package main

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

	"golang.org/x/net/context"
	"golang.org/x/oauth2"
)

// Retrieve a token, saves the token, then returns the generated client.
func getClient(config *oauth2.Config) (*http.Client, error) {
	// The file token.json stores the user's access and refresh tokens, and is
	// created automatically when the authorization flow completes for the first
	// time.
	tokFile := "token.json"
	tok, err := tokenFromFile(tokFile)
	if err != nil {
		tok, err = getTokenFromWeb(config)
		if err != nil {
			return nil, err
		}

		err = saveToken(tokFile, tok)
		if err != nil {
			return nil, err
		}
	}
	return config.Client(context.Background(), tok), nil
}

// Request a token from the web, then returns the retrieved token.
func getTokenFromWeb(config *oauth2.Config) (*oauth2.Token, error) {
	authURL := config.AuthCodeURL("state-token", oauth2.AccessTypeOffline)
	log.Printf("Go to the following link in your browser then type the authorization code: \n%v", authURL)

	var authCode string
	if _, err := fmt.Scan(&authCode); err != nil {
		return nil, err
	}

	tok, err := config.Exchange(context.TODO(), authCode)
	if err != nil {
		return nil, err
	}
	return tok, nil
}

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

// Saves a token to a file path.
func saveToken(path string, token *oauth2.Token) error {
	f, err := os.OpenFile(path, os.O_RDWR|os.O_CREATE|os.O_TRUNC, 0600)
	if err != nil {
		return err
	}
	defer f.Close()
	json.NewEncoder(f).Encode(token)

	return nil
}