Addrinfo#ipv6_to_ipv4

static VALUE
addrinfo_ipv6_to_ipv4(VALUE self)
{
    rb_addrinfo_t *rai = get_addrinfo(self);
    struct in6_addr *addr;
    int family = ai_get_afamily(rai);
    if (family != AF_INET6) return Qnil;
    addr = &rai->addr.in6.sin6_addr;
    if (IN6_IS_ADDR_V4MAPPED(addr) || IN6_IS_ADDR_V4COMPAT(addr)) {
        struct sockaddr_in sin4;
        INIT_SOCKADDR_IN(&sin4, sizeof(sin4));
        memcpy(&sin4.sin_addr, (char*)addr + sizeof(*addr) - sizeof(sin4.sin_addr), sizeof(sin4.sin_addr));
        return rsock_addrinfo_new((struct sockaddr *)&sin4, (socklen_t)sizeof(sin4),
                                  PF_INET, rai->socktype, rai->protocol,
                                  rai->canonname, rai->inspectname);
    }
    else {
        return Qnil;
    }
}