namespace AstrogateTunnel {


    private static Socket socket;
    private static int tun_fd;

    private static string ifname;
    private static InetSocketAddress server_address;
    private static InetSocketAddress client_address;

    private static InetAddress vpn_network;
    private static InetAddress vpn_netmask;
    private static InetAddress vpn_ip;
    private static int mtu = 1500;

    public static int main(string[] args) {
        int client_port = 0;
        int server_port = 0;
        if(args.length < 6 || !int.try_parse(args[3], out client_port) || !int.try_parse(args[5], out server_port)) {
            print(@"USAGE: $(args[0]) interface client_ip client_port server_ip server_port\n");
            return -1;
        }

        try {
            server_address = new InetSocketAddress.from_string(args[4], server_port);
            client_address = new InetSocketAddress.from_string(args[2], client_port);
            ifname = args[1];

            socket = new Socket(server_address.family, SocketType.DATAGRAM, SocketProtocol.UDP);
            socket.bind(client_address, false);

            // Open tun device
            tun_fd = handle_posix_error("opening tun device", Posix.open("/dev/net/tun", Posix.O_RDWR));

            // Create tunnel device
            var request = generate_ifreq();
            handle_posix_error("creating tun interface", Linux.ioctl(tun_fd, IfTun.Tun.SETIFF, &request));

            // Register with server
            var registration_message = new uint8[] { 'R' };
            socket.connect(server_address);
            socket.send(registration_message);

            print(@"Waiting for $(server_address)...\n");
            var ack_message = new uint8[1024];
            socket.receive(ack_message);

            if(ack_message[0] != 'A') {
                print("Received invalid reply from server. Exiting.\n");
                return -2;
            }

            var mtu_dat = new uint8[2];
            if(ack_message[1] == '4') {
                vpn_network = new InetAddress.from_bytes(ack_message[2:6], SocketFamily.IPV4);
                vpn_netmask = new InetAddress.from_bytes(ack_message[6:10], SocketFamily.IPV4);
                vpn_ip = new InetAddress.from_bytes(ack_message[10:14], SocketFamily.IPV4);
                mtu_dat = ack_message[14:16];
            }
            else if(ack_message[1] == '6') {
                vpn_network = new InetAddress.from_bytes(ack_message[2:18], SocketFamily.IPV6);
                vpn_netmask = new InetAddress.from_bytes(ack_message[18:34], SocketFamily.IPV6);
                vpn_ip = new InetAddress.from_bytes(ack_message[34:50], SocketFamily.IPV6);
                mtu_dat = ack_message[50:52];
            }
            else {
                print("Received unsupported network type from server. Exiting.\n");
                return -3;
            }

            uint16 mtu_val = 0;
            Memory.copy(&mtu_val, mtu_dat, sizeof(uint16));
            mtu = mtu_val.to_big_endian();

            print(@"Registered with Astrogate server at $server_address:\n");
            print(@"  Network: $vpn_network\n");
            print(@"  Subnet mask: $vpn_netmask\n");
            print(@"  Allocated IP: $vpn_ip\n");
            print(@"  MTU: $mtu\n\n");
            
            request.ifr_flags = 0;
            request.ifr_mtu = mtu;
            handle_posix_error(@"setting MTU for $ifname", Linux.ioctl(socket.fd, Linux.Network.SIOCSIFMTU, &request));

            request.ifr_flags |= Linux.Network.IfFlag.UP;
            handle_posix_error(@"bringing $ifname up", Linux.ioctl(socket.fd, Linux.Network.SIOCSIFFLAGS, &request));
            
            address_to_posix(vpn_ip, &request.ifr_addr);
            handle_posix_error(@"setting IP address for $ifname", Linux.ioctl(socket.fd, Linux.Network.SIOCSIFADDR, &request));

            address_to_posix(vpn_netmask, &request.ifr_addr);
            handle_posix_error(@"setting subnet mask for $ifname", Linux.ioctl(socket.fd, Linux.Network.SIOCSIFNETMASK, &request));

            address_to_posix(vpn_network, &request.ifr_addr);
            handle_posix_error(@"setting destination network for $ifname", Linux.ioctl(socket.fd, Linux.Network.SIOCSIFDSTADDR, &request));
        }
        catch(Error e) {
            print(@"Fatal error: $(e.message)\n");
            return -999;
        }

        new Thread<int>("udp listener", listen_udp);
        return listen_tun();
    }

    private int listen_udp() {
        while(true) {
            try {
                // Receive the next datagram
                var buffer = new uint8[mtu+5];
                var size = socket.receive(buffer);
                buffer.length = (int)size;

                if(buffer[0] == 'L') {
                    link_ip(buffer);
                    continue;
                }

                else if(buffer[0] == 'U') {
                    unlink_ip(buffer);
                    continue;
                }

                else if(buffer[0] != 'D') {
                    print("Ignoring invalid datagram from server\n");
                    continue;
                }

                handle_posix_error(@"piping datagram to $ifname", (int)Posix.write(tun_fd, buffer[1:buffer.length], buffer.length));
            }
            catch(Error e) {
                print(@"Exception handling incoming tunnel server datagram: $(e.message)\n");
            }
        }
    }

    private int listen_tun() {
        while(true) {
            var buffer = new uint8[mtu+4];
            var size = Posix.read(tun_fd, buffer, buffer.length);
            if(size < 0) {
                print(@"Got $(size) from Posix.read.\n");
                continue;
            }
            buffer.length = (int)size;

            try {
                var message = new uint8[buffer.length + 1];
                message[0] = 'D';
                Memory.copy(message[1:], buffer, buffer.length);
                socket.send(message);
            }
            catch (Error e) {
                print(@"Error forwarding datagram to server: $(e.message)\n");
            }
        }
    }

    private void link_ip(uint8[] buffer) throws Error {
        var address = new InetAddress.from_bytes(buffer[1:buffer.length], vpn_ip.family);
        print(@"Linking IP $(address)\n");
        var route = build_rtentry(address);

        var res = Linux.ioctl(socket.fd, Linux.Network.SIOCADDRT, &route);
        if(res < 0 && errno != Posix.EEXIST) {
            handle_posix_error(@"adding route to $address", res);
        }
        else if(errno == Posix.EEXIST) {
            print(@"Already linked to $(address)\n");
        }
    }

    private void unlink_ip(uint8[] buffer) throws Error {
        var address = new InetAddress.from_bytes(buffer[1:buffer.length], vpn_ip.family);
        print(@"Uninking IP $(address)\n");
        var route = build_rtentry(address);
        handle_posix_error(@"adding route to $address", Linux.ioctl(socket.fd, Linux.Network.SIOCDELRT, &route));
    }

    private Linux.Network.RtEntry build_rtentry(InetAddress address) {
        var route = Linux.Network.RtEntry();
        route.rt_dev = ifname;
        address_to_posix(address, &route.rt_dst);
        address_to_posix(new InetAddress.from_string("255.255.255.255"), &route.rt_genmask);
        route.rt_flags = Linux.Network.RtFlag.UP;
        return route;
    }

    private int handle_posix_error(string task, int result) throws Error {
        if(result < 0) {
            var code = Posix.errno;
            var message = Posix.strerror(code);
            throw new Error.literal(Quark.from_string("posix-error"), code, @"Got result $(result) while $(task): $message");
        }
        return result;
    }

    private void address_to_posix(InetAddress address, Posix.SockAddr *addr) {
        var addr_str = address.to_string();
        switch (address.family) {
            case SocketFamily.IPV4:
                var sin4 = Posix.SockAddrIn();
                sin4.sin_family = Posix.AF_INET;
                Posix.inet_pton(Posix.AF_INET, addr_str, &sin4.sin_addr.s_addr);
                Memory.copy(addr, &sin4, sizeof(Posix.SockAddr));
                break;
            case SocketFamily.IPV6:
                var sin6 = Posix.SockAddrIn();
                sin6.sin_family = Posix.AF_INET;
                Posix.inet_pton(Posix.AF_INET, addr_str, &sin6.sin_addr.s_addr);
                Memory.copy(addr, &sin6, sizeof(Posix.SockAddr));
                break;
            default:
                assert_not_reached();
        }
    }

    private Linux.Network.IfReq generate_ifreq() throws Error {
        var request = Linux.Network.IfReq();
        request.ifr_flags = IfTun.IfFlag.TUN;
        if(ifname.length > request.ifr_name.length) {
            error(@"Max length for interface name is $(request.ifr_name.length) characters, provided name is $(ifname.length) characters.");
        }
        for(int i = 0; i < ifname.length; i++){
            request.ifr_name[i] = ifname[i];
        }
        return request;
    }



}