Implement block work calculations, add a test. Small internal refactoring of BlockChain in preparation for bigger changes.

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

@@ -27,10 +27,10 @@ import static com.google.bitcoin.core.Utils.*;
 /**
  * A block is the foundation of the BitCoin system. It records a set of {@link Transaction}s together with
  * some data that links it into a place in the global block chain, and proves that a difficult calculation was done
- * over its contents. See the BitCoin technical paper for more detail on blocks.
+ * 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 more likely
- * you grab it from a downloaded {@link BlockChain}.
+ * 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}.
  */
 public class Block extends Message {
     private static final long serialVersionUID = 2738848929966035281L;
@@ -142,6 +142,21 @@ public class Block extends Message {
         return hash;
     }
 
+    /** The number that is one greater than the largest representable SHA-256 hash. */
+    static private BigInteger LARGEST_HASH = BigInteger.ONE.shiftLeft(256);
+
+    /**
+     * Returns the work represented by this block.<p>
+     *
+     * Work is defined as the number of tries needed to solve a block in the average case. Consider a difficulty
+     * target that covers 5% of all possible hash values. Then the work of the block will be 20. As the target gets
+     * lower, the amount of work goes up.
+     */
+    public BigInteger getWork() throws VerificationException {
+        BigInteger target = getDifficultyTargetAsInteger();
+        return LARGEST_HASH.divide(target.add(BigInteger.ONE));
+    }
+
     /**
      * Returns a multi-line string containing a description of the contents of the block. Use for debugging purposes
      * only.
@@ -183,6 +198,18 @@ public class Block extends Message {
         }
     }
 
+    /**
+     * Returns the difficulty target as a 256 bit value that can be compared to a SHA-256 hash. Inside a block the
+     * target is represented using a compact form. If this form decodes to a value that is out of bounds,
+     * an exception is thrown.
+     */
+    public BigInteger getDifficultyTargetAsInteger() throws VerificationException {
+        BigInteger target = Utils.decodeCompactBits(difficultyTarget);
+        if (target.compareTo(BigInteger.valueOf(0)) <= 0 || target.compareTo(params.proofOfWorkLimit) > 0)
+            throw new VerificationException("Difficulty target is bad");
+        return target;
+    }
+
     /** Returns true if the hash of the block is OK (lower than difficulty target). */
     private boolean checkProofOfWork(boolean throwException) throws VerificationException {
         // This part is key - it is what proves the block was as difficult to make as it claims
@@ -193,10 +220,7 @@ public class Block extends Message {
         //
         // To prevent this attack from being possible, elsewhere we check that the difficultyTarget
         // field is of the right value. This requires us to have the preceeding blocks.
-        BigInteger target = Utils.decodeCompactBits(difficultyTarget);
-        
-        if (target.compareTo(BigInteger.valueOf(0)) <= 0 || target.compareTo(params.proofOfWorkLimit) > 0)
-            throw new VerificationException("Difficulty target is bad");
+        BigInteger target = getDifficultyTargetAsInteger();
 
         BigInteger h = new BigInteger(1, getHash());
         if (h.compareTo(target) > 0) {
@@ -392,13 +416,6 @@ public class Block extends Message {
         return difficultyTarget;
     }
 
-    /**
-     * Returns the difficulty target as a 256 bit value that can be compared to a SHA-256 hash.
-     */
-    public BigInteger getDifficultyTargetBI() {
-        return Utils.decodeCompactBits(getDifficultyTarget());
-    }
-
     public void setDifficultyTarget(long compactForm) {
         this.difficultyTarget = compactForm;
         this.hash = null;

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

@@ -22,7 +22,6 @@ import java.util.Arrays;
 import java.util.LinkedList;
 
 import static com.google.bitcoin.core.Utils.LOG;
-import static com.google.bitcoin.core.Utils.bytesToHexString;
 
 /**
  * A BlockChain holds a series of {@link Block} objects, links them together, and knows how to verify that the
@@ -50,10 +49,7 @@ import static com.google.bitcoin.core.Utils.bytesToHexString;
  */
 public class BlockChain {
     // This is going away.
-    private final LinkedList<Block> blockChain = new LinkedList<Block>();
-
-    /** Each chain head that we saw so far. */
-    private final ArrayList<Block> chainHeads = new ArrayList<Block>();
+    private final LinkedList<Block> blockChain;
 
     private final NetworkParameters params;
     private final Wallet wallet;
@@ -63,6 +59,7 @@ public class BlockChain {
     private final ArrayList<Block> unconnectedBlocks = new ArrayList<Block>();
 
     public BlockChain(NetworkParameters params, Wallet wallet) {
+        blockChain = new LinkedList<Block>();
         blockChain.add(params.genesisBlock);
         this.params = params;
         this.wallet = wallet;
@@ -119,7 +116,7 @@ public class BlockChain {
             unconnectedBlocks.add(block);
             return false;
         }
-        checkDifficultyTransitions(block);
+        checkDifficultyTransitions(prev, block);
         // The block is OK so let's build the rest of the chain on it.
         block.prevBlock = prev;
         blockChain.add(block);
@@ -158,14 +155,13 @@ public class BlockChain {
     static private final int TARGET_SPACING = 10 * 60;
     static private final int INTERVAL = TARGET_TIMESPAN / TARGET_SPACING;
 
-    private void checkDifficultyTransitions(Block top) throws VerificationException {
-        Block prev = blockChain.getLast();
+    private void checkDifficultyTransitions(Block prev, Block next) throws VerificationException {
         // Is this supposed to be a difficulty transition point?
         if (blockChain.size() % INTERVAL != 0) {
             // No ... so check the difficulty didn't actually change.
-            if (top.getDifficultyTarget() != prev.getDifficultyTarget())
+            if (next.getDifficultyTarget() != prev.getDifficultyTarget())
                 throw new VerificationException("Unexpected change in difficulty at height " + blockChain.size() +
-                        ": " + Long.toHexString(top.getDifficultyTarget()) + " vs " +
+                        ": " + Long.toHexString(next.getDifficultyTarget()) + " vs " +
                         Long.toHexString(prev.getDifficultyTarget()));
             return;
         }
@@ -186,8 +182,8 @@ public class BlockChain {
             newDifficulty = params.proofOfWorkLimit;
         }
 
-        int accuracyBytes = (int) (top.getDifficultyTarget() >>> 24) - 3;
-        BigInteger receivedDifficulty = top.getDifficultyTargetBI();
+        int accuracyBytes = (int) (next.getDifficultyTarget() >>> 24) - 3;
+        BigInteger receivedDifficulty = next.getDifficultyTargetAsInteger();
 
         // The calculated difficulty is to a higher precision than received, so reduce here.
         BigInteger mask = BigInteger.valueOf(0xFFFFFFL).shiftLeft(accuracyBytes * 8);