package main

import (
	"context"
	"encoding/json"
	"fmt"
	"log"
	"net/http"
	"os"

	"golang.org/x/oauth2"
	"golang.org/x/oauth2/google"
	"google.golang.org/api/gmail/v1"
	"google.golang.org/api/option"
)

const TOKEN_FILE string = "creds/token.json"
const CREDENTIALS_FILE string = "creds/credentials.json"

func getClient(config *oauth2.Config) *http.Client {
	token_file := TOKEN_FILE
	token, err := tokenFromFile(token_file)
	if err != nil {
		token = getTokenFromWeb(config)
		saveToken(token_file, token)
	}
	return config.Client(context.Background(), token)
}

func getTokenFromWeb(config *oauth2.Config) *oauth2.Token {
	auth_url := config.AuthCodeURL("state-token", oauth2.AccessTypeOffline)
	fmt.Printf("Go to the following link in your browser, then type the " +
		"authorization code: \n%v\n", auth_url)
	
	var auth_code string
	if _, err := fmt.Scan(&auth_code) ; err != nil {
		log.Fatalf("Unable to read authorization code: %v", err)
	}

	token, err := config.Exchange(context.TODO(), auth_code)
	if err != nil {
		log.Fatalf("Unable to retrieve token from web: %v", err)
	}
	return token
}

func tokenFromFile(file string) (*oauth2.Token, error) {
	f, err := os.Open(file)
	if err != nil {
		return nil, err
	}
	defer f.Close()
	token := &oauth2.Token{}
	err = json.NewDecoder(f).Decode(token)
	return token, err
}

func saveToken(path string, token *oauth2.Token) {
	fmt.Printf("Saving credential file to: %s\n", path)
	file, err := os.OpenFile(path, os.O_RDWR|os.O_CREATE|os.O_TRUNC, 0600)
	if err != nil {
		log.Fatalf("Unable to cache oauth token: %v", err)
	}
	defer file.Close()
	json.NewEncoder(file).Encode(token)
}

func main() {
	ctx := context.Background()
	creds_b, err := os.ReadFile(CREDENTIALS_FILE)
	if err != nil {
		log.Fatalf("Unable to read client secret file: %v", err)
	}

	config, err := google.ConfigFromJSON(creds_b, gmail.GmailReadonlyScope)
	if err != nil {
		log.Fatalf("Unable to parse client secret file to config: %v", err)
	}
	client := getClient(config)

	service, err := gmail.NewService(ctx, option.WithHTTPClient(client))
	if err != nil {
		log.Fatalf("Unable to retrieve Gmail client: %v", err)
	}

	user := "me"
	r, err := service.Users.Labels.List(user).Do()
	if err != nil {
		log.Fatalf("Unable to retrieve labels: %v", err)
	}
	if len(r.Labels) == 0 {
		fmt.Println("No labels found")
		return
	}
	fmt.Println("Labels:")
	for _, label := range r.Labels {
		fmt.Printf("- %s\n", label.Name)
	}
}