Distance prime

#include<bits/stdc++.h>
#define ll long long
#define M 1000000
#define on(x) (marked[x / 64] & (1 << ((x % 64) / 2)))
#define mark(x) (marked[x / 64] |= (1 << ((x % 64) / 2)))
using namespace std;
int marked[M / 64 + 1];
vector<int>primes;
void sieve()
{
    for(int i = 3; i * i <= M; i += 2)
    {
        if(!on(i))
        {
            for(int j = i * i; j <= M; j += i + i)
                mark(j);
        }
    }
    primes.push_back(2);
    for(int i = 3; i <= M; i += 2)
    {
        if(!on(i))
            primes.push_back(i);
    }
}
vector<int> segsieve(int l, int r)
{
    vector<int>range_prime;
    bool is_prime[r - l + 1];
    for(int i = 0; i < r - l + 1; i++)
        is_prime[i] = true;
    if(l == 1)
        is_prime[0] = false;
    for(int i = 0; primes[i] * primes[i] <= r; i++)
    {
        int current_prime = primes[i];
        int base = (l / current_prime) * current_prime;
        if(base < l)
            base += current_prime;
        for(int j = base; j <= r; j += current_prime)
            is_prime[j - l] = false;
        if(base == current_prime)
            is_prime[base - l] = true;
    }
    for(int i = 0; i < r - l + 1; i++)
    {
        if(is_prime[i] == true)
            range_prime.push_back(i+l);
    }
    return range_prime;
}
int main()
{
    sieve();
    int l, u;
    while(cin >> l >> u)
    {
        vector<int>primes_in_range;
        primes_in_range = segsieve(l, u);

       int diff1 = INT_MIN, diff2 = INT_MAX;
       if(primes_in_range.size() <= 1)
            cout << "There are no adjacent primes." << endl;
       else
       {
           int a, b, c, d;
           for(int i = 1; i < primes_in_range.size(); i++)
           {
               if(primes_in_range[i] - primes_in_range[i -1] < diff2)
               {
                   b = primes_in_range[i];
                   a = primes_in_range[i - 1];
                   diff2 = primes_in_range[i] - primes_in_range[i - 1];
               }
               if(primes_in_range[i] - primes_in_range[i -1] > diff1)
               {
                   d = primes_in_range[i];
                   c = primes_in_range[i - 1];
                   diff1 = primes_in_range[i] - primes_in_range[i - 1];
               }
           }
           printf("%d,%d are closest, %d,%d are most distant.\n", a, b, c, d);
       }
    }
    return 0;
}