import java.util.List;
import java.io.FileWriter;
import java.io.BufferedWriter;
import java.nio.ByteBuffer;
import javax.smartcardio.*;

class ProxNRollReader {
  byte[] GET_UID = {(byte)0xFF, (byte)0xCA, (byte)0x00, (byte)0x00, (byte)0x00};

  public static void main(String[] args) {
    if (args.length < 1) {
      System.out.println("usage: java ProxNRollReader <file_to_output>");
      System.exit(1);
    }
    new ProxNRollReader().start(args[0]);
  }

  public void start(String fileToOutput) {
    FileWriter file;
    BufferedWriter out;
    try {
      file = new FileWriter(fileToOutput);
      out = new BufferedWriter(file);
    } catch (Exception e) {
      System.err.println("error opening file!");
      e.printStackTrace();
      return;
    }

    String currentUID = "";
    while (true) {
      try {
        TerminalFactory factory = TerminalFactory.getDefault();
        List<CardTerminal> terminals = factory.terminals().list();

        if (terminals.size() == 0) {
          System.err.println("still no readers.");
        }

        for (CardTerminal terminal: terminals) {
          if (terminal.isCardPresent()) {
            Card card = terminal.connect("*");
            CardChannel channel = card.getBasicChannel();

            String newUID = sendToChannel(GET_UID, channel);
            if (!newUID.equals("") && !newUID.equals(currentUID)) {
              System.out.println(newUID);
              out.write(newUID);
              out.newLine();
              out.flush();
            }
            currentUID = newUID;
          }
        }

        Thread.sleep(500);
      } catch (Exception e) {
        System.err.println("E: "+e);
        e.printStackTrace();
      }
    }
  }

  private String sendToChannel(byte[] cmd, CardChannel channel) {
    String res = "";
    byte[] baResp = new byte[258];
    ByteBuffer bufCmd = ByteBuffer.wrap(cmd);
    ByteBuffer bufResp = ByteBuffer.wrap(baResp);

    int output = 0;
    try {
      output = channel.transmit(bufCmd, bufResp);
    } catch (CardException ex) {
      System.err.println("Caught exception on channel");
    }

    for (int ii = 0; ii < output; ii++) {
      res += String.format("%02X", baResp[ii]);
    }

    return res;
  }
}