#define NO_THE_INDEX_COMPATIBILITY_MACROS
#include "cache.h"
#include "argv-array.h"
#include "repository.h"
#include "config.h"
#include "dir.h"
#include "tree.h"
#include "tree-walk.h"
#include "cache-tree.h"
#include "unpack-trees.h"
#include "progress.h"
#include "refs.h"
#include "attr.h"
#include "split-index.h"
#include "dir.h"
#include "submodule.h"
#include "submodule-config.h"
#include "fsmonitor.h"
#include "object-store.h"
#include "fetch-object.h"

/*
 * Error messages expected by scripts out of plumbing commands such as
 * read-tree.  Non-scripted Porcelain is not required to use these messages
 * and in fact are encouraged to reword them to better suit their particular
 * situation better.  See how "git checkout" and "git merge" replaces
 * them using setup_unpack_trees_porcelain(), for example.
 */
static const char *unpack_plumbing_errors[NB_UNPACK_TREES_ERROR_TYPES] = {
	/* ERROR_WOULD_OVERWRITE */
	"Entry '%s' would be overwritten by merge. Cannot merge.",

	/* ERROR_NOT_UPTODATE_FILE */
	"Entry '%s' not uptodate. Cannot merge.",

	/* ERROR_NOT_UPTODATE_DIR */
	"Updating '%s' would lose untracked files in it",

	/* ERROR_WOULD_LOSE_UNTRACKED_OVERWRITTEN */
	"Untracked working tree file '%s' would be overwritten by merge.",

	/* ERROR_WOULD_LOSE_UNTRACKED_REMOVED */
	"Untracked working tree file '%s' would be removed by merge.",

	/* ERROR_BIND_OVERLAP */
	"Entry '%s' overlaps with '%s'.  Cannot bind.",

	/* ERROR_SPARSE_NOT_UPTODATE_FILE */
	"Entry '%s' not uptodate. Cannot update sparse checkout.",

	/* ERROR_WOULD_LOSE_ORPHANED_OVERWRITTEN */
	"Working tree file '%s' would be overwritten by sparse checkout update.",

	/* ERROR_WOULD_LOSE_ORPHANED_REMOVED */
	"Working tree file '%s' would be removed by sparse checkout update.",

	/* ERROR_WOULD_LOSE_SUBMODULE */
	"Submodule '%s' cannot checkout new HEAD.",
};

#define ERRORMSG(o,type) \
	( ((o) && (o)->msgs[(type)]) \
	  ? ((o)->msgs[(type)])      \
	  : (unpack_plumbing_errors[(type)]) )

static const char *super_prefixed(const char *path)
{
	/*
	 * It is necessary and sufficient to have two static buffers
	 * here, as the return value of this function is fed to
	 * error() using the unpack_*_errors[] templates we see above.
	 */
	static struct strbuf buf[2] = {STRBUF_INIT, STRBUF_INIT};
	static int super_prefix_len = -1;
	static unsigned idx = ARRAY_SIZE(buf) - 1;

	if (super_prefix_len < 0) {
		const char *super_prefix = get_super_prefix();
		if (!super_prefix) {
			super_prefix_len = 0;
		} else {
			int i;
			for (i = 0; i < ARRAY_SIZE(buf); i++)
				strbuf_addstr(&buf[i], super_prefix);
			super_prefix_len = buf[0].len;
		}
	}

	if (!super_prefix_len)
		return path;

	if (++idx >= ARRAY_SIZE(buf))
		idx = 0;

	strbuf_setlen(&buf[idx], super_prefix_len);
	strbuf_addstr(&buf[idx], path);

	return buf[idx].buf;
}

void setup_unpack_trees_porcelain(struct unpack_trees_options *opts,
				  const char *cmd)
{
	int i;
	const char **msgs = opts->msgs;
	const char *msg;

	argv_array_init(&opts->msgs_to_free);

	if (!strcmp(cmd, "checkout"))
		msg = advice_commit_before_merge
		      ? _("Your local changes to the following files would be overwritten by checkout:\n%%s"
			  "Please commit your changes or stash them before you switch branches.")
		      : _("Your local changes to the following files would be overwritten by checkout:\n%%s");
	else if (!strcmp(cmd, "merge"))
		msg = advice_commit_before_merge
		      ? _("Your local changes to the following files would be overwritten by merge:\n%%s"
			  "Please commit your changes or stash them before you merge.")
		      : _("Your local changes to the following files would be overwritten by merge:\n%%s");
	else
		msg = advice_commit_before_merge
		      ? _("Your local changes to the following files would be overwritten by %s:\n%%s"
			  "Please commit your changes or stash them before you %s.")
		      : _("Your local changes to the following files would be overwritten by %s:\n%%s");
	msgs[ERROR_WOULD_OVERWRITE] = msgs[ERROR_NOT_UPTODATE_FILE] =
		argv_array_pushf(&opts->msgs_to_free, msg, cmd, cmd);

	msgs[ERROR_NOT_UPTODATE_DIR] =
		_("Updating the following directories would lose untracked files in them:\n%s");

	if (!strcmp(cmd, "checkout"))
		msg = advice_commit_before_merge
		      ? _("The following untracked working tree files would be removed by checkout:\n%%s"
			  "Please move or remove them before you switch branches.")
		      : _("The following untracked working tree files would be removed by checkout:\n%%s");
	else if (!strcmp(cmd, "merge"))
		msg = advice_commit_before_merge
		      ? _("The following untracked working tree files would be removed by merge:\n%%s"
			  "Please move or remove them before you merge.")
		      : _("The following untracked working tree files would be removed by merge:\n%%s");
	else
		msg = advice_commit_before_merge
		      ? _("The following untracked working tree files would be removed by %s:\n%%s"
			  "Please move or remove them before you %s.")
		      : _("The following untracked working tree files would be removed by %s:\n%%s");
	msgs[ERROR_WOULD_LOSE_UNTRACKED_REMOVED] =
		argv_array_pushf(&opts->msgs_to_free, msg, cmd, cmd);

	if (!strcmp(cmd, "checkout"))
		msg = advice_commit_before_merge
		      ? _("The following untracked working tree files would be overwritten by checkout:\n%%s"
			  "Please move or remove them before you switch branches.")
		      : _("The following untracked working tree files would be overwritten by checkout:\n%%s");
	else if (!strcmp(cmd, "merge"))
		msg = advice_commit_before_merge
		      ? _("The following untracked working tree files would be overwritten by merge:\n%%s"
			  "Please move or remove them before you merge.")
		      : _("The following untracked working tree files would be overwritten by merge:\n%%s");
	else
		msg = advice_commit_before_merge
		      ? _("The following untracked working tree files would be overwritten by %s:\n%%s"
			  "Please move or remove them before you %s.")
		      : _("The following untracked working tree files would be overwritten by %s:\n%%s");
	msgs[ERROR_WOULD_LOSE_UNTRACKED_OVERWRITTEN] =
		argv_array_pushf(&opts->msgs_to_free, msg, cmd, cmd);

	/*
	 * Special case: ERROR_BIND_OVERLAP refers to a pair of paths, we
	 * cannot easily display it as a list.
	 */
	msgs[ERROR_BIND_OVERLAP] = _("Entry '%s' overlaps with '%s'.  Cannot bind.");

	msgs[ERROR_SPARSE_NOT_UPTODATE_FILE] =
		_("Cannot update sparse checkout: the following entries are not up to date:\n%s");
	msgs[ERROR_WOULD_LOSE_ORPHANED_OVERWRITTEN] =
		_("The following working tree files would be overwritten by sparse checkout update:\n%s");
	msgs[ERROR_WOULD_LOSE_ORPHANED_REMOVED] =
		_("The following working tree files would be removed by sparse checkout update:\n%s");
	msgs[ERROR_WOULD_LOSE_SUBMODULE] =
		_("Cannot update submodule:\n%s");

	opts->show_all_errors = 1;
	/* rejected paths may not have a static buffer */
	for (i = 0; i < ARRAY_SIZE(opts->unpack_rejects); i++)
		opts->unpack_rejects[i].strdup_strings = 1;
}

void clear_unpack_trees_porcelain(struct unpack_trees_options *opts)
{
	argv_array_clear(&opts->msgs_to_free);
	memset(opts->msgs, 0, sizeof(opts->msgs));
}

static int do_add_entry(struct unpack_trees_options *o, struct cache_entry *ce,
			 unsigned int set, unsigned int clear)
{
	clear |= CE_HASHED;

	if (set & CE_REMOVE)
		set |= CE_WT_REMOVE;

	ce->ce_flags = (ce->ce_flags & ~clear) | set;
	return add_index_entry(&o->result, ce,
			       ADD_CACHE_OK_TO_ADD | ADD_CACHE_OK_TO_REPLACE);
}

static void add_entry(struct unpack_trees_options *o,
		      const struct cache_entry *ce,
		      unsigned int set, unsigned int clear)
{
	do_add_entry(o, dup_cache_entry(ce, &o->result), set, clear);
}

/*
 * add error messages on path <path>
 * corresponding to the type <e> with the message <msg>
 * indicating if it should be display in porcelain or not
 */
static int add_rejected_path(struct unpack_trees_options *o,
			     enum unpack_trees_error_types e,
			     const char *path)
{
	if (!o->show_all_errors)
		return error(ERRORMSG(o, e), super_prefixed(path));

	/*
	 * Otherwise, insert in a list for future display by
	 * display_error_msgs()
	 */
	string_list_append(&o->unpack_rejects[e], path);
	return -1;
}

/*
 * display all the error messages stored in a nice way
 */
static void display_error_msgs(struct unpack_trees_options *o)
{
	int e, i;
	int something_displayed = 0;
	for (e = 0; e < NB_UNPACK_TREES_ERROR_TYPES; e++) {
		struct string_list *rejects = &o->unpack_rejects[e];
		if (rejects->nr > 0) {
			struct strbuf path = STRBUF_INIT;
			something_displayed = 1;
			for (i = 0; i < rejects->nr; i++)
				strbuf_addf(&path, "\t%s\n", rejects->items[i].string);
			error(ERRORMSG(o, e), super_prefixed(path.buf));
			strbuf_release(&path);
		}
		string_list_clear(rejects, 0);
	}
	if (something_displayed)
		fprintf(stderr, _("Aborting\n"));
}

static int check_submodule_move_head(const struct cache_entry *ce,
				     const char *old_id,
				     const char *new_id,
				     struct unpack_trees_options *o)
{
	unsigned flags = SUBMODULE_MOVE_HEAD_DRY_RUN;
	const struct submodule *sub = submodule_from_ce(ce);

	if (!sub)
		return 0;

	if (o->reset)
		flags |= SUBMODULE_MOVE_HEAD_FORCE;

	if (submodule_move_head(ce->name, old_id, new_id, flags))
		return o->gently ? -1 :
				   add_rejected_path(o, ERROR_WOULD_LOSE_SUBMODULE, ce->name);
	return 0;
}

/*
 * Preform the loading of the repository's gitmodules file.  This function is
 * used by 'check_update()' to perform loading of the gitmodules file in two
 * differnt situations:
 * (1) before removing entries from the working tree if the gitmodules file has
 *     been marked for removal.  This situation is specified by 'state' == NULL.
 * (2) before checking out entries to the working tree if the gitmodules file
 *     has been marked for update.  This situation is specified by 'state' != NULL.
 */
static void load_gitmodules_file(struct index_state *index,
				 struct checkout *state)
{
	int pos = index_name_pos(index, GITMODULES_FILE, strlen(GITMODULES_FILE));

	if (pos >= 0) {
		struct cache_entry *ce = index->cache[pos];
		if (!state && ce->ce_flags & CE_WT_REMOVE) {
			repo_read_gitmodules(the_repository);
		} else if (state && (ce->ce_flags & CE_UPDATE)) {
			submodule_free(the_repository);
			checkout_entry(ce, state, NULL);
			repo_read_gitmodules(the_repository);
		}
	}
}

/*
 * Unlink the last component and schedule the leading directories for
 * removal, such that empty directories get removed.
 */
static void unlink_entry(const struct cache_entry *ce)
{
	const struct submodule *sub = submodule_from_ce(ce);
	if (sub) {
		/* state.force is set at the caller. */
		submodule_move_head(ce->name, "HEAD", NULL,
				    SUBMODULE_MOVE_HEAD_FORCE);
	}
	if (!check_leading_path(ce->name, ce_namelen(ce)))
		return;
	if (remove_or_warn(ce->ce_mode, ce->name))
		return;
	schedule_dir_for_removal(ce->name, ce_namelen(ce));
}

static struct progress *get_progress(struct unpack_trees_options *o)
{
	unsigned cnt = 0, total = 0;
	struct index_state *index = &o->result;

	if (!o->update || !o->verbose_update)
		return NULL;

	for (; cnt < index->cache_nr; cnt++) {
		const struct cache_entry *ce = index->cache[cnt];
		if (ce->ce_flags & (CE_UPDATE | CE_WT_REMOVE))
			total++;
	}

	return start_delayed_progress(_("Checking out files"), total);
}

static void setup_collided_checkout_detection(struct checkout *state,
					      struct index_state *index)
{
	int i;

	state->clone = 1;
	for (i = 0; i < index->cache_nr; i++)
		index->cache[i]->ce_flags &= ~CE_MATCHED;
}

static void report_collided_checkout(struct index_state *index)
{
	struct string_list list = STRING_LIST_INIT_NODUP;
	int i;

	for (i = 0; i < index->cache_nr; i++) {
		struct cache_entry *ce = index->cache[i];

		if (!(ce->ce_flags & CE_MATCHED))
			continue;

		string_list_append(&list, ce->name);
		ce->ce_flags &= ~CE_MATCHED;
	}

	list.cmp = fspathcmp;
	string_list_sort(&list);

	if (list.nr) {
		warning(_("the following paths have collided (e.g. case-sensitive paths\n"
			  "on a case-insensitive filesystem) and only one from the same\n"
			  "colliding group is in the working tree:\n"));

		for (i = 0; i < list.nr; i++)
			fprintf(stderr, "  '%s'\n", list.items[i].string);
	}

	string_list_clear(&list, 0);
}

static int check_updates(struct unpack_trees_options *o)
{
	unsigned cnt = 0;
	int errs = 0;
	struct progress *progress = NULL;
	struct index_state *index = &o->result;
	struct checkout state = CHECKOUT_INIT;
	int i;

	trace_performance_enter();
	state.force = 1;
	state.quiet = 1;
	state.refresh_cache = 1;
	state.istate = index;

	if (o->clone)
		setup_collided_checkout_detection(&state, index);

	progress = get_progress(o);

	if (o->update)
		git_attr_set_direction(GIT_ATTR_CHECKOUT);

	if (should_update_submodules() && o->update && !o->dry_run)
		load_gitmodules_file(index, NULL);

	for (i = 0; i < index->cache_nr; i++) {
		const struct cache_entry *ce = index->cache[i];

		if (ce->ce_flags & CE_WT_REMOVE) {
			display_progress(progress, ++cnt);
			if (o->update && !o->dry_run)
				unlink_entry(ce);
		}
	}
	remove_marked_cache_entries(index);
	remove_scheduled_dirs();

	if (should_update_submodules() && o->update && !o->dry_run)
		load_gitmodules_file(index, &state);

	enable_delayed_checkout(&state);
	if (repository_format_partial_clone && o->update && !o->dry_run) {
		/*
		 * Prefetch the objects that are to be checked out in the loop
		 * below.
		 */
		struct oid_array to_fetch = OID_ARRAY_INIT;
		int fetch_if_missing_store = fetch_if_missing;
		fetch_if_missing = 0;
		for (i = 0; i < index->cache_nr; i++) {
			struct cache_entry *ce = index->cache[i];
			if ((ce->ce_flags & CE_UPDATE) &&
			    !S_ISGITLINK(ce->ce_mode)) {
				if (!has_object_file(&ce->oid))
					oid_array_append(&to_fetch, &ce->oid);
			}
		}
		if (to_fetch.nr)
			fetch_objects(repository_format_partial_clone,
				      to_fetch.oid, to_fetch.nr);
		fetch_if_missing = fetch_if_missing_store;
		oid_array_clear(&to_fetch);
	}
	for (i = 0; i < index->cache_nr; i++) {
		struct cache_entry *ce = index->cache[i];

		if (ce->ce_flags & CE_UPDATE) {
			if (ce->ce_flags & CE_WT_REMOVE)
				BUG("both update and delete flags are set on %s",
				    ce->name);
			display_progress(progress, ++cnt);
			ce->ce_flags &= ~CE_UPDATE;
			if (o->update && !o->dry_run) {
				errs |= checkout_entry(ce, &state, NULL);
			}
		}
	}
	stop_progress(&progress);
	errs |= finish_delayed_checkout(&state);
	if (o->update)
		git_attr_set_direction(GIT_ATTR_CHECKIN);

	if (o->clone)
		report_collided_checkout(index);

	trace_performance_leave("check_updates");
	return errs != 0;
}

static int verify_uptodate_sparse(const struct cache_entry *ce,
				  struct unpack_trees_options *o);
static int verify_absent_sparse(const struct cache_entry *ce,
				enum unpack_trees_error_types,
				struct unpack_trees_options *o);

static int apply_sparse_checkout(struct index_state *istate,
				 struct cache_entry *ce,
				 struct unpack_trees_options *o)
{
	int was_skip_worktree = ce_skip_worktree(ce);

	if (ce->ce_flags & CE_NEW_SKIP_WORKTREE)
		ce->ce_flags |= CE_SKIP_WORKTREE;
	else
		ce->ce_flags &= ~CE_SKIP_WORKTREE;
	if (was_skip_worktree != ce_skip_worktree(ce)) {
		ce->ce_flags |= CE_UPDATE_IN_BASE;
		mark_fsmonitor_invalid(istate, ce);
		istate->cache_changed |= CE_ENTRY_CHANGED;
	}

	/*
	 * if (!was_skip_worktree && !ce_skip_worktree()) {
	 *	This is perfectly normal. Move on;
	 * }
	 */

	/*
	 * Merge strategies may set CE_UPDATE|CE_REMOVE outside checkout
	 * area as a result of ce_skip_worktree() shortcuts in
	 * verify_absent() and verify_uptodate().
	 * Make sure they don't modify worktree if they are already
	 * outside checkout area
	 */
	if (was_skip_worktree && ce_skip_worktree(ce)) {
		ce->ce_flags &= ~CE_UPDATE;

		/*
		 * By default, when CE_REMOVE is on, CE_WT_REMOVE is also
		 * on to get that file removed from both index and worktree.
		 * If that file is already outside worktree area, don't
		 * bother remove it.
		 */
		if (ce->ce_flags & CE_REMOVE)
			ce->ce_flags &= ~CE_WT_REMOVE;
	}

	if (!was_skip_worktree && ce_skip_worktree(ce)) {
		/*
		 * If CE_UPDATE is set, verify_uptodate() must be called already
		 * also stat info may have lost after merged_entry() so calling
		 * verify_uptodate() again may fail
		 */
		if (!(ce->ce_flags & CE_UPDATE) && verify_uptodate_sparse(ce, o))
			return -1;
		ce->ce_flags |= CE_WT_REMOVE;
		ce->ce_flags &= ~CE_UPDATE;
	}
	if (was_skip_worktree && !ce_skip_worktree(ce)) {
		if (verify_absent_sparse(ce, ERROR_WOULD_LOSE_UNTRACKED_OVERWRITTEN, o))
			return -1;
		ce->ce_flags |= CE_UPDATE;
	}
	return 0;
}

static inline int call_unpack_fn(const struct cache_entry * const *src,
				 struct unpack_trees_options *o)
{
	int ret = o->fn(src, o);
	if (ret > 0)
		ret = 0;
	return ret;
}

static void mark_ce_used(struct cache_entry *ce, struct unpack_trees_options *o)
{
	ce->ce_flags |= CE_UNPACKED;

	if (o->cache_bottom < o->src_index->cache_nr &&
	    o->src_index->cache[o->cache_bottom] == ce) {
		int bottom = o->cache_bottom;
		while (bottom < o->src_index->cache_nr &&
		       o->src_index->cache[bottom]->ce_flags & CE_UNPACKED)
			bottom++;
		o->cache_bottom = bottom;
	}
}

static void mark_all_ce_unused(struct index_state *index)
{
	int i;
	for (i = 0; i < index->cache_nr; i++)
		index->cache[i]->ce_flags &= ~(CE_UNPACKED | CE_ADDED | CE_NEW_SKIP_WORKTREE);
}

static int locate_in_src_index(const struct cache_entry *ce,
			       struct unpack_trees_options *o)
{
	struct index_state *index = o->src_index;
	int len = ce_namelen(ce);
	int pos = index_name_pos(index, ce->name, len);
	if (pos < 0)
		pos = -1 - pos;
	return pos;
}

/*
 * We call unpack_index_entry() with an unmerged cache entry
 * only in diff-index, and it wants a single callback.  Skip
 * the other unmerged entry with the same name.
 */
static void mark_ce_used_same_name(struct cache_entry *ce,
				   struct unpack_trees_options *o)
{
	struct index_state *index = o->src_index;
	int len = ce_namelen(ce);
	int pos;

	for (pos = locate_in_src_index(ce, o); pos < index->cache_nr; pos++) {
		struct cache_entry *next = index->cache[pos];
		if (len != ce_namelen(next) ||
		    memcmp(ce->name, next->name, len))
			break;
		mark_ce_used(next, o);
	}
}

static struct cache_entry *next_cache_entry(struct unpack_trees_options *o)
{
	const struct index_state *index = o->src_index;
	int pos = o->cache_bottom;

	while (pos < index->cache_nr) {
		struct cache_entry *ce = index->cache[pos];
		if (!(ce->ce_flags & CE_UNPACKED))
			return ce;
		pos++;
	}
	return NULL;
}

static void add_same_unmerged(const struct cache_entry *ce,
			      struct unpack_trees_options *o)
{
	struct index_state *index = o->src_index;
	int len = ce_namelen(ce);
	int pos = index_name_pos(index, ce->name, len);

	if (0 <= pos)
		die("programming error in a caller of mark_ce_used_same_name");
	for (pos = -pos - 1; pos < index->cache_nr; pos++) {
		struct cache_entry *next = index->cache[pos];
		if (len != ce_namelen(next) ||
		    memcmp(ce->name, next->name, len))
			break;
		add_entry(o, next, 0, 0);
		mark_ce_used(next, o);
	}
}

static int unpack_index_entry(struct cache_entry *ce,
			      struct unpack_trees_options *o)
{
	const struct cache_entry *src[MAX_UNPACK_TREES + 1] = { NULL, };
	int ret;

	src[0] = ce;

	mark_ce_used(ce, o);
	if (ce_stage(ce)) {
		if (o->skip_unmerged) {
			add_entry(o, ce, 0, 0);
			return 0;
		}
	}
	ret = call_unpack_fn(src, o);
	if (ce_stage(ce))
		mark_ce_used_same_name(ce, o);
	return ret;
}

static int find_cache_pos(struct traverse_info *, const struct name_entry *);

static void restore_cache_bottom(struct traverse_info *info, int bottom)
{
	struct unpack_trees_options *o = info->data;

	if (o->diff_index_cached)
		return;
	o->cache_bottom = bottom;
}

static int switch_cache_bottom(struct traverse_info *info)
{
	struct unpack_trees_options *o = info->data;
	int ret, pos;

	if (o->diff_index_cached)
		return 0;
	ret = o->cache_bottom;
	pos = find_cache_pos(info->prev, &info->name);

	if (pos < -1)
		o->cache_bottom = -2 - pos;
	else if (pos < 0)
		o->cache_bottom = o->src_index->cache_nr;
	return ret;
}

static inline int are_same_oid(struct name_entry *name_j, struct name_entry *name_k)
{
	return name_j->oid && name_k->oid && oideq(name_j->oid, name_k->oid);
}

static int all_trees_same_as_cache_tree(int n, unsigned long dirmask,
					struct name_entry *names,
					struct traverse_info *info)
{
	struct unpack_trees_options *o = info->data;
	int i;

	if (!o->merge || dirmask != ((1 << n) - 1))
		return 0;

	for (i = 1; i < n; i++)
		if (!are_same_oid(names, names + i))
			return 0;

	return cache_tree_matches_traversal(o->src_index->cache_tree, names, info);
}

static int index_pos_by_traverse_info(struct name_entry *names,
				      struct traverse_info *info)
{
	struct unpack_trees_options *o = info->data;
	int len = traverse_path_len(info, names);
	char *name = xmalloc(len + 1 /* slash */ + 1 /* NUL */);
	int pos;

	make_traverse_path(name, info, names);
	name[len++] = '/';
	name[len] = '\0';
	pos = index_name_pos(o->src_index, name, len);
	if (pos >= 0)
		BUG("This is a directory and should not exist in index");
	pos = -pos - 1;
	if (!starts_with(o->src_index->cache[pos]->name, name) ||
	    (pos > 0 && starts_with(o->src_index->cache[pos-1]->name, name)))
		BUG("pos must point at the first entry in this directory");
	free(name);
	return pos;
}

/*
 * Fast path if we detect that all trees are the same as cache-tree at this
 * path. We'll walk these trees in an iterative loop using cache-tree/index
 * instead of ODB since we already know what these trees contain.
 */
static int traverse_by_cache_tree(int pos, int nr_entries, int nr_names,
				  struct name_entry *names,
				  struct traverse_info *info)
{
	struct cache_entry *src[MAX_UNPACK_TREES + 1] = { NULL, };
	struct unpack_trees_options *o = info->data;
	struct cache_entry *tree_ce = NULL;
	int ce_len = 0;
	int i, d;

	if (!o->merge)
		BUG("We need cache-tree to do this optimization");

	/*
	 * Do what unpack_callback() and unpack_nondirectories() normally
	 * do. But we walk all paths in an iterative loop instead.
	 *
	 * D/F conflicts and higher stage entries are not a concern
	 * because cache-tree would be invalidated and we would never
	 * get here in the first place.
	 */
	for (i = 0; i < nr_entries; i++) {
		int new_ce_len, len, rc;

		src[0] = o->src_index->cache[pos + i];

		len = ce_namelen(src[0]);
		new_ce_len = cache_entry_size(len);

		if (new_ce_len > ce_len) {
			new_ce_len <<= 1;
			tree_ce = xrealloc(tree_ce, new_ce_len);
			memset(tree_ce, 0, new_ce_len);
			ce_len = new_ce_len;

			tree_ce->ce_flags = create_ce_flags(0);

			for (d = 1; d <= nr_names; d++)
				src[d] = tree_ce;
		}

		tree_ce->ce_mode = src[0]->ce_mode;
		tree_ce->ce_namelen = len;
		oidcpy(&tree_ce->oid, &src[0]->oid);
		memcpy(tree_ce->name, src[0]->name, len + 1);

		rc = call_unpack_fn((const struct cache_entry * const *)src, o);
		if (rc < 0) {
			free(tree_ce);
			return rc;
		}

		mark_ce_used(src[0], o);
	}
	free(tree_ce);
	if (o->debug_unpack)
		printf("Unpacked %d entries from %s to %s using cache-tree\n",
		       nr_entries,
		       o->src_index->cache[pos]->name,
		       o->src_index->cache[pos + nr_entries - 1]->name);
	return 0;
}

static int traverse_trees_recursive(int n, unsigned long dirmask,
				    unsigned long df_conflicts,
				    struct name_entry *names,
				    struct traverse_info *info)
{
	int i, ret, bottom;
	int nr_buf = 0;
	struct tree_desc t[MAX_UNPACK_TREES];
	void *buf[MAX_UNPACK_TREES];
	struct traverse_info newinfo;
	struct name_entry *p;
	int nr_entries;

	nr_entries = all_trees_same_as_cache_tree(n, dirmask, names, info);
	if (nr_entries > 0) {
		struct unpack_trees_options *o = info->data;
		int pos = index_pos_by_traverse_info(names, info);

		if (!o->merge || df_conflicts)
			BUG("Wrong condition to get here buddy");

		/*
		 * All entries up to 'pos' must have been processed
		 * (i.e. marked CE_UNPACKED) at this point. But to be safe,
		 * save and restore cache_bottom anyway to not miss
		 * unprocessed entries before 'pos'.
		 */
		bottom = o->cache_bottom;
		ret = traverse_by_cache_tree(pos, nr_entries, n, names, info);
		o->cache_bottom = bottom;
		return ret;
	}

	p = names;
	while (!p->mode)
		p++;

	newinfo = *info;
	newinfo.prev = info;
	newinfo.pathspec = info->pathspec;
	newinfo.name = *p;
	newinfo.pathlen += tree_entry_len(p) + 1;
	newinfo.df_conflicts |= df_conflicts;

	/*
	 * Fetch the tree from the ODB for each peer directory in the
	 * n commits.
	 *
	 * For 2- and 3-way traversals, we try to avoid hitting the
	 * ODB twice for the same OID.  This should yield a nice speed
	 * up in checkouts and merges when the commits are similar.
	 *
	 * We don't bother doing the full O(n^2) search for larger n,
	 * because wider traversals don't happen that often and we
	 * avoid the search setup.
	 *
	 * When 2 peer OIDs are the same, we just copy the tree
	 * descriptor data.  This implicitly borrows the buffer
	 * data from the earlier cell.
	 */
	for (i = 0; i < n; i++, dirmask >>= 1) {
		if (i > 0 && are_same_oid(&names[i], &names[i - 1]))
			t[i] = t[i - 1];
		else if (i > 1 && are_same_oid(&names[i], &names[i - 2]))
			t[i] = t[i - 2];
		else {
			const struct object_id *oid = NULL;
			if (dirmask & 1)
				oid = names[i].oid;
			buf[nr_buf++] = fill_tree_descriptor(t + i, oid);
		}
	}

	bottom = switch_cache_bottom(&newinfo);
	ret = traverse_trees(n, t, &newinfo);
	restore_cache_bottom(&newinfo, bottom);

	for (i = 0; i < nr_buf; i++)
		free(buf[i]);

	return ret;
}

/*
 * Compare the traverse-path to the cache entry without actually
 * having to generate the textual representation of the traverse
 * path.
 *
 * NOTE! This *only* compares up to the size of the traverse path
 * itself - the caller needs to do the final check for the cache
 * entry having more data at the end!
 */
static int do_compare_entry_piecewise(const struct cache_entry *ce, const struct traverse_info *info, const struct name_entry *n)
{
	int len, pathlen, ce_len;
	const char *ce_name;

	if (info->prev) {
		int cmp = do_compare_entry_piecewise(ce, info->prev,
						     &info->name);
		if (cmp)
			return cmp;
	}
	pathlen = info->pathlen;
	ce_len = ce_namelen(ce);

	/* If ce_len < pathlen then we must have previously hit "name == directory" entry */
	if (ce_len < pathlen)
		return -1;

	ce_len -= pathlen;
	ce_name = ce->name + pathlen;

	len = tree_entry_len(n);
	return df_name_compare(ce_name, ce_len, S_IFREG, n->path, len, n->mode);
}

static int do_compare_entry(const struct cache_entry *ce,
			    const struct traverse_info *info,
			    const struct name_entry *n)
{
	int len, pathlen, ce_len;
	const char *ce_name;
	int cmp;

	/*
	 * If we have not precomputed the traverse path, it is quicker
	 * to avoid doing so.  But if we have precomputed it,
	 * it is quicker to use the precomputed version.
	 */
	if (!info->traverse_path)
		return do_compare_entry_piecewise(ce, info, n);

	cmp = strncmp(ce->name, info->traverse_path, info->pathlen);
	if (cmp)
		return cmp;

	pathlen = info->pathlen;
	ce_len = ce_namelen(ce);

	if (ce_len < pathlen)
		return -1;

	ce_len -= pathlen;
	ce_name = ce->name + pathlen;

	len = tree_entry_len(n);
	return df_name_compare(ce_name, ce_len, S_IFREG, n->path, len, n->mode);
}

static int compare_entry(const struct cache_entry *ce, const struct traverse_info *info, const struct name_entry *n)
{
	int cmp = do_compare_entry(ce, info, n);
	if (cmp)
		return cmp;

	/*
	 * Even if the beginning compared identically, the ce should
	 * compare as bigger than a directory leading up to it!
	 */
	return ce_namelen(ce) > traverse_path_len(info, n);
}

static int ce_in_traverse_path(const struct cache_entry *ce,
			       const struct traverse_info *info)
{
	if (!info->prev)
		return 1;
	if (do_compare_entry(ce, info->prev, &info->name))
		return 0;
	/*
	 * If ce (blob) is the same name as the path (which is a tree
	 * we will be descending into), it won't be inside it.
	 */
	return (info->pathlen < ce_namelen(ce));
}

static struct cache_entry *create_ce_entry(const struct traverse_info *info,
	const struct name_entry *n,
	int stage,
	struct index_state *istate,
	int is_transient)
{
	int len = traverse_path_len(info, n);
	struct cache_entry *ce =
		is_transient ?
		make_empty_transient_cache_entry(len) :
		make_empty_cache_entry(istate, len);

	ce->ce_mode = create_ce_mode(n->mode);
	ce->ce_flags = create_ce_flags(stage);
	ce->ce_namelen = len;
	oidcpy(&ce->oid, n->oid);
	make_traverse_path(ce->name, info, n);

	return ce;
}

/*
 * Note that traverse_by_cache_tree() duplicates some logic in this function
 * without actually calling it. If you change the logic here you may need to
 * check and change there as well.
 */
static int unpack_nondirectories(int n, unsigned long mask,
				 unsigned long dirmask,
				 struct cache_entry **src,
				 const struct name_entry *names,
				 const struct traverse_info *info)
{
	int i;
	struct unpack_trees_options *o = info->data;
	unsigned long conflicts = info->df_conflicts | dirmask;

	/* Do we have *only* directories? Nothing to do */
	if (mask == dirmask && !src[0])
		return 0;

	/*
	 * Ok, we've filled in up to any potential index entry in src[0],
	 * now do the rest.
	 */
	for (i = 0; i < n; i++) {
		int stage;
		unsigned int bit = 1ul << i;
		if (conflicts & bit) {
			src[i + o->merge] = o->df_conflict_entry;
			continue;
		}
		if (!(mask & bit))
			continue;
		if (!o->merge)
			stage = 0;
		else if (i + 1 < o->head_idx)
			stage = 1;
		else if (i + 1 > o->head_idx)
			stage = 3;
		else
			stage = 2;

		/*
		 * If the merge bit is set, then the cache entries are
		 * discarded in the following block.  In this case,
		 * construct "transient" cache_entries, as they are
		 * not stored in the index.  otherwise construct the
		 * cache entry from the index aware logic.
		 */
		src[i + o->merge] = create_ce_entry(info, names + i, stage, &o->result, o->merge);
	}

	if (o->merge) {
		int rc = call_unpack_fn((const struct cache_entry * const *)src,
					o);
		for (i = 0; i < n; i++) {
			struct cache_entry *ce = src[i + o->merge];
			if (ce != o->df_conflict_entry)
				discard_cache_entry(ce);
		}
		return rc;
	}

	for (i = 0; i < n; i++)
		if (src[i] && src[i] != o->df_conflict_entry)
			if (do_add_entry(o, src[i], 0, 0))
				return -1;

	return 0;
}

static int unpack_failed(struct unpack_trees_options *o, const char *message)
{
	discard_index(&o->result);
	if (!o->gently && !o->exiting_early) {
		if (message)
			return error("%s", message);
		return -1;
	}
	return -1;
}

/*
 * The tree traversal is looking at name p.  If we have a matching entry,
 * return it.  If name p is a directory in the index, do not return
 * anything, as we will want to match it when the traversal descends into
 * the directory.
 */
static int find_cache_pos(struct traverse_info *info,
			  const struct name_entry *p)
{
	int pos;
	struct unpack_trees_options *o = info->data;
	struct index_state *index = o->src_index;
	int pfxlen = info->pathlen;
	int p_len = tree_entry_len(p);

	for (pos = o->cache_bottom; pos < index->cache_nr; pos++) {
		const struct cache_entry *ce = index->cache[pos];
		const char *ce_name, *ce_slash;
		int cmp, ce_len;

		if (ce->ce_flags & CE_UNPACKED) {
			/*
			 * cache_bottom entry is already unpacked, so
			 * we can never match it; don't check it
			 * again.
			 */
			if (pos == o->cache_bottom)
				++o->cache_bottom;
			continue;
		}
		if (!ce_in_traverse_path(ce, info)) {
			/*
			 * Check if we can skip future cache checks
			 * (because we're already past all possible
			 * entries in the traverse path).
			 */
			if (info->traverse_path) {
				if (strncmp(ce->name, info->traverse_path,
					    info->pathlen) > 0)
					break;
			}
			continue;
		}
		ce_name = ce->name + pfxlen;
		ce_slash = strchr(ce_name, '/');
		if (ce_slash)
			ce_len = ce_slash - ce_name;
		else
			ce_len = ce_namelen(ce) - pfxlen;
		cmp = name_compare(p->path, p_len, ce_name, ce_len);
		/*
		 * Exact match; if we have a directory we need to
		 * delay returning it.
		 */
		if (!cmp)
			return ce_slash ? -2 - pos : pos;
		if (0 < cmp)
			continue; /* keep looking */
		/*
		 * ce_name sorts after p->path; could it be that we
		 * have files under p->path directory in the index?
		 * E.g.  ce_name == "t-i", and p->path == "t"; we may
		 * have "t/a" in the index.
		 */
		if (p_len < ce_len && !memcmp(ce_name, p->path, p_len) &&
		    ce_name[p_len] < '/')
			continue; /* keep looking */
		break;
	}
	return -1;
}

static struct cache_entry *find_cache_entry(struct traverse_info *info,
					    const struct name_entry *p)
{
	int pos = find_cache_pos(info, p);
	struct unpack_trees_options *o = info->data;

	if (0 <= pos)
		return o->src_index->cache[pos];
	else
		return NULL;
}

static void debug_path(struct traverse_info *info)
{
	if (info->prev) {
		debug_path(info->prev);
		if (*info->prev->name.path)
			putchar('/');
	}
	printf("%s", info->name.path);
}

static void debug_name_entry(int i, struct name_entry *n)
{
	printf("ent#%d %06o %s\n", i,
	       n->path ? n->mode : 0,
	       n->path ? n->path : "(missing)");
}

static void debug_unpack_callback(int n,
				  unsigned long mask,
				  unsigned long dirmask,
				  struct name_entry *names,
				  struct traverse_info *info)
{
	int i;
	printf("* unpack mask %lu, dirmask %lu, cnt %d ",
	       mask, dirmask, n);
	debug_path(info);
	putchar('\n');
	for (i = 0; i < n; i++)
		debug_name_entry(i, names + i);
}

/*
 * Note that traverse_by_cache_tree() duplicates some logic in this function
 * without actually calling it. If you change the logic here you may need to
 * check and change there as well.
 */
static int unpack_callback(int n, unsigned long mask, unsigned long dirmask, struct name_entry *names, struct traverse_info *info)
{
	struct cache_entry *src[MAX_UNPACK_TREES + 1] = { NULL, };
	struct unpack_trees_options *o = info->data;
	const struct name_entry *p = names;

	/* Find first entry with a real name (we could use "mask" too) */
	while (!p->mode)
		p++;

	if (o->debug_unpack)
		debug_unpack_callback(n, mask, dirmask, names, info);

	/* Are we supposed to look at the index too? */
	if (o->merge) {
		while (1) {
			int cmp;
			struct cache_entry *ce;

			if (o->diff_index_cached)
				ce = next_cache_entry(o);
			else
				ce = find_cache_entry(info, p);

			if (!ce)
				break;
			cmp = compare_entry(ce, info, p);
			if (cmp < 0) {
				if (unpack_index_entry(ce, o) < 0)
					return unpack_failed(o, NULL);
				continue;
			}
			if (!cmp) {
				if (ce_stage(ce)) {
					/*
					 * If we skip unmerged index
					 * entries, we'll skip this
					 * entry *and* the tree
					 * entries associated with it!
					 */
					if (o->skip_unmerged) {
						add_same_unmerged(ce, o);
						return mask;
					}
				}
				src[0] = ce;
			}
			break;
		}
	}

	if (unpack_nondirectories(n, mask, dirmask, src, names, info) < 0)
		return -1;

	if (o->merge && src[0]) {
		if (ce_stage(src[0]))
			mark_ce_used_same_name(src[0], o);
		else
			mark_ce_used(src[0], o);
	}

	/* Now handle any directories.. */
	if (dirmask) {
		/* special case: "diff-index --cached" looking at a tree */
		if (o->diff_index_cached &&
		    n == 1 && dirmask == 1 && S_ISDIR(names->mode)) {
			int matches;
			matches = cache_tree_matches_traversal(o->src_index->cache_tree,
							       names, info);
			/*
			 * Everything under the name matches; skip the
			 * entire hierarchy.  diff_index_cached codepath
			 * special cases D/F conflicts in such a way that
			 * it does not do any look-ahead, so this is safe.
			 */
			if (matches) {
				o->cache_bottom += matches;
				return mask;
			}
		}

		if (traverse_trees_recursive(n, dirmask, mask & ~dirmask,
					     names, info) < 0)
			return -1;
		return mask;
	}

	return mask;
}

static int clear_ce_flags_1(struct index_state *istate,
			    struct cache_entry **cache, int nr,
			    struct strbuf *prefix,
			    int select_mask, int clear_mask,
			    struct exclude_list *el, int defval);

/* Whole directory matching */
static int clear_ce_flags_dir(struct index_state *istate,
			      struct cache_entry **cache, int nr,
			      struct strbuf *prefix,
			      char *basename,
			      int select_mask, int clear_mask,
			      struct exclude_list *el, int defval)
{
	struct cache_entry **cache_end;
	int dtype = DT_DIR;
	int ret = is_excluded_from_list(prefix->buf, prefix->len,
					basename, &dtype, el, istate);
	int rc;

	strbuf_addch(prefix, '/');

	/* If undecided, use matching result of parent dir in defval */
	if (ret < 0)
		ret = defval;

	for (cache_end = cache; cache_end != cache + nr; cache_end++) {
		struct cache_entry *ce = *cache_end;
		if (strncmp(ce->name, prefix->buf, prefix->len))
			break;
	}

	/*
	 * TODO: check el, if there are no patterns that may conflict
	 * with ret (iow, we know in advance the incl/excl
	 * decision for the entire directory), clear flag here without
	 * calling clear_ce_flags_1(). That function will call
	 * the expensive is_excluded_from_list() on every entry.
	 */
	rc = clear_ce_flags_1(istate, cache, cache_end - cache,
			      prefix,
			      select_mask, clear_mask,
			      el, ret);
	strbuf_setlen(prefix, prefix->len - 1);
	return rc;
}

/*
 * Traverse the index, find every entry that matches according to
 * o->el. Do "ce_flags &= ~clear_mask" on those entries. Return the
 * number of traversed entries.
 *
 * If select_mask is non-zero, only entries whose ce_flags has on of
 * those bits enabled are traversed.
 *
 * cache	: pointer to an index entry
 * prefix_len	: an offset to its path
 *
 * The current path ("prefix") including the trailing '/' is
 *   cache[0]->name[0..(prefix_len-1)]
 * Top level path has prefix_len zero.
 */
static int clear_ce_flags_1(struct index_state *istate,
			    struct cache_entry **cache, int nr,
			    struct strbuf *prefix,
			    int select_mask, int clear_mask,
			    struct exclude_list *el, int defval)
{
	struct cache_entry **cache_end = cache + nr;

	/*
	 * Process all entries that have the given prefix and meet
	 * select_mask condition
	 */
	while(cache != cache_end) {
		struct cache_entry *ce = *cache;
		const char *name, *slash;
		int len, dtype, ret;

		if (select_mask && !(ce->ce_flags & select_mask)) {
			cache++;
			continue;
		}

		if (prefix->len && strncmp(ce->name, prefix->buf, prefix->len))
			break;

		name = ce->name + prefix->len;
		slash = strchr(name, '/');

		/* If it's a directory, try whole directory match first */
		if (slash) {
			int processed;

			len = slash - name;
			strbuf_add(prefix, name, len);

			processed = clear_ce_flags_dir(istate, cache, cache_end - cache,
						       prefix,
						       prefix->buf + prefix->len - len,
						       select_mask, clear_mask,
						       el, defval);

			/* clear_c_f_dir eats a whole dir already? */
			if (processed) {
				cache += processed;
				strbuf_setlen(prefix, prefix->len - len);
				continue;
			}

			strbuf_addch(prefix, '/');
			cache += clear_ce_flags_1(istate, cache, cache_end - cache,
						  prefix,
						  select_mask, clear_mask, el, defval);
			strbuf_setlen(prefix, prefix->len - len - 1);
			continue;
		}

		/* Non-directory */
		dtype = ce_to_dtype(ce);
		ret = is_excluded_from_list(ce->name, ce_namelen(ce),
					    name, &dtype, el, istate);
		if (ret < 0)
			ret = defval;
		if (ret > 0)
			ce->ce_flags &= ~clear_mask;
		cache++;
	}
	return nr - (cache_end - cache);
}

static int clear_ce_flags(struct index_state *istate,
			  int select_mask, int clear_mask,
			  struct exclude_list *el)
{
	static struct strbuf prefix = STRBUF_INIT;

	strbuf_reset(&prefix);

	return clear_ce_flags_1(istate,
				istate->cache,
				istate->cache_nr,
				&prefix,
				select_mask, clear_mask,
				el, 0);
}

/*
 * Set/Clear CE_NEW_SKIP_WORKTREE according to $GIT_DIR/info/sparse-checkout
 */
static void mark_new_skip_worktree(struct exclude_list *el,
				   struct index_state *istate,
				   int select_flag, int skip_wt_flag)
{
	int i;

	/*
	 * 1. Pretend the narrowest worktree: only unmerged entries
	 * are checked out
	 */
	for (i = 0; i < istate->cache_nr; i++) {
		struct cache_entry *ce = istate->cache[i];

		if (select_flag && !(ce->ce_flags & select_flag))
			continue;

		if (!ce_stage(ce) && !(ce->ce_flags & CE_CONFLICTED))
			ce->ce_flags |= skip_wt_flag;
		else
			ce->ce_flags &= ~skip_wt_flag;
	}

	/*
	 * 2. Widen worktree according to sparse-checkout file.
	 * Matched entries will have skip_wt_flag cleared (i.e. "in")
	 */
	clear_ce_flags(istate, select_flag, skip_wt_flag, el);
}

static int verify_absent(const struct cache_entry *,
			 enum unpack_trees_error_types,
			 struct unpack_trees_options *);
/*
 * N-way merge "len" trees.  Returns 0 on success, -1 on failure to manipulate the
 * resulting index, -2 on failure to reflect the changes to the work tree.
 *
 * CE_ADDED, CE_UNPACKED and CE_NEW_SKIP_WORKTREE are used internally
 */
int unpack_trees(unsigned len, struct tree_desc *t, struct unpack_trees_options *o)
{
	int i, ret;
	static struct cache_entry *dfc;
	struct exclude_list el;

	if (len > MAX_UNPACK_TREES)
		die("unpack_trees takes at most %d trees", MAX_UNPACK_TREES);

	trace_performance_enter();
	memset(&el, 0, sizeof(el));
	if (!core_apply_sparse_checkout || !o->update)
		o->skip_sparse_checkout = 1;
	if (!o->skip_sparse_checkout) {
		char *sparse = git_pathdup("info/sparse-checkout");
		if (add_excludes_from_file_to_list(sparse, "", 0, &el, NULL) < 0)
			o->skip_sparse_checkout = 1;
		else
			o->el = &el;
		free(sparse);
	}

	memset(&o->result, 0, sizeof(o->result));
	o->result.initialized = 1;
	o->result.timestamp.sec = o->src_index->timestamp.sec;
	o->result.timestamp.nsec = o->src_index->timestamp.nsec;
	o->result.version = o->src_index->version;
	if (!o->src_index->split_index) {
		o->result.split_index = NULL;
	} else if (o->src_index == o->dst_index) {
		/*
		 * o->dst_index (and thus o->src_index) will be discarded
		 * and overwritten with o->result at the end of this function,
		 * so just use src_index's split_index to avoid having to
		 * create a new one.
		 */
		o->result.split_index = o->src_index->split_index;
		o->result.split_index->refcount++;
	} else {
		o->result.split_index = init_split_index(&o->result);
	}
	oidcpy(&o->result.oid, &o->src_index->oid);
	o->merge_size = len;
	mark_all_ce_unused(o->src_index);

	/*
	 * Sparse checkout loop #1: set NEW_SKIP_WORKTREE on existing entries
	 */
	if (!o->skip_sparse_checkout)
		mark_new_skip_worktree(o->el, o->src_index, 0, CE_NEW_SKIP_WORKTREE);

	if (!dfc)
		dfc = xcalloc(1, cache_entry_size(0));
	o->df_conflict_entry = dfc;

	if (len) {
		const char *prefix = o->prefix ? o->prefix : "";
		struct traverse_info info;

		setup_traverse_info(&info, prefix);
		info.fn = unpack_callback;
		info.data = o;
		info.show_all_errors = o->show_all_errors;
		info.pathspec = o->pathspec;

		if (o->prefix) {
			/*
			 * Unpack existing index entries that sort before the
			 * prefix the tree is spliced into.  Note that o->merge
			 * is always true in this case.
			 */
			while (1) {
				struct cache_entry *ce = next_cache_entry(o);
				if (!ce)
					break;
				if (ce_in_traverse_path(ce, &info))
					break;
				if (unpack_index_entry(ce, o) < 0)
					goto return_failed;
			}
		}

		trace_performance_enter();
		ret = traverse_trees(len, t, &info);
		trace_performance_leave("traverse_trees");
		if (ret < 0)
			goto return_failed;
	}

	/* Any left-over entries in the index? */
	if (o->merge) {
		while (1) {
			struct cache_entry *ce = next_cache_entry(o);
			if (!ce)
				break;
			if (unpack_index_entry(ce, o) < 0)
				goto return_failed;
		}
	}
	mark_all_ce_unused(o->src_index);

	if (o->trivial_merges_only && o->nontrivial_merge) {
		ret = unpack_failed(o, "Merge requires file-level merging");
		goto done;
	}

	if (!o->skip_sparse_checkout) {
		int empty_worktree = 1;

		/*
		 * Sparse checkout loop #2: set NEW_SKIP_WORKTREE on entries not in loop #1
		 * If the will have NEW_SKIP_WORKTREE, also set CE_SKIP_WORKTREE
		 * so apply_sparse_checkout() won't attempt to remove it from worktree
		 */
		mark_new_skip_worktree(o->el, &o->result, CE_ADDED, CE_SKIP_WORKTREE | CE_NEW_SKIP_WORKTREE);

		ret = 0;
		for (i = 0; i < o->result.cache_nr; i++) {
			struct cache_entry *ce = o->result.cache[i];

			/*
			 * Entries marked with CE_ADDED in merged_entry() do not have
			 * verify_absent() check (the check is effectively disabled
			 * because CE_NEW_SKIP_WORKTREE is set unconditionally).
			 *
			 * Do the real check now because we have had
			 * correct CE_NEW_SKIP_WORKTREE
			 */
			if (ce->ce_flags & CE_ADDED &&
			    verify_absent(ce, ERROR_WOULD_LOSE_UNTRACKED_OVERWRITTEN, o)) {
				if (!o->show_all_errors)
					goto return_failed;
				ret = -1;
			}

			if (apply_sparse_checkout(&o->result, ce, o)) {
				if (!o->show_all_errors)
					goto return_failed;
				ret = -1;
			}
			if (!ce_skip_worktree(ce))
				empty_worktree = 0;

		}
		if (ret < 0)
			goto return_failed;
		/*
		 * Sparse checkout is meant to narrow down checkout area
		 * but it does not make sense to narrow down to empty working
		 * tree. This is usually a mistake in sparse checkout rules.
		 * Do not allow users to do that.
		 */
		if (o->result.cache_nr && empty_worktree) {
			ret = unpack_failed(o, "Sparse checkout leaves no entry on working directory");
			goto done;
		}
	}

	ret = check_updates(o) ? (-2) : 0;
	if (o->dst_index) {
		move_index_extensions(&o->result, o->src_index);
		if (!ret) {
			if (git_env_bool("GIT_TEST_CHECK_CACHE_TREE", 0))
				cache_tree_verify(&o->result);
			if (!o->result.cache_tree)
				o->result.cache_tree = cache_tree();
			if (!cache_tree_fully_valid(o->result.cache_tree))
				cache_tree_update(&o->result,
						  WRITE_TREE_SILENT |
						  WRITE_TREE_REPAIR);
		}
		discard_index(o->dst_index);
		*o->dst_index = o->result;
	} else {
		discard_index(&o->result);
	}
	o->src_index = NULL;

done:
	trace_performance_leave("unpack_trees");
	clear_exclude_list(&el);
	return ret;

return_failed:
	if (o->show_all_errors)
		display_error_msgs(o);
	mark_all_ce_unused(o->src_index);
	ret = unpack_failed(o, NULL);
	if (o->exiting_early)
		ret = 0;
	goto done;
}

/* Here come the merge functions */

static int reject_merge(const struct cache_entry *ce,
			struct unpack_trees_options *o)
{
	return o->gently ? -1 :
		add_rejected_path(o, ERROR_WOULD_OVERWRITE, ce->name);
}

static int same(const struct cache_entry *a, const struct cache_entry *b)
{
	if (!!a != !!b)
		return 0;
	if (!a && !b)
		return 1;
	if ((a->ce_flags | b->ce_flags) & CE_CONFLICTED)
		return 0;
	return a->ce_mode == b->ce_mode &&
	       oideq(&a->oid, &b->oid);
}


/*
 * When a CE gets turned into an unmerged entry, we
 * want it to be up-to-date
 */
static int verify_uptodate_1(const struct cache_entry *ce,
			     struct unpack_trees_options *o,
			     enum unpack_trees_error_types error_type)
{
	struct stat st;

	if (o->index_only)
		return 0;

	/*
	 * CE_VALID and CE_SKIP_WORKTREE cheat, we better check again
	 * if this entry is truly up-to-date because this file may be
	 * overwritten.
	 */
	if ((ce->ce_flags & CE_VALID) || ce_skip_worktree(ce))
		; /* keep checking */
	else if (o->reset || ce_uptodate(ce))
		return 0;

	if (!lstat(ce->name, &st)) {
		int flags = CE_MATCH_IGNORE_VALID|CE_MATCH_IGNORE_SKIP_WORKTREE;
		unsigned changed = ie_match_stat(o->src_index, ce, &st, flags);

		if (submodule_from_ce(ce)) {
			int r = check_submodule_move_head(ce,
				"HEAD", oid_to_hex(&ce->oid), o);
			if (r)
				return o->gently ? -1 :
					add_rejected_path(o, error_type, ce->name);
			return 0;
		}

		if (!changed)
			return 0;
		/*
		 * Historic default policy was to allow submodule to be out
		 * of sync wrt the superproject index. If the submodule was
		 * not considered interesting above, we don't care here.
		 */
		if (S_ISGITLINK(ce->ce_mode))
			return 0;

		errno = 0;
	}
	if (errno == ENOENT)
		return 0;
	return o->gently ? -1 :
		add_rejected_path(o, error_type, ce->name);
}

int verify_uptodate(const struct cache_entry *ce,
		    struct unpack_trees_options *o)
{
	if (!o->skip_sparse_checkout && (ce->ce_flags & CE_NEW_SKIP_WORKTREE))
		return 0;
	return verify_uptodate_1(ce, o, ERROR_NOT_UPTODATE_FILE);
}

static int verify_uptodate_sparse(const struct cache_entry *ce,
				  struct unpack_trees_options *o)
{
	return verify_uptodate_1(ce, o, ERROR_SPARSE_NOT_UPTODATE_FILE);
}

/*
 * TODO: We should actually invalidate o->result, not src_index [1].
 * But since cache tree and untracked cache both are not copied to
 * o->result until unpacking is complete, we invalidate them on
 * src_index instead with the assumption that they will be copied to
 * dst_index at the end.
 *
 * [1] src_index->cache_tree is also used in unpack_callback() so if
 * we invalidate o->result, we need to update it to use
 * o->result.cache_tree as well.
 */
static void invalidate_ce_path(const struct cache_entry *ce,
			       struct unpack_trees_options *o)
{
	if (!ce)
		return;
	cache_tree_invalidate_path(o->src_index, ce->name);
	untracked_cache_invalidate_path(o->src_index, ce->name, 1);
}

/*
 * Check that checking out ce->sha1 in subdir ce->name is not
 * going to overwrite any working files.
 *
 * Currently, git does not checkout subprojects during a superproject
 * checkout, so it is not going to overwrite anything.
 */
static int verify_clean_submodule(const char *old_sha1,
				  const struct cache_entry *ce,
				  enum unpack_trees_error_types error_type,
				  struct unpack_trees_options *o)
{
	if (!submodule_from_ce(ce))
		return 0;

	return check_submodule_move_head(ce, old_sha1,
					 oid_to_hex(&ce->oid), o);
}

static int verify_clean_subdirectory(const struct cache_entry *ce,
				     enum unpack_trees_error_types error_type,
				     struct unpack_trees_options *o)
{
	/*
	 * we are about to extract "ce->name"; we would not want to lose
	 * anything in the existing directory there.
	 */
	int namelen;
	int i;
	struct dir_struct d;
	char *pathbuf;
	int cnt = 0;

	if (S_ISGITLINK(ce->ce_mode)) {
		struct object_id oid;
		int sub_head = resolve_gitlink_ref(ce->name, "HEAD", &oid);
		/*
		 * If we are not going to update the submodule, then
		 * we don't care.
		 */
		if (!sub_head && oideq(&oid, &ce->oid))
			return 0;
		return verify_clean_submodule(sub_head ? NULL : oid_to_hex(&oid),
					      ce, error_type, o);
	}

	/*
	 * First let's make sure we do not have a local modification
	 * in that directory.
	 */
	namelen = ce_namelen(ce);
	for (i = locate_in_src_index(ce, o);
	     i < o->src_index->cache_nr;
	     i++) {
		struct cache_entry *ce2 = o->src_index->cache[i];
		int len = ce_namelen(ce2);
		if (len < namelen ||
		    strncmp(ce->name, ce2->name, namelen) ||
		    ce2->name[namelen] != '/')
			break;
		/*
		 * ce2->name is an entry in the subdirectory to be
		 * removed.
		 */
		if (!ce_stage(ce2)) {
			if (verify_uptodate(ce2, o))
				return -1;
			add_entry(o, ce2, CE_REMOVE, 0);
			invalidate_ce_path(ce, o);
			mark_ce_used(ce2, o);
		}
		cnt++;
	}

	/*
	 * Then we need to make sure that we do not lose a locally
	 * present file that is not ignored.
	 */
	pathbuf = xstrfmt("%.*s/", namelen, ce->name);

	memset(&d, 0, sizeof(d));
	if (o->dir)
		d.exclude_per_dir = o->dir->exclude_per_dir;
	i = read_directory(&d, o->src_index, pathbuf, namelen+1, NULL);
	if (i)
		return o->gently ? -1 :
			add_rejected_path(o, ERROR_NOT_UPTODATE_DIR, ce->name);
	free(pathbuf);
	return cnt;
}

/*
 * This gets called when there was no index entry for the tree entry 'dst',
 * but we found a file in the working tree that 'lstat()' said was fine,
 * and we're on a case-insensitive filesystem.
 *
 * See if we can find a case-insensitive match in the index that also
 * matches the stat information, and assume it's that other file!
 */
static int icase_exists(struct unpack_trees_options *o, const char *name, int len, struct stat *st)
{
	const struct cache_entry *src;

	src = index_file_exists(o->src_index, name, len, 1);
	return src && !ie_match_stat(o->src_index, src, st, CE_MATCH_IGNORE_VALID|CE_MATCH_IGNORE_SKIP_WORKTREE);
}

static int check_ok_to_remove(const char *name, int len, int dtype,
			      const struct cache_entry *ce, struct stat *st,
			      enum unpack_trees_error_types error_type,
			      struct unpack_trees_options *o)
{
	const struct cache_entry *result;

	/*
	 * It may be that the 'lstat()' succeeded even though
	 * target 'ce' was absent, because there is an old
	 * entry that is different only in case..
	 *
	 * Ignore that lstat() if it matches.
	 */
	if (ignore_case && icase_exists(o, name, len, st))
		return 0;

	if (o->dir &&
	    is_excluded(o->dir, o->src_index, name, &dtype))
		/*
		 * ce->name is explicitly excluded, so it is Ok to
		 * overwrite it.
		 */
		return 0;
	if (S_ISDIR(st->st_mode)) {
		/*
		 * We are checking out path "foo" and
		 * found "foo/." in the working tree.
		 * This is tricky -- if we have modified
		 * files that are in "foo/" we would lose
		 * them.
		 */
		if (verify_clean_subdirectory(ce, error_type, o) < 0)
			return -1;
		return 0;
	}

	/*
	 * The previous round may already have decided to
	 * delete this path, which is in a subdirectory that
	 * is being replaced with a blob.
	 */
	result = index_file_exists(&o->result, name, len, 0);
	if (result) {
		if (result->ce_flags & CE_REMOVE)
			return 0;
	}

	return o->gently ? -1 :
		add_rejected_path(o, error_type, name);
}

/*
 * We do not want to remove or overwrite a working tree file that
 * is not tracked, unless it is ignored.
 */
static int verify_absent_1(const struct cache_entry *ce,
			   enum unpack_trees_error_types error_type,
			   struct unpack_trees_options *o)
{
	int len;
	struct stat st;

	if (o->index_only || o->reset || !o->update)
		return 0;

	len = check_leading_path(ce->name, ce_namelen(ce));
	if (!len)
		return 0;
	else if (len > 0) {
		char *path;
		int ret;

		path = xmemdupz(ce->name, len);
		if (lstat(path, &st))
			ret = error_errno("cannot stat '%s'", path);
		else {
			if (submodule_from_ce(ce))
				ret = check_submodule_move_head(ce,
								oid_to_hex(&ce->oid),
								NULL, o);
			else
				ret = check_ok_to_remove(path, len, DT_UNKNOWN, NULL,
							 &st, error_type, o);
		}
		free(path);
		return ret;
	} else if (lstat(ce->name, &st)) {
		if (errno != ENOENT)
			return error_errno("cannot stat '%s'", ce->name);
		return 0;
	} else {
		if (submodule_from_ce(ce))
			return check_submodule_move_head(ce, oid_to_hex(&ce->oid),
							 NULL, o);

		return check_ok_to_remove(ce->name, ce_namelen(ce),
					  ce_to_dtype(ce), ce, &st,
					  error_type, o);
	}
}

static int verify_absent(const struct cache_entry *ce,
			 enum unpack_trees_error_types error_type,
			 struct unpack_trees_options *o)
{
	if (!o->skip_sparse_checkout && (ce->ce_flags & CE_NEW_SKIP_WORKTREE))
		return 0;
	return verify_absent_1(ce, error_type, o);
}

static int verify_absent_sparse(const struct cache_entry *ce,
				enum unpack_trees_error_types error_type,
				struct unpack_trees_options *o)
{
	enum unpack_trees_error_types orphaned_error = error_type;
	if (orphaned_error == ERROR_WOULD_LOSE_UNTRACKED_OVERWRITTEN)
		orphaned_error = ERROR_WOULD_LOSE_ORPHANED_OVERWRITTEN;

	return verify_absent_1(ce, orphaned_error, o);
}

static int merged_entry(const struct cache_entry *ce,
			const struct cache_entry *old,
			struct unpack_trees_options *o)
{
	int update = CE_UPDATE;
	struct cache_entry *merge = dup_cache_entry(ce, &o->result);

	if (!old) {
		/*
		 * New index entries. In sparse checkout, the following
		 * verify_absent() will be delayed until after
		 * traverse_trees() finishes in unpack_trees(), then:
		 *
		 *  - CE_NEW_SKIP_WORKTREE will be computed correctly
		 *  - verify_absent() be called again, this time with
		 *    correct CE_NEW_SKIP_WORKTREE
		 *
		 * verify_absent() call here does nothing in sparse
		 * checkout (i.e. o->skip_sparse_checkout == 0)
		 */
		update |= CE_ADDED;
		merge->ce_flags |= CE_NEW_SKIP_WORKTREE;

		if (verify_absent(merge,
				  ERROR_WOULD_LOSE_UNTRACKED_OVERWRITTEN, o)) {
			discard_cache_entry(merge);
			return -1;
		}
		invalidate_ce_path(merge, o);

		if (submodule_from_ce(ce)) {
			int ret = check_submodule_move_head(ce, NULL,
							    oid_to_hex(&ce->oid),
							    o);
			if (ret)
				return ret;
		}

	} else if (!(old->ce_flags & CE_CONFLICTED)) {
		/*
		 * See if we can re-use the old CE directly?
		 * That way we get the uptodate stat info.
		 *
		 * This also removes the UPDATE flag on a match; otherwise
		 * we will end up overwriting local changes in the work tree.
		 */
		if (same(old, merge)) {
			copy_cache_entry(merge, old);
			update = 0;
		} else {
			if (verify_uptodate(old, o)) {
				discard_cache_entry(merge);
				return -1;
			}
			/* Migrate old flags over */
			update |= old->ce_flags & (CE_SKIP_WORKTREE | CE_NEW_SKIP_WORKTREE);
			invalidate_ce_path(old, o);
		}

		if (submodule_from_ce(ce)) {
			int ret = check_submodule_move_head(ce, oid_to_hex(&old->oid),
							    oid_to_hex(&ce->oid),
							    o);
			if (ret)
				return ret;
		}
	} else {
		/*
		 * Previously unmerged entry left as an existence
		 * marker by read_index_unmerged();
		 */
		invalidate_ce_path(old, o);
	}

	do_add_entry(o, merge, update, CE_STAGEMASK);
	return 1;
}

static int deleted_entry(const struct cache_entry *ce,
			 const struct cache_entry *old,
			 struct unpack_trees_options *o)
{
	/* Did it exist in the index? */
	if (!old) {
		if (verify_absent(ce, ERROR_WOULD_LOSE_UNTRACKED_REMOVED, o))
			return -1;
		return 0;
	}
	if (!(old->ce_flags & CE_CONFLICTED) && verify_uptodate(old, o))
		return -1;
	add_entry(o, ce, CE_REMOVE, 0);
	invalidate_ce_path(ce, o);
	return 1;
}

static int keep_entry(const struct cache_entry *ce,
		      struct unpack_trees_options *o)
{
	add_entry(o, ce, 0, 0);
	if (ce_stage(ce))
		invalidate_ce_path(ce, o);
	return 1;
}

#if DBRT_DEBUG
static void show_stage_entry(FILE *o,
			     const char *label, const struct cache_entry *ce)
{
	if (!ce)
		fprintf(o, "%s (missing)\n", label);
	else
		fprintf(o, "%s%06o %s %d\t%s\n",
			label,
			ce->ce_mode,
			oid_to_hex(&ce->oid),
			ce_stage(ce),
			ce->name);
}
#endif

int threeway_merge(const struct cache_entry * const *stages,
		   struct unpack_trees_options *o)
{
	const struct cache_entry *index;
	const struct cache_entry *head;
	const struct cache_entry *remote = stages[o->head_idx + 1];
	int count;
	int head_match = 0;
	int remote_match = 0;

	int df_conflict_head = 0;
	int df_conflict_remote = 0;

	int any_anc_missing = 0;
	int no_anc_exists = 1;
	int i;

	for (i = 1; i < o->head_idx; i++) {
		if (!stages[i] || stages[i] == o->df_conflict_entry)
			any_anc_missing = 1;
		else
			no_anc_exists = 0;
	}

	index = stages[0];
	head = stages[o->head_idx];

	if (head == o->df_conflict_entry) {
		df_conflict_head = 1;
		head = NULL;
	}

	if (remote == o->df_conflict_entry) {
		df_conflict_remote = 1;
		remote = NULL;
	}

	/*
	 * First, if there's a #16 situation, note that to prevent #13
	 * and #14.
	 */
	if (!same(remote, head)) {
		for (i = 1; i < o->head_idx; i++) {
			if (same(stages[i], head)) {
				head_match = i;
			}
			if (same(stages[i], remote)) {
				remote_match = i;
			}
		}
	}

	/*
	 * We start with cases where the index is allowed to match
	 * something other than the head: #14(ALT) and #2ALT, where it
	 * is permitted to match the result instead.
	 */
	/* #14, #14ALT, #2ALT */
	if (remote && !df_conflict_head && head_match && !remote_match) {
		if (index && !same(index, remote) && !same(index, head))
			return reject_merge(index, o);
		return merged_entry(remote, index, o);
	}
	/*
	 * If we have an entry in the index cache, then we want to
	 * make sure that it matches head.
	 */
	if (index && !same(index, head))
		return reject_merge(index, o);

	if (head) {
		/* #5ALT, #15 */
		if (same(head, remote))
			return merged_entry(head, index, o);
		/* #13, #3ALT */
		if (!df_conflict_remote && remote_match && !head_match)
			return merged_entry(head, index, o);
	}

	/* #1 */
	if (!head && !remote && any_anc_missing)
		return 0;

	/*
	 * Under the "aggressive" rule, we resolve mostly trivial
	 * cases that we historically had git-merge-one-file resolve.
	 */
	if (o->aggressive) {
		int head_deleted = !head;
		int remote_deleted = !remote;
		const struct cache_entry *ce = NULL;

		if (index)
			ce = index;
		else if (head)
			ce = head;
		else if (remote)
			ce = remote;
		else {
			for (i = 1; i < o->head_idx; i++) {
				if (stages[i] && stages[i] != o->df_conflict_entry) {
					ce = stages[i];
					break;
				}
			}
		}

		/*
		 * Deleted in both.
		 * Deleted in one and unchanged in the other.
		 */
		if ((head_deleted && remote_deleted) ||
		    (head_deleted && remote && remote_match) ||
		    (remote_deleted && head && head_match)) {
			if (index)
				return deleted_entry(index, index, o);
			if (ce && !head_deleted) {
				if (verify_absent(ce, ERROR_WOULD_LOSE_UNTRACKED_REMOVED, o))
					return -1;
			}
			return 0;
		}
		/*
		 * Added in both, identically.
		 */
		if (no_anc_exists && head && remote && same(head, remote))
			return merged_entry(head, index, o);

	}

	/* Below are "no merge" cases, which require that the index be
	 * up-to-date to avoid the files getting overwritten with
	 * conflict resolution files.
	 */
	if (index) {
		if (verify_uptodate(index, o))
			return -1;
	}

	o->nontrivial_merge = 1;

	/* #2, #3, #4, #6, #7, #9, #10, #11. */
	count = 0;
	if (!head_match || !remote_match) {
		for (i = 1; i < o->head_idx; i++) {
			if (stages[i] && stages[i] != o->df_conflict_entry) {
				keep_entry(stages[i], o);
				count++;
				break;
			}
		}
	}
#if DBRT_DEBUG
	else {
		fprintf(stderr, "read-tree: warning #16 detected\n");
		show_stage_entry(stderr, "head   ", stages[head_match]);
		show_stage_entry(stderr, "remote ", stages[remote_match]);
	}
#endif
	if (head) { count += keep_entry(head, o); }
	if (remote) { count += keep_entry(remote, o); }
	return count;
}

/*
 * Two-way merge.
 *
 * The rule is to "carry forward" what is in the index without losing
 * information across a "fast-forward", favoring a successful merge
 * over a merge failure when it makes sense.  For details of the
 * "carry forward" rule, please see <Documentation/git-read-tree.txt>.
 *
 */
int twoway_merge(const struct cache_entry * const *src,
		 struct unpack_trees_options *o)
{
	const struct cache_entry *current = src[0];
	const struct cache_entry *oldtree = src[1];
	const struct cache_entry *newtree = src[2];

	if (o->merge_size != 2)
		return error("Cannot do a twoway merge of %d trees",
			     o->merge_size);

	if (oldtree == o->df_conflict_entry)
		oldtree = NULL;
	if (newtree == o->df_conflict_entry)
		newtree = NULL;

	if (current) {
		if (current->ce_flags & CE_CONFLICTED) {
			if (same(oldtree, newtree) || o->reset) {
				if (!newtree)
					return deleted_entry(current, current, o);
				else
					return merged_entry(newtree, current, o);
			}
			return reject_merge(current, o);
		} else if ((!oldtree && !newtree) || /* 4 and 5 */
			 (!oldtree && newtree &&
			  same(current, newtree)) || /* 6 and 7 */
			 (oldtree && newtree &&
			  same(oldtree, newtree)) || /* 14 and 15 */
			 (oldtree && newtree &&
			  !same(oldtree, newtree) && /* 18 and 19 */
			  same(current, newtree))) {
			return keep_entry(current, o);
		} else if (oldtree && !newtree && same(current, oldtree)) {
			/* 10 or 11 */
			return deleted_entry(oldtree, current, o);
		} else if (oldtree && newtree &&
			 same(current, oldtree) && !same(current, newtree)) {
			/* 20 or 21 */
			return merged_entry(newtree, current, o);
		} else
			return reject_merge(current, o);
	}
	else if (newtree) {
		if (oldtree && !o->initial_checkout) {
			/*
			 * deletion of the path was staged;
			 */
			if (same(oldtree, newtree))
				return 1;
			return reject_merge(oldtree, o);
		}
		return merged_entry(newtree, current, o);
	}
	return deleted_entry(oldtree, current, o);
}

/*
 * Bind merge.
 *
 * Keep the index entries at stage0, collapse stage1 but make sure
 * stage0 does not have anything there.
 */
int bind_merge(const struct cache_entry * const *src,
	       struct unpack_trees_options *o)
{
	const struct cache_entry *old = src[0];
	const struct cache_entry *a = src[1];

	if (o->merge_size != 1)
		return error("Cannot do a bind merge of %d trees",
			     o->merge_size);
	if (a && old)
		return o->gently ? -1 :
			error(ERRORMSG(o, ERROR_BIND_OVERLAP),
			      super_prefixed(a->name),
			      super_prefixed(old->name));
	if (!a)
		return keep_entry(old, o);
	else
		return merged_entry(a, NULL, o);
}

/*
 * One-way merge.
 *
 * The rule is:
 * - take the stat information from stage0, take the data from stage1
 */
int oneway_merge(const struct cache_entry * const *src,
		 struct unpack_trees_options *o)
{
	const struct cache_entry *old = src[0];
	const struct cache_entry *a = src[1];

	if (o->merge_size != 1)
		return error("Cannot do a oneway merge of %d trees",
			     o->merge_size);

	if (!a || a == o->df_conflict_entry)
		return deleted_entry(old, old, o);

	if (old && same(old, a)) {
		int update = 0;
		if (o->reset && o->update && !ce_uptodate(old) && !ce_skip_worktree(old)) {
			struct stat st;
			if (lstat(old->name, &st) ||
			    ie_match_stat(o->src_index, old, &st, CE_MATCH_IGNORE_VALID|CE_MATCH_IGNORE_SKIP_WORKTREE))
				update |= CE_UPDATE;
		}
		if (o->update && S_ISGITLINK(old->ce_mode) &&
		    should_update_submodules() && !verify_uptodate(old, o))
			update |= CE_UPDATE;
		add_entry(o, old, update, 0);
		return 0;
	}
	return merged_entry(a, old, o);
}