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

3 files changed, 152 insertions, 1 deletions

diff --git a/org.eclipse.jgit.test/tst/org/eclipse/jgit/lib/BitmapIndexTest.java b/org.eclipse.jgit.test/tst/org/eclipse/jgit/lib/BitmapIndexTest.java
new file mode 100644
index 0000000000..ee4fa8bcc7
--- /dev/null
+++ b/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;
+		}
+	}
+}
\ No newline at end of file
diff --git a/org.eclipse.jgit/src/org/eclipse/jgit/internal/storage/file/BitmapIndexImpl.java b/org.eclipse.jgit/src/org/eclipse/jgit/internal/storage/file/BitmapIndexImpl.java
index 8d8c6a0455..5602158d2d 100644
--- a/org.eclipse.jgit/src/org/eclipse/jgit/internal/storage/file/BitmapIndexImpl.java
+++ b/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 @@ public class BitmapIndexImpl implements BitmapIndex {
 	@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 class BitmapIndexImpl implements BitmapIndex {
 		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) {
diff --git a/org.eclipse.jgit/src/org/eclipse/jgit/lib/BitmapIndex.java b/org.eclipse.jgit/src/org/eclipse/jgit/lib/BitmapIndex.java
index 97b5847f91..acaa6335d4 100644
--- a/org.eclipse.jgit/src/org/eclipse/jgit/lib/BitmapIndex.java
+++ b/org.eclipse.jgit/src/org/eclipse/jgit/lib/BitmapIndex.java
@@ -40,6 +40,58 @@ 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.
 	 */