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    * Copyright (C) 2008, Google Inc.
    * and other copyright owners as documented in the project's IP log.
    *
    * This program and the accompanying materials are made available
    * under the terms of the Eclipse Distribution License v1.0 which
    * accompanies this distribution, is reproduced below, and is
    * available at http://www.eclipse.org/org/documents/edl-v10.php
    *
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  package org.eclipse.jgit.treewalk;
  
  import java.io.IOException;
  
  import org.eclipse.jgit.dircache.DirCacheBuilder;
  import org.eclipse.jgit.errors.CorruptObjectException;
  import org.eclipse.jgit.lib.FileMode;
  import org.eclipse.jgit.lib.ObjectReader;
  import org.eclipse.jgit.lib.Repository;
  
  /**
   * Specialized TreeWalk to detect directory-file (D/F) name conflicts.
   * <p>
   * Due to the way a Git tree is organized the standard {@link TreeWalk} won't
   * easily find a D/F conflict when merging two or more trees together. In the
   * standard TreeWalk the file will be returned first, and then much later the
   * directory will be returned. This makes it impossible for the application to
   * efficiently detect and handle the conflict.
   * <p>
   * Using this walk implementation causes the directory to report earlier than
   * usual, at the same time as the non-directory entry. This permits the
   * application to handle the D/F conflict in a single step. The directory is
   * returned only once, so it does not get returned later in the iteration.
   * <p>
   * When a D/F conflict is detected {@link TreeWalk#isSubtree()} will return true
   * and {@link TreeWalk#enterSubtree()} will recurse into the subtree, no matter
   * which iterator originally supplied the subtree.
   * <p>
   * Because conflicted directories report early, using this walk implementation
   * to populate a {@link DirCacheBuilder} may cause the automatic resorting to
   * run and fix the entry ordering.
   * <p>
   * This walk implementation requires more CPU to implement a look-ahead and a
   * look-behind to merge a D/F pair together, or to skip a previously reported
   * directory. In typical Git repositories the look-ahead cost is 0 and the
   * look-behind doesn't trigger, as users tend not to create trees which contain
   * both "foo" as a directory and "foo.c" as a file.
   * <p>
   * In the worst-case however several thousand look-ahead steps per walk step may
   * be necessary, making the overhead quite significant. Since this worst-case
   * should never happen this walk implementation has made the time/space tradeoff
   * in favor of more-time/less-space, as that better suits the typical case.
   */
  public class NameConflictTreeWalk extends TreeWalk {
  	private static final int TREE_MODE = FileMode.TREE.getBits();
  
  	private boolean fastMinHasMatch;
  
  	private AbstractTreeIterator dfConflict;
  
  	/**
  	 * Create a new tree walker for a given repository.
  	 *
  	 * @param repo
  	 *            the repository the walker will obtain data from.
  	 */
 	public NameConflictTreeWalk(final Repository repo) {
 		super(repo);
 	}
 
 	/**
 	 * Create a new tree walker for a given repository.
 	 *
 	 * @param repo
 	 *            the repository the walker will obtain data from.
 	 * @param or
 	 *            the reader the walker will obtain tree data from.
 	 * @since 4.3
 	 */
 	public NameConflictTreeWalk(Repository repo, final ObjectReader or) {
 		super(repo, or);
 	}
 
 	/**
 	 * Create a new tree walker for a given repository.
 	 *
 	 * @param or
 	 *            the reader the walker will obtain tree data from.
 	 */
 	public NameConflictTreeWalk(final ObjectReader or) {
 		super(or);
 	}
 
 	@Override
 	AbstractTreeIterator min() throws CorruptObjectException {
 		for (;;) {
 			final AbstractTreeIterator minRef = fastMin();
 			if (fastMinHasMatch)
 				return minRef;
 
 			if (isTree(minRef)) {
 				if (skipEntry(minRef)) {
 					for (final AbstractTreeIterator t : trees) {
 						if (t.matches == minRef) {
 							t.next(1);
 							t.matches = null;
 						}
 					}
 					continue;
 				}
 				return minRef;
 			}
 
 			return combineDF(minRef);
 		}
 	}
 
 	private AbstractTreeIterator fastMin() {
 		fastMinHasMatch = true;
 
 		int i = 0;
 		AbstractTreeIterator minRef = trees[i];
 		while (minRef.eof() && ++i < trees.length)
 			minRef = trees[i];
 		if (minRef.eof())
 			return minRef;
 
 		boolean hasConflict = false;
 		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) {
 				if (fastMinHasMatch && isTree(minRef) && !isTree(t)
 						&& nameEqual(minRef, t)) {
 					// We used to be at a tree, but now we are at a file
 					// with the same name. Allow the file to match the
 					// tree anyway.
 					//
 					t.matches = minRef;
 					hasConflict = true;
 				} else {
 					fastMinHasMatch = false;
 					t.matches = t;
 					minRef = t;
 				}
 			} else if (cmp == 0) {
 				// Exact name/mode match is best.
 				//
 				t.matches = minRef;
 			} else if (fastMinHasMatch && isTree(t) && !isTree(minRef)
 					&& nameEqual(t, minRef)) {
 				// The minimum is a file (non-tree) but the next entry
 				// of this iterator is a tree whose name matches our file.
 				// This is a classic D/F conflict and commonly occurs like
 				// this, with no gaps in between the file and directory.
 				//
 				// Use the tree as the minimum instead (see combineDF).
 				//
 
 				for (int k = 0; k < i; k++) {
 					final AbstractTreeIterator p = trees[k];
 					if (p.matches == minRef)
 						p.matches = t;
 				}
 				t.matches = t;
 				minRef = t;
 				hasConflict = true;
 			} else
 				fastMinHasMatch = false;
 		}
 
 		if (hasConflict && fastMinHasMatch && dfConflict == null)
 			dfConflict = minRef;
 		return minRef;
 	}
 
 	private static boolean nameEqual(final AbstractTreeIterator a,
 			final AbstractTreeIterator b) {
 		return a.pathCompare(b, TREE_MODE) == 0;
 	}
 
 	private static boolean isTree(final AbstractTreeIterator p) {
 		return FileMode.TREE.equals(p.mode);
 	}
 
 	private boolean skipEntry(final AbstractTreeIterator minRef)
 			throws CorruptObjectException {
 		// A tree D/F may have been handled earlier. We need to
 		// not report this path if it has already been reported.
 		//
 		for (final AbstractTreeIterator t : trees) {
 			if (t.matches == minRef || t.first())
 				continue;
 
 			int stepsBack = 0;
 			for (;;) {
 				stepsBack++;
 				t.back(1);
 
 				final int cmp = t.pathCompare(minRef, 0);
 				if (cmp == 0) {
 					// We have already seen this "$path" before. Skip it.
 					//
 					t.next(stepsBack);
 					return true;
 				} else if (cmp < 0 || t.first()) {
 					// We cannot find "$path" in t; it will never appear.
 					//
 					t.next(stepsBack);
 					break;
 				}
 			}
 		}
 
 		// We have never seen the current path before.
 		//
 		return false;
 	}
 
 	private AbstractTreeIterator combineDF(final AbstractTreeIterator minRef)
 			throws CorruptObjectException {
 		// Look for a possible D/F conflict forward in the tree(s)
 		// as there may be a "$path/" which matches "$path". Make
 		// such entries match this entry.
 		//
 		AbstractTreeIterator treeMatch = null;
 		for (final AbstractTreeIterator t : trees) {
 			if (t.matches == minRef || t.eof())
 				continue;
 
 			for (;;) {
 				final int cmp = t.pathCompare(minRef, TREE_MODE);
 				if (cmp < 0) {
 					// The "$path/" may still appear later.
 					//
 					t.matchShift++;
 					t.next(1);
 					if (t.eof()) {
 						t.back(t.matchShift);
 						t.matchShift = 0;
 						break;
 					}
 				} else if (cmp == 0) {
 					// We have a conflict match here.
 					//
 					t.matches = minRef;
 					treeMatch = t;
 					break;
 				} else {
 					// A conflict match is not possible.
 					//
 					if (t.matchShift != 0) {
 						t.back(t.matchShift);
 						t.matchShift = 0;
 					}
 					break;
 				}
 			}
 		}
 
 		if (treeMatch != null) {
 			// If we do have a conflict use one of the directory
 			// matching iterators instead of the file iterator.
 			// This way isSubtree is true and isRecursive works.
 			//
 			for (final AbstractTreeIterator t : trees)
 				if (t.matches == minRef)
 					t.matches = treeMatch;
 
 			if (dfConflict == null)
 				dfConflict = treeMatch;
 
 			return treeMatch;
 		}
 
 		return minRef;
 	}
 
 	@Override
 	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) {
 				if (t.matchShift == 0)
 					t.next(1);
 				else {
 					t.back(t.matchShift);
 					t.matchShift = 0;
 				}
 				t.matches = null;
 			}
 		}
 
 		if (ch == dfConflict)
 			dfConflict = null;
 	}
 
 	@Override
 	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) {
 				if (t.matchShift == 0)
 					t.skip();
 				else {
 					t.back(t.matchShift);
 					t.matchShift = 0;
 				}
 				t.matches = null;
 			}
 		}
 
 		if (ch == dfConflict)
 			dfConflict = null;
 	}
 
 	void stopWalk() throws IOException {
 		if (!needsStopWalk()) {
 			return;
 		}
 
 		// Name conflicts make aborting early difficult. Multiple paths may
 		// exist between the file and directory versions of a name. To ensure
 		// the directory version is skipped over (as it was previously visited
 		// during the file version step) requires popping up the stack and
 		// finishing out each subtree that the walker dove into. Siblings in
 		// parents do not need to be recursed into, bounding the cost.
 		for (;;) {
 			AbstractTreeIterator t = min();
 			if (t.eof()) {
 				if (depth > 0) {
 					exitSubtree();
 					popEntriesEqual();
 					continue;
 				}
 				return;
 			}
 			currentHead = t;
 			skipEntriesEqual();
 		}
 	}
 
 	private boolean needsStopWalk() {
 		for (AbstractTreeIterator t : trees) {
 			if (t.needsStopWalk()) {
 				return true;
 			}
 		}
 		return false;
 	}
 
 	/**
 	 * True if the current entry is covered by a directory/file conflict.
 	 *
 	 * This means that for some prefix of the current entry's path, this walk
 	 * has detected a directory/file conflict. Also true if the current entry
 	 * itself is a directory/file conflict.
 	 *
 	 * Example: If this TreeWalk points to foo/bar/a.txt and this method returns
 	 * true then you know that either for path foo or for path foo/bar files and
 	 * folders were detected.
 	 *
 	 * @return <code>true</code> if the current entry is covered by a
 	 *         directory/file conflict, <code>false</code> otherwise
 	 */
 	public boolean isDirectoryFileConflict() {
 		return dfConflict != null;
 	}
 }