pastebin - collaborative debugging

#include <iostream>
#include <string>

using namespace std;

char generateChecksum(string s) {
    int checks = 0;
    for (int i=0; i <s.length(); i++) {
        int nr = (s[i] - 0x30);
        if (i % 3 == 0) checks += (7 * nr);
        if (i % 3 == 1) checks += (3 * nr);
        if (i % 3 == 2) checks += nr;
    }
    char ret = (checks % 10) + 0x30;
    return ret;
}

int main()
{
    string base     = "7881738905";
    string birth    = "";
    string expire   = "";

    cout    << "Geburts (yymmdd):";
    cin     >> birth;
    cout    << endl;

    cout    << "Ablauf (yymmdd):";
    cin     >> expire;
    cout    << endl;

    string subfinal = base + birth + generateChecksum(birth) + expire + generateChecksum(expire);
    string final    = base + "D << " + birth + generateChecksum(birth) + " < " + expire + generateChecksum(expire) + " <<<<<<< " + generateChecksum(subfinal);

    cout << "Personalausweisnummer:\n" << final;
}