C++ async

#include <iostream>
#include <future>
#include <chrono>
#include <cmath>
#include <iomanip>
using namespace std;
using namespace chrono;

//1
int rest_from_10deg(int deg) {                             //returns mod( 10^{deq}, 17 )
    int rests[16] = { 1, 10, 15, 14, 4, 6, 9, 5, 16, 7, 2, 3, 13, 11, 8, 12 };
    return rests[deg % 16];
}
int long_rest(vector<int>& lnum) {
    int resrest = 0, n = lnum.size();
    for (int i = n - 1; i >= 0; i--)
        resrest = (resrest + lnum[i] * rest_from_10deg(n - i - 1)) % 17;
    return resrest;
}
int long_rest_par(vector<int>& lnum, int ibeg, int iend) {  //ibeg <= iend are indices-borders of the exact interval in the whole number lnum
    int resrest = 0, n = lnum.size();
    for (int i = iend; i >= ibeg; i--)
        resrest += lnum[i] * rest_from_10deg(n - i - 1);
    return resrest % 17;
}
int multi_thread_long_rest(vector<int>& lnum, int nthr) {
    vector<future<int>> asyncs;
    int resrest = 0, batch = lnum.size() / nthr + !(lnum.size() % nthr);
    for (int i = 0; i < nthr - 1; i++)
        asyncs.push_back(async(long_rest_par, lnum, i * batch, (i + 1) * batch - 1));
    asyncs.push_back(async(long_rest_par, lnum, (nthr - 1) * batch, lnum.size() - 1));
    for (int i = 0; i < nthr; i++)
        resrest += asyncs[i].get();
    return resrest % 17;
}