Modified Dogecoin difficulty calculations to use next expected target to determine precision.

src/main/java/org/libdohj/params/AbstractDogecoinParams.java

@@ -211,7 +211,8 @@ public abstract class AbstractDogecoinParams extends NetworkParameters implement
 
         Block blockIntervalAgo = cursor.getHeader();
         long receivedTargetCompact = nextBlock.getDifficultyTarget();
-        long newTargetCompact = this.getNewDifficultyTarget(previousHeight, prev, blockIntervalAgo);
+        long newTargetCompact = this.getNewDifficultyTarget(previousHeight, prev,
+            nextBlock, blockIntervalAgo);
 
         if (newTargetCompact != receivedTargetCompact)
             throw new VerificationException("Network provided difficulty bits do not match what was calculated: " +
@@ -220,14 +221,17 @@ public abstract class AbstractDogecoinParams extends NetworkParameters implement
 
     /**
      * 
+     * @param previousHeight height of the block immediately before the retarget.
      * @param prev the block immediately before the retarget block.
+     * @param nextBlock the block the retarget happens at.
      * @param blockIntervalAgo The last retarget block.
      * @return New difficulty target as compact bytes.
      */
-    public long getNewDifficultyTarget(int previousHeight, final Block prev,
+    public long getNewDifficultyTarget(int previousHeight, final Block prev, final Block nextBlock,
         final Block blockIntervalAgo) {
         return this.getNewDifficultyTarget(previousHeight, prev.getTimeSeconds(),
-            blockIntervalAgo.getDifficultyTarget(), blockIntervalAgo.getTimeSeconds());
+            prev.getDifficultyTarget(), blockIntervalAgo.getTimeSeconds(),
+            nextBlock.getDifficultyTarget());
     }
 
     /**
@@ -236,10 +240,13 @@ public abstract class AbstractDogecoinParams extends NetworkParameters implement
      * @param previousBlockTime Time of the block immediately previous to the one we're calculating difficulty of.
      * @param lastDifficultyTarget Compact difficulty target of the last retarget block.
      * @param lastRetargetTime Time of the last difficulty retarget.
+     * @param nextDifficultyTarget The expected difficulty target of the next
+     * block, used for determining precision of the result.
      * @return New difficulty target as compact bytes.
      */
-    public long getNewDifficultyTarget(int previousHeight, long previousBlockTime,
+    protected long getNewDifficultyTarget(int previousHeight, long previousBlockTime,
-        final long lastDifficultyTarget, final long lastRetargetTime) {
+        final long lastDifficultyTarget, final long lastRetargetTime,
+        final long nextDifficultyTarget) {
         final int height = previousHeight + 1;
         final boolean digishieldAlgorithm = height >= this.getDigishieldBlockHeight();
         final int retargetTimespan = digishieldAlgorithm
@@ -287,7 +294,7 @@ public abstract class AbstractDogecoinParams extends NetworkParameters implement
             newTarget = this.getMaxTarget();
         }
 
-        int accuracyBytes = (int) (lastDifficultyTarget >>> 24) - 3;
+        int accuracyBytes = (int) (nextDifficultyTarget >>> 24) - 3;
 
         // The calculated difficulty is to a higher precision than received, so reduce here.
         BigInteger mask = BigInteger.valueOf(0xFFFFFFL).shiftLeft(accuracyBytes * 8);

src/test/java/org/libdohj/params/AbstractDogecoinParamsTest.java

@@ -18,6 +18,7 @@ package org.libdohj.params;
 import java.io.IOException;
 import org.bitcoinj.core.AltcoinBlock;
 import org.bitcoinj.core.Context;
+import org.bitcoinj.core.Sha256Hash;
 import org.bitcoinj.core.Util;
 import static org.junit.Assert.assertEquals;
 import org.junit.Before;
@@ -42,14 +43,20 @@ public class AbstractDogecoinParamsTest {
         long previousBlockTime = 1386475638; // Block 239
         long lastRetargetDifficulty = 0x1e0ffff0;
         long lastRetargetTime = 1386474927; // Block 1
-        long newDifficulty = params.getNewDifficultyTarget(previousHeight, previousBlockTime, lastRetargetDifficulty, lastRetargetTime);
+        long nextDifficulty = 0x1e00ffff;
+        long newDifficulty =
+            params.getNewDifficultyTarget(previousHeight, previousBlockTime,
+                lastRetargetDifficulty, lastRetargetTime, nextDifficulty);
         assertEquals(0x1e00ffff, newDifficulty);
 
         previousHeight = 9599;
         previousBlockTime = 1386954113;
         lastRetargetDifficulty = 0x1c1a1206;
         lastRetargetTime = 1386942008; // Block 9359
-        newDifficulty = params.getNewDifficultyTarget(previousHeight, previousBlockTime, lastRetargetDifficulty, lastRetargetTime);
+        nextDifficulty = 0x1c15ea59;
+        newDifficulty =
+            params.getNewDifficultyTarget(previousHeight, previousBlockTime,
+                lastRetargetDifficulty, lastRetargetTime, nextDifficulty);
         assertEquals(0x1c15ea59, newDifficulty);
     }
 
@@ -59,51 +66,73 @@ public class AbstractDogecoinParamsTest {
      */
     @Test
     public void shouldConstrainActualTime() {
-        int previousHeight = 719;
+        final int previousHeight = 719;
-        long previousBlockTime = 1386476362; // Block 719
+        final long previousBlockTime = 1386476362; // Block 719
-        long lastRetargetDifficulty = 0x1e00ffff;
+        final long lastRetargetDifficulty = 0x1e00ffff;
-        long lastRetargetTime = 1386475840; // Block 439
+        final long lastRetargetTime = 1386475840; // Block 479
-        long newDifficulty = params.getNewDifficultyTarget(previousHeight, previousBlockTime, lastRetargetDifficulty, lastRetargetTime);
+        final long nextDifficulty = 0x1d0ffff0; // Block 720
+        final long newDifficulty =
+            params.getNewDifficultyTarget(previousHeight, previousBlockTime,
+                lastRetargetDifficulty, lastRetargetTime, nextDifficulty);
         assertEquals(0x1d0ffff0, newDifficulty);
     }
 
     @Test
     public void shouldCalculateDigishieldDifficulty() {
-        int previousHeight = 145000;
+        final int previousHeight = 145000;
-        long previousBlockTime = 1395094679;
+        final long previousBlockTime = 1395094679;
-        long lastRetargetDifficulty = 0x1b499dfd;
+        final long lastRetargetDifficulty = 0x1b499dfd;
-        long lastRetargetTime = 1395094427;
+        final long lastRetargetTime = 1395094427;
-        long newDifficulty = params.getNewDifficultyTarget(previousHeight, previousBlockTime, lastRetargetDifficulty, lastRetargetTime);
+        final long nextDifficulty = 0x1b671062;
+        final long newDifficulty =
+            params.getNewDifficultyTarget(previousHeight, previousBlockTime,
+                lastRetargetDifficulty, lastRetargetTime, nextDifficulty);
         assertEquals(0x1b671062, newDifficulty);
     }
 
     @Test
     public void shouldCalculateDigishieldDifficultyRounding() {
         // Test case for correct rounding of modulated time
-        int previousHeight = 145001;
+        final int previousHeight = 145001;
-        long previousBlockTime = 1395094727;
+        final long previousBlockTime = 1395094727;
-        long lastRetargetDifficulty = 0x1b671062;
+        final long lastRetargetDifficulty = 0x1b671062;
-        long lastRetargetTime = 1395094679;
+        final long lastRetargetTime = 1395094679;
-        long newDifficulty = params.getNewDifficultyTarget(previousHeight, previousBlockTime, lastRetargetDifficulty, lastRetargetTime);
+        final long nextDifficulty = 0x1b6558a4;
+        final long newDifficulty =
+            params.getNewDifficultyTarget(previousHeight, previousBlockTime,
+                lastRetargetDifficulty, lastRetargetTime, nextDifficulty);
         assertEquals(0x1b6558a4, newDifficulty);
     }
 
     @Test
     public void shouldCalculateRetarget() throws IOException {
         // Do a more in-depth test for the first retarget
-        byte[] payload = Util.getBytes(getClass().getResourceAsStream("dogecoin_block239.bin"));
+        byte[] payload;
         AltcoinSerializer serializer = (AltcoinSerializer)params.getDefaultSerializer();
-        final AltcoinBlock block239 = (AltcoinBlock)serializer.makeBlock(payload);
+        final AltcoinBlock block239;
         final AltcoinBlock block479;
+        final AltcoinBlock block480;
         final AltcoinBlock block719;
+        final AltcoinBlock block720;
 
+        payload = Util.getBytes(getClass().getResourceAsStream("dogecoin_block239.bin"));
+        block239 = (AltcoinBlock)serializer.makeBlock(payload);
         payload = Util.getBytes(getClass().getResourceAsStream("dogecoin_block479.bin"));
         block479 = (AltcoinBlock)serializer.makeBlock(payload);
-
+        payload = Util.getBytes(getClass().getResourceAsStream("dogecoin_block480.bin"));
+        block480 = (AltcoinBlock)serializer.makeBlock(payload);
         payload = Util.getBytes(getClass().getResourceAsStream("dogecoin_block719.bin"));
         block719 = (AltcoinBlock)serializer.makeBlock(payload);
+        payload = Util.getBytes(getClass().getResourceAsStream("dogecoin_block720.bin"));
+        block720 = (AltcoinBlock)serializer.makeBlock(payload);
 
-        assertEquals(0x1e00ffff, params.getNewDifficultyTarget(479, block239, block479));
+        assertEquals(Sha256Hash.wrap("f9533416310fc4484cf43405a858b06afc9763ad401d267c1835d77e7d225a4e"), block239.getHash());
-        assertEquals(0x1d0ffff0, params.getNewDifficultyTarget(719, block479, block719));
+        assertEquals(Sha256Hash.wrap("ed83c923b532835f6597f70def42910aa9e06880e8a19b68f6b4a787f2b4b69f"), block479.getHash());
+        assertEquals(Sha256Hash.wrap("a0e6d1cdef02b394d31628c3281f67e8534bec74fda1a4294b58be80c3fdf3f3"), block480.getHash());
+        assertEquals(Sha256Hash.wrap("82e56e141ccfe019d475382d9a108ef86afeb297d95443dfd7250e57af805696"), block719.getHash());
+        assertEquals(Sha256Hash.wrap("6b34f1a7de1954beb0ddf100bb2b618ff0183b6ae2b4a9376721ef8e04ab3b39"), block720.getHash());
+
+        assertEquals(block480.getDifficultyTarget(), params.getNewDifficultyTarget(479, block479, block480, block239));
+        assertEquals(block720.getDifficultyTarget(), params.getNewDifficultyTarget(719, block719, block720, block479));
     }
 }