From 3955a64d9f0dd51a8fc720958e4c4fee3399b6ec Mon Sep 17 00:00:00 2001
From: Rob Watson <rob@netflux.io>
Date: Sun, 5 Sep 2021 11:49:59 +0200
Subject: [PATCH] poc: introduce AudioDownloader

---
 .gitignore      |   2 +-
 backend/main.go | 124 ++++++++++++++++++++++++++++++++++++------------
 2 files changed, 94 insertions(+), 32 deletions(-)

diff --git a/.gitignore b/.gitignore
index e9ff18a..40732b0 100644
--- a/.gitignore
+++ b/.gitignore
@@ -1 +1 @@
-*.m4a
+/backend/cache/
diff --git a/backend/main.go b/backend/main.go
index d0d392e..e43c77b 100644
--- a/backend/main.go
+++ b/backend/main.go
@@ -3,8 +3,12 @@ package main
 import (
 	"bytes"
 	"encoding/binary"
+	"fmt"
+	"io"
 	"log"
+	"os"
 	"os/exec"
+	"strconv"
 
 	"github.com/kkdai/youtube/v2"
 )
@@ -12,49 +16,107 @@ import (
 const (
 	ItagM4AAudio = 140
 	SizeOfInt16  = 2
+
+	DefaultAudioCodec = "pcm_s16le"
+	DefaultFormat     = "s16le"
+	DefaultSampleRate = 44100
 )
 
-// TODO: process as stream to avoid loading full buffer in memory
+type byteToIntConverter struct {
+	callback func(p []uint16)
+	buf      []uint16
+}
 
-func main() {
-	var client youtube.Client
-
-	videoID := "s_oJYdRlrv0"
-
-	video, err := client.GetVideo(videoID)
-	if err != nil {
-		log.Fatal(err)
+func (w byteToIntConverter) Write(p []byte) (int, error) {
+	if len(p)%2 != 0 {
+		panic("need to handle odd number of bytes")
 	}
 
-	format := video.Formats.FindByItag(ItagM4AAudio)
-	stream, _, err := client.GetStream(video, format)
-	if err != nil {
-		log.Fatal(err)
+	// grow the internal buffer if necessary:
+	exp := len(p) / SizeOfInt16
+	if len(w.buf) < exp {
+		w.buf = append(w.buf, make([]uint16, exp)...)
+
+	}
+	for i := 0; i < len(p); i += SizeOfInt16 {
+		w.buf[i/2] = binary.LittleEndian.Uint16(p[i : i+SizeOfInt16])
 	}
 
-	// TODO: force 44.1khz
-	cmd := exec.Command("ffmpeg", "-i", "-", "-f", "s16le", "-acodec", "pcm_s16le", "-")
-	cmd.Stdin = stream
+	w.callback(w.buf)
 
-	var out bytes.Buffer
-	cmd.Stdout = &out
+	return len(p), nil
+}
+
+type AudioDownloader struct {
+	videoID  string
+	ytClient youtube.Client
+}
+
+func NewAudioDownloader(videoID string) *AudioDownloader {
+	return &AudioDownloader{videoID: videoID}
+}
+
+func (d *AudioDownloader) ReadInts(fn func(p []uint16)) error {
+	cachePath := fmt.Sprintf("cache/%s.raw", d.videoID)
+
+	var reader io.Reader
+
+	fetch := true
+	if _, err := os.Stat(cachePath); err == nil {
+		if fptr, err := os.Open(cachePath); err == nil {
+			defer fptr.Close()
+			reader = fptr
+			fetch = false
+		} else {
+			log.Printf("warning: error opening cache file: %v", err)
+		}
+	}
+
+	if fetch {
+		log.Println("fetching audio stream from youtube...")
+
+		video, err := d.ytClient.GetVideo(d.videoID)
+		if err != nil {
+			return fmt.Errorf("error fetching video: %v", err)
+		}
+
+		format := video.Formats.FindByItag(ItagM4AAudio)
+		stream, _, err := d.ytClient.GetStream(video, format)
+		if err != nil {
+			return fmt.Errorf("error fetching stream: %v", err)
+		}
+
+		cacheFile, err := os.Create(cachePath)
+		if err != nil {
+			return fmt.Errorf("error creating cache file: %v", err)
+		}
+		reader = io.TeeReader(stream, cacheFile)
+	}
 
 	var errOut bytes.Buffer
+	cmd := exec.Command("ffmpeg", "-i", "-", "-f", DefaultFormat, "-ar", strconv.Itoa(DefaultSampleRate), "-acodec", DefaultAudioCodec, "-")
+	cmd.Stdin = reader
+	cmd.Stdout = byteToIntConverter{callback: fn}
 	cmd.Stderr = &errOut
 
-	if err = cmd.Run(); err != nil {
-		log.Fatalf("err = %v, stdErr = %s", err, errOut.String())
+	if err := cmd.Run(); err != nil {
+		log.Println(errOut.String())
+		return fmt.Errorf("error calling ffmpeg: %v", err)
 	}
 
-	log.Printf("byteLen = %d", out.Len())
-
-	// TODO: reflect.Slice to avoid copying?
-	data := make([]uint16, 0, out.Len()/SizeOfInt16)
-	for i := 0; i < out.Len(); i += SizeOfInt16 {
-		v := binary.LittleEndian.Uint16(out.Bytes()[i : i+SizeOfInt16])
-		data = append(data, v)
-	}
-
-	log.Printf("intLen = %d", len(data))
-	log.Printf("ints = %+v", data[0:8192])
+	return nil
+}
+
+func main() {
+	videoID := "s_oJYdRlrv0"
+	downloader := NewAudioDownloader(videoID)
+	err := downloader.ReadInts(func(p []uint16) {
+		// log.Printf("ints = %+v", p[0:16])
+	})
+
+	if err != nil {
+		log.Fatalf("error calling downloader: %v", err)
+	}
+
+	log.Println("done")
 }