mc-server-manager/shared-utils/source/shared_utils/server_protocol.d

module shared_utils.server_protocol;

import std.socket;
import std.algorithm;
import std.conv;
import std.array;
import std.range;
import std.bitmanip;
import std.json;
import std.stdio;
import std.datetime;

const ubyte SEGMENT_BITS = 0x7F;
const ubyte CONTINUE_BIT = 0x80;

/**
 * The data describing a minecraft server's status.
 */
struct ServerStatus {
    string name;
	bool online;
	int playersOnline;
	int maxPlayers;
	string[] playerNames;
    bool startupRequested = false;

    JSONValue toJsonObject() {
        JSONValue obj = JSONValue.emptyObject;
        obj.object["name"] = JSONValue(name);
        obj.object["online"] = JSONValue(online);
        obj.object["playersOnline"] = JSONValue(playersOnline);
        obj.object["maxPlayers"] = JSONValue(maxPlayers);
        obj.object["playerNames"] = JSONValue.emptyArray;
        foreach (name; playerNames) {
            obj.object["playerNames"].array ~= JSONValue(name);
        }
        obj.object["startupRequested"] = JSONValue(startupRequested);
        return obj;
    }
}

/**
 * Attempts to fetch the current server status by connecting to the server
 * with a status request packet. In the case of communication errors, the
 * server will be reported as offline.
 * Params:
 *   name = The name of the server (only used to add to the ServerStatus struct).
 *   ipAndPort = The combined IP:PORT string used to connect to the server.
 * Returns: The server status.
 */
ServerStatus fetchStatus(string name, string ipAndPort) {
    ptrdiff_t portIdx = countUntil(ipAndPort, ":");
    if (portIdx == -1) throw new Exception("Invalid IP address. Port is required.");
    string ip = ipAndPort[0..portIdx];
    ushort port = ipAndPort[portIdx + 1 .. $].to!ushort;
    writeln("Got IP " ~ ip ~ " and port ", port);
    Address address = new InternetAddress(ip, port);
    writeln("Got address: ", address);

    Socket socket = new TcpSocket();
    try {
        writeln("Socket created.");
        socket.setOption(SocketOptionLevel.SOCKET, SocketOption.RCVTIMEO, seconds(5));
        socket.setOption(SocketOptionLevel.SOCKET, SocketOption.SNDTIMEO, seconds(5));
        socket.connect(address);
        writeln("opened socket");
    } catch (SocketException e) {
        writeln(e);
        return ServerStatus(name, false, 0, -1, []);
    }
    writeln("About to send handshake packet...");
    socket.send(getHandshakePacket(763, ip, port, 1));
    writeln("About to send status packet...");
    socket.send(getStatusPacket());
    writeln("Sent status packet request.");
    
    ResponsePacket packet = readResponse(socket);
    writeln("Received packet response", packet.data.length);
    JSONValue statusJson = readStatusResponse(packet.data);
    return ServerStatus(
        name,
        true,
        cast(int) statusJson.object["players"].object["online"].integer,
        cast(int) statusJson.object["players"].object["max"].integer,
        []
    );
}

private struct ResponsePacket {
    int packetId;
    ubyte[] data;
}

private ResponsePacket readResponse(Socket socket) {
    ubyte[4096] buffer;
    ptrdiff_t initialBytesReceived = socket.receive(buffer);
    if (initialBytesReceived == Socket.ERROR || initialBytesReceived == 0) {
        throw new Exception("Couldn't read initial server response.");
    }
    ubyte[] bytes = buffer[0..initialBytesReceived].dup;
    int length = readVarInt(bytes);
    while (bytes.length < length) {
        ptrdiff_t bytesReceived = socket.receive(buffer);
        if (bytesReceived == Socket.ERROR || bytesReceived == 0) {
            throw new Exception("Failed to read more data from server.");
        }
        bytes ~= buffer[0..bytesReceived].dup;
    }

    ResponsePacket packet;
    packet.packetId = readVarInt(bytes);
    packet.data = bytes;
    return packet;
}

private ubyte[] getHandshakePacket(int protocolVersion, string ip, ushort port, int nextState) {
    Appender!(ubyte[]) dataApp;
    writeVarInt(dataApp, 0x00);
    writeVarInt(dataApp, protocolVersion);
    writeString(dataApp, ip);
    ubyte[2] shortData = nativeToBigEndian(port);
    dataApp ~= shortData[0];
    dataApp ~= shortData[1];
    writeVarInt(dataApp, nextState);
    size_t packetLength = dataApp[].length;
    Appender!(ubyte[]) packetLengthApp;
    writeVarInt(packetLengthApp, cast(int) packetLength);
    return packetLengthApp[] ~ dataApp[];
}

private ubyte[] getStatusPacket() {
    return [1, 0];
}

private JSONValue readStatusResponse(R)(ref R r) if (isInputRange!R && is(ElementType!R == ubyte)) {
    string jsonStr = readString(r);
    return parseJSON(jsonStr);
}

private int readVarInt(R)(ref R r) if (isInputRange!R && is(ElementType!R == ubyte)) {
    int value = 0;
    int position = 0;
    ubyte currentByte;
    while (true) {
        currentByte = r.front();
        r.popFront();
        value |= (currentByte & SEGMENT_BITS) << position;
        if ((currentByte & CONTINUE_BIT) == 0) break;
        position += 7;
        if (position >= 32) throw new Exception("VarInt is too big");
    }
    return value;
}

private string readString(R)(ref R r) if (isInputRange!R && is(ElementType!R == ubyte)) {
    int length = readVarInt(r);
    Appender!(ubyte[]) app;
    for (size_t i = 0; i < length; i++) {
        if (r.empty()) {
            writefln!"Range empty after %d instead of expected %d."(i, length);
            break;
        }
        app ~= r.front();
        r.popFront();
    }
    return cast(string) app[];
}

private void writeVarInt(R)(ref R r, int value) if (isOutputRange!(R, ubyte)) {
    while (true) {
        if ((value & ~SEGMENT_BITS) == 0) {
            r.put(cast(ubyte) value);
            return;
        }
        r.put(cast(ubyte) ((value & SEGMENT_BITS) | CONTINUE_BIT));
        value >>>= 7;
    }
}

private void writeString(R)(ref R r, string s) if (isOutputRange!(R, ubyte)) {
    writeVarInt(r, cast(int) s.length);
    ubyte[] bytes = cast(ubyte[]) s;
    foreach (b; bytes) r.put(b);
}

unittest {
    void testReadWriteInt(int value, ubyte[] bytes) {
        Appender!(ubyte[]) app;
        writeVarInt(app, value);
        ubyte[] data = app[];
        assert(data == bytes);
        int readValue = readVarInt(data);
        assert(readValue == value);
    }

    testReadWriteInt(0, [0]);
    testReadWriteInt(1, [1]);
    testReadWriteInt(2, [2]);
    testReadWriteInt(127, [127]);
    testReadWriteInt(128, [128, 1]);
    testReadWriteInt(255, [255, 1]);
    testReadWriteInt(25_565, [221, 199, 1]);
    testReadWriteInt(5181, [189, 40]);
    testReadWriteInt(5178, [186, 40]);
}