sieve.cc


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#include "sieve.hh"
#include <cmath>
#include <cstring>

[[nodiscard]] std::vector<int> sieve1(std::size_t size) {
  std::vector<bool> n(size, false);
  std::vector<int> r;

  for (std::size_t i = 2; i < n.size(); ++i) {
    if (n[i] == false) {
      r.emplace_back(i);
      for (std::size_t j = i + i; j < n.size(); j += i)
        n[j] = true;
    }
  }

  return r;
}

[[nodiscard]] std::vector<int> sieve2(std::size_t size) {
  std::vector<bool> n(size, false);
  std::vector<int> r{2};

  for (std::size_t i = 3; i < n.size(); i += 2) {
    if (n[i] == false) {
      r.emplace_back(i);
      for (std::size_t j = i + i; j < n.size(); j += i)
        n[j] = true;
    }
  }

  return r;
}

[[nodiscard]] std::vector<int> sieve3(std::size_t size) {
  std::vector<bool> n(size, false);
  std::vector<int> r{2};

  std::size_t s = sqrt(n.size());

  for (std::size_t i = 4; i <= s; i += 2)
    n[i] = true;

  for (std::size_t i = 3; i <= s; i += 2) {
    if (n[i] == false) {
      r.emplace_back(i);
      for (std::size_t j = i * i; j < n.size(); j += i)
        n[j] = true;
    }
  }

  s += s % 2 ? 2 : 1;
  for (std::size_t i = s; i < n.size(); i += 2)
    if (n[i] == false)
      r.emplace_back(i);

  return r;
}