#include "cache.h"
#include "builtin.h"
#include "parse-options.h"
#include "refs.h"
#include "commit.h"
#include "tree.h"
#include "tree-walk.h"
#include "cache-tree.h"
#include "unpack-trees.h"
#include "dir.h"
#include "run-command.h"
#include "merge-recursive.h"
#include "branch.h"
#include "diff.h"
#include "revision.h"
#include "remote.h"
#include "blob.h"
#include "xdiff-interface.h"
#include "ll-merge.h"
#include "resolve-undo.h"

static const char * const checkout_usage[] = {
	"git checkout [options] <branch>",
	"git checkout [options] [<branch>] -- <file>...",
	NULL,
};

struct checkout_opts {
	int quiet;
	int merge;
	int force;
	int writeout_stage;
	int writeout_error;

	const char *new_branch;
	int new_branch_log;
	enum branch_track track;
};

static int post_checkout_hook(struct commit *old, struct commit *new,
			      int changed)
{
	return run_hook(NULL, "post-checkout",
			sha1_to_hex(old ? old->object.sha1 : null_sha1),
			sha1_to_hex(new ? new->object.sha1 : null_sha1),
			changed ? "1" : "0", NULL);
	/* "new" can be NULL when checking out from the index before
	   a commit exists. */

}

static int update_some(const unsigned char *sha1, const char *base, int baselen,
		const char *pathname, unsigned mode, int stage, void *context)
{
	int len;
	struct cache_entry *ce;

	if (S_ISDIR(mode))
		return READ_TREE_RECURSIVE;

	len = baselen + strlen(pathname);
	ce = xcalloc(1, cache_entry_size(len));
	hashcpy(ce->sha1, sha1);
	memcpy(ce->name, base, baselen);
	memcpy(ce->name + baselen, pathname, len - baselen);
	ce->ce_flags = create_ce_flags(len, 0);
	ce->ce_mode = create_ce_mode(mode);
	add_cache_entry(ce, ADD_CACHE_OK_TO_ADD | ADD_CACHE_OK_TO_REPLACE);
	return 0;
}

static int read_tree_some(struct tree *tree, const char **pathspec)
{
	read_tree_recursive(tree, "", 0, 0, pathspec, update_some, NULL);

	/* update the index with the given tree's info
	 * for all args, expanding wildcards, and exit
	 * with any non-zero return code.
	 */
	return 0;
}

static int skip_same_name(struct cache_entry *ce, int pos)
{
	while (++pos < active_nr &&
	       !strcmp(active_cache[pos]->name, ce->name))
		; /* skip */
	return pos;
}

static int check_stage(int stage, struct cache_entry *ce, int pos)
{
	while (pos < active_nr &&
	       !strcmp(active_cache[pos]->name, ce->name)) {
		if (ce_stage(active_cache[pos]) == stage)
			return 0;
		pos++;
	}
	return error("path '%s' does not have %s version",
		     ce->name,
		     (stage == 2) ? "our" : "their");
}

static int check_all_stages(struct cache_entry *ce, int pos)
{
	if (ce_stage(ce) != 1 ||
	    active_nr <= pos + 2 ||
	    strcmp(active_cache[pos+1]->name, ce->name) ||
	    ce_stage(active_cache[pos+1]) != 2 ||
	    strcmp(active_cache[pos+2]->name, ce->name) ||
	    ce_stage(active_cache[pos+2]) != 3)
		return error("path '%s' does not have all three versions",
			     ce->name);
	return 0;
}

static int checkout_stage(int stage, struct cache_entry *ce, int pos,
			  struct checkout *state)
{
	while (pos < active_nr &&
	       !strcmp(active_cache[pos]->name, ce->name)) {
		if (ce_stage(active_cache[pos]) == stage)
			return checkout_entry(active_cache[pos], state, NULL);
		pos++;
	}
	return error("path '%s' does not have %s version",
		     ce->name,
		     (stage == 2) ? "our" : "their");
}

static int checkout_merged(int pos, struct checkout *state)
{
	struct cache_entry *ce = active_cache[pos];
	const char *path = ce->name;
	mmfile_t ancestor, ours, theirs;
	int status;
	unsigned char sha1[20];
	mmbuffer_t result_buf;

	if (ce_stage(ce) != 1 ||
	    active_nr <= pos + 2 ||
	    strcmp(active_cache[pos+1]->name, path) ||
	    ce_stage(active_cache[pos+1]) != 2 ||
	    strcmp(active_cache[pos+2]->name, path) ||
	    ce_stage(active_cache[pos+2]) != 3)
		return error("path '%s' does not have all 3 versions", path);

	read_mmblob(&ancestor, active_cache[pos]->sha1);
	read_mmblob(&ours, active_cache[pos+1]->sha1);
	read_mmblob(&theirs, active_cache[pos+2]->sha1);

	status = ll_merge(&result_buf, path, &ancestor,
			  &ours, "ours", &theirs, "theirs", 0);
	free(ancestor.ptr);
	free(ours.ptr);
	free(theirs.ptr);
	if (status < 0 || !result_buf.ptr) {
		free(result_buf.ptr);
		return error("path '%s': cannot merge", path);
	}

	/*
	 * NEEDSWORK:
	 * There is absolutely no reason to write this as a blob object
	 * and create a phony cache entry just to leak.  This hack is
	 * primarily to get to the write_entry() machinery that massages
	 * the contents to work-tree format and writes out which only
	 * allows it for a cache entry.  The code in write_entry() needs
	 * to be refactored to allow us to feed a <buffer, size, mode>
	 * instead of a cache entry.  Such a refactoring would help
	 * merge_recursive as well (it also writes the merge result to the
	 * object database even when it may contain conflicts).
	 */
	if (write_sha1_file(result_buf.ptr, result_buf.size,
			    blob_type, sha1))
		die("Unable to add merge result for '%s'", path);
	ce = make_cache_entry(create_ce_mode(active_cache[pos+1]->ce_mode),
			      sha1,
			      path, 2, 0);
	if (!ce)
		die("make_cache_entry failed for path '%s'", path);
	status = checkout_entry(ce, state, NULL);
	return status;
}

static int checkout_paths(struct tree *source_tree, const char **pathspec,
			  struct checkout_opts *opts)
{
	int pos;
	struct checkout state;
	static char *ps_matched;
	unsigned char rev[20];
	int flag;
	struct commit *head;
	int errs = 0;
	int stage = opts->writeout_stage;
	int merge = opts->merge;
	int newfd;
	struct lock_file *lock_file = xcalloc(1, sizeof(struct lock_file));

	newfd = hold_locked_index(lock_file, 1);
	if (read_cache_preload(pathspec) < 0)
		return error("corrupt index file");

	if (source_tree)
		read_tree_some(source_tree, pathspec);

	for (pos = 0; pathspec[pos]; pos++)
		;
	ps_matched = xcalloc(1, pos);

	for (pos = 0; pos < active_nr; pos++) {
		struct cache_entry *ce = active_cache[pos];
		match_pathspec(pathspec, ce->name, ce_namelen(ce), 0, ps_matched);
	}

	if (report_path_error(ps_matched, pathspec, 0))
		return 1;

	/* "checkout -m path" to recreate conflicted state */
	if (opts->merge)
		unmerge_cache(pathspec);

	/* Any unmerged paths? */
	for (pos = 0; pos < active_nr; pos++) {
		struct cache_entry *ce = active_cache[pos];
		if (match_pathspec(pathspec, ce->name, ce_namelen(ce), 0, NULL)) {
			if (!ce_stage(ce))
				continue;
			if (opts->force) {
				warning("path '%s' is unmerged", ce->name);
			} else if (stage) {
				errs |= check_stage(stage, ce, pos);
			} else if (opts->merge) {
				errs |= check_all_stages(ce, pos);
			} else {
				errs = 1;
				error("path '%s' is unmerged", ce->name);
			}
			pos = skip_same_name(ce, pos) - 1;
		}
	}
	if (errs)
		return 1;

	/* Now we are committed to check them out */
	memset(&state, 0, sizeof(state));
	state.force = 1;
	state.refresh_cache = 1;
	for (pos = 0; pos < active_nr; pos++) {
		struct cache_entry *ce = active_cache[pos];
		if (match_pathspec(pathspec, ce->name, ce_namelen(ce), 0, NULL)) {
			if (!ce_stage(ce)) {
				errs |= checkout_entry(ce, &state, NULL);
				continue;
			}
			if (stage)
				errs |= checkout_stage(stage, ce, pos, &state);
			else if (merge)
				errs |= checkout_merged(pos, &state);
			pos = skip_same_name(ce, pos) - 1;
		}
	}

	if (write_cache(newfd, active_cache, active_nr) ||
	    commit_locked_index(lock_file))
		die("unable to write new index file");

	resolve_ref("HEAD", rev, 0, &flag);
	head = lookup_commit_reference_gently(rev, 1);

	errs |= post_checkout_hook(head, head, 0);
	return errs;
}

static void show_local_changes(struct object *head)
{
	struct rev_info rev;
	/* I think we want full paths, even if we're in a subdirectory. */
	init_revisions(&rev, NULL);
	rev.abbrev = 0;
	rev.diffopt.output_format |= DIFF_FORMAT_NAME_STATUS;
	if (diff_setup_done(&rev.diffopt) < 0)
		die("diff_setup_done failed");
	add_pending_object(&rev, head, NULL);
	run_diff_index(&rev, 0);
}

static void describe_detached_head(char *msg, struct commit *commit)
{
	struct strbuf sb = STRBUF_INIT;
	struct pretty_print_context ctx = {0};
	parse_commit(commit);
	pretty_print_commit(CMIT_FMT_ONELINE, commit, &sb, &ctx);
	fprintf(stderr, "%s %s... %s\n", msg,
		find_unique_abbrev(commit->object.sha1, DEFAULT_ABBREV), sb.buf);
	strbuf_release(&sb);
}

static int reset_tree(struct tree *tree, struct checkout_opts *o, int worktree)
{
	struct unpack_trees_options opts;
	struct tree_desc tree_desc;

	memset(&opts, 0, sizeof(opts));
	opts.head_idx = -1;
	opts.update = worktree;
	opts.skip_unmerged = !worktree;
	opts.reset = 1;
	opts.merge = 1;
	opts.fn = oneway_merge;
	opts.verbose_update = !o->quiet;
	opts.src_index = &the_index;
	opts.dst_index = &the_index;
	parse_tree(tree);
	init_tree_desc(&tree_desc, tree->buffer, tree->size);
	switch (unpack_trees(1, &tree_desc, &opts)) {
	case -2:
		o->writeout_error = 1;
		/*
		 * We return 0 nevertheless, as the index is all right
		 * and more importantly we have made best efforts to
		 * update paths in the work tree, and we cannot revert
		 * them.
		 */
	case 0:
		return 0;
	default:
		return 128;
	}
}

struct branch_info {
	const char *name; /* The short name used */
	const char *path; /* The full name of a real branch */
	struct commit *commit; /* The named commit */
};

static void setup_branch_path(struct branch_info *branch)
{
	struct strbuf buf = STRBUF_INIT;

	strbuf_branchname(&buf, branch->name);
	if (strcmp(buf.buf, branch->name))
		branch->name = xstrdup(buf.buf);
	strbuf_splice(&buf, 0, 0, "refs/heads/", 11);
	branch->path = strbuf_detach(&buf, NULL);
}

static int merge_working_tree(struct checkout_opts *opts,
			      struct branch_info *old, struct branch_info *new)
{
	int ret;
	struct lock_file *lock_file = xcalloc(1, sizeof(struct lock_file));
	int newfd = hold_locked_index(lock_file, 1);

	if (read_cache_preload(NULL) < 0)
		return error("corrupt index file");

	resolve_undo_clear();
	if (opts->force) {
		ret = reset_tree(new->commit->tree, opts, 1);
		if (ret)
			return ret;
	} else {
		struct tree_desc trees[2];
		struct tree *tree;
		struct unpack_trees_options topts;

		memset(&topts, 0, sizeof(topts));
		topts.head_idx = -1;
		topts.src_index = &the_index;
		topts.dst_index = &the_index;

		topts.msgs.not_uptodate_file = "You have local changes to '%s'; cannot switch branches.";

		refresh_cache(REFRESH_QUIET);

		if (unmerged_cache()) {
			error("you need to resolve your current index first");
			return 1;
		}

		/* 2-way merge to the new branch */
		topts.initial_checkout = is_cache_unborn();
		topts.update = 1;
		topts.merge = 1;
		topts.gently = opts->merge && old->commit;
		topts.verbose_update = !opts->quiet;
		topts.fn = twoway_merge;
		topts.dir = xcalloc(1, sizeof(*topts.dir));
		topts.dir->flags |= DIR_SHOW_IGNORED;
		topts.dir->exclude_per_dir = ".gitignore";
		tree = parse_tree_indirect(old->commit ?
					   old->commit->object.sha1 :
					   (unsigned char *)EMPTY_TREE_SHA1_BIN);
		init_tree_desc(&trees[0], tree->buffer, tree->size);
		tree = parse_tree_indirect(new->commit->object.sha1);
		init_tree_desc(&trees[1], tree->buffer, tree->size);

		ret = unpack_trees(2, trees, &topts);
		if (ret == -1) {
			/*
			 * Unpack couldn't do a trivial merge; either
			 * give up or do a real merge, depending on
			 * whether the merge flag was used.
			 */
			struct tree *result;
			struct tree *work;
			struct merge_options o;
			if (!opts->merge)
				return 1;

			/*
			 * Without old->commit, the below is the same as
			 * the two-tree unpack we already tried and failed.
			 */
			if (!old->commit)
				return 1;

			/* Do more real merge */

			/*
			 * We update the index fully, then write the
			 * tree from the index, then merge the new
			 * branch with the current tree, with the old
			 * branch as the base. Then we reset the index
			 * (but not the working tree) to the new
			 * branch, leaving the working tree as the
			 * merged version, but skipping unmerged
			 * entries in the index.
			 */

			add_files_to_cache(NULL, NULL, 0);
			init_merge_options(&o);
			o.verbosity = 0;
			work = write_tree_from_memory(&o);

			ret = reset_tree(new->commit->tree, opts, 1);
			if (ret)
				return ret;
			o.branch1 = new->name;
			o.branch2 = "local";
			merge_trees(&o, new->commit->tree, work,
				old->commit->tree, &result);
			ret = reset_tree(new->commit->tree, opts, 0);
			if (ret)
				return ret;
		}
	}

	if (write_cache(newfd, active_cache, active_nr) ||
	    commit_locked_index(lock_file))
		die("unable to write new index file");

	if (!opts->force && !opts->quiet)
		show_local_changes(&new->commit->object);

	return 0;
}

static void report_tracking(struct branch_info *new)
{
	struct strbuf sb = STRBUF_INIT;
	struct branch *branch = branch_get(new->name);

	if (!format_tracking_info(branch, &sb))
		return;
	fputs(sb.buf, stdout);
	strbuf_release(&sb);
}

static void update_refs_for_switch(struct checkout_opts *opts,
				   struct branch_info *old,
				   struct branch_info *new)
{
	struct strbuf msg = STRBUF_INIT;
	const char *old_desc;
	if (opts->new_branch) {
		create_branch(old->name, opts->new_branch, new->name, 0,
			      opts->new_branch_log, opts->track);
		new->name = opts->new_branch;
		setup_branch_path(new);
	}

	old_desc = old->name;
	if (!old_desc && old->commit)
		old_desc = sha1_to_hex(old->commit->object.sha1);
	strbuf_addf(&msg, "checkout: moving from %s to %s",
		    old_desc ? old_desc : "(invalid)", new->name);

	if (new->path) {
		create_symref("HEAD", new->path, msg.buf);
		if (!opts->quiet) {
			if (old->path && !strcmp(new->path, old->path))
				fprintf(stderr, "Already on '%s'\n",
					new->name);
			else
				fprintf(stderr, "Switched to%s branch '%s'\n",
					opts->new_branch ? " a new" : "",
					new->name);
		}
	} else if (strcmp(new->name, "HEAD")) {
		update_ref(msg.buf, "HEAD", new->commit->object.sha1, NULL,
			   REF_NODEREF, DIE_ON_ERR);
		if (!opts->quiet) {
			if (old->path)
				fprintf(stderr, "Note: moving to '%s' which isn't a local branch\nIf you want to create a new branch from this checkout, you may do so\n(now or later) by using -b with the checkout command again. Example:\n  git checkout -b <new_branch_name>\n", new->name);
			describe_detached_head("HEAD is now at", new->commit);
		}
	}
	remove_branch_state();
	strbuf_release(&msg);
	if (!opts->quiet && (new->path || !strcmp(new->name, "HEAD")))
		report_tracking(new);
}

static int switch_branches(struct checkout_opts *opts, struct branch_info *new)
{
	int ret = 0;
	struct branch_info old;
	unsigned char rev[20];
	int flag;
	memset(&old, 0, sizeof(old));
	old.path = resolve_ref("HEAD", rev, 0, &flag);
	old.commit = lookup_commit_reference_gently(rev, 1);
	if (!(flag & REF_ISSYMREF))
		old.path = NULL;

	if (old.path && !prefixcmp(old.path, "refs/heads/"))
		old.name = old.path + strlen("refs/heads/");

	if (!new->name) {
		new->name = "HEAD";
		new->commit = old.commit;
		if (!new->commit)
			die("You are on a branch yet to be born");
		parse_commit(new->commit);
	}

	ret = merge_working_tree(opts, &old, new);
	if (ret)
		return ret;

	/*
	 * If we were on a detached HEAD, but have now moved to
	 * a new commit, we want to mention the old commit once more
	 * to remind the user that it might be lost.
	 */
	if (!opts->quiet && !old.path && old.commit && new->commit != old.commit)
		describe_detached_head("Previous HEAD position was", old.commit);

	update_refs_for_switch(opts, &old, new);

	ret = post_checkout_hook(old.commit, new->commit, 1);
	return ret || opts->writeout_error;
}

static int git_checkout_config(const char *var, const char *value, void *cb)
{
	return git_xmerge_config(var, value, cb);
}

static int interactive_checkout(const char *revision, const char **pathspec,
				struct checkout_opts *opts)
{
	return run_add_interactive(revision, "--patch=checkout", pathspec);
}

struct tracking_name_data {
	const char *name;
	char *remote;
	int unique;
};

static int check_tracking_name(const char *refname, const unsigned char *sha1,
			       int flags, void *cb_data)
{
	struct tracking_name_data *cb = cb_data;
	const char *slash;

	if (prefixcmp(refname, "refs/remotes/"))
		return 0;
	slash = strchr(refname + 13, '/');
	if (!slash || strcmp(slash + 1, cb->name))
		return 0;
	if (cb->remote) {
		cb->unique = 0;
		return 0;
	}
	cb->remote = xstrdup(refname);
	return 0;
}

static const char *unique_tracking_name(const char *name)
{
	struct tracking_name_data cb_data = { name, NULL, 1 };
	for_each_ref(check_tracking_name, &cb_data);
	if (cb_data.unique)
		return cb_data.remote;
	free(cb_data.remote);
	return NULL;
}

int cmd_checkout(int argc, const char **argv, const char *prefix)
{
	struct checkout_opts opts;
	unsigned char rev[20];
	const char *arg;
	struct branch_info new;
	struct tree *source_tree = NULL;
	char *conflict_style = NULL;
	int patch_mode = 0;
	int dwim_new_local_branch = 1;
	struct option options[] = {
		OPT__QUIET(&opts.quiet),
		OPT_STRING('b', NULL, &opts.new_branch, "new branch", "branch"),
		OPT_BOOLEAN('l', NULL, &opts.new_branch_log, "log for new branch"),
		OPT_SET_INT('t', "track",  &opts.track, "track",
			BRANCH_TRACK_EXPLICIT),
		OPT_SET_INT('2', "ours", &opts.writeout_stage, "stage",
			    2),
		OPT_SET_INT('3', "theirs", &opts.writeout_stage, "stage",
			    3),
		OPT_BOOLEAN('f', "force", &opts.force, "force"),
		OPT_BOOLEAN('m', "merge", &opts.merge, "merge"),
		OPT_STRING(0, "conflict", &conflict_style, "style",
			   "conflict style (merge or diff3)"),
		OPT_BOOLEAN('p', "patch", &patch_mode, "select hunks interactively"),
		{ OPTION_BOOLEAN, 0, "guess", &dwim_new_local_branch, NULL,
		  "second guess 'git checkout no-such-branch'",
		  PARSE_OPT_NOARG | PARSE_OPT_HIDDEN },
		OPT_END(),
	};
	int has_dash_dash;

	memset(&opts, 0, sizeof(opts));
	memset(&new, 0, sizeof(new));

	git_config(git_checkout_config, NULL);

	opts.track = BRANCH_TRACK_UNSPECIFIED;

	argc = parse_options(argc, argv, prefix, options, checkout_usage,
			     PARSE_OPT_KEEP_DASHDASH);

	if (patch_mode && (opts.track > 0 || opts.new_branch
			   || opts.new_branch_log || opts.merge || opts.force))
		die ("--patch is incompatible with all other options");

	/* --track without -b should DWIM */
	if (0 < opts.track && !opts.new_branch) {
		const char *argv0 = argv[0];
		if (!argc || !strcmp(argv0, "--"))
			die ("--track needs a branch name");
		if (!prefixcmp(argv0, "refs/"))
			argv0 += 5;
		if (!prefixcmp(argv0, "remotes/"))
			argv0 += 8;
		argv0 = strchr(argv0, '/');
		if (!argv0 || !argv0[1])
			die ("Missing branch name; try -b");
		opts.new_branch = argv0 + 1;
	}

	if (conflict_style) {
		opts.merge = 1; /* implied */
		git_xmerge_config("merge.conflictstyle", conflict_style, NULL);
	}

	if (opts.force && opts.merge)
		die("git checkout: -f and -m are incompatible");

	/*
	 * case 1: git checkout <ref> -- [<paths>]
	 *
	 *   <ref> must be a valid tree, everything after the '--' must be
	 *   a path.
	 *
	 * case 2: git checkout -- [<paths>]
	 *
	 *   everything after the '--' must be paths.
	 *
	 * case 3: git checkout <something> [<paths>]
	 *
	 *   With no paths, if <something> is a commit, that is to
	 *   switch to the branch or detach HEAD at it.  As a special case,
	 *   if <something> is A...B (missing A or B means HEAD but you can
	 *   omit at most one side), and if there is a unique merge base
	 *   between A and B, A...B names that merge base.
	 *
	 *   With no paths, if <something> is _not_ a commit, no -t nor -b
	 *   was given, and there is a tracking branch whose name is
	 *   <something> in one and only one remote, then this is a short-hand
	 *   to fork local <something> from that remote tracking branch.
	 *
	 *   Otherwise <something> shall not be ambiguous.
	 *   - If it's *only* a reference, treat it like case (1).
	 *   - If it's only a path, treat it like case (2).
	 *   - else: fail.
	 *
	 */
	if (argc) {
		if (!strcmp(argv[0], "--")) {       /* case (2) */
			argv++;
			argc--;
			goto no_reference;
		}

		arg = argv[0];
		has_dash_dash = (argc > 1) && !strcmp(argv[1], "--");

		if (!strcmp(arg, "-"))
			arg = "@{-1}";

		if (get_sha1_mb(arg, rev)) {
			if (has_dash_dash)          /* case (1) */
				die("invalid reference: %s", arg);
			if (!patch_mode &&
			    dwim_new_local_branch &&
			    opts.track == BRANCH_TRACK_UNSPECIFIED &&
			    !opts.new_branch &&
			    !check_filename(NULL, arg) &&
			    argc == 1) {
				const char *remote = unique_tracking_name(arg);
				if (!remote || get_sha1(remote, rev))
					goto no_reference;
				opts.new_branch = arg;
				arg = remote;
				/* DWIMmed to create local branch */
			}
			else
				goto no_reference;
		}

		/* we can't end up being in (2) anymore, eat the argument */
		argv++;
		argc--;

		new.name = arg;
		if ((new.commit = lookup_commit_reference_gently(rev, 1))) {
			setup_branch_path(&new);

			if ((check_ref_format(new.path) == CHECK_REF_FORMAT_OK) &&
			    resolve_ref(new.path, rev, 1, NULL))
				;
			else
				new.path = NULL;
			parse_commit(new.commit);
			source_tree = new.commit->tree;
		} else
			source_tree = parse_tree_indirect(rev);

		if (!source_tree)                   /* case (1): want a tree */
			die("reference is not a tree: %s", arg);
		if (!has_dash_dash) {/* case (3 -> 1) */
			/*
			 * Do not complain the most common case
			 *	git checkout branch
			 * even if there happen to be a file called 'branch';
			 * it would be extremely annoying.
			 */
			if (argc)
				verify_non_filename(NULL, arg);
		}
		else {
			argv++;
			argc--;
		}
	}

no_reference:

	if (opts.track == BRANCH_TRACK_UNSPECIFIED)
		opts.track = git_branch_track;

	if (argc) {
		const char **pathspec = get_pathspec(prefix, argv);

		if (!pathspec)
			die("invalid path specification");

		if (patch_mode)
			return interactive_checkout(new.name, pathspec, &opts);

		/* Checkout paths */
		if (opts.new_branch) {
			if (argc == 1) {
				die("git checkout: updating paths is incompatible with switching branches.\nDid you intend to checkout '%s' which can not be resolved as commit?", argv[0]);
			} else {
				die("git checkout: updating paths is incompatible with switching branches.");
			}
		}

		if (1 < !!opts.writeout_stage + !!opts.force + !!opts.merge)
			die("git checkout: --ours/--theirs, --force and --merge are incompatible when\nchecking out of the index.");

		return checkout_paths(source_tree, pathspec, &opts);
	}

	if (patch_mode)
		return interactive_checkout(new.name, NULL, &opts);

	if (opts.new_branch) {
		struct strbuf buf = STRBUF_INIT;
		if (strbuf_check_branch_ref(&buf, opts.new_branch))
			die("git checkout: we do not like '%s' as a branch name.",
			    opts.new_branch);
		if (!get_sha1(buf.buf, rev))
			die("git checkout: branch %s already exists", opts.new_branch);
		strbuf_release(&buf);
	}

	if (new.name && !new.commit) {
		die("Cannot switch branch to a non-commit.");
	}
	if (opts.writeout_stage)
		die("--ours/--theirs is incompatible with switching branches.");

	return switch_branches(&opts, &new);
}