static char string_compare(const char *string_1, const char *string_2) // compare 2 strings case insensitive
{
    int val;
    for (;; string_1++, string_2++)
    {
        val = tolower(*string_1) - tolower(*string_2); // check which character is bigger
        if (val != 0 || (*string_1 == '\0')) // stop when value is !0 or when string_1 is finished
        {
            return val;
        }
    }
}