Merge changes I39783eee,I874503ec,Ic942a8e4,I6ec2c3e8,I62cb5030, ...

* changes:
  PackBitmapIndex: Set distance threshold
  PackBitmapIndex: Not buffer inflated bitmap in BasePackBitmapIndex
  PackBitmapIndex: Remove convertedBitmaps in the Remapper
  PackBitmapIndex: Reduce memory usage in GC
  PackBitmapIndex: Add AddToBitmapWithCacheFilter class
  PackBitmapIndex: Add util methods and builder to BitmapCommit
  PackBitmapIndex: Move BitmapCommit to a top-level class
  Refactor: Make retriveCompressed an method of the Bitmap class

org.eclipse.jgit.test/tst/org/eclipse/jgit/internal/storage/pack/GcCommitSelectionTest.java

@@ -23,7 +23,6 @@ import java.util.Set;
 
 import org.eclipse.jgit.internal.storage.file.GcTestCase;
 import org.eclipse.jgit.internal.storage.file.PackBitmapIndexBuilder;
-import org.eclipse.jgit.internal.storage.pack.PackWriterBitmapPreparer.BitmapCommit;
 import org.eclipse.jgit.junit.TestRepository.BranchBuilder;
 import org.eclipse.jgit.junit.TestRepository.CommitBuilder;
 import org.eclipse.jgit.lib.Constants;

org.eclipse.jgit/src/org/eclipse/jgit/internal/revwalk/AddToBitmapWithCacheFilter.java

@@ -0,0 +1,91 @@
+/*
+ * Copyright (c) 2020, Google LLC  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
+ * http://www.eclipse.org/org/documents/edl-v10.php.
+ *
+ * SPDX-License-Identifier: BSD-3-Clause
+ */
+package org.eclipse.jgit.internal.revwalk;
+
+import org.eclipse.jgit.lib.BitmapIndex.Bitmap;
+import org.eclipse.jgit.lib.BitmapIndex.BitmapBuilder;
+import org.eclipse.jgit.lib.Constants;
+import org.eclipse.jgit.lib.AnyObjectId;
+import org.eclipse.jgit.revwalk.filter.RevFilter;
+import org.eclipse.jgit.revwalk.RevWalk;
+import org.eclipse.jgit.revwalk.RevCommit;
+import org.eclipse.jgit.revwalk.RevFlag;
+
+/**
+ * A RevFilter that adds the visited commits to {@code bitmap} as a side effect.
+ * <p>
+ * When the walk hits a commit that is the same as {@code cachedCommit} or is
+ * part of {@code bitmap}'s BitmapIndex, that entire bitmap is ORed into
+ * {@code bitmap} and the commit and its parents are marked as SEEN so that the
+ * walk does not have to visit its ancestors. This ensures the walk is very
+ * short if there is good bitmap coverage.
+ */
+public class AddToBitmapWithCacheFilter extends RevFilter {
+	private final AnyObjectId cachedCommit;
+
+	private final Bitmap cachedBitmap;
+
+	private final BitmapBuilder bitmap;
+
+	/**
+	 * Create a filter with a cached BitmapCommit that adds visited commits to
+	 * the given bitmap.
+	 *
+	 * @param cachedCommit
+	 *            the cached commit
+	 * @param cachedBitmap
+	 *            the bitmap corresponds to {@code cachedCommit}}
+	 * @param bitmap
+	 *            bitmap to write visited commits to
+	 */
+	public AddToBitmapWithCacheFilter(AnyObjectId cachedCommit,
+			Bitmap cachedBitmap,
+			BitmapBuilder bitmap) {
+		this.cachedCommit = cachedCommit;
+		this.cachedBitmap = cachedBitmap;
+		this.bitmap = bitmap;
+	}
+
+	/** {@inheritDoc} */
+	@Override
+	public final boolean include(RevWalk rw, RevCommit c) {
+		Bitmap visitedBitmap;
+
+		if (bitmap.contains(c)) {
+			// already included
+		} else if ((visitedBitmap = bitmap.getBitmapIndex()
+				.getBitmap(c)) != null) {
+			bitmap.or(visitedBitmap);
+		} else if (cachedCommit.equals(c)) {
+			bitmap.or(cachedBitmap);
+		} else {
+			bitmap.addObject(c, Constants.OBJ_COMMIT);
+			return true;
+		}
+
+		for (RevCommit p : c.getParents()) {
+			p.add(RevFlag.SEEN);
+		}
+		return false;
+	}
+
+	/** {@inheritDoc} */
+	@Override
+	public final RevFilter clone() {
+		throw new UnsupportedOperationException();
+	}
+
+	/** {@inheritDoc} */
+	@Override
+	public final boolean requiresCommitBody() {
+		return false;
+	}
+}
+

org.eclipse.jgit/src/org/eclipse/jgit/internal/storage/file/BasePackBitmapIndex.java

@@ -72,7 +72,6 @@ abstract class BasePackBitmapIndex extends PackBitmapIndex {
 				if (r instanceof EWAHCompressedBitmap) {
 					out = out.xor((EWAHCompressedBitmap) r);
 					out.trim();
-					bitmapContainer = out;
 					return out;
 				}
 				xb = (XorCompressedBitmap) r;

org.eclipse.jgit/src/org/eclipse/jgit/internal/storage/file/BitmapIndexImpl.java

@@ -252,6 +252,11 @@ public class BitmapIndexImpl implements BitmapIndex {
 			return bitmapIndex;
 		}
 
+		@Override
+		public EWAHCompressedBitmap retrieveCompressed() {
+			return build().retrieveCompressed();
+		}
+
 		private EWAHCompressedBitmap ewahBitmap(Bitmap other) {
 			if (other instanceof CompressedBitmap) {
 				CompressedBitmap b = (CompressedBitmap) other;
@@ -372,7 +377,8 @@ public class BitmapIndexImpl implements BitmapIndex {
 			};
 		}
 
-		EWAHCompressedBitmap getEwahCompressedBitmap() {
+		@Override
+		public EWAHCompressedBitmap retrieveCompressed() {
 			return bitmap;
 		}
 

org.eclipse.jgit/src/org/eclipse/jgit/internal/storage/file/PackBitmapIndexBuilder.java

@@ -11,17 +11,16 @@
 package org.eclipse.jgit.internal.storage.file;
 
 import java.text.MessageFormat;
+import java.util.ArrayList;
 import java.util.Collections;
-import java.util.Iterator;
+import java.util.LinkedList;
 import java.util.List;
-import java.util.NoSuchElementException;
 
 import org.eclipse.jgit.internal.JGitText;
-import org.eclipse.jgit.internal.storage.file.BitmapIndexImpl.CompressedBitmap;
+import org.eclipse.jgit.internal.storage.pack.BitmapCommit;
 import org.eclipse.jgit.internal.storage.pack.ObjectToPack;
 import org.eclipse.jgit.lib.AnyObjectId;
 import org.eclipse.jgit.lib.BitmapIndex.Bitmap;
-import org.eclipse.jgit.lib.BitmapIndex.BitmapBuilder;
 import org.eclipse.jgit.lib.Constants;
 import org.eclipse.jgit.lib.ObjectId;
 import org.eclipse.jgit.lib.ObjectIdOwnerMap;
@@ -41,8 +40,12 @@ public class PackBitmapIndexBuilder extends BasePackBitmapIndex {
 	private final EWAHCompressedBitmap blobs;
 	private final EWAHCompressedBitmap tags;
 	private final BlockList<PositionEntry> byOffset;
-	final BlockList<StoredBitmap>
+
-			byAddOrder = new BlockList<>();
+	private final LinkedList<StoredBitmap>
+			bitmapsToWriteXorBuffer = new LinkedList<>();
+
+	private List<StoredEntry> bitmapsToWrite = new ArrayList<>();
+
 	final ObjectIdOwnerMap<PositionEntry>
 			positionEntries = new ObjectIdOwnerMap<>();
 
@@ -134,16 +137,64 @@ public class PackBitmapIndexBuilder extends BasePackBitmapIndex {
 	 *            the flags to be stored with the bitmap
 	 */
 	public void addBitmap(AnyObjectId objectId, Bitmap bitmap, int flags) {
-		if (bitmap instanceof BitmapBuilder)
+		addBitmap(objectId, bitmap.retrieveCompressed(), flags);
-			bitmap = ((BitmapBuilder) bitmap).build();
+	}
 
-		EWAHCompressedBitmap compressed;
+	/**
-		if (bitmap instanceof CompressedBitmap)
+	 * Processes a commit and prepares its bitmap to write to the bitmap index
-			compressed = ((CompressedBitmap) bitmap).getEwahCompressedBitmap();
+	 * file.
-		else
+	 *
-			throw new IllegalArgumentException(bitmap.getClass().toString());
+	 * @param c
+	 *            the commit corresponds to the bitmap.
+	 * @param bitmap
+	 *            the bitmap to be written.
+	 * @param flags
+	 *            the flags of the commit.
+	 */
+	public void processBitmapForWrite(BitmapCommit c, Bitmap bitmap,
+			int flags) {
+		EWAHCompressedBitmap compressed = bitmap.retrieveCompressed();
+		compressed.trim();
+		StoredBitmap newest = new StoredBitmap(c, compressed, null, flags);
+
+		bitmapsToWriteXorBuffer.add(newest);
+		if (bitmapsToWriteXorBuffer.size() > MAX_XOR_OFFSET_SEARCH) {
+			bitmapsToWrite.add(
+					generateStoredEntry(bitmapsToWriteXorBuffer.pollFirst()));
+		}
 
-		addBitmap(objectId, compressed, flags);
+		if (c.isAddToIndex()) {
+			// The Bitmap map in the base class is used to make revwalks
+			// efficient, so only add bitmaps that keep it efficient without
+			// bloating memory.
+			addBitmap(c, bitmap, flags);
+		}
+	}
+
+	private StoredEntry generateStoredEntry(StoredBitmap bitmapToWrite) {
+		int bestXorOffset = 0;
+		EWAHCompressedBitmap bestBitmap = bitmapToWrite.getBitmap();
+
+		int offset = 1;
+		for (StoredBitmap curr : bitmapsToWriteXorBuffer) {
+			EWAHCompressedBitmap bitmap = curr.getBitmap()
+					.xor(bitmapToWrite.getBitmap());
+			if (bitmap.sizeInBytes() < bestBitmap.sizeInBytes()) {
+				bestBitmap = bitmap;
+				bestXorOffset = offset;
+			}
+			offset++;
+		}
+
+		PositionEntry entry = positionEntries.get(bitmapToWrite);
+		if (entry == null) {
+			throw new IllegalStateException();
+		}
+		bestBitmap.trim();
+		StoredEntry result = new StoredEntry(entry.namePosition, bestBitmap,
+				bestXorOffset, bitmapToWrite.getFlags());
+
+		return result;
 	}
 
 	/**
@@ -161,7 +212,6 @@ public class PackBitmapIndexBuilder extends BasePackBitmapIndex {
 		bitmap.trim();
 		StoredBitmap result = new StoredBitmap(objectId, bitmap, null, flags);
 		getBitmaps().add(result);
-		byAddOrder.add(result);
 	}
 
 	/** {@inheritDoc} */
@@ -247,15 +297,18 @@ public class PackBitmapIndexBuilder extends BasePackBitmapIndex {
 	/** {@inheritDoc} */
 	@Override
 	public int getBitmapCount() {
-		return getBitmaps().size();
+		return bitmapsToWriteXorBuffer.size() + bitmapsToWrite.size();
 	}
 
 	/**
 	 * Remove all the bitmaps entries added.
+	 *
+	 * @param size
+	 *            the expected number of bitmap entries to be written.
 	 */
-	public void clearBitmaps() {
+	public void resetBitmaps(int size) {
-		byAddOrder.clear();
 		getBitmaps().clear();
+		bitmapsToWrite = new ArrayList<>(size);
 	}
 
 	/** {@inheritDoc} */
@@ -265,64 +318,18 @@ public class PackBitmapIndexBuilder extends BasePackBitmapIndex {
 	}
 
 	/**
-	 * Get an iterator over the xor compressed entries.
+	 * Get list of xor compressed entries that need to be written.
 	 *
-	 * @return an iterator over the xor compressed entries.
+	 * @return a list of the xor compressed entries.
 	 */
-	public Iterable<StoredEntry> getCompressedBitmaps() {
+	public List<StoredEntry> getCompressedBitmaps() {
-		// Add order is from oldest to newest. The reverse add order is the
+		while (!bitmapsToWriteXorBuffer.isEmpty()) {
-		// output order.
+			bitmapsToWrite.add(
-		return () -> new Iterator<StoredEntry>() {
+					generateStoredEntry(bitmapsToWriteXorBuffer.pollFirst()));
-
+		}
-			private int index = byAddOrder.size() - 1;
-
-			@Override
-			public boolean hasNext() {
-				return index >= 0;
-			}
-
-			@Override
-			public StoredEntry next() {
-				if (!hasNext()) {
-					throw new NoSuchElementException();
-				}
-				StoredBitmap item = byAddOrder.get(index);
-				int bestXorOffset = 0;
-				EWAHCompressedBitmap bestBitmap = item.getBitmap();
-
-				// Attempt to compress the bitmap with an XOR of the
-				// previously written entries.
-				for (int i = 1; i <= MAX_XOR_OFFSET_SEARCH; i++) {
-					int curr = i + index;
-					if (curr >= byAddOrder.size()) {
-						break;
-					}
-
-					StoredBitmap other = byAddOrder.get(curr);
-					EWAHCompressedBitmap bitmap = other.getBitmap()
-							.xor(item.getBitmap());
-
-					if (bitmap.sizeInBytes() < bestBitmap.sizeInBytes()) {
-						bestBitmap = bitmap;
-						bestXorOffset = i;
-					}
-				}
-				index--;
-
-				PositionEntry entry = positionEntries.get(item);
-				if (entry == null) {
-					throw new IllegalStateException();
-				}
-				bestBitmap.trim();
-				return new StoredEntry(entry.namePosition, bestBitmap,
-						bestXorOffset, item.getFlags());
-			}
 
-			@Override
+		Collections.reverse(bitmapsToWrite);
-			public void remove() {
+		return bitmapsToWrite;
-				throw new UnsupportedOperationException();
-			}
-		};
 	}
 
 	/** Data object for the on disk representation of a bitmap entry. */

org.eclipse.jgit/src/org/eclipse/jgit/internal/storage/file/PackBitmapIndexRemapper.java

@@ -18,7 +18,6 @@ import org.eclipse.jgit.internal.storage.file.BasePackBitmapIndex.StoredBitmap;
 import org.eclipse.jgit.lib.AnyObjectId;
 import org.eclipse.jgit.lib.BitmapIndex;
 import org.eclipse.jgit.lib.ObjectId;
-import org.eclipse.jgit.lib.ObjectIdOwnerMap;
 
 import com.googlecode.javaewah.EWAHCompressedBitmap;
 import com.googlecode.javaewah.IntIterator;
@@ -34,7 +33,6 @@ public class PackBitmapIndexRemapper extends PackBitmapIndex
 
 	private final BasePackBitmapIndex oldPackIndex;
 	final PackBitmapIndex newPackIndex;
-	private final ObjectIdOwnerMap<StoredBitmap> convertedBitmaps;
 	private final BitSet inflated;
 	private final int[] prevToNewMapping;
 
@@ -65,7 +63,6 @@ public class PackBitmapIndexRemapper extends PackBitmapIndex
 	private PackBitmapIndexRemapper(PackBitmapIndex newPackIndex) {
 		this.oldPackIndex = null;
 		this.newPackIndex = newPackIndex;
-		this.convertedBitmaps = null;
 		this.inflated = null;
 		this.prevToNewMapping = null;
 	}
@@ -74,7 +71,6 @@ public class PackBitmapIndexRemapper extends PackBitmapIndex
 			BasePackBitmapIndex oldPackIndex, PackBitmapIndex newPackIndex) {
 		this.oldPackIndex = oldPackIndex;
 		this.newPackIndex = newPackIndex;
-		convertedBitmaps = new ObjectIdOwnerMap<>();
 		inflated = new BitSet(newPackIndex.getObjectCount());
 
 		prevToNewMapping = new int[oldPackIndex.getObjectCount()];
@@ -152,10 +148,6 @@ public class PackBitmapIndexRemapper extends PackBitmapIndex
 		if (bitmap != null || oldPackIndex == null)
 			return bitmap;
 
-		StoredBitmap stored = convertedBitmaps.get(objectId);
-		if (stored != null)
-			return stored.getBitmap();
-
 		StoredBitmap oldBitmap = oldPackIndex.getBitmaps().get(objectId);
 		if (oldBitmap == null)
 			return null;
@@ -168,8 +160,6 @@ public class PackBitmapIndexRemapper extends PackBitmapIndex
 			inflated.set(prevToNewMapping[i.next()]);
 		bitmap = inflated.toEWAHCompressedBitmap();
 		bitmap.trim();
-		convertedBitmaps.add(
-				new StoredBitmap(objectId, bitmap, null, oldBitmap.getFlags()));
 		return bitmap;
 	}
 

org.eclipse.jgit/src/org/eclipse/jgit/internal/storage/pack/BitmapCommit.java

@@ -0,0 +1,163 @@
+/*
+ * Copyright (c) 2020, Google LLC 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
+ * http://www.eclipse.org/org/documents/edl-v10.php.
+ *
+ * SPDX-License-Identifier: BSD-3-Clause
+ */
+package org.eclipse.jgit.internal.storage.pack;
+
+import org.eclipse.jgit.lib.AnyObjectId;
+import org.eclipse.jgit.lib.ObjectId;
+
+/**
+ * A commit object for which a bitmap index should be built.
+ */
+public final class BitmapCommit extends ObjectId {
+
+	private final boolean reuseWalker;
+
+	private final int flags;
+
+	private final boolean addToIndex;
+
+	BitmapCommit(AnyObjectId objectId, boolean reuseWalker, int flags) {
+		super(objectId);
+		this.reuseWalker = reuseWalker;
+		this.flags = flags;
+		this.addToIndex = false;
+	}
+
+	BitmapCommit(AnyObjectId objectId, boolean reuseWalker, int flags,
+				 boolean addToIndex) {
+		super(objectId);
+		this.reuseWalker = reuseWalker;
+		this.flags = flags;
+		this.addToIndex = addToIndex;
+	}
+
+	boolean isReuseWalker() {
+		return reuseWalker;
+	}
+
+	int getFlags() {
+		return flags;
+	}
+
+	/**
+	 * Whether corresponding bitmap should be added to PackBitmapIndexBuilder.
+	 *
+	 * @return true if the corresponding bitmap should be added to
+	 *         PackBitmapIndexBuilder.
+	 */
+	public boolean isAddToIndex() {
+		return addToIndex;
+	}
+
+	/**
+	 * Get a builder of BitmapCommit whose object id is {@code objId}.
+	 *
+	 * @param objId
+	 *            the object id of the BitmapCommit
+	 * @return a BitmapCommit builder with object id set.
+	 */
+	public static Builder newBuilder(AnyObjectId objId) {
+		return new Builder().setId(objId);
+	}
+
+	/**
+	 * Get a builder of BitmapCommit whose fields are copied from
+	 * {@code commit}.
+	 *
+	 * @param commit
+	 *            the bitmap commit the builder is copying from
+	 * @return a BitmapCommit build with fields copied from an existing bitmap
+	 *         commit.
+	 */
+	public static Builder copyFrom(BitmapCommit commit) {
+		return new Builder().setId(commit)
+				.setReuseWalker(commit.isReuseWalker())
+				.setFlags(commit.getFlags())
+				.setAddToIndex(commit.isAddToIndex());
+	}
+
+	/**
+	 * Builder of BitmapCommit.
+	 */
+	public static class Builder {
+		private AnyObjectId objectId;
+
+		private boolean reuseWalker;
+
+		private int flags;
+
+		private boolean addToIndex;
+
+		// Prevent default constructor.
+		private Builder() {
+		}
+
+		/**
+		 * Set objectId of the builder.
+		 *
+		 * @param objectId
+		 *            the object id of the BitmapCommit
+		 * @return the builder itself
+		 */
+		public Builder setId(AnyObjectId objectId) {
+			this.objectId = objectId;
+			return this;
+		}
+
+		/**
+		 * Set reuseWalker of the builder.
+		 *
+		 * @param reuseWalker
+		 *            whether the BitmapCommit should reuse bitmap walker when
+		 *            walking objects
+		 * @return the builder itself
+		 */
+		public Builder setReuseWalker(boolean reuseWalker) {
+			this.reuseWalker = reuseWalker;
+			return this;
+		}
+
+		/**
+		 * Set flags of the builder.
+		 *
+		 * @param flags
+		 *            the flags of the BitmapCommit
+		 * @return the builder itself
+		 */
+		public Builder setFlags(int flags) {
+			this.flags = flags;
+			return this;
+		}
+
+		/**
+		 * Set whether whether the bitmap of the BitmapCommit should be added to
+		 * PackBitmapIndexBuilder when building bitmap index file.
+		 *
+		 * @param addToIndex
+		 *            whether the bitmap of the BitmapCommit should be added to
+		 *            PackBitmapIndexBuilder when building bitmap index file
+		 * @return the builder itself
+		 */
+		public Builder setAddToIndex(boolean addToIndex) {
+			this.addToIndex = addToIndex;
+			return this;
+		}
+
+		/**
+		 * Builds BitmapCommit from the builder.
+		 *
+		 * @return the new BitmapCommit.
+		 */
+		public BitmapCommit build() {
+			return new BitmapCommit(objectId, reuseWalker, flags,
+					addToIndex);
+		}
+	}
+}

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

@@ -2313,14 +2313,14 @@ public class PackWriter implements AutoCloseable {
 		PackWriterBitmapPreparer bitmapPreparer = new PackWriterBitmapPreparer(
 				reader, writeBitmaps, pm, stats.interestingObjects, config);
 
-		Collection<PackWriterBitmapPreparer.BitmapCommit> selectedCommits = bitmapPreparer
+		Collection<BitmapCommit> selectedCommits = bitmapPreparer
 				.selectCommits(numCommits, excludeFromBitmapSelection);
 
 		beginPhase(PackingPhase.BUILDING_BITMAPS, pm, selectedCommits.size());
 
 		BitmapWalker walker = bitmapPreparer.newBitmapWalker();
 		AnyObjectId last = null;
-		for (PackWriterBitmapPreparer.BitmapCommit cmit : selectedCommits) {
+		for (BitmapCommit cmit : selectedCommits) {
 			if (!cmit.isReuseWalker()) {
 				walker = bitmapPreparer.newBitmapWalker();
 			}
@@ -2331,8 +2331,14 @@ public class PackWriter implements AutoCloseable {
 				throw new IllegalStateException(MessageFormat.format(
 						JGitText.get().bitmapMissingObject, cmit.name(),
 						last.name()));
-			last = cmit;
+			last = BitmapCommit.copyFrom(cmit).build();
-			writeBitmaps.addBitmap(cmit, bitmap.build(), cmit.getFlags());
+			writeBitmaps.processBitmapForWrite(cmit, bitmap.build(),
+					cmit.getFlags());
+
+			// The bitmap walker should stop when the walk hits the previous
+			// commit, which saves time.
+			walker.setPrevCommit(last);
+			walker.setPrevBitmap(bitmap);
 
 			pm.update(1);
 		}

org.eclipse.jgit/src/org/eclipse/jgit/internal/storage/pack/PackWriterBitmapPreparer.java

@@ -58,6 +58,8 @@ class PackWriterBitmapPreparer {
 
 	private static final int DAY_IN_SECONDS = 24 * 60 * 60;
 
+	private static final int DISTANCE_THRESHOLD = 2000;
+
 	private static final Comparator<RevCommit> ORDER_BY_REVERSE_TIMESTAMP = (
 			RevCommit a, RevCommit b) -> Integer
 					.signum(b.getCommitTime() - a.getCommitTime());
@@ -244,6 +246,7 @@ class PackWriterBitmapPreparer {
 					// This commit is selected.
 					// Calculate where to look for the next one.
 					int flags = nextFlg;
+					int currDist = distanceFromTip;
 					nextIn = nextSpan(distanceFromTip);
 					nextFlg = nextIn == distantCommitSpan
 							? PackBitmapIndex.FLAG_REUSE
@@ -279,8 +282,17 @@ class PackWriterBitmapPreparer {
 						longestAncestorChain = new ArrayList<>();
 						chains.add(longestAncestorChain);
 					}
-					longestAncestorChain.add(new BitmapCommit(c,
+
-							!longestAncestorChain.isEmpty(), flags));
+					// The commit bc should reuse bitmap walker from its close
+					// ancestor. And the bitmap of bc should only be added to
+					// PackBitmapIndexBuilder when it's an old enough
+					// commit,i.e. distance from tip should be greater than
+					// DISTANCE_THRESHOLD, to save memory.
+					BitmapCommit bc = BitmapCommit.newBuilder(c).setFlags(flags)
+							.setAddToIndex(currDist >= DISTANCE_THRESHOLD)
+							.setReuseWalker(!longestAncestorChain.isEmpty())
+							.build();
+					longestAncestorChain.add(bc);
 					writeBitmaps.addBitmap(c, bitmap, 0);
 				}
 
@@ -288,12 +300,12 @@ class PackWriterBitmapPreparer {
 					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));
 			}
+			writeBitmaps.resetBitmaps(selections.size()); // Remove the temporary commit bitmaps.
 
 			pm.endTask();
 			return selections;
@@ -467,28 +479,6 @@ class PackWriterBitmapPreparer {
 				new ObjectWalk(reader), bitmapIndex, null);
 	}
 
-	/**
-	 * A commit object for which a bitmap index should be built.
-	 */
-	static final class BitmapCommit extends ObjectId {
-		private final boolean reuseWalker;
-		private final int flags;
-
-		BitmapCommit(AnyObjectId objectId, boolean reuseWalker, int flags) {
-			super(objectId);
-			this.reuseWalker = reuseWalker;
-			this.flags = flags;
-		}
-
-		boolean isReuseWalker() {
-			return reuseWalker;
-		}
-
-		int getFlags() {
-			return flags;
-		}
-	}
-
 	/**
 	 * 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

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

@@ -14,6 +14,8 @@ import java.util.Iterator;
 
 import org.eclipse.jgit.internal.storage.file.PackBitmapIndex;
 
+import com.googlecode.javaewah.EWAHCompressedBitmap;
+
 /**
  * A compressed bitmap representation of the entire object graph.
  *
@@ -81,6 +83,14 @@ public interface BitmapIndex {
 		 */
 		@Override
 		Iterator<BitmapObject> iterator();
+
+		/**
+		 * Returns the corresponding raw compressed EWAH bitmap of the bitmap.
+		 * 
+		 * @return the corresponding {@code EWAHCompressedBitmap}
+		 * @since 5.8
+		 */
+		EWAHCompressedBitmap retrieveCompressed();
 	}
 
 	/**

org.eclipse.jgit/src/org/eclipse/jgit/revwalk/BitmapWalker.java

@@ -16,6 +16,7 @@ import java.util.Arrays;
 import org.eclipse.jgit.errors.IncorrectObjectTypeException;
 import org.eclipse.jgit.errors.MissingObjectException;
 import org.eclipse.jgit.internal.revwalk.AddToBitmapFilter;
+import org.eclipse.jgit.internal.revwalk.AddToBitmapWithCacheFilter;
 import org.eclipse.jgit.internal.revwalk.AddUnseenToBitmapFilter;
 import org.eclipse.jgit.lib.AnyObjectId;
 import org.eclipse.jgit.lib.BitmapIndex;
@@ -41,6 +42,11 @@ public final class BitmapWalker {
 
 	private long countOfBitmapIndexMisses;
 
+	// Cached bitmap and commit to save walk time.
+	private AnyObjectId prevCommit;
+
+	private Bitmap prevBitmap;
+
 	/**
 	 * Create a BitmapWalker.
 	 *
@@ -55,6 +61,28 @@ public final class BitmapWalker {
 		this.pm = (pm == null) ? NullProgressMonitor.INSTANCE : pm;
 	}
 
+	/**
+	 * Set the cached commit for the walker.
+	 *
+	 * @param prevCommit
+	 *            the cached commit.
+	 * @since 5.7
+	 */
+	public void setPrevCommit(AnyObjectId prevCommit) {
+		this.prevCommit = prevCommit;
+	}
+
+	/**
+	 * Set the bitmap associated with the cached commit for the walker.
+	 *
+	 * @param prevBitmap
+	 *            the bitmap associated with the cached commit.
+	 * @since 5.7
+	 */
+	public void setPrevBitmap(Bitmap prevBitmap) {
+		this.prevBitmap = prevBitmap;
+	}
+
 	/**
 	 * Return the number of objects that had to be walked because they were not covered by a
 	 * bitmap.
@@ -169,7 +197,10 @@ public final class BitmapWalker {
 		}
 
 		if (marked) {
-			if (seen == null) {
+			if (prevCommit != null) {
+				walker.setRevFilter(new AddToBitmapWithCacheFilter(prevCommit,
+						prevBitmap, bitmapResult));
+			} else if (seen == null) {
 				walker.setRevFilter(new AddToBitmapFilter(bitmapResult));
 			} else {
 				walker.setRevFilter(