#include <cassert>
#include <vector>

namespace {

inline std::size_t index(unsigned p) { return p - 2; }
inline bool is_prime(const std::vector<bool> &primes, unsigned p) {
  return primes[index(p)];
}

void discard_multiples(std::vector<bool> &primes, unsigned p) {
  for (auto k = 2; index(p * k) < primes.size(); k++) {
    primes[index(p * k)] = false;
  }
}

void discard_even(std::vector<bool> &primes) { discard_multiples(primes, 2); }

std::vector<unsigned> construct_primes(const std::vector<bool> &primes) {
  std::vector<unsigned> result;

  for (auto p = 0u; p < primes.size(); p++) {
    if (primes[p]) {
      result.push_back(p + 2);
    }
  }

  return result;
}

} // namespace

// Generate an array containing every prime number between 0 and the num
// specified (inclusive)
std::vector<unsigned> prime(unsigned n) {
  if (n < 2) {
    return {};
  }

  std::vector<bool> primes(n - 1, true);
  discard_even(primes);

  for (auto p = 3u; p <= n; p += 2) {
    if (!is_prime(primes, p)) {
      continue;
    }

    discard_multiples(primes, p);
  }

  return construct_primes(primes);
}

int main() {
  assert(prime(0) == std::vector<unsigned>{});
  assert(prime(1) == std::vector<unsigned>{});
  assert(prime(2) == std::vector<unsigned>{2});
  assert((prime(23) == std::vector<unsigned>{2, 3, 5, 7, 11, 13, 17, 19, 23}));
  assert(
      (prime(30) == std::vector<unsigned>{2, 3, 5, 7, 11, 13, 17, 19, 23, 29}));

  return 0;
}