PackWriter

/*
 * Copyright (C) 2008-2010, Google Inc.
 * Copyright (C) 2008, Marek Zawirski <marek.zawirski@gmail.com>
 * 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.internal.storage.pack;

import static org.eclipse.jgit.internal.storage.pack.StoredObjectRepresentation.PACK_DELTA;
import static org.eclipse.jgit.internal.storage.pack.StoredObjectRepresentation.PACK_WHOLE;
import static org.eclipse.jgit.lib.Constants.OBJECT_ID_LENGTH;
import static org.eclipse.jgit.lib.Constants.OBJ_BLOB;
import static org.eclipse.jgit.lib.Constants.OBJ_COMMIT;
import static org.eclipse.jgit.lib.Constants.OBJ_TAG;
import static org.eclipse.jgit.lib.Constants.OBJ_TREE;

import java.io.IOException;
import java.io.OutputStream;
import java.lang.ref.WeakReference;
import java.security.MessageDigest;
import java.text.MessageFormat;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.Collection;
import java.util.Collections;
import java.util.Comparator;
import java.util.HashSet;
import java.util.Iterator;
import java.util.List;
import java.util.Map;
import java.util.NoSuchElementException;
import java.util.Set;
import java.util.concurrent.ConcurrentHashMap;
import java.util.concurrent.ExecutionException;
import java.util.concurrent.Executor;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.Future;
import java.util.concurrent.TimeUnit;
import java.util.zip.CRC32;
import java.util.zip.CheckedOutputStream;
import java.util.zip.Deflater;
import java.util.zip.DeflaterOutputStream;

import org.eclipse.jgit.annotations.NonNull;
import org.eclipse.jgit.errors.CorruptObjectException;
import org.eclipse.jgit.errors.IncorrectObjectTypeException;
import org.eclipse.jgit.errors.LargeObjectException;
import org.eclipse.jgit.errors.MissingObjectException;
import org.eclipse.jgit.errors.StoredObjectRepresentationNotAvailableException;
import org.eclipse.jgit.internal.JGitText;
import org.eclipse.jgit.internal.storage.file.PackBitmapIndexBuilder;
import org.eclipse.jgit.internal.storage.file.PackBitmapIndexWriterV1;
import org.eclipse.jgit.internal.storage.file.PackIndexWriter;
import org.eclipse.jgit.lib.AnyObjectId;
import org.eclipse.jgit.lib.AsyncObjectSizeQueue;
import org.eclipse.jgit.lib.BatchingProgressMonitor;
import org.eclipse.jgit.lib.BitmapIndex;
import org.eclipse.jgit.lib.BitmapIndex.BitmapBuilder;
import org.eclipse.jgit.lib.BitmapObject;
import org.eclipse.jgit.lib.Constants;
import org.eclipse.jgit.lib.NullProgressMonitor;
import org.eclipse.jgit.lib.ObjectId;
import org.eclipse.jgit.lib.ObjectIdOwnerMap;
import org.eclipse.jgit.lib.ObjectIdSet;
import org.eclipse.jgit.lib.ObjectLoader;
import org.eclipse.jgit.lib.ObjectReader;
import org.eclipse.jgit.lib.ProgressMonitor;
import org.eclipse.jgit.lib.Repository;
import org.eclipse.jgit.lib.ThreadSafeProgressMonitor;
import org.eclipse.jgit.revwalk.AsyncRevObjectQueue;
import org.eclipse.jgit.revwalk.DepthWalk;
import org.eclipse.jgit.revwalk.ObjectWalk;
import org.eclipse.jgit.revwalk.RevCommit;
import org.eclipse.jgit.revwalk.RevFlag;
import org.eclipse.jgit.revwalk.RevObject;
import org.eclipse.jgit.revwalk.RevSort;
import org.eclipse.jgit.revwalk.RevTag;
import org.eclipse.jgit.revwalk.RevTree;
import org.eclipse.jgit.storage.pack.PackConfig;
import org.eclipse.jgit.storage.pack.PackStatistics;
import org.eclipse.jgit.transport.ObjectCountCallback;
import org.eclipse.jgit.transport.WriteAbortedException;
import org.eclipse.jgit.util.BlockList;
import org.eclipse.jgit.util.TemporaryBuffer;

/**
 * <p>
 * PackWriter class is responsible for generating pack files from specified set
 * of objects from repository. This implementation produce pack files in format
 * version 2.
 * </p>
 * <p>
 * Source of objects may be specified in two ways:
 * <ul>
 * <li>(usually) by providing sets of interesting and uninteresting objects in
 * repository - all interesting objects and their ancestors except uninteresting
 * objects and their ancestors will be included in pack, or</li>
 * <li>by providing iterator of {@link RevObject} specifying exact list and
 * order of objects in pack</li>
 * </ul>
 * <p>
 * Typical usage consists of creating an instance, configuring options,
 * preparing the list of objects by calling {@link #preparePack(Iterator)} or
 * {@link #preparePack(ProgressMonitor, Set, Set)}, and streaming with
 * {@link #writePack(ProgressMonitor, ProgressMonitor, OutputStream)}. If the
 * pack is being stored as a file the matching index can be written out after
 * writing the pack by {@link #writeIndex(OutputStream)}. An optional bitmap
 * index can be made by calling {@link #prepareBitmapIndex(ProgressMonitor)}
 * followed by {@link #writeBitmapIndex(OutputStream)}.
 * </p>
 * <p>
 * Class provide set of configurable options and {@link ProgressMonitor}
 * support, as operations may take a long time for big repositories. Deltas
 * searching algorithm is <b>NOT IMPLEMENTED</b> yet - this implementation
 * relies only on deltas and objects reuse.
 * </p>
 * <p>
 * This class is not thread safe. It is intended to be used in one thread as a
 * single pass to produce one pack. Invoking methods multiple times or out of
 * order is not supported as internal data structures are destroyed during
 * certain phases to save memory when packing large repositories.
 * </p>
 */
public class PackWriter implements AutoCloseable {
	private static final int PACK_VERSION_GENERATED = 2;

	/** Empty set of objects for {@code preparePack()}. */
	public static final Set<ObjectId> NONE = Collections.emptySet();

	private static final Map<WeakReference<PackWriter>, Boolean> instances =
			new ConcurrentHashMap<>();

	private static final Iterable<PackWriter> instancesIterable = new Iterable<PackWriter>() {
		@Override
		public Iterator<PackWriter> iterator() {
			return new Iterator<PackWriter>() {
				private final Iterator<WeakReference<PackWriter>> it =
						instances.keySet().iterator();
				private PackWriter next;

				@Override
				public boolean hasNext() {
					if (next != null)
						return true;
					while (it.hasNext()) {
						WeakReference<PackWriter> ref = it.next();
						next = ref.get();
						if (next != null)
							return true;
						it.remove();
					}
					return false;
				}

				@Override
				public PackWriter next() {
					if (hasNext()) {
						PackWriter result = next;
						next = null;
						return result;
					}
					throw new NoSuchElementException();
				}

				@Override
				public void remove() {
					throw new UnsupportedOperationException();
				}
			};
		}
	};

	/** @return all allocated, non-released PackWriters instances. */
	public static Iterable<PackWriter> getInstances() {
		return instancesIterable;
	}

	@SuppressWarnings("unchecked")
	BlockList<ObjectToPack> objectsLists[] = new BlockList[OBJ_TAG + 1];
	{
		objectsLists[OBJ_COMMIT] = new BlockList<>();
		objectsLists[OBJ_TREE] = new BlockList<>();
		objectsLists[OBJ_BLOB] = new BlockList<>();
		objectsLists[OBJ_TAG] = new BlockList<>();
	}

	private ObjectIdOwnerMap<ObjectToPack> objectsMap = new ObjectIdOwnerMap<>();

	// edge objects for thin packs
	private List<ObjectToPack> edgeObjects = new BlockList<>();

	// Objects the client is known to have already.
	private BitmapBuilder haveObjects;

	private List<CachedPack> cachedPacks = new ArrayList<>(2);

	private Set<ObjectId> tagTargets = NONE;

	private Set<? extends ObjectId> excludeFromBitmapSelection = NONE;

	private ObjectIdSet[] excludeInPacks;

	private ObjectIdSet excludeInPackLast;

	private Deflater myDeflater;

	private final ObjectReader reader;

	/** {@link #reader} recast to the reuse interface, if it supports it. */
	private final ObjectReuseAsIs reuseSupport;

	final PackConfig config;

	private final PackStatistics.Accumulator stats;

	private final MutableState state;

	private final WeakReference<PackWriter> selfRef;

	private PackStatistics.ObjectType.Accumulator typeStats;

	private List<ObjectToPack> sortedByName;

	private byte packcsum[];

	private boolean deltaBaseAsOffset;

	private boolean reuseDeltas;

	private boolean reuseDeltaCommits;

	private boolean reuseValidate;

	private boolean thin;

	private boolean useCachedPacks;

	private boolean useBitmaps;

	private boolean ignoreMissingUninteresting = true;

	private boolean pruneCurrentObjectList;

	private boolean shallowPack;

	private boolean canBuildBitmaps;

	private boolean indexDisabled;

	private int depth;

	private Collection<? extends ObjectId> unshallowObjects;

	private PackBitmapIndexBuilder writeBitmaps;

	private CRC32 crc32;

	private ObjectCountCallback callback;

	/**
	 * Create writer for specified repository.
	 * <p>
	 * Objects for packing are specified in {@link #preparePack(Iterator)} or
	 * {@link #preparePack(ProgressMonitor, Set, Set)}.
	 *
	 * @param repo
	 *            repository where objects are stored.
	 */
	public PackWriter(final Repository repo) {
		this(repo, repo.newObjectReader());
	}

	/**
	 * Create a writer to load objects from the specified reader.
	 * <p>
	 * Objects for packing are specified in {@link #preparePack(Iterator)} or
	 * {@link #preparePack(ProgressMonitor, Set, Set)}.
	 *
	 * @param reader
	 *            reader to read from the repository with.
	 */
	public PackWriter(final ObjectReader reader) {
		this(new PackConfig(), reader);
	}

	/**
	 * Create writer for specified repository.
	 * <p>
	 * Objects for packing are specified in {@link #preparePack(Iterator)} or
	 * {@link #preparePack(ProgressMonitor, Set, Set)}.
	 *
	 * @param repo
	 *            repository where objects are stored.
	 * @param reader
	 *            reader to read from the repository with.
	 */
	public PackWriter(final Repository repo, final ObjectReader reader) {
		this(new PackConfig(repo), reader);
	}

	/**
	 * Create writer with a specified configuration.
	 * <p>
	 * Objects for packing are specified in {@link #preparePack(Iterator)} or
	 * {@link #preparePack(ProgressMonitor, Set, Set)}.
	 *
	 * @param config
	 *            configuration for the pack writer.
	 * @param reader
	 *            reader to read from the repository with.
	 */
	public PackWriter(final PackConfig config, final ObjectReader reader) {
		this.config = config;
		this.reader = reader;
		if (reader instanceof ObjectReuseAsIs)
			reuseSupport = ((ObjectReuseAsIs) reader);
		else
			reuseSupport = null;

		deltaBaseAsOffset = config.isDeltaBaseAsOffset();
		reuseDeltas = config.isReuseDeltas();
		reuseValidate = true; // be paranoid by default
		stats = new PackStatistics.Accumulator();
		state = new MutableState();
		selfRef = new WeakReference<>(this);
		instances.put(selfRef, Boolean.TRUE);
	}

	/**
	 * Set the {@code ObjectCountCallback}.
	 * <p>
	 * It should be set before calling
	 * {@link #writePack(ProgressMonitor, ProgressMonitor, OutputStream)}.
	 *
	 * @param callback
	 *            the callback to set
	 *
	 * @return this object for chaining.
	 */
	public PackWriter setObjectCountCallback(ObjectCountCallback callback) {
		this.callback = callback;
		return this;
	}

	/**
	 * Records the set of shallow commits in the client.
	 *
	 * @param clientShallowCommits
	 *            the shallow commits in the client
	 */
	public void setClientShallowCommits(Set<ObjectId> clientShallowCommits) {
		stats.clientShallowCommits = Collections
				.unmodifiableSet(new HashSet<>(clientShallowCommits));
	}

	/**
	 * Check whether writer can store delta base as an offset (new style
	 * reducing pack size) or should store it as an object id (legacy style,
	 * compatible with old readers).
	 *
	 * Default setting: {@value PackConfig#DEFAULT_DELTA_BASE_AS_OFFSET}
	 *
	 * @return true if delta base is stored as an offset; false if it is stored
	 *         as an object id.
	 */
	public boolean isDeltaBaseAsOffset() {
		return deltaBaseAsOffset;
	}

	/**
	 * Set writer delta base format. Delta base can be written as an offset in a
	 * pack file (new approach reducing file size) or as an object id (legacy
	 * approach, compatible with old readers).
	 *
	 * Default setting: {@value PackConfig#DEFAULT_DELTA_BASE_AS_OFFSET}
	 *
	 * @param deltaBaseAsOffset
	 *            boolean indicating whether delta base can be stored as an
	 *            offset.
	 */
	public void setDeltaBaseAsOffset(boolean deltaBaseAsOffset) {
		this.deltaBaseAsOffset = deltaBaseAsOffset;
	}

	/**
	 * Check if the writer will reuse commits that are already stored as deltas.
	 *
	 * @return true if the writer would reuse commits stored as deltas, assuming
	 *         delta reuse is already enabled.
	 */
	public boolean isReuseDeltaCommits() {
		return reuseDeltaCommits;
	}

	/**
	 * Set the writer to reuse existing delta versions of commits.
	 *
	 * @param reuse
	 *            if true, the writer will reuse any commits stored as deltas.
	 *            By default the writer does not reuse delta commits.
	 */
	public void setReuseDeltaCommits(boolean reuse) {
		reuseDeltaCommits = reuse;
	}

	/**
	 * Check if the writer validates objects before copying them.
	 *
	 * @return true if validation is enabled; false if the reader will handle
	 *         object validation as a side-effect of it consuming the output.
	 */
	public boolean isReuseValidatingObjects() {
		return reuseValidate;
	}

	/**
	 * Enable (or disable) object validation during packing.
	 *
	 * @param validate
	 *            if true the pack writer will validate an object before it is
	 *            put into the output. This additional validation work may be
	 *            necessary to avoid propagating corruption from one local pack
	 *            file to another local pack file.
	 */
	public void setReuseValidatingObjects(boolean validate) {
		reuseValidate = validate;
	}

	/** @return true if this writer is producing a thin pack. */
	public boolean isThin() {
		return thin;
	}

	/**
	 * @param packthin
	 *            a boolean indicating whether writer may pack objects with
	 *            delta base object not within set of objects to pack, but
	 *            belonging to party repository (uninteresting/boundary) as
	 *            determined by set; this kind of pack is used only for
	 *            transport; true - to produce thin pack, false - otherwise.
	 */
	public void setThin(final boolean packthin) {
		thin = packthin;
	}

	/** @return true to reuse cached packs. If true index creation isn't available. */
	public boolean isUseCachedPacks() {
		return useCachedPacks;
	}

	/**
	 * @param useCached
	 *            if set to true and a cached pack is present, it will be
	 *            appended onto the end of a thin-pack, reducing the amount of
	 *            working set space and CPU used by PackWriter. Enabling this
	 *            feature prevents PackWriter from creating an index for the
	 *            newly created pack, so its only suitable for writing to a
	 *            network client, where the client will make the index.
	 */
	public void setUseCachedPacks(boolean useCached) {
		useCachedPacks = useCached;
	}

	/** @return true to use bitmaps for ObjectWalks, if available. */
	public boolean isUseBitmaps() {
		return useBitmaps;
	}

	/**
	 * @param useBitmaps
	 *            if set to true, bitmaps will be used when preparing a pack.
	 */
	public void setUseBitmaps(boolean useBitmaps) {
		this.useBitmaps = useBitmaps;
	}

	/** @return true if the index file cannot be created by this PackWriter. */
	public boolean isIndexDisabled() {
		return indexDisabled || !cachedPacks.isEmpty();
	}

	/**
	 * @param noIndex
	 *            true to disable creation of the index file.
	 */
	public void setIndexDisabled(boolean noIndex) {
		this.indexDisabled = noIndex;
	}

	/**
	 * @return true to ignore objects that are uninteresting and also not found
	 *         on local disk; false to throw a {@link MissingObjectException}
	 *         out of {@link #preparePack(ProgressMonitor, Set, Set)} if an
	 *         uninteresting object is not in the source repository. By default,
	 *         true, permitting gracefully ignoring of uninteresting objects.
	 */
	public boolean isIgnoreMissingUninteresting() {
		return ignoreMissingUninteresting;
	}

	/**
	 * @param ignore
	 *            true if writer should ignore non existing uninteresting
	 *            objects during construction set of objects to pack; false
	 *            otherwise - non existing uninteresting objects may cause
	 *            {@link MissingObjectException}
	 */
	public void setIgnoreMissingUninteresting(final boolean ignore) {
		ignoreMissingUninteresting = ignore;
	}

	/**
	 * Set the tag targets that should be hoisted earlier during packing.
	 * <p>
	 * Callers may put objects into this set before invoking any of the
	 * preparePack methods to influence where an annotated tag's target is
	 * stored within the resulting pack. Typically these will be clustered
	 * together, and hoisted earlier in the file even if they are ancient
	 * revisions, allowing readers to find tag targets with better locality.
	 *
	 * @param objects
	 *            objects that annotated tags point at.
	 */
	public void setTagTargets(Set<ObjectId> objects) {
		tagTargets = objects;
	}

	/**
	 * Configure this pack for a shallow clone.
	 *
	 * @param depth
	 *            maximum depth of history to return. 1 means return only the
	 *            "wants".
	 * @param unshallow
	 *            objects which used to be shallow on the client, but are being
	 *            extended as part of this fetch
	 */
	public void setShallowPack(int depth,
			Collection<? extends ObjectId> unshallow) {
		this.shallowPack = true;
		this.depth = depth;
		this.unshallowObjects = unshallow;
	}

	/**
	 * Returns objects number in a pack file that was created by this writer.
	 *
	 * @return number of objects in pack.
	 * @throws IOException
	 *             a cached pack cannot supply its object count.
	 */
	public long getObjectCount() throws IOException {
		if (stats.totalObjects == 0) {
			long objCnt = 0;

			objCnt += objectsLists[OBJ_COMMIT].size();
			objCnt += objectsLists[OBJ_TREE].size();
			objCnt += objectsLists[OBJ_BLOB].size();
			objCnt += objectsLists[OBJ_TAG].size();

			for (CachedPack pack : cachedPacks)
				objCnt += pack.getObjectCount();
			return objCnt;
		}
		return stats.totalObjects;
	}

	/**
	 * Returns the object ids in the pack file that was created by this writer.
	 * <p>
	 * This method can only be invoked after
	 * {@link #writePack(ProgressMonitor, ProgressMonitor, OutputStream)} has
	 * been invoked and completed successfully.
	 *
	 * @return set of objects in pack.
	 * @throws IOException
	 *             a cached pack cannot supply its object ids.
	 */
	public ObjectIdOwnerMap<ObjectIdOwnerMap.Entry> getObjectSet()
			throws IOException {
		if (!cachedPacks.isEmpty())
			throw new IOException(
					JGitText.get().cachedPacksPreventsListingObjects);

		if (writeBitmaps != null) {
			return writeBitmaps.getObjectSet();
		}

		ObjectIdOwnerMap<ObjectIdOwnerMap.Entry> r = new ObjectIdOwnerMap<>();
		for (BlockList<ObjectToPack> objList : objectsLists) {
			if (objList != null) {
				for (ObjectToPack otp : objList)
					r.add(new ObjectIdOwnerMap.Entry(otp) {
						// A new entry that copies the ObjectId
					});
			}
		}
		return r;
	}

	/**
	 * Add a pack index whose contents should be excluded from the result.
	 *
	 * @param idx
	 *            objects in this index will not be in the output pack.
	 */
	public void excludeObjects(ObjectIdSet idx) {
		if (excludeInPacks == null) {
			excludeInPacks = new ObjectIdSet[] { idx };
			excludeInPackLast = idx;
		} else {
			int cnt = excludeInPacks.length;
			ObjectIdSet[] newList = new ObjectIdSet[cnt + 1];
			System.arraycopy(excludeInPacks, 0, newList, 0, cnt);
			newList[cnt] = idx;
			excludeInPacks = newList;
		}
	}

	/**
	 * Prepare the list of objects to be written to the pack stream.
	 * <p>
	 * Iterator <b>exactly</b> determines which objects are included in a pack
	 * and order they appear in pack (except that objects order by type is not
	 * needed at input). This order should conform general rules of ordering
	 * objects in git - by recency and path (type and delta-base first is
	 * internally secured) and responsibility for guaranteeing this order is on
	 * a caller side. Iterator must return each id of object to write exactly
	 * once.
	 * </p>
	 *
	 * @param objectsSource
	 *            iterator of object to store in a pack; order of objects within
	 *            each type is important, ordering by type is not needed;
	 *            allowed types for objects are {@link Constants#OBJ_COMMIT},
	 *            {@link Constants#OBJ_TREE}, {@link Constants#OBJ_BLOB} and
	 *            {@link Constants#OBJ_TAG}; objects returned by iterator may be
	 *            later reused by caller as object id and type are internally
	 *            copied in each iteration.
	 * @throws IOException
	 *             when some I/O problem occur during reading objects.
	 */
	public void preparePack(@NonNull Iterator<RevObject> objectsSource)
			throws IOException {
		while (objectsSource.hasNext()) {
			addObject(objectsSource.next());
		}
	}

	/**
	 * Prepare the list of objects to be written to the pack stream.
	 * <p>
	 * Basing on these 2 sets, another set of objects to put in a pack file is
	 * created: this set consists of all objects reachable (ancestors) from
	 * interesting objects, except uninteresting objects and their ancestors.
	 * This method uses class {@link ObjectWalk} extensively to find out that
	 * appropriate set of output objects and their optimal order in output pack.
	 * Order is consistent with general git in-pack rules: sort by object type,
	 * recency, path and delta-base first.
	 * </p>
	 *
	 * @param countingMonitor
	 *            progress during object enumeration.
	 * @param want
	 *            collection of objects to be marked as interesting (start
	 *            points of graph traversal). Must not be {@code null}.
	 * @param have
	 *            collection of objects to be marked as uninteresting (end
	 *            points of graph traversal). Pass {@link #NONE} if all objects
	 *            reachable from {@code want} are desired, such as when serving
	 *            a clone.
	 * @throws IOException
	 *             when some I/O problem occur during reading objects.
	 */
	public void preparePack(ProgressMonitor countingMonitor,
			@NonNull Set<? extends ObjectId> want,
			@NonNull Set<? extends ObjectId> have) throws IOException {
		preparePack(countingMonitor, want, have, NONE, NONE);
	}

	/**
	 * Prepare the list of objects to be written to the pack stream.
	 * <p>
	 * Like {@link #preparePack(ProgressMonitor, Set, Set)} but also allows
	 * specifying commits that should not be walked past ("shallow" commits).
	 * The caller is responsible for filtering out commits that should not be
	 * shallow any more ("unshallow" commits as in {@link #setShallowPack}) from
	 * the shallow set.
	 *
	 * @param countingMonitor
	 *            progress during object enumeration.
	 * @param want
	 *            objects of interest, ancestors of which will be included in
	 *            the pack. Must not be {@code null}.
	 * @param have
	 *            objects whose ancestors (up to and including {@code shallow}
	 *            commits) do not need to be included in the pack because they
	 *            are already available from elsewhere. Must not be
	 *            {@code null}.
	 * @param shallow
	 *            commits indicating the boundary of the history marked with
	 *            {@code have}. Shallow commits have parents but those parents
	 *            are considered not to be already available. Parents of
	 *            {@code shallow} commits and earlier generations will be
	 *            included in the pack if requested by {@code want}. Must not be
	 *            {@code null}.
	 * @throws IOException
	 *             an I/O problem occurred while reading objects.
	 */
	public void preparePack(ProgressMonitor countingMonitor,
			@NonNull Set<? extends ObjectId> want,
			@NonNull Set<? extends ObjectId> have,
			@NonNull Set<? extends ObjectId> shallow) throws IOException {
		preparePack(countingMonitor, want, have, shallow, NONE);
	}

	/**
	 * Prepare the list of objects to be written to the pack stream.
	 * <p>
	 * Like {@link #preparePack(ProgressMonitor, Set, Set)} but also allows
	 * specifying commits that should not be walked past ("shallow" commits).
	 * The caller is responsible for filtering out commits that should not be
	 * shallow any more ("unshallow" commits as in {@link #setShallowPack}) from
	 * the shallow set.
	 *
	 * @param countingMonitor
	 *            progress during object enumeration.
	 * @param want
	 *            objects of interest, ancestors of which will be included in
	 *            the pack. Must not be {@code null}.
	 * @param have
	 *            objects whose ancestors (up to and including {@code shallow}
	 *            commits) do not need to be included in the pack because they
	 *            are already available from elsewhere. Must not be
	 *            {@code null}.
	 * @param shallow
	 *            commits indicating the boundary of the history marked with
	 *            {@code have}. Shallow commits have parents but those parents
	 *            are considered not to be already available. Parents of
	 *            {@code shallow} commits and earlier generations will be
	 *            included in the pack if requested by {@code want}. Must not be
	 *            {@code null}.
	 * @param noBitmaps
	 *            collection of objects to be excluded from bitmap commit
	 *            selection.
	 * @throws IOException
	 *             an I/O problem occurred while reading objects.
	 */
	public void preparePack(ProgressMonitor countingMonitor,
			@NonNull Set<? extends ObjectId> want,
			@NonNull Set<? extends ObjectId> have,
			@NonNull Set<? extends ObjectId> shallow,
			@NonNull Set<? extends ObjectId> noBitmaps) throws IOException {
		try (ObjectWalk ow = getObjectWalk()) {
			ow.assumeShallow(shallow);
			preparePack(countingMonitor, ow, want, have, noBitmaps);
		}
	}

	private ObjectWalk getObjectWalk() {
		return shallowPack ? new DepthWalk.ObjectWalk(reader, depth - 1)
				: new ObjectWalk(reader);
	}

	/**
	 * Prepare the list of objects to be written to the pack stream.
	 * <p>
	 * Basing on these 2 sets, another set of objects to put in a pack file is
	 * created: this set consists of all objects reachable (ancestors) from
	 * interesting objects, except uninteresting objects and their ancestors.
	 * This method uses class {@link ObjectWalk} extensively to find out that
	 * appropriate set of output objects and their optimal order in output pack.
	 * Order is consistent with general git in-pack rules: sort by object type,
	 * recency, path and delta-base first.
	 * </p>
	 *
	 * @param countingMonitor
	 *            progress during object enumeration.
	 * @param walk
	 *            ObjectWalk to perform enumeration.
	 * @param interestingObjects
	 *            collection of objects to be marked as interesting (start
	 *            points of graph traversal). Must not be {@code null}.
	 * @param uninterestingObjects
	 *            collection of objects to be marked as uninteresting (end
	 *            points of graph traversal). Pass {@link #NONE} if all objects
	 *            reachable from {@code want} are desired, such as when serving
	 *            a clone.
	 * @param noBitmaps
	 *            collection of objects to be excluded from bitmap commit
	 *            selection.
	 * @throws IOException
	 *             when some I/O problem occur during reading objects.
	 */
	public void preparePack(ProgressMonitor countingMonitor,
			@NonNull ObjectWalk walk,
			@NonNull Set<? extends ObjectId> interestingObjects,
			@NonNull Set<? extends ObjectId> uninterestingObjects,
			@NonNull Set<? extends ObjectId> noBitmaps)
			throws IOException {
		if (countingMonitor == null)
			countingMonitor = NullProgressMonitor.INSTANCE;
		if (shallowPack && !(walk instanceof DepthWalk.ObjectWalk))
			throw new IllegalArgumentException(
					JGitText.get().shallowPacksRequireDepthWalk);
		findObjectsToPack(countingMonitor, walk, interestingObjects,
				uninterestingObjects, noBitmaps);
	}

	/**
	 * Determine if the pack file will contain the requested object.
	 *
	 * @param id
	 *            the object to test the existence of.
	 * @return true if the object will appear in the output pack file.
	 * @throws IOException
	 *             a cached pack cannot be examined.
	 */
	public boolean willInclude(final AnyObjectId id) throws IOException {
		ObjectToPack obj = objectsMap.get(id);
		return obj != null && !obj.isEdge();
	}

	/**
	 * Lookup the ObjectToPack object for a given ObjectId.
	 *
	 * @param id
	 *            the object to find in the pack.
	 * @return the object we are packing, or null.
	 */
	public ObjectToPack get(AnyObjectId id) {
		ObjectToPack obj = objectsMap.get(id);
		return obj != null && !obj.isEdge() ? obj : null;
	}

	/**
	 * Computes SHA-1 of lexicographically sorted objects ids written in this
	 * pack, as used to name a pack file in repository.
	 *
	 * @return ObjectId representing SHA-1 name of a pack that was created.
	 */
	public ObjectId computeName() {
		final byte[] buf = new byte[OBJECT_ID_LENGTH];
		final MessageDigest md = Constants.newMessageDigest();
		for (ObjectToPack otp : sortByName()) {
			otp.copyRawTo(buf, 0);
			md.update(buf, 0, OBJECT_ID_LENGTH);
		}
		return ObjectId.fromRaw(md.digest());
	}

	/**
	 * Returns the index format version that will be written.
	 * <p>
	 * This method can only be invoked after
	 * {@link #writePack(ProgressMonitor, ProgressMonitor, OutputStream)} has
	 * been invoked and completed successfully.
	 *
	 * @return the index format version.
	 */
	public int getIndexVersion() {
		int indexVersion = config.getIndexVersion();
		if (indexVersion <= 0) {
			for (BlockList<ObjectToPack> objs : objectsLists)
				indexVersion = Math.max(indexVersion,
						PackIndexWriter.oldestPossibleFormat(objs));
		}
		return indexVersion;
	}

	/**
	 * Create an index file to match the pack file just written.
	 * <p>
	 * Called after
	 * {@link #writePack(ProgressMonitor, ProgressMonitor, OutputStream)}.
	 * <p>
	 * Writing an index is only required for local pack storage. Packs sent on
	 * the network do not need to create an index.
	 *
	 * @param indexStream
	 *            output for the index data. Caller is responsible for closing
	 *            this stream.
	 * @throws IOException
	 *             the index data could not be written to the supplied stream.
	 */
	public void writeIndex(final OutputStream indexStream) throws IOException {
		if (isIndexDisabled())
			throw new IOException(JGitText.get().cachedPacksPreventsIndexCreation);

		long writeStart = System.currentTimeMillis();
		final PackIndexWriter iw = PackIndexWriter.createVersion(
				indexStream, getIndexVersion());
		iw.write(sortByName(), packcsum);
		stats.timeWriting += System.currentTimeMillis() - writeStart;
	}

	/**
	 * Create a bitmap index file to match the pack file just written.
	 * <p>
	 * Called after {@link #prepareBitmapIndex(ProgressMonitor)}.
	 *
	 * @param bitmapIndexStream
	 *            output for the bitmap index data. Caller is responsible for
	 *            closing this stream.
	 * @throws IOException
	 *             the index data could not be written to the supplied stream.
	 */
	public void writeBitmapIndex(final OutputStream bitmapIndexStream)
			throws IOException {
		if (writeBitmaps == null)
			throw new IOException(JGitText.get().bitmapsMustBePrepared);

		long writeStart = System.currentTimeMillis();
		final PackBitmapIndexWriterV1 iw = new PackBitmapIndexWriterV1(bitmapIndexStream);
		iw.write(writeBitmaps, packcsum);
		stats.timeWriting += System.currentTimeMillis() - writeStart;
	}

	private List<ObjectToPack> sortByName() {
		if (sortedByName == null) {
			int cnt = 0;
			cnt += objectsLists[OBJ_COMMIT].size();
			cnt += objectsLists[OBJ_TREE].size();
			cnt += objectsLists[OBJ_BLOB].size();
			cnt += objectsLists[OBJ_TAG].size();

			sortedByName = new BlockList<>(cnt);
			sortedByName.addAll(objectsLists[OBJ_COMMIT]);
			sortedByName.addAll(objectsLists[OBJ_TREE]);
			sortedByName.addAll(objectsLists[OBJ_BLOB]);
			sortedByName.addAll(objectsLists[OBJ_TAG]);
			Collections.sort(sortedByName);
		}
		return sortedByName;
	}

	private void beginPhase(PackingPhase phase, ProgressMonitor monitor,
			long cnt) {
		state.phase = phase;
		String task;
		switch (phase) {
		case COUNTING:
			task = JGitText.get().countingObjects;
			break;
		case GETTING_SIZES:
			task = JGitText.get().searchForSizes;
			break;
		case FINDING_SOURCES:
			task = JGitText.get().searchForReuse;
			break;
		case COMPRESSING:
			task = JGitText.get().compressingObjects;
			break;
		case WRITING:
			task = JGitText.get().writingObjects;
			break;
		case BUILDING_BITMAPS:
			task = JGitText.get().buildingBitmaps;
			break;
		default:
 			throw new IllegalArgumentException(
 					MessageFormat.format(JGitText.get().illegalPackingPhase, phase));
 		}
 		monitor.beginTask(task, (int) cnt);
 	}

 	private void endPhase(ProgressMonitor monitor) {
 		monitor.endTask();
 	}

 	/**
 	 * Write the prepared pack to the supplied stream.
 	 * <p>
 	 * Called after
 	 * {@link #preparePack(ProgressMonitor, ObjectWalk, Set, Set, Set)} or
 	 * {@link #preparePack(ProgressMonitor, Set, Set)}.
 	 * <p>
 	 * Performs delta search if enabled and writes the pack stream.
 	 * <p>
 	 * All reused objects data checksum (Adler32/CRC32) is computed and
 	 * validated against existing checksum.
 	 *
 	 * @param compressMonitor
 	 *            progress monitor to report object compression work.
 	 * @param writeMonitor
 	 *            progress monitor to report the number of objects written.
 	 * @param packStream
 	 *            output stream of pack data. The stream should be buffered by
 	 *            the caller. The caller is responsible for closing the stream.
 	 * @throws IOException
 	 *             an error occurred reading a local object's data to include in
 	 *             the pack, or writing compressed object data to the output
 	 *             stream.
 	 * @throws WriteAbortedException
 	 *             the write operation is aborted by {@link ObjectCountCallback}
 	 *             .
 	 */
 	public void writePack(ProgressMonitor compressMonitor,
 			ProgressMonitor writeMonitor, OutputStream packStream)
 			throws IOException {
 		if (compressMonitor == null)
 			compressMonitor = NullProgressMonitor.INSTANCE;
 		if (writeMonitor == null)
 			writeMonitor = NullProgressMonitor.INSTANCE;

 		excludeInPacks = null;
 		excludeInPackLast = null;

 		boolean needSearchForReuse = reuseSupport != null && (
 				   reuseDeltas
 				|| config.isReuseObjects()
 				|| !cachedPacks.isEmpty());

 		if (compressMonitor instanceof BatchingProgressMonitor) {
 			long delay = 1000;
 			if (needSearchForReuse && config.isDeltaCompress())
 				delay = 500;
 			((BatchingProgressMonitor) compressMonitor).setDelayStart(
 					delay,
 					TimeUnit.MILLISECONDS);
 		}

 		if (needSearchForReuse)
 			searchForReuse(compressMonitor);
 		if (config.isDeltaCompress())
 			searchForDeltas(compressMonitor);

 		crc32 = new CRC32();
 		final PackOutputStream out = new PackOutputStream(
 			writeMonitor,
 			isIndexDisabled()
 				? packStream
 				: new CheckedOutputStream(packStream, crc32),
 			this);

 		long objCnt = getObjectCount();
 		stats.totalObjects = objCnt;
 		if (callback != null)
 			callback.setObjectCount(objCnt);
 		beginPhase(PackingPhase.WRITING, writeMonitor, objCnt);
 		long writeStart = System.currentTimeMillis();
 		try {
 			out.writeFileHeader(PACK_VERSION_GENERATED, objCnt);
 			out.flush();

 			writeObjects(out);
 			if (!edgeObjects.isEmpty() || !cachedPacks.isEmpty()) {
 				for (PackStatistics.ObjectType.Accumulator typeStat : stats.objectTypes) {
 					if (typeStat == null)
 						continue;
 					stats.thinPackBytes += typeStat.bytes;
 				}
 			}

 			stats.reusedPacks = Collections.unmodifiableList(cachedPacks);
 			for (CachedPack pack : cachedPacks) {
 				long deltaCnt = pack.getDeltaCount();
 				stats.reusedObjects += pack.getObjectCount();
 				stats.reusedDeltas += deltaCnt;
 				stats.totalDeltas += deltaCnt;
 				reuseSupport.copyPackAsIs(out, pack);
 			}
 			writeChecksum(out);
 			out.flush();
 		} finally {
 			stats.timeWriting = System.currentTimeMillis() - writeStart;
 			stats.depth = depth;

 			for (PackStatistics.ObjectType.Accumulator typeStat : stats.objectTypes) {
 				if (typeStat == null)
 					continue;
 				typeStat.cntDeltas += typeStat.reusedDeltas;
 				stats.reusedObjects += typeStat.reusedObjects;
 				stats.reusedDeltas += typeStat.reusedDeltas;
 				stats.totalDeltas += typeStat.cntDeltas;
 			}
 		}

 		stats.totalBytes = out.length();
 		reader.close();
 		endPhase(writeMonitor);
 	}

 	/**
 	 * @return description of what this PackWriter did in order to create the
 	 *         final pack stream. This should only be invoked after the calls to
 	 *         create the pack/index/bitmap have completed.
 	 */
 	public PackStatistics getStatistics() {
 		return new PackStatistics(stats);
 	}

 	/** @return snapshot of the current state of this PackWriter. */
 	public State getState() {
 		return state.snapshot();
 	}

 	/**
 	 * Release all resources used by this writer.
 	 */
 	@Override
 	public void close() {
 		reader.close();
 		if (myDeflater != null) {
 			myDeflater.end();
 			myDeflater = null;
 		}
 		instances.remove(selfRef);
 	}

 	private void searchForReuse(ProgressMonitor monitor) throws IOException {
 		long cnt = 0;
 		cnt += objectsLists[OBJ_COMMIT].size();
 		cnt += objectsLists[OBJ_TREE].size();
 		cnt += objectsLists[OBJ_BLOB].size();
 		cnt += objectsLists[OBJ_TAG].size();

 		long start = System.currentTimeMillis();
 		beginPhase(PackingPhase.FINDING_SOURCES, monitor, cnt);
 		if (cnt <= 4096) {
 			// For small object counts, do everything as one list.
 			BlockList<ObjectToPack> tmp = new BlockList<>((int) cnt);
 			tmp.addAll(objectsLists[OBJ_TAG]);
 			tmp.addAll(objectsLists[OBJ_COMMIT]);
 			tmp.addAll(objectsLists[OBJ_TREE]);
 			tmp.addAll(objectsLists[OBJ_BLOB]);
 			searchForReuse(monitor, tmp);
 			if (pruneCurrentObjectList) {
 				// If the list was pruned, we need to re-prune the main lists.
 				pruneEdgesFromObjectList(objectsLists[OBJ_COMMIT]);
 				pruneEdgesFromObjectList(objectsLists[OBJ_TREE]);
 				pruneEdgesFromObjectList(objectsLists[OBJ_BLOB]);
 				pruneEdgesFromObjectList(objectsLists[OBJ_TAG]);
 			}
 		} else {
 			searchForReuse(monitor, objectsLists[OBJ_TAG]);
 			searchForReuse(monitor, objectsLists[OBJ_COMMIT]);
 			searchForReuse(monitor, objectsLists[OBJ_TREE]);
 			searchForReuse(monitor, objectsLists[OBJ_BLOB]);
 		}
 		endPhase(monitor);
 		stats.timeSearchingForReuse = System.currentTimeMillis() - start;

 		if (config.isReuseDeltas() && config.getCutDeltaChains()) {
 			cutDeltaChains(objectsLists[OBJ_TREE]);
 			cutDeltaChains(objectsLists[OBJ_BLOB]);
 		}
 	}

 	private void searchForReuse(ProgressMonitor monitor, List<ObjectToPack> list)
 			throws IOException, MissingObjectException {
 		pruneCurrentObjectList = false;
 		reuseSupport.selectObjectRepresentation(this, monitor, list);
 		if (pruneCurrentObjectList)
 			pruneEdgesFromObjectList(list);
 	}

 	private void cutDeltaChains(BlockList<ObjectToPack> list)
 			throws IOException {
 		int max = config.getMaxDeltaDepth();
 		for (int idx = list.size() - 1; idx >= 0; idx--) {
 			int d = 0;
 			ObjectToPack b = list.get(idx).getDeltaBase();
 			while (b != null) {
 				if (d < b.getChainLength())
 					break;
 				b.setChainLength(++d);
 				if (d >= max && b.isDeltaRepresentation()) {
 					reselectNonDelta(b);
 					break;
 				}
 				b = b.getDeltaBase();
 			}
 		}
 		if (config.isDeltaCompress()) {
 			for (ObjectToPack otp : list)
 				otp.clearChainLength();
 		}
 	}

 	private void searchForDeltas(ProgressMonitor monitor)
 			throws MissingObjectException, IncorrectObjectTypeException,
 			IOException {
 		// Commits and annotated tags tend to have too many differences to
 		// really benefit from delta compression. Consequently just don't
 		// bother examining those types here.
 		//
 		ObjectToPack[] list = new ObjectToPack[
 				  objectsLists[OBJ_TREE].size()
 				+ objectsLists[OBJ_BLOB].size()
 				+ edgeObjects.size()];
 		int cnt = 0;
 		cnt = findObjectsNeedingDelta(list, cnt, OBJ_TREE);
 		cnt = findObjectsNeedingDelta(list, cnt, OBJ_BLOB);
 		if (cnt == 0)
 			return;
 		int nonEdgeCnt = cnt;

 		// Queue up any edge objects that we might delta against.  We won't
 		// be sending these as we assume the other side has them, but we need
 		// them in the search phase below.
 		//
 		for (ObjectToPack eo : edgeObjects) {
 			eo.setWeight(0);
 			list[cnt++] = eo;
 		}

 		// Compute the sizes of the objects so we can do a proper sort.
 		// We let the reader skip missing objects if it chooses. For
 		// some readers this can be a huge win. We detect missing objects
 		// by having set the weights above to 0 and allowing the delta
 		// search code to discover the missing object and skip over it, or
 		// abort with an exception if we actually had to have it.
 		//
 		final long sizingStart = System.currentTimeMillis();
 		beginPhase(PackingPhase.GETTING_SIZES, monitor, cnt);
 		AsyncObjectSizeQueue<ObjectToPack> sizeQueue = reader.getObjectSize(
 				Arrays.<ObjectToPack> asList(list).subList(0, cnt), false);
 		try {
 			final long limit = Math.min(
 					config.getBigFileThreshold(),
 					Integer.MAX_VALUE);
 			for (;;) {
 				try {
 					if (!sizeQueue.next())
 						break;
 				} catch (MissingObjectException notFound) {
 					monitor.update(1);
 					if (ignoreMissingUninteresting) {
 						ObjectToPack otp = sizeQueue.getCurrent();
 						if (otp != null && otp.isEdge()) {
 							otp.setDoNotDelta();
 							continue;
 						}

 						otp = objectsMap.get(notFound.getObjectId());
 						if (otp != null && otp.isEdge()) {
 							otp.setDoNotDelta();
 							continue;
 						}
 					}
 					throw notFound;
 				}

 				ObjectToPack otp = sizeQueue.getCurrent();
 				if (otp == null)
 					otp = objectsMap.get(sizeQueue.getObjectId());

 				long sz = sizeQueue.getSize();
 				if (DeltaIndex.BLKSZ < sz && sz < limit)
 					otp.setWeight((int) sz);
 				else
 					otp.setDoNotDelta(); // too small, or too big
 				monitor.update(1);
 			}
 		} finally {
 			sizeQueue.release();
 		}
 		endPhase(monitor);
 		stats.timeSearchingForSizes = System.currentTimeMillis() - sizingStart;

 		// Sort the objects by path hash so like files are near each other,
 		// and then by size descending so that bigger files are first. This
 		// applies "Linus' Law" which states that newer files tend to be the
 		// bigger ones, because source files grow and hardly ever shrink.
 		//
 		Arrays.sort(list, 0, cnt, new Comparator<ObjectToPack>() {
 			@Override
 			public int compare(ObjectToPack a, ObjectToPack b) {
 				int cmp = (a.isDoNotDelta() ? 1 : 0)
 						- (b.isDoNotDelta() ? 1 : 0);
 				if (cmp != 0)
 					return cmp;

 				cmp = a.getType() - b.getType();
 				if (cmp != 0)
 					return cmp;

 				cmp = (a.getPathHash() >>> 1) - (b.getPathHash() >>> 1);
 				if (cmp != 0)
 					return cmp;

 				cmp = (a.getPathHash() & 1) - (b.getPathHash() & 1);
 				if (cmp != 0)
 					return cmp;

 				cmp = (a.isEdge() ? 0 : 1) - (b.isEdge() ? 0 : 1);
 				if (cmp != 0)
 					return cmp;

 				return b.getWeight() - a.getWeight();
 			}
 		});

 		// Above we stored the objects we cannot delta onto the end.
 		// Remove them from the list so we don't waste time on them.
 		while (0 < cnt && list[cnt - 1].isDoNotDelta()) {
 			if (!list[cnt - 1].isEdge())
 				nonEdgeCnt--;
 			cnt--;
 		}
 		if (cnt == 0)
 			return;

 		final long searchStart = System.currentTimeMillis();
 		searchForDeltas(monitor, list, cnt);
 		stats.deltaSearchNonEdgeObjects = nonEdgeCnt;
 		stats.timeCompressing = System.currentTimeMillis() - searchStart;

 		for (int i = 0; i < cnt; i++)
 			if (!list[i].isEdge() && list[i].isDeltaRepresentation())
 				stats.deltasFound++;
 	}

 	private int findObjectsNeedingDelta(ObjectToPack[] list, int cnt, int type) {
 		for (ObjectToPack otp : objectsLists[type]) {
 			if (otp.isDoNotDelta()) // delta is disabled for this path
 				continue;
 			if (otp.isDeltaRepresentation()) // already reusing a delta
 				continue;
 			otp.setWeight(0);
 			list[cnt++] = otp;
 		}
 		return cnt;
 	}

 	private void reselectNonDelta(ObjectToPack otp) throws IOException {
 		otp.clearDeltaBase();
 		otp.clearReuseAsIs();
 		boolean old = reuseDeltas;
 		reuseDeltas = false;
 		reuseSupport.selectObjectRepresentation(this,
 				NullProgressMonitor.INSTANCE,
 				Collections.singleton(otp));
 		reuseDeltas = old;
 	}

 	private void searchForDeltas(final ProgressMonitor monitor,
 			final ObjectToPack[] list, final int cnt)
 			throws MissingObjectException, IncorrectObjectTypeException,
 			LargeObjectException, IOException {
 		int threads = config.getThreads();
 		if (threads == 0)
 			threads = Runtime.getRuntime().availableProcessors();
 		if (threads <= 1 || cnt <= config.getDeltaSearchWindowSize())
 			singleThreadDeltaSearch(monitor, list, cnt);
 		else
 			parallelDeltaSearch(monitor, list, cnt, threads);
 	}

 	private void singleThreadDeltaSearch(ProgressMonitor monitor,
 			ObjectToPack[] list, int cnt) throws IOException {
 		long totalWeight = 0;
 		for (int i = 0; i < cnt; i++) {
 			ObjectToPack o = list[i];
 			totalWeight += DeltaTask.getAdjustedWeight(o);
 		}

 		long bytesPerUnit = 1;
 		while (DeltaTask.MAX_METER <= (totalWeight / bytesPerUnit))
 			bytesPerUnit <<= 10;
 		int cost = (int) (totalWeight / bytesPerUnit);
 		if (totalWeight % bytesPerUnit != 0)
 			cost++;

 		beginPhase(PackingPhase.COMPRESSING, monitor, cost);
 		new DeltaWindow(config, new DeltaCache(config), reader,
 				monitor, bytesPerUnit,
 				list, 0, cnt).search();
 		endPhase(monitor);
 	}

 	private void parallelDeltaSearch(ProgressMonitor monitor,
 			ObjectToPack[] list, int cnt, int threads) throws IOException {
 		DeltaCache dc = new ThreadSafeDeltaCache(config);
 		ThreadSafeProgressMonitor pm = new ThreadSafeProgressMonitor(monitor);
 		DeltaTask.Block taskBlock = new DeltaTask.Block(threads, config,
 				reader, dc, pm,
 				list, 0, cnt);
 		taskBlock.partitionTasks();
 		beginPhase(PackingPhase.COMPRESSING, monitor, taskBlock.cost());
 		pm.startWorkers(taskBlock.tasks.size());

 		Executor executor = config.getExecutor();
 		final List<Throwable> errors =
 				Collections.synchronizedList(new ArrayList<Throwable>(threads));
 		if (executor instanceof ExecutorService) {
 			// Caller supplied us a service, use it directly.
 			runTasks((ExecutorService) executor, pm, taskBlock, errors);
 		} else if (executor == null) {
 			// Caller didn't give us a way to run the tasks, spawn up a
 			// temporary thread pool and make sure it tears down cleanly.
 			ExecutorService pool = Executors.newFixedThreadPool(threads);
 			try {
 				runTasks(pool, pm, taskBlock, errors);
 			} finally {
 				pool.shutdown();
 				for (;;) {
 					try {
 						if (pool.awaitTermination(60, TimeUnit.SECONDS))
 							break;
 					} catch (InterruptedException e) {
 						throw new IOException(
 								JGitText.get().packingCancelledDuringObjectsWriting);
 					}
 				}
 			}
 		} else {
 			// The caller gave us an executor, but it might not do
 			// asynchronous execution.  Wrap everything and hope it
 			// can schedule these for us.
 			for (final DeltaTask task : taskBlock.tasks) {
 				executor.execute(new Runnable() {
 					@Override
 					public void run() {
 						try {
 							task.call();
 						} catch (Throwable failure) {
 							errors.add(failure);
 						}
 					}
 				});
 			}
 			try {
 				pm.waitForCompletion();
 			} catch (InterruptedException ie) {
 				// We can't abort the other tasks as we have no handle.
 				// Cross our fingers and just break out anyway.
 				//
 				throw new IOException(
 						JGitText.get().packingCancelledDuringObjectsWriting);
 			}
 		}

 		// If any task threw an error, try to report it back as
 		// though we weren't using a threaded search algorithm.
 		//
 		if (!errors.isEmpty()) {
 			Throwable err = errors.get(0);
 			if (err instanceof Error)
 				throw (Error) err;
 			if (err instanceof RuntimeException)
 				throw (RuntimeException) err;
 			if (err instanceof IOException)
 				throw (IOException) err;

 			IOException fail = new IOException(err.getMessage());
 			fail.initCause(err);
 			throw fail;
 		}
 		endPhase(monitor);
 	}

 	private static void runTasks(ExecutorService pool,
 			ThreadSafeProgressMonitor pm,
 			DeltaTask.Block tb, List<Throwable> errors) throws IOException {
 		List<Future<?>> futures = new ArrayList<>(tb.tasks.size());
 		for (DeltaTask task : tb.tasks)
 			futures.add(pool.submit(task));

 		try {
 			pm.waitForCompletion();
 			for (Future<?> f : futures) {
 				try {
 					f.get();
 				} catch (ExecutionException failed) {
 					errors.add(failed.getCause());
 				}
 			}
 		} catch (InterruptedException ie) {
 			for (Future<?> f : futures)
 				f.cancel(true);
 			throw new IOException(
 					JGitText.get().packingCancelledDuringObjectsWriting);
 		}
 	}

 	private void writeObjects(PackOutputStream out) throws IOException {
 		writeObjects(out, objectsLists[OBJ_COMMIT]);
 		writeObjects(out, objectsLists[OBJ_TAG]);
 		writeObjects(out, objectsLists[OBJ_TREE]);
 		writeObjects(out, objectsLists[OBJ_BLOB]);
 	}

 	private void writeObjects(PackOutputStream out, List<ObjectToPack> list)
 			throws IOException {
 		if (list.isEmpty())
 			return;

 		typeStats = stats.objectTypes[list.get(0).getType()];
 		long beginOffset = out.length();

 		if (reuseSupport != null) {
 			reuseSupport.writeObjects(out, list);
 		} else {
 			for (ObjectToPack otp : list)
 				out.writeObject(otp);
 		}

 		typeStats.bytes += out.length() - beginOffset;
 		typeStats.cntObjects = list.size();
 	}

 	void writeObject(PackOutputStream out, ObjectToPack otp) throws IOException {
 		if (!otp.isWritten())
 			writeObjectImpl(out, otp);
 	}

 	private void writeObjectImpl(PackOutputStream out, ObjectToPack otp)
 			throws IOException {
 		if (otp.wantWrite()) {
 			// A cycle exists in this delta chain. This should only occur if a
 			// selected object representation disappeared during writing
 			// (for example due to a concurrent repack) and a different base
 			// was chosen, forcing a cycle. Select something other than a
 			// delta, and write this object.
 			reselectNonDelta(otp);
 		}
 		otp.markWantWrite();

 		while (otp.isReuseAsIs()) {
 			writeBase(out, otp.getDeltaBase());
 			if (otp.isWritten())
 				return; // Delta chain cycle caused this to write already.

 			crc32.reset();
 			otp.setOffset(out.length());
 			try {
 				reuseSupport.copyObjectAsIs(out, otp, reuseValidate);
 				out.endObject();
 				otp.setCRC((int) crc32.getValue());
 				typeStats.reusedObjects++;
 				if (otp.isDeltaRepresentation()) {
 					typeStats.reusedDeltas++;
 					typeStats.deltaBytes += out.length() - otp.getOffset();
 				}
 				return;
 			} catch (StoredObjectRepresentationNotAvailableException gone) {
 				if (otp.getOffset() == out.length()) {
 					otp.setOffset(0);
 					otp.clearDeltaBase();
 					otp.clearReuseAsIs();
 					reuseSupport.selectObjectRepresentation(this,
 							NullProgressMonitor.INSTANCE,
 							Collections.singleton(otp));
 					continue;
 				} else {
 					// Object writing already started, we cannot recover.
 					//
 					CorruptObjectException coe;
 					coe = new CorruptObjectException(otp, ""); //$NON-NLS-1$
 					coe.initCause(gone);
 					throw coe;
 				}
 			}
 		}

 		// If we reached here, reuse wasn't possible.
 		//
 		if (otp.isDeltaRepresentation())
 			writeDeltaObjectDeflate(out, otp);
 		else
 			writeWholeObjectDeflate(out, otp);
 		out.endObject();
 		otp.setCRC((int) crc32.getValue());
 	}

 	private void writeBase(PackOutputStream out, ObjectToPack base)
 			throws IOException {
 		if (base != null && !base.isWritten() && !base.isEdge())
 			writeObjectImpl(out, base);
 	}

 	private void writeWholeObjectDeflate(PackOutputStream out,
 			final ObjectToPack otp) throws IOException {
 		final Deflater deflater = deflater();
 		final ObjectLoader ldr = reader.open(otp, otp.getType());

 		crc32.reset();
 		otp.setOffset(out.length());
 		out.writeHeader(otp, ldr.getSize());

 		deflater.reset();
 		DeflaterOutputStream dst = new DeflaterOutputStream(out, deflater);
 		ldr.copyTo(dst);
 		dst.finish();
 	}

 	private void writeDeltaObjectDeflate(PackOutputStream out,
 			final ObjectToPack otp) throws IOException {
 		writeBase(out, otp.getDeltaBase());

 		crc32.reset();
 		otp.setOffset(out.length());

 		DeltaCache.Ref ref = otp.popCachedDelta();
 		if (ref != null) {
 			byte[] zbuf = ref.get();
 			if (zbuf != null) {
 				out.writeHeader(otp, otp.getCachedSize());
 				out.write(zbuf);
 				typeStats.cntDeltas++;
 				typeStats.deltaBytes += out.length() - otp.getOffset();
 				return;
 			}
 		}

 		try (TemporaryBuffer.Heap delta = delta(otp)) {
 			out.writeHeader(otp, delta.length());

 			Deflater deflater = deflater();
 			deflater.reset();
 			DeflaterOutputStream dst = new DeflaterOutputStream(out, deflater);
 			delta.writeTo(dst, null);
 			dst.finish();
 		}
 		typeStats.cntDeltas++;
 		typeStats.deltaBytes += out.length() - otp.getOffset();
 	}

 	private TemporaryBuffer.Heap delta(final ObjectToPack otp)
 			throws IOException {
 		DeltaIndex index = new DeltaIndex(buffer(otp.getDeltaBaseId()));
 		byte[] res = buffer(otp);

 		// We never would have proposed this pair if the delta would be
 		// larger than the unpacked version of the object. So using it
 		// as our buffer limit is valid: we will never reach it.
 		//
 		TemporaryBuffer.Heap delta = new TemporaryBuffer.Heap(res.length);
 		index.encode(delta, res);
 		return delta;
 	}

 	private byte[] buffer(AnyObjectId objId) throws IOException {
 		return buffer(config, reader, objId);
 	}

 	static byte[] buffer(PackConfig config, ObjectReader or, AnyObjectId objId)
 			throws IOException {
 		// PackWriter should have already pruned objects that
 		// are above the big file threshold, so our chances of
 		// the object being below it are very good. We really
 		// shouldn't be here, unless the implementation is odd.

 		return or.open(objId).getCachedBytes(config.getBigFileThreshold());
 	}

 	private Deflater deflater() {
 		if (myDeflater == null)
 			myDeflater = new Deflater(config.getCompressionLevel());
 		return myDeflater;
 	}

 	private void writeChecksum(PackOutputStream out) throws IOException {
 		packcsum = out.getDigest();
 		out.write(packcsum);
 	}

 	private void findObjectsToPack(@NonNull ProgressMonitor countingMonitor,
 			@NonNull ObjectWalk walker, @NonNull Set<? extends ObjectId> want,
 			@NonNull Set<? extends ObjectId> have,
 			@NonNull Set<? extends ObjectId> noBitmaps) throws IOException {
 		final long countingStart = System.currentTimeMillis();
 		beginPhase(PackingPhase.COUNTING, countingMonitor, ProgressMonitor.UNKNOWN);

 		stats.interestingObjects = Collections.unmodifiableSet(new HashSet<ObjectId>(want));
 		stats.uninterestingObjects = Collections.unmodifiableSet(new HashSet<ObjectId>(have));
 		excludeFromBitmapSelection = noBitmaps;

 		canBuildBitmaps = config.isBuildBitmaps()
 				&& !shallowPack
 				&& have.isEmpty()
 				&& (excludeInPacks == null || excludeInPacks.length == 0);
 		if (!shallowPack && useBitmaps) {
 			BitmapIndex bitmapIndex = reader.getBitmapIndex();
 			if (bitmapIndex != null) {
 				PackWriterBitmapWalker bitmapWalker = new PackWriterBitmapWalker(
 						walker, bitmapIndex, countingMonitor);
 				findObjectsToPackUsingBitmaps(bitmapWalker, want, have);
 				endPhase(countingMonitor);
 				stats.timeCounting = System.currentTimeMillis() - countingStart;
 				stats.bitmapIndexMisses = bitmapWalker.getCountOfBitmapIndexMisses();
 				return;
 			}
 		}

 		List<ObjectId> all = new ArrayList<>(want.size() + have.size());
 		all.addAll(want);
 		all.addAll(have);

 		final RevFlag include = walker.newFlag("include"); //$NON-NLS-1$
 		final RevFlag added = walker.newFlag("added"); //$NON-NLS-1$

 		walker.carry(include);

 		int haveEst = have.size();
 		if (have.isEmpty()) {
 			walker.sort(RevSort.COMMIT_TIME_DESC);
 		} else {
 			walker.sort(RevSort.TOPO);
 			if (thin)
 				walker.sort(RevSort.BOUNDARY, true);
 		}

 		List<RevObject> wantObjs = new ArrayList<>(want.size());
 		List<RevObject> haveObjs = new ArrayList<>(haveEst);
 		List<RevTag> wantTags = new ArrayList<>(want.size());

 		// Retrieve the RevWalk's versions of "want" and "have" objects to
 		// maintain any state previously set in the RevWalk.
 		AsyncRevObjectQueue q = walker.parseAny(all, true);
 		try {
 			for (;;) {
 				try {
 					RevObject o = q.next();
 					if (o == null)
 						break;
 					if (have.contains(o))
 						haveObjs.add(o);
 					if (want.contains(o)) {
 						o.add(include);
 						wantObjs.add(o);
 						if (o instanceof RevTag)
 							wantTags.add((RevTag) o);
 					}
 				} catch (MissingObjectException e) {
 					if (ignoreMissingUninteresting
 							&& have.contains(e.getObjectId()))
 						continue;
 					throw e;
 				}
 			}
 		} finally {
 			q.release();
 		}

 		if (!wantTags.isEmpty()) {
 			all = new ArrayList<>(wantTags.size());
 			for (RevTag tag : wantTags)
 				all.add(tag.getObject());
 			q = walker.parseAny(all, true);
 			try {
 				while (q.next() != null) {
 					// Just need to pop the queue item to parse the object.
 				}
 			} finally {
 				q.release();
 			}
 		}

 		if (walker instanceof DepthWalk.ObjectWalk) {
 			DepthWalk.ObjectWalk depthWalk = (DepthWalk.ObjectWalk) walker;
 			for (RevObject obj : wantObjs) {
 				depthWalk.markRoot(obj);
 			}
 			// Mark the tree objects associated with "have" commits as
 			// uninteresting to avoid writing redundant blobs. A normal RevWalk
 			// lazily propagates the "uninteresting" state from a commit to its
 			// tree during the walk, but DepthWalks can terminate early so
 			// preemptively propagate that state here.
 			for (RevObject obj : haveObjs) {
 				if (obj instanceof RevCommit) {
 					RevTree t = ((RevCommit) obj).getTree();
 					depthWalk.markUninteresting(t);
 				}
 			}

 			if (unshallowObjects != null) {
 				for (ObjectId id : unshallowObjects) {
 					depthWalk.markUnshallow(walker.parseAny(id));
 				}
 			}
 		} else {
 			for (RevObject obj : wantObjs)
 				walker.markStart(obj);
 		}
 		for (RevObject obj : haveObjs)
 			walker.markUninteresting(obj);

 		final int maxBases = config.getDeltaSearchWindowSize();
 		Set<RevTree> baseTrees = new HashSet<>();
 		BlockList<RevCommit> commits = new BlockList<>();
 		Set<ObjectId> roots = new HashSet<>();
 		RevCommit c;
 		while ((c = walker.next()) != null) {
 			if (exclude(c))
 				continue;
 			if (c.has(RevFlag.UNINTERESTING)) {
 				if (baseTrees.size() <= maxBases)
 					baseTrees.add(c.getTree());
 				continue;
 			}

 			commits.add(c);
 			if (c.getParentCount() == 0) {
 				roots.add(c.copy());
 			}
 			countingMonitor.update(1);
 		}
 		stats.rootCommits = Collections.unmodifiableSet(roots);

 		if (shallowPack) {
 			for (RevCommit cmit : commits) {
 				addObject(cmit, 0);
 			}
 		} else {
 			int commitCnt = 0;
 			boolean putTagTargets = false;
 			for (RevCommit cmit : commits) {
 				if (!cmit.has(added)) {
 					cmit.add(added);
 					addObject(cmit, 0);
 					commitCnt++;
 				}

 				for (int i = 0; i < cmit.getParentCount(); i++) {
 					RevCommit p = cmit.getParent(i);
 					if (!p.has(added) && !p.has(RevFlag.UNINTERESTING)
 							&& !exclude(p)) {
 						p.add(added);
 						addObject(p, 0);
 						commitCnt++;
 					}
 				}

 				if (!putTagTargets && 4096 < commitCnt) {
 					for (ObjectId id : tagTargets) {
 						RevObject obj = walker.lookupOrNull(id);
 						if (obj instanceof RevCommit
 								&& obj.has(include)
 								&& !obj.has(RevFlag.UNINTERESTING)
 								&& !obj.has(added)) {
 							obj.add(added);
 							addObject(obj, 0);
 						}
 					}
 					putTagTargets = true;
 				}
 			}
 		}
 		commits = null;

 		if (thin && !baseTrees.isEmpty()) {
 			BaseSearch bases = new BaseSearch(countingMonitor, baseTrees, //
 					objectsMap, edgeObjects, reader);
 			RevObject o;
 			while ((o = walker.nextObject()) != null) {
 				if (o.has(RevFlag.UNINTERESTING))
 					continue;
 				if (exclude(o))
 					continue;

 				int pathHash = walker.getPathHashCode();
 				byte[] pathBuf = walker.getPathBuffer();
 				int pathLen = walker.getPathLength();
 				bases.addBase(o.getType(), pathBuf, pathLen, pathHash);
 				addObject(o, pathHash);
 				countingMonitor.update(1);
 			}
 		} else {
 			RevObject o;
 			while ((o = walker.nextObject()) != null) {
 				if (o.has(RevFlag.UNINTERESTING))
 					continue;
 				if (exclude(o))
 					continue;
 				addObject(o, walker.getPathHashCode());
 				countingMonitor.update(1);
 			}
 		}

 		for (CachedPack pack : cachedPacks)
 			countingMonitor.update((int) pack.getObjectCount());
 		endPhase(countingMonitor);
 		stats.timeCounting = System.currentTimeMillis() - countingStart;
 		stats.bitmapIndexMisses = -1;
 	}

 	private void findObjectsToPackUsingBitmaps(
 			PackWriterBitmapWalker bitmapWalker, Set<? extends ObjectId> want,
 			Set<? extends ObjectId> have)
 			throws MissingObjectException, IncorrectObjectTypeException,
 			IOException {
 		BitmapBuilder haveBitmap = bitmapWalker.findObjects(have, null, true);
 		BitmapBuilder wantBitmap = bitmapWalker.findObjects(want, haveBitmap,
 				false);
 		BitmapBuilder needBitmap = wantBitmap.andNot(haveBitmap);

 		if (useCachedPacks && reuseSupport != null && !reuseValidate
 				&& (excludeInPacks == null || excludeInPacks.length == 0))
 			cachedPacks.addAll(
 					reuseSupport.getCachedPacksAndUpdate(needBitmap));

 		for (BitmapObject obj : needBitmap) {
 			ObjectId objectId = obj.getObjectId();
 			if (exclude(objectId)) {
 				needBitmap.remove(objectId);
 				continue;
 			}
 			addObject(objectId, obj.getType(), 0);
 		}

 		if (thin)
 			haveObjects = haveBitmap;
 	}

 	private static void pruneEdgesFromObjectList(List<ObjectToPack> list) {
 		final int size = list.size();
 		int src = 0;
 		int dst = 0;

 		for (; src < size; src++) {
 			ObjectToPack obj = list.get(src);
 			if (obj.isEdge())
 				continue;
 			if (dst != src)
 				list.set(dst, obj);
 			dst++;
 		}

 		while (dst < list.size())
 			list.remove(list.size() - 1);
 	}

 	/**
 	 * Include one object to the output file.
 	 * <p>
 	 * Objects are written in the order they are added. If the same object is
 	 * added twice, it may be written twice, creating a larger than necessary
 	 * file.
 	 *
 	 * @param object
 	 *            the object to add.
 	 * @throws IncorrectObjectTypeException
 	 *             the object is an unsupported type.
 	 */
 	public void addObject(final RevObject object)
 			throws IncorrectObjectTypeException {
 		if (!exclude(object))
 			addObject(object, 0);
 	}

 	private void addObject(final RevObject object, final int pathHashCode) {
 		addObject(object, object.getType(), pathHashCode);
 	}

 	private void addObject(
 			final AnyObjectId src, final int type, final int pathHashCode) {
 		final ObjectToPack otp;
 		if (reuseSupport != null)
 			otp = reuseSupport.newObjectToPack(src, type);
 		else
 			otp = new ObjectToPack(src, type);
 		otp.setPathHash(pathHashCode);
 		objectsLists[type].add(otp);
 		objectsMap.add(otp);
 	}

 	private boolean exclude(AnyObjectId objectId) {
 		if (excludeInPacks == null)
 			return false;
 		if (excludeInPackLast.contains(objectId))
 			return true;
 		for (ObjectIdSet idx : excludeInPacks) {
 			if (idx.contains(objectId)) {
 				excludeInPackLast = idx;
 				return true;
 			}
 		}
 		return false;
 	}

 	/**
 	 * Select an object representation for this writer.
 	 * <p>
 	 * An {@link ObjectReader} implementation should invoke this method once for
 	 * each representation available for an object, to allow the writer to find
 	 * the most suitable one for the output.
 	 *
 	 * @param otp
 	 *            the object being packed.
 	 * @param next
 	 *            the next available representation from the repository.
 	 */
 	public void select(ObjectToPack otp, StoredObjectRepresentation next) {
 		int nFmt = next.getFormat();

 		if (!cachedPacks.isEmpty()) {
 			if (otp.isEdge())
 				return;
 			if ((nFmt == PACK_WHOLE) | (nFmt == PACK_DELTA)) {
 				for (CachedPack pack : cachedPacks) {
 					if (pack.hasObject(otp, next)) {
 						otp.setEdge();
 						otp.clearDeltaBase();
 						otp.clearReuseAsIs();
 						pruneCurrentObjectList = true;
 						return;
 					}
 				}
 			}
 		}

 		if (nFmt == PACK_DELTA && reuseDeltas && reuseDeltaFor(otp)) {
 			ObjectId baseId = next.getDeltaBase();
 			ObjectToPack ptr = objectsMap.get(baseId);
 			if (ptr != null && !ptr.isEdge()) {
 				otp.setDeltaBase(ptr);
 				otp.setReuseAsIs();
 			} else if (thin && have(ptr, baseId)) {
 				otp.setDeltaBase(baseId);
 				otp.setReuseAsIs();
 			} else {
 				otp.clearDeltaBase();
 				otp.clearReuseAsIs();
 			}
 		} else if (nFmt == PACK_WHOLE && config.isReuseObjects()) {
 			int nWeight = next.getWeight();
 			if (otp.isReuseAsIs() && !otp.isDeltaRepresentation()) {
 				// We've chosen another PACK_WHOLE format for this object,
 				// choose the one that has the smaller compressed size.
 				//
 				if (otp.getWeight() <= nWeight)
 					return;
 			}
 			otp.clearDeltaBase();
 			otp.setReuseAsIs();
 			otp.setWeight(nWeight);
 		} else {
 			otp.clearDeltaBase();
 			otp.clearReuseAsIs();
 		}

 		otp.setDeltaAttempted(reuseDeltas & next.wasDeltaAttempted());
 		otp.select(next);
 	}

 	private final boolean have(ObjectToPack ptr, AnyObjectId objectId) {
 		return (ptr != null && ptr.isEdge())
 				|| (haveObjects != null && haveObjects.contains(objectId));
 	}

 	/**
 	 * Prepares the bitmaps to be written to the bitmap index file.
 	 * <p>
 	 * Bitmaps can be used to speed up fetches and clones by storing the entire
 	 * object graph at selected commits. Writing a bitmap index is an optional
 	 * feature that not all pack users may require.
 	 * <p>
 	 * Called after {@link #writeIndex(OutputStream)}.
 	 * <p>
 	 * To reduce memory internal state is cleared during this method, rendering
 	 * the PackWriter instance useless for anything further than a call to write
 	 * out the new bitmaps with {@link #writeBitmapIndex(OutputStream)}.
 	 *
 	 * @param pm
 	 *            progress monitor to report bitmap building work.
 	 * @return whether a bitmap index may be written.
 	 * @throws IOException
 	 *             when some I/O problem occur during reading objects.
 	 */
 	public boolean prepareBitmapIndex(ProgressMonitor pm) throws IOException {
 		if (!canBuildBitmaps || getObjectCount() > Integer.MAX_VALUE
 				|| !cachedPacks.isEmpty())
 			return false;

 		if (pm == null)
 			pm = NullProgressMonitor.INSTANCE;

 		int numCommits = objectsLists[OBJ_COMMIT].size();
 		List<ObjectToPack> byName = sortByName();
 		sortedByName = null;
 		objectsLists = null;
 		objectsMap = null;
 		writeBitmaps = new PackBitmapIndexBuilder(byName);
 		byName = null;

 		PackWriterBitmapPreparer bitmapPreparer = new PackWriterBitmapPreparer(
 				reader, writeBitmaps, pm, stats.interestingObjects, config);

 		Collection<PackWriterBitmapPreparer.BitmapCommit> selectedCommits = bitmapPreparer
 				.selectCommits(numCommits, excludeFromBitmapSelection);

 		beginPhase(PackingPhase.BUILDING_BITMAPS, pm, selectedCommits.size());

 		PackWriterBitmapWalker walker = bitmapPreparer.newBitmapWalker();
 		AnyObjectId last = null;
 		for (PackWriterBitmapPreparer.BitmapCommit cmit : selectedCommits) {
 			if (!cmit.isReuseWalker()) {
 				walker = bitmapPreparer.newBitmapWalker();
 			}
 			BitmapBuilder bitmap = walker.findObjects(
 					Collections.singleton(cmit), null, false);

 			if (last != null && cmit.isReuseWalker() && !bitmap.contains(last))
 				throw new IllegalStateException(MessageFormat.format(
 						JGitText.get().bitmapMissingObject, cmit.name(),
 						last.name()));
 			last = cmit;
 			writeBitmaps.addBitmap(cmit, bitmap.build(), cmit.getFlags());

 			pm.update(1);
 		}

 		endPhase(pm);
 		return true;
 	}

 	private boolean reuseDeltaFor(ObjectToPack otp) {
 		int type = otp.getType();
 		if ((type & 2) != 0) // OBJ_TREE(2) or OBJ_BLOB(3)
 			return true;
 		if (type == OBJ_COMMIT)
 			return reuseDeltaCommits;
 		if (type == OBJ_TAG)
 			return false;
 		return true;
 	}

 	/**
 	 * Summary of how PackWriter created the pack.
 	 *
 	 * @deprecated Use {@link PackStatistics} instead.
 	 */
 	@Deprecated
 	public static class Statistics {
 		/** Statistics about a single class of object. */
 		public static class ObjectType {
 			// All requests are forwarded to this object.
 			private PackStatistics.ObjectType objectType;

 			/**
 			 * Wraps an
 			 * {@link org.eclipse.jgit.storage.pack.PackStatistics.ObjectType}
 			 * instance to maintain backwards compatibility with existing API.
 			 *
 			 * @param type
 			 *            the wrapped instance
 			 */
 			public ObjectType(PackStatistics.ObjectType type) {
 				objectType = type;
 			}

 			/**
 			 * @return total number of objects output. This total includes the
 			 *         value of {@link #getDeltas()}.
 			 */
 			public long getObjects() {
 				return objectType.getObjects();
 			}

 			/**
 			 * @return total number of deltas output. This may be lower than the
 			 *         actual number of deltas if a cached pack was reused.
 			 */
 			public long getDeltas() {
 				return objectType.getDeltas();
 			}

 			/**
 			 * @return number of objects whose existing representation was
 			 *         reused in the output. This count includes
 			 *         {@link #getReusedDeltas()}.
 			 */
 			public long getReusedObjects() {
 				return objectType.getReusedObjects();
 			}

 			/**
 			 * @return number of deltas whose existing representation was reused
 			 *         in the output, as their base object was also output or
 			 *         was assumed present for a thin pack. This may be lower
 			 *         than the actual number of reused deltas if a cached pack
 			 *         was reused.
 			 */
 			public long getReusedDeltas() {
 				return objectType.getReusedDeltas();
 			}

 			/**
 			 * @return total number of bytes written. This size includes the
 			 *         object headers as well as the compressed data. This size
 			 *         also includes all of {@link #getDeltaBytes()}.
 			 */
 			public long getBytes() {
 				return objectType.getBytes();
 			}

 			/**
 			 * @return number of delta bytes written. This size includes the
 			 *         object headers for the delta objects.
 			 */
 			public long getDeltaBytes() {
 				return objectType.getDeltaBytes();
 			}
 		}

 		// All requests are forwarded to this object.
 		private PackStatistics statistics;

 		/**
 		 * Wraps a {@link PackStatistics} object to maintain backwards
 		 * compatibility with existing API.
 		 *
 		 * @param stats
 		 *            the wrapped PackStatitics object
 		 */
 		public Statistics(PackStatistics stats) {
 			statistics = stats;
 		}

 		/**
 		 * @return unmodifiable collection of objects to be included in the
 		 *         pack. May be null if the pack was hand-crafted in a unit
 		 *         test.
 		 */
 		public Set<ObjectId> getInterestingObjects() {
 			return statistics.getInterestingObjects();
 		}

 		/**
 		 * @return unmodifiable collection of objects that should be excluded
 		 *         from the pack, as the peer that will receive the pack already
 		 *         has these objects.
 		 */
 		public Set<ObjectId> getUninterestingObjects() {
 			return statistics.getUninterestingObjects();
 		}

 		/**
 		 * @return unmodifiable collection of the cached packs that were reused
 		 *         in the output, if any were selected for reuse.
 		 */
 		public Collection<CachedPack> getReusedPacks() {
 			return statistics.getReusedPacks();
 		}

 		/**
 		 * @return number of objects in the output pack that went through the
 		 *         delta search process in order to find a potential delta base.
 		 */
 		public int getDeltaSearchNonEdgeObjects() {
 			return statistics.getDeltaSearchNonEdgeObjects();
 		}

 		/**
 		 * @return number of objects in the output pack that went through delta
 		 *         base search and found a suitable base. This is a subset of
 		 *         {@link #getDeltaSearchNonEdgeObjects()}.
 		 */
 		public int getDeltasFound() {
 			return statistics.getDeltasFound();
 		}

 		/**
 		 * @return total number of objects output. This total includes the value
 		 *         of {@link #getTotalDeltas()}.
 		 */
 		public long getTotalObjects() {
 			return statistics.getTotalObjects();
 		}

 		/**
 		 * @return the count of objects that needed to be discovered through an
 		 *         object walk because they were not found in bitmap indices.
 		 *         Returns -1 if no bitmap indices were found.
 		 */
 		public long getBitmapIndexMisses() {
 			return statistics.getBitmapIndexMisses();
 		}

 		/**
 		 * @return total number of deltas output. This may be lower than the
 		 *         actual number of deltas if a cached pack was reused.
 		 */
 		public long getTotalDeltas() {
 			return statistics.getTotalDeltas();
 		}

 		/**
 		 * @return number of objects whose existing representation was reused in
 		 *         the output. This count includes {@link #getReusedDeltas()}.
 		 */
 		public long getReusedObjects() {
 			return statistics.getReusedObjects();
 		}

 		/**
 		 * @return number of deltas whose existing representation was reused in
 		 *         the output, as their base object was also output or was
 		 *         assumed present for a thin pack. This may be lower than the
 		 *         actual number of reused deltas if a cached pack was reused.
 		 */
 		public long getReusedDeltas() {
 			return statistics.getReusedDeltas();
 		}

 		/**
 		 * @return total number of bytes written. This size includes the pack
 		 *         header, trailer, thin pack, and reused cached pack(s).
 		 */
 		public long getTotalBytes() {
 			return statistics.getTotalBytes();
 		}

 		/**
 		 * @return size of the thin pack in bytes, if a thin pack was generated.
 		 *         A thin pack is created when the client already has objects
 		 *         and some deltas are created against those objects, or if a
 		 *         cached pack is being used and some deltas will reference
 		 *         objects in the cached pack. This size does not include the
 		 *         pack header or trailer.
 		 */
 		public long getThinPackBytes() {
 			return statistics.getThinPackBytes();
 		}

 		/**
 		 * @param typeCode
 		 *            object type code, e.g. OBJ_COMMIT or OBJ_TREE.
 		 * @return information about this type of object in the pack.
 		 */
 		public ObjectType byObjectType(int typeCode) {
 			return new ObjectType(statistics.byObjectType(typeCode));
 		}

 		/** @return true if the resulting pack file was a shallow pack. */
 		public boolean isShallow() {
 			return statistics.isShallow();
 		}

 		/** @return depth (in commits) the pack includes if shallow. */
 		public int getDepth() {
 			return statistics.getDepth();
 		}

 		/**
 		 * @return time in milliseconds spent enumerating the objects that need
 		 *         to be included in the output. This time includes any restarts
 		 *         that occur when a cached pack is selected for reuse.
 		 */
 		public long getTimeCounting() {
 			return statistics.getTimeCounting();
 		}

 		/**
 		 * @return time in milliseconds spent matching existing representations
 		 *         against objects that will be transmitted, or that the client
 		 *         can be assumed to already have.
 		 */
 		public long getTimeSearchingForReuse() {
 			return statistics.getTimeSearchingForReuse();
 		}

 		/**
 		 * @return time in milliseconds spent finding the sizes of all objects
 		 *         that will enter the delta compression search window. The
 		 *         sizes need to be known to better match similar objects
 		 *         together and improve delta compression ratios.
 		 */
 		public long getTimeSearchingForSizes() {
 			return statistics.getTimeSearchingForSizes();
 		}

 		/**
 		 * @return time in milliseconds spent on delta compression. This is
 		 *         observed wall-clock time and does not accurately track CPU
 		 *         time used when multiple threads were used to perform the
 		 *         delta compression.
 		 */
 		public long getTimeCompressing() {
 			return statistics.getTimeCompressing();
 		}

 		/**
 		 * @return time in milliseconds spent writing the pack output, from
 		 *         start of header until end of trailer. The transfer speed can
 		 *         be approximated by dividing {@link #getTotalBytes()} by this
 		 *         value.
 		 */
 		public long getTimeWriting() {
 			return statistics.getTimeWriting();
 		}

 		/** @return total time spent processing this pack. */
 		public long getTimeTotal() {
 			return statistics.getTimeTotal();
 		}

 		/**
 		 * @return get the average output speed in terms of bytes-per-second.
 		 *         {@code getTotalBytes() / (getTimeWriting() / 1000.0)}.
 		 */
 		public double getTransferRate() {
 			return statistics.getTransferRate();
 		}

 		/** @return formatted message string for display to clients. */
 		public String getMessage() {
 			return statistics.getMessage();
 		}
 	}

 	private class MutableState {
 		/** Estimated size of a single ObjectToPack instance. */
 		// Assume 64-bit pointers, since this is just an estimate.
 		private static final long OBJECT_TO_PACK_SIZE =
 				(2 * 8)               // Object header
 				+ (2 * 8) + (2 * 8)   // ObjectToPack fields
 				+ (8 + 8)             // PackedObjectInfo fields
 				+ 8                   // ObjectIdOwnerMap fields
 				+ 40                  // AnyObjectId fields
 				+ 8;                  // Reference in BlockList

 		private final long totalDeltaSearchBytes;

 		private volatile PackingPhase phase;

 		MutableState() {
 			phase = PackingPhase.COUNTING;
 			if (config.isDeltaCompress()) {
 				int threads = config.getThreads();
 				if (threads <= 0)
 					threads = Runtime.getRuntime().availableProcessors();
 				totalDeltaSearchBytes = (threads * config.getDeltaSearchMemoryLimit())
 						+ config.getBigFileThreshold();
 			} else
 				totalDeltaSearchBytes = 0;
 		}

 		State snapshot() {
 			long objCnt = 0;
 			BlockList<ObjectToPack>[] lists = objectsLists;
 			if (lists != null) {
 				objCnt += lists[OBJ_COMMIT].size();
 				objCnt += lists[OBJ_TREE].size();
 				objCnt += lists[OBJ_BLOB].size();
 				objCnt += lists[OBJ_TAG].size();
 				// Exclude CachedPacks.
 			}

 			long bytesUsed = OBJECT_TO_PACK_SIZE * objCnt;
 			PackingPhase curr = phase;
 			if (curr == PackingPhase.COMPRESSING)
 				bytesUsed += totalDeltaSearchBytes;
 			return new State(curr, bytesUsed);
 		}
 	}

 	/** Possible states that a PackWriter can be in. */
 	public static enum PackingPhase {
 		/** Counting objects phase. */
 		COUNTING,

 		/** Getting sizes phase. */
 		GETTING_SIZES,

 		/** Finding sources phase. */
 		FINDING_SOURCES,

 		/** Compressing objects phase. */
 		COMPRESSING,

 		/** Writing objects phase. */
 		WRITING,

 		/** Building bitmaps phase. */
 		BUILDING_BITMAPS;
 	}

 	/** Summary of the current state of a PackWriter. */
 	public class State {
 		private final PackingPhase phase;

 		private final long bytesUsed;

 		State(PackingPhase phase, long bytesUsed) {
 			this.phase = phase;
 			this.bytesUsed = bytesUsed;
 		}

 		/** @return the PackConfig used to build the writer. */
 		public PackConfig getConfig() {
 			return config;
 		}

 		/** @return the current phase of the writer. */
 		public PackingPhase getPhase() {
 			return phase;
 		}

 		/** @return an estimate of the total memory used by the writer. */
 		public long estimateBytesUsed() {
 			return bytesUsed;
 		}

 		@SuppressWarnings("nls")
 		@Override
 		public String toString() {
 			return "PackWriter.State[" + phase + ", memory=" + bytesUsed + "]";
 		}
 	}
 }