Implement ReverseDwordBytes. Resolves issue 100

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

@@ -16,17 +16,16 @@
 
 package com.google.bitcoin.core;
 
-import org.bouncycastle.crypto.digests.RIPEMD160Digest;
-
 import java.io.IOException;
 import java.io.OutputStream;
 import java.math.BigDecimal;
 import java.math.BigInteger;
 import java.security.MessageDigest;
 import java.security.NoSuchAlgorithmException;
-import java.util.Arrays;
 import java.util.Date;
 
+import org.bouncycastle.crypto.digests.RIPEMD160Digest;
+
 /**
  * A collection of various utility methods that are helpful for working with the BitCoin protocol.
  * To enable debug logging from the library, run with -Dbitcoinj.logging=true on your command line.
@@ -189,6 +188,27 @@ public class Utils {
             buf[i] = bytes[bytes.length - 1 - i];
         return buf;
     }
+    
+    /**
+     * Returns a copy of the given byte array with the bytes of each double-word (4 bytes) reversed.
+     * 
+     * @param bytes length must be divisible by 4.
+     * @param trimLength trim output to this length.  If positive, must be divisible by 4.
+     */
+    public static byte[] reverseDwordBytes(byte[] bytes, int trimLength) {
+        assert bytes.length % 4 == 0;
+        assert trimLength < 0 || trimLength % 4 == 0;
+        
+        byte[] rev = new byte[trimLength >= 0 && bytes.length > trimLength ? trimLength : bytes.length];
+        
+        for (int i = 0; i < rev.length; i += 4) {
+            System.arraycopy(bytes, i, rev, i , 4);
+            for (int j = 0; j < 4; j++) {
+                rev[i + j] = bytes[i + 3 - j];
+            }
+        }
+        return rev;
+}
 
     public static long readUint32(byte[] bytes, int offset) {
         return ((bytes[offset++] & 0xFFL) << 0) |

tests/com/google/bitcoin/core/UtilsTest.java

@@ -21,6 +21,7 @@ import static junit.framework.Assert.fail;
 
 import static com.google.bitcoin.core.Utils.*;
 
+import org.junit.Assert;
 import org.junit.Test;
 
 public class UtilsTest {
@@ -51,4 +52,17 @@ public class UtilsTest {
         assertEquals("1.23", bitcoinValueToFriendlyString(toNanoCoins(1, 23)));
         assertEquals("-1.23", bitcoinValueToFriendlyString(toNanoCoins(1, 23).negate()));
     }
+    
+    @Test
+    public void testReverseBytes() {
+        Assert.assertArrayEquals(new byte[] {1,2,3,4,5}, Utils.reverseBytes(new byte[] {5,4,3,2,1}));
+    }
+
+    @Test
+    public void testReverseDwordBytes() {
+        Assert.assertArrayEquals(new byte[] {1,2,3,4,5,6,7,8}, Utils.reverseDwordBytes(new byte[] {4,3,2,1,8,7,6,5}, -1));
+        Assert.assertArrayEquals(new byte[] {1,2,3,4}, Utils.reverseDwordBytes(new byte[] {4,3,2,1,8,7,6,5}, 4));
+        Assert.assertArrayEquals(new byte[0], Utils.reverseDwordBytes(new byte[] {4,3,2,1,8,7,6,5}, 0));
+        Assert.assertArrayEquals(new byte[0], Utils.reverseDwordBytes(new byte[0], 0));
+    }
 }