Fix TreeWalk.idEqual when both trees are missing the path

[egit/graphgui.git] / org.spearce.jgit / src / org / spearce / jgit / treewalk / TreeWalk.java

/*
 * Copyright (C) 2008, Shawn O. Pearce <spearce@spearce.org>
 *
 * 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 Git Development Community 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.spearce.jgit.treewalk;

import java.io.IOException;
import java.util.Collections;

import org.spearce.jgit.errors.CorruptObjectException;
import org.spearce.jgit.errors.IncorrectObjectTypeException;
import org.spearce.jgit.errors.MissingObjectException;
import org.spearce.jgit.errors.StopWalkException;
import org.spearce.jgit.lib.AnyObjectId;
import org.spearce.jgit.lib.Constants;
import org.spearce.jgit.lib.FileMode;
import org.spearce.jgit.lib.MutableObjectId;
import org.spearce.jgit.lib.ObjectId;
import org.spearce.jgit.lib.Repository;
import org.spearce.jgit.lib.WindowCursor;
import org.spearce.jgit.revwalk.RevTree;
import org.spearce.jgit.treewalk.filter.PathFilterGroup;
import org.spearce.jgit.treewalk.filter.TreeFilter;
import org.spearce.jgit.util.RawParseUtils;

/**
 * Walks one or more {@link AbstractTreeIterator}s in parallel.
 * <p>
 * This class can perform n-way differences across as many trees as necessary.
 * <p>
 * A TreeWalk instance can only be used once to generate results. Running a
 * second time requires creating a new TreeWalk instance, or invoking
 * {@link #reset()} and adding new trees before starting again. Resetting an
 * existing instance may be faster for some applications as some internal
 * buffers may be recycled.
 * <p>
 * TreeWalk instances are not thread-safe. Applications must either restrict
 * usage of a TreeWalk instance to a single thread, or implement their own
 * synchronization at a higher level.
 * <p>
 * Multiple simultaneous TreeWalk instances per {@link Repository} are
 * permitted, even from concurrent threads.
 */
public class TreeWalk {
        /**
         * Open a tree walk and filter to exactly one path.
         * <p>
         * The returned tree walk is already positioned on the requested path, so
         * the caller should not need to invoke {@link #next()} unless they are
         * looking for a possible directory/file name conflict.
         * 
         * @param db
         *            repository to read tree object data from.
         * @param path
         *            single path to advance the tree walk instance into.
         * @param trees
         *            one or more trees to walk through.
         * @return a new tree walk configured for exactly this one path; null if no
         *         path was found in any of the trees.
         * @throws IOException
         *             reading a pack file or loose object failed.
         * @throws CorruptObjectException
         *             an tree object could not be read as its data stream did not
         *             appear to be a tree, or could not be inflated.
         * @throws IncorrectObjectTypeException
         *             an object we expected to be a tree was not a tree.
         * @throws MissingObjectException
         *             a tree object was not found.
         */
        public static TreeWalk forPath(final Repository db, final String path,
                        final AnyObjectId[] trees) throws MissingObjectException,
                        IncorrectObjectTypeException, CorruptObjectException, IOException {
                final TreeWalk r = new TreeWalk(db);
                r.setFilter(PathFilterGroup.createFromStrings(Collections
                                .singleton(path)));
                r.setRecursive(r.getFilter().shouldBeRecursive());
                r.reset(trees);
                return r.next() ? r : null;
        }

        /**
         * Open a tree walk and filter to exactly one path.
         * <p>
         * The returned tree walk is already positioned on the requested path, so
         * the caller should not need to invoke {@link #next()} unless they are
         * looking for a possible directory/file name conflict.
         * 
         * @param db
         *            repository to read tree object data from.
         * @param path
         *            single path to advance the tree walk instance into.
         * @param tree
         *            the single tree to walk through.
         * @return a new tree walk configured for exactly this one path; null if no
         *         path was found in any of the trees.
         * @throws IOException
         *             reading a pack file or loose object failed.
         * @throws CorruptObjectException
         *             an tree object could not be read as its data stream did not
         *             appear to be a tree, or could not be inflated.
         * @throws IncorrectObjectTypeException
         *             an object we expected to be a tree was not a tree.
         * @throws MissingObjectException
         *             a tree object was not found.
         */
        public static TreeWalk forPath(final Repository db, final String path,
                        final RevTree tree) throws MissingObjectException,
                        IncorrectObjectTypeException, CorruptObjectException, IOException {
                return forPath(db, path, new ObjectId[] { tree });
        }

        private final Repository db;

        private final MutableObjectId idBuffer = new MutableObjectId();

        private final WindowCursor curs = new WindowCursor();

        private TreeFilter filter;

        AbstractTreeIterator[] trees;

        private boolean recursive;

        private boolean postOrderTraversal;

        private int depth;

        private boolean advance;

        private boolean postChildren;

        AbstractTreeIterator currentHead;

        /**
         * Create a new tree walker for a given repository.
         * 
         * @param repo
         *            the repository the walker will obtain data from.
         */
        public TreeWalk(final Repository repo) {
                db = repo;
                filter = TreeFilter.ALL;
                trees = new AbstractTreeIterator[] { new EmptyTreeIterator() };
        }

        /**
         * Get the repository this tree walker is reading from.
         * 
         * @return the repository configured when the walker was created.
         */
        public Repository getRepository() {
                return db;
        }

        /**
         * Get the currently configured filter.
         * 
         * @return the current filter. Never null as a filter is always needed.
         */
        public TreeFilter getFilter() {
                return filter;
        }

        /**
         * Set the tree entry filter for this walker.
         * <p>
         * Multiple filters may be combined by constructing an arbitrary tree of
         * <code>AndTreeFilter</code> or <code>OrTreeFilter</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
         * TreeWalk instances. Every TreeWalk 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 TreeFilter#clone()}
         * to create a unique filter tree for this TreeWalk instance.
         * 
         * @param newFilter
         *            the new filter. If null the special {@link TreeFilter#ALL}
         *            filter will be used instead, as it matches every entry.
         * @see org.spearce.jgit.treewalk.filter.AndTreeFilter
         * @see org.spearce.jgit.treewalk.filter.OrTreeFilter
         */
        public void setFilter(final TreeFilter newFilter) {
                filter = newFilter != null ? newFilter : TreeFilter.ALL;
        }

        /**
         * Is this walker automatically entering into subtrees?
         * <p>
         * If the walker is recursive then the caller will not see a subtree node
         * and instead will only receive file nodes in all relevant subtrees.
         * 
         * @return true if automatically entering subtrees is enabled.
         */
        public boolean isRecursive() {
                return recursive;
        }

        /**
         * Set the walker to enter (or not enter) subtrees automatically.
         * <p>
         * If recursive mode is enabled the walker will hide subtree nodes from the
         * calling application and will produce only file level nodes. If a tree
         * (directory) is deleted then all of the file level nodes will appear to be
         * deleted, recursively, through as many levels as necessary to account for
         * all entries.
         * 
         * @param b
         *            true to skip subtree nodes and only obtain files nodes.
         */
        public void setRecursive(final boolean b) {
                recursive = b;
        }

        /**
         * Does this walker return a tree entry after it exits the subtree?
         * <p>
         * If post order traversal is enabled then the walker will return a subtree
         * after it has returned the last entry within that subtree. This may cause
         * a subtree to be seen by the application twice if {@link #isRecursive()}
         * is false, as the application will see it once, call
         * {@link #enterSubtree()}, and then see it again as it leaves the subtree.
         * <p>
         * If an application does not enable {@link #isRecursive()} and it does not
         * call {@link #enterSubtree()} then the tree is returned only once as none
         * of the children were processed.
         *
         * @return true if subtrees are returned after entries within the subtree.
         */
        public boolean isPostOrderTraversal() {
                return postOrderTraversal;
        }

        /**
         * Set the walker to return trees after their children.
         *
         * @param b
         *            true to get trees after their children.
         * @see #isPostOrderTraversal()
         */
        public void setPostOrderTraversal(final boolean b) {
                postOrderTraversal = b;
        }

        /** Reset this walker so new tree iterators can be added to it. */
        public void reset() {
                trees = new AbstractTreeIterator[0];
                advance = false;
                depth = 0;
        }

        /**
         * Reset this walker to run over a single existing tree.
         *
         * @param id
         *            the tree we need to parse. The walker will execute over this
         *            single tree if the reset is successful.
         * @throws MissingObjectException
         *             the given tree object does not exist in this repository.
         * @throws IncorrectObjectTypeException
         *             the given object id does not denote a tree, but instead names
         *             some other non-tree type of object. Note that commits are not
         *             trees, even if they are sometimes called a "tree-ish".
         * @throws CorruptObjectException
         *             the object claimed to be a tree, but its contents did not
         *             appear to be a tree. The repository may have data corruption.
         * @throws IOException
         *             a loose object or pack file could not be read.
         */
        public void reset(final AnyObjectId id) throws MissingObjectException,
                        IncorrectObjectTypeException, CorruptObjectException, IOException {
                if (trees.length == 1) {
                        AbstractTreeIterator o = trees[0];
                        while (o.parent != null)
                                o = o.parent;
                        if (o instanceof CanonicalTreeParser) {
                                o.matches = null;
                                o.matchShift = 0;
                                ((CanonicalTreeParser) o).reset(db, id, curs);
                                trees[0] = o;
                        } else {
                                trees[0] = parserFor(id);
                        }
                } else {
                        trees = new AbstractTreeIterator[] { parserFor(id) };
                }

                advance = false;
                depth = 0;
        }

        /**
         * Reset this walker to run over a set of existing trees.
         * 
         * @param ids
         *            the trees we need to parse. The walker will execute over this
         *            many parallel trees if the reset is successful.
         * @throws MissingObjectException
         *             the given tree object does not exist in this repository.
         * @throws IncorrectObjectTypeException
         *             the given object id does not denote a tree, but instead names
         *             some other non-tree type of object. Note that commits are not
         *             trees, even if they are sometimes called a "tree-ish".
         * @throws CorruptObjectException
         *             the object claimed to be a tree, but its contents did not
         *             appear to be a tree. The repository may have data corruption.
         * @throws IOException
         *             a loose object or pack file could not be read.
         */
        public void reset(final AnyObjectId[] ids) throws MissingObjectException,
                        IncorrectObjectTypeException, CorruptObjectException, IOException {
                final int oldLen = trees.length;
                final int newLen = ids.length;
                final AbstractTreeIterator[] r = newLen == oldLen ? trees
                                : new AbstractTreeIterator[newLen];
                for (int i = 0; i < newLen; i++) {
                        AbstractTreeIterator o;

                        if (i < oldLen) {
                                o = trees[i];
                                while (o.parent != null)
                                        o = o.parent;
                                if (o instanceof CanonicalTreeParser) {
                                        o.matches = null;
                                        o.matchShift = 0;
                                        ((CanonicalTreeParser) o).reset(db, ids[i], curs);
                                        r[i] = o;
                                        continue;
                                }
                        }

                        o = parserFor(ids[i]);
                        r[i] = o;
                }

                trees = r;
                advance = false;
                depth = 0;
        }

        /**
         * Add an already existing tree object for walking.
         * <p>
         * The position of this tree is returned to the caller, in case the caller
         * has lost track of the order they added the trees into the walker.
         * 
         * @param id
         *            identity of the tree object the caller wants walked.
         * @return position of this tree within the walker.
         * @throws MissingObjectException
         *             the given tree object does not exist in this repository.
         * @throws IncorrectObjectTypeException
         *             the given object id does not denote a tree, but instead names
         *             some other non-tree type of object. Note that commits are not
         *             trees, even if they are sometimes called a "tree-ish".
         * @throws CorruptObjectException
         *             the object claimed to be a tree, but its contents did not
         *             appear to be a tree. The repository may have data corruption.
         * @throws IOException
         *             a loose object or pack file could not be read.
         */
        public int addTree(final ObjectId id) throws MissingObjectException,
                        IncorrectObjectTypeException, CorruptObjectException, IOException {
                return addTree(parserFor(id));
        }

        /**
         * Add an already created tree iterator for walking.
         * <p>
         * The position of this tree is returned to the caller, in case the caller
         * has lost track of the order they added the trees into the walker.
         * 
         * @param p
         *            an iterator to walk over. The iterator should be new, with no
         *            parent, and should still be positioned before the first entry.
         * @return position of this tree within the walker.
         * @throws CorruptObjectException
         *             the iterator was unable to obtain its first entry, due to
         *             possible data corruption within the backing data store.
         */
        public int addTree(final AbstractTreeIterator p)
                        throws CorruptObjectException {
                final int n = trees.length;
                final AbstractTreeIterator[] newTrees = new AbstractTreeIterator[n + 1];

                System.arraycopy(trees, 0, newTrees, 0, n);
                newTrees[n] = p;
                p.matches = null;
                p.matchShift = 0;

                trees = newTrees;
                return n;
        }

        /**
         * Get the number of trees known to this walker.
         * 
         * @return the total number of trees this walker is iterating over.
         */
        public int getTreeCount() {
                return trees.length;
        }

        /**
         * Advance this walker to the next relevant entry.
         * 
         * @return true if there is an entry available; false if all entries have
         *         been walked and the walk of this set of tree iterators is over.
         * @throws MissingObjectException
         *             {@link #isRecursive()} was enabled, a subtree was found, but
         *             the subtree object does not exist in this repository. The
         *             repository may be missing objects.
         * @throws IncorrectObjectTypeException
         *             {@link #isRecursive()} was enabled, a subtree was found, and
         *             the subtree id does not denote a tree, but instead names some
         *             other non-tree type of object. The repository may have data
         *             corruption.
         * @throws CorruptObjectException
         *             the contents of a tree did not appear to be a tree. The
         *             repository may have data corruption.
         * @throws IOException
         *             a loose object or pack file could not be read.
         */
        public boolean next() throws MissingObjectException,
                        IncorrectObjectTypeException, CorruptObjectException, IOException {
                try {
                        if (advance) {
                                advance = false;
                                postChildren = false;
                                popEntriesEqual();
                        }

                        for (;;) {
                                final AbstractTreeIterator t = min();
                                if (t.eof()) {
                                        if (depth > 0) {
                                                exitSubtree();
                                                if (postOrderTraversal) {
                                                        advance = true;
                                                        postChildren = true;
                                                        return true;
                                                }
                                                popEntriesEqual();
                                                continue;
                                        }
                                        return false;
                                }

                                currentHead = t;
                                if (!filter.include(this)) {
                                        skipEntriesEqual();
                                        continue;
                                }

                                if (recursive && FileMode.TREE.equals(t.mode)) {
                                        enterSubtree();
                                        continue;
                                }

                                advance = true;
                                return true;
                        }
                } catch (StopWalkException stop) {
                        for (final AbstractTreeIterator t : trees)
                                t.stopWalk();
                        return false;
                }
        }

        /**
         * Obtain the tree iterator for the current entry.
         * <p>
         * Entering into (or exiting out of) a subtree causes the current tree
         * iterator instance to be changed for the nth tree. This allows the tree
         * iterators to manage only one list of items, with the diving handled by
         * recursive trees.
         *
         * @param <T>
         *            type of the tree iterator expected by the caller.
         * @param nth
         *            tree to obtain the current iterator of.
         * @param clazz
         *            type of the tree iterator expected by the caller.
         * @return r the current iterator of the requested type; null if the tree
         *         has no entry to match the current path.
         */
        public <T extends AbstractTreeIterator> T getTree(final int nth,
                        final Class<T> clazz) {
                final AbstractTreeIterator t = trees[nth];
                return t.matches == currentHead ? (T) t : null;
        }

        /**
         * Obtain the raw {@link FileMode} bits for the current entry.
         * <p>
         * Every added tree supplies mode bits, even if the tree does not contain
         * the current entry. In the latter case {@link FileMode#MISSING}'s mode
         * bits (0) are returned.
         * 
         * @param nth
         *            tree to obtain the mode bits from.
         * @return mode bits for the current entry of the nth tree.
         * @see FileMode#fromBits(int)
         */
        public int getRawMode(final int nth) {
                final AbstractTreeIterator t = trees[nth];
                return t.matches == currentHead ? t.mode : 0;
        }

        /**
         * Obtain the {@link FileMode} for the current entry.
         * <p>
         * Every added tree supplies a mode, even if the tree does not contain the
         * current entry. In the latter case {@link FileMode#MISSING} is returned.
         * 
         * @param nth
         *            tree to obtain the mode from.
         * @return mode for the current entry of the nth tree.
         */
        public FileMode getFileMode(final int nth) {
                return FileMode.fromBits(getRawMode(nth));
        }

        /**
         * Obtain the ObjectId for the current entry.
         * <p>
         * Using this method to compare ObjectId values between trees of this walker
         * is very inefficient. Applications should try to use
         * {@link #idEqual(int, int)} or {@link #getObjectId(MutableObjectId, int)}
         * whenever possible.
         * <p>
         * Every tree supplies an object id, even if the tree does not contain the
         * current entry. In the latter case {@link ObjectId#zeroId()} is returned.
         * 
         * @param nth
         *            tree to obtain the object identifier from.
         * @return object identifier for the current tree entry.
         * @see #getObjectId(MutableObjectId, int)
         * @see #idEqual(int, int)
         */
        public ObjectId getObjectId(final int nth) {
                final AbstractTreeIterator t = trees[nth];
                return t.matches == currentHead ? t.getEntryObjectId() : ObjectId
                                .zeroId();
        }

        /**
         * Obtain the ObjectId for the current entry.
         * <p>
         * Every tree supplies an object id, even if the tree does not contain the
         * current entry. In the latter case {@link ObjectId#zeroId()} is supplied.
         * <p>
         * Applications should try to use {@link #idEqual(int, int)} when possible
         * as it avoids conversion overheads.
         *
         * @param out
         *            buffer to copy the object id into.
         * @param nth
         *            tree to obtain the object identifier from.
         * @see #idEqual(int, int)
         */
        public void getObjectId(final MutableObjectId out, final int nth) {
                final AbstractTreeIterator t = trees[nth];
                if (t.matches == currentHead)
                        t.getEntryObjectId(out);
                else
                        out.clear();
        }

        /**
         * Compare two tree's current ObjectId values for equality.
         * 
         * @param nthA
         *            first tree to compare the object id from.
         * @param nthB
         *            second tree to compare the object id from.
         * @return result of
         *         <code>getObjectId(nthA).equals(getObjectId(nthB))</code>.
         * @see #getObjectId(int)
         */
        public boolean idEqual(final int nthA, final int nthB) {
                final AbstractTreeIterator ch = currentHead;
                final AbstractTreeIterator a = trees[nthA];
                final AbstractTreeIterator b = trees[nthB];
                if (a.matches == ch && b.matches == ch)
                        return a.idEqual(b);
                if (a.matches != ch && b.matches != ch) {
                        // If neither tree matches the current path node then neither
                        // tree has this entry. In such case the ObjectId is zero(),
                        // and zero() is always equal to zero().
                        //
                        return true;
                }
                return false;
        }

        /**
         * Get the current entry's name within its parent tree.
         * <p>
         * This method is not very efficient and is primarily meant for debugging
         * and final output generation. Applications should try to avoid calling it,
         * and if invoked do so only once per interesting entry, where the name is
         * absolutely required for correct function.
         *
         * @return name of the current entry within the parent tree (or directory).
         *         The name never includes a '/'.
         */
        public String getNameString() {
                final AbstractTreeIterator t = currentHead;
                final int off = t.pathOffset;
                final int end = t.pathLen;
                return RawParseUtils.decode(Constants.CHARSET, t.path, off, end);
        }

        /**
         * Get the current entry's complete path.
         * <p>
         * This method is not very efficient and is primarily meant for debugging
         * and final output generation. Applications should try to avoid calling it,
         * and if invoked do so only once per interesting entry, where the name is
         * absolutely required for correct function.
         * 
         * @return complete path of the current entry, from the root of the
         *         repository. If the current entry is in a subtree there will be at
         *         least one '/' in the returned string.
         */
        public String getPathString() {
                return pathOf(currentHead);
        }

        /**
         * Test if the supplied path matches the current entry's path.
         * <p>
         * This method tests that the supplied path is exactly equal to the current
         * entry, or is one of its parent directories. It is faster to use this
         * method then to use {@link #getPathString()} to first create a String
         * object, then test <code>startsWith</code> or some other type of string
         * match function.
         * 
         * @param p
         *            path buffer to test. Callers should ensure the path does not
         *            end with '/' prior to invocation.
         * @param pLen
         *            number of bytes from <code>buf</code> to test.
         * @return < 0 if p is before the current path; 0 if p matches the current
         *         path; 1 if the current path is past p and p will never match
         *         again on this tree walk.
         */
        public int isPathPrefix(final byte[] p, final int pLen) {
                final AbstractTreeIterator t = currentHead;
                final byte[] c = t.path;
                final int cLen = t.pathLen;
                int ci;

                for (ci = 0; ci < cLen && ci < pLen; ci++) {
                        final int c_value = (c[ci] & 0xff) - (p[ci] & 0xff);
                        if (c_value != 0)
                                return c_value;
                }

                if (ci < cLen) {
                        // Ran out of pattern but we still had current data.
                        // If c[ci] == '/' then pattern matches the subtree.
                        // Otherwise we cannot be certain so we return -1.
                        //
                        return c[ci] == '/' ? 0 : -1;
                }

                if (ci < pLen) {
                        // Ran out of current, but we still have pattern data.
                        // If p[ci] == '/' then pattern matches this subtree,
                        // otherwise we cannot be certain so we return -1.
                        //
                        return p[ci] == '/' ? 0 : -1;
                }

                // Both strings are identical.
                //
                return 0;
        }

        /**
         * Is the current entry a subtree?
         * <p>
         * This method is faster then testing the raw mode bits of all trees to see
         * if any of them are a subtree. If at least one is a subtree then this
         * method will return true.
         * 
         * @return true if {@link #enterSubtree()} will work on the current node.
         */
        public boolean isSubtree() {
                return FileMode.TREE.equals(currentHead.mode);
        }

        /**
         * Is the current entry a subtree returned after its children?
         *
         * @return true if the current node is a tree that has been returned after
         *         its children were already processed.
         * @see #isPostOrderTraversal()
         */
        public boolean isPostChildren() {
                return postChildren && isSubtree();
        }

        /**
         * Enter into the current subtree.
         * <p>
         * If the current entry is a subtree this method arranges for its children
         * to be returned before the next sibling following the subtree is returned.
         * 
         * @throws MissingObjectException
         *             a subtree was found, but the subtree object does not exist in
         *             this repository. The repository may be missing objects.
         * @throws IncorrectObjectTypeException
         *             a subtree was found, and the subtree id does not denote a
         *             tree, but instead names some other non-tree type of object.
         *             The repository may have data corruption.
         * @throws CorruptObjectException
         *             the contents of a tree did not appear to be a tree. The
         *             repository may have data corruption.
         * @throws IOException
         *             a loose object or pack file could not be read.
         */
        public void enterSubtree() throws MissingObjectException,
                        IncorrectObjectTypeException, CorruptObjectException, IOException {
                final AbstractTreeIterator ch = currentHead;
                final AbstractTreeIterator[] tmp = new AbstractTreeIterator[trees.length];
                for (int i = 0; i < trees.length; i++) {
                        final AbstractTreeIterator t = trees[i];
                        final AbstractTreeIterator n;
                        if (t.matches == ch && !t.eof() && FileMode.TREE.equals(t.mode))
                                n = t.createSubtreeIterator(db, idBuffer, curs);
                        else
                                n = new EmptyTreeIterator(t);
                        tmp[i] = n;
                }
                depth++;
                advance = false;
                System.arraycopy(tmp, 0, trees, 0, trees.length);
        }

        AbstractTreeIterator min() throws CorruptObjectException {
                int i = 0;
                AbstractTreeIterator minRef = trees[i];
                while (minRef.eof() && ++i < trees.length)
                        minRef = trees[i];
                if (minRef.eof())
                        return minRef;

                minRef.matches = minRef;
                while (++i < trees.length) {
                        final AbstractTreeIterator t = trees[i];
                        if (t.eof())
                                continue;
                        final int cmp = t.pathCompare(minRef);
                        if (cmp < 0) {
                                t.matches = t;
                                minRef = t;
                        } else if (cmp == 0) {
                                t.matches = minRef;
                        }
                }

                return minRef;
        }

        void popEntriesEqual() throws CorruptObjectException {
                final AbstractTreeIterator ch = currentHead;
                for (int i = 0; i < trees.length; i++) {
                        final AbstractTreeIterator t = trees[i];
                        if (t.matches == ch) {
                                t.next(1);
                                t.matches = null;
                        }
                }
        }

        void skipEntriesEqual() throws CorruptObjectException {
                final AbstractTreeIterator ch = currentHead;
                for (int i = 0; i < trees.length; i++) {
                        final AbstractTreeIterator t = trees[i];
                        if (t.matches == ch) {
                                t.skip();
                                t.matches = null;
                        }
                }
        }

        private void exitSubtree() {
                depth--;
                for (int i = 0; i < trees.length; i++)
                        trees[i] = trees[i].parent;

                AbstractTreeIterator minRef = null;
                for (final AbstractTreeIterator t : trees) {
                        if (t.matches != t)
                                continue;
                        if (minRef == null || t.pathCompare(minRef) < 0)
                                minRef = t;
                }
                currentHead = minRef;
        }

        private CanonicalTreeParser parserFor(final AnyObjectId id)
                        throws IncorrectObjectTypeException, IOException {
                final CanonicalTreeParser p = new CanonicalTreeParser();
                p.reset(db, id, curs);
                return p;
        }

        static String pathOf(final AbstractTreeIterator t) {
                return RawParseUtils.decode(Constants.CHARSET, t.path, 0, t.pathLen);
        }
}