Fully implement SHA-1 abbreviations

ObjectReader implementations are now responsible for creating the
unique abbreviation of an ObjectId, or for resolving an abbreviation
back to its full form.  In this latter case the reader can offer up
multiple candidates to the caller, who may be able to disambiguate
them based on context.

Repository.resolve() doesn't take multiple candidates into account
right now, but it could in the future by looking for a remaining
^0 or ^{commit} suffix and take an expansion if there is only one
commit that matches the input abbreviation.  It could also use
the distance from an annotated tag to resolve "tag-NNN-gcommit"
style strings that are often output by `git describe`.

Change-Id: Icd3250adc8177ae05278b858933afdca0cbbdb56
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>

org.eclipse.jgit.pgm/src/org/eclipse/jgit/pgm/AbstractFetchCommand.java

@@ -47,41 +47,47 @@
 
 package org.eclipse.jgit.pgm;
 
+import java.io.IOException;
 import java.io.PrintWriter;
 
-import org.kohsuke.args4j.Option;
 import org.eclipse.jgit.lib.Constants;
 import org.eclipse.jgit.lib.ObjectId;
+import org.eclipse.jgit.lib.ObjectReader;
 import org.eclipse.jgit.lib.RefUpdate;
 import org.eclipse.jgit.transport.FetchResult;
 import org.eclipse.jgit.transport.TrackingRefUpdate;
 import org.eclipse.jgit.transport.Transport;
+import org.kohsuke.args4j.Option;
 
 abstract class AbstractFetchCommand extends TextBuiltin {
 	@Option(name = "--verbose", aliases = { "-v" }, usage = "usage_beMoreVerbose")
 	private boolean verbose;
 
 	protected void showFetchResult(final Transport tn, final FetchResult r) {
-		boolean shownURI = false;
-		for (final TrackingRefUpdate u : r.getTrackingRefUpdates()) {
-			if (!verbose && u.getResult() == RefUpdate.Result.NO_CHANGE)
-				continue;
-
-			final char type = shortTypeOf(u.getResult());
-			final String longType = longTypeOf(u);
-			final String src = abbreviateRef(u.getRemoteName(), false);
-			final String dst = abbreviateRef(u.getLocalName(), true);
-
-			if (!shownURI) {
-				out.format(CLIText.get().fromURI, tn.getURI());
+		ObjectReader reader = db.newObjectReader();
+		try {
+			boolean shownURI = false;
+			for (final TrackingRefUpdate u : r.getTrackingRefUpdates()) {
+				if (!verbose && u.getResult() == RefUpdate.Result.NO_CHANGE)
+					continue;
+
+				final char type = shortTypeOf(u.getResult());
+				final String longType = longTypeOf(reader, u);
+				final String src = abbreviateRef(u.getRemoteName(), false);
+				final String dst = abbreviateRef(u.getLocalName(), true);
+
+				if (!shownURI) {
+					out.format(CLIText.get().fromURI, tn.getURI());
+					out.println();
+					shownURI = true;
+				}
+
+				out.format(" %c %-17s %-10s -> %s", type, longType, src, dst);
 				out.println();
-				shownURI = true;
 			}
-
-			out.format(" %c %-17s %-10s -> %s", type, longType, src, dst);
-			out.println();
+		} finally {
+			reader.release();
 		}
-
 		showRemoteMessages(r.getMessages());
 	}
 
@@ -116,7 +122,7 @@ abstract class AbstractFetchCommand extends TextBuiltin {
 		writer.flush();
 	}
 
-	private String longTypeOf(final TrackingRefUpdate u) {
+	private String longTypeOf(ObjectReader reader, final TrackingRefUpdate u) {
 		final RefUpdate.Result r = u.getResult();
 		if (r == RefUpdate.Result.LOCK_FAILURE)
 			return "[lock fail]";
@@ -136,14 +142,14 @@ abstract class AbstractFetchCommand extends TextBuiltin {
 		}
 
 		if (r == RefUpdate.Result.FORCED) {
-			final String aOld = u.getOldObjectId().abbreviate(db).name();
-			final String aNew = u.getNewObjectId().abbreviate(db).name();
+			final String aOld = safeAbbreviate(reader, u.getOldObjectId());
+			final String aNew = safeAbbreviate(reader, u.getNewObjectId());
 			return aOld + "..." + aNew;
 		}
 
 		if (r == RefUpdate.Result.FAST_FORWARD) {
-			final String aOld = u.getOldObjectId().abbreviate(db).name();
-			final String aNew = u.getNewObjectId().abbreviate(db).name();
+			final String aOld = safeAbbreviate(reader, u.getOldObjectId());
+			final String aNew = safeAbbreviate(reader, u.getNewObjectId());
 			return aOld + ".." + aNew;
 		}
 
@@ -152,6 +158,14 @@ abstract class AbstractFetchCommand extends TextBuiltin {
 		return "[" + r.name() + "]";
 	}
 
+	private String safeAbbreviate(ObjectReader reader, ObjectId id) {
+		try {
+			return reader.abbreviate(id).name();
+		} catch (IOException cannotAbbreviate) {
+			return id.name();
+		}
+	}
+
 	private static char shortTypeOf(final RefUpdate.Result r) {
 		if (r == RefUpdate.Result.LOCK_FAILURE)
 			return '!';

org.eclipse.jgit.pgm/src/org/eclipse/jgit/pgm/Branch.java

@@ -53,6 +53,7 @@ import java.util.Map.Entry;
 
 import org.eclipse.jgit.lib.Constants;
 import org.eclipse.jgit.lib.ObjectId;
+import org.eclipse.jgit.lib.ObjectReader;
 import org.eclipse.jgit.lib.Ref;
 import org.eclipse.jgit.lib.RefComparator;
 import org.eclipse.jgit.lib.RefRename;
@@ -182,9 +183,16 @@ class Branch extends TextBuiltin {
 				addRef("(no branch)", head);
 			addRefs(refs, Constants.R_HEADS, !remote);
 			addRefs(refs, Constants.R_REMOTES, remote);
-			for (final Entry<String, Ref> e : printRefs.entrySet()) {
-				final Ref ref = e.getValue();
-				printHead(e.getKey(), current.equals(ref.getName()), ref);
+
+			ObjectReader reader = db.newObjectReader();
+			try {
+				for (final Entry<String, Ref> e : printRefs.entrySet()) {
+					final Ref ref = e.getValue();
+					printHead(reader, e.getKey(),
+							current.equals(ref.getName()), ref);
+				}
+			} finally {
+				reader.release();
 			}
 		}
 	}
@@ -205,8 +213,8 @@ class Branch extends TextBuiltin {
 		maxNameLength = Math.max(maxNameLength, name.length());
 	}
 
-	private void printHead(final String ref, final boolean isCurrent,
-			final Ref refObj) throws Exception {
+	private void printHead(final ObjectReader reader, final String ref,
+			final boolean isCurrent, final Ref refObj) throws Exception {
 		out.print(isCurrent ? '*' : ' ');
 		out.print(' ');
 		out.print(ref);
@@ -214,7 +222,7 @@ class Branch extends TextBuiltin {
 			final int spaces = maxNameLength - ref.length() + 1;
 			out.format("%" + spaces + "s", "");
 			final ObjectId objectId = refObj.getObjectId();
-			out.print(objectId.abbreviate(db).name());
+			out.print(reader.abbreviate(objectId).name());
 			out.print(' ');
 			out.print(rw.parseCommit(objectId).getShortMessage());
 		}

org.eclipse.jgit.pgm/src/org/eclipse/jgit/pgm/Push.java

@@ -43,14 +43,15 @@
 
 package org.eclipse.jgit.pgm;
 
+import java.io.IOException;
 import java.text.MessageFormat;
 import java.util.ArrayList;
 import java.util.Collection;
 import java.util.List;
 
-import org.kohsuke.args4j.Argument;
-import org.kohsuke.args4j.Option;
 import org.eclipse.jgit.lib.Constants;
+import org.eclipse.jgit.lib.ObjectId;
+import org.eclipse.jgit.lib.ObjectReader;
 import org.eclipse.jgit.lib.Ref;
 import org.eclipse.jgit.lib.TextProgressMonitor;
 import org.eclipse.jgit.transport.PushResult;
@@ -59,6 +60,8 @@ import org.eclipse.jgit.transport.RemoteRefUpdate;
 import org.eclipse.jgit.transport.Transport;
 import org.eclipse.jgit.transport.URIish;
 import org.eclipse.jgit.transport.RemoteRefUpdate.Status;
+import org.kohsuke.args4j.Argument;
+import org.kohsuke.args4j.Option;
 
 @Command(common = true, usage = "usage_UpdateRemoteRepositoryFromLocalRefs")
 class Push extends TextBuiltin {
@@ -132,11 +135,17 @@ class Push extends TextBuiltin {
 			} finally {
 				transport.close();
 			}
-			printPushResult(uri, result);
+
+			ObjectReader reader = db.newObjectReader();
+			try {
+				printPushResult(reader, uri, result);
+			} finally {
+				reader.release();
+			}
 		}
 	}
 
-	private void printPushResult(final URIish uri,
+	private void printPushResult(final ObjectReader reader, final URIish uri,
 			final PushResult result) {
 		shownURI = false;
 		boolean everythingUpToDate = true;
@@ -145,7 +154,7 @@ class Push extends TextBuiltin {
 		for (final RemoteRefUpdate rru : result.getRemoteUpdates()) {
 			if (rru.getStatus() == Status.UP_TO_DATE) {
 				if (verbose)
-					printRefUpdateResult(uri, result, rru);
+					printRefUpdateResult(reader, uri, result, rru);
 			} else
 				everythingUpToDate = false;
 		}
@@ -153,14 +162,14 @@ class Push extends TextBuiltin {
 		for (final RemoteRefUpdate rru : result.getRemoteUpdates()) {
 			// ...then successful updates...
 			if (rru.getStatus() == Status.OK)
-				printRefUpdateResult(uri, result, rru);
+				printRefUpdateResult(reader, uri, result, rru);
 		}
 
 		for (final RemoteRefUpdate rru : result.getRemoteUpdates()) {
 			// ...finally, others (problematic)
 			if (rru.getStatus() != Status.OK
 					&& rru.getStatus() != Status.UP_TO_DATE)
-				printRefUpdateResult(uri, result, rru);
+				printRefUpdateResult(reader, uri, result, rru);
 		}
 
 		AbstractFetchCommand.showRemoteMessages(result.getMessages());
@@ -168,8 +177,8 @@ class Push extends TextBuiltin {
 			out.println(CLIText.get().everythingUpToDate);
 	}
 
-	private void printRefUpdateResult(final URIish uri,
-			final PushResult result, final RemoteRefUpdate rru) {
+	private void printRefUpdateResult(final ObjectReader reader,
+			final URIish uri, final PushResult result, final RemoteRefUpdate rru) {
 		if (!shownURI) {
 			shownURI = true;
 			out.println(MessageFormat.format(CLIText.get().pushTo, uri));
@@ -194,10 +203,10 @@ class Push extends TextBuiltin {
 				} else {
 					boolean fastForward = rru.isFastForward();
 					final char flag = fastForward ? ' ' : '+';
-					final String summary = oldRef.getObjectId().abbreviate(db)
-							.name()
+					final String summary = safeAbbreviate(reader, oldRef
+							.getObjectId())
 							+ (fastForward ? ".." : "...")
-							+ rru.getNewObjectId().abbreviate(db).name();
+							+ safeAbbreviate(reader, rru.getNewObjectId());
 					final String message = fastForward ? null : CLIText.get().forcedUpdate;
 					printUpdateLine(flag, summary, srcRef, remoteName, message);
 				}
@@ -220,8 +229,8 @@ class Push extends TextBuiltin {
 
 		case REJECTED_REMOTE_CHANGED:
 			final String message = MessageFormat.format(
-					CLIText.get().remoteRefObjectChangedIsNotExpectedOne
-					, rru.getExpectedOldObjectId().abbreviate(db).name());
+					CLIText.get().remoteRefObjectChangedIsNotExpectedOne,
+					safeAbbreviate(reader, rru.getExpectedOldObjectId()));
 			printUpdateLine('!', "[rejected]", srcRef, remoteName, message);
 			break;
 
@@ -243,6 +252,14 @@ class Push extends TextBuiltin {
 		}
 	}
 
+	private String safeAbbreviate(ObjectReader reader, ObjectId id) {
+		try {
+			return reader.abbreviate(id).name();
+		} catch (IOException cannotAbbreviate) {
+			return id.name();
+		}
+	}
+
 	private void printUpdateLine(final char flag, final String summary,
 			final String srcRef, final String destRef, final String message) {
 		out.format(" %c %-17s", flag, summary);

org.eclipse.jgit.test/tst/org/eclipse/jgit/diff/DiffFormatterTest.java

@@ -74,6 +74,7 @@ public class DiffFormatterTest extends RepositoryTestCase {
 		testDb = new TestRepository(db);
 		df = new DiffFormatter(DisabledOutputStream.INSTANCE);
 		df.setRepository(db);
+		df.setAbbreviationLength(8);
 	}
 
 	public void testCreateFileHeader_Modify() throws Exception {
@@ -159,8 +160,8 @@ public class DiffFormatterTest extends RepositoryTestCase {
 
 	private String makeDiffHeader(String pathA, String pathB, ObjectId aId,
 			ObjectId bId) {
-		String a = aId.abbreviate(db).name();
-		String b = bId.abbreviate(db).name();
+		String a = aId.abbreviate(8).name();
+		String b = bId.abbreviate(8).name();
 		return DIFF + "a/" + pathA + " " + "b/" + pathB + "\n" + //
 				"index " + a + ".." + b + " " + REGULAR_FILE + "\n" + //
 				"--- a/" + pathA + "\n" + //
@@ -169,8 +170,8 @@ public class DiffFormatterTest extends RepositoryTestCase {
 
 	private String makeDiffHeaderModeChange(String pathA, String pathB,
 			ObjectId aId, ObjectId bId, String modeA, String modeB) {
-		String a = aId.abbreviate(db).name();
-		String b = bId.abbreviate(db).name();
+		String a = aId.abbreviate(8).name();
+		String b = bId.abbreviate(8).name();
 		return DIFF + "a/" + pathA + " " + "b/" + pathB + "\n" + //
 				"old mode " + modeA + "\n" + //
 				"new mode " + modeB + "\n" + //
@@ -182,5 +183,4 @@ public class DiffFormatterTest extends RepositoryTestCase {
 	private ObjectId blob(String content) throws Exception {
 		return testDb.blob(content).copy();
 	}
-
 }

org.eclipse.jgit.test/tst/org/eclipse/jgit/lib/AbbreviatedObjectIdTest.java

@@ -348,4 +348,30 @@ public class AbbreviatedObjectIdTest extends TestCase {
 		assertTrue(a.prefixCompare(i3) > 0);
 		assertFalse(i3.startsWith(a));
 	}
+
+	public void testIsId() {
+		// These are all too short.
+		assertFalse(AbbreviatedObjectId.isId(""));
+		assertFalse(AbbreviatedObjectId.isId("a"));
+
+		// These are too long.
+		assertFalse(AbbreviatedObjectId.isId(ObjectId.fromString(
+				"7b6e8067ec86acef9a4184b43210d583b6d2f99a").name()
+				+ "0"));
+		assertFalse(AbbreviatedObjectId.isId(ObjectId.fromString(
+				"7b6e8067ec86acef9a4184b43210d583b6d2f99a").name()
+				+ "c0ffee"));
+
+		// These contain non-hex characters.
+		assertFalse(AbbreviatedObjectId.isId("01notahexstring"));
+
+		// These should all work.
+		assertTrue(AbbreviatedObjectId.isId("ab"));
+		assertTrue(AbbreviatedObjectId.isId("abc"));
+		assertTrue(AbbreviatedObjectId.isId("abcd"));
+		assertTrue(AbbreviatedObjectId.isId("abcd0"));
+		assertTrue(AbbreviatedObjectId.isId("abcd09"));
+		assertTrue(AbbreviatedObjectId.isId(ObjectId.fromString(
+				"7b6e8067ec86acef9a4184b43210d583b6d2f99a").name()));
+	}
 }

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

@@ -0,0 +1,199 @@
+/*
+ * Copyright (C) 2010, Google Inc.
+ * and other copyright owners as documented in the project's IP log.
+ *
+ * This program and the accompanying materials are made available
+ * under the terms of the Eclipse Distribution License v1.0 which
+ * accompanies this distribution, is reproduced below, and is
+ * available at http://www.eclipse.org/org/documents/edl-v10.php
+ *
+ * All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or
+ * without modification, are permitted provided that the following
+ * conditions are met:
+ *
+ * - Redistributions of source code must retain the above copyright
+ *   notice, this list of conditions and the following disclaimer.
+ *
+ * - Redistributions in binary form must reproduce the above
+ *   copyright notice, this list of conditions and the following
+ *   disclaimer in the documentation and/or other materials provided
+ *   with the distribution.
+ *
+ * - Neither the name of the Eclipse Foundation, Inc. nor the
+ *   names of its contributors may be used to endorse or promote
+ *   products derived from this software without specific prior
+ *   written permission.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND
+ * CONTRIBUTORS "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES,
+ * INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
+ * OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
+ * ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR
+ * CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+ * SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
+ * NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;
+ * LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
+ * CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT,
+ * STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
+ * ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF
+ * ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+ */
+
+package org.eclipse.jgit.storage.file;
+
+import static org.eclipse.jgit.lib.Constants.OBJECT_ID_LENGTH;
+import static org.eclipse.jgit.lib.Constants.OBJECT_ID_STRING_LENGTH;
+
+import java.io.BufferedOutputStream;
+import java.io.ByteArrayOutputStream;
+import java.io.File;
+import java.io.FileOutputStream;
+import java.io.IOException;
+import java.io.OutputStream;
+import java.util.ArrayList;
+import java.util.Collection;
+import java.util.List;
+
+import org.eclipse.jgit.junit.LocalDiskRepositoryTestCase;
+import org.eclipse.jgit.junit.TestRepository;
+import org.eclipse.jgit.lib.AbbreviatedObjectId;
+import org.eclipse.jgit.lib.ObjectId;
+import org.eclipse.jgit.lib.ObjectReader;
+import org.eclipse.jgit.revwalk.RevBlob;
+import org.eclipse.jgit.transport.PackedObjectInfo;
+
+public class AbbreviationTest extends LocalDiskRepositoryTestCase {
+	private FileRepository db;
+
+	private ObjectReader reader;
+
+	private TestRepository<FileRepository> test;
+
+	public void setUp() throws Exception {
+		super.setUp();
+		db = createBareRepository();
+		reader = db.newObjectReader();
+		test = new TestRepository<FileRepository>(db);
+	}
+
+	public void tearDown() throws Exception {
+		if (reader != null)
+			reader.release();
+	}
+
+	public void testAbbreviateOnEmptyRepository() throws IOException {
+		ObjectId id = id("9d5b926ed164e8ee88d3b8b1e525d699adda01ba");
+
+		assertEquals(id.abbreviate(2), reader.abbreviate(id, 2));
+		assertEquals(id.abbreviate(7), reader.abbreviate(id, 7));
+		assertEquals(id.abbreviate(8), reader.abbreviate(id, 8));
+		assertEquals(id.abbreviate(10), reader.abbreviate(id, 10));
+		assertEquals(id.abbreviate(16), reader.abbreviate(id, 16));
+
+		assertEquals(AbbreviatedObjectId.fromObjectId(id), //
+				reader.abbreviate(id, OBJECT_ID_STRING_LENGTH));
+
+		Collection<ObjectId> matches;
+
+		matches = reader.resolve(reader.abbreviate(id, 8));
+		assertNotNull(matches);
+		assertEquals(0, matches.size());
+
+		matches = reader.resolve(AbbreviatedObjectId.fromObjectId(id));
+		assertNotNull(matches);
+		assertEquals(1, matches.size());
+		assertEquals(id, matches.iterator().next());
+	}
+
+	public void testAbbreviateLooseBlob() throws Exception {
+		ObjectId id = test.blob("test");
+
+		assertEquals(id.abbreviate(2), reader.abbreviate(id, 2));
+		assertEquals(id.abbreviate(7), reader.abbreviate(id, 7));
+		assertEquals(id.abbreviate(8), reader.abbreviate(id, 8));
+		assertEquals(id.abbreviate(10), reader.abbreviate(id, 10));
+		assertEquals(id.abbreviate(16), reader.abbreviate(id, 16));
+
+		Collection<ObjectId> matches = reader.resolve(reader.abbreviate(id, 8));
+		assertNotNull(matches);
+		assertEquals(1, matches.size());
+		assertEquals(id, matches.iterator().next());
+
+		assertEquals(id, db.resolve(reader.abbreviate(id, 8).name()));
+	}
+
+	public void testAbbreviatePackedBlob() throws Exception {
+		RevBlob id = test.blob("test");
+		test.branch("master").commit().add("test", id).child();
+		test.packAndPrune();
+		assertTrue(reader.has(id));
+
+		assertEquals(id.abbreviate(7), reader.abbreviate(id, 7));
+		assertEquals(id.abbreviate(8), reader.abbreviate(id, 8));
+		assertEquals(id.abbreviate(10), reader.abbreviate(id, 10));
+		assertEquals(id.abbreviate(16), reader.abbreviate(id, 16));
+
+		Collection<ObjectId> matches = reader.resolve(reader.abbreviate(id, 8));
+		assertNotNull(matches);
+		assertEquals(1, matches.size());
+		assertEquals(id, matches.iterator().next());
+
+		assertEquals(id, db.resolve(reader.abbreviate(id, 8).name()));
+	}
+
+	@SuppressWarnings("unchecked")
+	public void testAbbreviateIsActuallyUnique() throws Exception {
+		// This test is far more difficult. We have to manually craft
+		// an input that contains collisions at a particular prefix,
+		// but this is computationally difficult. Instead we force an
+		// index file to have what we want.
+		//
+
+		ObjectId id = id("9d5b926ed164e8ee88d3b8b1e525d699adda01ba");
+		byte[] idBuf = toByteArray(id);
+		List<PackedObjectInfo> objects = new ArrayList<PackedObjectInfo>();
+		for (int i = 0; i < 256; i++) {
+			idBuf[9] = (byte) i;
+			objects.add(new PackedObjectInfo(ObjectId.fromRaw(idBuf)));
+		}
+
+		String packName = "pack-" + id.name();
+		File packDir = new File(db.getObjectDatabase().getDirectory(), "pack");
+		File idxFile = new File(packDir, packName + ".idx");
+		File packFile = new File(packDir, packName + ".pack");
+		packDir.mkdir();
+		OutputStream dst = new BufferedOutputStream(new FileOutputStream(
+				idxFile));
+		try {
+			PackIndexWriter writer = new PackIndexWriterV2(dst);
+			writer.write(objects, new byte[OBJECT_ID_LENGTH]);
+		} finally {
+			dst.close();
+		}
+		new FileOutputStream(packFile).close();
+		db.openPack(packFile, idxFile);
+
+		assertEquals(id.abbreviate(20), reader.abbreviate(id, 2));
+
+		Collection<ObjectId> matches = reader.resolve(id.abbreviate(8));
+		assertNotNull(matches);
+		assertEquals(objects.size(), matches.size());
+		for (PackedObjectInfo info : objects)
+			assertTrue("contains " + info.name(), matches.contains(info));
+
+		assertNull("cannot resolve", db.resolve(id.abbreviate(8).name()));
+		assertEquals(id, db.resolve(id.abbreviate(20).name()));
+	}
+
+	private static ObjectId id(String name) {
+		return ObjectId.fromString(name);
+	}
+
+	private static byte[] toByteArray(ObjectId id) throws IOException {
+		ByteArrayOutputStream buf = new ByteArrayOutputStream(OBJECT_ID_LENGTH);
+		id.copyRawTo(buf);
+		return buf.toByteArray();
+	}
+}

org.eclipse.jgit/src/org/eclipse/jgit/diff/DiffFormatter.java

@@ -63,6 +63,7 @@ import org.eclipse.jgit.lib.Constants;
 import org.eclipse.jgit.lib.CoreConfig;
 import org.eclipse.jgit.lib.FileMode;
 import org.eclipse.jgit.lib.ObjectLoader;
+import org.eclipse.jgit.lib.ObjectReader;
 import org.eclipse.jgit.lib.Repository;
 import org.eclipse.jgit.patch.FileHeader;
 import org.eclipse.jgit.patch.HunkHeader;
@@ -100,7 +101,7 @@ public class DiffFormatter {
 	public DiffFormatter(OutputStream out) {
 		this.out = out;
 		setContext(3);
-		setAbbreviationLength(8);
+		setAbbreviationLength(7);
 	}
 
 	/** @return the stream we are outputting data to. */
@@ -327,10 +328,18 @@ public class DiffFormatter {
 		o.write(encode("+++ " + newName + '\n'));
 	}
 
-	private String format(AbbreviatedObjectId oldId) {
-		if (oldId.isComplete() && db != null)
-			oldId = oldId.toObjectId().abbreviate(db, abbreviationLength);
-		return oldId.name();
+	private String format(AbbreviatedObjectId id) {
+		if (id.isComplete() && db != null) {
+			ObjectReader reader = db.newObjectReader();
+			try {
+				id = reader.abbreviate(id.toObjectId(), abbreviationLength);
+			} catch (IOException cannotAbbreviate) {
+				// Ignore this. We'll report the full identity.
+			} finally {
+				reader.release();
+			}
+		}
+		return id.name();
 	}
 
 	private static String quotePath(String name) {

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

@@ -62,6 +62,27 @@ import org.eclipse.jgit.util.RawParseUtils;
  * efficient for matching against an object.
  */
 public final class AbbreviatedObjectId {
+	/**
+	 * Test a string of characters to verify it is a hex format.
+	 * <p>
+	 * If true the string can be parsed with {@link #fromString(String)}.
+	 *
+	 * @param id
+	 *            the string to test.
+	 * @return true if the string can converted into an AbbreviatedObjectId.
+	 */
+	public static final boolean isId(final String id) {
+		if (id.length() < 2 || Constants.OBJECT_ID_STRING_LENGTH < id.length())
+			return false;
+		try {
+			for (int i = 0; i < id.length(); i++)
+				RawParseUtils.parseHexInt4((byte) id.charAt(i));
+			return true;
+		} catch (ArrayIndexOutOfBoundsException e) {
+			return false;
+		}
+	}
+
 	/**
 	 * Convert an AbbreviatedObjectId from hex characters (US-ASCII).
 	 *
@@ -205,7 +226,7 @@ public final class AbbreviatedObjectId {
 	 *         &gt;0 if this abbreviation names an object that is after
 	 *         <code>other</code>.
 	 */
-	public int prefixCompare(final AnyObjectId other) {
+	public final int prefixCompare(final AnyObjectId other) {
 		int cmp;
 
 		cmp = NB.compareUInt32(w1, mask(1, other.w1));
@@ -227,6 +248,83 @@ public final class AbbreviatedObjectId {
 		return NB.compareUInt32(w5, mask(5, other.w5));
 	}
 
+	/**
+	 * Compare this abbreviation to a network-byte-order ObjectId.
+	 *
+	 * @param bs
+	 *            array containing the other ObjectId in network byte order.
+	 * @param p
+	 *            position within {@code bs} to start the compare at. At least
+	 *            20 bytes, starting at this position are required.
+	 * @return &lt;0 if this abbreviation names an object that is less than
+	 *         <code>other</code>; 0 if this abbreviation exactly matches the
+	 *         first {@link #length()} digits of <code>other.name()</code>;
+	 *         &gt;0 if this abbreviation names an object that is after
+	 *         <code>other</code>.
+	 */
+	public final int prefixCompare(final byte[] bs, final int p) {
+		int cmp;
+
+		cmp = NB.compareUInt32(w1, mask(1, NB.decodeInt32(bs, p)));
+		if (cmp != 0)
+			return cmp;
+
+		cmp = NB.compareUInt32(w2, mask(2, NB.decodeInt32(bs, p + 4)));
+		if (cmp != 0)
+			return cmp;
+
+		cmp = NB.compareUInt32(w3, mask(3, NB.decodeInt32(bs, p + 8)));
+		if (cmp != 0)
+			return cmp;
+
+		cmp = NB.compareUInt32(w4, mask(4, NB.decodeInt32(bs, p + 12)));
+		if (cmp != 0)
+			return cmp;
+
+		return NB.compareUInt32(w5, mask(5, NB.decodeInt32(bs, p + 16)));
+	}
+
+	/**
+	 * Compare this abbreviation to a network-byte-order ObjectId.
+	 *
+	 * @param bs
+	 *            array containing the other ObjectId in network byte order.
+	 * @param p
+	 *            position within {@code bs} to start the compare at. At least 5
+	 *            ints, starting at this position are required.
+	 * @return &lt;0 if this abbreviation names an object that is less than
+	 *         <code>other</code>; 0 if this abbreviation exactly matches the
+	 *         first {@link #length()} digits of <code>other.name()</code>;
+	 *         &gt;0 if this abbreviation names an object that is after
+	 *         <code>other</code>.
+	 */
+	public final int prefixCompare(final int[] bs, final int p) {
+		int cmp;
+
+		cmp = NB.compareUInt32(w1, mask(1, bs[p]));
+		if (cmp != 0)
+			return cmp;
+
+		cmp = NB.compareUInt32(w2, mask(2, bs[p + 1]));
+		if (cmp != 0)
+			return cmp;
+
+		cmp = NB.compareUInt32(w3, mask(3, bs[p + 2]));
+		if (cmp != 0)
+			return cmp;
+
+		cmp = NB.compareUInt32(w4, mask(4, bs[p + 3]));
+		if (cmp != 0)
+			return cmp;
+
+		return NB.compareUInt32(w5, mask(5, bs[p + 4]));
+	}
+
+	/** @return value for a fan-out style map, only valid of length >= 2. */
+	public final int getFirstByte() {
+		return w1 >>> 24;
+	}
+
 	private int mask(final int word, final int v) {
 		return mask(nibbles, word, v);
 	}

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

@@ -463,32 +463,17 @@ public abstract class AnyObjectId implements Comparable {
 	}
 
 	/**
-	 * Return unique abbreviation (prefix) of this object SHA-1.
-	 * <p>
-	 * This method is a utility for <code>abbreviate(repo, 8)</code>.
+	 * Return an abbreviation (prefix) of this object SHA-1.
 	 *
-	 * @param repo
-	 *            repository for checking uniqueness within.
-	 * @return SHA-1 abbreviation.
-	 */
-	public AbbreviatedObjectId abbreviate(final Repository repo) {
-		return abbreviate(repo, 8);
-	}
-
-	/**
-	 * Return unique abbreviation (prefix) of this object SHA-1.
-	 * <p>
-	 * Current implementation is not guaranteeing uniqueness, it just returns
-	 * fixed-length prefix of SHA-1 string.
+	 * This implementation does not guaranteeing uniqueness. Callers should
+	 * instead use {@link ObjectReader#abbreviate(AnyObjectId, int)} to obtain a
+	 * unique abbreviation within the scope of a particular object database.
 	 *
-	 * @param repo
-	 *            repository for checking uniqueness within.
 	 * @param len
-	 *            minimum length of the abbreviated string.
+	 *            length of the abbreviated string.
 	 * @return SHA-1 abbreviation.
 	 */
-	public AbbreviatedObjectId abbreviate(final Repository repo, final int len) {
-		// TODO implement checking for uniqueness
+	public AbbreviatedObjectId abbreviate(final int len) {
 		final int a = AbbreviatedObjectId.mask(len, 1, w1);
 		final int b = AbbreviatedObjectId.mask(len, 2, w2);
 		final int c = AbbreviatedObjectId.mask(len, 3, w3);

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

@@ -44,8 +44,10 @@
 package org.eclipse.jgit.lib;
 
 import java.io.IOException;
+import java.util.ArrayList;
 import java.util.Collection;
 import java.util.Iterator;
+import java.util.List;
 
 import org.eclipse.jgit.errors.IncorrectObjectTypeException;
 import org.eclipse.jgit.errors.MissingObjectException;
@@ -74,6 +76,104 @@ public abstract class ObjectReader {
 	 */
 	public abstract ObjectReader newReader();
 
+	/**
+	 * Obtain a unique abbreviation (prefix) of an object SHA-1.
+	 *
+	 * This method uses a reasonable default for the minimum length. Callers who
+	 * don't care about the minimum length should prefer this method.
+	 *
+	 * The returned abbreviation would expand back to the argument ObjectId when
+	 * passed to {@link #resolve(AbbreviatedObjectId)}, assuming no new objects
+	 * are added to this repository between calls.
+	 *
+	 * @param objectId
+	 *            object identity that needs to be abbreviated.
+	 * @return SHA-1 abbreviation.
+	 * @throws IOException
+	 *             the object store cannot be read.
+	 */
+	public AbbreviatedObjectId abbreviate(AnyObjectId objectId)
+			throws IOException {
+		return abbreviate(objectId, 7);
+	}
+
+	/**
+	 * Obtain a unique abbreviation (prefix) of an object SHA-1.
+	 *
+	 * The returned abbreviation would expand back to the argument ObjectId when
+	 * passed to {@link #resolve(AbbreviatedObjectId)}, assuming no new objects
+	 * are added to this repository between calls.
+	 *
+	 * The default implementation of this method abbreviates the id to the
+	 * minimum length, then resolves it to see if there are multiple results.
+	 * When multiple results are found, the length is extended by 1 and resolve
+	 * is tried again.
+	 *
+	 * @param objectId
+	 *            object identity that needs to be abbreviated.
+	 * @param len
+	 *            minimum length of the abbreviated string. Must be in the range
+	 *            [2, {@value Constants#OBJECT_ID_STRING_LENGTH}].
+	 * @return SHA-1 abbreviation. If no matching objects exist in the
+	 *         repository, the abbreviation will match the minimum length.
+	 * @throws IOException
+	 *             the object store cannot be read.
+	 */
+	public AbbreviatedObjectId abbreviate(AnyObjectId objectId, int len)
+			throws IOException {
+		if (len == Constants.OBJECT_ID_STRING_LENGTH)
+			return AbbreviatedObjectId.fromObjectId(objectId);
+
+		AbbreviatedObjectId abbrev = objectId.abbreviate(len);
+		Collection<ObjectId> matches = resolve(abbrev);
+		while (1 < matches.size() && len < Constants.OBJECT_ID_STRING_LENGTH) {
+			abbrev = objectId.abbreviate(++len);
+			List<ObjectId> n = new ArrayList<ObjectId>(8);
+			for (ObjectId candidate : matches) {
+				if (abbrev.prefixCompare(candidate) == 0)
+					n.add(candidate);
+			}
+			if (1 < n.size())
+				matches = n;
+			else
+				matches = resolve(abbrev);
+		}
+		return abbrev;
+	}
+
+	/**
+	 * Resolve an abbreviated ObjectId to its full form.
+	 *
+	 * This method searches for an ObjectId that begins with the abbreviation,
+	 * and returns at least some matching candidates.
+	 *
+	 * If the returned collection is empty, no objects start with this
+	 * abbreviation. The abbreviation doesn't belong to this repository, or the
+	 * repository lacks the necessary objects to complete it.
+	 *
+	 * If the collection contains exactly one member, the abbreviation is
+	 * (currently) unique within this database. There is a reasonably high
+	 * probability that the returned id is what was previously abbreviated.
+	 *
+	 * If the collection contains 2 or more members, the abbreviation is not
+	 * unique. In this case the implementation is only required to return at
+	 * least 2 candidates to signal the abbreviation has conflicts. User
+	 * friendly implementations should return as many candidates as reasonably
+	 * possible, as the caller may be able to disambiguate further based on
+	 * context. However since databases can be very large (e.g. 10 million
+	 * objects) returning 625,000 candidates for the abbreviation "0" is simply
+	 * unreasonable, so implementors should draw the line at around 256 matches.
+	 *
+	 * @param id
+	 *            abbreviated id to resolve to a complete identity. The
+	 *            abbreviation must have a length of at least 2.
+	 * @return candidates that begin with the abbreviated identity.
+	 * @throws IOException
+	 *             the object store cannot be read.
+	 */
+	public abstract Collection<ObjectId> resolve(AbbreviatedObjectId id)
+			throws IOException;
+
 	/**
 	 * Does the requested object exist in this database?
 	 *

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

@@ -51,6 +51,7 @@ import java.io.File;
 import java.io.FileNotFoundException;
 import java.io.FileOutputStream;
 import java.io.IOException;
+import java.util.Collection;
 import java.util.Collections;
 import java.util.HashMap;
 import java.util.HashSet;
@@ -373,6 +374,8 @@ public abstract class Repository {
 	 * Currently supported is combinations of these.
 	 * <ul>
 	 * <li>SHA-1 - a SHA-1</li>
+	 * <li>SHA-1 abbreviation - a leading prefix of a SHA-1. At least the first
+	 * two bytes must be supplied.</li>
 	 * <li>refs/... - a ref name</li>
 	 * <li>ref^n - nth parent reference</li>
 	 * <li>ref~n - distance via parent reference</li>
@@ -383,8 +386,8 @@ public abstract class Repository {
 	 *
 	 * Not supported is:
 	 * <ul>
+	 * <li>tag-NNN-gcommit - a non tagged revision from git describe</li>
 	 * <li>timestamps in reflogs, ref@{full or relative timestamp}</li>
-	 * <li>abbreviated SHA-1's</li>
 	 * </ul>
 	 *
 	 * @param revstr
@@ -569,8 +572,24 @@ public abstract class Repository {
 	private ObjectId resolveSimple(final String revstr) throws IOException {
 		if (ObjectId.isId(revstr))
 			return ObjectId.fromString(revstr);
-		final Ref r = getRefDatabase().getRef(revstr);
-		return r != null ? r.getObjectId() : null;
+
+		Ref r = getRefDatabase().getRef(revstr);
+		if (r != null)
+			return r.getObjectId();
+
+		if (AbbreviatedObjectId.isId(revstr)) {
+			AbbreviatedObjectId id = AbbreviatedObjectId.fromString(revstr);
+			ObjectReader reader = newObjectReader();
+			try {
+				Collection<ObjectId> matches = reader.resolve(id);
+				if (matches.size() == 1)
+					return matches.iterator().next();
+			} finally {
+				reader.release();
+			}
+		}
+
+		return null;
 	}
 
 	/** Increment the use counter by one, requiring a matched {@link #close()}. */

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

@@ -46,7 +46,9 @@ package org.eclipse.jgit.storage.file;
 
 import java.io.File;
 import java.io.IOException;
+import java.util.Set;
 
+import org.eclipse.jgit.lib.AbbreviatedObjectId;
 import org.eclipse.jgit.lib.AnyObjectId;
 import org.eclipse.jgit.lib.Constants;
 import org.eclipse.jgit.lib.ObjectDatabase;
@@ -143,6 +145,17 @@ class CachedObjectDirectory extends FileObjectDatabase {
 		return alts;
 	}
 
+	@Override
+	void resolve(Set<ObjectId> matches, AbbreviatedObjectId id)
+			throws IOException {
+		// In theory we could accelerate the loose object scan using our
+		// unpackedObjects map, but its not worth the huge code complexity.
+		// Scanning a single loose directory is fast enough, and this is
+		// unlikely to be called anyway.
+		//
+		wrapped.resolve(matches, id);
+	}
+
 	@Override
 	boolean tryAgain1() {
 		return wrapped.tryAgain1();

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

@@ -45,9 +45,12 @@ package org.eclipse.jgit.storage.file;
 
 import java.io.File;
 import java.io.IOException;
+import java.util.Set;
 
+import org.eclipse.jgit.lib.AbbreviatedObjectId;
 import org.eclipse.jgit.lib.AnyObjectId;
 import org.eclipse.jgit.lib.ObjectDatabase;
+import org.eclipse.jgit.lib.ObjectId;
 import org.eclipse.jgit.lib.ObjectLoader;
 import org.eclipse.jgit.lib.ObjectReader;
 import org.eclipse.jgit.storage.pack.ObjectToPack;
@@ -97,6 +100,9 @@ abstract class FileObjectDatabase extends ObjectDatabase {
 		return false;
 	}
 
+	abstract void resolve(Set<ObjectId> matches, AbbreviatedObjectId id)
+			throws IOException;
+
 	/**
 	 * Open an object from this database.
 	 * <p>

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

@@ -65,8 +65,10 @@ import org.eclipse.jgit.JGitText;
 import org.eclipse.jgit.errors.PackMismatchException;
 import org.eclipse.jgit.events.ConfigChangedEvent;
 import org.eclipse.jgit.events.ConfigChangedListener;
+import org.eclipse.jgit.lib.AbbreviatedObjectId;
 import org.eclipse.jgit.lib.AnyObjectId;
 import org.eclipse.jgit.lib.Config;
+import org.eclipse.jgit.lib.Constants;
 import org.eclipse.jgit.lib.CoreConfig;
 import org.eclipse.jgit.lib.ObjectDatabase;
 import org.eclipse.jgit.lib.ObjectId;
@@ -100,6 +102,9 @@ public class ObjectDirectory extends FileObjectDatabase implements
 		ConfigChangedListener {
 	private static final PackList NO_PACKS = new PackList(-1, -1, new PackFile[0]);
 
+	/** Maximum number of candidates offered as resolutions of abbreviation. */
+	private static final int RESOLVE_ABBREV_LIMIT = 256;
+
 	private final Config config;
 
 	private final File objects;
@@ -280,6 +285,48 @@ public class ObjectDirectory extends FileObjectDatabase implements
 		return false;
 	}
 
+	void resolve(Set<ObjectId> matches, AbbreviatedObjectId id)
+			throws IOException {
+		PackList pList = packList.get();
+		if (pList == null)
+			pList = scanPacks(pList);
+		for (PackFile p : pList.packs) {
+			try {
+				p.resolve(matches, id, RESOLVE_ABBREV_LIMIT);
+			} catch (IOException e) {
+				// Assume the pack is corrupted.
+				//
+				removePack(p);
+			}
+			if (matches.size() > RESOLVE_ABBREV_LIMIT)
+				return;
+		}
+
+		String fanOut = id.name().substring(0, 2);
+		String[] entries = new File(getDirectory(), fanOut).list();
+		if (entries != null) {
+			for (String e : entries) {
+				if (e.length() != Constants.OBJECT_ID_STRING_LENGTH - 2)
+					continue;
+				try {
+					ObjectId entId = ObjectId.fromString(fanOut + e);
+					if (id.prefixCompare(entId) == 0)
+						matches.add(entId);
+				} catch (IllegalArgumentException notId) {
+					continue;
+				}
+				if (matches.size() > RESOLVE_ABBREV_LIMIT)
+					return;
+			}
+		}
+
+		for (AlternateHandle alt : myAlternates()) {
+			alt.db.resolve(matches, id);
+			if (matches.size() > RESOLVE_ABBREV_LIMIT)
+				return;
+		}
+	}
+
 	ObjectLoader openObject1(final WindowCursor curs,
 			final AnyObjectId objectId) throws IOException {
 		PackList pList = packList.get();

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

@@ -56,6 +56,7 @@ import java.util.Arrays;
 import java.util.Collections;
 import java.util.Comparator;
 import java.util.Iterator;
+import java.util.Set;
 import java.util.zip.CRC32;
 import java.util.zip.DataFormatException;
 import java.util.zip.Inflater;
@@ -66,6 +67,7 @@ import org.eclipse.jgit.errors.MissingObjectException;
 import org.eclipse.jgit.errors.PackInvalidException;
 import org.eclipse.jgit.errors.PackMismatchException;
 import org.eclipse.jgit.errors.StoredObjectRepresentationNotAvailableException;
+import org.eclipse.jgit.lib.AbbreviatedObjectId;
 import org.eclipse.jgit.lib.AnyObjectId;
 import org.eclipse.jgit.lib.Constants;
 import org.eclipse.jgit.lib.ObjectId;
@@ -209,6 +211,11 @@ public class PackFile implements Iterable<PackIndex.MutableEntry> {
 		return 0 < offset && !isCorrupt(offset) ? load(curs, offset) : null;
 	}
 
+	void resolve(Set<ObjectId> matches, AbbreviatedObjectId id, int matchLimit)
+			throws IOException {
+		idx().resolve(matches, id, matchLimit);
+	}
+
 	/**
 	 * Close the resources utilized by this repository
 	 */

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

@@ -50,9 +50,11 @@ import java.io.FileNotFoundException;
 import java.io.IOException;
 import java.text.MessageFormat;
 import java.util.Iterator;
+import java.util.Set;
 
 import org.eclipse.jgit.JGitText;
 import org.eclipse.jgit.errors.MissingObjectException;
+import org.eclipse.jgit.lib.AbbreviatedObjectId;
 import org.eclipse.jgit.lib.AnyObjectId;
 import org.eclipse.jgit.lib.MutableObjectId;
 import org.eclipse.jgit.lib.ObjectId;
@@ -249,6 +251,9 @@ public abstract class PackIndex implements Iterable<PackIndex.MutableEntry> {
 	 */
 	abstract boolean hasCRC32Support();
 
+	abstract void resolve(Set<ObjectId> matches, AbbreviatedObjectId id,
+			int matchLimit) throws IOException;
+
 	/**
 	 * Represent mutable entry of pack index consisting of object id and offset
 	 * in pack (both mutable).

org.eclipse.jgit/src/org/eclipse/jgit/storage/file/PackIndexV1.java

@@ -51,8 +51,10 @@ import java.io.InputStream;
 import java.util.Arrays;
 import java.util.Iterator;
 import java.util.NoSuchElementException;
+import java.util.Set;
 
 import org.eclipse.jgit.errors.CorruptObjectException;
+import org.eclipse.jgit.lib.AbbreviatedObjectId;
 import org.eclipse.jgit.lib.AnyObjectId;
 import org.eclipse.jgit.lib.Constants;
 import org.eclipse.jgit.lib.ObjectId;
@@ -129,7 +131,7 @@ class PackIndexV1 extends PackIndex {
 
 		base = levelOne > 0 ? idxHeader[levelOne - 1] : 0;
 		final int p = (int) (nthPosition - base);
-		final int dataIdx = ((4 + Constants.OBJECT_ID_LENGTH) * p) + 4;
+		final int dataIdx = idOffset(p);
 		return ObjectId.fromRaw(idxdata[levelOne], dataIdx);
 	}
 
@@ -142,7 +144,7 @@ class PackIndexV1 extends PackIndex {
 		int low = 0;
 		do {
 			final int mid = (low + high) >>> 1;
-			final int pos = ((4 + Constants.OBJECT_ID_LENGTH) * mid) + 4;
+			final int pos = idOffset(mid);
 			final int cmp = objId.compareTo(data, pos);
 			if (cmp < 0)
 				high = mid;
@@ -172,6 +174,41 @@ class PackIndexV1 extends PackIndex {
 		return new IndexV1Iterator();
 	}
 
+	@Override
+	void resolve(Set<ObjectId> matches, AbbreviatedObjectId id, int matchLimit)
+			throws IOException {
+		byte[] data = idxdata[id.getFirstByte()];
+		if (data == null)
+			return;
+		int max = data.length / (4 + Constants.OBJECT_ID_LENGTH);
+		int high = max;
+		int low = 0;
+		do {
+			int p = (low + high) >>> 1;
+			final int cmp = id.prefixCompare(data, idOffset(p));
+			if (cmp < 0)
+				high = p;
+			else if (cmp == 0) {
+				// We may have landed in the middle of the matches.  Move
+				// backwards to the start of matches, then walk forwards.
+				//
+				while (0 < p && id.prefixCompare(data, idOffset(p - 1)) == 0)
+					p--;
+				for (; p < max && id.prefixCompare(data, idOffset(p)) == 0; p++) {
+					matches.add(ObjectId.fromRaw(data, idOffset(p)));
+					if (matches.size() > matchLimit)
+						break;
+				}
+				return;
+			} else
+				low = p + 1;
+		} while (low < high);
+	}
+
+	private static int idOffset(int mid) {
+		return ((4 + Constants.OBJECT_ID_LENGTH) * mid) + 4;
+	}
+
 	private class IndexV1Iterator extends EntriesIterator {
 		private int levelOne;
 

org.eclipse.jgit/src/org/eclipse/jgit/storage/file/PackIndexV2.java

@@ -48,9 +48,11 @@ import java.io.InputStream;
 import java.util.Arrays;
 import java.util.Iterator;
 import java.util.NoSuchElementException;
+import java.util.Set;
 
 import org.eclipse.jgit.JGitText;
 import org.eclipse.jgit.errors.MissingObjectException;
+import org.eclipse.jgit.lib.AbbreviatedObjectId;
 import org.eclipse.jgit.lib.AnyObjectId;
 import org.eclipse.jgit.lib.Constants;
 import org.eclipse.jgit.lib.ObjectId;
@@ -221,6 +223,41 @@ class PackIndexV2 extends PackIndex {
 		return new EntriesIteratorV2();
 	}
 
+	@Override
+	void resolve(Set<ObjectId> matches, AbbreviatedObjectId id, int matchLimit)
+			throws IOException {
+		int[] data = names[id.getFirstByte()];
+		int max = offset32[id.getFirstByte()].length >>> 2;
+		int high = max;
+		if (high == 0)
+			return;
+		int low = 0;
+		do {
+			int p = (low + high) >>> 1;
+			final int cmp = id.prefixCompare(data, idOffset(p));
+			if (cmp < 0)
+				high = p;
+			else if (cmp == 0) {
+				// We may have landed in the middle of the matches.  Move
+				// backwards to the start of matches, then walk forwards.
+				//
+				while (0 < p && id.prefixCompare(data, idOffset(p - 1)) == 0)
+					p--;
+				for (; p < max && id.prefixCompare(data, idOffset(p)) == 0; p++) {
+					matches.add(ObjectId.fromRaw(data, idOffset(p)));
+					if (matches.size() > matchLimit)
+						break;
+				}
+				return;
+			} else
+				low = p + 1;
+		} while (low < high);
+	}
+
+	private static int idOffset(int p) {
+		return (p << 2) + p; // p * 5
+	}
+
 	private int binarySearchLevelTwo(final AnyObjectId objId, final int levelOne) {
 		final int[] data = names[levelOne];
 		int high = offset32[levelOne].length >>> 2;

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

@@ -45,15 +45,20 @@
 package org.eclipse.jgit.storage.file;
 
 import java.io.IOException;
+import java.util.Collection;
+import java.util.Collections;
+import java.util.HashSet;
 import java.util.zip.DataFormatException;
 import java.util.zip.Inflater;
 
 import org.eclipse.jgit.errors.IncorrectObjectTypeException;
 import org.eclipse.jgit.errors.MissingObjectException;
 import org.eclipse.jgit.errors.StoredObjectRepresentationNotAvailableException;
+import org.eclipse.jgit.lib.AbbreviatedObjectId;
 import org.eclipse.jgit.lib.AnyObjectId;
 import org.eclipse.jgit.lib.Constants;
 import org.eclipse.jgit.lib.InflaterCache;
+import org.eclipse.jgit.lib.ObjectId;
 import org.eclipse.jgit.lib.ObjectLoader;
 import org.eclipse.jgit.lib.ObjectReader;
 import org.eclipse.jgit.lib.ProgressMonitor;
@@ -83,6 +88,16 @@ final class WindowCursor extends ObjectReader implements ObjectReuseAsIs {
 		return new WindowCursor(db);
 	}
 
+	@Override
+	public Collection<ObjectId> resolve(AbbreviatedObjectId id)
+			throws IOException {
+		if (id.isComplete())
+			return Collections.singleton(id.toObjectId());
+		HashSet<ObjectId> matches = new HashSet<ObjectId>(4);
+		db.resolve(matches, id);
+		return matches;
+	}
+
 	public boolean has(AnyObjectId objectId) throws IOException {
 		return db.has(objectId);
 	}

org.eclipse.jgit/src/org/eclipse/jgit/transport/RemoteRefUpdate.java

@@ -353,8 +353,8 @@ public class RemoteRefUpdate {
 	@Override
 	public String toString() {
 		return "RemoteRefUpdate[remoteName=" + remoteName + ", " + status
-				+ ", " + (expectedOldObjectId!=null?expectedOldObjectId.abbreviate(localDb).name() :"(null)")
-				+ "..." + (newObjectId != null ? newObjectId.abbreviate(localDb).name() : "(null)")
+				+ ", " + (expectedOldObjectId!=null ? expectedOldObjectId.name() : "(null)")
+				+ "..." + (newObjectId != null ? newObjectId.name() : "(null)")
 				+ (fastForward ? ", fastForward" : "")
 				+ ", srcRef=" + srcRef + (forceUpdate ? ", forceUpdate" : "") + ", message=" + (message != null ? "\""
 				+ message + "\"" : "null") + "]";