bignum library - test2.cpp - find mersenne primes

//a bignum library written in C++, it can (in theory) calculate with 4 billion digit long numbers
//this is mostly for demonstration purposes, if you need performance, use something like GNU arbitrary precision library
//Implemented by: http://www.youtube.com/user/thecplusplusguy
//this might be an older version, I have to check
//this one find the first 50 mersenne prime (LOL, if you have enough time :)
#include <iostream>
#include <fstream>
#include "bignum.h"
#include <string>

bool is_prime(int cur)
{
    int tmp=3;
//  std::cout << cur << std::endl;
    while(tmp*tmp<cur)
    {
        if((cur%tmp)==0)
            return false;
        tmp+=2;
    }
    return true;
}

void nextprime(int& cur)
{
    if(cur==2)
    {
        cur=3;
        return;
    }
    cur+=2;
    while(!is_prime(cur))
        cur+=2;
}

int main()
{
    bignum bn(1);
    bignum tmp;
    bignum tmp2;
    int i=0;
    int j=3;
    int j2=0;
    int diff;
    int mult;
    while(i<50)
    {
        diff=j-j2;
        mult=1;
        mult<<=diff;
        bn*=mult;
        tmp=bn-1;
        tmp2=4;
        std::cout << 2 << " " << 4 << std::endl;    //manually this one
        for(int k=0;k<j-2;k++)
        {
//          std::cout << (tmp2*tmp2) << std::endl;
            tmp2=((tmp2*tmp2)-2)%tmp;
//          std::cout << bn << " " << k << " " << tmp2 << std::endl;
        }
        if(tmp2==0)
        {
            std::cout << j << " " << tmp << std::endl;
            i++;
        }
        j2=j;
        nextprime(j);

    }
    std::cout << std::endl;
    return 0;
}