MultiPackIndex: add getObjectCount() method

When we build a chain a multipack indexes, the midx position of an
object is the position in its midx plus the object count of its
base. We need the midx to provide this count, because due to
duplicates, it doesn't necessarily match the sum of objects in the
covered packs.

Add a getter to the MultiPackIndex to get the count of objects in the index.

Change-Id: I8bec9d4aa4079cc0291ef35efe75ccfbd656727e

Author: ifradeo

org.eclipse.jgit.test/tst/org/eclipse/jgit/internal/storage/midx/MultiPackIndexTest.java

@@ -373,6 +373,56 @@ public void jgit_findPosition() throws IOException {
 		assertNull(midx.find(ObjectId.zeroId()));
 	}
 
+	@Test
+	public void jgit_getObjectCount() throws IOException {
+		PackIndex idxOne = FakeIndexFactory.indexOf(List.of(
+				new FakeIndexFactory.IndexObject(
+						"0000000000000000000000000000000000000001", 500),
+				new FakeIndexFactory.IndexObject(
+						"0000000000000000000000000000000000000005", 12),
+				new FakeIndexFactory.IndexObject(
+						"0000000000000000000000000000000000000010", 1500)));
+		PackIndex idxTwo = FakeIndexFactory.indexOf(List.of(
+				new FakeIndexFactory.IndexObject(
+						"0000000000000000000000000000000000000002", 501),
+				new FakeIndexFactory.IndexObject(
+						"0000000000000000000000000000000000000003", 13),
+				new FakeIndexFactory.IndexObject(
+						"0000000000000000000000000000000000000015", 1501)));
+		PackIndex idxThree = FakeIndexFactory.indexOf(List.of(
+				new FakeIndexFactory.IndexObject(
+						"0000000000000000000000000000000000000004", 502),
+				new FakeIndexFactory.IndexObject(
+						"0000000000000000000000000000000000000007", 14),
+				new FakeIndexFactory.IndexObject(
+						"0000000000000000000000000000000000000012", 1502)));
+
+		Map<String, PackIndex> packs = Map.of("p1", idxOne, "p2", idxTwo, "p3",
+				idxThree);
+		MultiPackIndexWriter writer = new MultiPackIndexWriter();
+		ByteArrayOutputStream out = new ByteArrayOutputStream();
+		writer.write(NullProgressMonitor.INSTANCE, out, packs);
+
+		MultiPackIndex midx = MultiPackIndexLoader
+				.read(new ByteArrayInputStream(out.toByteArray()));
+		assertEquals(9, midx.getObjectCount());
+	}
+
+	@Test
+	public void jgit_getObjectCount_emtpy() throws IOException {
+		PackIndex idxOne = FakeIndexFactory.indexOf(List.of());
+		PackIndex idxTwo = FakeIndexFactory.indexOf(List.of());
+
+		Map<String, PackIndex> packs = Map.of("p1", idxOne, "p2", idxTwo);
+		MultiPackIndexWriter writer = new MultiPackIndexWriter();
+		ByteArrayOutputStream out = new ByteArrayOutputStream();
+		writer.write(NullProgressMonitor.INSTANCE, out, packs);
+		MultiPackIndex midx = MultiPackIndexLoader
+				.read(new ByteArrayInputStream(out.toByteArray()));
+
+		assertEquals(0, midx.getObjectCount());
+	}
+
 	private static PackIndex indexWith(String... oids) {
 		List<FakeIndexFactory.IndexObject> idxObjs = new ArrayList<>(
 				oids.length);

org.eclipse.jgit/src/org/eclipse/jgit/internal/storage/midx/MultiPackIndex.java

@@ -65,6 +65,16 @@ public interface MultiPackIndex {
 	 */
 	int findPosition(AnyObjectId objectId);
 
+	/**
+	 * Number of objects in this midx
+	 * <p>
+	 * This number doesn't match with the sum of objects in each covered pack
+	 * because midx removes duplicates.
+	 *
+	 * @return number of objects in this midx
+	 */
+	int getObjectCount();
+
 	/**
 	 * Find objects matching the prefix abbreviation.
 	 *

org.eclipse.jgit/src/org/eclipse/jgit/internal/storage/midx/MultiPackIndexV1.java

@@ -65,6 +65,11 @@ public int findPosition(AnyObjectId oid) {
 		return idx.findMultiPackIndexPosition(oid);
 	}
 
+	@Override
+	public int getObjectCount() {
+		return idx.getObjectCount();
+	}
+
 	@Override
 	@Nullable
 	public PackOffset find(AnyObjectId objectId) {
@@ -290,5 +295,9 @@ private int idOffset(int position) {
 		long getMemorySize() {
 			return 4L + byteArrayLengh(oidLookup) + (FANOUT * 4);
 		}
+
+		int getObjectCount() {
+			return fanoutTable[FANOUT - 1];
+		}
 	}
 }