ByteBufUtil#decodeHexDump

Motivation:
ByteBufUtil provides a hexDump method. For debugging purposes it is often useful to decode that hex dump to get the original content, but no such method exists.

Modifications:
- Add ByteBufUtil#decodeHexDump

Result:
ByteBufUtil#decodeHexDump is available to make debugging easier.

buffer/src/main/java/io/netty/buffer/ByteBufUtil.java

@@ -21,6 +21,7 @@
 import io.netty.util.Recycler;
 import io.netty.util.Recycler.Handle;
 import io.netty.util.concurrent.FastThreadLocal;
+import io.netty.util.internal.EmptyArrays;
 import io.netty.util.internal.PlatformDependent;
 import io.netty.util.internal.StringUtil;
 import io.netty.util.internal.SystemPropertyUtil;
@@ -124,6 +125,13 @@ public static String hexDump(byte[] array, int fromIndex, int length) {
         return HexUtil.hexDump(array, fromIndex, length);
     }
 
+    /**
+     * Decodes a string generated by {@link #hexDump(byte[])}
+     */
+    public static byte[] decodeHexDump(CharSequence hexDump) {
+        return HexUtil.decodeHexDump(hexDump, 0, hexDump.length());
+    }
+
     /**
      * Calculates the hash code of the specified buffer.  This method is
      * useful when implementing a new buffer type.
@@ -869,6 +877,25 @@ private static String hexDump(ByteBuf buffer, int fromIndex, int length) {
             return new String(buf);
         }
 
+        private static byte[] decodeHexDump(CharSequence hexDump, int fromIndex, int length) {
+            if (length < 0) {
+                throw new IllegalArgumentException("length: " + length);
+            }
+            if (length == 0) {
+                return EmptyArrays.EMPTY_BYTES;
+            }
+
+            int endIndex = fromIndex + length - 1;
+            byte[] bytes = new byte[length >>> 1];
+
+            for (; fromIndex < endIndex; fromIndex += 2) {
+                bytes[fromIndex >>> 1] = (byte) ((Character.digit(hexDump.charAt(fromIndex), 16) << 4) +
+                                                  Character.digit(hexDump.charAt(fromIndex + 1), 16));
+            }
+
+            return bytes;
+        }
+
         private static String hexDump(byte[] array, int fromIndex, int length) {
             if (length < 0) {
               throw new IllegalArgumentException("length: " + length);

buffer/src/test/java/io/netty/buffer/ByteBufUtilTest.java

@@ -23,12 +23,32 @@
 import java.util.Random;
 
 import static io.netty.buffer.Unpooled.unreleasableBuffer;
+import static org.junit.Assert.assertArrayEquals;
 import static org.junit.Assert.assertEquals;
 import static org.junit.Assert.assertFalse;
 import static org.junit.Assert.assertTrue;
 import static org.junit.Assert.fail;
 
 public class ByteBufUtilTest {
+    @Test
+    public void decodeHexEvenLength() {
+        decodeHex(256);
+    }
+
+    @Test
+    public void decodeHexOddLength() {
+        decodeHex(257);
+    }
+
+    private static void decodeHex(int len) {
+        byte[] b = new byte[len];
+        Random rand = new Random();
+        rand.nextBytes(b);
+        String hexDump = ByteBufUtil.hexDump(b);
+        byte[] decodedBytes = ByteBufUtil.decodeHexDump(hexDump);
+        assertArrayEquals(b, decodedBytes);
+    }
+
     @Test
     public void equalsBufferSubsections() {
         byte[] b1 = new byte[128];