Support getting specific entry number in reflog

The number specified is interpreted as relative to the
last entry in the reflog.

Change-Id: Ie4dd03370bb0d475a0e89d3015113ca98920100f

org.eclipse.jgit.test/tst/org/eclipse/jgit/storage/file/ReflogReaderTest.java

@@ -229,6 +229,42 @@ public class ReflogReaderTest extends SampleDataRepositoryTestCase {
 		assertEquals("new/work", checkout.getFromBranch());
 	}
 
+	@Test
+	public void testSpecificEntryNumber() throws Exception {
+		setupReflog("logs/refs/heads/master", twoLine);
+
+		ReflogReader reader = new ReflogReader(db, "refs/heads/master");
+		ReflogEntry e = reader.getReverseEntry(0);
+		assertEquals(
+				ObjectId.fromString("c6734895958052a9dbc396cff4459dc1a25029ab"),
+				e.getOldId());
+		assertEquals(
+				ObjectId.fromString("54794942a18a237c57a80719afed44bb78172b10"),
+				e.getNewId());
+		assertEquals("Same A U Thor", e.getWho().getName());
+		assertEquals("same.author@example.com", e.getWho().getEmailAddress());
+		assertEquals(60, e.getWho().getTimeZoneOffset());
+		assertEquals("2009-05-22T22:36:42", iso(e.getWho()));
+		assertEquals(
+				"rebase finished: refs/heads/rr/renamebranch5 onto c6e3b9fe2da0293f11eae202ec35fb343191a82d",
+				e.getComment());
+
+		e = reader.getReverseEntry(1);
+		assertEquals(
+				ObjectId.fromString("0000000000000000000000000000000000000000"),
+				e.getOldId());
+		assertEquals(
+				ObjectId.fromString("c6734895958052a9dbc396cff4459dc1a25029ab"),
+				e.getNewId());
+		assertEquals("A U Thor", e.getWho().getName());
+		assertEquals("thor@committer.au", e.getWho().getEmailAddress());
+		assertEquals(-60, e.getWho().getTimeZoneOffset());
+		assertEquals("2009-05-22T20:36:41", iso(e.getWho()));
+		assertEquals("branch: Created from rr/renamebranchv4", e.getComment());
+
+		assertNull(reader.getReverseEntry(3));
+	}
+
 	private void setupReflog(String logName, byte[] data)
 			throws FileNotFoundException, IOException {
 				File logfile = new File(db.getDirectory(), logName);

org.eclipse.jgit/resources/org/eclipse/jgit/JGitText.properties

@@ -366,7 +366,7 @@ receivingObjects=Receiving objects
 refAlreadyExists=Ref {0} already exists
 refNotResolved=Ref {0} can not be resolved
 refUpdateReturnCodeWas=RefUpdate return code was: {0}
-reflogEntryNotFound=Entry {0} not found  in reflog for ''{1}'', only {2} entries exist
+reflogEntryNotFound=Entry {0} not found  in reflog for ''{1}''
 remoteConfigHasNoURIAssociated=Remote config "{0}" has no URIs associated
 remoteDoesNotHaveSpec=Remote does not have {0} available for fetch.
 remoteDoesNotSupportSmartHTTPPush=remote does not support smart HTTP push

org.eclipse.jgit/src/org/eclipse/jgit/lib/Repository.java

@@ -630,14 +630,13 @@ public abstract class Repository {
 					JGitText.get().invalidReflogRevision, time));
 
 		ReflogReader reader = new ReflogReader(this, ref.getName());
-		List<ReflogEntry> entries = reader.getReverseEntries(number + 1);
-		if (number >= entries.size())
+		ReflogEntry entry = reader.getReverseEntry(number);
+		if (entry == null)
 			throw new RevisionSyntaxException(MessageFormat.format(
 					JGitText.get().reflogEntryNotFound,
-					Integer.valueOf(number), ref.getName(),
-					Integer.valueOf(entries.size())));
+					Integer.valueOf(number), ref.getName()));
 
-		return rw.parseCommit(entries.get(number).getNewId());
+		return rw.parseCommit(entry.getNewId());
 	}
 
 	private ObjectId resolveAbbreviation(final String revstr) throws IOException,

org.eclipse.jgit/src/org/eclipse/jgit/storage/file/ReflogReader.java

@@ -77,8 +77,7 @@ public class ReflogReader {
 	 * @throws IOException
 	 */
 	public ReflogEntry getLastEntry() throws IOException {
-		List<ReflogEntry> entries = getReverseEntries(1);
-		return entries.size() > 0 ? entries.get(0) : null;
+		return getReverseEntry(0);
 	}
 
 	/**
@@ -89,9 +88,39 @@ public class ReflogReader {
 		return getReverseEntries(Integer.MAX_VALUE);
 	}
 
+	/**
+	 * Get specific entry in the reflog relative to the last entry which is
+	 * considered entry zero.
+	 *
+	 * @param number
+	 * @return reflog entry or null if not found
+	 * @throws IOException
+	 */
+	public ReflogEntry getReverseEntry(int number) throws IOException {
+		if (number < 0)
+			throw new IllegalArgumentException();
+
+		final byte[] log;
+		try {
+			log = IO.readFully(logName);
+		} catch (FileNotFoundException e) {
+			return null;
+		}
+
+		int rs = RawParseUtils.prevLF(log, log.length);
+		int current = 0;
+		while (rs >= 0) {
+			rs = RawParseUtils.prevLF(log, rs);
+			if (number == current)
+				return new ReflogEntry(log, rs < 0 ? 0 : rs + 2);
+			current++;
+		}
+		return null;
+	}
+
 	/**
 	 * @param max
-	 *            max numer of entries to read
+	 *            max number of entries to read
 	 * @return all reflog entries in reverse order
 	 * @throws IOException
 	 */