I need an optimal algorithm to find the largest divisor of a number N. Preferably in C++ or C#

Question

I am currently using the following code but its very slow for large numbers

        static int divisor(int number)
        {
            int i;
                    


        

Answer 1

One of the industry standard methods for finding factors of large numbers is the Quadratic Sieve algorithm.

Have a read of this:

http://en.wikipedia.org/wiki/Quadratic_sieve

P.S. you should also consider how big your numbers are. Different factorisation algorithms tend to perform well for a certain size, and not for others. As noted in the QS wiki article, this method is generally the fastest for numbers less than about 100 decimal digits.