Grasscutter/src/main/java/emu/grasscutter/command/commands/CoopCommand.java

package emu.grasscutter.command.commands;

import emu.grasscutter.Grasscutter;
import emu.grasscutter.command.Command;
import emu.grasscutter.command.CommandHandler;
import emu.grasscutter.game.player.Player;

import java.util.List;

@Command(label = "coop", usage = {"[<host UID>]"}, permission = "server.coop", permissionTargeted = "server.coop.others")
public final class CoopCommand implements CommandHandler {

    @Override
    public void execute(Player sender, Player targetPlayer, List<String> args) {
        Player host = sender;
        switch (args.size()) {
            case 0:  // Summon target to self
                if (sender == null) { // Console doesn't have a self to summon to
                    sendUsageMessage(sender);
                    return;
                }
                break;
            case 1:  // Summon target to argument
                try {
                    int hostId = Integer.parseInt(args.get(0));
                    host = Grasscutter.getGameServer().getPlayerByUid(hostId);
                    if (host == null) {
                        CommandHandler.sendTranslatedMessage(sender, "commands.execution.player_offline_error");
                        return;
                    }
                    break;
                } catch (NumberFormatException ignored) {
                    CommandHandler.sendTranslatedMessage(sender, "commands.generic.invalid.uid");
                    return;
                }
            default:
                sendUsageMessage(sender);
                return;
        }

        // There's no target==host check but this just places them in multiplayer in their own world which seems fine.
        if (targetPlayer.isInMultiplayer()) {
            targetPlayer.getServer().getMultiplayerSystem().leaveCoop(targetPlayer);
        }
        host.getServer().getMultiplayerSystem().applyEnterMp(targetPlayer, host.getUid());
        targetPlayer.getServer().getMultiplayerSystem().applyEnterMpReply(host, targetPlayer.getUid(), true);
        CommandHandler.sendTranslatedMessage(sender, "commands.coop.success", targetPlayer.getNickname(), host.getNickname());
    }
}