revwalk: Introduce bitmap-based object reachability checker

Change-Id: I0b1a2bd21f98894862aab339f8c2e4a417897b89
Signed-off-by: Ivan Frade <ifrade@google.com>

org.eclipse.jgit.test/tst/org/eclipse/jgit/revwalk/BitmappedObjectReachabilityTest.java

@@ -0,0 +1,31 @@
+/*
+ * 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
+ * https://www.eclipse.org/org/documents/edl-v10.php.
+ *
+ * SPDX-License-Identifier: BSD-3-Clause
+ */
+package org.eclipse.jgit.revwalk;
+
+import org.eclipse.jgit.internal.storage.file.FileRepository;
+import org.eclipse.jgit.internal.storage.file.GC;
+import org.eclipse.jgit.junit.TestRepository;
+
+public class BitmappedObjectReachabilityTest
+	extends ObjectReachabilityTestCase {
+
+	@Override
+	ObjectReachabilityChecker getChecker(
+			TestRepository<FileRepository> repository) throws Exception {
+		// GC generates the bitmaps
+		GC gc = new GC(repository.getRepository());
+		gc.setAuto(false);
+		gc.gc();
+
+		return new BitmappedObjectReachabilityChecker(
+				repository.getRevWalk().toObjectWalkWithSameObjects());
+	}
+
+}

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

@@ -0,0 +1,79 @@
+/*
+ * 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
+ * https://www.eclipse.org/org/documents/edl-v10.php.
+ *
+ * SPDX-License-Identifier: BSD-3-Clause
+ */
+package org.eclipse.jgit.revwalk;
+
+import java.io.IOException;
+import java.util.ArrayList;
+import java.util.Arrays;
+import java.util.Collection;
+import java.util.Iterator;
+import java.util.List;
+import java.util.Optional;
+import java.util.stream.Stream;
+
+import org.eclipse.jgit.errors.IncorrectObjectTypeException;
+import org.eclipse.jgit.errors.MissingObjectException;
+import org.eclipse.jgit.lib.BitmapIndex.BitmapBuilder;
+
+/**
+ * Checks if all objects are reachable from certain starting points using
+ * bitmaps.
+ */
+public class BitmappedObjectReachabilityChecker
+		implements ObjectReachabilityChecker {
+
+	private ObjectWalk walk;
+
+	/**
+	 * New instance of the reachability checker using a existing walk.
+	 *
+	 * @param walk
+	 *            ObjectWalk instance to reuse. Caller retains ownership.
+	 */
+	public BitmappedObjectReachabilityChecker(ObjectWalk walk) {
+		this.walk = walk;
+	}
+
+	/**
+	 * {@inheritDoc}
+	 *
+	 * This implementation tries to shortcut the check adding starters
+	 * incrementally. Ordering the starters by relevance can improve performance
+	 * in the average case.
+	 */
+	@Override
+	public Optional<RevObject> areAllReachable(Collection<RevObject> targets,
+			Stream<RevObject> starters) throws IOException {
+
+		try {
+			List<RevObject> remainingTargets = new ArrayList<>(targets);
+			BitmapWalker bitmapWalker = new BitmapWalker(walk,
+					walk.getObjectReader().getBitmapIndex(), null);
+
+			Iterator<RevObject> starterIt = starters.iterator();
+			BitmapBuilder seen = null;
+			while (starterIt.hasNext()) {
+				List<RevObject> asList = Arrays.asList(starterIt.next());
+				BitmapBuilder visited = bitmapWalker.findObjects(asList, seen,
+						true);
+				seen = seen == null ? visited : seen.or(visited);
+
+				remainingTargets.removeIf(seen::contains);
+				if (remainingTargets.isEmpty()) {
+					return Optional.empty();
+				}
+			}
+
+			return Optional.of(remainingTargets.get(0));
+		} catch (MissingObjectException | IncorrectObjectTypeException e) {
+			throw new IllegalStateException(e);
+		}
+	}
+}