jgit/org.eclipse.jgit/src/org/eclipse/jgit/storage/pack/PackWriter.java

/*
 * 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.storage.pack;

import static org.eclipse.jgit.storage.pack.StoredObjectRepresentation.PACK_DELTA;
import static org.eclipse.jgit.storage.pack.StoredObjectRepresentation.PACK_WHOLE;

import java.io.IOException;
import java.io.OutputStream;
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.HashMap;
import java.util.HashSet;
import java.util.Iterator;
import java.util.LinkedList;
import java.util.List;
import java.util.Map;
import java.util.Set;
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.Deflater;
import java.util.zip.DeflaterOutputStream;

import org.eclipse.jgit.JGitText;
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.lib.AnyObjectId;
import org.eclipse.jgit.lib.AsyncObjectSizeQueue;
import org.eclipse.jgit.lib.Constants;
import org.eclipse.jgit.lib.NullProgressMonitor;
import org.eclipse.jgit.lib.ObjectId;
import org.eclipse.jgit.lib.ObjectIdSubclassMap;
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.ObjectWalk;
import org.eclipse.jgit.revwalk.RevCommit;
import org.eclipse.jgit.revwalk.RevFlag;
import org.eclipse.jgit.revwalk.RevFlagSet;
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.file.PackIndexWriter;
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>
 * Typical usage consists of creating instance intended for some pack,
 * configuring options, preparing the list of objects by calling
 * {@link #preparePack(Iterator)} or
 * {@link #preparePack(ProgressMonitor, Collection, Collection)}, and finally
 * producing the stream with {@link #writePack(ProgressMonitor, ProgressMonitor, 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, with
 * one instance per created pack. Subsequent calls to writePack result in
 * undefined behavior.
 * </p>
 */
public class PackWriter {
	private static final int PACK_VERSION_GENERATED = 2;

	@SuppressWarnings("unchecked")
	private final List<ObjectToPack> objectsLists[] = new List[Constants.OBJ_TAG + 1];
	{
		objectsLists[0] = Collections.<ObjectToPack> emptyList();
		objectsLists[Constants.OBJ_COMMIT] = new ArrayList<ObjectToPack>();
		objectsLists[Constants.OBJ_TREE] = new ArrayList<ObjectToPack>();
		objectsLists[Constants.OBJ_BLOB] = new ArrayList<ObjectToPack>();
		objectsLists[Constants.OBJ_TAG] = new ArrayList<ObjectToPack>();
	}

	private final ObjectIdSubclassMap<ObjectToPack> objectsMap = new ObjectIdSubclassMap<ObjectToPack>();

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

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

	private Set<ObjectId> tagTargets = Collections.emptySet();

	private Deflater myDeflater;

	private final ObjectReader reader;

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

	private final PackConfig config;

	private final Statistics stats;

	private List<ObjectToPack> sortedByName;

	private byte packcsum[];

	private boolean deltaBaseAsOffset;

	private boolean reuseDeltas;

	private boolean reuseDeltaCommits;

	private boolean thin;

	private boolean useCachedPacks;

	private boolean ignoreMissingUninteresting = true;

	/**
	 * Create writer for specified repository.
	 * <p>
	 * Objects for packing are specified in {@link #preparePack(Iterator)} or
	 * {@link #preparePack(ProgressMonitor, Collection, Collection)}.
	 *
	 * @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, Collection, Collection)}.
	 *
	 * @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, Collection, Collection)}.
	 *
	 * @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, Collection, Collection)}.
	 *
	 * @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();
		stats = new Statistics();
	}

	/**
	 * 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;
	}

	/** @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 ignore objects that are uninteresting and also not found
	 *         on local disk; false to throw a {@link MissingObjectException}
	 *         out of {@link #preparePack(ProgressMonitor, Collection, Collection)} 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;
	}

	/**
	 * Returns objects number in a pack file that was created by this writer.
	 *
	 * @return number of objects in pack.
	 */
	public long getObjectsNumber() {
		return stats.totalObjects;
	}

	/**
	 * 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(final 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).
	 * @param have
	 *            collection of objects to be marked as uninteresting (end
	 *            points of graph traversal).
	 * @throws IOException
	 *             when some I/O problem occur during reading objects.
	 */
	public void preparePack(ProgressMonitor countingMonitor,
			final Collection<? extends ObjectId> want,
			final Collection<? extends ObjectId> have) throws IOException {
		ObjectWalk ow = new ObjectWalk(reader);
		preparePack(countingMonitor, ow, want, have);
	}

	/**
	 * 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).
	 * @param uninterestingObjects
	 *            collection of objects to be marked as uninteresting (end
	 *            points of graph traversal).
	 * @throws IOException
	 *             when some I/O problem occur during reading objects.
	 */
	public void preparePack(ProgressMonitor countingMonitor,
			final ObjectWalk walk,
			final Collection<? extends ObjectId> interestingObjects,
			final Collection<? extends ObjectId> uninterestingObjects)
			throws IOException {
		if (countingMonitor == null)
			countingMonitor = NullProgressMonitor.INSTANCE;
		findObjectsToPack(countingMonitor, walk, interestingObjects,
				uninterestingObjects);
	}

	/**
	 * 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);
		if (obj != null && !obj.isEdge())
			return true;

		Set<ObjectId> toFind = Collections.singleton(id.toObjectId());
		for (CachedPack pack : cachedPacks) {
			if (pack.hasObject(toFind).contains(id))
				return true;
		}

		return false;
	}

	/**
	 * 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[Constants.OBJECT_ID_LENGTH];
		final MessageDigest md = Constants.newMessageDigest();
		for (ObjectToPack otp : sortByName()) {
			otp.copyRawTo(buf, 0);
			md.update(buf, 0, Constants.OBJECT_ID_LENGTH);
		}
		return ObjectId.fromRaw(md.digest());
	}

	/**
	 * Create an index file to match the pack file just written.
	 * <p>
	 * This method can only be invoked after {@link #preparePack(Iterator)} or
	 * {@link #preparePack(ProgressMonitor, Collection, Collection)} has been
	 * invoked and completed successfully. Writing a corresponding index is an
	 * optional feature that not all pack users may require.
	 *
	 * @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 (!cachedPacks.isEmpty())
			throw new IOException(JGitText.get().cachedPacksPreventsIndexCreation);

		final List<ObjectToPack> list = sortByName();
		final PackIndexWriter iw;
		int indexVersion = config.getIndexVersion();
		if (indexVersion <= 0)
			iw = PackIndexWriter.createOldestPossible(indexStream, list);
		else
			iw = PackIndexWriter.createVersion(indexStream, indexVersion);
		iw.write(list, packcsum);
	}

	private List<ObjectToPack> sortByName() {
		if (sortedByName == null) {
			int cnt = 0;
			for (List<ObjectToPack> list : objectsLists)
				cnt += list.size();
			sortedByName = new ArrayList<ObjectToPack>(cnt);
			for (List<ObjectToPack> list : objectsLists) {
				for (ObjectToPack otp : list)
					sortedByName.add(otp);
			}
			Collections.sort(sortedByName);
		}
		return sortedByName;
	}

	/**
	 * Write the prepared pack to the supplied stream.
	 * <p>
	 * At first, this method collects and sorts objects to pack, then deltas
	 * search is performed if set up accordingly, finally pack stream is
	 * written.
	 * </p>
	 * <p>
	 * All reused objects data checksum (Adler32/CRC32) is computed and
	 * validated against existing checksum.
	 * </p>
	 *
	 * @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.
	 */
	public void writePack(ProgressMonitor compressMonitor,
			ProgressMonitor writeMonitor, OutputStream packStream)
			throws IOException {
		if (compressMonitor == null)
			compressMonitor = NullProgressMonitor.INSTANCE;
		if (writeMonitor == null)
			writeMonitor = NullProgressMonitor.INSTANCE;

		if ((reuseDeltas || config.isReuseObjects()) && reuseSupport != null)
			searchForReuse(compressMonitor);
		if (config.isDeltaCompress())
			searchForDeltas(compressMonitor);

		final PackOutputStream out = new PackOutputStream(writeMonitor,
				packStream, this);

		long objCnt = 0;
		for (List<ObjectToPack> list : objectsLists)
			objCnt += list.size();
		for (CachedPack pack : cachedPacks)
			objCnt += pack.getObjectCount();
		stats.totalObjects = objCnt;

		writeMonitor.beginTask(JGitText.get().writingObjects, (int) objCnt);
		long writeStart = System.currentTimeMillis();

		long headerStart = out.length();
		out.writeFileHeader(PACK_VERSION_GENERATED, objCnt);
		out.flush();
		long headerEnd = out.length();

		writeObjects(out);
		if (!edgeObjects.isEmpty() || !cachedPacks.isEmpty())
			stats.thinPackBytes = out.length() - (headerEnd - headerStart);

		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();
		stats.timeWriting = System.currentTimeMillis() - writeStart;
		stats.totalBytes = out.length();
		stats.reusedPacks = Collections.unmodifiableList(cachedPacks);

		reader.release();
		writeMonitor.endTask();
	}

	/**
	 * @return description of what this PackWriter did in order to create the
	 *         final pack stream. The object is only available to callers after
	 *         {@link #writePack(ProgressMonitor, ProgressMonitor, OutputStream)}
	 */
	public Statistics getStatistics() {
		return stats;
	}

	/** Release all resources used by this writer. */
	public void release() {
		reader.release();
		if (myDeflater != null) {
			myDeflater.end();
			myDeflater = null;
		}
	}

	private void searchForReuse(ProgressMonitor monitor) throws IOException {
		int cnt = 0;
		for (List<ObjectToPack> list : objectsLists)
			cnt += list.size();
		long start = System.currentTimeMillis();
		monitor.beginTask(JGitText.get().searchForReuse, cnt);
		for (List<ObjectToPack> list : objectsLists)
			reuseSupport.selectObjectRepresentation(this, monitor, list);
		monitor.endTask();
		stats.timeSearchingForReuse = System.currentTimeMillis() - start;
	}

	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[Constants.OBJ_TREE].size()
				+ objectsLists[Constants.OBJ_BLOB].size()
				+ edgeObjects.size()];
		int cnt = 0;
		cnt = findObjectsNeedingDelta(list, cnt, Constants.OBJ_TREE);
		cnt = findObjectsNeedingDelta(list, cnt, Constants.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();
		monitor.beginTask(JGitText.get().searchForSizes, cnt);
		AsyncObjectSizeQueue<ObjectToPack> sizeQueue = reader.getObjectSize(
				Arrays.<ObjectToPack> asList(list).subList(0, cnt), false);
		try {
			final long limit = config.getBigFileThreshold();
			for (;;) {
				monitor.update(1);

				try {
					if (!sizeQueue.next())
						break;
				} catch (MissingObjectException notFound) {
					if (ignoreMissingUninteresting) {
						ObjectToPack otp = sizeQueue.getCurrent();
						if (otp != null && otp.isEdge()) {
							otp.setDoNotDelta(true);
							continue;
						}

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

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

				long sz = sizeQueue.getSize();
				if (limit <= sz || Integer.MAX_VALUE <= sz)
					otp.setDoNotDelta(true); // too big, avoid costly files

				else if (sz <= DeltaIndex.BLKSZ)
					otp.setDoNotDelta(true); // too small, won't work

				else
					otp.setWeight((int) sz);
			}
		} finally {
			sizeQueue.release();
		}
		monitor.endTask();
		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>() {
			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();
		monitor.beginTask(JGitText.get().compressingObjects, nonEdgeCnt);
		searchForDeltas(monitor, list, cnt);
		monitor.endTask();
		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.isReuseAsIs()) // already reusing a representation
				continue;
			if (otp.isDoNotDelta()) // delta is disabled for this path
				continue;
			otp.setWeight(0);
			list[cnt++] = otp;
		}
		return cnt;
	}

	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 <= 2 * config.getDeltaSearchWindowSize()) {
			DeltaCache dc = new DeltaCache(config);
			DeltaWindow dw = new DeltaWindow(config, dc, reader);
			dw.search(monitor, list, 0, cnt);
			return;
		}

		final DeltaCache dc = new ThreadSafeDeltaCache(config);
		final ThreadSafeProgressMonitor pm = new ThreadSafeProgressMonitor(monitor);

		// Guess at the size of batch we want. Because we don't really
		// have a way for a thread to steal work from another thread if
		// it ends early, we over partition slightly so the work units
		// are a bit smaller.
		//
		int estSize = cnt / (threads * 2);
		if (estSize < 2 * config.getDeltaSearchWindowSize())
			estSize = 2 * config.getDeltaSearchWindowSize();

		final List<DeltaTask> myTasks = new ArrayList<DeltaTask>(threads * 2);
		for (int i = 0; i < cnt;) {
			final int start = i;
			final int batchSize;

			if (cnt - i < estSize) {
				// If we don't have enough to fill the remaining block,
				// schedule what is left over as a single block.
				//
				batchSize = cnt - i;
			} else {
				// Try to split the block at the end of a path.
				//
				int end = start + estSize;
				while (end < cnt) {
					ObjectToPack a = list[end - 1];
					ObjectToPack b = list[end];
					if (a.getPathHash() == b.getPathHash())
						end++;
					else
						break;
				}
				batchSize = end - start;
			}
			i += batchSize;
			myTasks.add(new DeltaTask(config, reader, dc, pm, batchSize, start, list));
		}
		pm.startWorkers(myTasks.size());

		final Executor executor = config.getExecutor();
		final List<Throwable> errors = Collections
				.synchronizedList(new ArrayList<Throwable>());
		if (executor instanceof ExecutorService) {
			// Caller supplied us a service, use it directly.
			//
			runTasks((ExecutorService) executor, pm, myTasks, 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, myTasks, 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 : myTasks) {
				executor.execute(new Runnable() {
					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;
		}
	}

	private void runTasks(ExecutorService pool, ThreadSafeProgressMonitor pm,
			List<DeltaTask> tasks, List<Throwable> errors) throws IOException {
		List<Future<?>> futures = new ArrayList<Future<?>>(tasks.size());
		for (DeltaTask task : 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 {
		for (List<ObjectToPack> list : objectsLists)
				writeObjects(out, list);
	}

	private void writeObjects(PackOutputStream out, List<ObjectToPack> list)
			throws IOException {
		if (reuseSupport != null) {
			reuseSupport.writeObjects(out, list);
		} else {
			for (ObjectToPack otp : list)
				out.writeObject(otp);
		}
	}

	void writeObject(PackOutputStream out, ObjectToPack otp) throws IOException {
		if (otp.isWritten())
			return; // We shouldn't be here.

		otp.markWantWrite();
		if (otp.isDeltaRepresentation())
			writeBaseFirst(out, otp);

		out.resetCRC32();
		otp.setOffset(out.length());

		while (otp.isReuseAsIs()) {
			try {
				reuseSupport.copyObjectAsIs(out, otp);
				out.endObject();
				otp.setCRC(out.getCRC32());
				stats.reusedObjects++;
				if (otp.isDeltaRepresentation()) {
					stats.totalDeltas++;
					stats.reusedDeltas++;
				}
				return;
			} catch (StoredObjectRepresentationNotAvailableException gone) {
				if (otp.getOffset() == out.length()) {
					redoSearchForReuse(otp);
					continue;
				} else {
					// Object writing already started, we cannot recover.
					//
					CorruptObjectException coe;
					coe = new CorruptObjectException(otp, "");
					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(out.getCRC32());
	}

	private void writeBaseFirst(PackOutputStream out, final ObjectToPack otp)
			throws IOException {
		ObjectToPack baseInPack = otp.getDeltaBase();
		if (baseInPack != null) {
			if (!baseInPack.isWritten()) {
				if (baseInPack.wantWrite()) {
					// There is a cycle. Our caller is trying to write the
					// object we want as a base, and called us. Turn off
					// delta reuse so we can find another form.
					//
					reuseDeltas = false;
					redoSearchForReuse(otp);
					reuseDeltas = true;
				} else {
					writeObject(out, baseInPack);
				}
			}
		} else if (!thin) {
			// This should never occur, the base isn't in the pack and
			// the pack isn't allowed to reference base outside objects.
			// Write the object as a whole form, even if that is slow.
			//
			otp.clearDeltaBase();
			otp.clearReuseAsIs();
		}
	}

	private void redoSearchForReuse(final ObjectToPack otp) throws IOException,
			MissingObjectException {
		otp.clearDeltaBase();
		otp.clearReuseAsIs();
		reuseSupport.selectObjectRepresentation(this,
				NullProgressMonitor.INSTANCE, Collections.singleton(otp));
	}

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

		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 {
		DeltaCache.Ref ref = otp.popCachedDelta();
		if (ref != null) {
			byte[] zbuf = ref.get();
			if (zbuf != null) {
				out.writeHeader(otp, otp.getCachedSize());
				out.write(zbuf);
				return;
			}
		}

		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();
		stats.totalDeltas++;
	}

	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(final ProgressMonitor countingMonitor,
			final ObjectWalk walker, final Collection<? extends ObjectId> want,
			Collection<? extends ObjectId> have)
			throws MissingObjectException, IOException,
			IncorrectObjectTypeException {
		final long countingStart = System.currentTimeMillis();
		countingMonitor.beginTask(JGitText.get().countingObjects,
				ProgressMonitor.UNKNOWN);

		if (have == null)
			have = Collections.emptySet();

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

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

		final Map<ObjectId, CachedPack> tipToPack = new HashMap<ObjectId, CachedPack>();
		final RevFlag inCachedPack = walker.newFlag("inCachedPack");
		final RevFlag include = walker.newFlag("include");
		final RevFlag added = walker.newFlag("added");

		final RevFlagSet keepOnRestart = new RevFlagSet();
		keepOnRestart.add(inCachedPack);

		walker.setRetainBody(false);
		walker.carry(include);

		int haveEst = have.size();
		if (have.isEmpty()) {
			walker.sort(RevSort.COMMIT_TIME_DESC);
			if (useCachedPacks && reuseSupport != null) {
				Set<ObjectId> need = new HashSet<ObjectId>(want);
				List<CachedPack> shortCircuit = new LinkedList<CachedPack>();

				for (CachedPack pack : reuseSupport.getCachedPacks()) {
					if (need.containsAll(pack.getTips())) {
						need.removeAll(pack.getTips());
						shortCircuit.add(pack);
					}

					for (ObjectId id : pack.getTips()) {
						tipToPack.put(id, pack);
						all.add(id);
					}
				}

				if (need.isEmpty() && !shortCircuit.isEmpty()) {
					cachedPacks.addAll(shortCircuit);
					for (CachedPack pack : shortCircuit)
						countingMonitor.update((int) pack.getObjectCount());
					countingMonitor.endTask();
					stats.timeCounting = System.currentTimeMillis() - countingStart;
					return;
				}

				haveEst += tipToPack.size();
			}
		} else {
			walker.sort(RevSort.TOPO);
			if (thin)
				walker.sort(RevSort.BOUNDARY, true);
		}

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

		AsyncRevObjectQueue q = walker.parseAny(all, true);
		try {
			for (;;) {
				try {
					RevObject o = q.next();
					if (o == null)
						break;

					if (tipToPack.containsKey(o))
						o.add(inCachedPack);

					if (have.contains(o)) {
						haveObjs.add(o);
					} else 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<ObjectId>(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();
			}
		}

		for (RevObject obj : wantObjs)
			walker.markStart(obj);
		for (RevObject obj : haveObjs)
			walker.markUninteresting(obj);

		int typesToPrune = 0;
		final int maxBases = config.getDeltaSearchWindowSize();
		Set<RevTree> baseTrees = new HashSet<RevTree>();
		List<RevCommit> commits = new ArrayList<RevCommit>();
		RevCommit c;
		while ((c = walker.next()) != null) {
			if (c.has(inCachedPack)) {
				CachedPack pack = tipToPack.get(c);
				if (includesAllTips(pack, include, walker)) {
					useCachedPack(walker, keepOnRestart, //
							wantObjs, haveObjs, pack);
					commits = new ArrayList<RevCommit>();

					countingMonitor.endTask();
					countingMonitor.beginTask(JGitText.get().countingObjects,
							ProgressMonitor.UNKNOWN);
					continue;
				}
			}

			if (c.has(RevFlag.UNINTERESTING)) {
				if (baseTrees.size() <= maxBases)
					baseTrees.add(c.getTree());
				continue;
			}

			commits.add(c);
			countingMonitor.update(1);
		}

		if (objectsLists[Constants.OBJ_COMMIT] instanceof ArrayList) {
			ArrayList<ObjectToPack> list = (ArrayList<ObjectToPack>) objectsLists[Constants.OBJ_COMMIT];
			list.ensureCapacity(list.size() + commits.size());
		}

		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)) {
					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;

		for (CachedPack p : cachedPacks) {
			for (ObjectId d : p.hasObject(objectsLists[Constants.OBJ_COMMIT])) {
				if (baseTrees.size() <= maxBases)
					baseTrees.add(walker.lookupCommit(d).getTree());
				objectsMap.get(d).setEdge();
				typesToPrune |= 1 << Constants.OBJ_COMMIT;
			}
		}

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

		for (CachedPack p : cachedPacks) {
			for (ObjectId d : p.hasObject(objectsLists[Constants.OBJ_TREE])) {
				objectsMap.get(d).setEdge();
				typesToPrune |= 1 << Constants.OBJ_TREE;
			}
			for (ObjectId d : p.hasObject(objectsLists[Constants.OBJ_BLOB])) {
				objectsMap.get(d).setEdge();
				typesToPrune |= 1 << Constants.OBJ_BLOB;
			}
			for (ObjectId d : p.hasObject(objectsLists[Constants.OBJ_TAG])) {
				objectsMap.get(d).setEdge();
				typesToPrune |= 1 << Constants.OBJ_TAG;
			}
		}

		if (typesToPrune != 0) {
			pruneObjectList(typesToPrune, Constants.OBJ_COMMIT);
			pruneObjectList(typesToPrune, Constants.OBJ_TREE);
			pruneObjectList(typesToPrune, Constants.OBJ_BLOB);
			pruneObjectList(typesToPrune, Constants.OBJ_TAG);
		}

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

	private void pruneObjectList(int typesToPrune, int typeCode) {
		if ((typesToPrune & (1 << typeCode)) == 0)
			return;

		final List<ObjectToPack> list = objectsLists[typeCode];
		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(dst);
	}

	private void useCachedPack(ObjectWalk walker, RevFlagSet keepOnRestart,
			List<RevObject> wantObj, List<RevObject> baseObj, CachedPack pack)
			throws MissingObjectException, IncorrectObjectTypeException,
			IOException {
		cachedPacks.add(pack);
		for (ObjectId id : pack.getTips())
			baseObj.add(walker.lookupOrNull(id));

		setThin(true);
		walker.resetRetain(keepOnRestart);
		walker.sort(RevSort.TOPO);
		walker.sort(RevSort.BOUNDARY, true);

		for (RevObject id : wantObj)
			walker.markStart(id);
		for (RevObject id : baseObj)
			walker.markUninteresting(id);
	}

	private static boolean includesAllTips(CachedPack pack, RevFlag include,
			ObjectWalk walker) {
		for (ObjectId id : pack.getTips()) {
			if (!walker.lookupOrNull(id).has(include))
				return false;
		}
		return true;
	}

	/**
	 * 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 {
		addObject(object, 0);
	}

	private void addObject(final RevObject object, final int pathHashCode)
			throws IncorrectObjectTypeException {
		final ObjectToPack otp;
		if (reuseSupport != null)
			otp = reuseSupport.newObjectToPack(object);
		else
			otp = new ObjectToPack(object);
		otp.setPathHash(pathHashCode);

		try {
			objectsLists[object.getType()].add(otp);
		} catch (ArrayIndexOutOfBoundsException x) {
			throw new IncorrectObjectTypeException(object,
					JGitText.get().incorrectObjectType_COMMITnorTREEnorBLOBnorTAG);
		} catch (UnsupportedOperationException x) {
			// index pointing to "dummy" empty list
			throw new IncorrectObjectTypeException(object,
					JGitText.get().incorrectObjectType_COMMITnorTREEnorBLOBnorTAG);
		}
		objectsMap.add(otp);
	}

	/**
	 * 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();
		int nWeight;
		if (otp.isReuseAsIs()) {
			// We've already chosen to reuse a packed form, if next
			// cannot beat that break out early.
			//
			if (PACK_WHOLE < nFmt)
				return; // next isn't packed
			else if (PACK_DELTA < nFmt && otp.isDeltaRepresentation())
				return; // next isn't a delta, but we are

			nWeight = next.getWeight();
			if (otp.getWeight() <= nWeight)
				return; // next would be bigger
		} else
			nWeight = next.getWeight();

		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();
				otp.setWeight(nWeight);
			} else if (thin && ptr != null && ptr.isEdge()) {
				otp.setDeltaBase(baseId);
				otp.setReuseAsIs();
				otp.setWeight(nWeight);
			} else {
				otp.clearDeltaBase();
				otp.clearReuseAsIs();
			}
		} else if (nFmt == PACK_WHOLE && config.isReuseObjects()) {
			otp.clearDeltaBase();
			otp.setReuseAsIs();
			otp.setWeight(nWeight);
		} else {
			otp.clearDeltaBase();
			otp.clearReuseAsIs();
		}

		otp.select(next);
	}

	private boolean reuseDeltaFor(ObjectToPack otp) {
		switch (otp.getType()) {
		case Constants.OBJ_COMMIT:
			return reuseDeltaCommits;
		case Constants.OBJ_TREE:
			return true;
		case Constants.OBJ_BLOB:
			return true;
		case Constants.OBJ_TAG:
			return false;
		default:
			return true;
		}
	}

	/** Summary of how PackWriter created the pack. */
	public static class Statistics {
		Set<ObjectId> interestingObjects;

		Set<ObjectId> uninterestingObjects;

		Collection<CachedPack> reusedPacks;

		int deltaSearchNonEdgeObjects;

		int deltasFound;

		long totalObjects;

		long totalDeltas;

		long reusedObjects;

		long reusedDeltas;

		long totalBytes;

		long thinPackBytes;

		long timeCounting;

		long timeSearchingForReuse;

		long timeSearchingForSizes;

		long timeCompressing;

		long timeWriting;

		/**
		 * @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 interestingObjects;
		}

		/**
		 * @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 uninterestingObjects;
		}

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

		/**
		 * @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 deltaSearchNonEdgeObjects;
		}

		/**
		 * @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 deltasFound;
		}

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

		/**
		 * @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 totalDeltas;
		}

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

		/**
		 * @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 reusedDeltas;
		}

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

		/**
		 * @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 thinPackBytes;
		}

		/**
		 * @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 timeCounting;
		}

		/**
		 * @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 timeSearchingForReuse;
		}

		/**
		 * @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 timeSearchingForSizes;
		}

		/**
		 * @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 timeCompressing;
		}

		/**
		 * @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 timeWriting;
		}

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

		/** @return formatted message string for display to clients. */
		public String getMessage() {
			return MessageFormat.format(JGitText.get().packWriterStatistics, //
					totalObjects, totalDeltas, //
					reusedObjects, reusedDeltas);
		}
	}
}