IOTA-Enabled Phone Booth

package main

import (
	"context"
	"encoding/json"
	"fmt"
	"io/ioutil"
	"net/http"
	"os"
	"time"

	"github.com/pkg/errors"
	"golang.org/x/oauth2"
	"golang.org/x/oauth2/google"
	"google.golang.org/api/calendar/v3"
)

const minimumSlotWindow = 30 * time.Minute
const minimumSlotWindowAmount = 30

type CalendarService struct {
	srv *calendar.Service
}

func (c *CalendarService) FindNextFreeSlot(calendarID string) (time.Time, error) {
	t := time.Now().Format(time.RFC3339)
	events, err := c.srv.Events.List(calendarID).ShowDeleted(false).
		SingleEvents(true).TimeMin(t).MaxResults(2500).OrderBy("startTime").Do()
	if err != nil {
		return time.Now(), errors.Wrap(err, "unable to retrieve next ten of the user's events")
	}

	if len(events.Items) == 0 {
		nextFreeSlot := time.Now().Truncate(minimumSlotWindow)
		return nextFreeSlot, nil
	}

	last := events.Items[len(events.Items)-1]
	et, err := time.Parse(time.RFC3339, last.End.DateTime)
	if err != nil {
		return time.Now(), errors.Wrap(err, "failed to parse last event end date")
	}

	nextFreeSlot := et.Truncate(minimumSlotWindow)
	if nextFreeSlot.Before(et) {
		nextFreeSlot = nextFreeSlot.Add(minimumSlotWindow)
	}
	return nextFreeSlot, nil
}

func (c *CalendarService) EnqueueEvent(calendarID string, value int64, label string) error {
	nextStart, err := c.FindNextFreeSlot(calendarID)
	if err != nil {
		return errors.Wrap(err, "cannot find the next free slot")
	}

	value = value - (value % minimumSlotWindowAmount)
	nextEnd := nextStart.Add(time.Duration(value) * time.Minute)

	if value == 0 {
		return nil
	}

	newEvent := calendar.Event{
		Summary: label,
		Start: &calendar.EventDateTime{DateTime: nextStart.Format(time.RFC3339)},
		End: &calendar.EventDateTime{DateTime: nextEnd.Format(time.RFC3339)},
	}

	_, err = c.srv.Events.Insert(calendarID, &newEvent).Do()
	return err
}

func NewCalendarService() (*CalendarService, error) {
	b, err := ioutil.ReadFile("credentials.json")
	if err != nil {
		return nil, errors.Wrap(err, "unable to read client secret file")
	}

	// If modifying these scopes, delete your previously saved token.json.
	config, err := google.ConfigFromJSON(b, calendar.CalendarScope)
	if err != nil {
		return nil, errors.Wrap(err, "unable to parse client secret file to config")
	}

	client, err := getClient(config)
	if err != nil {
		return nil, errors.Wrap(err, "unable to create client")
	}

	srv, err := calendar.New(client)
	if err != nil {
		return nil, errors.Wrap(err, "unable to retrieve Calendar client")
	}

	return &CalendarService{
		srv: srv,
	}, nil
}

// 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, errors.Wrap(err, "unable to get token from WEB")
		}
		err = saveToken(tokFile, tok)
		if err != nil {
			return nil, errors.Wrap(err, "unable to save token to file")
		}
	}
	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)
	fmt.Printf("Go to the following link in your browser then type the "+
		"authorization code: \n%v\n", authURL)

	var authCode string
	if _, err := fmt.Scan(&authCode); err != nil {
		return nil, errors.Wrap(err, "unable to read authorization code")
	}

	tok, err := config.Exchange(context.TODO(), authCode)
	if err != nil {
		return nil, errors.Wrap(err, "unable to retrieve token from web")
	}
	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 {
	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 {
		return errors.Wrap(err, "unable to cache oauth token")
	}
	defer f.Close()
	json.NewEncoder(f).Encode(token)
	return nil
}