Prime number calculation fun

Question

We\'re having a bit of fun here at work. It all started with one of the guys setting up a Hackintosh and we were wondering whether it was faster than a Windows Box of (nearl

Answer 1

Here is a fast and simple solution:

• Finding primes less than 100000; 9592 were found in 5 ms
• Finding primes less than 1000000; 78498 were found in 20 ms
• Finding primes less than 10000000; 664579 were found in 143 ms
• Finding primes less than 100000000; 5761455 were found in 2024 ms

• Finding primes less than 1000000000; 50847534 were found in 23839 ms

  //returns number of primes less than n
  private static int getNumberOfPrimes(final int n)
  {
      if(n < 2) 
          return 0;
      BitSet candidates = new BitSet(n - 1);
      candidates.set(0, false);
      candidates.set(1, false);
      candidates.set(2, n);
      for(int i = 2; i < n; i++)
          if(candidates.get(i))
              for(int j = i + i; j < n; j += i)
                  if(candidates.get(j) && j % i == 0)
                      candidates.set(j, false);            
      return candidates.cardinality();
  }