package calendar

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

	"golang.org/x/oauth2"
	"golang.org/x/oauth2/google"
	"google.golang.org/api/calendar/v3"
	"google.golang.org/api/option"
)

const tokFile = "token.json"

type Service struct {
	*calendar.Service
}

func NewService(ctx context.Context, credJson string) *Service {
	b := []byte(credJson)

	config, err := google.ConfigFromJSON(b, calendar.CalendarReadonlyScope)
	if err != nil {
		log.Fatalf("Unable to parse client secret file to config: %v", err)
	}
	client := getClient(ctx, config)

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

	return &Service{srv}
}

// TODO: implement create event, list events in a period, delete event

// TODO: Delete this. It is just for Proof of Concept
func (s Service) ListNextEvents() (*calendar.Events, error) {
	t := time.Now().Format(time.RFC3339)
	events, err := s.Events.List("primary").ShowDeleted(false).
		SingleEvents(true).TimeMin(t).MaxResults(10).OrderBy("startTime").Do()
	if err != nil {
		return nil, fmt.Errorf("unable to retrieve next ten of the user's events: %w", err)
	}
	if len(events.Items) == 0 {
		return nil, errors.New("no upcoming events found")
	} else {
		// TODO: return a custom type
		return events, nil
	}
}

// Retrieve a token, saves the token, then returns the generated client.
func getClient(ctx context.Context, config *oauth2.Config) *http.Client {
	// 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.
	tok, err := tokenFromFile(tokFile)
	if err != nil {
		tok = getTokenFromWeb(config)
		saveToken(tokFile, tok)
	}
	return config.Client(ctx, tok)
}

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

	// TODO: write a callback endpoint that will parse the auth code from the URL
	var authCode string
	fmt.Print("Type the authorization code: \n> ")
	if _, err := fmt.Scan(&authCode); err != nil {
		log.Fatalf("Unable to read authorization code: %v", err)
	}

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

// 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) {
	fmt.Printf("Saving credential file to: %s\n", path)
	f, 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 f.Close()
	json.NewEncoder(f).Encode(token)
}