org.eclipse.jgit/src/org/eclipse/jgit/internal/storage/pack/PackWriterBitmapPreparer.java - jgit - Git at Google

 /*
  * Copyright (C) 2012, Google Inc. and others
  *
  * This program and the accompanying materials are made available under the
  * terms of the Eclipse Distribution License v. 1.0 which is available at
  * https://www.eclipse.org/org/documents/edl-v10.php.
  *
  * SPDX-License-Identifier: BSD-3-Clause
  */

 package org.eclipse.jgit.internal.storage.pack;

 import static org.eclipse.jgit.internal.storage.file.PackBitmapIndex.FLAG_REUSE;
 import static org.eclipse.jgit.revwalk.RevFlag.SEEN;

 import java.io.IOException;
 import java.util.ArrayList;
 import java.util.Collection;
 import java.util.Collections;
 import java.util.Comparator;
 import java.util.HashSet;
 import java.util.Iterator;
 import java.util.List;
 import java.util.Set;

 import org.eclipse.jgit.errors.IncorrectObjectTypeException;
 import org.eclipse.jgit.errors.MissingObjectException;
 import org.eclipse.jgit.internal.JGitText;
 import org.eclipse.jgit.internal.revwalk.AddUnseenToBitmapFilter;
 import org.eclipse.jgit.internal.storage.file.BitmapIndexImpl;
 import org.eclipse.jgit.internal.storage.file.BitmapIndexImpl.CompressedBitmap;
 import org.eclipse.jgit.internal.storage.file.PackBitmapIndex;
 import org.eclipse.jgit.internal.storage.file.PackBitmapIndexBuilder;
 import org.eclipse.jgit.internal.storage.file.PackBitmapIndexRemapper;
 import org.eclipse.jgit.lib.AnyObjectId;
 import org.eclipse.jgit.lib.BitmapIndex.BitmapBuilder;
 import org.eclipse.jgit.lib.Constants;
 import org.eclipse.jgit.lib.ObjectId;
 import org.eclipse.jgit.lib.ObjectReader;
 import org.eclipse.jgit.lib.ProgressMonitor;
 import org.eclipse.jgit.revwalk.BitmapWalker;
 import org.eclipse.jgit.revwalk.ObjectWalk;
 import org.eclipse.jgit.revwalk.RevCommit;
 import org.eclipse.jgit.revwalk.RevObject;
 import org.eclipse.jgit.revwalk.RevWalk;
 import org.eclipse.jgit.revwalk.filter.RevFilter;
 import org.eclipse.jgit.storage.pack.PackConfig;
 import org.eclipse.jgit.util.BlockList;
 import org.eclipse.jgit.util.SystemReader;

 import com.googlecode.javaewah.EWAHCompressedBitmap;

 /**
  * Helper class for the {@link PackWriter} to select commits for which to build
  * pack index bitmaps.
  */
 class PackWriterBitmapPreparer {

 	private static final int DAY_IN_SECONDS = 24 * 60 * 60;

 	private static final Comparator<RevCommit> ORDER_BY_REVERSE_TIMESTAMP = (
 			RevCommit a, RevCommit b) -> Integer
 					.signum(b.getCommitTime() - a.getCommitTime());

 	private final ObjectReader reader;
 	private final ProgressMonitor pm;
 	private final Set<? extends ObjectId> want;
 	private final PackBitmapIndexBuilder writeBitmaps;
 	private final BitmapIndexImpl commitBitmapIndex;
 	private final PackBitmapIndexRemapper bitmapRemapper;
 	private final BitmapIndexImpl bitmapIndex;

 	private final int contiguousCommitCount;
 	private final int recentCommitCount;
 	private final int recentCommitSpan;
 	private final int distantCommitSpan;
 	private final int excessiveBranchCount;
 	private final long inactiveBranchTimestamp;

 	PackWriterBitmapPreparer(ObjectReader reader,
 			PackBitmapIndexBuilder writeBitmaps, ProgressMonitor pm,
 			Set<? extends ObjectId> want, PackConfig config)
 					throws IOException {
 		this.reader = reader;
 		this.writeBitmaps = writeBitmaps;
 		this.pm = pm;
 		this.want = want;
 		this.commitBitmapIndex = new BitmapIndexImpl(writeBitmaps);
 		this.bitmapRemapper = PackBitmapIndexRemapper.newPackBitmapIndex(
 				reader.getBitmapIndex(), writeBitmaps);
 		this.bitmapIndex = new BitmapIndexImpl(bitmapRemapper);
 		this.contiguousCommitCount = config.getBitmapContiguousCommitCount();
 		this.recentCommitCount = config.getBitmapRecentCommitCount();
 		this.recentCommitSpan = config.getBitmapRecentCommitSpan();
 		this.distantCommitSpan = config.getBitmapDistantCommitSpan();
 		this.excessiveBranchCount = config.getBitmapExcessiveBranchCount();
 		long now = SystemReader.getInstance().getCurrentTime();
 		long ageInSeconds = config.getBitmapInactiveBranchAgeInDays()
 				* DAY_IN_SECONDS;
 		this.inactiveBranchTimestamp = (now / 1000) - ageInSeconds;
 	}

 	/**
 	 * Returns the commit objects for which bitmap indices should be built.
 	 *
 	 * @param expectedCommitCount
 	 *            count of commits in the pack
 	 * @param excludeFromBitmapSelection
 	 *            commits that should be excluded from bitmap selection
 	 * @return commit objects for which bitmap indices should be built
 	 * @throws IncorrectObjectTypeException
 	 *             if any of the processed objects is not a commit
 	 * @throws IOException
 	 *             on errors reading pack or index files
 	 * @throws MissingObjectException
 	 *             if an expected object is missing
 	 */
 	Collection<BitmapCommit> selectCommits(int expectedCommitCount,
 			Set<? extends ObjectId> excludeFromBitmapSelection)
 			throws IncorrectObjectTypeException, IOException,
 			MissingObjectException {
 		/*
 		 * Thinking of bitmap indices as a cache, if we find bitmaps at or at a
 		 * close ancestor to 'old' and 'new' when calculating old..new, then all
 		 * objects can be calculated with minimal graph walking. A distribution
 		 * that favors creating bitmaps for the most recent commits maximizes
 		 * the cache hits for clients that are close to HEAD, which is the
 		 * majority of calculations performed.
 		 */
 		try (RevWalk rw = new RevWalk(reader);
 				RevWalk rw2 = new RevWalk(reader)) {
 			pm.beginTask(JGitText.get().selectingCommits,
 					ProgressMonitor.UNKNOWN);
 			rw.setRetainBody(false);
 			CommitSelectionHelper selectionHelper = captureOldAndNewCommits(rw,
 					expectedCommitCount, excludeFromBitmapSelection);
 			pm.endTask();

 			// Add reused bitmaps from the previous GC pack's bitmap indices.
 			// Currently they are always fully reused, even if their spans don't
 			// match this run's PackConfig values.
 			int newCommits = selectionHelper.getCommitCount();
 			BlockList<BitmapCommit> selections = new BlockList<>(
 					selectionHelper.reusedCommits.size()
 							+ newCommits / recentCommitSpan + 1);
 			for (BitmapCommit reuse : selectionHelper.reusedCommits) {
 				selections.add(reuse);
 			}

 			if (newCommits == 0) {
 				for (AnyObjectId id : selectionHelper.newWants) {
 					selections.add(new BitmapCommit(id, false, 0));
 				}
 				return selections;
 			}

 			pm.beginTask(JGitText.get().selectingCommits, newCommits);
 			int totalWants = want.size();
 			BitmapBuilder seen = commitBitmapIndex.newBitmapBuilder();
 			seen.or(selectionHelper.reusedCommitsBitmap);
 			rw2.setRetainBody(false);
 			rw2.setRevFilter(new NotInBitmapFilter(seen));

 			// For each branch, do a revwalk to enumerate its commits. Exclude
 			// both reused commits and any commits seen in a previous branch.
 			// Then iterate through all new commits from oldest to newest,
 			// selecting well-spaced commits in this branch.
 			for (RevCommit rc : selectionHelper.newWantsByNewest) {
 				BitmapBuilder tipBitmap = commitBitmapIndex.newBitmapBuilder();
 				rw2.markStart((RevCommit) rw2.peel(rw2.parseAny(rc)));
 				RevCommit rc2;
 				while ((rc2 = rw2.next()) != null) {
 					tipBitmap.addObject(rc2, Constants.OBJ_COMMIT);
 				}
 				int cardinality = tipBitmap.cardinality();
 				seen.or(tipBitmap);

 				// Within this branch, keep ordered lists of commits
 				// representing chains in its history, where each chain is a
 				// "sub-branch". Ordering commits by these chains makes for
 				// fewer differences between consecutive selected commits, which
 				// in turn provides better compression/on the run-length
 				// encoding of the XORs between them.
 				List<List<BitmapCommit>> chains = new ArrayList<>();

 				// Mark the current branch as inactive if its tip commit isn't
 				// recent and there are an excessive number of branches, to
 				// prevent memory bloat of computing too many bitmaps for stale
 				// branches.
 				boolean isActiveBranch = true;
 				if (totalWants > excessiveBranchCount && !isRecentCommit(rc)) {
 					isActiveBranch = false;
 				}

 				// Insert bitmaps at the offsets suggested by the
 				// nextSelectionDistance() heuristic. Only reuse bitmaps created
 				// for more distant commits.
 				int index = -1;
 				int nextIn = nextSpan(cardinality);
 				int nextFlg = nextIn == distantCommitSpan
 						? PackBitmapIndex.FLAG_REUSE
 						: 0;

 				// For the current branch, iterate through all commits from
 				// oldest to newest.
 				for (RevCommit c : selectionHelper) {
 					// Optimization: if we have found all the commits for this
 					// branch, stop searching
 					int distanceFromTip = cardinality - index - 1;
 					if (distanceFromTip == 0) {
 						break;
 					}

 					// Ignore commits that are not in this branch
 					if (!tipBitmap.contains(c)) {
 						continue;
 					}

 					index++;
 					nextIn--;
 					pm.update(1);

 					// Always pick the items in wants, prefer merge commits.
 					if (selectionHelper.newWants.remove(c)) {
 						if (nextIn > 0) {
 							nextFlg = 0;
 						}
 					} else {
 						boolean stillInSpan = nextIn >= 0;
 						boolean isMergeCommit = c.getParentCount() > 1;
 						// Force selection if:
 						// a) we have exhausted the window looking for merges
 						// b) we are in the top commits of an active branch
 						// c) we are at a branch tip
 						boolean mustPick = (nextIn <= -recentCommitSpan)
 								|| (isActiveBranch
 										&& (distanceFromTip <= contiguousCommitCount))
 								|| (distanceFromTip == 1); // most recent commit
 						if (!mustPick && (stillInSpan || !isMergeCommit)) {
 							continue;
 						}
 					}

 					// This commit is selected.
 					// Calculate where to look for the next one.
 					int flags = nextFlg;
 					nextIn = nextSpan(distanceFromTip);
 					nextFlg = nextIn == distantCommitSpan
 							? PackBitmapIndex.FLAG_REUSE
 							: 0;

 					// Create the commit bitmap for the current commit
 					BitmapBuilder bitmap = commitBitmapIndex.newBitmapBuilder();
 					rw.reset();
 					rw.markStart(c);
 					rw.setRevFilter(new AddUnseenToBitmapFilter(
 							selectionHelper.reusedCommitsBitmap, bitmap));
 					while (rw.next() != null) {
 						// The filter adds the reachable commits to bitmap.
 					}

 					// Sort the commits by independent chains in this branch's
 					// history, yielding better compression when building
 					// bitmaps.
 					List<BitmapCommit> longestAncestorChain = null;
 					for (List<BitmapCommit> chain : chains) {
 						BitmapCommit mostRecentCommit = chain
 								.get(chain.size() - 1);
 						if (bitmap.contains(mostRecentCommit)) {
 							if (longestAncestorChain == null
 									|| longestAncestorChain.size() < chain
 											.size()) {
 								longestAncestorChain = chain;
 							}
 						}
 					}

 					if (longestAncestorChain == null) {
 						longestAncestorChain = new ArrayList<>();
 						chains.add(longestAncestorChain);
 					}
 					longestAncestorChain.add(new BitmapCommit(c,
 							!longestAncestorChain.isEmpty(), flags));
 					writeBitmaps.addBitmap(c, bitmap, 0);
 				}

 				for (List<BitmapCommit> chain : chains) {
 					selections.addAll(chain);
 				}
 			}
 			writeBitmaps.clearBitmaps(); // Remove the temporary commit bitmaps.

 			// Add the remaining peeledWant
 			for (AnyObjectId remainingWant : selectionHelper.newWants) {
 				selections.add(new BitmapCommit(remainingWant, false, 0));
 			}

 			pm.endTask();
 			return selections;
 		}
 	}

 	private boolean isRecentCommit(RevCommit revCommit) {
 		return revCommit.getCommitTime() > inactiveBranchTimestamp;
 	}

 	/**
 	 * A RevFilter that excludes the commits named in a bitmap from the walk.
 	 * <p>
 	 * If a commit is in {@code bitmap} then that commit is not emitted by the
 	 * walk and its parents are marked as SEEN so the walk can skip them.  The
 	 * bitmaps passed in have the property that the parents of any commit in
 	 * {@code bitmap} are also in {@code bitmap}, so marking the parents as
 	 * SEEN speeds up the RevWalk by saving it from walking down blind alleys
 	 * and does not change the commits emitted.
 	 */
 	private static class NotInBitmapFilter extends RevFilter {
 		private final BitmapBuilder bitmap;

 		NotInBitmapFilter(BitmapBuilder bitmap) {
 			this.bitmap = bitmap;
 		}

 		@Override
 		public final boolean include(RevWalk rw, RevCommit c) {
 			if (!bitmap.contains(c)) {
 				return true;
 			}
 			for (RevCommit p : c.getParents()) {
 				p.add(SEEN);
 			}
 			return false;
 		}

 		@Override
 		public final NotInBitmapFilter clone() {
 			throw new UnsupportedOperationException();
 		}

 		@Override
 		public final boolean requiresCommitBody() {
 			return false;
 		}
 	}

 	/**
 	 * Records which of the {@code wants} can be found in the previous GC pack's
 	 * bitmap indices and which are new.
 	 *
 	 * @param rw
 	 *            a {@link RevWalk} to find reachable objects in this repository
 	 * @param expectedCommitCount
 	 *            expected count of commits. The actual count may be less due to
 	 *            unreachable garbage.
 	 * @param excludeFromBitmapSelection
 	 *            commits that should be excluded from bitmap selection
 	 * @return a {@link CommitSelectionHelper} capturing which commits are
 	 *         covered by a previous pack's bitmaps and which new commits need
 	 *         bitmap coverage
 	 * @throws IncorrectObjectTypeException
 	 *             if any of the processed objects is not a commit
 	 * @throws IOException
 	 *             on errors reading pack or index files
 	 * @throws MissingObjectException
 	 *             if an expected object is missing
 	 */
 	private CommitSelectionHelper captureOldAndNewCommits(RevWalk rw,
 			int expectedCommitCount,
 			Set<? extends ObjectId> excludeFromBitmapSelection)
 			throws IncorrectObjectTypeException, IOException,
 			MissingObjectException {
 		// Track bitmaps and commits from the previous GC pack bitmap indices.
 		BitmapBuilder reuse = commitBitmapIndex.newBitmapBuilder();
 		List<BitmapCommit> reuseCommits = new ArrayList<>();
 		for (PackBitmapIndexRemapper.Entry entry : bitmapRemapper) {
 			// More recent commits did not have the reuse flag set, so skip them
 			if ((entry.getFlags() & FLAG_REUSE) != FLAG_REUSE) {
 				continue;
 			}
 			RevObject ro = rw.peel(rw.parseAny(entry));
 			if (!(ro instanceof RevCommit)) {
 				continue;
 			}

 			RevCommit rc = (RevCommit) ro;
 			reuseCommits.add(new BitmapCommit(rc, false, entry.getFlags()));
 			if (!reuse.contains(rc)) {
 				EWAHCompressedBitmap bitmap = bitmapRemapper.ofObjectType(
 						bitmapRemapper.getBitmap(rc), Constants.OBJ_COMMIT);
 				reuse.or(new CompressedBitmap(bitmap, commitBitmapIndex));
 			}
 		}

 		// Add branch tips that are not represented in a previous pack's bitmap
 		// indices. Set up a RevWalk to find new commits not in the old packs.
 		List<RevCommit> newWantsByNewest = new ArrayList<>(want.size());
 		Set<RevCommit> newWants = new HashSet<>(want.size());
 		for (AnyObjectId objectId : want) {
 			RevObject ro = rw.peel(rw.parseAny(objectId));
 			if (!(ro instanceof RevCommit) || reuse.contains(ro)
 					|| excludeFromBitmapSelection.contains(ro)) {
 				continue;
 			}

 			RevCommit rc = (RevCommit) ro;
 			rw.markStart(rc);
 			newWants.add(rc);
 			newWantsByNewest.add(rc);
 		}

 		// Create a list of commits in reverse order (older to newer) that are
 		// not in the previous bitmap indices and are reachable.
 		rw.setRevFilter(new NotInBitmapFilter(reuse));
 		RevCommit[] commits = new RevCommit[expectedCommitCount];
 		int pos = commits.length;
 		RevCommit rc;
 		while ((rc = rw.next()) != null && pos > 0) {
 			commits[--pos] = rc;
 			pm.update(1);
 		}

 		// Sort the new wants by reverse commit time.
 		Collections.sort(newWantsByNewest, ORDER_BY_REVERSE_TIMESTAMP);
 		return new CommitSelectionHelper(newWants, commits, pos,
 				newWantsByNewest, reuse, reuseCommits);
 	}

 	/*-
 	 * Returns the desired distance to the next bitmap based on the distance
 	 * from the tip commit. Only differentiates recent from distant spans,
 	 * selectCommits() handles the contiguous commits at the tip for active
 	 * or inactive branches.
 	 *
 	 * A graph of this function looks like this, where
 	 * the X axis is the distance from the tip commit and the Y axis is the
 	 * bitmap selection distance.
 	 *
 	 * 5000                ____...
 	 *                    /
 	 *                  /
 	 *                /
 	 *              /
 	 *  100  _____/
 	 *       0  20100  25000
 	 *
 	 * Linear scaling between 20100 and 25000 prevents spans >100 for distances
 	 * <20000 (otherwise, a span of 5000 would be returned for a distance of
 	 * 21000, and the range 16000-20000 would have no selections).
 	 */
 	int nextSpan(int distanceFromTip) {
 		if (distanceFromTip < 0) {
 			throw new IllegalArgumentException();
 		}

 		// Commits more toward the start will have more bitmaps.
 		if (distanceFromTip <= recentCommitCount) {
 			return recentCommitSpan;
 		}

 		int next = Math.min(distanceFromTip - recentCommitCount,
 				distantCommitSpan);
 		return Math.max(next, recentCommitSpan);
 	}

 	BitmapWalker newBitmapWalker() {
 		return new BitmapWalker(
 				new ObjectWalk(reader), bitmapIndex, null);
 	}

 	/**
 	 * Container for state used in the first phase of selecting commits, which
 	 * walks all of the reachable commits via the branch tips that are not
 	 * covered by a previous pack's bitmaps ({@code newWants}) and stores them
 	 * in {@code newCommitsByOldest}. {@code newCommitsByOldest} is initialized
 	 * with an expected size of all commits, but may be smaller if some commits
 	 * are unreachable and/or some commits are covered by a previous pack's
 	 * bitmaps. {@code commitStartPos} will contain a positive offset to either
 	 * the root commit or the oldest commit not covered by previous bitmaps.
 	 */
 	private static final class CommitSelectionHelper implements Iterable<RevCommit> {
 		final Set<? extends ObjectId> newWants;

 		final List<RevCommit> newWantsByNewest;
 		final BitmapBuilder reusedCommitsBitmap;

 		final List<BitmapCommit> reusedCommits;

 		final RevCommit[] newCommitsByOldest;

 		final int newCommitStartPos;

 		CommitSelectionHelper(Set<? extends ObjectId> newWants,
 				RevCommit[] commitsByOldest, int commitStartPos,
 				List<RevCommit> newWantsByNewest,
 				BitmapBuilder reusedCommitsBitmap,
 				List<BitmapCommit> reuse) {
 			this.newWants = newWants;
 			this.newCommitsByOldest = commitsByOldest;
 			this.newCommitStartPos = commitStartPos;
 			this.newWantsByNewest = newWantsByNewest;
 			this.reusedCommitsBitmap = reusedCommitsBitmap;
 			this.reusedCommits = reuse;
 		}

 		@Override
 		public Iterator<RevCommit> iterator() {
 			// Member variables referenced by this iterator will have synthetic
 			// accessors generated for them if they are made private.
 			return new Iterator<RevCommit>() {
 				int pos = newCommitStartPos;

 				@Override
 				public boolean hasNext() {
 					return pos < newCommitsByOldest.length;
 				}

 				@Override
 				public RevCommit next() {
 					return newCommitsByOldest[pos++];
 				}

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

 		int getCommitCount() {
 			return newCommitsByOldest.length - newCommitStartPos;
 		}
 	}
 }
	/*
	* Copyright (C) 2012, Google Inc. and others
	*
	* This program and the accompanying materials are made available under the
	* terms of the Eclipse Distribution License v. 1.0 which is available at
	* https://www.eclipse.org/org/documents/edl-v10.php.
	*
	* SPDX-License-Identifier: BSD-3-Clause
	*/

	package org.eclipse.jgit.internal.storage.pack;

	import static org.eclipse.jgit.internal.storage.file.PackBitmapIndex.FLAG_REUSE;
	import static org.eclipse.jgit.revwalk.RevFlag.SEEN;

	import java.io.IOException;
	import java.util.ArrayList;
	import java.util.Collection;
	import java.util.Collections;
	import java.util.Comparator;
	import java.util.HashSet;
	import java.util.Iterator;
	import java.util.List;
	import java.util.Set;

	import org.eclipse.jgit.errors.IncorrectObjectTypeException;
	import org.eclipse.jgit.errors.MissingObjectException;
	import org.eclipse.jgit.internal.JGitText;
	import org.eclipse.jgit.internal.revwalk.AddUnseenToBitmapFilter;
	import org.eclipse.jgit.internal.storage.file.BitmapIndexImpl;
	import org.eclipse.jgit.internal.storage.file.BitmapIndexImpl.CompressedBitmap;
	import org.eclipse.jgit.internal.storage.file.PackBitmapIndex;
	import org.eclipse.jgit.internal.storage.file.PackBitmapIndexBuilder;
	import org.eclipse.jgit.internal.storage.file.PackBitmapIndexRemapper;
	import org.eclipse.jgit.lib.AnyObjectId;
	import org.eclipse.jgit.lib.BitmapIndex.BitmapBuilder;
	import org.eclipse.jgit.lib.Constants;
	import org.eclipse.jgit.lib.ObjectId;
	import org.eclipse.jgit.lib.ObjectReader;
	import org.eclipse.jgit.lib.ProgressMonitor;
	import org.eclipse.jgit.revwalk.BitmapWalker;
	import org.eclipse.jgit.revwalk.ObjectWalk;
	import org.eclipse.jgit.revwalk.RevCommit;
	import org.eclipse.jgit.revwalk.RevObject;
	import org.eclipse.jgit.revwalk.RevWalk;
	import org.eclipse.jgit.revwalk.filter.RevFilter;
	import org.eclipse.jgit.storage.pack.PackConfig;
	import org.eclipse.jgit.util.BlockList;
	import org.eclipse.jgit.util.SystemReader;

	import com.googlecode.javaewah.EWAHCompressedBitmap;

	/**
	* Helper class for the {@link PackWriter} to select commits for which to build
	* pack index bitmaps.
	*/
	class PackWriterBitmapPreparer {

	private static final int DAY_IN_SECONDS = 24 * 60 * 60;

	private static final Comparator<RevCommit> ORDER_BY_REVERSE_TIMESTAMP = (
	RevCommit a, RevCommit b) -> Integer
	.signum(b.getCommitTime() - a.getCommitTime());

	private final ObjectReader reader;
	private final ProgressMonitor pm;
	private final Set<? extends ObjectId> want;
	private final PackBitmapIndexBuilder writeBitmaps;
	private final BitmapIndexImpl commitBitmapIndex;
	private final PackBitmapIndexRemapper bitmapRemapper;
	private final BitmapIndexImpl bitmapIndex;

	private final int contiguousCommitCount;
	private final int recentCommitCount;
	private final int recentCommitSpan;
	private final int distantCommitSpan;
	private final int excessiveBranchCount;
	private final long inactiveBranchTimestamp;

	PackWriterBitmapPreparer(ObjectReader reader,
	PackBitmapIndexBuilder writeBitmaps, ProgressMonitor pm,
	Set<? extends ObjectId> want, PackConfig config)
	throws IOException {
	this.reader = reader;
	this.writeBitmaps = writeBitmaps;
	this.pm = pm;
	this.want = want;
	this.commitBitmapIndex = new BitmapIndexImpl(writeBitmaps);
	this.bitmapRemapper = PackBitmapIndexRemapper.newPackBitmapIndex(
	reader.getBitmapIndex(), writeBitmaps);
	this.bitmapIndex = new BitmapIndexImpl(bitmapRemapper);
	this.contiguousCommitCount = config.getBitmapContiguousCommitCount();
	this.recentCommitCount = config.getBitmapRecentCommitCount();
	this.recentCommitSpan = config.getBitmapRecentCommitSpan();
	this.distantCommitSpan = config.getBitmapDistantCommitSpan();
	this.excessiveBranchCount = config.getBitmapExcessiveBranchCount();
	long now = SystemReader.getInstance().getCurrentTime();
	long ageInSeconds = config.getBitmapInactiveBranchAgeInDays()
	* DAY_IN_SECONDS;
	this.inactiveBranchTimestamp = (now / 1000) - ageInSeconds;
	}

	/**
	* Returns the commit objects for which bitmap indices should be built.
	*
	* @param expectedCommitCount
	* count of commits in the pack
	* @param excludeFromBitmapSelection
	* commits that should be excluded from bitmap selection
	* @return commit objects for which bitmap indices should be built
	* @throws IncorrectObjectTypeException
	* if any of the processed objects is not a commit
	* @throws IOException
	* on errors reading pack or index files
	* @throws MissingObjectException
	* if an expected object is missing
	*/
	Collection<BitmapCommit> selectCommits(int expectedCommitCount,
	Set<? extends ObjectId> excludeFromBitmapSelection)
	throws IncorrectObjectTypeException, IOException,
	MissingObjectException {
	/*
	* Thinking of bitmap indices as a cache, if we find bitmaps at or at a
	* close ancestor to 'old' and 'new' when calculating old..new, then all
	* objects can be calculated with minimal graph walking. A distribution
	* that favors creating bitmaps for the most recent commits maximizes
	* the cache hits for clients that are close to HEAD, which is the
	* majority of calculations performed.
	*/
	try (RevWalk rw = new RevWalk(reader);
	RevWalk rw2 = new RevWalk(reader)) {
	pm.beginTask(JGitText.get().selectingCommits,
	ProgressMonitor.UNKNOWN);
	rw.setRetainBody(false);
	CommitSelectionHelper selectionHelper = captureOldAndNewCommits(rw,
	expectedCommitCount, excludeFromBitmapSelection);
	pm.endTask();

	// Add reused bitmaps from the previous GC pack's bitmap indices.
	// Currently they are always fully reused, even if their spans don't
	// match this run's PackConfig values.
	int newCommits = selectionHelper.getCommitCount();
	BlockList<BitmapCommit> selections = new BlockList<>(
	selectionHelper.reusedCommits.size()
	+ newCommits / recentCommitSpan + 1);
	for (BitmapCommit reuse : selectionHelper.reusedCommits) {
	selections.add(reuse);
	}

	if (newCommits == 0) {
	for (AnyObjectId id : selectionHelper.newWants) {
	selections.add(new BitmapCommit(id, false, 0));
	}
	return selections;
	}

	pm.beginTask(JGitText.get().selectingCommits, newCommits);
	int totalWants = want.size();
	BitmapBuilder seen = commitBitmapIndex.newBitmapBuilder();
	seen.or(selectionHelper.reusedCommitsBitmap);
	rw2.setRetainBody(false);
	rw2.setRevFilter(new NotInBitmapFilter(seen));

	// For each branch, do a revwalk to enumerate its commits. Exclude
	// both reused commits and any commits seen in a previous branch.
	// Then iterate through all new commits from oldest to newest,
	// selecting well-spaced commits in this branch.
	for (RevCommit rc : selectionHelper.newWantsByNewest) {
	BitmapBuilder tipBitmap = commitBitmapIndex.newBitmapBuilder();
	rw2.markStart((RevCommit) rw2.peel(rw2.parseAny(rc)));
	RevCommit rc2;
	while ((rc2 = rw2.next()) != null) {
	tipBitmap.addObject(rc2, Constants.OBJ_COMMIT);
	}
	int cardinality = tipBitmap.cardinality();
	seen.or(tipBitmap);

	// Within this branch, keep ordered lists of commits
	// representing chains in its history, where each chain is a
	// "sub-branch". Ordering commits by these chains makes for
	// fewer differences between consecutive selected commits, which
	// in turn provides better compression/on the run-length
	// encoding of the XORs between them.
	List<List<BitmapCommit>> chains = new ArrayList<>();

	// Mark the current branch as inactive if its tip commit isn't
	// recent and there are an excessive number of branches, to
	// prevent memory bloat of computing too many bitmaps for stale
	// branches.
	boolean isActiveBranch = true;
	if (totalWants > excessiveBranchCount && !isRecentCommit(rc)) {
	isActiveBranch = false;
	}

	// Insert bitmaps at the offsets suggested by the
	// nextSelectionDistance() heuristic. Only reuse bitmaps created
	// for more distant commits.
	int index = -1;
	int nextIn = nextSpan(cardinality);
	int nextFlg = nextIn == distantCommitSpan
	? PackBitmapIndex.FLAG_REUSE
	: 0;

	// For the current branch, iterate through all commits from
	// oldest to newest.
	for (RevCommit c : selectionHelper) {
	// Optimization: if we have found all the commits for this
	// branch, stop searching
	int distanceFromTip = cardinality - index - 1;
	if (distanceFromTip == 0) {
	break;
	}

	// Ignore commits that are not in this branch
	if (!tipBitmap.contains(c)) {
	continue;
	}

	index++;
	nextIn--;
	pm.update(1);

	// Always pick the items in wants, prefer merge commits.
	if (selectionHelper.newWants.remove(c)) {
	if (nextIn > 0) {
	nextFlg = 0;
	}
	} else {
	boolean stillInSpan = nextIn >= 0;
	boolean isMergeCommit = c.getParentCount() > 1;
	// Force selection if:
	// a) we have exhausted the window looking for merges
	// b) we are in the top commits of an active branch
	// c) we are at a branch tip
	boolean mustPick = (nextIn <= -recentCommitSpan)
	\|\| (isActiveBranch
	&& (distanceFromTip <= contiguousCommitCount))
	\|\| (distanceFromTip == 1); // most recent commit
	if (!mustPick && (stillInSpan \|\| !isMergeCommit)) {
	continue;
	}
	}

	// This commit is selected.
	// Calculate where to look for the next one.
	int flags = nextFlg;
	nextIn = nextSpan(distanceFromTip);
	nextFlg = nextIn == distantCommitSpan
	? PackBitmapIndex.FLAG_REUSE
	: 0;

	// Create the commit bitmap for the current commit
	BitmapBuilder bitmap = commitBitmapIndex.newBitmapBuilder();
	rw.reset();
	rw.markStart(c);
	rw.setRevFilter(new AddUnseenToBitmapFilter(
	selectionHelper.reusedCommitsBitmap, bitmap));
	while (rw.next() != null) {
	// The filter adds the reachable commits to bitmap.
	}

	// Sort the commits by independent chains in this branch's
	// history, yielding better compression when building
	// bitmaps.
	List<BitmapCommit> longestAncestorChain = null;
	for (List<BitmapCommit> chain : chains) {
	BitmapCommit mostRecentCommit = chain
	.get(chain.size() - 1);
	if (bitmap.contains(mostRecentCommit)) {
	if (longestAncestorChain == null
	\|\| longestAncestorChain.size() < chain
	.size()) {
	longestAncestorChain = chain;
	}
	}
	}

	if (longestAncestorChain == null) {
	longestAncestorChain = new ArrayList<>();
	chains.add(longestAncestorChain);
	}
	longestAncestorChain.add(new BitmapCommit(c,
	!longestAncestorChain.isEmpty(), flags));
	writeBitmaps.addBitmap(c, bitmap, 0);
	}

	for (List<BitmapCommit> chain : chains) {
	selections.addAll(chain);
	}
	}
	writeBitmaps.clearBitmaps(); // Remove the temporary commit bitmaps.

	// Add the remaining peeledWant
	for (AnyObjectId remainingWant : selectionHelper.newWants) {
	selections.add(new BitmapCommit(remainingWant, false, 0));
	}

	pm.endTask();
	return selections;
	}
	}

	private boolean isRecentCommit(RevCommit revCommit) {
	return revCommit.getCommitTime() > inactiveBranchTimestamp;
	}

	/**
	* A RevFilter that excludes the commits named in a bitmap from the walk.
	* <p>
	* If a commit is in {@code bitmap} then that commit is not emitted by the
	* walk and its parents are marked as SEEN so the walk can skip them. The
	* bitmaps passed in have the property that the parents of any commit in
	* {@code bitmap} are also in {@code bitmap}, so marking the parents as
	* SEEN speeds up the RevWalk by saving it from walking down blind alleys
	* and does not change the commits emitted.
	*/
	private static class NotInBitmapFilter extends RevFilter {
	private final BitmapBuilder bitmap;

	NotInBitmapFilter(BitmapBuilder bitmap) {
	this.bitmap = bitmap;
	}

	@Override
	public final boolean include(RevWalk rw, RevCommit c) {
	if (!bitmap.contains(c)) {
	return true;
	}
	for (RevCommit p : c.getParents()) {
	p.add(SEEN);
	}
	return false;
	}

	@Override
	public final NotInBitmapFilter clone() {
	throw new UnsupportedOperationException();
	}

	@Override
	public final boolean requiresCommitBody() {
	return false;
	}
	}

	/**
	* Records which of the {@code wants} can be found in the previous GC pack's
	* bitmap indices and which are new.
	*
	* @param rw
	* a {@link RevWalk} to find reachable objects in this repository
	* @param expectedCommitCount
	* expected count of commits. The actual count may be less due to
	* unreachable garbage.
	* @param excludeFromBitmapSelection
	* commits that should be excluded from bitmap selection
	* @return a {@link CommitSelectionHelper} capturing which commits are
	* covered by a previous pack's bitmaps and which new commits need
	* bitmap coverage
	* @throws IncorrectObjectTypeException
	* if any of the processed objects is not a commit
	* @throws IOException
	* on errors reading pack or index files
	* @throws MissingObjectException
	* if an expected object is missing
	*/
	private CommitSelectionHelper captureOldAndNewCommits(RevWalk rw,
	int expectedCommitCount,
	Set<? extends ObjectId> excludeFromBitmapSelection)
	throws IncorrectObjectTypeException, IOException,
	MissingObjectException {
	// Track bitmaps and commits from the previous GC pack bitmap indices.
	BitmapBuilder reuse = commitBitmapIndex.newBitmapBuilder();
	List<BitmapCommit> reuseCommits = new ArrayList<>();
	for (PackBitmapIndexRemapper.Entry entry : bitmapRemapper) {
	// More recent commits did not have the reuse flag set, so skip them
	if ((entry.getFlags() & FLAG_REUSE) != FLAG_REUSE) {
	continue;
	}
	RevObject ro = rw.peel(rw.parseAny(entry));
	if (!(ro instanceof RevCommit)) {
	continue;
	}

	RevCommit rc = (RevCommit) ro;
	reuseCommits.add(new BitmapCommit(rc, false, entry.getFlags()));
	if (!reuse.contains(rc)) {
	EWAHCompressedBitmap bitmap = bitmapRemapper.ofObjectType(
	bitmapRemapper.getBitmap(rc), Constants.OBJ_COMMIT);
	reuse.or(new CompressedBitmap(bitmap, commitBitmapIndex));
	}
	}

	// Add branch tips that are not represented in a previous pack's bitmap
	// indices. Set up a RevWalk to find new commits not in the old packs.
	List<RevCommit> newWantsByNewest = new ArrayList<>(want.size());
	Set<RevCommit> newWants = new HashSet<>(want.size());
	for (AnyObjectId objectId : want) {
	RevObject ro = rw.peel(rw.parseAny(objectId));
	if (!(ro instanceof RevCommit) \|\| reuse.contains(ro)
	\|\| excludeFromBitmapSelection.contains(ro)) {
	continue;
	}

	RevCommit rc = (RevCommit) ro;
	rw.markStart(rc);
	newWants.add(rc);
	newWantsByNewest.add(rc);
	}

	// Create a list of commits in reverse order (older to newer) that are
	// not in the previous bitmap indices and are reachable.
	rw.setRevFilter(new NotInBitmapFilter(reuse));
	RevCommit[] commits = new RevCommit[expectedCommitCount];
	int pos = commits.length;
	RevCommit rc;
	while ((rc = rw.next()) != null && pos > 0) {
	commits[--pos] = rc;
	pm.update(1);
	}

	// Sort the new wants by reverse commit time.
	Collections.sort(newWantsByNewest, ORDER_BY_REVERSE_TIMESTAMP);
	return new CommitSelectionHelper(newWants, commits, pos,
	newWantsByNewest, reuse, reuseCommits);
	}

	/*-
	* Returns the desired distance to the next bitmap based on the distance
	* from the tip commit. Only differentiates recent from distant spans,
	* selectCommits() handles the contiguous commits at the tip for active
	* or inactive branches.
	*
	* A graph of this function looks like this, where
	* the X axis is the distance from the tip commit and the Y axis is the
	* bitmap selection distance.
	*
	* 5000 ____...
	* /
	* /
	* /
	* /
	* 100 _____/
	* 0 20100 25000
	*
	* Linear scaling between 20100 and 25000 prevents spans >100 for distances
	* <20000 (otherwise, a span of 5000 would be returned for a distance of
	* 21000, and the range 16000-20000 would have no selections).
	*/
	int nextSpan(int distanceFromTip) {
	if (distanceFromTip < 0) {
	throw new IllegalArgumentException();
	}

	// Commits more toward the start will have more bitmaps.
	if (distanceFromTip <= recentCommitCount) {
	return recentCommitSpan;
	}

	int next = Math.min(distanceFromTip - recentCommitCount,
	distantCommitSpan);
	return Math.max(next, recentCommitSpan);
	}

	BitmapWalker newBitmapWalker() {
	return new BitmapWalker(
	new ObjectWalk(reader), bitmapIndex, null);
	}

	/**
	* Container for state used in the first phase of selecting commits, which
	* walks all of the reachable commits via the branch tips that are not
	* covered by a previous pack's bitmaps ({@code newWants}) and stores them
	* in {@code newCommitsByOldest}. {@code newCommitsByOldest} is initialized
	* with an expected size of all commits, but may be smaller if some commits
	* are unreachable and/or some commits are covered by a previous pack's
	* bitmaps. {@code commitStartPos} will contain a positive offset to either
	* the root commit or the oldest commit not covered by previous bitmaps.
	*/
	private static final class CommitSelectionHelper implements Iterable<RevCommit> {
	final Set<? extends ObjectId> newWants;

	final List<RevCommit> newWantsByNewest;
	final BitmapBuilder reusedCommitsBitmap;

	final List<BitmapCommit> reusedCommits;

	final RevCommit[] newCommitsByOldest;

	final int newCommitStartPos;

	CommitSelectionHelper(Set<? extends ObjectId> newWants,
	RevCommit[] commitsByOldest, int commitStartPos,
	List<RevCommit> newWantsByNewest,
	BitmapBuilder reusedCommitsBitmap,
	List<BitmapCommit> reuse) {
	this.newWants = newWants;
	this.newCommitsByOldest = commitsByOldest;
	this.newCommitStartPos = commitStartPos;
	this.newWantsByNewest = newWantsByNewest;
	this.reusedCommitsBitmap = reusedCommitsBitmap;
	this.reusedCommits = reuse;
	}

	@Override
	public Iterator<RevCommit> iterator() {
	// Member variables referenced by this iterator will have synthetic
	// accessors generated for them if they are made private.
	return new Iterator<RevCommit>() {
	int pos = newCommitStartPos;

	@Override
	public boolean hasNext() {
	return pos < newCommitsByOldest.length;
	}

	@Override
	public RevCommit next() {
	return newCommitsByOldest[pos++];
	}

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

	int getCommitCount() {
	return newCommitsByOldest.length - newCommitStartPos;
	}
	}
	}