package main

import (
	"fmt"
	"io"
	"log"
	"net/http"
	"os"
	"strings"
	"time"

	"github.com/kkdai/youtube/v2"
)

const (
	ContentTypeAudioM4A = "audio/m4a"
	ItagM4AAudio        = 140
	DefaultHTTPBindAddr = "0.0.0.0:8888"
	DefaultTimeout      = 5 * time.Second
)

type AudioDownloader struct {
	videoID  string
	ytClient youtube.Client
	reader   io.Reader
}

func NewAudioDownloader(videoID string) (*AudioDownloader, error) {
	var (
		reader   io.Reader
		ytClient youtube.Client
	)

	cachePath := fmt.Sprintf("cache/%s.m4a", videoID)
	fetch := true

	if _, err := os.Stat(cachePath); err == nil {
		if fptr, err := os.Open(cachePath); err == nil {
			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 := ytClient.GetVideo(videoID)
		if err != nil {
			return nil, fmt.Errorf("error fetching video: %v", err)
		}

		format := video.Formats.FindByItag(ItagM4AAudio)
		log.Printf("M4A format expected to contain %d bytes", format.ContentLength)
		stream, _, err := ytClient.GetStream(video, format)
		if err != nil {
			return nil, fmt.Errorf("error fetching stream: %v", err)
		}

		// TODO: only allow the cached file to be accessed after it has been
		// successfully downloaded.
		cacheFile, err := os.Create(cachePath)
		if err != nil {
			return nil, fmt.Errorf("error creating cache file: %v", err)
		}
		reader = io.TeeReader(stream, cacheFile)
	}

	return &AudioDownloader{
		videoID:  videoID,
		ytClient: ytClient,
		reader:   reader,
	}, nil
}

func (d *AudioDownloader) Read(p []byte) (int, error) {
	return d.reader.Read(p)
}

func (d *AudioDownloader) Close() error {
	if rc, ok := d.reader.(io.ReadCloser); ok {
		return rc.Close()
	}
	return nil
}

func handleRequest(w http.ResponseWriter, r *http.Request) {
	if r.Method != http.MethodGet {
		w.WriteHeader(http.StatusMethodNotAllowed)
		w.Write([]byte("method not allowed"))
		return
	}

	if !strings.HasPrefix(r.URL.Path, "/api/audio") {
		w.WriteHeader(http.StatusNotFound)
		w.Write([]byte("page not found"))
		return
	}

	videoID := r.URL.Query().Get("video_id")
	if videoID == "" {
		w.WriteHeader(http.StatusBadRequest)
		w.Write([]byte("no video ID provided"))
		return
	}

	downloader, err := NewAudioDownloader(videoID)
	if err != nil {
		log.Printf("downloader error: %v", err)
		w.WriteHeader(http.StatusInternalServerError)
		w.Write([]byte("could not download video"))
		return
	}
	defer downloader.Close()

	w.Header().Add("Content-Type", ContentTypeAudioM4A)
	w.WriteHeader(http.StatusOK)

	n, err := io.Copy(w, downloader)
	if err != nil {
		log.Printf("error writing response: %v", err)
		return
	}

	log.Printf("wrote %d bytes for video ID %s", n, videoID)
}

func main() {
	srv := http.Server{
		ReadTimeout:  DefaultTimeout,
		WriteTimeout: DefaultTimeout,
		Addr:         DefaultHTTPBindAddr,
		Handler:      http.HandlerFunc(handleRequest),
	}

	log.Fatal(srv.ListenAndServe())
}