Split note leaf buckets at 256 elements

Leaf level notes trees are split into a new fan-out tree if an
insertion occurs and the tree already contains >= 256 notes in it.

The splitting may occur multiple times if all of the notes have the
same prefix; in the worst case this produces a tree path such as
"00/00/00/00/00/00/00/00/00/00/00/00/00/00/00/00/00/00/00/be" if all
of the notes begin with zeros.

Change-Id: I2d7d98f35108def9ec49936ddbdc34b13822a3c7
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>

org.eclipse.jgit.test/tst/org/eclipse/jgit/notes/NoteMapTest.java

@@ -47,6 +47,7 @@ import java.io.IOException;
 
 import org.eclipse.jgit.junit.TestRepository;
 import org.eclipse.jgit.lib.CommitBuilder;
+import org.eclipse.jgit.lib.Constants;
 import org.eclipse.jgit.lib.MutableObjectId;
 import org.eclipse.jgit.lib.ObjectInserter;
 import org.eclipse.jgit.lib.ObjectReader;
@@ -361,6 +362,46 @@ public class NoteMapTest extends RepositoryTestCase {
 				.forPath(reader, "zoo-animals.txt", n.getTree()).getObjectId(0));
 	}
 
+	public void testLeafSplitsWhenFull() throws Exception {
+		RevBlob data1 = tr.blob("data1");
+		MutableObjectId idBuf = new MutableObjectId();
+
+		RevCommit r = tr.commit() //
+				.add(data1.name(), data1) //
+				.create();
+		tr.parseBody(r);
+
+		NoteMap map = NoteMap.read(reader, r);
+		for (int i = 0; i < 254; i++) {
+			idBuf.setByte(Constants.OBJECT_ID_LENGTH - 1, i);
+			map.set(idBuf, data1);
+		}
+
+		RevCommit n = commitNoteMap(map);
+		TreeWalk tw = new TreeWalk(reader);
+		tw.reset(n.getTree());
+		while (tw.next())
+			assertFalse("no fan-out subtree", tw.isSubtree());
+
+		for (int i = 254; i < 256; i++) {
+			idBuf.setByte(Constants.OBJECT_ID_LENGTH - 1, i);
+			map.set(idBuf, data1);
+		}
+		idBuf.setByte(Constants.OBJECT_ID_LENGTH - 2, 1);
+		map.set(idBuf, data1);
+		n = commitNoteMap(map);
+
+		// The 00 bucket is fully split.
+		String path = fanout(38, idBuf.name());
+		tw = TreeWalk.forPath(reader, path, n.getTree());
+		assertNotNull("has " + path, tw);
+
+		// The other bucket is not.
+		path = fanout(2, data1.name());
+		tw = TreeWalk.forPath(reader, path, n.getTree());
+		assertNotNull("has " + path, tw);
+	}
+
 	public void testRemoveDeletesTreeFanout2_38() throws Exception {
 		RevBlob a = tr.blob("a");
 		RevBlob data1 = tr.blob("data1");

org.eclipse.jgit/src/org/eclipse/jgit/notes/FanoutBucket.java

@@ -228,6 +228,24 @@ class FanoutBucket extends InMemoryNoteBucket {
 		return sz;
 	}
 
+	@Override
+	InMemoryNoteBucket append(Note note) {
+		int cell = cell(note);
+		InMemoryNoteBucket b = (InMemoryNoteBucket) table[cell];
+
+		if (b == null) {
+			LeafBucket n = new LeafBucket(prefixLen + 2);
+			table[cell] = n.append(note);
+			cnt++;
+
+		} else {
+			InMemoryNoteBucket n = b.append(note);
+			if (n != b)
+				table[cell] = n;
+		}
+		return this;
+	}
+
 	private int cell(AnyObjectId id) {
 		return id.getByte(prefixLen >> 1);
 	}

org.eclipse.jgit/src/org/eclipse/jgit/notes/InMemoryNoteBucket.java

@@ -68,4 +68,6 @@ abstract class InMemoryNoteBucket extends NoteBucket {
 	InMemoryNoteBucket(int prefixLen) {
 		this.prefixLen = prefixLen;
 	}
+
+	abstract InMemoryNoteBucket append(Note note);
 }

org.eclipse.jgit/src/org/eclipse/jgit/notes/LeafBucket.java

@@ -73,6 +73,8 @@ import org.eclipse.jgit.lib.TreeFormatter;
  * A LeafBucket must be parsed from a tree object by {@link NoteParser}.
  */
 class LeafBucket extends InMemoryNoteBucket {
+	static final int MAX_SIZE = 256;
+
 	/** All note blobs in this bucket, sorted sequentially. */
 	private Note[] notes;
 
@@ -142,13 +144,18 @@ class LeafBucket extends InMemoryNoteBucket {
 			}
 
 		} else if (noteData != null) {
-			growIfFull();
-			p = -(p + 1);
-			if (p < cnt)
-				System.arraycopy(notes, p, notes, p + 1, cnt - p);
-			notes[p] = new Note(noteOn, noteData.copy());
-			cnt++;
-			return this;
+			if (shouldSplit()) {
+				return split().set(noteOn, noteData, or);
+
+			} else {
+				growIfFull();
+				p = -(p + 1);
+				if (p < cnt)
+					System.arraycopy(notes, p, notes, p + 1, cnt - p);
+				notes[p] = new Note(noteOn, noteData.copy());
+				cnt++;
+				return this;
+			}
 
 		} else {
 			return this;
@@ -193,6 +200,18 @@ class LeafBucket extends InMemoryNoteBucket {
 		notes[cnt++] = new Note(noteOn, noteData.copy());
 	}
 
+	@Override
+	InMemoryNoteBucket append(Note note) {
+		if (shouldSplit()) {
+			return split().append(note);
+
+		} else {
+			growIfFull();
+			notes[cnt++] = note;
+			return this;
+		}
+	}
+
 	private void growIfFull() {
 		if (notes.length == cnt) {
 			Note[] n = new Note[notes.length * 2];
@@ -200,4 +219,16 @@ class LeafBucket extends InMemoryNoteBucket {
 			notes = n;
 		}
 	}
+
+	private boolean shouldSplit() {
+		return MAX_SIZE <= cnt && prefixLen + 2 < OBJECT_ID_STRING_LENGTH;
+	}
+
+	private InMemoryNoteBucket split() {
+		FanoutBucket n = new FanoutBucket(prefixLen);
+		for (int i = 0; i < cnt; i++)
+			n.append(notes[i]);
+		n.nonNotes = nonNotes;
+		return n;
+	}
 }