/*
    * Copyright (C) 2007, Robin Rosenberg <robin.rosenberg@dewire.com>
    * Copyright (C) 2008, Shawn O. Pearce <spearce@spearce.org>
    * Copyright (C) 2014, Gustaf Lundh <gustaf.lundh@sonymobile.com> 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.revwalk;
  
  import java.io.IOException;
  import java.text.MessageFormat;
  import java.util.ArrayList;
  import java.util.Collection;
  import java.util.EnumSet;
  import java.util.Iterator;
  import java.util.List;
  
  import org.eclipse.jgit.annotations.NonNull;
  import org.eclipse.jgit.annotations.Nullable;
  import org.eclipse.jgit.errors.CorruptObjectException;
  import org.eclipse.jgit.errors.IncorrectObjectTypeException;
  import org.eclipse.jgit.errors.LargeObjectException;
  import org.eclipse.jgit.errors.MissingObjectException;
  import org.eclipse.jgit.errors.RevWalkException;
  import org.eclipse.jgit.internal.JGitText;
  import org.eclipse.jgit.lib.AnyObjectId;
  import org.eclipse.jgit.lib.AsyncObjectLoaderQueue;
  import org.eclipse.jgit.lib.Constants;
  import org.eclipse.jgit.lib.MutableObjectId;
  import org.eclipse.jgit.lib.ObjectId;
  import org.eclipse.jgit.lib.ObjectIdOwnerMap;
  import org.eclipse.jgit.lib.ObjectLoader;
  import org.eclipse.jgit.lib.ObjectReader;
  import org.eclipse.jgit.lib.Repository;
  import org.eclipse.jgit.revwalk.filter.RevFilter;
  import org.eclipse.jgit.treewalk.filter.TreeFilter;
  import org.eclipse.jgit.util.References;
  
  /**
   * Walks a commit graph and produces the matching commits in order.
   * <p>
   * A RevWalk instance can only be used once to generate results. Running a
   * second time requires creating a new RevWalk instance, or invoking
   * {@link #reset()} before starting again. Resetting an existing instance may be
   * faster for some applications as commit body parsing can be avoided on the
   * later invocations.
   * <p>
   * RevWalk instances are not thread-safe. Applications must either restrict
   * usage of a RevWalk instance to a single thread, or implement their own
   * synchronization at a higher level.
   * <p>
   * Multiple simultaneous RevWalk instances per
   * {@link org.eclipse.jgit.lib.Repository} are permitted, even from concurrent
   * threads. Equality of {@link org.eclipse.jgit.revwalk.RevCommit}s from two
   * different RevWalk instances is never true, even if their
   * {@link org.eclipse.jgit.lib.ObjectId}s are equal (and thus they describe the
   * same commit).
   * <p>
   * The offered iterator is over the list of RevCommits described by the
   * configuration of this instance. Applications should restrict themselves to
   * using either the provided Iterator or {@link #next()}, but never use both on
   * the same RevWalk at the same time. The Iterator may buffer RevCommits, while
   * {@link #next()} does not.
   */
  public class RevWalk implements Iterable<RevCommit>, AutoCloseable {
  	private static final int MB = 1 << 20;
  
  	/**
  	 * Set on objects whose important header data has been loaded.
  	 * <p>
  	 * For a RevCommit this indicates we have pulled apart the tree and parent
  	 * references from the raw bytes available in the repository and translated
  	 * those to our own local RevTree and RevCommit instances. The raw buffer is
  	 * also available for message and other header filtering.
  	 * <p>
  	 * For a RevTag this indicates we have pulled part the tag references to
  	 * find out who the tag refers to, and what that object's type is.
  	 */
  	static final int PARSED = 1 << 0;
  
  	/**
  	 * Set on RevCommit instances added to our {@link #pending} queue.
  	 * <p>
  	 * We use this flag to avoid adding the same commit instance twice to our
  	 * queue, especially if we reached it by more than one path.
  	 */
  	static final int SEEN = 1 << 1;
  
  	/**
  	 * Set on RevCommit instances the caller does not want output.
  	 * <p>
  	 * We flag commits as uninteresting if the caller does not want commits
  	 * reachable from a commit given to {@link #markUninteresting(RevCommit)}.
  	 * This flag is always carried into the commit's parents and is a key part
 	 * of the "rev-list B --not A" feature; A is marked UNINTERESTING.
 	 */
 	static final int UNINTERESTING = 1 << 2;
 
 	/**
 	 * Set on a RevCommit that can collapse out of the history.
 	 * <p>
 	 * If the {@link #treeFilter} concluded that this commit matches his
 	 * parents' for all of the paths that the filter is interested in then we
 	 * mark the commit REWRITE. Later we can rewrite the parents of a REWRITE
 	 * child to remove chains of REWRITE commits before we produce the child to
 	 * the application.
 	 *
 	 * @see RewriteGenerator
 	 */
 	static final int REWRITE = 1 << 3;
 
 	/**
 	 * Temporary mark for use within generators or filters.
 	 * <p>
 	 * This mark is only for local use within a single scope. If someone sets
 	 * the mark they must unset it before any other code can see the mark.
 	 */
 	static final int TEMP_MARK = 1 << 4;
 
 	/**
 	 * Temporary mark for use within {@link TopoSortGenerator}.
 	 * <p>
 	 * This mark indicates the commit could not produce when it wanted to, as at
 	 * least one child was behind it. Commits with this flag are delayed until
 	 * all children have been output first.
 	 */
 	static final int TOPO_DELAY = 1 << 5;
 
 	/**
 	 * Temporary mark for use within {@link TopoNonIntermixSortGenerator}.
 	 * <p>
 	 * This mark indicates the commit has been queued for emission in
 	 * {@link TopoSortGenerator} and can be produced. This mark is removed when
 	 * the commit has been produced.
 	 */
 	static final int TOPO_QUEUED = 1 << 6;
 
 	/** Number of flag bits we keep internal for our own use. See above flags. */
 	static final int RESERVED_FLAGS = 7;
 
 	private static final int APP_FLAGS = -1 & ~((1 << RESERVED_FLAGS) - 1);
 
 	final ObjectReader reader;
 
 	private final boolean closeReader;
 
 	final MutableObjectId idBuffer;
 
 	ObjectIdOwnerMap<RevObject> objects;
 
 	int freeFlags = APP_FLAGS;
 
 	private int delayFreeFlags;
 
 	private int retainOnReset;
 
 	int carryFlags = UNINTERESTING;
 
 	final ArrayList<RevCommit> roots;
 
 	AbstractRevQueue queue;
 
 	Generator pending;
 
 	private final EnumSet<RevSort> sorting;
 
 	private RevFilter filter;
 
 	private TreeFilter treeFilter;
 
 	private boolean retainBody = true;
 
 	private boolean rewriteParents = true;
 
 	private boolean firstParent;
 
 	boolean shallowCommitsInitialized;
 
 	/**
 	 * Create a new revision walker for a given repository.
 	 *
 	 * @param repo
 	 *            the repository the walker will obtain data from. An
 	 *            ObjectReader will be created by the walker, and will be closed
 	 *            when the walker is closed.
 	 */
 	public RevWalk(Repository repo) {
 		this(repo.newObjectReader(), true);
 	}
 
 	/**
 	 * Create a new revision walker for a given repository.
 	 * <p>
 	 *
 	 * @param or
 	 *            the reader the walker will obtain data from. The reader is not
 	 *            closed when the walker is closed (but is closed by {@link
 	 *            #dispose()}.
 	 */
 	public RevWalk(ObjectReader or) {
 		this(or, false);
 	}
 
 	private RevWalk(ObjectReader or, boolean closeReader) {
 		reader = or;
 		idBuffer = new MutableObjectId();
 		objects = new ObjectIdOwnerMap<>();
 		roots = new ArrayList<>();
 		queue = new DateRevQueue(false);
 		pending = new StartGenerator(this);
 		sorting = EnumSet.of(RevSort.NONE);
 		filter = RevFilter.ALL;
 		treeFilter = TreeFilter.ALL;
 		this.closeReader = closeReader;
 	}
 
 	/**
 	 * Get the reader this walker is using to load objects.
 	 *
 	 * @return the reader this walker is using to load objects.
 	 */
 	public ObjectReader getObjectReader() {
 		return reader;
 	}
 
 	/**
 	 * Get a reachability checker for commits over this revwalk.
 	 *
 	 * @return the most efficient reachability checker for this repository.
 	 * @throws IOException
 	 *             if it cannot open any of the underlying indices.
 	 *
 	 * @since 5.4
 	 */
 	public ReachabilityChecker createReachabilityChecker() throws IOException {
 		if (reader.getBitmapIndex() != null) {
 			return new BitmappedReachabilityChecker(this);
 		}
 
 		return new PedestrianReachabilityChecker(true, this);
 	}
 
 	/**
 	 * {@inheritDoc}
 	 * <p>
 	 * Release any resources used by this walker's reader.
 	 * <p>
 	 * A walker that has been released can be used again, but may need to be
 	 * released after the subsequent usage.
 	 *
 	 * @since 4.0
 	 */
 	@Override
 	public void close() {
 		if (closeReader) {
 			reader.close();
 		}
 	}
 
 	/**
 	 * Mark a commit to start graph traversal from.
 	 * <p>
 	 * Callers are encouraged to use {@link #parseCommit(AnyObjectId)} to obtain
 	 * the commit reference, rather than {@link #lookupCommit(AnyObjectId)}, as
 	 * this method requires the commit to be parsed before it can be added as a
 	 * root for the traversal.
 	 * <p>
 	 * The method will automatically parse an unparsed commit, but error
 	 * handling may be more difficult for the application to explain why a
 	 * RevCommit is not actually a commit. The object pool of this walker would
 	 * also be 'poisoned' by the non-commit RevCommit.
 	 *
 	 * @param c
 	 *            the commit to start traversing from. The commit passed must be
 	 *            from this same revision walker.
 	 * @throws org.eclipse.jgit.errors.MissingObjectException
 	 *             the commit supplied is not available from the object
 	 *             database. This usually indicates the supplied commit is
 	 *             invalid, but the reference was constructed during an earlier
 	 *             invocation to {@link #lookupCommit(AnyObjectId)}.
 	 * @throws org.eclipse.jgit.errors.IncorrectObjectTypeException
 	 *             the object was not parsed yet and it was discovered during
 	 *             parsing that it is not actually a commit. This usually
 	 *             indicates the caller supplied a non-commit SHA-1 to
 	 *             {@link #lookupCommit(AnyObjectId)}.
 	 * @throws java.io.IOException
 	 *             a pack file or loose object could not be read.
 	 */
 	public void markStart(RevCommit c) throws MissingObjectException,
 			IncorrectObjectTypeException, IOException {
 		if ((c.flags & SEEN) != 0)
 			return;
 		if ((c.flags & PARSED) == 0)
 			c.parseHeaders(this);
 		c.flags |= SEEN;
 		roots.add(c);
 		queue.add(c);
 	}
 
 	/**
 	 * Mark commits to start graph traversal from.
 	 *
 	 * @param list
 	 *            commits to start traversing from. The commits passed must be
 	 *            from this same revision walker.
 	 * @throws org.eclipse.jgit.errors.MissingObjectException
 	 *             one of the commits supplied is not available from the object
 	 *             database. This usually indicates the supplied commit is
 	 *             invalid, but the reference was constructed during an earlier
 	 *             invocation to {@link #lookupCommit(AnyObjectId)}.
 	 * @throws org.eclipse.jgit.errors.IncorrectObjectTypeException
 	 *             the object was not parsed yet and it was discovered during
 	 *             parsing that it is not actually a commit. This usually
 	 *             indicates the caller supplied a non-commit SHA-1 to
 	 *             {@link #lookupCommit(AnyObjectId)}.
 	 * @throws java.io.IOException
 	 *             a pack file or loose object could not be read.
 	 */
 	public void markStart(Collection<RevCommit> list)
 			throws MissingObjectException, IncorrectObjectTypeException,
 			IOException {
 		for (RevCommit c : list)
 			markStart(c);
 	}
 
 	/**
 	 * Mark a commit to not produce in the output.
 	 * <p>
 	 * Uninteresting commits denote not just themselves but also their entire
 	 * ancestry chain, back until the merge base of an uninteresting commit and
 	 * an otherwise interesting commit.
 	 * <p>
 	 * Callers are encouraged to use {@link #parseCommit(AnyObjectId)} to obtain
 	 * the commit reference, rather than {@link #lookupCommit(AnyObjectId)}, as
 	 * this method requires the commit to be parsed before it can be added as a
 	 * root for the traversal.
 	 * <p>
 	 * The method will automatically parse an unparsed commit, but error
 	 * handling may be more difficult for the application to explain why a
 	 * RevCommit is not actually a commit. The object pool of this walker would
 	 * also be 'poisoned' by the non-commit RevCommit.
 	 *
 	 * @param c
 	 *            the commit to start traversing from. The commit passed must be
 	 *            from this same revision walker.
 	 * @throws org.eclipse.jgit.errors.MissingObjectException
 	 *             the commit supplied is not available from the object
 	 *             database. This usually indicates the supplied commit is
 	 *             invalid, but the reference was constructed during an earlier
 	 *             invocation to {@link #lookupCommit(AnyObjectId)}.
 	 * @throws org.eclipse.jgit.errors.IncorrectObjectTypeException
 	 *             the object was not parsed yet and it was discovered during
 	 *             parsing that it is not actually a commit. This usually
 	 *             indicates the caller supplied a non-commit SHA-1 to
 	 *             {@link #lookupCommit(AnyObjectId)}.
 	 * @throws java.io.IOException
 	 *             a pack file or loose object could not be read.
 	 */
 	public void markUninteresting(RevCommit c)
 			throws MissingObjectException, IncorrectObjectTypeException,
 			IOException {
 		c.flags |= UNINTERESTING;
 		carryFlagsImpl(c);
 		markStart(c);
 	}
 
 	/**
 	 * Determine if a commit is reachable from another commit.
 	 * <p>
 	 * A commit <code>base</code> is an ancestor of <code>tip</code> if we
 	 * can find a path of commits that leads from <code>tip</code> and ends at
 	 * <code>base</code>.
 	 * <p>
 	 * This utility function resets the walker, inserts the two supplied
 	 * commits, and then executes a walk until an answer can be obtained.
 	 * Currently allocated RevFlags that have been added to RevCommit instances
 	 * will be retained through the reset.
 	 *
 	 * @param base
 	 *            commit the caller thinks is reachable from <code>tip</code>.
 	 * @param tip
 	 *            commit to start iteration from, and which is most likely a
 	 *            descendant (child) of <code>base</code>.
 	 * @return true if there is a path directly from <code>tip</code> to
 	 *         <code>base</code> (and thus <code>base</code> is fully merged
 	 *         into <code>tip</code>); false otherwise.
 	 * @throws org.eclipse.jgit.errors.MissingObjectException
 	 *             one or more of the next commit's parents are not available
 	 *             from the object database, but were thought to be candidates
 	 *             for traversal. This usually indicates a broken link.
 	 * @throws org.eclipse.jgit.errors.IncorrectObjectTypeException
 	 *             one or more of the next commit's parents are not actually
 	 *             commit objects.
 	 * @throws java.io.IOException
 	 *             a pack file or loose object could not be read.
 	 */
 	public boolean isMergedInto(RevCommit"../../../../org/eclipse/jgit/revwalk/RevCommit.html#RevCommit">RevCommit base, RevCommit tip)
 			throws MissingObjectException, IncorrectObjectTypeException,
 			IOException {
 		final RevFilter oldRF = filter;
 		final TreeFilter oldTF = treeFilter;
 		try {
 			finishDelayedFreeFlags();
 			reset(~freeFlags & APP_FLAGS);
 			filter = RevFilter.MERGE_BASE;
 			treeFilter = TreeFilter.ALL;
 			markStart(tip);
 			markStart(base);
 			RevCommit mergeBase;
 			while ((mergeBase = next()) != null) {
 				if (References.isSameObject(mergeBase, base)) {
 					return true;
 				}
 			}
 			return false;
 		} finally {
 			filter = oldRF;
 			treeFilter = oldTF;
 		}
 	}
 
 	/**
 	 * Pop the next most recent commit.
 	 *
 	 * @return next most recent commit; null if traversal is over.
 	 * @throws org.eclipse.jgit.errors.MissingObjectException
 	 *             one or more of the next commit's parents are not available
 	 *             from the object database, but were thought to be candidates
 	 *             for traversal. This usually indicates a broken link.
 	 * @throws org.eclipse.jgit.errors.IncorrectObjectTypeException
 	 *             one or more of the next commit's parents are not actually
 	 *             commit objects.
 	 * @throws java.io.IOException
 	 *             a pack file or loose object could not be read.
 	 */
 	public RevCommit next() throws MissingObjectException,
 			IncorrectObjectTypeException, IOException {
 		return pending.next();
 	}
 
 	/**
 	 * Obtain the sort types applied to the commits returned.
 	 *
 	 * @return the sorting strategies employed. At least one strategy is always
 	 *         used, but that strategy may be
 	 *         {@link org.eclipse.jgit.revwalk.RevSort#NONE}.
 	 */
 	public EnumSet<RevSort> getRevSort() {
 		return sorting.clone();
 	}
 
 	/**
 	 * Check whether the provided sorting strategy is enabled.
 	 *
 	 * @param sort
 	 *            a sorting strategy to look for.
 	 * @return true if this strategy is enabled, false otherwise
 	 */
 	public boolean hasRevSort(RevSort sort) {
 		return sorting.contains(sort);
 	}
 
 	/**
 	 * Select a single sorting strategy for the returned commits.
 	 * <p>
 	 * Disables all sorting strategies, then enables only the single strategy
 	 * supplied by the caller.
 	 *
 	 * @param s
 	 *            a sorting strategy to enable.
 	 */
 	public void sort(RevSort s) {
 		assertNotStarted();
 		sorting.clear();
 		sorting.add(s);
 	}
 
 	/**
 	 * Add or remove a sorting strategy for the returned commits.
 	 * <p>
 	 * Multiple strategies can be applied at once, in which case some strategies
 	 * may take precedence over others. As an example,
 	 * {@link org.eclipse.jgit.revwalk.RevSort#TOPO} must take precedence over
 	 * {@link org.eclipse.jgit.revwalk.RevSort#COMMIT_TIME_DESC}, otherwise it
 	 * cannot enforce its ordering.
 	 *
 	 * @param s
 	 *            a sorting strategy to enable or disable.
 	 * @param use
 	 *            true if this strategy should be used, false if it should be
 	 *            removed.
 	 */
 	public void sort(RevSort s, boolean use) {
 		assertNotStarted();
 		if (use)
 			sorting.add(s);
 		else
 			sorting.remove(s);
 
 		if (sorting.size() > 1)
 			sorting.remove(RevSort.NONE);
 		else if (sorting.isEmpty())
 			sorting.add(RevSort.NONE);
 	}
 
 	/**
 	 * Get the currently configured commit filter.
 	 *
 	 * @return the current filter. Never null as a filter is always needed.
 	 */
 	@NonNull
 	public RevFilter getRevFilter() {
 		return filter;
 	}
 
 	/**
 	 * Set the commit filter for this walker.
 	 * <p>
 	 * Multiple filters may be combined by constructing an arbitrary tree of
 	 * <code>AndRevFilter</code> or <code>OrRevFilter</code> instances to
 	 * describe the boolean expression required by the application. Custom
 	 * filter implementations may also be constructed by applications.
 	 * <p>
 	 * Note that filters are not thread-safe and may not be shared by concurrent
 	 * RevWalk instances. Every RevWalk must be supplied its own unique filter,
 	 * unless the filter implementation specifically states it is (and always
 	 * will be) thread-safe. Callers may use
 	 * {@link org.eclipse.jgit.revwalk.filter.RevFilter#clone()} to create a
 	 * unique filter tree for this RevWalk instance.
 	 *
 	 * @param newFilter
 	 *            the new filter. If null the special
 	 *            {@link org.eclipse.jgit.revwalk.filter.RevFilter#ALL} filter
 	 *            will be used instead, as it matches every commit.
 	 * @see org.eclipse.jgit.revwalk.filter.AndRevFilter
 	 * @see org.eclipse.jgit.revwalk.filter.OrRevFilter
 	 */
 	public void setRevFilter(RevFilter newFilter) {
 		assertNotStarted();
 		filter = newFilter != null ? newFilter : RevFilter.ALL;
 	}
 
 	/**
 	 * Get the tree filter used to simplify commits by modified paths.
 	 *
 	 * @return the current filter. Never null as a filter is always needed. If
 	 *         no filter is being applied
 	 *         {@link org.eclipse.jgit.treewalk.filter.TreeFilter#ALL} is
 	 *         returned.
 	 */
 	@NonNull
 	public TreeFilter getTreeFilter() {
 		return treeFilter;
 	}
 
 	/**
 	 * Set the tree filter used to simplify commits by modified paths.
 	 * <p>
 	 * If null or {@link org.eclipse.jgit.treewalk.filter.TreeFilter#ALL} the
 	 * path limiter is removed. Commits will not be simplified.
 	 * <p>
 	 * If non-null and not
 	 * {@link org.eclipse.jgit.treewalk.filter.TreeFilter#ALL} then the tree
 	 * filter will be installed. Commits will have their ancestry simplified to
 	 * hide commits that do not contain tree entries matched by the filter,
 	 * unless {@code setRewriteParents(false)} is called.
 	 * <p>
 	 * Usually callers should be inserting a filter graph including
 	 * {@link org.eclipse.jgit.treewalk.filter.TreeFilter#ANY_DIFF} along with
 	 * one or more {@link org.eclipse.jgit.treewalk.filter.PathFilter}
 	 * instances.
 	 *
 	 * @param newFilter
 	 *            new filter. If null the special
 	 *            {@link org.eclipse.jgit.treewalk.filter.TreeFilter#ALL} filter
 	 *            will be used instead, as it matches everything.
 	 * @see org.eclipse.jgit.treewalk.filter.PathFilter
 	 */
 	public void setTreeFilter(TreeFilter newFilter) {
 		assertNotStarted();
 		treeFilter = newFilter != null ? newFilter : TreeFilter.ALL;
 	}
 
 	/**
 	 * Set whether to rewrite parent pointers when filtering by modified paths.
 	 * <p>
 	 * By default, when {@link #setTreeFilter(TreeFilter)} is called with non-
 	 * null and non-{@link org.eclipse.jgit.treewalk.filter.TreeFilter#ALL}
 	 * filter, commits will have their ancestry simplified and parents rewritten
 	 * to hide commits that do not match the filter.
 	 * <p>
 	 * This behavior can be bypassed by passing false to this method.
 	 *
 	 * @param rewrite
 	 *            whether to rewrite parents; defaults to true.
 	 * @since 3.4
 	 */
 	public void setRewriteParents(boolean rewrite) {
 		rewriteParents = rewrite;
 	}
 
 	boolean getRewriteParents() {
 		return rewriteParents;
 	}
 
 	/**
 	 * Should the body of a commit or tag be retained after parsing its headers?
 	 * <p>
 	 * Usually the body is always retained, but some application code might not
 	 * care and would prefer to discard the body of a commit as early as
 	 * possible, to reduce memory usage.
 	 * <p>
 	 * True by default on {@link org.eclipse.jgit.revwalk.RevWalk} and false by
 	 * default for {@link org.eclipse.jgit.revwalk.ObjectWalk}.
 	 *
 	 * @return true if the body should be retained; false it is discarded.
 	 */
 	public boolean isRetainBody() {
 		return retainBody;
 	}
 
 	/**
 	 * Set whether or not the body of a commit or tag is retained.
 	 * <p>
 	 * If a body of a commit or tag is not retained, the application must call
 	 * {@link #parseBody(RevObject)} before the body can be safely accessed
 	 * through the type specific access methods.
 	 * <p>
 	 * True by default on {@link org.eclipse.jgit.revwalk.RevWalk} and false by
 	 * default for {@link org.eclipse.jgit.revwalk.ObjectWalk}.
 	 *
 	 * @param retain
 	 *            true to retain bodies; false to discard them early.
 	 */
 	public void setRetainBody(boolean retain) {
 		retainBody = retain;
 	}
 
 	/**
 	 * @return whether only first-parent links should be followed when walking.
 	 *
 	 * @since 5.5
 	 */
 	public boolean isFirstParent() {
 		return firstParent;
 	}
 
 	/**
 	 * Set whether or not only first parent links should be followed.
 	 * <p>
 	 * If set, second- and higher-parent links are not traversed at all.
 	 * <p>
 	 * This must be called prior to {@link #markStart(RevCommit)}.
 	 *
 	 * @param enable
 	 *            true to walk only first-parent links.
 	 *
 	 * @since 5.5
 	 */
 	public void setFirstParent(boolean enable) {
 		assertNotStarted();
 		assertNoCommitsMarkedStart();
 		firstParent = enable;
 		queue = new DateRevQueue(firstParent);
 		pending = new StartGenerator(this);
 	}
 
 	/**
 	 * Locate a reference to a blob without loading it.
 	 * <p>
 	 * The blob may or may not exist in the repository. It is impossible to tell
 	 * from this method's return value.
 	 *
 	 * @param id
 	 *            name of the blob object.
 	 * @return reference to the blob object. Never null.
 	 */
 	@NonNull
 	public RevBlob lookupBlob(AnyObjectId id) {
 		RevBlob c = (RevBlob) objects.get(id);
 		if (c == null) {
 			c = new RevBlob(id);
 			objects.add(c);
 		}
 		return c;
 	}
 
 	/**
 	 * Locate a reference to a tree without loading it.
 	 * <p>
 	 * The tree may or may not exist in the repository. It is impossible to tell
 	 * from this method's return value.
 	 *
 	 * @param id
 	 *            name of the tree object.
 	 * @return reference to the tree object. Never null.
 	 */
 	@NonNull
 	public RevTree lookupTree(AnyObjectId id) {
 		RevTree c = (RevTree) objects.get(id);
 		if (c == null) {
 			c = new RevTree(id);
 			objects.add(c);
 		}
 		return c;
 	}
 
 	/**
 	 * Locate a reference to a commit without loading it.
 	 * <p>
 	 * The commit may or may not exist in the repository. It is impossible to
 	 * tell from this method's return value.
 	 * <p>
 	 * See {@link #parseHeaders(RevObject)} and {@link #parseBody(RevObject)}
 	 * for loading contents.
 	 *
 	 * @param id
 	 *            name of the commit object.
 	 * @return reference to the commit object. Never null.
 	 */
 	@NonNull
 	public RevCommit lookupCommit(AnyObjectId id) {
 		RevCommit c = (RevCommit) objects.get(id);
 		if (c == null) {
 			c = createCommit(id);
 			objects.add(c);
 		}
 		return c;
 	}
 
 	/**
 	 * Locate a reference to a tag without loading it.
 	 * <p>
 	 * The tag may or may not exist in the repository. It is impossible to tell
 	 * from this method's return value.
 	 *
 	 * @param id
 	 *            name of the tag object.
 	 * @return reference to the tag object. Never null.
 	 */
 	@NonNull
 	public RevTag lookupTag(AnyObjectId id) {
 		RevTag c = (RevTag) objects.get(id);
 		if (c == null) {
 			c = new RevTag(id);
 			objects.add(c);
 		}
 		return c;
 	}
 
 	/**
 	 * Locate a reference to any object without loading it.
 	 * <p>
 	 * The object may or may not exist in the repository. It is impossible to
 	 * tell from this method's return value.
 	 *
 	 * @param id
 	 *            name of the object.
 	 * @param type
 	 *            type of the object. Must be a valid Git object type.
 	 * @return reference to the object. Never null.
 	 */
 	@NonNull
 	public RevObject lookupAny(AnyObjectId id, int type) {
 		RevObject r = objects.get(id);
 		if (r == null) {
 			switch (type) {
 			case Constants.OBJ_COMMIT:
 				r = createCommit(id);
 				break;
 			case Constants.OBJ_TREE:
 				r = new RevTree(id);
 				break;
 			case Constants.OBJ_BLOB:
 				r = new RevBlob(id);
 				break;
 			case Constants.OBJ_TAG:
 				r = new RevTag(id);
 				break;
 			default:
 				throw new IllegalArgumentException(MessageFormat.format(
 						JGitText.get().invalidGitType, Integer.valueOf(type)));
 			}
 			objects.add(r);
 		}
 		return r;
 	}
 
 	/**
 	 * Locate an object that was previously allocated in this walk.
 	 *
 	 * @param id
 	 *            name of the object.
 	 * @return reference to the object if it has been previously located;
 	 *         otherwise null.
 	 */
 	public RevObject lookupOrNull(AnyObjectId id) {
 		return objects.get(id);
 	}
 
 	/**
 	 * Locate a reference to a commit and immediately parse its content.
 	 * <p>
 	 * Unlike {@link #lookupCommit(AnyObjectId)} this method only returns
 	 * successfully if the commit object exists, is verified to be a commit, and
 	 * was parsed without error.
 	 *
 	 * @param id
 	 *            name of the commit object.
 	 * @return reference to the commit object. Never null.
 	 * @throws org.eclipse.jgit.errors.MissingObjectException
 	 *             the supplied commit does not exist.
 	 * @throws org.eclipse.jgit.errors.IncorrectObjectTypeException
 	 *             the supplied id is not a commit or an annotated tag.
 	 * @throws java.io.IOException
 	 *             a pack file or loose object could not be read.
 	 */
 	@NonNull
 	public RevCommit parseCommit(AnyObjectId id)
 			throws MissingObjectException, IncorrectObjectTypeException,
 			IOException {
 		RevObject c = peel(parseAny(id));
 		if (!(c instanceof RevCommit))
 			throw new IncorrectObjectTypeException(id.toObjectId(),
 					Constants.TYPE_COMMIT);
 		return (RevCommit) c;
 	}
 
 	/**
 	 * Locate a reference to a tree.
 	 * <p>
 	 * This method only returns successfully if the tree object exists, is
 	 * verified to be a tree.
 	 *
 	 * @param id
 	 *            name of the tree object, or a commit or annotated tag that may
 	 *            reference a tree.
 	 * @return reference to the tree object. Never null.
 	 * @throws org.eclipse.jgit.errors.MissingObjectException
 	 *             the supplied tree does not exist.
 	 * @throws org.eclipse.jgit.errors.IncorrectObjectTypeException
 	 *             the supplied id is not a tree, a commit or an annotated tag.
 	 * @throws java.io.IOException
 	 *             a pack file or loose object could not be read.
 	 */
 	@NonNull
 	public RevTree parseTree(AnyObjectId id)
 			throws MissingObjectException, IncorrectObjectTypeException,
 			IOException {
 		RevObject c = peel(parseAny(id));
 
 		final RevTree t;
 		if (c instanceof RevCommit)
 			t = ((RevCommit) c).getTree();
 		else if (!(c instanceof RevTree))
 			throw new IncorrectObjectTypeException(id.toObjectId(),
 					Constants.TYPE_TREE);
 		else
 			t = (RevTree) c;
 		parseHeaders(t);
 		return t;
 	}
 
 	/**
 	 * Locate a reference to an annotated tag and immediately parse its content.
 	 * <p>
 	 * Unlike {@link #lookupTag(AnyObjectId)} this method only returns
 	 * successfully if the tag object exists, is verified to be a tag, and was
 	 * parsed without error.
 	 *
 	 * @param id
 	 *            name of the tag object.
 	 * @return reference to the tag object. Never null.
 	 * @throws org.eclipse.jgit.errors.MissingObjectException
 	 *             the supplied tag does not exist.
 	 * @throws org.eclipse.jgit.errors.IncorrectObjectTypeException
 	 *             the supplied id is not a tag or an annotated tag.
 	 * @throws java.io.IOException
 	 *             a pack file or loose object could not be read.
 	 */
 	@NonNull
 	public RevTag parseTag(AnyObjectId id) throws MissingObjectException,
 			IncorrectObjectTypeException, IOException {
 		RevObject c = parseAny(id);
 		if (!(c instanceof RevTag))
 			throw new IncorrectObjectTypeException(id.toObjectId(),
 					Constants.TYPE_TAG);
 		return (RevTag) c;
 	}
 
 	/**
 	 * Locate a reference to any object and immediately parse its headers.
 	 * <p>
 	 * This method only returns successfully if the object exists and was parsed
 	 * without error. Parsing an object can be expensive as the type must be
 	 * determined. For blobs this may mean the blob content was unpacked
 	 * unnecessarily, and thrown away.
 	 *
 	 * @param id
 	 *            name of the object.
 	 * @return reference to the object. Never null.
 	 * @throws org.eclipse.jgit.errors.MissingObjectException
 	 *             the supplied does not exist.
 	 * @throws java.io.IOException
 	 *             a pack file or loose object could not be read.
 	 */
 	@NonNull
 	public RevObject parseAny(AnyObjectId id)
 			throws MissingObjectException, IOException {
 		RevObject r = objects.get(id);
 		if (r == null)
 			r = parseNew(id, reader.open(id));
 		else
 			parseHeaders(r);
 		return r;
 	}
 
 	private RevObject parseNew(AnyObjectId id, ObjectLoader ldr)
 			throws LargeObjectException, CorruptObjectException,
 			MissingObjectException, IOException {
 		RevObject r;
 		int type = ldr.getType();
 		switch (type) {
 		case Constants.OBJ_COMMIT: {
 			final RevCommit c = createCommit(id);
 			c.parseCanonical(this, getCachedBytes(c, ldr));
 			r = c;
 			break;
 		}
 		case Constants.OBJ_TREE: {
 			r = new RevTree(id);
 			r.flags |= PARSED;
 			break;
 		}
 		case Constants.OBJ_BLOB: {
 			r = new RevBlob(id);
 			r.flags |= PARSED;
 			break;
 		}
 		case Constants.OBJ_TAG: {
 			final RevTagk/RevTag.html#RevTag">RevTag t = new RevTag(id);
 			t.parseCanonical(this, getCachedBytes(t, ldr));
 			r = t;
 			break;
 		}
 		default:
 			throw new IllegalArgumentException(MessageFormat.format(
 					JGitText.get().badObjectType, Integer.valueOf(type)));
 		}
 		objects.add(r);
 		return r;
 	}
 
 	byte[] getCachedBytes(RevObject obj) throws LargeObjectException,
 			MissingObjectException, IncorrectObjectTypeException, IOException {
 		return getCachedBytes(obj, reader.open(obj, obj.getType()));
 	}
 
 	byte[] getCachedBytes(RevObject obj, ObjectLoader ldr)
 			throws LargeObjectException, MissingObjectException, IOException {
 		try {
 			return ldr.getCachedBytes(5 * MB);
 		} catch (LargeObjectException tooBig) {
 			tooBig.setObjectId(obj);
 			throw tooBig;
 		}
 	}
 
 	/**
 	 * Asynchronous object parsing.
 	 *
 	 * @param objectIds
 	 *            objects to open from the object store. The supplied collection
 	 *            must not be modified until the queue has finished.
 	 * @param reportMissing
 	 *            if true missing objects are reported by calling failure with a
 	 *            MissingObjectException. This may be more expensive for the
 	 *            implementation to guarantee. If false the implementation may
 	 *            choose to report MissingObjectException, or silently skip over
 	 *            the object with no warning.
 	 * @return queue to read the objects from.
 	 */
 	public <T extends ObjectId> AsyncRevObjectQueue parseAny(
 			Iterable<T> objectIds, boolean reportMissing) {
 		List<T> need = new ArrayList<>();
 		List<RevObject> have = new ArrayList<>();
 		for (T id : objectIds) {
 			RevObject r = objects.get(id);
 			if (r != null && (r.flags & PARSED) != 0)
 				have.add(r);
 			else
 				need.add(id);
 		}
 
 		final Iterator<RevObject> objItr = have.iterator();
 		if (need.isEmpty()) {
 			return new AsyncRevObjectQueue() {
 				@Override
 				public RevObject next() {
 					return objItr.hasNext() ? objItr.next() : null;
 				}
 
 				@Override
 				public boolean cancel(boolean mayInterruptIfRunning) {
 					return true;
 				}
 
 				@Override
 				public void release() {
 					// In-memory only, no action required.
 				}
 			};
 		}
 
 		final AsyncObjectLoaderQueue<T> lItr = reader.open(need, reportMissing);
 		return new AsyncRevObjectQueue() {
 			@Override
 			public RevObject next() throws MissingObjectException,
 					IncorrectObjectTypeException, IOException {
 				if (objItr.hasNext())
 					return objItr.next();
 				if (!lItr.next())
 					return null;
 
 				ObjectId id = lItr.getObjectId();
 				ObjectLoader ldr = lItr.open();
 				RevObject r = objects.get(id);
 				if (r == null)
 					r = parseNew(id, ldr);
 				else if (r instanceof RevCommit) {
 					byte[] raw = ldr.getCachedBytes();
 					((RevCommit) r).parseCanonical(RevWalk.this, raw);
 				} else if (r instanceof RevTag) {
 					byte[] raw = ldr.getCachedBytes();
 					((RevTag) r).parseCanonical(RevWalk.this, raw);
 				} else
 					r.flags |= PARSED;
 				return r;
 			}
 
 			@Override
 			public boolean cancel(boolean mayInterruptIfRunning) {
 				return lItr.cancel(mayInterruptIfRunning);
 			}
 
 			@Override
 			public void release() {
 				lItr.release();
 			}
 		};
 	}
 
 	/**
 	 * Ensure the object's critical headers have been parsed.
 	 * <p>
 	 * This method only returns successfully if the object exists and was parsed
 	 * without error.
 	 *
 	 * @param obj
 	 *            the object the caller needs to be parsed.
 	 * @throws org.eclipse.jgit.errors.MissingObjectException
 	 *             the supplied does not exist.
 	 * @throws java.io.IOException
 	 *             a pack file or loose object could not be read.
 	 */
 	public void parseHeaders(RevObject obj)
 			throws MissingObjectException, IOException {
 		if ((obj.flags & PARSED) == 0)
 			obj.parseHeaders(this);
 	}
 
 	/**
 	 * Ensure the object's full body content is available.
 	 * <p>
 	 * This method only returns successfully if the object exists and was parsed
 	 * without error.
 	 *
 	 * @param obj
 	 *            the object the caller needs to be parsed.
 	 * @throws org.eclipse.jgit.errors.MissingObjectException
 	 *             the supplied does not exist.
 	 * @throws java.io.IOException
 	 *             a pack file or loose object could not be read.
 	 */
 	public void parseBody(RevObject obj)
 			throws MissingObjectException, IOException {
 		obj.parseBody(this);
 	}
 
 	/**
 	 * Peel back annotated tags until a non-tag object is found.
 	 *
 	 * @param obj
 	 *            the starting object.
 	 * @return If {@code obj} is not an annotated tag, {@code obj}. Otherwise
 	 *         the first non-tag object that {@code obj} references. The
 	 *         returned object's headers have been parsed.
 	 * @throws org.eclipse.jgit.errors.MissingObjectException
 	 *             a referenced object cannot be found.
 	 * @throws java.io.IOException
 	 *             a pack file or loose object could not be read.
 	 */
 	public RevObject="../../../../org/eclipse/jgit/revwalk/RevObject.html#RevObject">RevObject peel(RevObject obj) throws MissingObjectException,
 			IOException {
 		while (obj instanceof RevTag) {
 			parseHeaders(obj);
 			obj = ((RevTag) obj).getObject();
 		}
 		parseHeaders(obj);
 		return obj;
 	}
 
 	/**
 	 * Create a new flag for application use during walking.
 	 * <p>
 	 * Applications are only assured to be able to create 24 unique flags on any
 	 * given revision walker instance. Any flags beyond 24 are offered only if
 	 * the implementation has extra free space within its internal storage.
 	 *
 	 * @param name
 	 *            description of the flag, primarily useful for debugging.
 	 * @return newly constructed flag instance.
 	 * @throws java.lang.IllegalArgumentException
 	 *             too many flags have been reserved on this revision walker.
 	 */
 	public RevFlag newFlag(String name) {
 		final int m = allocFlag();
 		return new RevFlag(this, name, m);
 	}
 
 	int allocFlag() {
 		if (freeFlags == 0)
 			throw new IllegalArgumentException(MessageFormat.format(
 					JGitText.get().flagsAlreadyCreated,
 					Integer.valueOf(32 - RESERVED_FLAGS)));
 		final int m = Integer.lowestOneBit(freeFlags);
 		freeFlags &= ~m;
 		return m;
 	}
 
 	/**
 	 * Automatically carry a flag from a child commit to its parents.
 	 * <p>
 	 * A carried flag is copied from the child commit onto its parents when the
 	 * child commit is popped from the lowest level of walk's internal graph.
 	 *
 	 * @param flag
 	 *            the flag to carry onto parents, if set on a descendant.
 	 */
 	public void carry(RevFlag flag) {
 		if ((freeFlags & flag.mask) != 0)
 			throw new IllegalArgumentException(MessageFormat.format(JGitText.get().flagIsDisposed, flag.name));
 		if (flag.walker != this)
 			throw new IllegalArgumentException(MessageFormat.format(JGitText.get().flagNotFromThis, flag.name));
 		carryFlags |= flag.mask;
 	}
 
 	/**
 	 * Automatically carry flags from a child commit to its parents.
 	 * <p>
 	 * A carried flag is copied from the child commit onto its parents when the
 	 * child commit is popped from the lowest level of walk's internal graph.
 	 *
 	 * @param set
 	 *            the flags to carry onto parents, if set on a descendant.
 	 */
 	public void carry(Collection<RevFlag> set) {
 		for (RevFlag flag : set)
 			carry(flag);
 	}
 
 	/**
 	 * Preserve a RevFlag during all {@code reset} methods.
 	 * <p>
 	 * Calling {@code retainOnReset(flag)} avoids needing to pass the flag
 	 * during each {@code resetRetain()} invocation on this instance.
 	 * <p>
 	 * Clearing flags marked retainOnReset requires disposing of the flag with
 	 * {@code #disposeFlag(RevFlag)} or disposing of the entire RevWalk by
 	 * {@code #dispose()}.
 	 *
 	 * @param flag
 	 *            the flag to retain during all resets.
 	 * @since 3.6
 	 */
 	public final void retainOnReset(RevFlag flag) {
 		if ((freeFlags & flag.mask) != 0)
 			throw new IllegalArgumentException(MessageFormat.format(JGitText.get().flagIsDisposed, flag.name));
 		if (flag.walker != this)
 			throw new IllegalArgumentException(MessageFormat.format(JGitText.get().flagNotFromThis, flag.name));
 		retainOnReset |= flag.mask;
 	}
 
 	/**
 	 * Preserve a set of RevFlags during all {@code reset} methods.
 	 * <p>
 	 * Calling {@code retainOnReset(set)} avoids needing to pass the flags
 	 * during each {@code resetRetain()} invocation on this instance.
 	 * <p>
 	 * Clearing flags marked retainOnReset requires disposing of the flag with
 	 * {@code #disposeFlag(RevFlag)} or disposing of the entire RevWalk by
 	 * {@code #dispose()}.
 	 *
 	 * @param flags
 	 *            the flags to retain during all resets.
 	 * @since 3.6
 	 */
 	public final void retainOnReset(Collection<RevFlag> flags) {
 		for (RevFlag f : flags)
 			retainOnReset(f);
 	}
 
 	/**
 	 * Allow a flag to be recycled for a different use.
 	 * <p>
 	 * Recycled flags always come back as a different Java object instance when
 	 * assigned again by {@link #newFlag(String)}.
 	 * <p>
 	 * If the flag was previously being carried, the carrying request is
 	 * removed. Disposing of a carried flag while a traversal is in progress has
 	 * an undefined behavior.
 	 *
 	 * @param flag
 	 *            the to recycle.
 	 */
 	public void disposeFlag(RevFlag flag) {
 		freeFlag(flag.mask);
 	}
 
 	void freeFlag(int mask) {
 		retainOnReset &= ~mask;
 		if (isNotStarted()) {
 			freeFlags |= mask;
 			carryFlags &= ~mask;
 		} else {
 			delayFreeFlags |= mask;
 		}
 	}
 
 	private void finishDelayedFreeFlags() {
 		if (delayFreeFlags != 0) {
 			freeFlags |= delayFreeFlags;
 			carryFlags &= ~delayFreeFlags;
 			delayFreeFlags = 0;
 		}
 	}
 
 	/**
 	 * Resets internal state and allows this instance to be used again.
 	 * <p>
 	 * Unlike {@link #dispose()} previously acquired RevObject (and RevCommit)
 	 * instances are not invalidated. RevFlag instances are not invalidated, but
 	 * are removed from all RevObjects.
 	 */
 	public final void reset() {
 		reset(0);
 	}
 
 	/**
 	 * Resets internal state and allows this instance to be used again.
 	 * <p>
 	 * Unlike {@link #dispose()} previously acquired RevObject (and RevCommit)
 	 * instances are not invalidated. RevFlag instances are not invalidated, but
 	 * are removed from all RevObjects.
 	 *
 	 * @param retainFlags
 	 *            application flags that should <b>not</b> be cleared from
 	 *            existing commit objects.
 	 */
 	public final void resetRetain(RevFlagSet retainFlags) {
 		reset(retainFlags.mask);
 	}
 
 	/**
 	 * Resets internal state and allows this instance to be used again.
 	 * <p>
 	 * Unlike {@link #dispose()} previously acquired RevObject (and RevCommit)
 	 * instances are not invalidated. RevFlag instances are not invalidated, but
 	 * are removed from all RevObjects.
 	 * <p>
 	 * See {@link #retainOnReset(RevFlag)} for an alternative that does not
 	 * require passing the flags during each reset.
 	 *
 	 * @param retainFlags
 	 *            application flags that should <b>not</b> be cleared from
 	 *            existing commit objects.
 	 */
 	public final void resetRetain(RevFlag... retainFlags) {
 		int mask = 0;
 		for (RevFlag flag : retainFlags)
 			mask |= flag.mask;
 		reset(mask);
 	}
 
 	/**
 	 * Resets internal state and allows this instance to be used again.
 	 * <p>
 	 * Unlike {@link #dispose()} previously acquired RevObject (and RevCommit)
 	 * instances are not invalidated. RevFlag instances are not invalidated, but
 	 * are removed from all RevObjects. The value of {@code firstParent} is
 	 * retained.
 	 *
 	 * @param retainFlags
 	 *            application flags that should <b>not</b> be cleared from
 	 *            existing commit objects.
 	 */
 	protected void reset(int retainFlags) {
 		finishDelayedFreeFlags();
 		retainFlags |= PARSED | retainOnReset;
 		final int clearFlags = ~retainFlags;
 
 		final FIFORevQueueRevQueue.html#FIFORevQueue">FIFORevQueue q = new FIFORevQueue();
 		for (RevCommit c : roots) {
 			if ((c.flags & clearFlags) == 0)
 				continue;
 			c.flags &= retainFlags;
 			c.reset();
 			q.add(c);
 		}
 
 		for (;;) {
 			final RevCommit c = q.next();
 			if (c == null)
 				break;
 			if (c.parents == null)
 				continue;
 			for (RevCommit p : c.parents) {
 				if ((p.flags & clearFlags) == 0)
 					continue;
 				p.flags &= retainFlags;
 				p.reset();
 				q.add(p);
 			}
 		}
 
 		roots.clear();
 		queue = new DateRevQueue(firstParent);
 		pending = new StartGenerator(this);
 	}
 
 	/**
 	 * Dispose all internal state and invalidate all RevObject instances.
 	 * <p>
 	 * All RevObject (and thus RevCommit, etc.) instances previously acquired
 	 * from this RevWalk are invalidated by a dispose call. Applications must
 	 * not retain or use RevObject instances obtained prior to the dispose call.
 	 * All RevFlag instances are also invalidated, and must not be reused.
 	 */
 	public void dispose() {
 		reader.close();
 		freeFlags = APP_FLAGS;
 		delayFreeFlags = 0;
 		retainOnReset = 0;
 		carryFlags = UNINTERESTING;
 		firstParent = false;
 		objects.clear();
 		roots.clear();
 		queue = new DateRevQueue(firstParent);
 		pending = new StartGenerator(this);
 		shallowCommitsInitialized = false;
 	}
 
 	/**
 	 * Like {@link #next()}, but if a checked exception is thrown during the
 	 * walk it is rethrown as a {@link RevWalkException}.
 	 *
 	 * @throws RevWalkException if an {@link IOException} was thrown.
 	 * @return next most recent commit; null if traversal is over.
 	 */
 	@Nullable
 	private RevCommit nextForIterator() {
 		try {
 			return next();
 		} catch (IOException e) {
 			throw new RevWalkException(e);
 		}
 	}
 
 	/**
 	 * {@inheritDoc}
 	 * <p>
 	 * Returns an Iterator over the commits of this walker.
 	 * <p>
 	 * The returned iterator is only useful for one walk. If this RevWalk gets
 	 * reset a new iterator must be obtained to walk over the new results.
 	 * <p>
 	 * Applications must not use both the Iterator and the {@link #next()} API
 	 * at the same time. Pick one API and use that for the entire walk.
 	 * <p>
 	 * If a checked exception is thrown during the walk (see {@link #next()}) it
 	 * is rethrown from the Iterator as a {@link RevWalkException}.
 	 *
 	 * @see RevWalkException
 	 */
 	@Override
 	public Iterator<RevCommit> iterator() {
 		RevCommit first = nextForIterator();
 
 		return new Iterator<RevCommit>() {
 			RevCommit next = first;
 
 			@Override
 			public boolean hasNext() {
 				return next != null;
 			}
 
 			@Override
 			public RevCommit next() {
 				RevCommit r = next;
 				next = nextForIterator();
 				return r;
 			}
 
 			@Override
 			public void remove() {
 				throw new UnsupportedOperationException();
 			}
 		};
 	}
 
 	/**
 	 * Throws an exception if we have started producing output.
 	 */
 	protected void assertNotStarted() {
 		if (isNotStarted())
 			return;
 		throw new IllegalStateException(JGitText.get().outputHasAlreadyBeenStarted);
 	}
 
 	/**
 	 * Throws an exception if any commits have been marked as start.
 	 * <p>
 	 * If {@link #markStart(RevCommit)} has already been called,
 	 * {@link #reset()} can be called to satisfy this condition.
 	 *
 	 * @since 5.5
 	 */
 	protected void assertNoCommitsMarkedStart() {
 		if (roots.isEmpty())
 			return;
 		throw new IllegalStateException(
 				JGitText.get().commitsHaveAlreadyBeenMarkedAsStart);
 	}
 
 	private boolean isNotStarted() {
 		return pending instanceof StartGenerator;
 	}
 
 	/**
 	 * Create and return an {@link org.eclipse.jgit.revwalk.ObjectWalk} using
 	 * the same objects.
 	 * <p>
 	 * Prior to using this method, the caller must reset this RevWalk to clean
 	 * any flags that were used during the last traversal.
 	 * <p>
 	 * The returned ObjectWalk uses the same ObjectReader, internal object pool,
 	 * and free RevFlags. Once the ObjectWalk is created, this RevWalk should
 	 * not be used anymore.
 	 *
 	 * @return a new walk, using the exact same object pool.
 	 */
 	public ObjectWalk toObjectWalkWithSameObjects() {
 		ObjectWalk ow = new ObjectWalk(reader);
 		RevWalk rw = ow;
 		rw.objects = objects;
 		rw.freeFlags = freeFlags;
 		return ow;
 	}
 
 	/**
 	 * Construct a new unparsed commit for the given object.
 	 *
 	 * @param id
 	 *            the object this walker requires a commit reference for.
 	 * @return a new unparsed reference for the object.
 	 */
 	protected RevCommit createCommit(AnyObjectId id) {
 		return new RevCommit(id);
 	}
 
 	void carryFlagsImpl(RevCommit c) {
 		final int carry = c.flags & carryFlags;
 		if (carry != 0)
 			RevCommit.carryFlags(c, carry);
 	}
 
 	/**
 	 * Assume additional commits are shallow (have no parents).
 	 * <p>
 	 * This method is a No-op if the collection is empty.
 	 *
 	 * @param ids
 	 *            commits that should be treated as shallow commits, in addition
 	 *            to any commits already known to be shallow by the repository.
 	 * @since 3.3
 	 */
 	public void assumeShallow(Collection<? extends ObjectId> ids) {
 		for (ObjectId id : ids)
 			lookupCommit(id).parents = RevCommit.NO_PARENTS;
 	}
 
 	/**
 	 * Reads the "shallow" file and applies it by setting the parents of shallow
 	 * commits to an empty array.
 	 * <p>
 	 * There is a sequencing problem if the first commit being parsed is a
 	 * shallow commit, since {@link RevCommit#parseCanonical(RevWalk, byte[])}
 	 * calls this method before its callers add the new commit to the
 	 * {@link RevWalk#objects} map. That means a call from this method to
 	 * {@link #lookupCommit(AnyObjectId)} fails to find that commit and creates
 	 * a new one, which is promptly discarded.
 	 * <p>
 	 * To avoid that, {@link RevCommit#parseCanonical(RevWalk, byte[])} passes
 	 * its commit to this method, so that this method can apply the shallow
 	 * state to it directly and avoid creating the duplicate commit object.
 	 *
 	 * @param rc
 	 *            the initial commit being parsed
 	 * @throws IOException
 	 *             if the shallow commits file can't be read
 	 */
 	void initializeShallowCommits(RevCommit rc) throws IOException {
 		if (shallowCommitsInitialized) {
 			throw new IllegalStateException(
 					JGitText.get().shallowCommitsAlreadyInitialized);
 		}
 
 		shallowCommitsInitialized = true;
 
 		if (reader == null) {
 			return;
 		}
 
 		for (ObjectId id : reader.getShallowCommits()) {
 			if (id.equals(rc.getId())) {
 				rc.parents = RevCommit.NO_PARENTS;
 			} else {
 				lookupCommit(id).parents = RevCommit.NO_PARENTS;
 			}
 		}
 	}
 }