content: package com.comphenix.tinyprotocol;
  
  import io.netty.channel.Channel;
  import io.netty.channel.ChannelDuplexHandler;
  import io.netty.channel.ChannelFuture;
  import io.netty.channel.ChannelHandlerContext;
  import io.netty.channel.ChannelInboundHandlerAdapter;
  import io.netty.channel.ChannelInitializer;
  import io.netty.channel.ChannelPipeline;
  import io.netty.channel.ChannelPromise;
  
  import java.lang.reflect.Field;
  import java.util.Collections;
  import java.util.List;
  import java.util.Map;
  import java.util.NoSuchElementException;
  import java.util.Set;
  import java.util.concurrent.atomic.AtomicInteger;
  import java.util.logging.Level;
  
  import org.bukkit.Bukkit;
  import org.bukkit.entity.Player;
  import org.bukkit.event.EventHandler;
  import org.bukkit.event.EventPriority;
  import org.bukkit.event.HandlerList;
  import org.bukkit.event.Listener;
  import org.bukkit.event.player.PlayerLoginEvent;
  import org.bukkit.event.server.PluginDisableEvent;
  import org.bukkit.plugin.Plugin;
  import org.bukkit.scheduler.BukkitRunnable;
  
  import com.comphenix.tinyprotocol.Reflection.FieldAccessor;
  import com.comphenix.tinyprotocol.Reflection.MethodInvoker;
  import com.google.common.collect.Lists;
  import com.google.common.collect.MapMaker;
  import com.mojang.authlib.GameProfile;
  
  /**
   * Represents a very tiny alternative to ProtocolLib.
   * <p>
   * It now supports intercepting packets during login and status ping (such as OUT_SERVER_PING)!
   * 
   * @author Kristian
   */
  public abstract class TinyProtocol {
  	private static final AtomicInteger ID = new AtomicInteger(0);
  
  	// Required Minecraft classes
  	private static final Class<?> entityPlayerClass = Reflection.getClass("{nms}.EntityPlayer", "net.minecraft.server.level.EntityPlayer");
  	private static final Class<?> playerConnectionClass = Reflection.getClass("{nms}.PlayerConnection", "net.minecraft.server.network.PlayerConnection");
  	private static final Class<?> networkManagerClass = Reflection.getClass("{nms}.NetworkManager", "net.minecraft.network.NetworkManager");
  
  	// Used in order to lookup a channel
  	private static final MethodInvoker getPlayerHandle = Reflection.getMethod("{obc}.entity.CraftPlayer", "getHandle");
  	private static final FieldAccessor<?> getConnection = Reflection.getField(entityPlayerClass, null, playerConnectionClass);
  	private static final FieldAccessor<?> getManager = Reflection.getField(playerConnectionClass, null, networkManagerClass);
  	private static final FieldAccessor<Channel> getChannel = Reflection.getField(networkManagerClass, Channel.class, 0);
  
  	// Looking up ServerConnection
  	private static final Class<Object> minecraftServerClass = Reflection.getUntypedClass("{nms}.MinecraftServer", "net.minecraft.server.MinecraftServer");
  	private static final Class<Object> serverConnectionClass = Reflection.getUntypedClass("{nms}.ServerConnection", "net.minecraft.server.network.ServerConnection");
  	private static final FieldAccessor<Object> getMinecraftServer = Reflection.getField("{obc}.CraftServer", minecraftServerClass, 0);
  	private static final FieldAccessor<Object> getServerConnection = Reflection.getField(minecraftServerClass, serverConnectionClass, 0);
  
  	// Packets we have to intercept
  	private static final Class<?> PACKET_LOGIN_IN_START = Reflection.getClass("{nms}.PacketLoginInStart", "net.minecraft.network.protocol.login.PacketLoginInStart");
  	private static final FieldAccessor<GameProfile> getGameProfile = Reflection.getField(PACKET_LOGIN_IN_START, GameProfile.class, 0);
  
  	// Speedup channel lookup
  	private Map<String, Channel> channelLookup = new MapMaker().weakValues().makeMap();
  	private Listener listener;
  
  	// Channels that have already been removed
  	private Set<Channel> uninjectedChannels = Collections.newSetFromMap(new MapMaker().weakKeys().<Channel, Boolean>makeMap());
  
  	// List of network markers
  	private List<Object> networkManagers;
  
  	// Injected channel handlers
  	private List<Channel> serverChannels = new ArrayList<>();
  	private ChannelInboundHandlerAdapter serverChannelHandler;
  	private ChannelInitializer<Channel> beginInitProtocol;
  	private ChannelInitializer<Channel> endInitProtocol;
  
  	// Current handler name
  	private String handlerName;
  
  	protected volatile boolean closed;
  	protected Plugin plugin;
  
  	/**
  	 * Construct a new instance of TinyProtocol, and start intercepting packets for all connected clients and future clients.
  	 * <p>
  	 * You can construct multiple instances per plugin.
  	 * 
  	 * @param plugin - the plugin.
  	 */
  	public TinyProtocol(final Plugin plugin) {
  		this.plugin = plugin;
  
  		// Compute handler name
  		this.handlerName = getHandlerName();
  
  		// Prepare existing players
  		registerBukkitEvents();
  
  		try {
  			registerChannelHandler();
  			registerPlayers(plugin);
  		} catch (IllegalArgumentException ex) {
  			// Damn you, late bind
  			plugin.getLogger().info("[TinyProtocol] Delaying server channel injection due to late bind.");
  
  			new BukkitRunnable() {
  				@Override
  				public void run() {
  					registerChannelHandler();
  					registerPlayers(plugin);
  					plugin.getLogger().info("[TinyProtocol] Late bind injection successful.");
  				}
  			}.runTask(plugin);
  		}
  	}
  
  	private void createServerChannelHandler() {
  		// Handle connected channels
  		endInitProtocol = new ChannelInitializer<Channel>() {
  
  			@Override
  			protected void initChannel(Channel channel) throws Exception {
  				try {
  					// This can take a while, so we need to stop the main thread from interfering
  					synchronized (networkManagers) {
  						// Stop injecting channels
  						if (!closed) {
  							channel.eventLoop().submit(() -> injectChannelInternal(channel));
  						}
  					}
  				} catch (Exception e) {
  					plugin.getLogger().log(Level.SEVERE, "Cannot inject incomming channel " + channel, e);
  				}
  			}
  
  		};
  
  		// This is executed before Minecraft's channel handler
  		beginInitProtocol = new ChannelInitializer<Channel>() {
  
  			@Override
  			protected void initChannel(Channel channel) throws Exception {
  				channel.pipeline().addLast(endInitProtocol);
  			}
  
  		};
  
  		serverChannelHandler = new ChannelInboundHandlerAdapter() {
  	
  			@Override
  			public void channelRead(ChannelHandlerContext ctx, Object msg) throws Exception {
  				Channel channel = (Channel) msg;
  
  				// Prepare to initialize ths channel
  				channel.pipeline().addFirst(beginInitProtocol);
  				ctx.fireChannelRead(msg);
  			}
  
  		};
  	}
  
  	/**
  	 * Register bukkit events.
  	 */
  	private void registerBukkitEvents() {
  		listener = new Listener() {
  
  			@EventHandler(priority = EventPriority.LOWEST)
  			public final void onPlayerLogin(PlayerLoginEvent e) {
  				if (closed)
  					return;
  
  				Channel channel = getChannel(e.getPlayer());
  
  				// Don't inject players that have been explicitly uninjected
  				if (!uninjectedChannels.contains(channel)) {
  					injectPlayer(e.getPlayer());
  				}
  			}
  
  			@EventHandler
  			public final void onPluginDisable(PluginDisableEvent e) {
  				if (e.getPlugin().equals(plugin)) {
  					close();
  				}
  			}
  
  		};
  
  		plugin.getServer().getPluginManager().registerEvents(listener, plugin);
  	}
  
  	@SuppressWarnings("unchecked")
  	private void registerChannelHandler() {
  		Object mcServer = getMinecraftServer.get(Bukkit.getServer());
  		Object serverConnection = getServerConnection.get(mcServer);
  		boolean looking = true;
  
  		try {
  			Field field = Reflection.getParameterizedField(serverConnectionClass, List.class, networkManagerClass);
  			field.setAccessible(true);
  
  			networkManagers = (List<Object>) field.get(serverConnection);
  		} catch (Exception ex) {
  			plugin.getLogger().info("Encountered an exception checking list fields" + ex);
  			MethodInvoker method = Reflection.getTypedMethod(serverConnectionClass, null, List.class, serverConnectionClass);
  
  			networkManagers = (List<Object>) method.invoke(null, serverConnection);
  		}
  
  		if (networkManagers == null) {
  			throw new IllegalArgumentException("Failed to obtain list of network managers");
  		}
  		// We need to synchronize against this list
  		createServerChannelHandler();
  
  		// Find the correct list, or implicitly throw an exception
  		for (int i = 0; looking; i++) {
  			List<Object> list = Reflection.getField(serverConnection.getClass(), List.class, i).get(serverConnection);
  
  			for (Object item : list) {
  				if (!ChannelFuture.class.isInstance(item))
  					break;
  
  				// Channel future that contains the server connection
  				Channel serverChannel = ((ChannelFuture) item).channel();
  
  				serverChannels.add(serverChannel);
  				serverChannel.pipeline().addFirst(serverChannelHandler);
  				looking = false;
  			}
  		}
  	}
  
  	private void unregisterChannelHandler() {
  		if (serverChannelHandler == null)
  			return;
  
  		for (Channel serverChannel : serverChannels) {
  			final ChannelPipeline pipeline = serverChannel.pipeline();
  
  			// Remove channel handler
  			serverChannel.eventLoop().execute(new Runnable() {
  
  				@Override
  				public void run() {
  					try {
  						pipeline.remove(serverChannelHandler);
  					} catch (NoSuchElementException e) {
  						// That's fine
  					}
  				}
  
  			});
  		}
  	}
  
  	private void registerPlayers(Plugin plugin) {
  		for (Player player : plugin.getServer().getOnlinePlayers()) {
  			injectPlayer(player);
  		}
  	}
  
  	/**
  	 * Invoked when the server is starting to send a packet to a player.
  	 * <p>
  	 * Note that this is not executed on the main thread.
  	 * 
  	 * @param receiver - the receiving player, NULL for early login/status packets.
  	 * @param channel - the channel that received the packet. Never NULL.
  	 * @param packet - the packet being sent.
  	 * @return The packet to send instead, or NULL to cancel the transmission.
  	 */
  	public Object onPacketOutAsync(Player receiver, Channel channel, Object packet) {
  		return packet;
  	}
  
  	/**
  	 * Invoked when the server has received a packet from a given player.
  	 * <p>
  	 * Use {@link Channel#remoteAddress()} to get the remote address of the client.
  	 * 
  	 * @param sender - the player that sent the packet, NULL for early login/status packets.
  	 * @param channel - channel that received the packet. Never NULL.
  	 * @param packet - the packet being received.
  	 * @return The packet to recieve instead, or NULL to cancel.
  	 */
  	public Object onPacketInAsync(Player sender, Channel channel, Object packet) {
  		return packet;
  	}
  
  	/**
  	 * Send a packet to a particular player.
  	 * <p>
  	 * Note that {@link #onPacketOutAsync(Player, Channel, Object)} will be invoked with this packet.
  	 * 
  	 * @param player - the destination player.
  	 * @param packet - the packet to send.
  	 */
  	public void sendPacket(Player player, Object packet) {
  		sendPacket(getChannel(player), packet);
  	}
  
  	/**
  	 * Send a packet to a particular client.
  	 * <p>
  	 * Note that {@link #onPacketOutAsync(Player, Channel, Object)} will be invoked with this packet.
  	 * 
  	 * @param channel - client identified by a channel.
  	 * @param packet - the packet to send.
  	 */
  	public void sendPacket(Channel channel, Object packet) {
  		channel.pipeline().writeAndFlush(packet);
  	}
  
  	/**
  	 * Pretend that a given packet has been received from a player.
  	 * <p>
  	 * Note that {@link #onPacketInAsync(Player, Channel, Object)} will be invoked with this packet.
  	 * 
  	 * @param player - the player that sent the packet.
  	 * @param packet - the packet that will be received by the server.
  	 */
  	public void receivePacket(Player player, Object packet) {
  		receivePacket(getChannel(player), packet);
  	}
  
  	/**
  	 * Pretend that a given packet has been received from a given client.
  	 * <p>
  	 * Note that {@link #onPacketInAsync(Player, Channel, Object)} will be invoked with this packet.
  	 * 
  	 * @param channel - client identified by a channel.
  	 * @param packet - the packet that will be received by the server.
  	 */
  	public void receivePacket(Channel channel, Object packet) {
  		channel.pipeline().context("encoder").fireChannelRead(packet);
  	}
  
  	/**
  	 * Retrieve the name of the channel injector, default implementation is "tiny-" + plugin name + "-" + a unique ID.
  	 * <p>
  	 * Note that this method will only be invoked once. It is no longer necessary to override this to support multiple instances.
  	 * 
  	 * @return A unique channel handler name.
  	 */
  	protected String getHandlerName() {
  		return "tiny-" + plugin.getName() + "-" + ID.incrementAndGet();
  	}
  
  	/**
  	 * Add a custom channel handler to the given player's channel pipeline, allowing us to intercept sent and received packets.
  	 * <p>
  	 * This will automatically be called when a player has logged in.
  	 * 
  	 * @param player - the player to inject.
  	 */
  	public void injectPlayer(Player player) {
  		injectChannelInternal(getChannel(player)).player = player;
  	}
  
  	/**
  	 * Add a custom channel handler to the given channel.
  	 * 
  	 * @param channel - the channel to inject.
  	 * @return The intercepted channel, or NULL if it has already been injected.
  	 */
  	public void injectChannel(Channel channel) {
  		injectChannelInternal(channel);
  	}
  
  	/**
  	 * Add a custom channel handler to the given channel.
  	 * 
  	 * @param channel - the channel to inject.
  	 * @return The packet interceptor.
  	 */
  	private PacketInterceptor injectChannelInternal(Channel channel) {
  		try {
  			PacketInterceptor interceptor = (PacketInterceptor) channel.pipeline().get(handlerName);
  
  			// Inject our packet interceptor
  			if (interceptor == null) {
  				interceptor = new PacketInterceptor();
  				channel.pipeline().addBefore("packet_handler", handlerName, interceptor);
  				uninjectedChannels.remove(channel);
  			}
  
  			return interceptor;
  		} catch (IllegalArgumentException e) {
  			// Try again
  			return (PacketInterceptor) channel.pipeline().get(handlerName);
  		}
  	}
  
  	/**
  	 * Retrieve the Netty channel associated with a player. This is cached.
  	 * 
  	 * @param player - the player.
  	 * @return The Netty channel.
  	 */
  	public Channel getChannel(Player player) {
  		Channel channel = channelLookup.get(player.getName());
  
  		// Lookup channel again
  		if (channel == null) {
  			Object connection = getConnection.get(getPlayerHandle.invoke(player));
  			Object manager = getManager.get(connection);
  
  			channelLookup.put(player.getName(), channel = getChannel.get(manager));
  		}
  
  		return channel;
  	}
  
  	/**
  	 * Uninject a specific player.
  	 * 
  	 * @param player - the injected player.
  	 */
  	public void uninjectPlayer(Player player) {
  		uninjectChannel(getChannel(player));
  	}
  
  	/**
  	 * Uninject a specific channel.
  	 * <p>
  	 * This will also disable the automatic channel injection that occurs when a player has properly logged in.
  	 * 
  	 * @param channel - the injected channel.
  	 */
  	public void uninjectChannel(final Channel channel) {
  		// No need to guard against this if we're closing
  		if (!closed) {
  			uninjectedChannels.add(channel);
  		}
  
  		// See ChannelInjector in ProtocolLib, line 590
  		channel.eventLoop().execute(new Runnable() {
  
  			@Override
  			public void run() {
  				channel.pipeline().remove(handlerName);
  			}
  
  		});
  	}
  
  	/**
  	 * Determine if the given player has been injected by TinyProtocol.
  	 * 
  	 * @param player - the player.
  	 * @return TRUE if it is, FALSE otherwise.
  	 */
  	public boolean hasInjected(Player player) {
  		return hasInjected(getChannel(player));
  	}
  
  	/**
  	 * Determine if the given channel has been injected by TinyProtocol.
  	 * 
  	 * @param channel - the channel.
  	 * @return TRUE if it is, FALSE otherwise.
  	 */
  	public boolean hasInjected(Channel channel) {
  		return channel.pipeline().get(handlerName) != null;
  	}
  
  	/**
  	 * Cease listening for packets. This is called automatically when your plugin is disabled.
  	 */
  	public final void close() {
  		if (!closed) {
  			closed = true;
  
  			// Remove our handlers
  			for (Player player : plugin.getServer().getOnlinePlayers()) {
  				uninjectPlayer(player);
  			}
  
  			// Clean up Bukkit
  			HandlerList.unregisterAll(listener);
  			unregisterChannelHandler();
  		}
  	}
  
  	/**
  	 * Channel handler that is inserted into the player's channel pipeline, allowing us to intercept sent and received packets.
  	 * 
  	 * @author Kristian
  	 */
  	private final class PacketInterceptor extends ChannelDuplexHandler {
  		// Updated by the login event
  		public volatile Player player;
  
  		@Override
  		public void channelRead(ChannelHandlerContext ctx, Object msg) throws Exception {
  			// Intercept channel
  			final Channel channel = ctx.channel();
  			handleLoginStart(channel, msg);
  
  			try {
  				msg = onPacketInAsync(player, channel, msg);
  			} catch (Exception e) {
  				plugin.getLogger().log(Level.SEVERE, "Error in onPacketInAsync().", e);
  			}
  
  			if (msg != null) {
  				super.channelRead(ctx, msg);
  			}
  		}
  
  		@Override
  		public void write(ChannelHandlerContext ctx, Object msg, ChannelPromise promise) throws Exception {
  			try {
  				msg = onPacketOutAsync(player, ctx.channel(), msg);
  			} catch (Exception e) {
  				plugin.getLogger().log(Level.SEVERE, "Error in onPacketOutAsync().", e);
  			}
  
  			if (msg != null) {
  				super.write(ctx, msg, promise);
  			}
  		}
  
  		private void handleLoginStart(Channel channel, Object packet) {
  			if (PACKET_LOGIN_IN_START.isInstance(packet)) {
  				GameProfile profile = getGameProfile.get(packet);
  				channelLookup.put(profile.getName(), channel);
  			}
  		}
  	}
  }
  