import java.io.IOException;
import java.net.InetSocketAddress;
import java.net.ServerSocket;
import java.nio.ByteBuffer;
import java.nio.channels.SelectionKey;
import java.nio.channels.Selector;
import java.nio.channels.ServerSocketChannel;
import java.nio.channels.SocketChannel;
import java.util.Iterator;
import java.util.Set;


public class Server {
 
    public static void main(String[] args) throws IOException {
        // Open the selector
        Selector selector = Selector.open();
        // Create our input channel
        ServerSocketChannel serverSocket = ServerSocketChannel.open();
        serverSocket.bind(new InetSocketAddress("localhost", 5001));
        serverSocket.configureBlocking(false);
        // Register our channel to the selector
        serverSocket.register(selector, SelectionKey.OP_ACCEPT);
 
        // Loop on selector
        while (true) {
            System.out.println("Listening for new connections or new messages.");
            // Block here
            selector.select();
            System.out.println("\tActivity on selector!");
           
            // Key(s) are ready
            Set<SelectionKey> selectedKeys = selector.selectedKeys();
            // Loop over ready keys
            Iterator<SelectionKey> iter = selectedKeys.iterator();
            while (iter.hasNext()) {
                // Grab current key
                SelectionKey key = iter.next();
                
                // Optional
                if(key.isValid() == false) { 
                    continue; 
                }

                // New connection on serverSocket
                if (key.isAcceptable()) {
                    register(selector, serverSocket);
                }
 
                // Previous connection has data to read
                if (key.isReadable()) {
                    readAndRespond(key);
                }

                // Remove it from our set
                iter.remove();
            }
        }
    }

    private static void register(Selector selector, ServerSocketChannel serverSocket) throws IOException {
        // Grab the incoming socket from the serverSocket
        SocketChannel client = serverSocket.accept();
        // Configure it to be a new channel and key that our selector should monitor
        client.configureBlocking(false);
        client.register(selector, SelectionKey.OP_READ);
        System.out.println("\t\tNew client registered.");
    }
 
    private static void readAndRespond(SelectionKey key) throws IOException {
        // Create a buffer to read into
        ByteBuffer buffer = ByteBuffer.allocate(256);

        // Grab the socket from the key 
        SocketChannel client = (SocketChannel) key.channel();
        // Read from it
        int bytesRead = client.read(buffer);
        //Handle a closed connection
        if (bytesRead == -1) {
            client.close();
            System.out.println("\t\tClient disconnected.");
        } else {
            // Return their message to them
            System.out.println("\t\tReceived: " + new String(buffer.array()));
            // Flip the buffer to now write
            buffer.flip();
            client.write(buffer);
            // Clear the buffer
            buffer.clear();
        }
                                
    }
 
    
}