dca0"
+	"dcbot/util"
+
+	"github.com/bwmarrin/discordgo"
+)
+
+////////////////////////////////
+// Helper functions.
+////////////////////////////////
+// The first return value contains the voice channel ID, if it was found. If it
+// was not found, it is set to "".
+// The second return value indicates whether the voice channel was found.
+func GetUserVoiceChannel(g *discordgo.Guild, userID string) (string, bool) {
+	for _, vs := range g.VoiceStates {
+		if vs.UserID == userID {
+			return vs.ChannelID, true
+		}
+	}
+	return "", false
+}
+
+////////////////////////////////
+// Structs.
+////////////////////////////////
+type Playback struct {
+	Track
+	CmdCh  chan dca0.Command
+	RespCh chan dca0.Response
+	Paused bool
+	Loop   bool // Whether playback is looping right now.
+}
+
+type Track struct {
+	Title    string // Title, if any.
+	Url      string // Short URL, for example from YouTube.
+	MediaUrl string // Long URL of the associated media file.
+}
+
+// All methods of Client are thread safe, however manual locking is required
+// when accessing any fields.
+type Client struct {
+	sync.RWMutex
+
+	// The discordgo session.
+	s *discordgo.Session
+
+	// TextChannelID and VoiceChannelID indicate the current channels through
+	// which the bot should send text / audio. They may be set to "".
+	TextChannelID  string
+	VoiceChannelID string
+
+	// Current audio playback.
+	Playback *Playback
+	// Queue.
+	Queue []*Track
+}
+
+func NewClient(s *discordgo.Session) *Client {
+	return &Client{
+		s: s,
+	}
+}
+
+func (c *Client) Messagef(format string, a ...interface{}) {
+	c.RLock()
+	if c.TextChannelID == "" {
+		fmt.Printf(format+"\n", a...)
+	} else {
+		c.s.ChannelMessageSend(c.TextChannelID, fmt.Sprintf(format, a...))
+	}
+	c.RUnlock()
+}
+
+// Updates the text channel and voice channel IDs. May set them to "" if there
+// are none associated with the message.
+func (c *Client) UpdateChannels(g *discordgo.Guild, m *discordgo.Message) {
+	c.Lock()
+	c.TextChannelID = m.ChannelID
+
+	vc, _ := GetUserVoiceChannel(g, m.Author.ID)
+	c.VoiceChannelID = vc
+	c.Unlock()
+}
+
+func (c *Client) GetTextChannelID() string {
+	c.RLock()
+	ret := c.TextChannelID
+	c.RUnlock()
+	return ret
+}
+
+func (c *Client) GetVoiceChannelID() string {
+	c.RLock()
+	ret := c.TextChannelID
+	c.RUnlock()
+	return ret
+}
+
+// Returns a COPY of c.Playback. Modifications to the returned Playback struct
+// are NOT preserved, as it is a copy.
+func (c *Client) GetPlaybackInfo() (p Playback, ok bool) {
+	c.RLock()
+	ret := c.Playback
+	c.RUnlock()
+	if ret == nil {
+		return Playback{}, false
+	} else {
+		return *ret, true
+	}
+}
+
+func (c *Client) QueueLen() int {
+	c.RLock()
+	l := len(c.Queue)
+	c.RUnlock()
+	return l
+}
+
+// Similarly to GetPlaybackInfo, this function returns a COPY. Any modifications
+// are NOT preserved.
+// ok field returns false if the index is out of bounds.
+func (c *Client) QueueAt(i int) (t Track, ok bool) {
+	l := c.QueueLen()
+	if i >= l {
+		return Track{}, false
+	}
+	c.RLock()
+	ret := c.Queue[i]
+	c.RUnlock()
+	return *ret, true
+}
+
+func (c *Client) QueuePushBack(t *Track) {
+	c.Lock()
+	c.Queue = append(c.Queue, t)
+	c.Unlock()
+}
+
+func (c *Client) QueuePushFront(t *Track) {
+	c.Lock()
+	c.Queue = append([]*Track{t}, c.Queue...)
+	c.Unlock()
+}
+
+func (c *Client) QueuePopFront() (t Track, ok bool) {
+	t, ok = c.QueueAt(0)
+	if ok {
+		c.Lock()
+		c.Queue = c.Queue[1:]
+		c.Unlock()
+	}
+	return t, ok
+}
+
+// Deletes a single item at any position.
+// Returns false if i was out of bounds.
+func (c *Client) QueueDelete(i int) bool {
+	if i >= c.QueueLen() {
+		return false
+	}
+	c.Lock()
+	c.Queue = append(c.Queue[:i], c.Queue[i+1:]...)
+	c.Unlock()
+	return true
+}
+
+// Swaps two items in the queue.
+// Returns false if a or b is out of bounds.
+func (c *Client) QueueSwap(a, b int) bool {
+	if a == b {
+		return true
+	}
+	l := c.QueueLen()
+	if a >= l || b >= l {
+		return false
+	}
+	c.Lock()
+	c.Queue[a], c.Queue[b] = c.Queue[b], c.Queue[a]
+	c.Unlock()
+	return true
+}
+
+func (c *Client) QueueClear() {
+	c.Lock()
+	c.Queue = nil
+	c.Unlock()
+}
+
+func (c *Client) QueueFront() (t Track, ok bool) {
+	c.Lock()
+	defer c.Unlock()
+	if len(c.Queue) == 0 {
+		return Track{}, false
+	}
+	ret := *c.Queue[0]
+	return ret, true
+}
+
+////////////////////////////////
+// Global variables.
+////////////////////////////////
+var clients map[string]*Client // Guild ID to client
+var mClients sync.Mutex
+
+var cfg Config
+
+////////////////////////////////
+// Main program.
+////////////////////////////////
+func main() {
+	if err := ReadConfig(&cfg); err != nil {
+		fmt.Println(err)
+		if err := WriteDefaultConfig(); err != nil {
+			fmt.Println("Failed to create the default configuration file:", err)
+			return
+		}
+		fmt.Println("Wrote the default configuration to " + configFile + ".")
+		fmt.Println("You will have to manually configure the token by editing " + configFile + ".")
+		return
+	}
+
+	if cfg.Token == tokenDefaultString {
+		fmt.Println("Please set your bot token in " + configFile + " first.")
+		return
+	}
+
+	// Check if all binary dependencies are installed correctly.
+	const notInstalledErrMsg = "Unable to find %s in the specified path '%s', please make sure it's installed correctly.\nYou can manually set its path by editing %s\n"
+	if !util.CheckInstalled(cfg.YtdlPath, "--version") {
+		fmt.Printf(notInstalledErrMsg, "youtube-dl", cfg.YtdlPath, configFile)
+		return
+	}
+	if !util.CheckInstalled(cfg.FfmpegPath, "-version") {
+		fmt.Printf(notInstalledErrMsg, "ffmpeg", cfg.FfmpegPath, configFile)
+		return
+	}
+
+	// Initialize client map.
+	clients = make(map[string]*Client)
+
+	// Initialize bot.
+	dg, err := discordgo.New("Bot " + cfg.Token)
+	if err != nil {
+		fmt.Println("Error creating Discord session:", err)
+		return
+	}
+
+	dg.AddHandler(ready)
+	dg.AddHandler(banAdd)
+	dg.AddHandler(messageCreate)
+
+	// What information we need about guilds.
+	dg.Identify.Intents = discordgo.IntentsGuilds | discordgo.IntentsGuildMessages | discordgo.IntentsGuildVoiceStates | discordgo.IntentsGuildBans
+
+	// Open the websocket and begin listening.
+	err = dg.Open()
+	if err != nil {
+		fmt.Println("Error opening Discord session:", err)
+		return
+	}
+
+	// Wait here until Ctrl+c or other term signal is received.
+	fmt.Println("Bot is now running. Press Ctrl+c to exit.")
+	sc := make(chan os.Signal, 1)
+	signal.Notify(sc, syscall.SIGINT, syscall.SIGTERM, os.Interrupt, os.Kill)
+	<-sc
+
+	fmt.Println("\nSignal received, closing Discord session.")
+
+	// Cleanly close down the Discord session.
+	dg.Close()
+}
+
+func ready(s *discordgo.Session, event *discordgo.Ready) {
+	u := s.State.User
+	fmt.Println("Logged in as", u.Username+"#"+u.Discriminator+".")
+	s.UpdateListeningStatus(cfg.Prefix + "help")
+}
+
+func banAdd(s *discordgo.Session, event *discordgo.GuildBanAdd) {
+	fmt.Println(event)
+}
+
+func messageCreate(s *discordgo.Session, m *discordgo.MessageCreate) {
+	// Ignore messages from the bot itself.
+	if m.Author.ID == s.State.User.ID {
+		return
+	}
+
+	var g *discordgo.Guild
+	g, err := s.State.Guild(m.GuildID)
+	if err != nil {
+		// Could not find guild.
+		s.ChannelMessageSend(m.ChannelID, "This bot only works in guilds (servers).")
+		return
+	}
+
+	var c *Client
+	mClients.Lock()
+	{
+		var ok bool
+		if c, ok = clients[m.GuildID]; !ok {
+			c = NewClient(s)
+			clients[m.GuildID] = c
+		}
+	}
+	mClients.Unlock()
+	// Update the text and voice channels associated with the client.
+	c.UpdateChannels(g, m.Message)
+
+	args, ok := CmdGetArgs(m.Content)
+	if !ok {
+		// Not a command.
+		return
+	}
+
+	if len(args) == 0 {
+		c.Messagef("No command specified. Type `%shelp` for help.", cfg.Prefix)
+		return
+	}
+
+	switch args[0] {
+	case "help":
+		commandHelp(c)
+	case "play":
+		commandPlay(s, g, c, args[1:])
+	case "seek":
+		commandSeek(c, args[1:])
+	case "pos":
+		commandPos(c)
+	case "loop":
+		commandLoop(c)
+	case "add":
+		commandAdd(c, args[1:], false)
+	case "queue":
+		commandQueue(c)
+	case "pause":
+		commandPause(c)
+	case "stop":
+		commandStop(c)
+	case "skip":
+		commandSkip(c)
+	case "delete":
+		commandDelete(c, args[1:])
+	case "swap":
+		commandSwap(c, args[1:])
+	case "shuffle":
+		commandShuffle(c)
+	}
+}
diff --git a/util/util.go b/util/util.go
new file mode 100644
index 0000000..8014e0b
--- /dev/null
+++ b/util/util.go
@@ -0,0 +1,15 @@
+package util
+
+import (
+	"os/exec"
+)
+
+// Not using "command -v" because it doesn't work with Windows.
+// testArg will usually be something like --version.
+func CheckInstalled(program, testArg string) bool {
+	cmd := exec.Command(program, testArg)
+	if err := cmd.Run(); err != nil {
+		return false
+	}
+	return true
+}
diff --git a/ytdl/error.go b/ytdl/error.go
new file mode 100644
index 0000000..159f410
--- /dev/null
+++ b/ytdl/error.go
@@ -0,0 +1,26 @@
+package ytdl
+
+type Error struct {
+	// `Input` is the input given to youtube-dl when the error occurred.
+	// If `Input` is set to "", it is ignored.
+	Input string
+	// `Err` is the underlying error.
+	Err error
+}
+
+// `input` may be set to "", in which case it is ignored when outputting the
+// error message.
+func newError(input string, err error) *Error {
+	return &Error{
+		Input: input,
+		Err:   err,
+	}
+}
+
+func (e *Error) Error() string {
+	if e.Input == "" {
+		return "ytdl: " + e.Err.Error()
+	} else {
+		return "ytdl['" + e.Input + "']: " + e.Err.Error()
+	}
+}
diff --git a/ytdl/extractor.go b/ytdl/extractor.go
new file mode 100644
index 0000000..b183f84
--- /dev/null
+++ b/ytdl/extractor.go
@@ -0,0 +1,165 @@
+package ytdl
+
+import (
+	"bytes"
+	"encoding/json"
+	"errors"
+	"os/exec"
+)
+
+type Extractor struct {
+	DefaultSearch string
+	YtdlPath      string
+}
+
+func NewExtractor(ytdlPath string) *Extractor {
+	return &Extractor{
+		DefaultSearch: "ytsearch",
+		YtdlPath:      ytdlPath,
+	}
+}
+
+var (
+	errInvalidMetadata = errors.New("invalid metadata received from youtube-dl")
+)
+
+type Metadata map[string]interface{}
+
+/*// `input` can be a URL or a search query.
+// Returns a slice with size 1 if the input is a single media file. Returns a
+// larger slice if the input is a playlist.
+// Progress sends a struct{} every time a single item has been added.
+func (e *Extractor) GetMetadata(input string, progress chan<- struct{}) ([]Metadata, error) {
+	cmd := exec.Command(e.YtdlPath, "--default-search", e.DefaultSearch, "-j", input)
+	var out bytes.Buffer
+	cmd.Stdout = &out
+	if err := cmd.Start(); err != nil {
+		return nil, newError(input, err)
+	}
+
+	defer close(progress)
+
+	var m sync.Mutex
+	var done bool
+	var ret []Metadata
+	var ytdlErr error
+	var killedYtdl bool
+	go func() {
+		killYtdl := func(err error) {
+			m.Lock()
+			ytdlErr = err
+			cmd.Process.Signal(os.Interrupt)
+			killedYtdl = true
+			m.Unlock()
+		}
+
+		dec := json.NewDecoder(&out)
+		m.Lock()
+		d := done
+		m.Unlock()
+		for !d && dec.More() {
+			var meta interface{}
+			if err := dec.Decode(&meta); err != nil {
+				killYtdl(newError(input, err))
+			}
+			progress <- struct{}{}
+
+			if m, ok := meta.(map[string]interface{}); ok {
+				ret = append(ret, m)
+			} else {
+				killYtdl(newError(input, errInvalidMetadata))
+			}
+		}
+	}()
+
+	err := cmd.Wait()
+	m.Lock()
+	done = true
+	m.Unlock()
+	if ytdlErr != nil {
+		return nil, ytdlErr
+	}
+	if err != nil && !killedYtdl {
+		return nil, newError(input, err)
+	}
+	return ret, nil
+}*/
+
+
+// `input` can be a URL or a search query.
+// Returns a slice with size 1 if the input is a single media file. Returns a
+// larger slice if the input is a playlist.
+func (e *Extractor) GetMetadata(input string) ([]Metadata, error) {
+	cmd := exec.Command(e.YtdlPath, "--default-search", e.DefaultSearch, "-j", input)
+	var out bytes.Buffer
+	cmd.Stdout = &out
+	if err := cmd.Run(); err != nil {
+		return nil, newError(input, err)
+	}
+
+	var ret []Metadata
+	dec := json.NewDecoder(&out)
+	for dec.More() {
+		var meta interface{}
+		if err := dec.Decode(&meta); err != nil {
+			return nil, newError(input, err)
+		}
+
+		if m, ok := meta.(map[string]interface{}); ok {
+			ret = append(ret, m)
+		} else {
+			return nil, newError(input, errInvalidMetadata)
+		}
+	}
+	return ret, nil
+}
+
+// Returns the best available audio-only format. If the given URL links directly
+// to a media file, it just returns that URL.
+func GetAudioURL(meta Metadata) (string, error) {
+	// This function has a lot of code, but what it does is actually pretty
+	// simple. We just have to do a lot to ensure that there are no integrity
+	// issues with the metadata.
+	// 'iSomething' stands for 'interface something' here.
+
+	iExtractor, ok := meta["extractor"]
+	if !ok {
+		// Extractor not specified.
+		return "", newError("", errInvalidMetadata)
+	}
+	if extractor, ok := iExtractor.(string); ok {
+		if extractor == "generic" {
+			url, ok := meta["url"]
+			if u := url.(string); ok {
+				return u, nil
+			} else {
+				return "", newError("", errors.New("unable to get any audio or video URL"))
+			}
+		}
+
+		iFormats, ok := meta["formats"]
+		if !ok {
+			return "", newError("", errors.New("no format selection available and no raw URL specified"))
+		}
+		// Get the best audio format.
+		if formats, ok := iFormats.([]interface{}); ok {
+			// In youtube-dl, the last format is always the best one. Here we're
+			// looking for the last (=best) format that contains no video.
+			for i := len(formats) - 1; i >= 0; i-- {
+				iFormat := formats[i]
+				format, ok := iFormat.(map[string]interface{})
+				if !ok {
+					return "", newError("", errInvalidMetadata)
+				}
+				if format["vcodec"].(string) == "none" {
+					return format["url"].(string), nil
+				}
+			}
+			return "", newError("", errors.New("unable to find any audio-only format"))
+		} else {
+			return "", newError("", errInvalidMetadata)
+		}
+	} else {
+		return "", newError("", errInvalidMetadata)
+	}
+}