Enable MagicNumber checkstyle module

Genymobile requires MagicNumber to be enabled.

However, it produces many false-positives, especially with some bitwise
operations (widely used in this project):

    x = value & 0xff;
    x = value & 0xf000;
    x = value >> 16;

Extracting 16, 0xff and 0xf000 to separate constants would be insane in
these examples.

It also consider many numbers as "magic" in tests, where we don't want
to extract them to a separate constant:

    int sum = add(12, 34);
    assertEquals(46, sum);

Therefore, handle these specific cases by disabling the check locally
through @SuppressWarnings annotations.

app/src/main/java/com/genymobile/gnirehtet/Binary.java

@@ -16,6 +16,7 @@
 
 package com.genymobile.gnirehtet;
 
+@SuppressWarnings("checkstyle:MagicNumber")
 public final class Binary {
 
     private Binary() {

app/src/main/java/com/genymobile/gnirehtet/Forwarder.java

@@ -39,6 +39,9 @@ public class Forwarder {
 
     private static final int BUFSIZE = 4096;
 
+    private static final byte[] DUMMY_ADDRESS = {42, 42, 42, 42};
+    private static final int DUMMY_PORT = 4242;
+
     private final FileDescriptor vpnFileDescriptor;
     private final Tunnel tunnel;
 
@@ -143,8 +146,8 @@ private void wakeUpReadWorkaround() {
             public void run() {
                 try {
                     DatagramSocket socket = new DatagramSocket();
-                    InetAddress addr = InetAddress.getByAddress(new byte[] {42, 42, 42, 42});
-                    DatagramPacket packet = new DatagramPacket(new byte[0], 0, addr, 4242);
+                    InetAddress dummyAddr = InetAddress.getByAddress(DUMMY_ADDRESS);
+                    DatagramPacket packet = new DatagramPacket(new byte[0], 0, dummyAddr, DUMMY_PORT);
                     socket.send(packet);
                 } catch (IOException e) {
                     // ignore

app/src/main/java/com/genymobile/gnirehtet/GnirehtetService.java

@@ -89,6 +89,7 @@ private void startVpn(VpnConfiguration config) {
         startForwarding();
     }
 
+    @SuppressWarnings("checkstyle:MagicNumber")
     private void setupVpn(VpnConfiguration config) {
         Builder builder = new Builder();
         builder.addAddress(VPN_ADDRESS, 32);
@@ -114,6 +115,7 @@ private void setupVpn(VpnConfiguration config) {
         setAsUndernlyingNetwork();
     }
 
+    @SuppressWarnings("checkstyle:MagicNumber")
     private void setAsUndernlyingNetwork() {
         if (Build.VERSION.SDK_INT >= 22) {
             Network vpnNetwork = findVpnNetwork();

app/src/main/java/com/genymobile/gnirehtet/IPPacketOutputStream.java

@@ -25,6 +25,7 @@
 /**
  * Wrapper for writing one IP packet at a time to an {@link OutputStream}.
  */
+@SuppressWarnings("checkstyle:MagicNumber")
 public class IPPacketOutputStream extends OutputStream {
 
     private static final String TAG = IPPacketOutputStream.class.getSimpleName();

app/src/main/java/com/genymobile/gnirehtet/RelayTunnelProvider.java

@@ -25,6 +25,8 @@
  */
 public class RelayTunnelProvider {
 
+    private static final int DELAY_BETWEEN_ATTEMPTS_MS = 5000;
+
     private final VpnService vpnService;
     private RelayTunnel tunnel;
     private boolean first = true;
@@ -36,8 +38,7 @@ public RelayTunnelProvider(VpnService vpnService) {
     public synchronized RelayTunnel getCurrentTunnel() throws IOException, InterruptedException {
         if (tunnel == null) {
             if (!first) {
-                // add delay between attempts
-                Thread.sleep(5000);
+                Thread.sleep(DELAY_BETWEEN_ATTEMPTS_MS);
             } else {
                 first = false;
             }

app/src/test/java/com/genymobile/gnirehtet/TestIPPacketOutputSteam.java

@@ -25,6 +25,7 @@
 import java.nio.ByteBuffer;
 import java.util.Arrays;
 
+@SuppressWarnings("checkstyle:MagicNumber")
 public class TestIPPacketOutputSteam {
 
     private ByteBuffer createMockPacket() {

config/checkstyle/checkstyle.xml

@@ -129,6 +129,11 @@ page at http://checkstyle.sourceforge.net/config.html -->
     </module>
     <module name="IllegalInstantiation" />
     <module name="InnerAssignment" />
+    <module name="MagicNumber">
+      <property name="severity" value="info" />
+      <property name="ignoreHashCodeMethod" value="true" />
+      <property name="ignoreAnnotation" value="true" />
+    </module>
     <module name="MissingSwitchDefault" />
     <module name="SimplifyBooleanExpression" />
     <module name="SimplifyBooleanReturn" />

relay/src/main/java/com/genymobile/relay/Binary.java

@@ -18,6 +18,7 @@
 
 import java.nio.ByteBuffer;
 
+@SuppressWarnings("checkstyle:MagicNumber")
 public final class Binary {
 
     private Binary() {

relay/src/main/java/com/genymobile/relay/DatagramBuffer.java

@@ -34,6 +34,7 @@
  *  head     tail
  * </pre>
  */
+@SuppressWarnings("checkstyle:MagicNumber")
 public class DatagramBuffer {
 
     private static final String TAG = DatagramBuffer.class.getSimpleName();

relay/src/main/java/com/genymobile/relay/IPv4Header.java

@@ -18,6 +18,7 @@
 
 import java.nio.ByteBuffer;
 
+@SuppressWarnings("checkstyle:MagicNumber")
 public class IPv4Header {
 
     public enum Protocol {
@@ -44,6 +45,8 @@ static Protocol fromNumber(int number) {
         }
     }
 
+    private static final int MIN_IPV4_HEADER_LENGTH = 20;
+
     private ByteBuffer raw;
     private byte version;
     private int headerLength;
@@ -53,7 +56,7 @@ static Protocol fromNumber(int number) {
     private int destination;
 
     public IPv4Header(ByteBuffer raw) {
-        assert raw.limit() >= 20 : "IPv4 headers length must be at least 20 bytes";
+        assert raw.limit() >= MIN_IPV4_HEADER_LENGTH : "IPv4 headers length must be at least 20 bytes";
         this.raw = raw;
 
         byte versionAndIHL = raw.get(0);

relay/src/main/java/com/genymobile/relay/IPv4Packet.java

@@ -23,6 +23,7 @@ public class IPv4Packet {
 
     private static final String TAG = IPv4Packet.class.getSimpleName();
 
+    @SuppressWarnings("checkstyle:MagicNumber")
     public static final int MAX_PACKET_LENGTH = 1 << 16; // packet length is stored on 16 bits
 
     private final ByteBuffer raw;

relay/src/main/java/com/genymobile/relay/IPv4PacketBuffer.java

@@ -28,6 +28,7 @@ public int readFrom(ReadableByteChannel channel) throws IOException {
         return channel.read(buffer);
     }
 
+    @SuppressWarnings("checkstyle:MagicNumber")
     private int getAvailablePacketLength() {
         int length = IPv4Header.readLength(buffer);
         assert length == -1 || IPv4Header.readVersion(buffer) == 4 : "This function must not be called when the packet is not IPv4";

relay/src/main/java/com/genymobile/relay/Net.java

@@ -49,6 +49,7 @@ public static InetAddress toInetAddress(byte[] raw) {
         }
     }
 
+    @SuppressWarnings("checkstyle:MagicNumber")
     public static InetAddress toInetAddress(int ipAddr) {
         byte[] ip = {
                 (byte) (ipAddr >>> 24),

relay/src/main/java/com/genymobile/relay/TCPHeader.java

@@ -18,6 +18,7 @@
 
 import java.nio.ByteBuffer;
 
+@SuppressWarnings("checkstyle:MagicNumber")
 public class TCPHeader implements TransportHeader {
 
     public static final int FLAG_FIN = 1 << 0;

relay/src/main/java/com/genymobile/relay/UDPHeader.java

@@ -18,6 +18,7 @@
 
 import java.nio.ByteBuffer;
 
+@SuppressWarnings("checkstyle:MagicNumber")
 public class UDPHeader implements TransportHeader {
 
     private static final int UDP_HEADER_LENGTH = 8;

relay/src/test/java/com/genymobile/relay/DatagramBufferTest.java

@@ -25,6 +25,7 @@
 import java.nio.channels.Channels;
 import java.nio.channels.WritableByteChannel;
 
+@SuppressWarnings("checkstyle:MagicNumber")
 public class DatagramBufferTest {
 
     private static ByteBuffer createDatagram(int size) {

relay/src/test/java/com/genymobile/relay/IPv4HeaderTest.java

@@ -21,6 +21,7 @@
 
 import java.nio.ByteBuffer;
 
+@SuppressWarnings("checkstyle:MagicNumber")
 public class IPv4HeaderTest {
 
     @Test

relay/src/test/java/com/genymobile/relay/IPv4PacketBufferTest.java

@@ -25,6 +25,7 @@
 import java.nio.channels.Channels;
 import java.nio.channels.ReadableByteChannel;
 
+@SuppressWarnings("checkstyle:MagicNumber")
 public class IPv4PacketBufferTest {
 
     private static ByteBuffer createMockPacket() {

relay/src/test/java/com/genymobile/relay/IPv4PacketTest.java

@@ -21,6 +21,7 @@
 
 import java.nio.ByteBuffer;
 
+@SuppressWarnings("checkstyle:MagicNumber")
 public class IPv4PacketTest {
 
     private static ByteBuffer createMockPacket() {

relay/src/test/java/com/genymobile/relay/InetAddressTest.java

@@ -21,6 +21,7 @@
 
 import java.net.InetAddress;
 
+@SuppressWarnings("checkstyle:MagicNumber")
 public class InetAddressTest {
 
     @Test

relay/src/test/java/com/genymobile/relay/PacketizerTest.java

@@ -25,6 +25,7 @@
 import java.nio.channels.Channels;
 import java.nio.channels.ReadableByteChannel;
 
+@SuppressWarnings("checkstyle:MagicNumber")
 public class PacketizerTest {
 
     private static ByteBuffer createMockPacket() {

relay/src/test/java/com/genymobile/relay/StreamBufferTest.java

@@ -26,6 +26,7 @@
 import java.nio.channels.Channels;
 import java.nio.channels.WritableByteChannel;
 
+@SuppressWarnings("checkstyle:MagicNumber")
 public class StreamBufferTest {
 
     private static ByteBuffer createChunk() {

relay/src/test/java/com/genymobile/relay/TCPHeaderTest.java

@@ -21,6 +21,7 @@
 
 import java.nio.ByteBuffer;
 
+@SuppressWarnings("checkstyle:MagicNumber")
 public class TCPHeaderTest {
 
     private static ByteBuffer createMockPacket() {

relay/src/test/java/com/genymobile/relay/UDPHeaderTest.java

@@ -21,6 +21,7 @@
 
 import java.nio.ByteBuffer;
 
+@SuppressWarnings("checkstyle:MagicNumber")
 public class UDPHeaderTest {
 
     private static ByteBuffer createMockHeaders() {