ScriptPattern: simplify isWitnessCommitment() and extractWitnessCommi…

…tmentHash()

It now operates on bytes, rather than chunks. This also adds a bit of JavaDoc
and a couple of tests.

core/src/main/java/org/bitcoinj/script/ScriptPattern.java

@@ -258,25 +258,23 @@ public static boolean isOpReturn(Script script) {
     /**
      * Returns whether this script matches the pattern for a segwit commitment (in an output of the coinbase
      * transaction).
+     * See <a href="https://github.com/bitcoin/bips/blob/master/bip-0141.mediawiki#commitment-structure">BIP141</a>.
      */
     public static boolean isWitnessCommitment(Script script) {
-        List<ScriptChunk> chunks = script.chunks;
-        if (chunks.size() < 2)
-            return false;
-        if (!chunks.get(0).equalsOpCode(ScriptOpCodes.OP_RETURN))
-            return false;
-        byte[] chunkData = chunks.get(1).data;
-        if (chunkData == null || chunkData.length != 36)
-            return false;
-        if (!Arrays.equals(Arrays.copyOfRange(chunkData, 0, 4), SEGWIT_COMMITMENT_HEADER))
-            return false;
-        return true;
+        byte[] bytes = script.getProgram();
+        return bytes.length >= 38
+                && bytes[0] == ScriptOpCodes.OP_RETURN
+                && bytes[1] == 36 // length byte
+                && Arrays.equals(Arrays.copyOfRange(bytes, 2, 6), SEGWIT_COMMITMENT_HEADER);
     }
 
     /**
      * Retrieves the hash from a segwit commitment (in an output of the coinbase transaction).
+     * You will want to guard calls to this method with {@link #isWitnessCommitment(Script)}.
+     * See <a href="https://github.com/bitcoin/bips/blob/master/bip-0141.mediawiki#commitment-structure">BIP141</a>.
      */
     public static Sha256Hash extractWitnessCommitmentHash(Script script) {
-        return Sha256Hash.wrap(Arrays.copyOfRange(script.chunks.get(1).data, 4, 36));
+        byte[] hash = Arrays.copyOfRange(script.getProgram(), 6, 38);
+        return Sha256Hash.wrap(hash);
     }
 }

core/src/test/java/org/bitcoinj/script/ScriptPatternTest.java

@@ -21,11 +21,14 @@
 import com.google.common.collect.Lists;
 
 import org.bitcoinj.core.ECKey;
+import org.bitcoinj.core.Sha256Hash;
+import org.bitcoinj.core.Utils;
 import org.junit.Test;
 
 import java.util.List;
 
 import static org.bitcoinj.script.ScriptOpCodes.OP_CHECKMULTISIG;
+import static org.junit.Assert.assertEquals;
 import static org.junit.Assert.assertFalse;
 import static org.junit.Assert.assertTrue;
 
@@ -93,4 +96,55 @@ public void testCreateOpReturnScript() {
                 ScriptBuilder.createOpReturnScript(new byte[10])
         ));
     }
+
+    @Test
+    public void isWitnessCommitment() {
+        // OP_RETURN <1-byte length 36> <4-byte commitment header> <32 bytes commitment>
+        String hex = "6a24aa21a9ed0000000000000000000000000000000000000000000000000000000000000000";
+        Script script = new Script(Utils.HEX.decode(hex));
+        assertTrue(ScriptPattern.isWitnessCommitment(script));
+        assertEquals(Sha256Hash.ZERO_HASH, ScriptPattern.extractWitnessCommitmentHash(script));
+    }
+
+    @Test
+    public void isWitnessCommitment_tooShort() {
+        // OP_RETURN <1-byte length 35> <4-byte commitment header> <31 bytes commitment>
+        String hex = "6a23aa21a9ed00000000000000000000000000000000000000000000000000000000000000";
+        Script script = new Script(Utils.HEX.decode(hex));
+        assertFalse(ScriptPattern.isWitnessCommitment(script));
+    }
+
+    @Test
+    public void isWitnessCommitment_tooLong() {
+        // OP_RETURN <1-byte length 37> <4-byte commitment header> <33 bytes commitment>
+        String hex = "6a25aa21a9ed000000000000000000000000000000000000000000000000000000000000000000";
+        Script script = new Script(Utils.HEX.decode(hex));
+        assertFalse(ScriptPattern.isWitnessCommitment(script));
+    }
+
+    @Test
+    public void isWitnessCommitment_noOpReturn() {
+        // OP_NOP <1-byte length 36> <4-byte commitment header> <32 bytes commitment>
+        String hex = "6124aa21a9ed0000000000000000000000000000000000000000000000000000000000000000";
+        Script script = new Script(Utils.HEX.decode(hex));
+        assertFalse(ScriptPattern.isWitnessCommitment(script));
+    }
+
+    @Test
+    public void isWitnessCommitment_wrongCommitmentHeader() {
+        // OP_RETURN <1-byte length 36> <4-byte commitment header> <32 bytes commitment>
+        String hex = "6a24ffffffff0000000000000000000000000000000000000000000000000000000000000000";
+        Script script = new Script(Utils.HEX.decode(hex));
+        assertFalse(ScriptPattern.isWitnessCommitment(script));
+    }
+
+    @Test
+    public void extractWitnessCommitmentHash() {
+        // OP_RETURN <1-byte length 36> <4-byte commitment header> <32 bytes commitment>
+        String hex = "6a24aa21a9ed0000000000000000000000000000000000000000000000000000000000000000";
+        Script script = new Script(Utils.HEX.decode(hex));
+        Sha256Hash hash = ScriptPattern.extractWitnessCommitmentHash(script);
+        assertEquals("0000000000000000000000000000000000000000000000000000000000000000",
+                Utils.HEX.encode(hash.getBytes()));
+    }
 }