PackBitmapIndex: Add AddToBitmapWithCacheFilter class

Add a new revwalk filter, AddToBitmapWithCachedFilter. This filter updates
a client-provided {@code BitmapBuilder} as a side effect of a revwalk.
Similar to {@code AddToBitmapFilter}, it short circuits the walk when it
encounters a commit which is included in the provided bitmap's BitmapIndex.
It also short circuits the walk if it encounters the client-provided
cached commit.

Change-Id: I62cb503016f4d3995d648d92b82baab7f93549a9
Signed-off-by: Yunjie Li <yunjieli@google.com>

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;
+	}
+}
+