/*
 * Copyright (C) 2007, Dave Watson <dwatson@mimvista.com>
 * Copyright (C) 2009-2010, Google Inc.
 * Copyright (C) 2007, Robin Rosenberg <robin.rosenberg@dewire.com>
 * Copyright (C) 2006, Shawn O. Pearce <spearce@spearce.org>
 * and other copyright owners as documented in the project's IP log.
 *
 * This program and the accompanying materials are made available
 * under the terms of the Eclipse Distribution License v1.0 which
 * accompanies this distribution, is reproduced below, and is
 * available at http://www.eclipse.org/org/documents/edl-v10.php
 *
 * All rights reserved.
 *
 * Redistribution and use in source and binary forms, with or
 * without modification, are permitted provided that the following
 * conditions are met:
 *
 * - Redistributions of source code must retain the above copyright
 *   notice, this list of conditions and the following disclaimer.
 *
 * - Redistributions in binary form must reproduce the above
 *   copyright notice, this list of conditions and the following
 *   disclaimer in the documentation and/or other materials provided
 *   with the distribution.
 *
 * - Neither the name of the Eclipse Foundation, Inc. nor the
 *   names of its contributors may be used to endorse or promote
 *   products derived from this software without specific prior
 *   written permission.
 *
 * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND
 * CONTRIBUTORS "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES,
 * INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
 * OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
 * ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR
 * CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
 * SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
 * NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;
 * LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
 * CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT,
 * STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
 * ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF
 * ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 */

package org.eclipse.jgit.internal.storage.file;

import static java.nio.charset.StandardCharsets.UTF_8;
import static org.eclipse.jgit.lib.Constants.HEAD;
import static org.eclipse.jgit.lib.Constants.LOGS;
import static org.eclipse.jgit.lib.Constants.OBJECT_ID_STRING_LENGTH;
import static org.eclipse.jgit.lib.Constants.PACKED_REFS;
import static org.eclipse.jgit.lib.Constants.R_HEADS;
import static org.eclipse.jgit.lib.Constants.R_REFS;
import static org.eclipse.jgit.lib.Constants.R_TAGS;
import static org.eclipse.jgit.lib.Ref.Storage.LOOSE;
import static org.eclipse.jgit.lib.Ref.Storage.NEW;
import static org.eclipse.jgit.lib.Ref.Storage.PACKED;

import java.io.BufferedReader;
import java.io.File;
import java.io.FileInputStream;
import java.io.FileNotFoundException;
import java.io.IOException;
import java.io.InputStreamReader;
import java.io.InterruptedIOException;
import java.nio.file.DirectoryNotEmptyException;
import java.nio.file.Files;
import java.nio.file.Path;
import java.security.DigestInputStream;
import java.security.MessageDigest;
import java.text.MessageFormat;
import java.util.Arrays;
import java.util.Collection;
import java.util.Collections;
import java.util.HashMap;
import java.util.LinkedList;
import java.util.List;
import java.util.Map;
import java.util.concurrent.atomic.AtomicInteger;
import java.util.concurrent.atomic.AtomicReference;
import java.util.concurrent.locks.ReentrantLock;
import java.util.stream.Stream;

import org.eclipse.jgit.annotations.NonNull;
import org.eclipse.jgit.annotations.Nullable;
import org.eclipse.jgit.errors.InvalidObjectIdException;
import org.eclipse.jgit.errors.LockFailedException;
import org.eclipse.jgit.errors.MissingObjectException;
import org.eclipse.jgit.errors.ObjectWritingException;
import org.eclipse.jgit.events.RefsChangedEvent;
import org.eclipse.jgit.internal.JGitText;
import org.eclipse.jgit.lib.ConfigConstants;
import org.eclipse.jgit.lib.Constants;
import org.eclipse.jgit.lib.ObjectId;
import org.eclipse.jgit.lib.ObjectIdRef;
import org.eclipse.jgit.lib.Ref;
import org.eclipse.jgit.lib.RefComparator;
import org.eclipse.jgit.lib.RefDatabase;
import org.eclipse.jgit.lib.RefUpdate;
import org.eclipse.jgit.lib.RefWriter;
import org.eclipse.jgit.lib.Repository;
import org.eclipse.jgit.lib.SymbolicRef;
import org.eclipse.jgit.revwalk.RevObject;
import org.eclipse.jgit.revwalk.RevTag;
import org.eclipse.jgit.revwalk.RevWalk;
import org.eclipse.jgit.util.FS;
import org.eclipse.jgit.util.FileUtils;
import org.eclipse.jgit.util.IO;
import org.eclipse.jgit.util.RawParseUtils;
import org.eclipse.jgit.util.RefList;
import org.eclipse.jgit.util.RefMap;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;

Traditional file system based RefDatabase. 

This is the classical reference database representation for a Git repository.
References are stored in two formats: loose, and packed.
 Loose references are stored as individual files within the refs/ directory. The file name matches the reference name and the file contents is the current ObjectId in string form. 
 Packed references are stored in a single text file named packed-refs. In the packed format, each reference is stored on its own line. This file reduces the number of files needed for large reference spaces, reducing the overall size of a Git repository on disk. /**
 * Traditional file system based {@link org.eclipse.jgit.lib.RefDatabase}.
 * <p>
 * This is the classical reference database representation for a Git repository.
 * References are stored in two formats: loose, and packed.
 * <p>
 * Loose references are stored as individual files within the {@code refs/}
 * directory. The file name matches the reference name and the file contents is
 * the current {@link org.eclipse.jgit.lib.ObjectId} in string form.
 * <p>
 * Packed references are stored in a single text file named {@code packed-refs}.
 * In the packed format, each reference is stored on its own line. This file
 * reduces the number of files needed for large reference spaces, reducing the
 * overall size of a Git repository on disk.
 */
public class RefDirectory extends RefDatabase {
	private final static Logger LOG = LoggerFactory
			.getLogger(RefDirectory.class);

	
Magic string denoting the start of a symbolic reference file. /** Magic string denoting the start of a symbolic reference file. */
	public static final String SYMREF = "ref: "; //$NON-NLS-1$

	
Magic string denoting the header of a packed-refs file. /** Magic string denoting the header of a packed-refs file. */
	public static final String PACKED_REFS_HEADER = "# pack-refs with:"; //$NON-NLS-1$

	
If in the header, denotes the file has peeled data. /** If in the header, denotes the file has peeled data. */
	public static final String PACKED_REFS_PEELED = " peeled"; //$NON-NLS-1$

	
The names of the additional refs supported by this class /** The names of the additional refs supported by this class */
	private static final String[] additionalRefsNames = new String[] {
			Constants.MERGE_HEAD, Constants.FETCH_HEAD, Constants.ORIG_HEAD,
			Constants.CHERRY_PICK_HEAD };

	@SuppressWarnings("boxing")
	private static final List<Integer> RETRY_SLEEP_MS =
			Collections.unmodifiableList(Arrays.asList(0, 100, 200, 400, 800, 1600));

	private final FileRepository parent;

	private final File gitDir;

	final File refsDir;

	final File packedRefsFile;

	final File logsDir;

	final File logsRefsDir;

	
Immutable sorted list of loose references.

Symbolic references in this collection are stored unresolved, that is
their target appears to be a new reference with no ObjectId. These are
converted into resolved references during a get operation, ensuring the
live value is always returned.
/**
	 * Immutable sorted list of loose references.
	 * <p>
	 * Symbolic references in this collection are stored unresolved, that is
	 * their target appears to be a new reference with no ObjectId. These are
	 * converted into resolved references during a get operation, ensuring the
	 * live value is always returned.
	 */
	private final AtomicReference<RefList<LooseRef>> looseRefs = new AtomicReference<>();

	
Immutable sorted list of packed references. /** Immutable sorted list of packed references. */
	final AtomicReference<PackedRefList> packedRefs = new AtomicReference<>();

	
Lock for coordinating operations within a single process that may contend on the packed-refs file. 
 All operations that write packed-refs must still acquire a LockFile on packedRefsFile, even after they have acquired this lock, since there may be multiple RefDirectory instances or other processes operating on the same repo on disk. 
 This lock exists so multiple threads in the same process can wait in a fair queue without trying, failing, and retrying to acquire the on-disk lock. If RepositoryCache is used, this lock instance will be used by all threads. /**
	 * Lock for coordinating operations within a single process that may contend
	 * on the {@code packed-refs} file.
	 * <p>
	 * All operations that write {@code packed-refs} must still acquire a
	 * {@link LockFile} on {@link #packedRefsFile}, even after they have acquired
	 * this lock, since there may be multiple {@link RefDirectory} instances or
	 * other processes operating on the same repo on disk.
	 * <p>
	 * This lock exists so multiple threads in the same process can wait in a fair
	 * queue without trying, failing, and retrying to acquire the on-disk lock. If
	 * {@code RepositoryCache} is used, this lock instance will be used by all
	 * threads.
	 */
	final ReentrantLock inProcessPackedRefsLock = new ReentrantLock(true);

	
Number of modifications made to this database.

This counter is incremented when a change is made, or detected from the
filesystem during a read operation.
/**
	 * Number of modifications made to this database.
	 * <p>
	 * This counter is incremented when a change is made, or detected from the
	 * filesystem during a read operation.
	 */
	private final AtomicInteger modCnt = new AtomicInteger();

	
Last modCnt that we sent to listeners. 
 This value is compared to modCnt, and a notification is sent to the listeners only when it differs. /**
	 * Last {@link #modCnt} that we sent to listeners.
	 * <p>
	 * This value is compared to {@link #modCnt}, and a notification is sent to
	 * the listeners only when it differs.
	 */
	private final AtomicInteger lastNotifiedModCnt = new AtomicInteger();

	private List<Integer> retrySleepMs = RETRY_SLEEP_MS;

	RefDirectory(FileRepository db) {
		final FS fs = db.getFS();
		parent = db;
		gitDir = db.getDirectory();
		refsDir = fs.resolve(gitDir, R_REFS);
		logsDir = fs.resolve(gitDir, LOGS);
		logsRefsDir = fs.resolve(gitDir, LOGS + '/' + R_REFS);
		packedRefsFile = fs.resolve(gitDir, PACKED_REFS);

		looseRefs.set(RefList.<LooseRef> emptyList());
		packedRefs.set(NO_PACKED_REFS);
	}

	Repository getRepository() {
		return parent;
	}

	ReflogWriter newLogWriter(boolean force) {
		return new ReflogWriter(this, force);
	}

	
Locate the log file on disk for a single reference name.
Params:
name – 
           name of the ref, relative to the Git repository top level
           directory (so typically starts with refs/).
Returns:
the log file location./**
	 * Locate the log file on disk for a single reference name.
	 *
	 * @param name
	 *            name of the ref, relative to the Git repository top level
	 *            directory (so typically starts with refs/).
	 * @return the log file location.
	 */
	public File logFor(String name) {
		if (name.startsWith(R_REFS)) {
			name = name.substring(R_REFS.length());
			return new File(logsRefsDir, name);
		}
		return new File(logsDir, name);
	}

	
{@inheritDoc} /** {@inheritDoc} */
	@Override
	public void create() throws IOException {
		FileUtils.mkdir(refsDir);
		FileUtils.mkdir(new File(refsDir, R_HEADS.substring(R_REFS.length())));
		FileUtils.mkdir(new File(refsDir, R_TAGS.substring(R_REFS.length())));
		newLogWriter(false).create();
	}

	
{@inheritDoc} /** {@inheritDoc} */
	@Override
	public void close() {
		clearReferences();
	}

	private void clearReferences() {
		looseRefs.set(RefList.<LooseRef> emptyList());
		packedRefs.set(NO_PACKED_REFS);
	}

	
{@inheritDoc} /** {@inheritDoc} */
	@Override
	public void refresh() {
		super.refresh();
		clearReferences();
	}

	
{@inheritDoc} /** {@inheritDoc} */
	@Override
	public boolean isNameConflicting(String name) throws IOException {
		RefList<Ref> packed = getPackedRefs();
		RefList<LooseRef> loose = getLooseRefs();

		// Cannot be nested within an existing reference.
		int lastSlash = name.lastIndexOf('/');
		while (0 < lastSlash) {
			String needle = name.substring(0, lastSlash);
			if (loose.contains(needle) || packed.contains(needle))
				return true;
			lastSlash = name.lastIndexOf('/', lastSlash - 1);
		}

		// Cannot be the container of an existing reference.
		String prefix = name + '/';
		int idx;

		idx = -(packed.find(prefix) + 1);
		if (idx < packed.size() && packed.get(idx).getName().startsWith(prefix))
			return true;

		idx = -(loose.find(prefix) + 1);
		if (idx < loose.size() && loose.get(idx).getName().startsWith(prefix))
			return true;

		return false;
	}

	private RefList<LooseRef> getLooseRefs() {
		final RefList<LooseRef> oldLoose = looseRefs.get();

		LooseScanner scan = new LooseScanner(oldLoose);
		scan.scan(ALL);

		RefList<LooseRef> loose;
		if (scan.newLoose != null) {
			loose = scan.newLoose.toRefList();
			if (looseRefs.compareAndSet(oldLoose, loose))
				modCnt.incrementAndGet();
		} else
			loose = oldLoose;
		return loose;
	}

	@Nullable
	private Ref readAndResolve(String name, RefList<Ref> packed) throws IOException {
		try {
			Ref ref = readRef(name, packed);
			if (ref != null) {
				ref = resolve(ref, 0, null, null, packed);
			}
			return ref;
		} catch (IOException e) {
			if (name.contains("/") //$NON-NLS-1$
					|| !(e.getCause() instanceof InvalidObjectIdException)) {
				throw e;
			}

			// While looking for a ref outside of refs/ (e.g., 'config'), we
			// found a non-ref file (e.g., a config file) instead.  Treat this
			// as a ref-not-found condition.
			return null;
		}
	}

	
{@inheritDoc} /** {@inheritDoc} */
	@Override
	public Ref exactRef(String name) throws IOException {
		try {
			return readAndResolve(name, getPackedRefs());
		} finally {
			fireRefsChanged();
		}
	}

	
{@inheritDoc} /** {@inheritDoc} */
	@Override
	@NonNull
	public Map<String, Ref> exactRef(String... refs) throws IOException {
		try {
			RefList<Ref> packed = getPackedRefs();
			Map<String, Ref> result = new HashMap<>(refs.length);
			for (String name : refs) {
				Ref ref = readAndResolve(name, packed);
				if (ref != null) {
					result.put(name, ref);
				}
			}
			return result;
		} finally {
			fireRefsChanged();
		}
	}

	
{@inheritDoc} /** {@inheritDoc} */
	@Override
	@Nullable
	public Ref firstExactRef(String... refs) throws IOException {
		try {
			RefList<Ref> packed = getPackedRefs();
			for (String name : refs) {
				Ref ref = readAndResolve(name, packed);
				if (ref != null) {
					return ref;
				}
			}
			return null;
		} finally {
			fireRefsChanged();
		}
	}

	
{@inheritDoc} /** {@inheritDoc} */
	@Override
	public Map<String, Ref> getRefs(String prefix) throws IOException {
		final RefList<LooseRef> oldLoose = looseRefs.get();
		LooseScanner scan = new LooseScanner(oldLoose);
		scan.scan(prefix);
		final RefList<Ref> packed = getPackedRefs();

		RefList<LooseRef> loose;
		if (scan.newLoose != null) {
			scan.newLoose.sort();
			loose = scan.newLoose.toRefList();
			if (looseRefs.compareAndSet(oldLoose, loose))
				modCnt.incrementAndGet();
		} else
			loose = oldLoose;
		fireRefsChanged();

		RefList.Builder<Ref> symbolic = scan.symbolic;
		for (int idx = 0; idx < symbolic.size();) {
			final Ref symbolicRef = symbolic.get(idx);
			final Ref resolvedRef = resolve(symbolicRef, 0, prefix, loose, packed);
			if (resolvedRef != null && resolvedRef.getObjectId() != null) {
				symbolic.set(idx, resolvedRef);
				idx++;
			} else {
				// A broken symbolic reference, we have to drop it from the
				// collections the client is about to receive. Should be a
				// rare occurrence so pay a copy penalty.
				symbolic.remove(idx);
				final int toRemove = loose.find(symbolicRef.getName());
				if (0 <= toRemove)
					loose = loose.remove(toRemove);
			}
		}
		symbolic.sort();

		return new RefMap(prefix, packed, upcast(loose), symbolic.toRefList());
	}

	
{@inheritDoc} /** {@inheritDoc} */
	@Override
	public List<Ref> getAdditionalRefs() throws IOException {
		List<Ref> ret = new LinkedList<>();
		for (String name : additionalRefsNames) {
			Ref r = exactRef(name);
			if (r != null)
				ret.add(r);
		}
		return ret;
	}

	@SuppressWarnings("unchecked")
	private RefList<Ref> upcast(RefList<? extends Ref> loose) {
		return (RefList<Ref>) loose;
	}

	private class LooseScanner {
		private final RefList<LooseRef> curLoose;

		private int curIdx;

		final RefList.Builder<Ref> symbolic = new RefList.Builder<>(4);

		RefList.Builder<LooseRef> newLoose;

		LooseScanner(RefList<LooseRef> curLoose) {
			this.curLoose = curLoose;
		}

		void scan(String prefix) {
			if (ALL.equals(prefix)) {
				scanOne(HEAD);
				scanTree(R_REFS, refsDir);

				// If any entries remain, they are deleted, drop them.
				if (newLoose == null && curIdx < curLoose.size())
					newLoose = curLoose.copy(curIdx);

			} else if (prefix.startsWith(R_REFS) && prefix.endsWith("/")) { //$NON-NLS-1$
				curIdx = -(curLoose.find(prefix) + 1);
				File dir = new File(refsDir, prefix.substring(R_REFS.length()));
				scanTree(prefix, dir);

				// Skip over entries still within the prefix; these have
				// been removed from the directory.
				while (curIdx < curLoose.size()) {
					if (!curLoose.get(curIdx).getName().startsWith(prefix))
						break;
					if (newLoose == null)
						newLoose = curLoose.copy(curIdx);
					curIdx++;
				}

				// Keep any entries outside of the prefix space, we
				// do not know anything about their status.
				if (newLoose != null) {
					while (curIdx < curLoose.size())
						newLoose.add(curLoose.get(curIdx++));
				}
			}
		}

		private boolean scanTree(String prefix, File dir) {
			final String[] entries = dir.list(LockFile.FILTER);
			if (entries == null) // not a directory or an I/O error
				return false;
			if (0 < entries.length) {
				for (int i = 0; i < entries.length; ++i) {
					String e = entries[i];
					File f = new File(dir, e);
					if (f.isDirectory())
						entries[i] += '/';
				}
				Arrays.sort(entries);
				for (String name : entries) {
					if (name.charAt(name.length() - 1) == '/')
						scanTree(prefix + name, new File(dir, name));
					else
						scanOne(prefix + name);
				}
			}
			return true;
		}

		private void scanOne(String name) {
			LooseRef cur;

			if (curIdx < curLoose.size()) {
				do {
					cur = curLoose.get(curIdx);
					int cmp = RefComparator.compareTo(cur, name);
					if (cmp < 0) {
						// Reference is not loose anymore, its been deleted.
						// Skip the name in the new result list.
						if (newLoose == null)
							newLoose = curLoose.copy(curIdx);
						curIdx++;
						cur = null;
						continue;
					}

					if (cmp > 0) // Newly discovered loose reference.
						cur = null;
					break;
				} while (curIdx < curLoose.size());
			} else
				cur = null; // Newly discovered loose reference.

			LooseRef n;
			try {
				n = scanRef(cur, name);
			} catch (IOException notValid) {
				n = null;
			}

			if (n != null) {
				if (cur != n && newLoose == null)
					newLoose = curLoose.copy(curIdx);
				if (newLoose != null)
					newLoose.add(n);
				if (n.isSymbolic())
					symbolic.add(n);
			} else if (cur != null) {
				// Tragically, this file is no longer a loose reference.
				// Kill our cached entry of it.
				if (newLoose == null)
					newLoose = curLoose.copy(curIdx);
			}

			if (cur != null)
				curIdx++;
		}
	}

	
{@inheritDoc} /** {@inheritDoc} */
	@Override
	public Ref peel(Ref ref) throws IOException {
		final Ref leaf = ref.getLeaf();
		if (leaf.isPeeled() || leaf.getObjectId() == null)
			return ref;

		ObjectIdRef newLeaf = doPeel(leaf);

		// Try to remember this peeling in the cache, so we don't have to do
		// it again in the future, but only if the reference is unchanged.
		if (leaf.getStorage().isLoose()) {
			RefList<LooseRef> curList = looseRefs.get();
			int idx = curList.find(leaf.getName());
			if (0 <= idx && curList.get(idx) == leaf) {
				LooseRef asPeeled = ((LooseRef) leaf).peel(newLeaf);
				RefList<LooseRef> newList = curList.set(idx, asPeeled);
				looseRefs.compareAndSet(curList, newList);
			}
		}

		return recreate(ref, newLeaf);
	}

	private ObjectIdRef doPeel(Ref leaf) throws MissingObjectException,
			IOException {
		try (RevWalk rw = new RevWalk(getRepository())) {
			RevObject obj = rw.parseAny(leaf.getObjectId());
			if (obj instanceof RevTag) {
				return new ObjectIdRef.PeeledTag(leaf.getStorage(), leaf
						.getName(), leaf.getObjectId(), rw.peel(obj).copy());
			} else {
				return new ObjectIdRef.PeeledNonTag(leaf.getStorage(), leaf
						.getName(), leaf.getObjectId());
			}
		}
	}

	private static Ref recreate(Ref old, ObjectIdRef leaf) {
		if (old.isSymbolic()) {
			Ref dst = recreate(old.getTarget(), leaf);
			return new SymbolicRef(old.getName(), dst);
		}
		return leaf;
	}

	void storedSymbolicRef(RefDirectoryUpdate u, FileSnapshot snapshot,
			String target) {
		putLooseRef(newSymbolicRef(snapshot, u.getRef().getName(), target));
		fireRefsChanged();
	}

	
{@inheritDoc} /** {@inheritDoc} */
	@Override
	public RefDirectoryUpdate newUpdate(String name, boolean detach)
			throws IOException {
		boolean detachingSymbolicRef = false;
		final RefList<Ref> packed = getPackedRefs();
		Ref ref = readRef(name, packed);
		if (ref != null)
			ref = resolve(ref, 0, null, null, packed);
		if (ref == null)
			ref = new ObjectIdRef.Unpeeled(NEW, name, null);
		else {
			detachingSymbolicRef = detach && ref.isSymbolic();
		}
		RefDirectoryUpdate refDirUpdate = new RefDirectoryUpdate(this, ref);
		if (detachingSymbolicRef)
			refDirUpdate.setDetachingSymbolicRef();
		return refDirUpdate;
	}

	
{@inheritDoc} /** {@inheritDoc} */
	@Override
	public RefDirectoryRename newRename(String fromName, String toName)
			throws IOException {
		RefDirectoryUpdate from = newUpdate(fromName, false);
		RefDirectoryUpdate to = newUpdate(toName, false);
		return new RefDirectoryRename(from, to);
	}

	
{@inheritDoc} /** {@inheritDoc} */
	@Override
	public PackedBatchRefUpdate newBatchUpdate() {
		return new PackedBatchRefUpdate(this);
	}

	
{@inheritDoc} /** {@inheritDoc} */
	@Override
	public boolean performsAtomicTransactions() {
		return true;
	}

	void stored(RefDirectoryUpdate update, FileSnapshot snapshot) {
		final ObjectId target = update.getNewObjectId().copy();
		final Ref leaf = update.getRef().getLeaf();
		putLooseRef(new LooseUnpeeled(snapshot, leaf.getName(), target));
	}

	private void putLooseRef(LooseRef ref) {
		RefList<LooseRef> cList, nList;
		do {
			cList = looseRefs.get();
			nList = cList.put(ref);
		} while (!looseRefs.compareAndSet(cList, nList));
		modCnt.incrementAndGet();
		fireRefsChanged();
	}

	void delete(RefDirectoryUpdate update) throws IOException {
		Ref dst = update.getRef();
		if (!update.isDetachingSymbolicRef()) {
			dst = dst.getLeaf();
		}
		String name = dst.getName();

		// Write the packed-refs file using an atomic update. We might
		// wind up reading it twice, before and after the lock, to ensure
		// we don't miss an edit made externally.
		final PackedRefList packed = getPackedRefs();
		if (packed.contains(name)) {
			inProcessPackedRefsLock.lock();
			try {
				LockFile lck = lockPackedRefsOrThrow();
				try {
					PackedRefList cur = readPackedRefs();
					int idx = cur.find(name);
					if (0 <= idx) {
						commitPackedRefs(lck, cur.remove(idx), packed, true);
					}
				} finally {
					lck.unlock();
				}
			} finally {
				inProcessPackedRefsLock.unlock();
			}
		}

		RefList<LooseRef> curLoose, newLoose;
		do {
			curLoose = looseRefs.get();
			int idx = curLoose.find(name);
			if (idx < 0)
				break;
			newLoose = curLoose.remove(idx);
		} while (!looseRefs.compareAndSet(curLoose, newLoose));

		int levels = levelsIn(name) - 2;
		delete(logFor(name), levels);
		if (dst.getStorage().isLoose()) {
			update.unlock();
			delete(fileFor(name), levels);
		}

		modCnt.incrementAndGet();
		fireRefsChanged();
	}

	
Adds a set of refs to the set of packed-refs. Only non-symbolic refs are
added. If a ref with the given name already existed in packed-refs it is
updated with the new value. Each loose ref which was added to the
packed-ref file is deleted. If a given ref can't be locked it will not be
added to the pack file.
Params:
refs – 
           the refs to be added. Must be fully qualified.
Throws:
IOException – /**
	 * Adds a set of refs to the set of packed-refs. Only non-symbolic refs are
	 * added. If a ref with the given name already existed in packed-refs it is
	 * updated with the new value. Each loose ref which was added to the
	 * packed-ref file is deleted. If a given ref can't be locked it will not be
	 * added to the pack file.
	 *
	 * @param refs
	 *            the refs to be added. Must be fully qualified.
	 * @throws java.io.IOException
	 */
	public void pack(List<String> refs) throws IOException {
		pack(refs, Collections.emptyMap());
	}

	PackedRefList pack(Map<String, LockFile> heldLocks) throws IOException {
		return pack(heldLocks.keySet(), heldLocks);
	}

	private PackedRefList pack(Collection<String> refs,
			Map<String, LockFile> heldLocks) throws IOException {
		for (LockFile ol : heldLocks.values()) {
			ol.requireLock();
		}
		if (refs.isEmpty()) {
			return null;
		}
		FS fs = parent.getFS();

		// Lock the packed refs file and read the content
		inProcessPackedRefsLock.lock();
		try {
			LockFile lck = lockPackedRefsOrThrow();
			try {
				final PackedRefList packed = getPackedRefs();
				RefList<Ref> cur = readPackedRefs();

				// Iterate over all refs to be packed
				boolean dirty = false;
				for (String refName : refs) {
					Ref oldRef = readRef(refName, cur);
					if (oldRef == null) {
						continue; // A non-existent ref is already correctly packed.
					}
					if (oldRef.isSymbolic()) {
						continue; // can't pack symbolic refs
					}
					// Add/Update it to packed-refs
					Ref newRef = peeledPackedRef(oldRef);
					if (newRef == oldRef) {
						// No-op; peeledPackedRef returns the input ref only if it's already
						// packed, and readRef returns a packed ref only if there is no
						// loose ref.
						continue;
					}

					dirty = true;
					int idx = cur.find(refName);
					if (idx >= 0) {
						cur = cur.set(idx, newRef);
					} else {
						cur = cur.add(idx, newRef);
					}
				}
				if (!dirty) {
					// All requested refs were already packed accurately
					return packed;
				}

				// The new content for packed-refs is collected. Persist it.
				PackedRefList result = commitPackedRefs(lck, cur, packed,
						false);

				// Now delete the loose refs which are now packed
				for (String refName : refs) {
					// Lock the loose ref
					File refFile = fileFor(refName);
					if (!fs.exists(refFile)) {
						continue;
					}

					LockFile rLck = heldLocks.get(refName);
					boolean shouldUnlock;
					if (rLck == null) {
						rLck = new LockFile(refFile);
						if (!rLck.lock()) {
							continue;
						}
						shouldUnlock = true;
					} else {
						shouldUnlock = false;
					}

					try {
						LooseRef currentLooseRef = scanRef(null, refName);
						if (currentLooseRef == null || currentLooseRef.isSymbolic()) {
							continue;
						}
						Ref packedRef = cur.get(refName);
						ObjectId clr_oid = currentLooseRef.getObjectId();
						if (clr_oid != null
								&& clr_oid.equals(packedRef.getObjectId())) {
							RefList<LooseRef> curLoose, newLoose;
							do {
								curLoose = looseRefs.get();
								int idx = curLoose.find(refName);
								if (idx < 0) {
									break;
								}
								newLoose = curLoose.remove(idx);
							} while (!looseRefs.compareAndSet(curLoose, newLoose));
							int levels = levelsIn(refName) - 2;
							delete(refFile, levels, rLck);
						}
					} finally {
						if (shouldUnlock) {
							rLck.unlock();
						}
					}
				}
				// Don't fire refsChanged. The refs have not change, only their
				// storage.
				return result;
			} finally {
				lck.unlock();
			}
		} finally {
			inProcessPackedRefsLock.unlock();
		}
	}

	@Nullable
	LockFile lockPackedRefs() throws IOException {
		LockFile lck = new LockFile(packedRefsFile);
		for (int ms : getRetrySleepMs()) {
			sleep(ms);
			if (lck.lock()) {
				return lck;
			}
		}
		return null;
	}

	private LockFile lockPackedRefsOrThrow() throws IOException {
		LockFile lck = lockPackedRefs();
		if (lck == null) {
			throw new LockFailedException(packedRefsFile);
		}
		return lck;
	}

	
Make sure a ref is peeled and has the Storage PACKED. If the given ref has this attributes simply return it. Otherwise create a new peeled ObjectIdRef where Storage is set to PACKED. 
Params:
f – 
Throws:
MissingObjectException – 
IOException – 
Returns:
a ref for Storage PACKED having the same name, id, peeledId as f/**
	 * Make sure a ref is peeled and has the Storage PACKED. If the given ref
	 * has this attributes simply return it. Otherwise create a new peeled
	 * {@link ObjectIdRef} where Storage is set to PACKED.
	 *
	 * @param f
	 * @return a ref for Storage PACKED having the same name, id, peeledId as f
	 * @throws MissingObjectException
	 * @throws IOException
	 */
	private Ref peeledPackedRef(Ref f)
			throws MissingObjectException, IOException {
		if (f.getStorage().isPacked() && f.isPeeled()) {
			return f;
		}
		if (!f.isPeeled()) {
			f = peel(f);
		}
		ObjectId peeledObjectId = f.getPeeledObjectId();
		if (peeledObjectId != null) {
			return new ObjectIdRef.PeeledTag(PACKED, f.getName(),
					f.getObjectId(), peeledObjectId);
		} else {
			return new ObjectIdRef.PeeledNonTag(PACKED, f.getName(),
					f.getObjectId());
		}
	}

	void log(boolean force, RefUpdate update, String msg, boolean deref)
			throws IOException {
		newLogWriter(force).log(update, msg, deref);
	}

	private Ref resolve(final Ref ref, int depth, String prefix,
			RefList<LooseRef> loose, RefList<Ref> packed) throws IOException {
		if (ref.isSymbolic()) {
			Ref dst = ref.getTarget();

			if (MAX_SYMBOLIC_REF_DEPTH <= depth)
				return null; // claim it doesn't exist

			// If the cached value can be assumed to be current due to a
			// recent scan of the loose directory, use it.
			if (loose != null && dst.getName().startsWith(prefix)) {
				int idx;
				if (0 <= (idx = loose.find(dst.getName())))
					dst = loose.get(idx);
				else if (0 <= (idx = packed.find(dst.getName())))
					dst = packed.get(idx);
				else
					return ref;
			} else {
				dst = readRef(dst.getName(), packed);
				if (dst == null)
					return ref;
			}

			dst = resolve(dst, depth + 1, prefix, loose, packed);
			if (dst == null)
				return null;
			return new SymbolicRef(ref.getName(), dst);
		}
		return ref;
	}

	PackedRefList getPackedRefs() throws IOException {
		boolean trustFolderStat = getRepository().getConfig().getBoolean(
				ConfigConstants.CONFIG_CORE_SECTION,
				ConfigConstants.CONFIG_KEY_TRUSTFOLDERSTAT, true);

		final PackedRefList curList = packedRefs.get();
		if (trustFolderStat && !curList.snapshot.isModified(packedRefsFile)) {
			return curList;
		}

		final PackedRefList newList = readPackedRefs();
		if (packedRefs.compareAndSet(curList, newList)
				&& !curList.id.equals(newList.id)) {
			modCnt.incrementAndGet();
		}
		return newList;
	}

	private PackedRefList readPackedRefs() throws IOException {
		int maxStaleRetries = 5;
		int retries = 0;
		while (true) {
			final FileSnapshot snapshot = FileSnapshot.save(packedRefsFile);
			final MessageDigest digest = Constants.newMessageDigest();
			try (BufferedReader br = new BufferedReader(new InputStreamReader(
					new DigestInputStream(new FileInputStream(packedRefsFile),
							digest),
					UTF_8))) {
				try {
					return new PackedRefList(parsePackedRefs(br), snapshot,
							ObjectId.fromRaw(digest.digest()));
				} catch (IOException e) {
					if (FileUtils.isStaleFileHandleInCausalChain(e)
							&& retries < maxStaleRetries) {
						if (LOG.isDebugEnabled()) {
							LOG.debug(MessageFormat.format(
									JGitText.get().packedRefsHandleIsStale,
									Integer.valueOf(retries)), e);
						}
						retries++;
						continue;
					}
					throw e;
				}
			} catch (FileNotFoundException noPackedRefs) {
				if (packedRefsFile.exists()) {
					throw noPackedRefs;
				}
				// Ignore it and leave the new list empty.
				return NO_PACKED_REFS;
			}
		}
	}

	private RefList<Ref> parsePackedRefs(BufferedReader br)
			throws IOException {
		RefList.Builder<Ref> all = new RefList.Builder<>();
		Ref last = null;
		boolean peeled = false;
		boolean needSort = false;

		String p;
		while ((p = br.readLine()) != null) {
			if (p.charAt(0) == '#') {
				if (p.startsWith(PACKED_REFS_HEADER)) {
					p = p.substring(PACKED_REFS_HEADER.length());
					peeled = p.contains(PACKED_REFS_PEELED);
				}
				continue;
			}

			if (p.charAt(0) == '^') {
				if (last == null)
					throw new IOException(JGitText.get().peeledLineBeforeRef);

				ObjectId id = ObjectId.fromString(p.substring(1));
				last = new ObjectIdRef.PeeledTag(PACKED, last.getName(), last
						.getObjectId(), id);
				all.set(all.size() - 1, last);
				continue;
			}

			int sp = p.indexOf(' ');
			if (sp < 0) {
				throw new IOException(MessageFormat.format(
						JGitText.get().packedRefsCorruptionDetected,
						packedRefsFile.getAbsolutePath()));
			}
			ObjectId id = ObjectId.fromString(p.substring(0, sp));
			String name = copy(p, sp + 1, p.length());
			ObjectIdRef cur;
			if (peeled)
				cur = new ObjectIdRef.PeeledNonTag(PACKED, name, id);
			else
				cur = new ObjectIdRef.Unpeeled(PACKED, name, id);
			if (last != null && RefComparator.compareTo(last, cur) > 0)
				needSort = true;
			all.add(cur);
			last = cur;
		}

		if (needSort)
			all.sort();
		return all.toRefList();
	}

	private static String copy(String src, int off, int end) {
		// Don't use substring since it could leave a reference to the much
		// larger existing string. Force construction of a full new object.
		return new StringBuilder(end - off).append(src, off, end).toString();
	}

	PackedRefList commitPackedRefs(final LockFile lck, final RefList<Ref> refs,
			final PackedRefList oldPackedList, boolean changed)
			throws IOException {
		// Can't just return packedRefs.get() from this method; it might have been
		// updated again after writePackedRefs() returns.
		AtomicReference<PackedRefList> result = new AtomicReference<>();
		new RefWriter(refs) {
			@Override
			protected void writeFile(String name, byte[] content)
					throws IOException {
				lck.setFSync(true);
				lck.setNeedSnapshot(true);
				try {
					lck.write(content);
				} catch (IOException ioe) {
					throw new ObjectWritingException(MessageFormat.format(JGitText.get().unableToWrite, name), ioe);
				}
				try {
					lck.waitForStatChange();
				} catch (InterruptedException e) {
					lck.unlock();
					throw new ObjectWritingException(MessageFormat.format(JGitText.get().interruptedWriting, name));
				}
				if (!lck.commit())
					throw new ObjectWritingException(MessageFormat.format(JGitText.get().unableToWrite, name));

				byte[] digest = Constants.newMessageDigest().digest(content);
				PackedRefList newPackedList = new PackedRefList(
						refs, lck.getCommitSnapshot(), ObjectId.fromRaw(digest));

				// This thread holds the file lock, so no other thread or process should
				// be able to modify the packed-refs file on disk. If the list changed,
				// it means something is very wrong, so throw an exception.
				//
				// However, we can't use a naive compareAndSet to check whether the
				// update was successful, because another thread might _read_ the
				// packed refs file that was written out by this thread while holding
				// the lock, and update the packedRefs reference to point to that. So
				// compare the actual contents instead.
				PackedRefList afterUpdate = packedRefs.updateAndGet(
						p -> p.id.equals(oldPackedList.id) ? newPackedList : p);
				if (!afterUpdate.id.equals(newPackedList.id)) {
					throw new ObjectWritingException(
							MessageFormat.format(JGitText.get().unableToWrite, name));
				}
				if (changed) {
					modCnt.incrementAndGet();
				}
				result.set(newPackedList);
			}
		}.writePackedRefs();
		return result.get();
	}

	private Ref readRef(String name, RefList<Ref> packed) throws IOException {
		final RefList<LooseRef> curList = looseRefs.get();
		final int idx = curList.find(name);
		if (0 <= idx) {
			final LooseRef o = curList.get(idx);
			final LooseRef n = scanRef(o, name);
			if (n == null) {
				if (looseRefs.compareAndSet(curList, curList.remove(idx)))
					modCnt.incrementAndGet();
				return packed.get(name);
			}

			if (o == n)
				return n;
			if (looseRefs.compareAndSet(curList, curList.set(idx, n)))
				modCnt.incrementAndGet();
			return n;
		}

		final LooseRef n = scanRef(null, name);
		if (n == null)
			return packed.get(name);

		// check whether the found new ref is the an additional ref. These refs
		// should not go into looseRefs
		for (String additionalRefsName : additionalRefsNames) {
			if (name.equals(additionalRefsName)) {
				return n;
			}
		}

		if (looseRefs.compareAndSet(curList, curList.add(idx, n)))
			modCnt.incrementAndGet();
		return n;
	}

	LooseRef scanRef(LooseRef ref, String name) throws IOException {
		final File path = fileFor(name);
		FileSnapshot currentSnapshot = null;

		if (ref != null) {
			currentSnapshot = ref.getSnapShot();
			if (!currentSnapshot.isModified(path))
				return ref;
			name = ref.getName();
		}

		final int limit = 4096;
		final byte[] buf;
		FileSnapshot otherSnapshot = FileSnapshot.save(path);
		try {
			buf = IO.readSome(path, limit);
		} catch (FileNotFoundException noFile) {
			if (path.exists() && path.isFile()) {
				throw noFile;
			}
			return null; // doesn't exist or no file; not a reference.
		}

		int n = buf.length;
		if (n == 0)
			return null; // empty file; not a reference.

		if (isSymRef(buf, n)) {
			if (n == limit)
				return null; // possibly truncated ref

			// trim trailing whitespace
			while (0 < n && Character.isWhitespace(buf[n - 1]))
				n--;
			if (n < 6) {
				String content = RawParseUtils.decode(buf, 0, n);
				throw new IOException(MessageFormat.format(JGitText.get().notARef, name, content));
			}
			final String target = RawParseUtils.decode(buf, 5, n);
			if (ref != null && ref.isSymbolic()
					&& ref.getTarget().getName().equals(target)) {
				assert(currentSnapshot != null);
				currentSnapshot.setClean(otherSnapshot);
				return ref;
			}
			return newSymbolicRef(otherSnapshot, name, target);
		}

		if (n < OBJECT_ID_STRING_LENGTH)
			return null; // impossibly short object identifier; not a reference.

		final ObjectId id;
		try {
			id = ObjectId.fromString(buf, 0);
			if (ref != null && !ref.isSymbolic()
					&& id.equals(ref.getTarget().getObjectId())) {
				assert(currentSnapshot != null);
				currentSnapshot.setClean(otherSnapshot);
				return ref;
			}

		} catch (IllegalArgumentException notRef) {
			while (0 < n && Character.isWhitespace(buf[n - 1]))
				n--;
			String content = RawParseUtils.decode(buf, 0, n);

			throw new IOException(MessageFormat.format(JGitText.get().notARef,
					name, content), notRef);
		}
		return new LooseUnpeeled(otherSnapshot, name, id);
	}

	private static boolean isSymRef(byte[] buf, int n) {
		if (n < 6)
			return false;
		return /**/buf[0] == 'r' //
				&& buf[1] == 'e' //
				&& buf[2] == 'f' //
				&& buf[3] == ':' //
				&& buf[4] == ' ';
	}

	
Detect if we are in a clone command execution
Throws:
IOException – 
Returns:
true if we are currently cloning a repository/**
	 * Detect if we are in a clone command execution
	 *
	 * @return {@code true} if we are currently cloning a repository
	 * @throws IOException
	 */
	boolean isInClone() throws IOException {
		return hasDanglingHead() && !packedRefsFile.exists() && !hasLooseRef();
	}

	private boolean hasDanglingHead() throws IOException {
		Ref head = exactRef(Constants.HEAD);
		if (head != null) {
			ObjectId id = head.getObjectId();
			return id == null || id.equals(ObjectId.zeroId());
		}
		return false;
	}

	private boolean hasLooseRef() throws IOException {
		try (Stream<Path> stream = Files.walk(refsDir.toPath())) {
			return stream.anyMatch(Files::isRegularFile);
		}
	}

	
If the parent should fire listeners, fires them. /** If the parent should fire listeners, fires them. */
	void fireRefsChanged() {
		final int last = lastNotifiedModCnt.get();
		final int curr = modCnt.get();
		if (last != curr && lastNotifiedModCnt.compareAndSet(last, curr) && last != 0)
			parent.fireEvent(new RefsChangedEvent());
	}

	
Create a reference update to write a temporary reference.
Throws:
IOException – 
            a temporary name cannot be allocated.
Returns:
an update for a new temporary reference./**
	 * Create a reference update to write a temporary reference.
	 *
	 * @return an update for a new temporary reference.
	 * @throws IOException
	 *             a temporary name cannot be allocated.
	 */
	RefDirectoryUpdate newTemporaryUpdate() throws IOException {
		File tmp = File.createTempFile("renamed_", "_ref", refsDir); //$NON-NLS-1$ //$NON-NLS-2$
		String name = Constants.R_REFS + tmp.getName();
		Ref ref = new ObjectIdRef.Unpeeled(NEW, name, null);
		return new RefDirectoryUpdate(this, ref);
	}

	
Locate the file on disk for a single reference name.
Params:
name – 
           name of the ref, relative to the Git repository top level
           directory (so typically starts with refs/).
Returns:
the loose file location./**
	 * Locate the file on disk for a single reference name.
	 *
	 * @param name
	 *            name of the ref, relative to the Git repository top level
	 *            directory (so typically starts with refs/).
	 * @return the loose file location.
	 */
	File fileFor(String name) {
		if (name.startsWith(R_REFS)) {
			name = name.substring(R_REFS.length());
			return new File(refsDir, name);
		}
		return new File(gitDir, name);
	}

	static int levelsIn(String name) {
		int count = 0;
		for (int p = name.indexOf('/'); p >= 0; p = name.indexOf('/', p + 1))
			count++;
		return count;
	}

	static void delete(File file, int depth) throws IOException {
		delete(file, depth, null);
	}

	private static void delete(File file, int depth, LockFile rLck)
			throws IOException {
		if (!file.delete() && file.isFile()) {
			throw new IOException(MessageFormat.format(
					JGitText.get().fileCannotBeDeleted, file));
		}

		if (rLck != null) {
			rLck.unlock(); // otherwise cannot delete dir below
		}
		File dir = file.getParentFile();
		for (int i = 0; i < depth; ++i) {
			try {
				Files.deleteIfExists(dir.toPath());
			} catch (DirectoryNotEmptyException e) {
				// Don't log; normal case when there are other refs with the
				// same prefix
				break;
			} catch (IOException e) {
				LOG.warn(MessageFormat.format(JGitText.get().unableToRemovePath,
						dir), e);
				break;
			}
			dir = dir.getParentFile();
		}
	}

	
Get times to sleep while retrying a possibly contentious operation.
 For retrying an operation that might have high contention, such as locking the packed-refs file, the caller may implement a retry loop using the returned values: 
for (int toSleepMs : getRetrySleepMs()) {
  sleep(toSleepMs);
  if (isSuccessful(doSomething())) {
    return success;
  }
}
return failure;

The first value in the returned iterable is 0, and the caller should treat
a fully-consumed iterator as a timeout.
Returns:
iterable of times, in milliseconds, that the caller should sleep
        before attempting an operation./**
	 * Get times to sleep while retrying a possibly contentious operation.
	 * <p>
	 * For retrying an operation that might have high contention, such as locking
	 * the {@code packed-refs} file, the caller may implement a retry loop using
	 * the returned values:
	 *
	 * <pre>
	 * for (int toSleepMs : getRetrySleepMs()) {
	 *   sleep(toSleepMs);
	 *   if (isSuccessful(doSomething())) {
	 *     return success;
	 *   }
	 * }
	 * return failure;
	 * </pre>
	 *
	 * The first value in the returned iterable is 0, and the caller should treat
	 * a fully-consumed iterator as a timeout.
	 *
	 * @return iterable of times, in milliseconds, that the caller should sleep
	 *         before attempting an operation.
	 */
	Iterable<Integer> getRetrySleepMs() {
		return retrySleepMs;
	}

	void setRetrySleepMs(List<Integer> retrySleepMs) {
		if (retrySleepMs == null || retrySleepMs.isEmpty()
				|| retrySleepMs.get(0).intValue() != 0) {
			throw new IllegalArgumentException();
		}
		this.retrySleepMs = retrySleepMs;
	}

	
Sleep with Thread.sleep(long), converting InterruptedException to InterruptedIOException. 
Params:
ms – 
           time to sleep, in milliseconds; zero or negative is a no-op.
Throws:
InterruptedIOException – 
            if sleeping was interrupted./**
	 * Sleep with {@link Thread#sleep(long)}, converting {@link
	 * InterruptedException} to {@link InterruptedIOException}.
	 *
	 * @param ms
	 *            time to sleep, in milliseconds; zero or negative is a no-op.
	 * @throws InterruptedIOException
	 *             if sleeping was interrupted.
	 */
	static void sleep(long ms) throws InterruptedIOException {
		if (ms <= 0) {
			return;
		}
		try {
			Thread.sleep(ms);
		} catch (InterruptedException e) {
			InterruptedIOException ie = new InterruptedIOException();
			ie.initCause(e);
			throw ie;
		}
	}

	static class PackedRefList extends RefList<Ref> {

		private final FileSnapshot snapshot;

		private final ObjectId id;

		private PackedRefList(RefList<Ref> src, FileSnapshot s, ObjectId i) {
			super(src);
			snapshot = s;
			id = i;
		}
	}

	private static final PackedRefList NO_PACKED_REFS = new PackedRefList(
			RefList.emptyList(), FileSnapshot.MISSING_FILE,
			ObjectId.zeroId());

	private static LooseSymbolicRef newSymbolicRef(FileSnapshot snapshot,
			String name, String target) {
		Ref dst = new ObjectIdRef.Unpeeled(NEW, target, null);
		return new LooseSymbolicRef(snapshot, name, dst);
	}

	private static interface LooseRef extends Ref {
		FileSnapshot getSnapShot();

		LooseRef peel(ObjectIdRef newLeaf);
	}

	private final static class LoosePeeledTag extends ObjectIdRef.PeeledTag
			implements LooseRef {
		private final FileSnapshot snapShot;

		LoosePeeledTag(FileSnapshot snapshot, @NonNull String refName,
				@NonNull ObjectId id, @NonNull ObjectId p) {
			super(LOOSE, refName, id, p);
			this.snapShot = snapshot;
		}

		@Override
		public FileSnapshot getSnapShot() {
			return snapShot;
		}

		@Override
		public LooseRef peel(ObjectIdRef newLeaf) {
			return this;
		}
	}

	private final static class LooseNonTag extends ObjectIdRef.PeeledNonTag
			implements LooseRef {
		private final FileSnapshot snapShot;

		LooseNonTag(FileSnapshot snapshot, @NonNull String refName,
				@NonNull ObjectId id) {
			super(LOOSE, refName, id);
			this.snapShot = snapshot;
		}

		@Override
		public FileSnapshot getSnapShot() {
			return snapShot;
		}

		@Override
		public LooseRef peel(ObjectIdRef newLeaf) {
			return this;
		}
	}

	private final static class LooseUnpeeled extends ObjectIdRef.Unpeeled
			implements LooseRef {
		private FileSnapshot snapShot;

		LooseUnpeeled(FileSnapshot snapShot, @NonNull String refName,
				@NonNull ObjectId id) {
			super(LOOSE, refName, id);
			this.snapShot = snapShot;
		}

		@Override
		public FileSnapshot getSnapShot() {
			return snapShot;
		}

		@NonNull
		@Override
		public ObjectId getObjectId() {
			ObjectId id = super.getObjectId();
			assert id != null; // checked in constructor
			return id;
		}

		@Override
		public LooseRef peel(ObjectIdRef newLeaf) {
			ObjectId peeledObjectId = newLeaf.getPeeledObjectId();
			ObjectId objectId = getObjectId();
			if (peeledObjectId != null) {
				return new LoosePeeledTag(snapShot, getName(),
						objectId, peeledObjectId);
			} else {
				return new LooseNonTag(snapShot, getName(),
						objectId);
			}
		}
	}

	private final static class LooseSymbolicRef extends SymbolicRef implements
			LooseRef {
		private final FileSnapshot snapShot;

		LooseSymbolicRef(FileSnapshot snapshot, @NonNull String refName,
				@NonNull Ref target) {
			super(refName, target);
			this.snapShot = snapshot;
		}

		@Override
		public FileSnapshot getSnapShot() {
			return snapShot;
		}

		@Override
		public LooseRef peel(ObjectIdRef newLeaf) {
			// We should never try to peel the symbolic references.
			throw new UnsupportedOperationException();
		}
	}
}