Allow resets of RevWalk while retaining some application flags

[egit/zawir.git] / org.spearce.jgit / src / org / spearce / jgit / revwalk / RevWalk.java

/*
 *  Copyright (C) 2008  Shawn Pearce <spearce@spearce.org>
 *
 *  This library is free software; you can redistribute it and/or
 *  modify it under the terms of the GNU General Public
 *  License, version 2, as published by the Free Software Foundation.
 *
 *  This library is distributed in the hope that it will be useful,
 *  but WITHOUT ANY WARRANTY; without even the implied warranty of
 *  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
 *  General Public License for more details.
 *
 *  You should have received a copy of the GNU General Public
 *  License along with this library; if not, write to the Free Software
 *  Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301
 */
package org.spearce.jgit.revwalk;

import java.io.IOException;
import java.util.ArrayList;
import java.util.Collection;
import java.util.EnumSet;
import java.util.Iterator;

import org.spearce.jgit.errors.IncorrectObjectTypeException;
import org.spearce.jgit.errors.MissingObjectException;
import org.spearce.jgit.errors.RevWalkException;
import org.spearce.jgit.lib.AnyObjectId;
import org.spearce.jgit.lib.Constants;
import org.spearce.jgit.lib.MutableObjectId;
import org.spearce.jgit.lib.ObjectId;
import org.spearce.jgit.lib.ObjectIdSubclassMap;
import org.spearce.jgit.lib.ObjectLoader;
import org.spearce.jgit.lib.Repository;
import org.spearce.jgit.lib.WindowCursor;
import org.spearce.jgit.revwalk.filter.RevFilter;
import org.spearce.jgit.treewalk.filter.TreeFilter;

/**
 * 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 Repository} are permitted,
 * even from concurrent threads. Equality of {@link RevCommit}s from two
 * different RevWalk instances is never true, even if their {@link 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> {
        /**
         * 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;

        /** Number of flag bits we keep internal for our own use. See above flags. */
        static final int RESERVED_FLAGS = 6;

        private static final int APP_FLAGS = -1 & ~((1 << RESERVED_FLAGS) - 1);

        final Repository db;

        final WindowCursor curs;

        final MutableObjectId idBuffer;

        private final ObjectIdSubclassMap<RevObject> objects;

        private int freeFlags = APP_FLAGS;

        int carryFlags;

        private final ArrayList<RevCommit> roots;

        AbstractRevQueue queue;

        Generator pending;

        private final EnumSet<RevSort> sorting;

        private RevFilter filter;

        private TreeFilter treeFilter;

        /**
         * Create a new revision walker for a given repository.
         * 
         * @param repo
         *            the repository the walker will obtain data from.
         */
        public RevWalk(final Repository repo) {
                db = repo;
                curs = new WindowCursor();
                idBuffer = new MutableObjectId();
                objects = new ObjectIdSubclassMap<RevObject>();
                roots = new ArrayList<RevCommit>();
                queue = new FIFORevQueue();
                pending = new StartGenerator(this);
                sorting = EnumSet.of(RevSort.NONE);
                filter = RevFilter.ALL;
                treeFilter = TreeFilter.ALL;
        }

        /**
         * Get the repository this walker loads objects from.
         * 
         * @return the repository this walker was created to read.
         */
        public Repository getRepository() {
                return db;
        }

        /**
         * 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 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 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 IOException
         *             a pack file or loose object could not be read.
         */
        public void markStart(final RevCommit c) throws MissingObjectException,
                        IncorrectObjectTypeException, IOException {
                if ((c.flags & SEEN) != 0)
                        return;
                if ((c.flags & PARSED) == 0)
                        c.parse(this);
                c.flags |= SEEN;
                roots.add(c);
                queue.add(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 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 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 IOException
         *             a pack file or loose object could not be read.
         */
        public void markUninteresting(final RevCommit c)
                        throws MissingObjectException, IncorrectObjectTypeException,
                        IOException {
                c.flags |= UNINTERESTING;
                c.carryFlags(UNINTERESTING);
                markStart(c);
        }

        /**
         * Pop the next most recent commit.
         * 
         * @return next most recent commit; null if traversal is over.
         * @throws MissingObjectException
         *             one or 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 IncorrectObjectTypeException
         *             one or or more of the next commit's parents are not actually
         *             commit objects.
         * @throws 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 RevSort#NONE}.
         */
        public EnumSet<RevSort> getRevSort() {
                return sorting.clone();
        }

        /**
         * 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 RevSort#TOPO} must
         * take precedence over {@link 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(final RevSort s, final boolean use) {
                assertNotStarted();
                if (use)
                        sorting.add(s);
                else
                        sorting.remove(s);

                if (sorting.size() > 1)
                        sorting.remove(RevSort.NONE);
                else if (sorting.size() == 0)
                        sorting.add(RevSort.NONE);
        }

        /**
         * Get the currently configured commit filter.
         * 
         * @return the current filter. Never null as a filter is always needed.
         */
        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 RevFilter#clone()} to create
         * a unique filter tree for this RevWalk instance.
         * 
         * @param newFilter
         *            the new filter. If null the special {@link RevFilter#ALL}
         *            filter will be used instead, as it matches every commit.
         * @see org.spearce.jgit.revwalk.filter.AndRevFilter
         * @see org.spearce.jgit.revwalk.filter.OrRevFilter
         */
        public void setRevFilter(final 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 TreeFilter#ALL} is returned.
         */
        public TreeFilter getTreeFilter() {
                return treeFilter;
        }

        /**
         * Set the tree filter used to simplify commits by modified paths.
         * <p>
         * If null or {@link TreeFilter#ALL} the path limiter is removed. Commits
         * will not be simplified.
         * <p>
         * If non-null and not {@link TreeFilter#ALL} then the tree filter will be
         * installed and commits will have their ancestry simplified to hide commits
         * that do not contain tree entries matched by the filter.
         * <p>
         * Usually callers should be inserting a filter graph including
         * {@link TreeFilter#ANY_DIFF} along with one or more
         * {@link org.spearce.jgit.treewalk.filter.PathFilter} instances.
         * 
         * @param newFilter
         *            new filter. If null the special {@link TreeFilter#ALL} filter
         *            will be used instead, as it matches everything.
         * @see org.spearce.jgit.treewalk.filter.PathFilter
         */
        public void setTreeFilter(final TreeFilter newFilter) {
                assertNotStarted();
                treeFilter = newFilter != null ? newFilter : TreeFilter.ALL;
        }

        /**
         * 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.
         */
        public RevTree lookupTree(final 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.
         * 
         * @param id
         *            name of the commit object.
         * @return reference to the commit object. Never null.
         */
        public RevCommit lookupCommit(final AnyObjectId id) {
                RevCommit c = (RevCommit) objects.get(id);
                if (c == null) {
                        c = createCommit(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.
         */
        public RevObject lookupAny(final AnyObjectId id, final 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("invalid git type: " + type);
                        }
                        objects.add(r);
                }
                return r;
        }

        /**
         * 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 MissingObjectException
         *             the supplied commit does not exist.
         * @throws IncorrectObjectTypeException
         *             the supplied id is not a commit or an annotated tag.
         * @throws IOException
         *             a pack file or loose object could not be read.
         */
        public RevCommit parseCommit(final AnyObjectId id)
                        throws MissingObjectException, IncorrectObjectTypeException,
                        IOException {
                RevObject c = parseAny(id);
                while (c instanceof RevTag) {
                        c = ((RevTag) c).getObject();
                        parse(c);
                }
                return (RevCommit) c;
        }

        /**
         * Locate a reference to any object and immediately parse its content.
         * <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 MissingObjectException
         *             the supplied does not exist.
         * @throws IOException
         *             a pack file or loose object could not be read.
         */
        public RevObject parseAny(final AnyObjectId id)
                        throws MissingObjectException, IOException {
                RevObject r = objects.get(id);
                if (r == null) {
                        final ObjectLoader ldr = db.openObject(id);
                        if (ldr == null)
                                throw new MissingObjectException(id.toObjectId(), "unknown");
                        final byte[] data = ldr.getCachedBytes();
                        final int type = ldr.getType();
                        switch (type) {
                        case Constants.OBJ_COMMIT: {
                                final RevCommit c = createCommit(ldr.getId());
                                c.parseCanonical(this, data);
                                r = c;
                                break;
                        }
                        case Constants.OBJ_TREE: {
                                r = new RevTree(ldr.getId());
                                break;
                        }
                        case Constants.OBJ_BLOB: {
                                r = new RevBlob(ldr.getId());
                                break;
                        }
                        case Constants.OBJ_TAG: {
                                final RevTag t = new RevTag(ldr.getId());
                                t.parseCanonical(this, data);
                                r = t;
                                break;
                        }
                        default:
                                throw new IllegalArgumentException("Bad object type: " + type);
                        }
                        objects.add(r);
                } else if ((r.flags & PARSED) == 0)
                        r.parse(this);
                return r;
        }

        /**
         * Ensure the object's content has 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 MissingObjectException
         *             the supplied does not exist.
         * @throws IOException
         *             a pack file or loose object could not be read.
         */
        public void parse(final RevObject obj) throws MissingObjectException,
                        IOException {
                if ((obj.flags & PARSED) != 0)
                        return;
                obj.parse(this);
        }

        /**
         * 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 IllegalArgumentException
         *             too many flags have been reserved on this revision walker.
         */
        public RevFlag newFlag(final String name) {
                if (freeFlags == 0)
                        throw new IllegalArgumentException(32 - RESERVED_FLAGS
                                        + " flags already created.");
                final int m = Integer.lowestOneBit(freeFlags);
                freeFlags &= ~m;
                return new RevFlag(this, name, 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(final RevFlag flag) {
                if ((freeFlags & flag.mask) != 0)
                        throw new IllegalArgumentException(flag.name + " is disposed.");
                if (flag.walker != this)
                        throw new IllegalArgumentException(flag.name + " not from this.");
                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(final Collection<RevFlag> set) {
                for (final RevFlag flag : set)
                        carry(flag);
        }

        /**
         * 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(final RevFlag flag) {
                freeFlags |= flag.mask;
                carryFlags &= ~flag.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.
         */
        public 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 void resetRetain(final 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.
         * 
         * @param retainFlags
         *            application flags that should <b>not</b> be cleared from
         *            existing commit objects.
         */
        public void resetRetain(final RevFlag... retainFlags) {
                int mask = 0;
                for (final RevFlag flag : retainFlags)
                        mask |= flag.mask;
                reset(mask);
        }

        private void reset(int retainFlags) {
                retainFlags |= PARSED;

                final FIFORevQueue q = new FIFORevQueue();
                for (final RevCommit c : roots) {
                        if ((c.flags & SEEN) == 0)
                                continue;
                        c.flags &= retainFlags;
                        c.reset();
                        q.add(c);
                }

                for (;;) {
                        final RevCommit c = q.next();
                        if (c == null)
                                break;
                        for (final RevCommit p : c.parents) {
                                if ((p.flags & SEEN) == 0)
                                        continue;
                                p.flags &= retainFlags;
                                q.add(p);
                        }
                }

                curs.release();
                roots.clear();
                queue = new FIFORevQueue();
                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() {
                freeFlags = APP_FLAGS;
                carryFlags = 0;
                objects.clear();
                curs.release();
                roots.clear();
                queue = new FIFORevQueue();
                pending = new StartGenerator(this);
        }

        /**
         * 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}.
         * 
         * @return an iterator over this walker's commits.
         * @see RevWalkException
         */
        public Iterator<RevCommit> iterator() {
                final RevCommit first;
                try {
                        first = RevWalk.this.next();
                } catch (MissingObjectException e) {
                        throw new RevWalkException(e);
                } catch (IncorrectObjectTypeException e) {
                        throw new RevWalkException(e);
                } catch (IOException e) {
                        throw new RevWalkException(e);
                }

                return new Iterator<RevCommit>() {
                        RevCommit next = first;

                        public boolean hasNext() {
                                return next != null;
                        }

                        public RevCommit next() {
                                try {
                                        final RevCommit r = next;
                                        next = RevWalk.this.next();
                                        return r;
                                } catch (MissingObjectException e) {
                                        throw new RevWalkException(e);
                                } catch (IncorrectObjectTypeException e) {
                                        throw new RevWalkException(e);
                                } catch (IOException e) {
                                        throw new RevWalkException(e);
                                }
                        }

                        public void remove() {
                                throw new UnsupportedOperationException();
                        }
                };
        }

        /** Throws an exception if we have started producing output. */
        protected void assertNotStarted() {
                if (pending instanceof StartGenerator)
                        return;
                throw new IllegalStateException("Output has already been started.");
        }

        /**
         * 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(final AnyObjectId id) {
                return new RevCommit(id);
        }
}