package main

import (
	"crypto/sha1"
	"encoding/hex"
	"fmt"
	"os"
	"os/exec"
	"regexp"
	"strconv"
	"strings"
	"time"
)

// Function getOTP gerates a time based OTP and returns a 64bit Integer
// or an error, if any.
func getOTP() (int64, error) {

	currentTime := time.Now()

	UTCTime := time.Date(
		currentTime.Year(),
		currentTime.Month(),
		currentTime.Day(),
		currentTime.Hour(),
		0,
		0,
		0,
		time.UTC)

	// Converting timestamp to string
	timestampString := strconv.FormatInt(UTCTime.Unix(), 10)

	// Create a SHA1 instance
	shaHash := sha1.New()

	// Write timestamp string to SHA1 instance
	shaHash.Write([]byte(timestampString))

	// Convert SHA1 Hash to String
	shaHashString := hex.EncodeToString(shaHash.Sum(nil))

	// Creating a Regular Expression instance to find digits
	// from the SHA1 hash
	regEx := regexp.MustCompile("\\d")

	// Extracting 8 digits
	digitsArr := regEx.FindAllString(shaHashString, -1)[0:8]

	return strconv.ParseInt(
		strings.Join(digitsArr[0:8], ""), // Join array to form a string
		10,                               // base 10 numeral
		64)                               // Int64
}

// Function launch simply launches a hardcoded command/application
func launch() {
	// Create a command instance to be executed.
	cmd := exec.Command("app.dll")

	// Run the command.
	err := cmd.Run()
	if err != nil {
		fmt.Println("An error occured!")
		os.Exit(0)
	}
}

// Function main is the entry point of our application.
func main() {
	// Print the message to ask for OTP
	fmt.Println("Enter OTP to launch")
	fmt.Println("---------------------")

	var otp int64 // To store entered OTP

	_, err := fmt.Scan(&otp) // Enter the OTP

	// Check if we've got any errors.
	if err != nil {
		fmt.Println("An error occured!")
		// Quit in case of any error.
		os.Exit(0)
	}

	// Find the correct OTP that we need to match
	// the user input against.
	realOTP, _ := getOTP()

	// Compare both real OTP and user's input
	if otp == realOTP {
		fmt.Println("Launching ...")
		// Launch the application if OTP is correct.
		launch()
	} else {
		fmt.Println("Wrong OTP!")
	}
}