#include <iostream>
#include <string>
#include <stdlib.h>
using namespace std;

int main()
{
    string s;
    char ss[10];
    short l, i;
    int ostatok = 0, mnojitel = 1;
    cin >> s, ss;
    while (i < s.length() / 8)
    {
        l = s.copy(ss, 8, i*8);
        ss[9] = '\0';
        ostatok = (ostatok * 100000000 + atoi(ss)) % 7;
        i++;
    }
    for (l = 0; l < s.length() - i*8; l++)
        mnojitel *= 10;
    l = s.copy(ss, s.length() - i*8, i*8);
    ss[s.length() - i*8] = '\0';
    ostatok = (ostatok * mnojitel + atoi(ss)) % 7;
    cout << ostatok << endl;

    return 0;
}

It's work on my PC and give true results for input data from "Test Cases"