example_api.go

const (
    apiAddress = ":8081"
)

func main() {
	mux := setupMux()
	http.ListenAndServe(apiAddress, mux)
}

func setupMux() *http.ServeMux {
	mux := http.NewServeMux()
	mux.HandleFunc("/check-is-prime", isPrimeHandler)
	return mux
}

func isPrimeHandler(w http.ResponseWriter, r *http.Request) {
	number := r.URL.Query().Get("number")
	n, err := strconv.Atoi(number)
	if err != nil {
		http.Error(w, "invalid number", http.StatusBadRequest)
		return
	}

	fmt.Fprint(w, strconv.FormatBool(isPrime(int64(n))))
}

func isPrime(n int64) bool {
	return big.NewInt(n).ProbablyPrime(0)
}