/*
    * Copyright (C) 2010, Chris Aniszczyk <caniszczyk@gmail.com>
    * Copyright (C) 2011, Matthias Sohn <matthias.sohn@sap.com>
    * and other copyright owners as documented in the project's IP log.
    *
    * This program and the accompanying materials are made available
    * under the terms of the Eclipse Distribution License v1.0 which
    * accompanies this distribution, is reproduced below, and is
    * available at http://www.eclipse.org/org/documents/edl-v10.php
   *
   * All rights reserved.
   *
   * Redistribution and use in source and binary forms, with or
   * without modification, are permitted provided that the following
   * conditions are met:
   *
   * - Redistributions of source code must retain the above copyright
   *   notice, this list of conditions and the following disclaimer.
   *
   * - Redistributions in binary form must reproduce the above
   *   copyright notice, this list of conditions and the following
   *   disclaimer in the documentation and/or other materials provided
   *   with the distribution.
   *
   * - Neither the name of the Eclipse Foundation, Inc. nor the
   *   names of its contributors may be used to endorse or promote
   *   products derived from this software without specific prior
   *   written permission.
   *
   * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND
   * CONTRIBUTORS "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES,
   * INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
   * OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
   * ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR
   * CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
   * SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
   * NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;
   * LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
   * CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT,
   * STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
   * ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF
   * ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
   */
  package org.eclipse.jgit.api;
  
  import java.io.IOException;
  import java.text.MessageFormat;
  import java.util.ArrayList;
  import java.util.EnumSet;
  import java.util.HashSet;
  import java.util.LinkedList;
  import java.util.List;
  import java.util.Set;
  
  import org.eclipse.jgit.api.CheckoutResult.Status;
  import org.eclipse.jgit.api.errors.CheckoutConflictException;
  import org.eclipse.jgit.api.errors.GitAPIException;
  import org.eclipse.jgit.api.errors.InvalidRefNameException;
  import org.eclipse.jgit.api.errors.JGitInternalException;
  import org.eclipse.jgit.api.errors.RefAlreadyExistsException;
  import org.eclipse.jgit.api.errors.RefNotFoundException;
  import org.eclipse.jgit.dircache.DirCache;
  import org.eclipse.jgit.dircache.DirCacheCheckout;
  import org.eclipse.jgit.dircache.DirCacheCheckout.CheckoutMetadata;
  import org.eclipse.jgit.dircache.DirCacheEditor;
  import org.eclipse.jgit.dircache.DirCacheEditor.PathEdit;
  import org.eclipse.jgit.dircache.DirCacheEntry;
  import org.eclipse.jgit.dircache.DirCacheIterator;
  import org.eclipse.jgit.errors.AmbiguousObjectException;
  import org.eclipse.jgit.errors.UnmergedPathException;
  import org.eclipse.jgit.events.WorkingTreeModifiedEvent;
  import org.eclipse.jgit.internal.JGitText;
  import org.eclipse.jgit.lib.AnyObjectId;
  import org.eclipse.jgit.lib.Constants;
  import org.eclipse.jgit.lib.CoreConfig.EolStreamType;
  import org.eclipse.jgit.lib.FileMode;
  import org.eclipse.jgit.lib.ObjectId;
  import org.eclipse.jgit.lib.ObjectReader;
  import org.eclipse.jgit.lib.Ref;
  import org.eclipse.jgit.lib.RefUpdate;
  import org.eclipse.jgit.lib.RefUpdate.Result;
  import org.eclipse.jgit.lib.Repository;
  import org.eclipse.jgit.revwalk.RevCommit;
  import org.eclipse.jgit.revwalk.RevTree;
  import org.eclipse.jgit.revwalk.RevWalk;
  import org.eclipse.jgit.treewalk.TreeWalk;
  import org.eclipse.jgit.treewalk.filter.PathFilterGroup;
  
  /**
   * Checkout a branch to the working tree.
   * <p>
   * Examples (<code>git</code> is a {@link Git} instance):
   * <p>
   * Check out an existing branch:
   *
   * <pre>
   * git.checkout().setName(&quot;feature&quot;).call();
   * </pre>
   * <p>
  * Check out paths from the index:
  *
  * <pre>
  * git.checkout().addPath(&quot;file1.txt&quot;).addPath(&quot;file2.txt&quot;).call();
  * </pre>
  * <p>
  * Check out a path from a commit:
  *
  * <pre>
  * git.checkout().setStartPoint(&quot;HEAD&circ;&quot;).addPath(&quot;file1.txt&quot;).call();
  * </pre>
  *
  * <p>
  * Create a new branch and check it out:
  *
  * <pre>
  * git.checkout().setCreateBranch(true).setName(&quot;newbranch&quot;).call();
  * </pre>
  * <p>
  * Create a new tracking branch for a remote branch and check it out:
  *
  * <pre>
  * git.checkout().setCreateBranch(true).setName(&quot;stable&quot;)
  * 		.setUpstreamMode(SetupUpstreamMode.SET_UPSTREAM)
  * 		.setStartPoint(&quot;origin/stable&quot;).call();
  * </pre>
  *
  * @see <a
  *      href="http://www.kernel.org/pub/software/scm/git/docs/git-checkout.html"
  *      >Git documentation about Checkout</a>
  */
 public class CheckoutCommand extends GitCommand<Ref> {
 
 	/**
 	 * Stage to check out, see {@link CheckoutCommand#setStage(Stage)}.
 	 */
 	public static enum Stage {
 		/**
 		 * Base stage (#1)
 		 */
 		BASE(DirCacheEntry.STAGE_1),
 
 		/**
 		 * Ours stage (#2)
 		 */
 		OURS(DirCacheEntry.STAGE_2),
 
 		/**
 		 * Theirs stage (#3)
 		 */
 		THEIRS(DirCacheEntry.STAGE_3);
 
 		private final int number;
 
 		private Stage(int number) {
 			this.number = number;
 		}
 	}
 
 	private String name;
 
 	private boolean force = false;
 
 	private boolean createBranch = false;
 
 	private boolean orphan = false;
 
 	private CreateBranchCommand.SetupUpstreamMode upstreamMode;
 
 	private String startPoint = null;
 
 	private RevCommit startCommit;
 
 	private Stage checkoutStage = null;
 
 	private CheckoutResult status;
 
 	private List<String> paths;
 
 	private boolean checkoutAllPaths;
 
 	private Set<String> actuallyModifiedPaths;
 
 	/**
 	 * @param repo
 	 */
 	protected CheckoutCommand(Repository repo) {
 		super(repo);
 		this.paths = new LinkedList<>();
 	}
 
 	/**
 	 * @throws RefAlreadyExistsException
 	 *             when trying to create (without force) a branch with a name
 	 *             that already exists
 	 * @throws RefNotFoundException
 	 *             if the start point or branch can not be found
 	 * @throws InvalidRefNameException
 	 *             if the provided name is <code>null</code> or otherwise
 	 *             invalid
 	 * @throws CheckoutConflictException
 	 *             if the checkout results in a conflict
 	 * @return the newly created branch
 	 */
 	@Override
 	public Ref call() throws GitAPIException, RefAlreadyExistsException,
 			RefNotFoundException, InvalidRefNameException,
 			CheckoutConflictException {
 		checkCallable();
 		try {
 			processOptions();
 			if (checkoutAllPaths || !paths.isEmpty()) {
 				checkoutPaths();
 				status = new CheckoutResult(Status.OK, paths);
 				setCallable(false);
 				return null;
 			}
 
 			if (createBranch) {
 				try (Git git = new Git(repo)) {
 					CreateBranchCommand command = git.branchCreate();
 					command.setName(name);
 					if (startCommit != null)
 						command.setStartPoint(startCommit);
 					else
 						command.setStartPoint(startPoint);
 					if (upstreamMode != null)
 						command.setUpstreamMode(upstreamMode);
 					command.call();
 				}
 			}
 
 			Ref headRef = repo.exactRef(Constants.HEAD);
 			if (headRef == null) {
 				// TODO Git CLI supports checkout from unborn branch, we should
 				// also allow this
 				throw new UnsupportedOperationException(
 						JGitText.get().cannotCheckoutFromUnbornBranch);
 			}
 			String shortHeadRef = getShortBranchName(headRef);
 			String refLogMessage = "checkout: moving from " + shortHeadRef; //$NON-NLS-1$
 			ObjectId branch;
 			if (orphan) {
 				if (startPoint == null && startCommit == null) {
 					Result r = repo.updateRef(Constants.HEAD).link(
 							getBranchName());
 					if (!EnumSet.of(Result.NEW, Result.FORCED).contains(r))
 						throw new JGitInternalException(MessageFormat.format(
 								JGitText.get().checkoutUnexpectedResult,
 								r.name()));
 					this.status = CheckoutResult.NOT_TRIED_RESULT;
 					return repo.exactRef(Constants.HEAD);
 				}
 				branch = getStartPointObjectId();
 			} else {
 				branch = repo.resolve(name);
 				if (branch == null)
 					throw new RefNotFoundException(MessageFormat.format(
 							JGitText.get().refNotResolved, name));
 			}
 
 			RevCommit headCommit = null;
 			RevCommit newCommit = null;
 			try (RevWalk revWalk = new RevWalk(repo)) {
 				AnyObjectId headId = headRef.getObjectId();
 				headCommit = headId == null ? null
 						: revWalk.parseCommit(headId);
 				newCommit = revWalk.parseCommit(branch);
 			}
 			RevTree headTree = headCommit == null ? null : headCommit.getTree();
 			DirCacheCheckout dco;
 			DirCache dc = repo.lockDirCache();
 			try {
 				dco = new DirCacheCheckout(repo, headTree, dc,
 						newCommit.getTree());
 				dco.setFailOnConflict(true);
 				try {
 					dco.checkout();
 				} catch (org.eclipse.jgit.errors.CheckoutConflictException e) {
 					status = new CheckoutResult(Status.CONFLICTS,
 							dco.getConflicts());
 					throw new CheckoutConflictException(dco.getConflicts(), e);
 				}
 			} finally {
 				dc.unlock();
 			}
 			Ref ref = repo.findRef(name);
 			if (ref != null && !ref.getName().startsWith(Constants.R_HEADS))
 				ref = null;
 			String toName = Repository.shortenRefName(name);
 			RefUpdate refUpdate = repo.updateRef(Constants.HEAD, ref == null);
 			refUpdate.setForceUpdate(force);
 			refUpdate.setRefLogMessage(refLogMessage + " to " + toName, false); //$NON-NLS-1$
 			Result updateResult;
 			if (ref != null)
 				updateResult = refUpdate.link(ref.getName());
 			else if (orphan) {
 				updateResult = refUpdate.link(getBranchName());
 				ref = repo.exactRef(Constants.HEAD);
 			} else {
 				refUpdate.setNewObjectId(newCommit);
 				updateResult = refUpdate.forceUpdate();
 			}
 
 			setCallable(false);
 
 			boolean ok = false;
 			switch (updateResult) {
 			case NEW:
 				ok = true;
 				break;
 			case NO_CHANGE:
 			case FAST_FORWARD:
 			case FORCED:
 				ok = true;
 				break;
 			default:
 				break;
 			}
 
 			if (!ok)
 				throw new JGitInternalException(MessageFormat.format(JGitText
 						.get().checkoutUnexpectedResult, updateResult.name()));
 
 
 			if (!dco.getToBeDeleted().isEmpty()) {
 				status = new CheckoutResult(Status.NONDELETED,
 						dco.getToBeDeleted(),
 						new ArrayList<>(dco.getUpdated().keySet()),
 						dco.getRemoved());
 			} else
 				status = new CheckoutResult(new ArrayList<>(dco
 						.getUpdated().keySet()), dco.getRemoved());
 
 			return ref;
 		} catch (IOException ioe) {
 			throw new JGitInternalException(ioe.getMessage(), ioe);
 		} finally {
 			if (status == null)
 				status = CheckoutResult.ERROR_RESULT;
 		}
 	}
 
 	private String getShortBranchName(Ref headRef) {
 		if (headRef.isSymbolic()) {
 			return Repository.shortenRefName(headRef.getTarget().getName());
 		}
 		// Detached HEAD. Every non-symbolic ref in the ref database has an
 		// object id, so this cannot be null.
 		ObjectId id = headRef.getObjectId();
 		if (id == null) {
 			throw new NullPointerException();
 		}
 		return id.getName();
 	}
 
 	/**
 	 * Add a single slash-separated path to the list of paths to check out. To
 	 * check out all paths, use {@link #setAllPaths(boolean)}.
 	 * <p>
 	 * If this option is set, neither the {@link #setCreateBranch(boolean)} nor
 	 * {@link #setName(String)} option is considered. In other words, these
 	 * options are exclusive.
 	 *
 	 * @param path
 	 *            path to update in the working tree and index (with
 	 *            <code>/</code> as separator)
 	 * @return {@code this}
 	 */
 	public CheckoutCommand addPath(String path) {
 		checkCallable();
 		this.paths.add(path);
 		return this;
 	}
 
 	/**
 	 * Add multiple slash-separated paths to the list of paths to check out. To
 	 * check out all paths, use {@link #setAllPaths(boolean)}.
 	 * <p>
 	 * If this option is set, neither the {@link #setCreateBranch(boolean)} nor
 	 * {@link #setName(String)} option is considered. In other words, these
 	 * options are exclusive.
 	 *
 	 * @param p
 	 *            paths to update in the working tree and index (with
 	 *            <code>/</code> as separator)
 	 * @return {@code this}
 	 * @since 4.6
 	 */
 	public CheckoutCommand addPaths(List<String> p) {
 		checkCallable();
 		this.paths.addAll(p);
 		return this;
 	}
 
 	/**
 	 * Set whether to checkout all paths.
 	 * <p>
 	 * This options should be used when you want to do a path checkout on the
 	 * entire repository and so calling {@link #addPath(String)} is not possible
 	 * since empty paths are not allowed.
 	 * <p>
 	 * If this option is set, neither the {@link #setCreateBranch(boolean)} nor
 	 * {@link #setName(String)} option is considered. In other words, these
 	 * options are exclusive.
 	 *
 	 * @param all
 	 *            <code>true</code> to checkout all paths, <code>false</code>
 	 *            otherwise
 	 * @return {@code this}
 	 * @since 2.0
 	 */
 	public CheckoutCommand setAllPaths(boolean all) {
 		checkoutAllPaths = all;
 		return this;
 	}
 
 	/**
 	 * Checkout paths into index and working directory, firing a
 	 * {@link WorkingTreeModifiedEvent} if the working tree was modified.
 	 *
 	 * @return this instance
 	 * @throws IOException
 	 * @throws RefNotFoundException
 	 */
 	protected CheckoutCommand checkoutPaths() throws IOException,
 			RefNotFoundException {
 		actuallyModifiedPaths = new HashSet<>();
 		DirCache dc = repo.lockDirCache();
 		try (RevWalk revWalk = new RevWalk(repo);
 				TreeWalk treeWalk = new TreeWalk(repo,
 						revWalk.getObjectReader())) {
 			treeWalk.setRecursive(true);
 			if (!checkoutAllPaths)
 				treeWalk.setFilter(PathFilterGroup.createFromStrings(paths));
 			if (isCheckoutIndex())
 				checkoutPathsFromIndex(treeWalk, dc);
 			else {
 				RevCommit commit = revWalk.parseCommit(getStartPointObjectId());
 				checkoutPathsFromCommit(treeWalk, dc, commit);
 			}
 		} finally {
 			try {
 				dc.unlock();
 			} finally {
 				WorkingTreeModifiedEvent event = new WorkingTreeModifiedEvent(
 						actuallyModifiedPaths, null);
 				actuallyModifiedPaths = null;
 				if (!event.isEmpty()) {
 					repo.fireEvent(event);
 				}
 			}
 		}
 		return this;
 	}
 
 	private void checkoutPathsFromIndex(TreeWalk treeWalk, DirCache dc)
 			throws IOException {
 		DirCacheIterator dci = new DirCacheIterator(dc);
 		treeWalk.addTree(dci);
 
 		String previousPath = null;
 
 		final ObjectReader r = treeWalk.getObjectReader();
 		DirCacheEditor editor = dc.editor();
 		while (treeWalk.next()) {
 			String path = treeWalk.getPathString();
 			// Only add one edit per path
 			if (path.equals(previousPath))
 				continue;
 
 			final EolStreamType eolStreamType = treeWalk.getEolStreamType();
 			final String filterCommand = treeWalk
 					.getFilterCommand(Constants.ATTR_FILTER_TYPE_SMUDGE);
 			editor.add(new PathEdit(path) {
 				@Override
 				public void apply(DirCacheEntry ent) {
 					int stage = ent.getStage();
 					if (stage > DirCacheEntry.STAGE_0) {
 						if (checkoutStage != null) {
 							if (stage == checkoutStage.number) {
 								checkoutPath(ent, r, new CheckoutMetadata(
 										eolStreamType, filterCommand));
 								actuallyModifiedPaths.add(path);
 							}
 						} else {
 							UnmergedPathException e = new UnmergedPathException(
 									ent);
 							throw new JGitInternalException(e.getMessage(), e);
 						}
 					} else {
 						checkoutPath(ent, r, new CheckoutMetadata(eolStreamType,
 								filterCommand));
 						actuallyModifiedPaths.add(path);
 					}
 				}
 			});
 
 			previousPath = path;
 		}
 		editor.commit();
 	}
 
 	private void checkoutPathsFromCommit(TreeWalk treeWalk, DirCache dc,
 			RevCommit commit) throws IOException {
 		treeWalk.addTree(commit.getTree());
 		final ObjectReader r = treeWalk.getObjectReader();
 		DirCacheEditor editor = dc.editor();
 		while (treeWalk.next()) {
 			final ObjectId blobId = treeWalk.getObjectId(0);
 			final FileMode mode = treeWalk.getFileMode(0);
 			final EolStreamType eolStreamType = treeWalk.getEolStreamType();
 			final String filterCommand = treeWalk
 					.getFilterCommand(Constants.ATTR_FILTER_TYPE_SMUDGE);
 			final String path = treeWalk.getPathString();
 			editor.add(new PathEdit(path) {
 				@Override
 				public void apply(DirCacheEntry ent) {
 					ent.setObjectId(blobId);
 					ent.setFileMode(mode);
 					checkoutPath(ent, r,
 							new CheckoutMetadata(eolStreamType, filterCommand));
 					actuallyModifiedPaths.add(path);
 				}
 			});
 		}
 		editor.commit();
 	}
 
 	private void checkoutPath(DirCacheEntry entry, ObjectReader reader,
 			CheckoutMetadata checkoutMetadata) {
 		try {
 			DirCacheCheckout.checkoutEntry(repo, entry, reader, true,
 					checkoutMetadata);
 		} catch (IOException e) {
 			throw new JGitInternalException(MessageFormat.format(
 					JGitText.get().checkoutConflictWithFile,
 					entry.getPathString()), e);
 		}
 	}
 
 	private boolean isCheckoutIndex() {
 		return startCommit == null && startPoint == null;
 	}
 
 	private ObjectId getStartPointObjectId() throws AmbiguousObjectException,
 			RefNotFoundException, IOException {
 		if (startCommit != null)
 			return startCommit.getId();
 
 		String startPointOrHead = (startPoint != null) ? startPoint
 				: Constants.HEAD;
 		ObjectId result = repo.resolve(startPointOrHead);
 		if (result == null)
 			throw new RefNotFoundException(MessageFormat.format(
 					JGitText.get().refNotResolved, startPointOrHead));
 		return result;
 	}
 
 	private void processOptions() throws InvalidRefNameException,
 			RefAlreadyExistsException, IOException {
 		if (((!checkoutAllPaths && paths.isEmpty()) || orphan)
 				&& (name == null || !Repository
 						.isValidRefName(Constants.R_HEADS + name)))
 			throw new InvalidRefNameException(MessageFormat.format(JGitText
 					.get().branchNameInvalid, name == null ? "<null>" : name)); //$NON-NLS-1$
 
 		if (orphan) {
 			Ref refToCheck = repo.exactRef(getBranchName());
 			if (refToCheck != null)
 				throw new RefAlreadyExistsException(MessageFormat.format(
 						JGitText.get().refAlreadyExists, name));
 		}
 	}
 
 	private String getBranchName() {
 		if (name.startsWith(Constants.R_REFS))
 			return name;
 
 		return Constants.R_HEADS + name;
 	}
 
 	/**
 	 * Specify the name of the branch or commit to check out, or the new branch
 	 * name.
 	 * <p>
 	 * When only checking out paths and not switching branches, use
 	 * {@link #setStartPoint(String)} or {@link #setStartPoint(RevCommit)} to
 	 * specify from which branch or commit to check out files.
 	 * <p>
 	 * When {@link #setCreateBranch(boolean)} is set to <code>true</code>, use
 	 * this method to set the name of the new branch to create and
 	 * {@link #setStartPoint(String)} or {@link #setStartPoint(RevCommit)} to
 	 * specify the start point of the branch.
 	 *
 	 * @param name
 	 *            the name of the branch or commit
 	 * @return this instance
 	 */
 	public CheckoutCommand setName(String name) {
 		checkCallable();
 		this.name = name;
 		return this;
 	}
 
 	/**
 	 * Specify whether to create a new branch.
 	 * <p>
 	 * If <code>true</code> is used, the name of the new branch must be set
 	 * using {@link #setName(String)}. The commit at which to start the new
 	 * branch can be set using {@link #setStartPoint(String)} or
 	 * {@link #setStartPoint(RevCommit)}; if not specified, HEAD is used. Also
 	 * see {@link #setUpstreamMode} for setting up branch tracking.
 	 *
 	 * @param createBranch
 	 *            if <code>true</code> a branch will be created as part of the
 	 *            checkout and set to the specified start point
 	 * @return this instance
 	 */
 	public CheckoutCommand setCreateBranch(boolean createBranch) {
 		checkCallable();
 		this.createBranch = createBranch;
 		return this;
 	}
 
 	/**
 	 * Specify whether to create a new orphan branch.
 	 * <p>
 	 * If <code>true</code> is used, the name of the new orphan branch must be
 	 * set using {@link #setName(String)}. The commit at which to start the new
 	 * orphan branch can be set using {@link #setStartPoint(String)} or
 	 * {@link #setStartPoint(RevCommit)}; if not specified, HEAD is used.
 	 *
 	 * @param orphan
 	 *            if <code>true</code> a orphan branch will be created as part
 	 *            of the checkout to the specified start point
 	 * @return this instance
 	 * @since 3.3
 	 */
 	public CheckoutCommand setOrphan(boolean orphan) {
 		checkCallable();
 		this.orphan = orphan;
 		return this;
 	}
 
 	/**
 	 * Specify to force the ref update in case of a branch switch.
 	 *
 	 * @param force
 	 *            if <code>true</code> and the branch with the given name
 	 *            already exists, the start-point of an existing branch will be
 	 *            set to a new start-point; if false, the existing branch will
 	 *            not be changed
 	 * @return this instance
 	 */
 	public CheckoutCommand setForce(boolean force) {
 		checkCallable();
 		this.force = force;
 		return this;
 	}
 
 	/**
 	 * Set the name of the commit that should be checked out.
 	 * <p>
 	 * When checking out files and this is not specified or <code>null</code>,
 	 * the index is used.
 	 * <p>
 	 * When creating a new branch, this will be used as the start point. If not
 	 * specified or <code>null</code>, the current HEAD is used.
 	 *
 	 * @param startPoint
 	 *            commit name to check out
 	 * @return this instance
 	 */
 	public CheckoutCommand setStartPoint(String startPoint) {
 		checkCallable();
 		this.startPoint = startPoint;
 		this.startCommit = null;
 		checkOptions();
 		return this;
 	}
 
 	/**
 	 * Set the commit that should be checked out.
 	 * <p>
 	 * When creating a new branch, this will be used as the start point. If not
 	 * specified or <code>null</code>, the current HEAD is used.
 	 * <p>
 	 * When checking out files and this is not specified or <code>null</code>,
 	 * the index is used.
 	 *
 	 * @param startCommit
 	 *            commit to check out
 	 * @return this instance
 	 */
 	public CheckoutCommand setStartPoint(RevCommit startCommit) {
 		checkCallable();
 		this.startCommit = startCommit;
 		this.startPoint = null;
 		checkOptions();
 		return this;
 	}
 
 	/**
 	 * When creating a branch with {@link #setCreateBranch(boolean)}, this can
 	 * be used to configure branch tracking.
 	 *
 	 * @param mode
 	 *            corresponds to the --track/--no-track options; may be
 	 *            <code>null</code>
 	 * @return this instance
 	 */
 	public CheckoutCommand setUpstreamMode(
 			CreateBranchCommand.SetupUpstreamMode mode) {
 		checkCallable();
 		this.upstreamMode = mode;
 		return this;
 	}
 
 	/**
 	 * When checking out the index, check out the specified stage (ours or
 	 * theirs) for unmerged paths.
 	 * <p>
 	 * This can not be used when checking out a branch, only when checking out
 	 * the index.
 	 *
 	 * @param stage
 	 *            the stage to check out
 	 * @return this
 	 */
 	public CheckoutCommand setStage(Stage stage) {
 		checkCallable();
 		this.checkoutStage = stage;
 		checkOptions();
 		return this;
 	}
 
 	/**
 	 * @return the result, never <code>null</code>
 	 */
 	public CheckoutResult getResult() {
 		if (status == null)
 			return CheckoutResult.NOT_TRIED_RESULT;
 		return status;
 	}
 
 	private void checkOptions() {
 		if (checkoutStage != null && !isCheckoutIndex())
 			throw new IllegalStateException(
 					JGitText.get().cannotCheckoutOursSwitchBranch);
 	}
 }