Add ignoreMissingUninteresting option to PackWriter

[egit.git] / org.spearce.jgit / src / org / spearce / jgit / lib / PackWriter.java

/*
 * Copyright (C) 2008, Marek Zawirski <marek.zawirski@gmail.com>
 *
 * 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 Git Development Community 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.spearce.jgit.lib;

import java.io.IOException;
import java.io.OutputStream;
import java.security.DigestOutputStream;
import java.security.MessageDigest;
import java.util.ArrayList;
import java.util.Collection;
import java.util.Collections;
import java.util.Iterator;
import java.util.LinkedList;
import java.util.List;
import java.util.zip.Deflater;
import java.util.zip.DeflaterOutputStream;

import org.spearce.jgit.errors.IncorrectObjectTypeException;
import org.spearce.jgit.errors.MissingObjectException;
import org.spearce.jgit.revwalk.ObjectWalk;
import org.spearce.jgit.revwalk.RevFlag;
import org.spearce.jgit.revwalk.RevObject;
import org.spearce.jgit.revwalk.RevSort;
import org.spearce.jgit.util.CountingOutputStream;
import org.spearce.jgit.util.NB;

/**
 * <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 through accessors methods and finally call
 * {@link #writePack(Iterator)} or
 * {@link #writePack(Collection, Collection, boolean, boolean)} with objects
 * specification, to generate a pack stream.
 * </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 {
        /**
         * Title of {@link ProgressMonitor} task used during counting objects to
         * pack.
         *
         * @see #writePack(Collection, Collection, boolean, boolean)
         */
        public static final String COUNTING_OBJECTS_PROGRESS = "Counting objects to pack";

        /**
         * Title of {@link ProgressMonitor} task used during searching for objects
         * reuse or delta reuse.
         *
         * @see #writePack(Iterator)
         * @see #writePack(Collection, Collection, boolean, boolean)
         */
        public static final String SEARCHING_REUSE_PROGRESS = "Searching for delta and object reuse";

        /**
         * Title of {@link ProgressMonitor} task used during writing out pack
         * (objects)
         *
         * @see #writePack(Iterator)
         * @see #writePack(Collection, Collection, boolean, boolean)
         */
        public static final String WRITING_OBJECTS_PROGRESS = "Writing objects";

        /**
         * Default value of deltas reuse option.
         *
         * @see #setReuseDeltas(boolean)
         */
        public static final boolean DEFAULT_REUSE_DELTAS = true;

        /**
         * Default value of objects reuse option.
         *
         * @see #setReuseObjects(boolean)
         */
        public static final boolean DEFAULT_REUSE_OBJECTS = true;

        /**
         * Default value of delta base as offset option.
         *
         * @see #setDeltaBaseAsOffset(boolean)
         */
        public static final boolean DEFAULT_DELTA_BASE_AS_OFFSET = false;

        /**
         * Default value of maximum delta chain depth.
         *
         * @see #setMaxDeltaDepth(int)
         */
        public static final int DEFAULT_MAX_DELTA_DEPTH = 50;

        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 final ObjectIdSubclassMap<ObjectId> edgeObjects = new ObjectIdSubclassMap<ObjectId>();

        private final Repository db;

        private final DigestOutputStream out;

        private final CountingOutputStream countingOut;

        private final Deflater deflater;

        private final ProgressMonitor monitor;

        private final byte[] buf = new byte[16384]; // 16 KB

        private final WindowCursor windowCursor = new WindowCursor();

        private boolean reuseDeltas = DEFAULT_REUSE_DELTAS;

        private boolean reuseObjects = DEFAULT_REUSE_OBJECTS;

        private boolean deltaBaseAsOffset = DEFAULT_DELTA_BASE_AS_OFFSET;

        private int maxDeltaDepth = DEFAULT_MAX_DELTA_DEPTH;

        private boolean thin;

        /**
         * Create writer for specified repository, that will write a pack to
         * provided output stream. Objects for packing are specified in
         * {@link #writePack(Iterator)} or
         * {@link #writePack(Collection, Collection, boolean, boolean)}.
         *
         * @param repo
         *            repository where objects are stored.
         * @param out
         *            output stream of pack data; no buffering is guaranteed by
         *            writer.
         * @param monitor
         *            operations progress monitor, used within
         *            {@link #writePack(Iterator)} or
         *            {@link #writePack(Collection, Collection, boolean, boolean)}.
         */
        public PackWriter(final Repository repo, final OutputStream out,
                        final ProgressMonitor monitor) {
                this.db = repo;
                this.monitor = monitor;
                this.countingOut = new CountingOutputStream(out);
                this.out = new DigestOutputStream(countingOut, Constants
                                .newMessageDigest());
                this.deflater = new Deflater(db.getConfig().getCore().getCompression());
        }

        /**
         * Check whether object is configured to reuse deltas existing in
         * repository.
         * <p>
         * Default setting: {@value #DEFAULT_REUSE_DELTAS}
         * </p>
         *
         * @return true if object is configured to reuse deltas; false otherwise.
         */
        public boolean isReuseDeltas() {
                return reuseDeltas;
        }

        /**
         * Set reuse deltas configuration option for this writer. When enabled,
         * writer will search for delta representation of object in repository and
         * use it if possible. Normally, only deltas with base to another object
         * existing in set of objects to pack will be used. Exception is however
         * thin-pack (see
         * {@link #writePack(Collection, Collection, boolean, boolean)} and
         * {@link #writePack(Iterator)}) where base object must exist on other side
         * machine.
         * <p>
         * When raw delta data is directly copied from a pack file, checksum is
         * computed to verify data.
         * </p>
         * <p>
         * Default setting: {@value #DEFAULT_REUSE_DELTAS}
         * </p>
         *
         * @param reuseDeltas
         *            boolean indicating whether or not try to reuse deltas.
         */
        public void setReuseDeltas(boolean reuseDeltas) {
                this.reuseDeltas = reuseDeltas;
        }

        /**
         * Checks whether object is configured to reuse existing objects
         * representation in repository.
         * <p>
         * Default setting: {@value #DEFAULT_REUSE_OBJECTS}
         * </p>
         *
         * @return true if writer is configured to reuse objects representation from
         *         pack; false otherwise.
         */
        public boolean isReuseObjects() {
                return reuseObjects;
        }

        /**
         * Set reuse objects configuration option for this writer. If enabled,
         * writer searches for representation in a pack file. If possible,
         * compressed data is directly copied from such a pack file. Data checksum
         * is verified.
         * <p>
         * Default setting: {@value #DEFAULT_REUSE_OBJECTS}
         * </p>
         *
         * @param reuseObjects
         *            boolean indicating whether or not writer should reuse existing
         *            objects representation.
         */
        public void setReuseObjects(boolean reuseObjects) {
                this.reuseObjects = reuseObjects;
        }

        /**
         * 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).
         * <p>
         * Default setting: {@value #DEFAULT_DELTA_BASE_AS_OFFSET}
         * </p>
         *
         * @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).
         * <p>
         * Default setting: {@value #DEFAULT_DELTA_BASE_AS_OFFSET}
         * </p>
         *
         * @param deltaBaseAsOffset
         *            boolean indicating whether delta base can be stored as an
         *            offset.
         */
        public void setDeltaBaseAsOffset(boolean deltaBaseAsOffset) {
                this.deltaBaseAsOffset = deltaBaseAsOffset;
        }

        /**
         * Get maximum depth of delta chain set up for this writer. Generated chains
         * are not longer than this value.
         * <p>
         * Default setting: {@value #DEFAULT_MAX_DELTA_DEPTH}
         * </p>
         *
         * @return maximum delta chain depth.
         */
        public int getMaxDeltaDepth() {
                return maxDeltaDepth;
        }

        /**
         * Set up maximum depth of delta chain for this writer. Generated chains are
         * not longer than this value. Too low value causes low compression level,
         * while too big makes unpacking (reading) longer.
         * <p>
         * Default setting: {@value #DEFAULT_MAX_DELTA_DEPTH}
         * </p>
         *
         * @param maxDeltaDepth
         *            maximum delta chain depth.
         */
        public void setMaxDeltaDepth(int maxDeltaDepth) {
                this.maxDeltaDepth = maxDeltaDepth;
        }

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

        /**
         * Write pack to output stream according to current writer configuration for
         * provided source iterator of objects.
         * <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>
         * <p>
         * When iterator returns object that has {@link RevFlag#UNINTERESTING} flag,
         * this object won't be included in an output pack. Instead, it is recorded
         * as edge-object (known to remote repository) for thin-pack. In such a case
         * writer may pack objects with delta base object not within set of objects
         * to pack, but belonging to party repository - those marked with
         * {@link RevFlag#UNINTERESTING} flag. This type of pack is used only for
         * transport.
         * </p>
         * <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. {@link ProgressMonitor} tasks {@value #SEARCHING_REUSE_PROGRESS}
         * (only if resueDeltas or reuseObjects is enabled) and
         * {@value #WRITING_OBJECTS_PROGRESS} are updated during packing.
         * </p>
         * <p>
         * All reused objects data checksum (Adler32/CRC32) is computed and
         * validated against existing checksum.
         * </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; if object returned by iterator has
         *            {@link RevFlag#UNINTERESTING} flag set, it won't be included
         *            in a pack, but is considered as edge-object for thin-pack.
         * @throws IOException
         *             when some I/O problem occur during reading objects for pack
         *             or writing pack stream.
         */
        public void writePack(final Iterator<RevObject> objectsSource)
                        throws IOException {
                while (objectsSource.hasNext()) {
                        addObject(objectsSource.next());
                }
                writePackInternal();
        }

        /**
         * Write pack to output stream according to current writer configuration for
         * provided sets of interesting and uninteresting objects.
         * <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>
         * <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. {@link ProgressMonitor} tasks
         * {@value #COUNTING_OBJECTS_PROGRESS}, {@value #SEARCHING_REUSE_PROGRESS}
         * (only if resueDeltas or reuseObjects is enabled) and
         * {@value #WRITING_OBJECTS_PROGRESS} are updated during packing.
         * </p>
         * <p>
         * All reused objects data checksum (Adler32/CRC32) is computed and
         * validated against existing checksum.
         * </p>
         *
         * @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).
         * @param thin
         *            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.
         * @param ignoreMissingUninteresting
         *            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}
         * @throws IOException
         *             when some I/O problem occur during reading objects for pack
         *             or writing pack stream.
         */
        public void writePack(final Collection<ObjectId> interestingObjects,
                        final Collection<ObjectId> uninterestingObjects,
                        final boolean thin, final boolean ignoreMissingUninteresting)
                        throws IOException {
                ObjectWalk walker = setUpWalker(interestingObjects,
                                uninterestingObjects, thin, ignoreMissingUninteresting);
                findObjectsToPack(walker);
                writePackInternal();
        }

        /**
         * 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 ArrayList<ObjectToPack> sorted = new ArrayList<ObjectToPack>(
                                objectsMap.size());
                for (List<ObjectToPack> list : objectsLists) {
                        for (ObjectToPack otp : list)
                                sorted.add(otp);
                }

                final MessageDigest md = Constants.newMessageDigest();
                Collections.sort(sorted);
                for (ObjectToPack otp : sorted) {
                        otp.copyRawTo(buf, 0);
                        md.update(buf, 0, Constants.OBJECT_ID_LENGTH);
                }
                return ObjectId.fromRaw(md.digest());
        }

        private void writePackInternal() throws IOException {
                if (reuseDeltas || reuseObjects)
                        searchForReuse();

                monitor.beginTask(WRITING_OBJECTS_PROGRESS, getObjectsNumber());
                writeHeader();
                writeObjects();
                writeChecksum();

                out.flush();
                windowCursor.release();
                monitor.endTask();
        }

        private void searchForReuse() throws IOException {
                monitor.beginTask(SEARCHING_REUSE_PROGRESS, getObjectsNumber());
                final Collection<PackedObjectLoader> reuseLoaders = new LinkedList<PackedObjectLoader>();

                for (List<ObjectToPack> list : objectsLists) {
                        for (ObjectToPack otp : list) {
                                if (monitor.isCancelled())
                                        throw new IOException(
                                                        "Packing cancelled during objects writing");
                                reuseLoaders.clear();
                                db.openObjectInAllPacks(otp, reuseLoaders, windowCursor);
                                if (reuseDeltas) {
                                        selectDeltaReuseForObject(otp, reuseLoaders);
                                }
                                // delta reuse is preferred over object reuse
                                if (reuseObjects && !otp.hasReuseLoader()) {
                                        selectObjectReuseForObject(otp, reuseLoaders);
                                }
                                monitor.update(1);
                        }
                }

                monitor.endTask();
        }

        private void selectDeltaReuseForObject(final ObjectToPack otp,
                        final Collection<PackedObjectLoader> loaders) throws IOException {
                PackedObjectLoader bestLoader = null;
                ObjectId bestBase = null;

                for (PackedObjectLoader loader : loaders) {
                        ObjectId idBase = loader.getDeltaBase();
                        if (idBase == null)
                                continue;
                        ObjectToPack otpBase = objectsMap.get(idBase);

                        // only if base is in set of objects to write or thin-pack's edge
                        if ((otpBase != null || (thin && edgeObjects.get(idBase) != null))
                        // select smallest possible delta if > 1 available
                                        && isBetterDeltaReuseLoader(bestLoader, loader)) {
                                bestLoader = loader;
                                bestBase = (otpBase != null ? otpBase : idBase);
                        }
                }

                if (bestLoader != null) {
                        otp.setReuseLoader(bestLoader);
                        otp.setDeltaBase(bestBase);
                }
        }

        private static boolean isBetterDeltaReuseLoader(
                        PackedObjectLoader currentLoader, PackedObjectLoader loader)
                        throws IOException {
                if (currentLoader == null)
                        return true;
                if (loader.getRawSize() < currentLoader.getRawSize())
                        return true;
                return (loader.getRawSize() == currentLoader.getRawSize()
                                && loader.supportsFastCopyRawData() && !currentLoader
                                .supportsFastCopyRawData());
        }

        private void selectObjectReuseForObject(final ObjectToPack otp,
                        final Collection<PackedObjectLoader> loaders) {
                for (final PackedObjectLoader loader : loaders) {
                        if (loader instanceof WholePackedObjectLoader) {
                                otp.setReuseLoader(loader);
                                return;
                        }
                }
        }

        private void writeHeader() throws IOException {
                out.write(Constants.PACK_SIGNATURE);

                NB.encodeInt32(buf, 0, PACK_VERSION_GENERATED);
                out.write(buf, 0, 4);

                NB.encodeInt32(buf, 0, getObjectsNumber());
                out.write(buf, 0, 4);
        }

        private void writeObjects() throws IOException {
                for (List<ObjectToPack> list : objectsLists) {
                        for (ObjectToPack otp : list) {
                                if (monitor.isCancelled())
                                        throw new IOException(
                                                        "Packing cancelled during objects writing");
                                if (!otp.isWritten())
                                        writeObject(otp);
                        }
                }
        }

        private void writeObject(final ObjectToPack otp) throws IOException {
                otp.markWantWrite();
                if (otp.isDeltaRepresentation()) {
                        ObjectToPack deltaBase = otp.getDeltaBase();
                        assert deltaBase != null || thin;
                        if (deltaBase != null && !deltaBase.isWritten()) {
                                if (deltaBase.wantWrite()) {
                                        otp.clearDeltaBase(); // cycle detected
                                        otp.disposeLoader();
                                } else {
                                        writeObject(deltaBase);
                                }
                        }

                        otp.updateDeltaDepth();
                        if (otp.getDeltaDepth() > maxDeltaDepth) {
                                otp.clearDeltaBase();
                                otp.disposeLoader();
                        }
                }

                assert !otp.isWritten();

                otp.markWritten(countingOut.getCount());
                if (otp.isDeltaRepresentation())
                        writeDeltaObject(otp);
                else
                        writeWholeObject(otp);

                monitor.update(1);
        }

        private void writeWholeObject(final ObjectToPack otp) throws IOException {
                if (otp.hasReuseLoader()) {
                        final PackedObjectLoader loader = otp.getReuseLoader();
                        writeObjectHeader(loader.getType(), loader.getSize());
                        loader.copyRawData(out, buf);
                        otp.disposeLoader();
                } else {
                        final ObjectLoader loader = db.openObject(windowCursor, otp);
                        final DeflaterOutputStream deflaterOut = new DeflaterOutputStream(
                                        out, deflater);
                        writeObjectHeader(loader.getType(), loader.getSize());
                        deflaterOut.write(loader.getCachedBytes());
                        deflaterOut.finish();
                        deflater.reset();
                }
        }

        private void writeDeltaObject(final ObjectToPack otp) throws IOException {
                final PackedObjectLoader loader = otp.getReuseLoader();
                if (deltaBaseAsOffset && otp.getDeltaBase() != null) {
                        writeObjectHeader(Constants.OBJ_OFS_DELTA, loader.getRawSize());

                        final ObjectToPack deltaBase = otp.getDeltaBase();
                        long offsetDiff = otp.getOffset() - deltaBase.getOffset();
                        int pos = buf.length - 1;
                        buf[pos] = (byte) (offsetDiff & 0x7F);
                        while ((offsetDiff >>= 7) > 0) {
                                buf[--pos] = (byte) (0x80 | (--offsetDiff & 0x7F));
                        }

                        out.write(buf, pos, buf.length - pos);
                } else {
                        writeObjectHeader(Constants.OBJ_REF_DELTA, loader.getRawSize());
                        otp.getDeltaBaseId().copyRawTo(buf, 0);
                        out.write(buf, 0, Constants.OBJECT_ID_LENGTH);
                }
                loader.copyRawData(out, buf);
                otp.disposeLoader();
        }

        private void writeObjectHeader(final int objectType, long dataLength)
                        throws IOException {
                long nextLength = dataLength >>> 4;
                int size = 0;
                buf[size++] = (byte) ((nextLength > 0 ? 0x80 : 0x00)
                                | (objectType << 4) | (dataLength & 0x0F));
                dataLength = nextLength;
                while (dataLength > 0) {
                        nextLength >>>= 7;
                        buf[size++] = (byte) ((nextLength > 0 ? 0x80 : 0x00) | (dataLength & 0x7F));
                        dataLength = nextLength;
                }
                out.write(buf, 0, size);
        }

        private void writeChecksum() throws IOException {
                out.on(false);
                final byte checksum[] = out.getMessageDigest().digest();
                out.write(checksum);
        }

        private ObjectWalk setUpWalker(
                        final Collection<ObjectId> interestingObjects,
                        final Collection<ObjectId> uninterestingObjects,
                        final boolean thin, final boolean ignoreMissingUninteresting)
                        throws MissingObjectException, IOException,
                        IncorrectObjectTypeException {
                final ObjectWalk walker = new ObjectWalk(db);
                walker.sort(RevSort.TOPO, true);
                walker.sort(RevSort.COMMIT_TIME_DESC, true);
                if (thin)
                        walker.sort(RevSort.BOUNDARY);

                for (ObjectId id : interestingObjects) {
                        RevObject o = walker.parseAny(id);
                        walker.markStart(o);
                }
                for (ObjectId id : uninterestingObjects) {
                        final RevObject o;
                        try {
                                o = walker.parseAny(id);
                        } catch (MissingObjectException x) {
                                if (ignoreMissingUninteresting)
                                        continue;
                                throw x;
                        }
                        walker.markUninteresting(o);
                }
                return walker;
        }

        private void findObjectsToPack(final ObjectWalk walker)
                        throws MissingObjectException, IncorrectObjectTypeException,
                        IOException {
                monitor.beginTask(COUNTING_OBJECTS_PROGRESS, ProgressMonitor.UNKNOWN);
                RevObject o;

                while ((o = walker.next()) != null) {
                        addObject(o);
                        monitor.update(1);
                }
                while ((o = walker.nextObject()) != null) {
                        addObject(o);
                        monitor.update(1);
                }
                monitor.endTask();
        }

        private void addObject(RevObject object)
                        throws IncorrectObjectTypeException {
                if (object.has(RevFlag.UNINTERESTING)) {
                        edgeObjects.add(object);
                        thin = true;
                        return;
                }

                final ObjectToPack otp = new ObjectToPack(object);
                try {
                        objectsLists[object.getType()].add(otp);
                } catch (ArrayIndexOutOfBoundsException x) {
                        throw new IncorrectObjectTypeException(object,
                                        "COMMIT nor TREE nor BLOB nor TAG");
                } catch (UnsupportedOperationException x) {
                        // index pointing to "dummy" empty list
                        throw new IncorrectObjectTypeException(object,
                                        "COMMIT nor TREE nor BLOB nor TAG");
                }
                objectsMap.add(otp);
        }

        /**
         * Class holding information about object that is going to be packed by
         * {@link PackWriter}. Information include object representation in a
         * pack-file and object status.
         *
         */
        static class ObjectToPack extends ObjectId {
                private ObjectId deltaBase;

                private PackedObjectLoader reuseLoader;

                private long offset = -1;

                private int deltaDepth;

                private boolean wantWrite;

                /**
                 * Construct object for specified object id. <br/> By default object is
                 * marked as not written and non-delta packed (as a whole object).
                 *
                 * @param src
                 *            object id of object for packing
                 */
                ObjectToPack(AnyObjectId src) {
                        super(src);
                }

                /**
                 * @return delta base object id if object is going to be packed in delta
                 *         representation; null otherwise - if going to be packed as a
                 *         whole object.
                 */
                ObjectId getDeltaBaseId() {
                        return deltaBase;
                }

                /**
                 * @return delta base object to pack if object is going to be packed in
                 *         delta representation and delta is specified as object to
                 *         pack; null otherwise - if going to be packed as a whole
                 *         object or delta base is specified only as id.
                 */
                ObjectToPack getDeltaBase() {
                        if (deltaBase instanceof ObjectToPack)
                                return (ObjectToPack) deltaBase;
                        return null;
                }

                /**
                 * Set delta base for the object. Delta base set by this method is used
                 * by {@link PackWriter} to write object - determines its representation
                 * in a created pack.
                 *
                 * @param deltaBase
                 *            delta base object or null if object should be packed as a
                 *            whole object.
                 *
                 */
                void setDeltaBase(ObjectId deltaBase) {
                        this.deltaBase = deltaBase;
                }

                void clearDeltaBase() {
                        this.deltaBase = null;
                }

                /**
                 * @return true if object is going to be written as delta; false
                 *         otherwise.
                 */
                boolean isDeltaRepresentation() {
                        return deltaBase != null;
                }

                /**
                 * Check if object is already written in a pack. This information is
                 * used to achieve delta-base precedence in a pack file.
                 *
                 * @return true if object is already written; false otherwise.
                 */
                boolean isWritten() {
                        return offset != -1;
                }

                /**
                 * @return offset in pack when object has been already written, or -1 if
                 *         it has not been written yet
                 */
                long getOffset() {
                        return offset;
                }

                /**
                 * Mark object as written. This information is used to achieve
                 * delta-base precedence in a pack file.
                 *
                 * @param offset
                 *            offset where written object starts
                 */
                void markWritten(long offset) {
                        this.offset = offset;
                }

                PackedObjectLoader getReuseLoader() {
                        return reuseLoader;
                }

                boolean hasReuseLoader() {
                        return reuseLoader != null;
                }

                void setReuseLoader(PackedObjectLoader reuseLoader) {
                        this.reuseLoader = reuseLoader;
                }

                void disposeLoader() {
                        this.reuseLoader = null;
                }

                int getDeltaDepth() {
                        return deltaDepth;
                }

                void updateDeltaDepth() {
                        if (deltaBase instanceof ObjectToPack)
                                deltaDepth = ((ObjectToPack) deltaBase).deltaDepth + 1;
                        else if (deltaBase != null)
                                deltaDepth = 1;
                }

                boolean wantWrite() {
                        return wantWrite;
                }

                void markWantWrite() {
                        this.wantWrite = true;
                }
        }
}