Starcraft CD-Key Validator

sckey.c

int validate(const char *key) {
    int magic = 3;
    int count = 0;
    for (char c; (c = *key); key++) {
        if (c < '0' || c > '9') continue;
        int v = c - '0';
        if (++count == 13) {
            // final character.
            return magic % 10 == v;
        } else {
            v ^= magic * 2;
            magic += v;
        }
    }
    return 0;
}

int main(int argc, char **argv) {
    if (argc == 1) {
        // self-test.
        if (!validate("0000-00000-0003")) return -1;
        if (!validate("1234-56789-0123")) return -1;
        if (!validate("1337-42069-0008")) return -1;
        if (!validate("1998-00000-1997")) return -1;
        if (!validate("3333-33333-3333")) return -1;
        return 0;
    }

    int ret = 0;
    for (int i = 1; i < argc; i++) {
        if (!validate(argv[i])) ret++;
    }
    return ret;
}