prime.cpp

constexpr Sieve of Eratosthenes example prime.cpp

// prime calculation based on https://gist.github.com/rongjiecomputer/d52f34d27a21b8c9c9e82ca85b806640
// add increase limits or it wont compile for sieveSize = 1000000 -fconstexpr-loop-limit=2000000 -fconstexpr-ops-limit=335544320
// use debug build.
// release build will optimize most of the loop away.

#include <chrono>
#include <iostream>
#include <numeric>
#include <span>
#include <stdio.h>
using namespace std::chrono;

constexpr auto sieveSize = 1000000;
struct PrimeTable
{
  constexpr
  PrimeTable ()
      : sieve ()
  {
    sieve[0] = sieve[1] = false;
    for (size_t i = 2; i < sieveSize; i++)
      sieve[i] = true;
    for (size_t i = 2; i < sieveSize; i++)
      {
        if (sieve[i])
          for (size_t j = i * i; j < sieveSize; j += 2*i)
            sieve[j] = false;
      }
  }
  bool sieve[sieveSize];
};

int
main ()
{
  auto passes = 0;
  auto tStart = steady_clock::now ();
  constexpr auto tableCheck = PrimeTable{};
  auto tableSpan = std::span{ tableCheck.sieve };
  std::cout << "prim count: " << std::count (tableSpan.begin (), tableSpan.end (), true) << std::endl;
  std::cout << "acutal primes : " << 78498 << std::endl;
  while (duration_cast<seconds> (steady_clock::now () - tStart).count () < 5)
    {
      constexpr auto table = PrimeTable{};
      if (table.sieve[0] == false) passes++;
    }
  std::cout << "passes: " << passes << std::endl;
  std::cout << "duration: " << duration_cast<seconds> (steady_clock::now () - tStart).count () << std::endl;
}