Begrænsnig af primtals program

Kode 

#include <iostream>
#include <deque>
#include <cmath>
#include <fstream>
#include <iterator>

std::deque<unsigned long> read_primes() {
  std::deque<unsigned long> P;
  std::ifstream primes("Primtal.txt");
  if (primes){
    copy(std::istream_iterator<unsigned long>(primes),
         std::istream_iterator<unsigned long>(),
         back_inserter(P));
    P.pop_front();
  }
  else {
    primes.close();
    P.push_back(3);
    std::ofstream primes("Primtal.txt",
                         std::ios::out|
                         std::ios::app);
    primes << "2" << std::endl;
    primes << P.back() << std::endl;
  }
  primes.close();
  return P;
}
int main() {
  std::deque<unsigned long>P;
  unsigned long long i;
  double root;
  bool is_prime;
  std::cout << "Laeser Primtal Fra filen... ";
  P = read_primes();
  std::cout << "\\n\\nFaerdig!\\n" << std::endl;
  std::cout << "Primtallet " << P.size()+1 << " laest fra filen!\\n" << std::endl;
  std::cout << "Tryk enter for at starte skrivningen";
  std::cin.get();
  std::cout << "\\nVent venlgit mens jeg skriver filen. Dette kan tage noget tid...\\n\\n" << "Skriver primtalene mellem " << P.back() << " og 18446744073709551615 til filen... ";
  std::ofstream primes("Primtal.txt",std::ios::out|std::ios::app);
  for (unsigned long long test = P.back() + 2; test != 1; test += 2) {
    root = sqrt(test);
    for (i = 0, is_prime = true; P[i] <= root && is_prime == true; i++)
      if (test % P[i] == 0) is_prime = false;
    if (is_prime == true && test <= 0xffffffff) P.push_back(test);
    if (is_prime == true) primes << P.back() << std::endl;
  }
  primes.close();
  std::cout << "Faerdig!" << std::endl;
  std::cout << "Lavet af Kasper Pedersen!" << std::endl;
  std::cout << P.size()+1 << " Primtal der var fundet mellem 0 og 18446744073709551615";
  std::cin.get();
}