Add MessageNano.messageNanoEquals().

Allows two messages to be compared directly for equality without
generating an equals method for every generated message.

(Ports CL58125010)

Change-Id: I92ab5088539d1fd722fee7b5e28a8c825926c3b6

java/src/main/java/com/google/protobuf/nano/MessageNano.java

@@ -31,6 +31,7 @@
 package com.google.protobuf.nano;
 
 import java.io.IOException;
+import java.util.Arrays;
 
 /**
  * Abstract interface implemented by Protocol Message objects.
@@ -150,6 +151,31 @@ public abstract class MessageNano {
         }
     }
 
+    /**
+     * Compares two {@code MessageNano}s and returns true if the message's are the same class and
+     * have serialized form equality (i.e. all of the field values are the same).
+     */
+    public static final boolean messageNanoEquals(MessageNano a, MessageNano b) {
+        if (a == b) {
+            return true;
+        }
+        if (a == null || b == null) {
+            return false;
+        }
+        if (a.getClass() != b.getClass()) {
+          return false;
+        }
+        final int serializedSize = a.getSerializedSize();
+        if (b.getSerializedSize() != serializedSize) {
+            return false;
+        }
+        final byte[] aByteArray = new byte[serializedSize];
+        final byte[] bByteArray = new byte[serializedSize];
+        toByteArray(a, aByteArray, 0, serializedSize);
+        toByteArray(b, bByteArray, 0, serializedSize);
+        return Arrays.equals(aByteArray, bByteArray);
+    }
+
     /**
      * Returns a string that is (mostly) compatible with ProtoBuffer's TextFormat. Note that groups
      * (which are deprecated) are not serialized with the correct field name.

java/src/test/java/com/google/protobuf/NanoTest.java

@@ -3212,6 +3212,9 @@ public class NanoTest extends TestCase {
     TestAllTypesNano a = createMessageForHashCodeEqualsTest();
     TestAllTypesNano aEquivalent = createMessageForHashCodeEqualsTest();
 
+    assertTrue(MessageNano.messageNanoEquals(a, aEquivalent));
+    assertFalse(MessageNano.messageNanoEquals(a, new TestAllTypesNano()));
+
     // Null and empty array for repeated fields equality:
     TestAllTypesNano b = createMessageForHashCodeEqualsTest();
     b.repeatedBool = null;