import java.util.HashMap;
import java.util.Map;

public class Arp {

    private int ip;

    private int mac;

    private Map<Integer, Integer> arptable;

    // for simplicity sake the ARP packet format is assumed to be <ip-address>-<mac-address>
    private static final String PACKET_DELIMITER = "-";

    private static final String SPOOF_ALERT_STATUS = "-1";
    private static final String IP_ALREADY_IN_USE_STATUS = "-2";
    private static final String NOT_FOUND = "-3";

    // constructor
    public Arp(int mac, int ip) {
        this.mac = mac;
        this.ip = ip;
        arptable = new HashMap<>();
        arptable.put(ip, mac);
    }

    // This function returns a spoofed ARP packet:
    // The argument passed to this function is the IP address that you want to
    // impersonate.
    public String spoofArp(int spoofIP) {
        return spoofIP + PACKET_DELIMITER + mac;
    }

    // Receive a message and provide the response. This function returns either
    // a packet, or a status code.
    public String receiveArp(String message) {
        String[] tokens = message.split(PACKET_DELIMITER);
        int receivedIp = Integer.parseInt(tokens[0].trim());
        int receivedMac = -1;
        if(tokens.length > 1) {
            receivedMac = Integer.parseInt(tokens[1].trim());
        }
        if(receivedMac == mac && receivedIp != ip) {
            return SPOOF_ALERT_STATUS;
        }

        Integer knownCachedMac = arptable.get(receivedIp);
        if(knownCachedMac != null && receivedMac != -1 && knownCachedMac != receivedMac) {
            return IP_ALREADY_IN_USE_STATUS;
        }

        if(knownCachedMac == null) {
            // cache
            if(receivedMac == -1) {
                message = NOT_FOUND;
            }
            else {
                arptable.put(receivedIp, receivedMac);
            }
        }
        else {
            message = receivedIp + PACKET_DELIMITER + knownCachedMac;
        }

        return message;
    }

    public static void main(String[] args) {
        Arp arp = new Arp(1, 1);
        String spoofPacket = arp.spoofArp(2);
        if(!spoofPacket.equals("2-1")) {
            System.out.println("Improper spoofing, expected 2-1 but received " + spoofPacket);
        }

        String receivedMessage = arp.receiveArp("6-1");
        if(!receivedMessage.equals(SPOOF_ALERT_STATUS)) {
            System.out.println("expected a spoofing alert but didn't arrive");
        }

        receivedMessage = arp.receiveArp("2-2");
        if(!receivedMessage.equals("2-2")) {
            System.out.println("expected the same message to be returned");
        }

        receivedMessage = arp.receiveArp("2-");
        if(!receivedMessage.equals("2-2")) {
            System.out.println("expected cached message to be returned");
        }

        receivedMessage = arp.receiveArp("2-5");
        if(!receivedMessage.equals(IP_ALREADY_IN_USE_STATUS)) {
            System.out.println("expected IP_ALREADY_IN_USE_STATUS");
        }
    }
}