BitmapIndex: Add interface to track bitmaps found (or not)

We want to know what objects had bitmaps in the walk of the
request. We can check their position in the history and evaluate our
bitmap selection algorithm.

Introduce a listener interface to the BitmapIndex to report which
getBitmap() calls returned a bitmap (or not) and a method to the
bitmap index to set the listener.

Change-Id: Iac8fcc1539ddd2dd450e8a1cf5a5b1089679c378

org.eclipse.jgit.test/tst/org/eclipse/jgit/lib/BitmapIndexTest.java

@@ -0,0 +1,85 @@
+/*
+ * Copyright (C) 2024, 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.lib;
+
+import static org.junit.Assert.assertEquals;
+import static org.junit.Assert.assertNotNull;
+
+import org.eclipse.jgit.internal.storage.file.FileRepository;
+import org.eclipse.jgit.internal.storage.file.GC;
+import org.eclipse.jgit.junit.LocalDiskRepositoryTestCase;
+import org.eclipse.jgit.junit.TestRepository;
+import org.eclipse.jgit.revwalk.RevCommit;
+import org.eclipse.jgit.revwalk.RevWalk;
+import org.junit.Before;
+import org.junit.Test;
+
+public class BitmapIndexTest extends LocalDiskRepositoryTestCase {
+
+  private static final String MAIN = "refs/heads/main";
+
+  TestRepository<FileRepository> repo;
+
+  RevCommit tipWithBitmap;
+
+  @Override
+  @Before
+  public void setUp() throws Exception {
+    super.setUp();
+    FileRepository db = createWorkRepository();
+    repo = new TestRepository<>(db);
+
+    RevCommit base = repo.commit().create();
+    RevCommit one = repo.commit().parent(base).create();
+    tipWithBitmap = repo.commit().parent(one).create();
+    repo.update(MAIN, tipWithBitmap);
+
+    GC gc = new GC(repo.getRepository());
+    gc.setAuto(false);
+    gc.gc().get();
+
+    assertNotNull(repo.getRevWalk().getObjectReader().getBitmapIndex());
+  }
+
+
+  @Test
+  public void listener_getBitmap_counted() throws Exception {
+    try (RevWalk rw = repo.getRevWalk();
+        ObjectReader or = rw.getObjectReader()) {
+      BitmapLookupCounter counter = new BitmapLookupCounter();
+      BitmapIndex bitmapIndex = or.getBitmapIndex();
+      bitmapIndex.addBitmapLookupListener(counter);
+
+      bitmapIndex.getBitmap(tipWithBitmap);
+      bitmapIndex.getBitmap(tipWithBitmap);
+      bitmapIndex.getBitmap(ObjectId.zeroId());
+
+      assertEquals(2, counter.bitmapFound);
+      assertEquals(1, counter.bitmapNotFound);
+    }
+  }
+
+	private static class BitmapLookupCounter
+			implements BitmapIndex.BitmapLookupListener {
+		int bitmapFound = 0;
+
+		int bitmapNotFound = 0;
+
+		@Override
+		public void onBitmapFound(AnyObjectId oid) {
+			bitmapFound += 1;
+		}
+
+		@Override
+		public void onBitmapNotFound(AnyObjectId oid) {
+			bitmapNotFound += 1;
+		}
+	}
+}

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

@@ -38,6 +38,8 @@ public class BitmapIndexImpl implements BitmapIndex {
 
 	final int indexObjectCount;
 
+	private BitmapLookupListener listener = BitmapLookupListener.NOOP;
+
 	/**
 	 * Creates a BitmapIndex that is back by Compressed bitmaps.
 	 *
@@ -57,8 +59,11 @@ PackBitmapIndex getPackBitmapIndex() {
 	@Override
 	public CompressedBitmap getBitmap(AnyObjectId objectId) {
 		EWAHCompressedBitmap compressed = packIndex.getBitmap(objectId);
-		if (compressed == null)
+		if (compressed == null) {
+			listener.onBitmapNotFound(objectId);
 			return null;
+		}
+		listener.onBitmapFound(objectId);
 		return new CompressedBitmap(compressed, this);
 	}
 
@@ -67,6 +72,15 @@ public CompressedBitmapBuilder newBitmapBuilder() {
 		return new CompressedBitmapBuilder(this);
 	}
 
+	@Override
+	public void addBitmapLookupListener(BitmapLookupListener listener) {
+		if (listener == null) {
+			throw new IllegalArgumentException(
+					"Use NOOP instance for no listener"); // @NON-NLS-1@
+		}
+		this.listener = listener;
+	}
+
 	int findPosition(AnyObjectId objectId) {
 		int position = packIndex.findPosition(objectId);
 		if (position < 0) {

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

@@ -39,6 +39,58 @@ public interface BitmapIndex {
 	 */
 	BitmapBuilder newBitmapBuilder();
 
+	/**
+	 * Report the results of {@link #getBitmap(AnyObjectId)}
+	 *
+	 * @since 6.8
+	 */
+	interface BitmapLookupListener {
+
+		/**
+		 * This object has a bitmap in the index
+		 *
+		 * @param oid
+		 *            object id
+		 */
+		void onBitmapFound(AnyObjectId oid);
+
+		/**
+		 * This object does not have a bitmap in the index
+		 *
+		 * @param oid
+		 *            object id
+		 */
+		void onBitmapNotFound(AnyObjectId oid);
+
+		/**
+		 * No-op instance
+		 */
+		BitmapLookupListener NOOP = new BitmapLookupListener() {
+			@Override
+			public void onBitmapFound(AnyObjectId oid) {
+				// Nothing to do
+			}
+
+			@Override
+			public void onBitmapNotFound(AnyObjectId oid) {
+				// Nothing to do
+			}
+		};
+	}
+
+	/**
+	 * Report to this listener whether {@link #getBitmap(AnyObjectId)} finds a
+	 * commit.
+	 *
+	 * @param listener
+	 *            instance listening to lookup events in the index. Never null.
+	 *            Set to {@link BitmapLookupListener#NOOP} to disable.
+	 * @since 6.8
+	 */
+	default void addBitmapLookupListener(BitmapLookupListener listener) {
+		// Empty implementation for API compatibility
+	}
+
 	/**
 	 * A bitmap representation of ObjectIds that can be iterated to return the
 	 * underlying {@code ObjectId}s or operated on with other {@code Bitmap}s.