Correct AuxPoW index calculation to match unsigned integer maths used…

… by reference client.

Add unit tests for AuxPoW index calculation.

src/main/java/org/bitcoinj/core/AuxPoW.java

@@ -355,7 +355,10 @@ protected boolean checkProofOfWork(Sha256Hash hashAuxBlock,
 
         if (getChainMerkleBranch().getIndex() != getExpectedIndex(nonce, ((AuxPoWNetworkParameters) params).getChainID(), getChainMerkleBranch().size())) {
             if (throwException) {
-                throw new VerificationException("Aux POW wrong index");
+                throw new VerificationException("Aux POW wrong index in chain merkle branch for chain ID "
+                    + ((AuxPoWNetworkParameters) params).getChainID() + ". Was "
+                    + getChainMerkleBranch().getIndex() + ", expected "
+                    + getExpectedIndex(nonce, ((AuxPoWNetworkParameters) params).getChainID(), getChainMerkleBranch().size()));
             }
             return false;
         }
@@ -402,12 +405,20 @@ protected static int getExpectedIndex(final long nonce, final int chainId, final
         // Choose a pseudo-random slot in the chain merkle tree
         // but have it be fixed for a size/nonce/chain combination.
 
-        long rand = nonce;
+        // We do most of the maths with a signed 32 bit integer, as the operation is
+        // the same as the 32 unsigned integer that the reference version uses
+        int rand = (int) nonce;
         rand = rand * 1103515245 + 12345;
         rand += chainId;
         rand = rand * 1103515245 + 12345;
 
-        return (int) (rand % (1L << merkleHeight));
+        // At this point, we need to flip the value to its positive version,
+        // so we switch to a 64 bit signed integer for the last calculations
+        long longRand = rand & 0xffffffffl;
+
+        longRand %= (1 << merkleHeight);
+
+        return (int) longRand;
     }
 
     public Transaction getTransaction() {

src/test/java/org/bitcoinj/core/DogecoinBlockTest.java

@@ -122,6 +122,31 @@ public void shouldParseBlock371337() throws IOException {
         assertTrue(auxpow.checkProofOfWork(block.getHash(), block.getDifficultyTargetAsInteger(), false));
     }
 
+    /**
+     * Confirm parsing of block with a nonce value above Integer.MAX_VALUE.
+     * See https://github.com/rnicoll/libdohj/pull/7
+     * 
+     * @throws IOException 
+     */
+    @Test
+    public void shouldParseBlock748634() throws IOException {
+        byte[] payload = Util.getBytes(getClass().getResourceAsStream("dogecoin_block748634.bin"));
+        AltcoinSerializer serializer = (AltcoinSerializer)params.getDefaultSerializer();
+        final AltcoinBlock block = (AltcoinBlock)serializer.makeBlock(payload);
+        assertEquals("bd98a06391115285265c04984e8505229739f6ffa5d498929a91fbe7c281ea7b", block.getHashAsString());
+        assertEquals(0, block.getNonce());
+
+        // Check block version values
+        assertEquals(2, block.getVersion());
+        assertEquals(98, block.getChainID());
+        assertTrue(block.getVersionFlags().get(0));
+
+        final AuxPoW auxpow = block.getAuxPoW();
+        assertNotNull(auxpow);
+
+        assertTrue(auxpow.checkProofOfWork(block.getHash(), block.getDifficultyTargetAsInteger(), true));
+    }
+
     /**
      * Confirm parsing of block with a nonce value above Integer.MAX_VALUE.
      * See https://github.com/rnicoll/libdohj/issues/5