Add seekPastPrefix method to RefCursor

This method will be used by the follow-up change. This useful if we want
to go over all the changes after a specific ref.

For example, the new method allows us to create a follow-up that would
go over all the refs until we reach a specific ref (e.g refs/changes/),
and then we use seekPastPrefix(refs/changes/) to read the rest of the refs,
thus basically we return all refs except a specific prefix.

When seeking past a prefix, the previous condition that created the
RefCursor still applies. E.g, if the cursor was created by
seekRefsWithPrefix, we can skip some refs but we will not return refs
that are not starting with this prefix.

Signed-off-by: Gal Paikin <paiking@google.com>
Change-Id: I2c02e89c877fe90da8619cb8a4a9a0c865f238ef

3 files changed, 57 insertions, 0 deletions

diff --git a/org.eclipse.jgit/src/org/eclipse/jgit/internal/storage/reftable/MergedReftable.java b/org.eclipse.jgit/src/org/eclipse/jgit/internal/storage/reftable/MergedReftable.java
index a78f4d24da..e210acf058 100644
--- a/org.eclipse.jgit/src/org/eclipse/jgit/internal/storage/reftable/MergedReftable.java
+++ b/org.eclipse.jgit/src/org/eclipse/jgit/internal/storage/reftable/MergedReftable.java
@@ -11,6 +11,7 @@
 package org.eclipse.jgit.internal.storage.reftable;
 
 import java.io.IOException;
+import java.util.ArrayList;
 import java.util.List;
 import java.util.PriorityQueue;
 
@@ -215,6 +216,23 @@ public class MergedReftable extends Reftable {
 			}
 		}
 
+		@Override
+		public void seekPastPrefix(String prefixName) throws IOException {
+			List<RefQueueEntry> entriesToAdd = new ArrayList<>();
+			entriesToAdd.addAll(queue);
+			if (head != null) {
+				entriesToAdd.add(head);
+			}
+
+			head = null;
+			queue.clear();
+
+			for(RefQueueEntry entry : entriesToAdd){
+				entry.rc.seekPastPrefix(prefixName);
+				add(entry);
+			}
+		}
+
 		private RefQueueEntry poll() {
 			RefQueueEntry e = head;
 			if (e != null) {
diff --git a/org.eclipse.jgit/src/org/eclipse/jgit/internal/storage/reftable/RefCursor.java b/org.eclipse.jgit/src/org/eclipse/jgit/internal/storage/reftable/RefCursor.java
index d96648eb50..5e2c350883 100644
--- a/org.eclipse.jgit/src/org/eclipse/jgit/internal/storage/reftable/RefCursor.java
+++ b/org.eclipse.jgit/src/org/eclipse/jgit/internal/storage/reftable/RefCursor.java
@@ -29,6 +29,19 @@ public abstract class RefCursor implements AutoCloseable {
 	public abstract boolean next() throws IOException;
 
 	/**
+	 * Seeks forward to the first ref record lexicographically beyond
+	 * {@code prefixName} that doesn't start with {@code prefixName}. If there are
+	 * no more results, skipping some refs won't add new results. E.g if we create a
+	 * RefCursor that returns only results with a specific prefix, skipping that
+	 * prefix won't give results that are not part of the original prefix.
+	 *
+	 * @param prefixName prefix that should be skipped. All previous refs before it
+	 *                   will be skipped.
+	 * @throws java.io.IOException references cannot be read.
+	 */
+	public abstract void seekPastPrefix(String prefixName) throws IOException;
+
+	/**
 	 * Get reference at the current position.
 	 *
 	 * @return reference at the current position.
diff --git a/org.eclipse.jgit/src/org/eclipse/jgit/internal/storage/reftable/ReftableReader.java b/org.eclipse.jgit/src/org/eclipse/jgit/internal/storage/reftable/ReftableReader.java
index 095276f57b..9e2ae91608 100644
--- a/org.eclipse.jgit/src/org/eclipse/jgit/internal/storage/reftable/ReftableReader.java
+++ b/org.eclipse.jgit/src/org/eclipse/jgit/internal/storage/reftable/ReftableReader.java
@@ -509,6 +509,21 @@ public class ReftableReader extends Reftable implements AutoCloseable {
 		}
 
 		@Override
+		public void seekPastPrefix(String prefixName) throws IOException {
+			initRefIndex();
+			byte[] key = prefixName.getBytes(UTF_8);
+			ByteBuffer byteBuffer = ByteBuffer.allocate(key.length + 1);
+			byteBuffer.put(key);
+			// Add the representation of the last byte lexicographically. Based on how UTF_8
+			// representation works, this byte will be bigger lexicographically than any
+			// UTF_8 character when translated into bytes, since 0xFF can never be a part of
+			// a UTF_8 string.
+			byteBuffer.put((byte) 0xFF);
+
+			block = seek(REF_BLOCK_TYPE, byteBuffer.array(), refIndex, 0, refEnd);
+		}
+
+		@Override
 		public Ref getRef() {
 			return ref;
 		}
@@ -682,6 +697,17 @@ public class ReftableReader extends Reftable implements AutoCloseable {
 		}
 
 		@Override
+		/**
+		 * The implementation here would not be efficient complexity-wise since it
+		 * expected that there are a small number of refs that match the same object id.
+		 * In such case it's better to not even use this method (as the caller might
+		 * expect it to be efficient).
+		 */
+		public void seekPastPrefix(String prefixName) throws IOException {
+		    throw new UnsupportedOperationException();
+		}
+
+		@Override
 		public Ref getRef() {
 			return ref;
 		}