Optimize Block.cloneAsHeader() which was taking about 25% of the CPU during chain download on Android. The previous implementation was lazy (serialize and deserialize), the new implementation is much faster and not much more complex.

The profiles are now dominated by checking difficulty transitions.

src/com/google/bitcoin/core/Block.java

@@ -33,7 +33,7 @@ import static com.google.bitcoin.core.Utils.*;
  * over its contents. See the BitCoin technical paper for more detail on blocks.<p>
  *
  * To get a block, you can either build one from the raw bytes you can get from another implementation,
- * or request one specifically using {@link Peer#getBlock(byte[])}, or grab one from a downloaded {@link BlockChain}.
+ * or request one specifically using {@link Peer#getBlock(Sha256Hash)}, or grab one from a downloaded {@link BlockChain}.
  */
 public class Block extends Message {
     private static final Logger log = LoggerFactory.getLogger(Block.class);
@@ -49,12 +49,13 @@ public class Block extends Message {
 
     // For unit testing. If not zero, use this instead of the current time.
     static long fakeClock = 0;
+
+    // Fields defined as part of the protocol format.
     private long version;
     private Sha256Hash prevBlockHash;
     private Sha256Hash merkleRoot;
     private long time;
     private long difficultyTarget;  // "nBits"
-
     private long nonce;
 
     /** If null, it means this object holds only the headers. */
@@ -62,7 +63,7 @@ public class Block extends Message {
     /** Stores the hash of the block. If null, getHash() will recalculate it. */
     private transient Sha256Hash hash;
 
-    /** Special case constructor, used for the genesis node and unit tests. */
+    /** Special case constructor, used for the genesis node, cloneAsHeader and unit tests. */
     Block(NetworkParameters params) {
         super(params);
         // Set up a few basic things. We are not complete after this though.
@@ -169,14 +170,16 @@ public class Block extends Message {
 
     /** Returns a copy of the block, but without any transactions. */
     public Block cloneAsHeader() {
-        try {
-            Block block = new Block(params, bitcoinSerialize());
-            block.transactions = null;
-            return block;
-        } catch (ProtocolException e) {
-            // Should not be able to happen unless our state is internally inconsistent.
-            throw new RuntimeException(e);
-        }
+        Block block = new Block(params);
+        block.nonce = nonce;
+        block.prevBlockHash = prevBlockHash.clone();
+        block.merkleRoot = getMerkleRoot().clone();
+        block.version = version;
+        block.time = time;
+        block.difficultyTarget = difficultyTarget;
+        block.transactions = null;
+        block.hash = null;
+        return block;
     }
 
     /**

src/com/google/bitcoin/core/NetworkParameters.java

@@ -111,7 +111,7 @@ public class NetworkParameters implements Serializable {
         n.genesisBlock.setDifficultyTarget(0x1d07fff8L);
         n.genesisBlock.setNonce(384568319);
         String genesisHash = n.genesisBlock.getHashAsString();
-        assert genesisHash.equals("00000007199508e34a9ff81e6ec0c477a4cccff2a4767a8eee39c11db367b008");
+        assert genesisHash.equals("00000007199508e34a9ff81e6ec0c477a4cccff2a4767a8eee39c11db367b008") : genesisHash;
         return n;
     }
 

src/com/google/bitcoin/core/Sha256Hash.java

@@ -73,4 +73,9 @@ public class Sha256Hash implements Serializable {
     public byte[] getBytes() {
         return bytes;
     }
+
+    @Override
+    public Sha256Hash clone() {
+        return new Sha256Hash(bytes);
+    }
 }