From b1ce4bc2b0e93e9c82888e724983b350ea8f2844 Mon Sep 17 00:00:00 2001
From: Rob Watson <rob@netflux.io>
Date: Sun, 5 Sep 2021 07:24:51 +0200
Subject: [PATCH] poc: decode m4a to PCM with ffmpeg

---
 backend/main.go | 41 +++++++++++++++++++++++++++++------------
 1 file changed, 29 insertions(+), 12 deletions(-)

diff --git a/backend/main.go b/backend/main.go
index 745b678..d0d392e 100644
--- a/backend/main.go
+++ b/backend/main.go
@@ -1,14 +1,20 @@
 package main
 
 import (
-	"io"
+	"bytes"
+	"encoding/binary"
 	"log"
-	"os"
+	"os/exec"
 
 	"github.com/kkdai/youtube/v2"
 )
 
-const ItagM4AAudio = 140
+const (
+	ItagM4AAudio = 140
+	SizeOfInt16  = 2
+)
+
+// TODO: process as stream to avoid loading full buffer in memory
 
 func main() {
 	var client youtube.Client
@@ -26,18 +32,29 @@ func main() {
 		log.Fatal(err)
 	}
 
-	file, err := os.Create("out.m4a")
-	if err != nil {
-		log.Fatal(err)
+	// TODO: force 44.1khz
+	cmd := exec.Command("ffmpeg", "-i", "-", "-f", "s16le", "-acodec", "pcm_s16le", "-")
+	cmd.Stdin = stream
+
+	var out bytes.Buffer
+	cmd.Stdout = &out
+
+	var errOut bytes.Buffer
+	cmd.Stderr = &errOut
+
+	if err = cmd.Run(); err != nil {
+		log.Fatalf("err = %v, stdErr = %s", err, errOut.String())
 	}
 
-	defer file.Close()
+	log.Printf("byteLen = %d", out.Len())
 
-	_, err = io.Copy(file, stream)
-
-	if err != nil {
-		log.Fatal(err)
+	// 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.Println("done")
+	log.Printf("intLen = %d", len(data))
+	log.Printf("ints = %+v", data[0:8192])
 }