Basic pipelines architecture + changed API

See https://blog.golang.org/pipelines for the concurrency usage.
I changed also the `id` parameter to allow multiple entry.

Example: `youtube-dl -ids
https://www.youtube.com/watch\?v\=lWEbEtr_Vng,https://www.youtube.com/watch\?v\=HpNluHOAJFA\&list\=RDHpNluHOAJFA`

Should fix #17.

api/concurrency.go

@@ -0,0 +1,30 @@
+package api
+
+import "sync"
+
+//DownloadStreams download a batch of elements asynchronouslly
+func DownloadStreams(maxOperations int, format, outputPath string, bitrate uint, urls []string) <-chan error {
+
+	var wg sync.WaitGroup
+	wg.Add(len(urls))
+
+	ch := make(chan error, maxOperations)
+	for _, url := range urls {
+		go func(url string) {
+			defer wg.Done()
+
+			if ID, err := GetVideoId(url); err != nil {
+				ch <- err
+			} else {
+				ch <- APIGetVideoStream(format, ID, outputPath, bitrate)
+			}
+		}(url)
+	}
+
+	go func() {
+		wg.Wait()
+		close(ch)
+	}()
+
+	return ch
+}

ytd.go

@@ -13,6 +13,7 @@ import (
 	"os"
 	"runtime"
 	"runtime/pprof"
+	"strings"
 
 	"github.com/Ch3ck/youtube-dl/api"
 	"github.com/Sirupsen/logrus"
@@ -21,24 +22,28 @@ import (
 const (
 
 	//BANNER for ytd which prints the help info
-	BANNER = "ytd -id 'videoId' -format mp3 -bitrate 123  -path ~/Downloads/ videoUrl %s\n"
+	BANNER = "ytd -ids 'videoId,videoId2' -format mp3 -bitrate 123  -path ~/Downloads/ videoUrl %s\n"
 	//VERSION which prints the ytd version.
 	VERSION = "v0.1"
 )
 
 var (
-	id      string
+	ids     string
 	version bool
 	format  string
 	path    string
 	bitrate uint
 )
 
+const (
+	defaultMaxDownloads = 5
+)
+
 var cpuprofile = flag.String("cpuprofile", "", "write cpu profile to file")
 
 func init() {
 	// parse flags
-	flag.StringVar(&id, "id", "", "Youtube Video ID")
+	flag.StringVar(&ids, "ids", "", "Youtube Video IDs. Separated then by using a comma.")
 	flag.StringVar(&format, "format", "", "File Format(mp3, webm, flv)")
 	flag.StringVar(&path, "path", ".", "Output Path")
 	flag.BoolVar(&version, "version", false, "print version and exit")
@@ -57,7 +62,6 @@ func init() {
 }
 
 func main() {
-	var ID string
 	if *cpuprofile != "" {
 		f, err := os.Create(*cpuprofile)
 		if err != nil {
@@ -74,17 +78,23 @@ func main() {
 	if len(os.Args) == 1 {
 		usageAndExit(BANNER, -1)
 	}
+
 	//Get Video Id
-	if id == "" {
+	if ids == "" {
 		url := os.Args[1]
-		ID, _ = api.GetVideoId(url)
+		startProcessing([]string{url})
 	} else {
-		ID, _ = api.GetVideoId(id)
+		startProcessing(strings.Split(ids, ","))
 	}
+}
 
-	//Extract Video data and decode
-	if err := api.APIGetVideoStream(format, ID, path, bitrate); err != nil {
-		logrus.Errorf("Error decoding Video stream: %v", err)
+func startProcessing(urls []string) {
+	ch := api.DownloadStreams(defaultMaxDownloads, format, path, bitrate, urls)
+	for err := range ch {
+		//Extract Video data and decode
+		if err != nil {
+			logrus.Errorf("Error decoding Video stream: %v", err)
+		}
 	}
 }