package main

import (
	"crypto/sha256"
	"io"
	"log"
	"net/http"
	"os"
	"strconv"
	"strings"
	"time"

	"github.com/joho/godotenv"
	"gopkg.in/gomail.v2"
)

/* Get a website HTML. */
func getWebsite(url string) []byte {
	response, err := http.Get(url)
	if err != nil {
		log.Fatal("Error: ", err)
	}
	defer response.Body.Close()

	body, err := io.ReadAll(response.Body)
	if err != nil {
		log.Fatal("Error reading response body: ", err)
	}

	return body
}

/* Convert data to sha256 hash. */
func stringToHash(data []byte) string {
	hash := sha256.Sum256(data)
	return string(hash[:])
}

/* Send an email. */
func sendEmail(identity string, username string, password string, host string, recipient []string, port int) {
	message := gomail.NewMessage()
	message.SetHeader("From", identity)
	message.SetHeader("To", strings.Join(recipient, ", "))
	message.SetHeader("Subject", "Changes on website")
	message.SetBody("text/plain", "A changed has been found on the website.")

	dialer := gomail.NewDialer(host, port, username, password)

	if err := dialer.DialAndSend(message); err != nil {
		log.Fatal(err)
	}
}

/* Environnement variable checker */
func checkEnvVariables(keys []string) {
	for _, key := range keys {
		_, i := os.LookupEnv(key)
		if !i {
			log.Fatal(key, " doesn't exist.")
		}
	}
}

/* Take a map of string * string and returns a list of its values */
func valuesOfMapStringToListString(from map[string]string) []string {
	res := make([]string, 0, len(from))

	for _, value := range from {
		res = append(res, value)
	}

	return res
}

func main() {
	// Loading environment variables
	godotenv.Load()
	keys := map[string]string{
		"url":              "WEBSITE",
		"smtp_host":        "SMTP_HOST",
		"smtp_from":        "SMTP_FROM",
		"smtp_user":        "SMTP_USERNAME",
		"smtp_pass":        "SMTP_PASSWORD",
		"smtp_port":        "SMTP_PORT",
		"recipient_adress": "RECIPIENT_ADRESS",
	}
	checkEnvVariables(valuesOfMapStringToListString(keys))

	url := os.Getenv(keys["url"])
	smtp_from := os.Getenv(keys["smtp_from"])
	smtp_user := os.Getenv(keys["smtp_user"])
	smtp_pass := os.Getenv(keys["smtp_pass"])
	smtp_host := os.Getenv(keys["smtp_host"])
	recipient_adress := os.Getenv(keys["recipient_adress"])
	smtp_port, err := strconv.Atoi(os.Getenv(keys["smtp_port"]))
	if err != nil {
		log.Fatal("SMTP port is invalid.")
	}

	last_hash := ""
	sleepDuration := 24
	log.Default().Print("Checking: ", url)
	log.Default().Print("Running each ", sleepDuration, "h, starting now.")
	for {
		body := getWebsite(url)
		hash := stringToHash(body)
		if last_hash != hash && len(last_hash) > 0 {
			sendEmail(
				smtp_from,
				smtp_user,
				smtp_pass,
				smtp_host,
				strings.Split(recipient_adress, ","),
				smtp_port,
			)
		}
		last_hash = hash

		/* log.Default().Print("Next loop in ", sleepDuration, "h.") */
		time.Sleep(time.Duration(sleepDuration) * time.Hour)
	}
}