package de.measite.minidns;

import java.io.IOException;
import java.io.InputStream;
import java.io.InputStreamReader;
import java.io.LineNumberReader;
import java.lang.reflect.Method;
import java.net.DatagramPacket;
import java.net.DatagramSocket;
import java.net.InetAddress;
import java.security.NoSuchAlgorithmException;
import java.security.SecureRandom;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.HashSet;
import java.util.LinkedHashMap;
import java.util.Map.Entry;
import java.util.Random;
import java.util.logging.Level;
import java.util.logging.Logger;

import de.measite.minidns.Record.CLASS;
import de.measite.minidns.Record.TYPE;

/**
 * A minimal DNS client for SRV/A/AAAA/NS and CNAME lookups, with IDN support.
 * This circumvents the missing javax.naming package on android.
 */
public class Client {

    private static final Logger LOGGER = Logger.getLogger(Client.class.getName());

    /**
     * The internal random class for sequence generation.
     */
    protected Random random;

    /**
     * The buffer size for dns replies.
     */
    protected int bufferSize = 1500;

    /**
     * DNS timeout.
     */
    protected int timeout = 5000;

    /**
     * The internal DNS cache.
     */
    protected LinkedHashMap<Question, DNSMessage> cache;

    /**
     * Maximum acceptable ttl.
     */
    protected long maxTTL = 60 * 60 * 1000;

    /**
     * Create a new DNS client.
     */
    public Client() {
        try {
            random = SecureRandom.getInstance("SHA1PRNG");
        } catch (NoSuchAlgorithmException e1) {
            random = new SecureRandom();
        }
        setCacheSize(10);
    }

    /**
     * Query a nameserver for a single entry.
     * @param name The DNS name to request.
     * @param type The DNS type to request (SRV, A, AAAA, ...).
     * @param clazz The class of the request (usually IN for Internet).
     * @param host The DNS server host.
     * @param port The DNS server port.
     * @return 
     * @throws IOException On IO Errors.
     */
    public DNSMessage query(String name, TYPE type, CLASS clazz, String host, int port)
        throws IOException
    {
        Question q = new Question(name, type, clazz);
        return query(q, host, port);
    }

    /**
     * Query a nameserver for a single entry.
     * @param name The DNS name to request.
     * @param type The DNS type to request (SRV, A, AAAA, ...).
     * @param clazz The class of the request (usually IN for Internet).
     * @param host The DNS server host.
     * @return 
     * @throws IOException On IO Errors.
     */
    public DNSMessage query(String name, TYPE type, CLASS clazz, String host)
        throws IOException
    {
        Question q = new Question(name, type, clazz);
        return query(q, host);
    }

    /**
     * Query the system nameserver for a single entry.
     * @param name The DNS name to request.
     * @param type The DNS type to request (SRV, A, AAAA, ...).
     * @param clazz The class of the request (usually IN for Internet).
     * @return The DNSMessage reply or null.
     */
    public DNSMessage query(String name, TYPE type, CLASS clazz)
    {
        Question q = new Question(name, type, clazz);
        return query(q);
    }

    /**
     * Query a specific server for one entry.
     * @param q The question section of the DNS query.
     * @param host The dns server host.
     * @throws IOException On IOErrors.
     */
    public DNSMessage query(Question q, String host) throws IOException {
        return query(q, host, 53);
    }

    /**
     * Query a specific server for one entry.
     * @param q The question section of the DNS query.
     * @param host The dns server host.
     * @param port the dns port.
     * @throws IOException On IOErrors.
     */
    public DNSMessage query(Question q, String host, int port) throws IOException {
        DNSMessage dnsMessage = (cache == null) ? null : cache.get(q);
        if (dnsMessage != null && dnsMessage.getReceiveTimestamp() > 0l) {
            // check the ttl
            long ttl = maxTTL;
            for (Record r : dnsMessage.getAnswers()) {
                ttl = Math.min(ttl, r.ttl);
            }
            for (Record r : dnsMessage.getAdditionalResourceRecords()) {
                ttl = Math.min(ttl, r.ttl);
            }
            if (dnsMessage.getReceiveTimestamp() + ttl <
                System.currentTimeMillis()) {
                return dnsMessage;
            }
        }
        DNSMessage message = new DNSMessage();
        message.setQuestions(new Question[]{q});
        message.setRecursionDesired(true);
        message.setId(random.nextInt());
        byte[] buf = message.toArray();
        try (DatagramSocket socket = new DatagramSocket()) {
            DatagramPacket packet = new DatagramPacket(buf, buf.length,
                    InetAddress.getByName(host), port);
            socket.setSoTimeout(timeout);
            socket.send(packet);
            packet = new DatagramPacket(new byte[bufferSize], bufferSize);
            socket.receive(packet);
            dnsMessage = DNSMessage.parse(packet.getData());
            if (dnsMessage.getId() != message.getId()) {
                return null;
            }
            for (Record record : dnsMessage.getAnswers()) {
                if (record.isAnswer(q)) {
                    if (cache != null) {
                        cache.put(q, dnsMessage);
                    }
                    break;
                }
            }
            return dnsMessage;
        }
    }

    /**
     * Query the system DNS server for one entry.
     * @param q The question section of the DNS query.
     */
    public DNSMessage query(Question q) {
        // While this query method does in fact re-use query(Question, String)
        // we still do a cache lookup here in order to avoid unnecessary
        // findDNS()calls, which are expensive on Android. Note that we do not
        // put the results back into the Cache, as this is already done by
        // query(Question, String).
        DNSMessage message = cache.get(q);
        if (message != null) {
            return message;
        }
        String dnsServer[] = findDNS();
        for (String dns : dnsServer) {
            try {
                message = query(q, dns);
                if (message == null) {
                    continue;
                }
                if (message.getResponseCode() !=
                    DNSMessage.RESPONSE_CODE.NO_ERROR) {
                    continue;
                }
                for (Record record: message.getAnswers()) {
                    if (record.isAnswer(q)) {
                        return message;
                    }
                }
            } catch (IOException ioe) {
                LOGGER.log(Level.FINE, "IOException in query", ioe);
            }
        }
        return null;
    }

    /**
     * Retrieve a list of currently configured DNS servers.
     * @return The server array.
     */
    public String[] findDNS() {
        String[] result = findDNSByReflection();
        if (result != null) {
            LOGGER.fine("Got DNS servers via reflection: " + Arrays.toString(result));
            return result;
        }

        result = findDNSByExec();
        if (result != null) {
            LOGGER.fine("Got DNS servers via exec: " + Arrays.toString(result));
            return result;
        }

        // fallback for ipv4 and ipv6 connectivity
        // see https://developers.google.com/speed/public-dns/docs/using
        LOGGER.fine("No DNS found? Using fallback [8.8.8.8, [2001:4860:4860::8888]]");

        return new String[]{"8.8.8.8", "[2001:4860:4860::8888]"};
    }

    /**
     * Try to retrieve the list of dns server by executing getprop.
     * @return Array of servers, or null on failure.
     */
    protected String[] findDNSByExec() {
        try {
            Process process = Runtime.getRuntime().exec("getprop");
            InputStream inputStream = process.getInputStream();
            LineNumberReader lnr = new LineNumberReader(
                new InputStreamReader(inputStream));
            String line = null;
            HashSet<String> server = new HashSet<String>(6);
            while ((line = lnr.readLine()) != null) {
                int split = line.indexOf("]: [");
                if (split == -1) {
                    continue;
                }
                String property = line.substring(1, split);
                String value = line.substring(split + 4, line.length() - 1);
                if (property.endsWith(".dns") || property.endsWith(".dns1") ||
                    property.endsWith(".dns2") || property.endsWith(".dns3") ||
                    property.endsWith(".dns4")) {

                    // normalize the address

                    InetAddress ip = InetAddress.getByName(value);

                    if (ip == null) continue;

                    value = ip.getHostAddress();

                    if (value == null) continue;
                    if (value.length() == 0) continue;

                    server.add(value);
                }
            }
            if (server.size() > 0) {
                return server.toArray(new String[server.size()]);
            }
        } catch (IOException e) {
            LOGGER.log(Level.WARNING, "Exception in findDNSByExec", e);
        }
        return null;
    }

    /**
     * Try to retrieve the list of dns server by calling SystemProperties.
     * @return Array of servers, or null on failure.
     */
    protected String[] findDNSByReflection() {
        try {
            Class<?> SystemProperties =
                    Class.forName("android.os.SystemProperties");
            Method method = SystemProperties.getMethod("get",
                    new Class[] { String.class });

            ArrayList<String> servers = new ArrayList<String>(5);

            for (String propKey : new String[] {
                    "net.dns1", "net.dns2", "net.dns3", "net.dns4"}) {

                String value = (String)method.invoke(null, propKey);

                if (value == null) continue;
                if (value.length() == 0) continue;
                if (servers.contains(value)) continue;

                InetAddress ip = InetAddress.getByName(value);

                if (ip == null) continue;

                value = ip.getHostAddress();

                if (value == null) continue;
                if (value.length() == 0) continue;
                if (servers.contains(value)) continue;

                servers.add(value);
            }

            if (servers.size() > 0) {
                return servers.toArray(new String[servers.size()]);
            }
        } catch (Exception e) {
            // we might trigger some problems this way
            LOGGER.log(Level.WARNING, "Exception in findDNSByReflection", e);
        }
        return null;
    }

    /**
     * Configure the cache size (default 10).
     * @param maximumSize The new cache size or 0 to disable.
     */
    @SuppressWarnings("serial")
    public void setCacheSize(final int maximumSize) {
        if (maximumSize == 0) {
            this.cache = null;
        } else {
            LinkedHashMap<Question,DNSMessage> old = cache;
            cache = new LinkedHashMap<Question,DNSMessage>() {
                @Override
                protected boolean removeEldestEntry(
                        Entry<Question, DNSMessage> eldest) {
                    return size() > maximumSize;
                }
            };
            if (old != null) {
                cache.putAll(old);
            }
        }
    }

    /**
     * Flush the DNS cache.
     */
    public void flushCache() {
        if (cache != null) {
            cache.clear();
        }
    }

    /**
     * Get the current maximum record ttl.
     * @return The maximum record ttl.
     */
    public long getMaxTTL() {
        return maxTTL;
    }

    /**
     * Set the maximum record ttl.
     * @param maxTTL The new maximum ttl.
     */
    public void setMaxTTL(long maxTTL) {
        this.maxTTL = maxTTL;
    }

}