Untitled

Efficiently calculating the total number of divisors of integers in a range
typedef unsigned long long ull;
void countDivisors(){
    ull PF_idx=0, PF=0, ans=1, N=0, power;
    for(ull i=2; i<MAX; ++i){
        if (i<SIEVE_SIZE and isPrime[i]) factors[i]=2;
        else{
        PF_idx=0;
        PF=primes[PF_idx];
        ans=1;
        N=i;
        while(N!=1 and (PF*PF<=N)){
            power = 0;
            while(N%PF==0){ N/=PF; ++power;}
            ans*=(power+1);
            PF = primes[++PF_idx];
        }
        if (N!=1) ans*=2;
        factors[i] = ans;
        }
    }
}

for (ull i = 1; i < MAX; ++i) {
    for (ull j = i; j < MAX; j += i) {
        factors[j] += i;
    }
}