package main

import (
	"crypto/hmac"
	"crypto/sha256"
	"fmt"
	"hash"
	"html/template"
	"log"
	"net/http"
	"strconv"
	"strings"
)

var TPL *template.Template

const (
	SECRET                = "QDFuO2FyRHhGKEg5QlI4XDMlM2sK"
	COUNTER_COOKIE string = "counter"
)

func displayPage(res http.ResponseWriter, req *http.Request) {
	count, cookie := updateCounter(req)
	http.SetCookie(res, cookie)
	TPL.Execute(res, count)
}

func updateCounter(req *http.Request) (uint64, *http.Cookie) {
	count := fromCookie(req)
	count++
	val := toCookieVal(count)
	cookie := &http.Cookie{
		Name:  COUNTER_COOKIE,
		Value: val,
	}
	return count, cookie
}

func fromCookie(req *http.Request) uint64 {
	cookie, err := req.Cookie(COUNTER_COOKIE)
	if err != nil {
		return 0
	}
	sp := strings.Split(cookie.Value, "|")
	if len(sp) != 2 {
		return 0
	}
	c, err := strconv.Atoi(sp[0])
	if err != nil {
		return 0
	}
	if !checkMac(sp[0], sp[1]) {
		return 0
	}
	return uint64(c)
}

func checkMac(val string, macStr string) bool {
	var mac []byte
	fmt.Sscanf(macStr, "%x", &mac)
	return hmac.Equal(mac, cookieHash(val).Sum(nil))
}

func cookieHash(val string) hash.Hash {
	mac := hmac.New(sha256.New, []byte(SECRET))
	mac.Write([]byte(val))
	return mac
}

func toCookieVal(count uint64) string {
	val := strconv.FormatUint(count, 10)
	return val + "|" + fmt.Sprintf("%x", cookieHash(val).Sum(nil))
}

func main() {
	var err error
	TPL, err = template.ParseFiles("tpl.html")
	if err != nil {
		log.Fatalln(err)
	}
	http.HandleFunc("/", displayPage)
	http.ListenAndServe(":9000", nil)
}