Are You A Prime Number

Question

I\'ve been interested in the problem of finding a better prime number recognizer for years. I realize this is a huge area of academic research and study - my interest in this i

Answer 1

Here's my algorithm, Complexity remains O(n^0.5) but i managed to remove some expensive operations in the code...

The algorithm's slowest part is the modulus operation, i've managed to eliminate sqrt or doing i * i <= n

This way i save precious cycles...its based on the fact that sum of odd numbers is always a perfect square.

Since we are iterating over odd numbers anyway, why not exploit it? :)

int isPrime(int n)
{
    int squares = 1;
    int odd = 3;

    if( ((n & 1) == 0) || (n < 9)) return (n == 2) || ((n > 1) && (n & 1));
    else
    {
        for( ;squares <= n; odd += 2)
        {
            if( n % odd == 0) 
                return 0;
            squares+=odd;
        }
        return 1;
    }
}