Transaction: Add methods for calculating transaction weight and virtual transaction size.

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

@@ -307,6 +307,29 @@ public class Transaction extends ChildMessage {
         return cachedWTxId;
     }
 
+    /** Gets the transaction weight as defined in BIP141. */
+    public int getWeight() {
+        if (!hasWitnesses())
+            return getMessageSize() * 4;
+        try (final ByteArrayOutputStream stream = new UnsafeByteArrayOutputStream(length)) {
+            bitcoinSerializeToStream(stream, false);
+            final int baseSize = stream.size();
+            stream.reset();
+            bitcoinSerializeToStream(stream, true);
+            final int totalSize = stream.size();
+            return baseSize * 3 + totalSize;
+        } catch (IOException e) {
+            throw new RuntimeException(e); // cannot happen
+        }
+    }
+
+    /** Gets the virtual transaction size as defined in BIP141. */
+    public int getVsize() {
+        if (!hasWitnesses())
+            return getMessageSize();
+        return (getWeight() + 3) / 4; // round up
+    }
+
     /**
      * Gets the sum of the inputs, regardless of who owns them.
      */
@@ -863,8 +886,13 @@ public class Transaction extends ChildMessage {
         final Coin fee = getFee();
         if (fee != null) {
             final int size = unsafeBitcoinSerialize().length;
-            s.append(indent).append("   fee  ").append(fee.multiply(1000).divide(size).toFriendlyString()).append("/kB, ")
+            final int vsize = getVsize();
-                    .append(fee.toFriendlyString()).append(" for ").append(size).append(" bytes\n");
+            s.append(indent).append("   fee  ").append(fee.multiply(1000).divide(size).toFriendlyString())
+                    .append("/kB – ");
+            s.append(fee.toFriendlyString()).append(" for ");
+            if (size != vsize)
+                s.append(vsize).append(" virtual bytes, ");
+            s.append(size).append(" bytes\n");
         }
         return s.toString();
     }

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

@@ -735,4 +735,14 @@ public class TransactionTest {
             uint32ToByteStreamLE(getLockTime(), stream);
         }
     }
+
+    @Test
+    public void getWeightAndVsize() {
+        // example from https://en.bitcoin.it/wiki/Weight_units
+        String txHex = "0100000000010115e180dc28a2327e687facc33f10f2a20da717e5548406f7ae8b4c811072f85603000000171600141d7cd6c75c2e86f4cbf98eaed221b30bd9a0b928ffffffff019caef505000000001976a9141d7cd6c75c2e86f4cbf98eaed221b30bd9a0b92888ac02483045022100f764287d3e99b1474da9bec7f7ed236d6c81e793b20c4b5aa1f3051b9a7daa63022016a198031d5554dbb855bdbe8534776a4be6958bd8d530dc001c32b828f6f0ab0121038262a6c6cec93c2d3ecd6c6072efea86d02ff8e3328bbd0242b20af3425990ac00000000";
+        Transaction tx = new Transaction(UNITTEST, HEX.decode(txHex));
+        assertEquals(218, tx.getMessageSize());
+        assertEquals(542, tx.getWeight());
+        assertEquals(136, tx.getVsize());
+    }
 }