[PATCH 00/17] Add directory rename detection to merge-ort

@@ -1359,7 +1359,23 @@ static void get_provisional_directory_renames(struct merge_options *opt,
 
 static void handle_directory_level_conflicts(struct merge_options *opt)
 {
-	die("Not yet implemented!");
+	struct hashmap_iter iter;
+	struct strmap_entry *entry;
+	struct string_list duplicated = STRING_LIST_INIT_NODUP;
+	struct strmap *side1_dir_renames = &opt->priv->renames.dir_renames[1];
+	struct strmap *side2_dir_renames = &opt->priv->renames.dir_renames[2];
+	int i;
+
+	strmap_for_each_entry(side1_dir_renames, &iter, entry) {
+		if (strmap_contains(side2_dir_renames, entry->key))
+			string_list_append(&duplicated, entry->key);
+	}
+
+	for (i=0; i<duplicated.nr; ++i) {
+		strmap_remove(side1_dir_renames, duplicated.items[i].string, 0);
+		strmap_remove(side2_dir_renames, duplicated.items[i].string, 0);
+	}
+	string_list_clear(&duplicated, 0);
 }
 
 /*** Function Grouping: functions related to regular rename detection ***/

@@ -311,8 +311,12 @@ static void free_strmap_strings(struct strmap *map)
 static void clear_or_reinit_internal_opts(struct merge_options_internal *opti,
 					  int reinitialize)
 {
+	struct rename_info *renames = &opti->renames;
+	int i;
 	void (*strmap_func)(struct strmap *, int) =
 		reinitialize ? strmap_partial_clear : strmap_clear;
+	void (*strset_func)(struct strset *) =
+		reinitialize ? strset_partial_clear : strset_clear;
 
 	/*
 	 * We marked opti->paths with strdup_strings = 0, so that we

@@ -342,6 +346,23 @@ static void clear_or_reinit_internal_opts(struct merge_options_internal *opti,
 	string_list_clear(&opti->paths_to_free, 0);
 	opti->paths_to_free.strdup_strings = 0;
 
+	/* Free memory used by various renames maps */
+	for (i = MERGE_SIDE1; i <= MERGE_SIDE2; ++i) {
+		struct hashmap_iter iter;
+		struct strmap_entry *entry;
+
+		strset_func(&renames->dirs_removed[i]);
+
+		strmap_for_each_entry(&renames->dir_rename_count[i],
+				      &iter, entry) {
+			struct strintmap *counts = entry->value;
+			strintmap_clear(counts);
+		}
+		strmap_func(&renames->dir_rename_count[i], 1);
+
+		strmap_func(&renames->dir_renames[i], 0);
+	}
+
 	if (!reinitialize) {
 		struct hashmap_iter iter;
 		struct strmap_entry *e;

@@ -2447,6 +2468,9 @@ static struct commit *make_virtual_commit(struct repository *repo,
 
 static void merge_start(struct merge_options *opt, struct merge_result *result)
 {
+	struct rename_info *renames;
+	int i;
+
 	/* Sanity checks on opt */
 	assert(opt->repo);
 

@@ -2481,6 +2505,17 @@ static void merge_start(struct merge_options *opt, struct merge_result *result)
 	/* Initialization of opt->priv, our internal merge data */
 	opt->priv = xcalloc(1, sizeof(*opt->priv));
 
+	/* Initialization of various renames fields */
+	renames = &opt->priv->renames;
+	for (i = MERGE_SIDE1; i <= MERGE_SIDE2; i++) {
+		strset_init_with_options(&renames->dirs_removed[i],
+					 NULL, 0);
+		strmap_init_with_options(&renames->dir_rename_count[i],
+					 NULL, 1);
+		strmap_init_with_options(&renames->dir_renames[i],
+					 NULL, 0);
+	}
+
 	/*
 	 * Although we initialize opt->priv->paths with strdup_strings=0,
 	 * that's just to avoid making yet another copy of an allocated

@@ -1144,6 +1144,19 @@ struct collision_info {
 	unsigned reported_already:1;
 };
 
+/*
+ * Return a new string that replaces the beginning portion (which matches
+ * rename_info->key), with rename_info->util.new_dir.  In perl-speak:
+ *   new_path_name = (old_path =~ s/rename_info->key/rename_info->value/);
+ * NOTE:
+ *   Caller must ensure that old_path starts with rename_info->key + '/'.
+ */
+static char *apply_dir_rename(struct strmap_entry *rename_info,
+			      const char *old_path)
+{
+	die("Not yet implemented!");
+}
+
 static void get_renamed_dir_portion(const char *old_path, const char *new_path,
 				    char **old_dir, char **new_dir)
 {

@@ -1383,11 +1396,64 @@ static void handle_directory_level_conflicts(struct merge_options *opt)
 	string_list_clear(&duplicated, 0);
 }
 
+static struct strmap_entry *check_dir_renamed(const char *path,
+					      struct strmap *dir_renames)
+{
+	die("Not yet implemented!");
+}
+
 static void compute_collisions(struct strmap *collisions,
 			       struct strmap *dir_renames,
 			       struct diff_queue_struct *pairs)
 {
-	die("Not yet implemented.");
+	int i;
+
+	strmap_init_with_options(collisions, NULL, 0);
+	if (strmap_empty(dir_renames))
+		return;
+
+	/*
+	 * Multiple files can be mapped to the same path due to directory
+	 * renames done by the other side of history.  Since that other
+	 * side of history could have merged multiple directories into one,
+	 * if our side of history added the same file basename to each of
+	 * those directories, then all N of them would get implicitly
+	 * renamed by the directory rename detection into the same path,
+	 * and we'd get an add/add/.../add conflict, and all those adds
+	 * from *this* side of history.  This is not representable in the
+	 * index, and users aren't going to easily be able to make sense of
+	 * it.  So we need to provide a good warning about what's
+	 * happening, and fall back to no-directory-rename detection
+	 * behavior for those paths.
+	 *
+	 * See testcases 9e and all of section 5 from t6043 for examples.
+	 */
+	for (i = 0; i < pairs->nr; ++i) {
+		struct strmap_entry *rename_info;
+		struct collision_info *collision_info;
+		char *new_path;
+		struct diff_filepair *pair = pairs->queue[i];
+
+		if (pair->status != 'A' && pair->status != 'R')
+			continue;
+		rename_info = check_dir_renamed(pair->two->path, dir_renames);
+		if (!rename_info)
+			continue;
+
+		new_path = apply_dir_rename(rename_info, pair->two->path);
+		assert(new_path);
+		collision_info = strmap_get(collisions, new_path);
+		if (collision_info) {
+			free(new_path);
+		} else {
+			collision_info = xcalloc(1,
+						 sizeof(struct collision_info));
+			string_list_init(&collision_info->source_files, 0);
+			strmap_put(collisions, new_path, collision_info);
+		}
+		string_list_insert(&collision_info->source_files,
+				   pair->two->path);
+	}
 }
 
 static char *check_for_directory_rename(struct merge_options *opt,

@@ -52,14 +52,42 @@ enum merge_side {
 };
 
 struct rename_info {
+	/*
+	 * All variables that are arrays of size 3 correspond to data tracked
+	 * for the sides in enum merge_side.  Index 0 is almost always unused
+	 * because we often only need to track information for MERGE_SIDE1 and
+	 * MERGE_SIDE2 (MERGE_BASE can't have rename information since renames
+	 * are determined relative to what changed since the MERGE_BASE).
+	 */
+
 	/*
 	 * pairs: pairing of filenames from diffcore_rename()
-	 *
-	 * Index 1 and 2 correspond to sides 1 & 2 as used in
-	 * conflict_info.stages.  Index 0 unused.
 	 */
 	struct diff_queue_struct pairs[3];
 
+	/*
+	 * dirs_removed: directories removed on a given side of history.
+	 */
+	struct strset dirs_removed[3];
+
+	/*
+	 * dir_rename_count: tracking where parts of a directory were renamed to
+	 *
+	 * When files in a directory are renamed, they may not all go to the
+	 * same location.  Each strmap here tracks:
+	 *      old_dir => {new_dir => int}
+	 * That is, dir_rename_count[side] is a strmap to a strintmap.
+	 */
+	struct strmap dir_rename_count[3];
+
+	/*
+	 * dir_renames: computed directory renames
+	 *
+	 * This is a map of old_dir => new_dir and is derived in part from
+	 * dir_rename_count.
+	 */
+	struct strmap dir_renames[3];
+
 	/*
 	 * needed_limit: value needed for inexact rename detection to run
 	 *

@@ -1282,6 +1282,21 @@ static void get_renamed_dir_portion(const char *old_path, const char *new_path,
 	*new_dir = xstrndup(new_path, end_of_new - new_path);
 }
 
+/*
+ * See if there is a directory rename for path, and if there are any file
+ * level conflicts on the given side for the renamed location.  If there is
+ * a rename and there are no conflicts, return the new name.  Otherwise,
+ * return NULL.
+ */
+static char *handle_path_level_conflicts(struct merge_options *opt,
+					 const char *path,
+					 unsigned side_index,
+					 struct strmap_entry *rename_info,
+					 struct strmap *collisions)
+{
+	die("Not yet implemented");
+}
+
 static void increment_count(struct strmap *dir_rename_count,
 			    char *old_dir,
 			    char *new_dir)

@@ -1497,7 +1512,57 @@ static char *check_for_directory_rename(struct merge_options *opt,
 					struct strmap *collisions,
 					int *clean_merge)
 {
-	die("Not yet implemented.");
+	char *new_path = NULL;
+	struct strmap_entry *rename_info;
+	struct strmap_entry *otherinfo = NULL;
+	const char *new_dir;
+
+	if (strmap_empty(dir_renames))
+		return new_path;
+	rename_info = check_dir_renamed(path, dir_renames);
+	if (!rename_info)
+		return new_path;
+	/* old_dir = rename_info->key; */
+	new_dir = rename_info->value;
+
+	/*
+	 * This next part is a little weird.  We do not want to do an
+	 * implicit rename into a directory we renamed on our side, because
+	 * that will result in a spurious rename/rename(1to2) conflict.  An
+	 * example:
+	 *   Base commit: dumbdir/afile, otherdir/bfile
+	 *   Side 1:      smrtdir/afile, otherdir/bfile
+	 *   Side 2:      dumbdir/afile, dumbdir/bfile
+	 * Here, while working on Side 1, we could notice that otherdir was
+	 * renamed/merged to dumbdir, and change the diff_filepair for
+	 * otherdir/bfile into a rename into dumbdir/bfile.  However, Side
+	 * 2 will notice the rename from dumbdir to smrtdir, and do the
+	 * transitive rename to move it from dumbdir/bfile to
+	 * smrtdir/bfile.  That gives us bfile in dumbdir vs being in
+	 * smrtdir, a rename/rename(1to2) conflict.  We really just want
+	 * the file to end up in smrtdir.  And the way to achieve that is
+	 * to not let Side1 do the rename to dumbdir, since we know that is
+	 * the source of one of our directory renames.
+	 *
+	 * That's why otherinfo and dir_rename_exclusions is here.
+	 *
+	 * As it turns out, this also prevents N-way transient rename
+	 * confusion; See testcases 9c and 9d of t6043.
+	 */
+	otherinfo = strmap_get_entry(dir_rename_exclusions, new_dir);
+	if (otherinfo) {
+		path_msg(opt, rename_info->key, 1,
+			 _("WARNING: Avoiding applying %s -> %s rename "
+			   "to %s, because %s itself was renamed."),
+			 rename_info->key, new_dir, path, new_dir);
+		return NULL;
+	}
+
+	new_path = handle_path_level_conflicts(opt, path, side_index,
+					       rename_info, collisions);
+	*clean_merge &= (new_path != NULL);
+
+	return new_path;
 }
 
 static void apply_directory_rename_modifications(struct merge_options *opt,

@@ -532,6 +532,27 @@ static void setup_path_info(struct merge_options *opt,
 	result->util = mi;
 }
 
+static void collect_rename_info(struct merge_options *opt,
+				struct name_entry *names,
+				const char *dirname,
+				const char *fullname,
+				unsigned filemask,
+				unsigned dirmask,
+				unsigned match_mask)
+{
+	struct rename_info *renames = &opt->priv->renames;
+
+	/* Update dirs_removed, as needed */
+	if (dirmask == 1 || dirmask == 3 || dirmask == 5) {
+		/* absent_mask = 0x07 - dirmask; sides = absent_mask/2 */
+		unsigned sides = (0x07 - dirmask)/2;
+		if (sides & 1)
+			strset_add(&renames->dirs_removed[1], fullname);
+		if (sides & 2)
+			strset_add(&renames->dirs_removed[2], fullname);
+	}
+}
+
 static int collect_merge_info_callback(int n,
 				       unsigned long mask,
 				       unsigned long dirmask,

@@ -632,6 +653,12 @@ static int collect_merge_info_callback(int n,
 		return mask;
 	}
 
+	/*
+	 * Gather additional information used in rename detection.
+	 */
+	collect_rename_info(opt, names, dirname, fullpath,
+			    filemask, dirmask, match_mask);
+
 	/*
 	 * Record information about the path so we can resolve later in
 	 * process_entries.

@@ -1139,6 +1139,18 @@ static int handle_content_merge(struct merge_options *opt,
 
 /*** Function Grouping: functions related to directory rename detection ***/
 
+static void get_provisional_directory_renames(struct merge_options *opt,
+					      unsigned side,
+					      int *clean)
+{
+	die("Not yet implemented!");
+}
+
+static void handle_directory_level_conflicts(struct merge_options *opt)
+{
+	die("Not yet implemented!");
+}
+
 /*** Function Grouping: functions related to regular rename detection ***/
 
 static int process_renames(struct merge_options *opt,

@@ -1504,13 +1516,24 @@ static int detect_and_process_renames(struct merge_options *opt,
 {
 	struct diff_queue_struct combined;
 	struct rename_info *renames = &opt->priv->renames;
-	int s, clean = 1;
+	int need_dir_renames, s, clean = 1;
 
 	memset(&combined, 0, sizeof(combined));
 
 	detect_regular_renames(opt, merge_base, side1, MERGE_SIDE1);
 	detect_regular_renames(opt, merge_base, side2, MERGE_SIDE2);
 
+	need_dir_renames =
+	  !opt->priv->call_depth &&
+	  (opt->detect_directory_renames == MERGE_DIRECTORY_RENAMES_TRUE ||
+	   opt->detect_directory_renames == MERGE_DIRECTORY_RENAMES_CONFLICT);
+
+	if (need_dir_renames) {
+		for (s = MERGE_SIDE1; s <= MERGE_SIDE2; s++)
+			get_provisional_directory_renames(opt, s, &clean);
+		handle_directory_level_conflicts(opt);
+	}
+
 	ALLOC_GROW(combined.queue,
 		   renames->pairs[1].nr + renames->pairs[2].nr,
 		   combined.alloc);

@@ -1139,6 +1139,110 @@ static int handle_content_merge(struct merge_options *opt,
 
 /*** Function Grouping: functions related to directory rename detection ***/
 
+MAYBE_UNUSED
+static void get_renamed_dir_portion(const char *old_path, const char *new_path,
+				    char **old_dir, char **new_dir)
+{
+	char *end_of_old, *end_of_new;
+
+	/* Default return values: NULL, meaning no rename */
+	*old_dir = NULL;
+	*new_dir = NULL;
+
+	/*
+	 * For
+	 *    "a/b/c/d/e/foo.c" -> "a/b/some/thing/else/e/foo.c"
+	 * the "e/foo.c" part is the same, we just want to know that
+	 *    "a/b/c/d" was renamed to "a/b/some/thing/else"
+	 * so, for this example, this function returns "a/b/c/d" in
+	 * *old_dir and "a/b/some/thing/else" in *new_dir.
+	 */
+
+	/*
+	 * If the basename of the file changed, we don't care.  We want
+	 * to know which portion of the directory, if any, changed.
+	 */
+	end_of_old = strrchr(old_path, '/');
+	end_of_new = strrchr(new_path, '/');
+
+	/*
+	 * If end_of_old is NULL, old_path wasn't in a directory, so there
+	 * could not be a directory rename (our rule elsewhere that a
+	 * directory which still exists is not considered to have been
+	 * renamed means the root directory can never be renamed -- because
+	 * the root directory always exists).
+	 */
+	if (end_of_old == NULL)
+		return; /* Note: *old_dir and *new_dir are still NULL */
+
+	/*
+	 * If new_path contains no directory (end_of_new is NULL), then we
+	 * have a rename of old_path's directory to the root directory.
+	 */
+	if (end_of_new == NULL) {
+		*old_dir = xstrndup(old_path, end_of_old - old_path);
+		*new_dir = xstrdup("");
+		return;
+	}
+
+	/* Find the first non-matching character traversing backwards */
+	while (*--end_of_new == *--end_of_old &&
+	       end_of_old != old_path &&
+	       end_of_new != new_path)
+		; /* Do nothing; all in the while loop */
+
+	/*
+	 * If both got back to the beginning of their strings, then the
+	 * directory didn't change at all, only the basename did.
+	 */
+	if (end_of_old == old_path && end_of_new == new_path &&
+	    *end_of_old == *end_of_new)
+		return; /* Note: *old_dir and *new_dir are still NULL */
+
+	/*
+	 * If end_of_new got back to the beginning of its string, and
+	 * end_of_old got back to the beginning of some subdirectory, then
+	 * we have a rename/merge of a subdirectory into the root, which
+	 * needs slightly special handling.
+	 *
+	 * Note: There is no need to consider the opposite case, with a
+	 * rename/merge of the root directory into some subdirectory
+	 * because as noted above the root directory always exists so it
+	 * cannot be considered to be renamed.
+	 */
+	if (end_of_new == new_path &&
+	    end_of_old != old_path && end_of_old[-1] == '/') {
+		*old_dir = xstrndup(old_path, --end_of_old - old_path);
+		*new_dir = xstrdup("");
+		return;
+	}
+
+	/*
+	 * We've found the first non-matching character in the directory
+	 * paths.  That means the current characters we were looking at
+	 * were part of the first non-matching subdir name going back from
+	 * the end of the strings.  Get the whole name by advancing both
+	 * end_of_old and end_of_new to the NEXT '/' character.  That will
+	 * represent the entire directory rename.
+	 *
+	 * The reason for the increment is cases like
+	 *    a/b/star/foo/whatever.c -> a/b/tar/foo/random.c
+	 * After dropping the basename and going back to the first
+	 * non-matching character, we're now comparing:
+	 *    a/b/s          and         a/b/
+	 * and we want to be comparing:
+	 *    a/b/star/      and         a/b/tar/
+	 * but without the pre-increment, the one on the right would stay
+	 * a/b/.
+	 */
+	end_of_old = strchr(++end_of_old, '/');
+	end_of_new = strchr(++end_of_new, '/');
+
+	/* Copy the old and new directories into *old_dir and *new_dir. */
+	*old_dir = xstrndup(old_path, end_of_old - old_path);
+	*new_dir = xstrndup(new_path, end_of_new - new_path);
+}
+
 static void compute_rename_counts(struct diff_queue_struct *pairs,
 				  struct strmap *dir_rename_count,
 				  struct strset *dirs_removed)

@@ -171,12 +171,15 @@ struct merge_options_internal {
 	struct rename_info renames;
 
 	/*
-	 * current_dir_name: temporary var used in collect_merge_info_callback()
+	 * current_dir_name, toplevel_dir: temporary vars
 	 *
-	 * Used to set merged_info.directory_name; see documentation for that
-	 * variable and the requirements placed on that field.
+	 * These are used in collect_merge_info_callback(), and will set the
+	 * various merged_info.directory_name for the various paths we get;
+	 * see documentation for that variable and the requirements placed on
+	 * that field.
 	 */
 	const char *current_dir_name;
+	const char *toplevel_dir;
 
 	/* call_depth: recursion level counter for merging merge bases */
 	int call_depth;

@@ -734,10 +737,10 @@ static int collect_merge_info(struct merge_options *opt,
 	int ret;
 	struct tree_desc t[3];
 	struct traverse_info info;
-	const char *toplevel_dir_placeholder = "";
 
-	opt->priv->current_dir_name = toplevel_dir_placeholder;
-	setup_traverse_info(&info, toplevel_dir_placeholder);
+	opt->priv->toplevel_dir = "";
+	opt->priv->current_dir_name = opt->priv->toplevel_dir;
+	setup_traverse_info(&info, opt->priv->toplevel_dir);
 	info.fn = collect_merge_info_callback;
 	info.data = opt;
 	info.show_all_errors = 1;

@@ -1139,11 +1139,69 @@ static int handle_content_merge(struct merge_options *opt,
 
 /*** Function Grouping: functions related to directory rename detection ***/
 
+static void compute_rename_counts(struct diff_queue_struct *pairs,
+				  struct strmap *dir_rename_count,
+				  struct strset *dirs_removed)
+{
+	die("Not yet implemented!");
+}
+
 static void get_provisional_directory_renames(struct merge_options *opt,
 					      unsigned side,
 					      int *clean)
 {
-	die("Not yet implemented!");
+	struct hashmap_iter iter;
+	struct strmap_entry *entry;
+	struct rename_info *renames = &opt->priv->renames;
+
+	compute_rename_counts(&renames->pairs[side],
+			      &renames->dir_rename_count[side],
+			      &renames->dirs_removed[side]);
+	/*
+	 * Collapse
+	 *    dir_rename_count: old_directory -> {new_directory -> count}
+	 * down to
+	 *    dir_renames: old_directory -> best_new_directory
+	 * where best_new_directory is the one with the unique highest count.
+	 */
+	strmap_for_each_entry(&renames->dir_rename_count[side], &iter, entry) {
+		const char *source_dir = entry->key;
+		struct strintmap *counts = entry->value;
+		struct hashmap_iter count_iter;
+		struct strmap_entry *count_entry;
+		int max = 0;
+		int bad_max = 0;
+		const char *best = NULL;
+
+		strintmap_for_each_entry(counts, &count_iter, count_entry) {
+			const char *target_dir = count_entry->key;
+			intptr_t count = (intptr_t)count_entry->value;
+
+			if (count == max)
+				bad_max = max;
+			else if (count > max) {
+				max = count;
+				best = target_dir;
+			}
+		}
+
+		if (max == 0)
+			continue;
+
+		if (bad_max == max) {
+			path_msg(opt, source_dir, 0,
+			       _("CONFLICT (directory rename split): "
+				 "Unclear where to rename %s to; it was "
+				 "renamed to multiple other directories, with "
+				 "no destination getting a majority of the "
+				 "files."),
+			       source_dir);
+			*clean &= 0;
+		} else {
+			strmap_put(&renames->dir_renames[side],
+				   source_dir, (void*)best);
+		}
+	}
 }
 
 static void handle_directory_level_conflicts(struct merge_options *opt)

@@ -1139,6 +1139,11 @@ static int handle_content_merge(struct merge_options *opt,
 
 /*** Function Grouping: functions related to directory rename detection ***/
 
+struct collision_info {
+	struct string_list source_files;
+	unsigned reported_already:1;
+};
+
 static void get_renamed_dir_portion(const char *old_path, const char *new_path,
 				    char **old_dir, char **new_dir)
 {

@@ -1378,6 +1383,31 @@ static void handle_directory_level_conflicts(struct merge_options *opt)
 	string_list_clear(&duplicated, 0);
 }
 
+static void compute_collisions(struct strmap *collisions,
+			       struct strmap *dir_renames,
+			       struct diff_queue_struct *pairs)
+{
+	die("Not yet implemented.");
+}
+
+static char *check_for_directory_rename(struct merge_options *opt,
+					const char *path,
+					unsigned side_index,
+					struct strmap *dir_renames,
+					struct strmap *dir_rename_exclusions,
+					struct strmap *collisions,
+					int *clean_merge)
+{
+	die("Not yet implemented.");
+}
+
+static void apply_directory_rename_modifications(struct merge_options *opt,
+						 struct diff_filepair *pair,
+						 char *new_path)
+{
+	die("Not yet implemented.");
+}
+
 /*** Function Grouping: functions related to regular rename detection ***/
 
 static int process_renames(struct merge_options *opt,

@@ -1703,22 +1733,44 @@ static void detect_regular_renames(struct merge_options *opt,
  */
 static int collect_renames(struct merge_options *opt,
 			   struct diff_queue_struct *result,
-			   unsigned side_index)
+			   unsigned side_index,
+			   struct strmap *dir_renames_for_side,
+			   struct strmap *rename_exclusions)
 {
 	int i, clean = 1;
+	struct strmap collisions;
 	struct diff_queue_struct *side_pairs;
+	struct hashmap_iter iter;
+	struct strmap_entry *entry;
 	struct rename_info *renames = &opt->priv->renames;
 
 	side_pairs = &renames->pairs[side_index];
+	compute_collisions(&collisions, dir_renames_for_side, side_pairs);
 
 	for (i = 0; i < side_pairs->nr; ++i) {
 		struct diff_filepair *p = side_pairs->queue[i];
+		char *new_path; /* non-NULL only with directory renames */
 
-		if (p->status != 'R') {
+		if (p->status != 'A' && p->status != 'R') {
 			diff_free_filepair(p);
 			continue;
 		}
 
+		new_path = check_for_directory_rename(opt, p->two->path,
+						      side_index,
+						      dir_renames_for_side,
+						      rename_exclusions,
+						      &collisions,
+						      &clean);
+
+		if (p->status != 'R' && !new_path) {
+			diff_free_filepair(p);
+			continue;
+		}
+
+		if (new_path)
+			apply_directory_rename_modifications(opt, p, new_path);
+
 		/*
 		 * p->score comes back from diffcore_rename_extended() with
 		 * the similarity of the renamed file.  The similarity is

@@ -1733,6 +1785,20 @@ static int collect_renames(struct merge_options *opt,
 		result->queue[result->nr++] = p;
 	}
 
+	/* Free each value in the collisions map */
+	strmap_for_each_entry(&collisions, &iter, entry) {
+		struct collision_info *info = entry->value;
+		string_list_clear(&info->source_files, 0);
+	}
+	/*
+	 * In compute_collisions(), we set collisions.strdup_strings to 0
+	 * so that we wouldn't have to make another copy of the new_path
+	 * allocated by apply_dir_rename().  But now that we've used them
+	 * and have no other references to these strings, it is time to
+	 * deallocate them.
+	 */
+	free_strmap_strings(&collisions);
+	strmap_clear(&collisions, 1);
 	return clean;
 }
 

@@ -1764,8 +1830,12 @@ static int detect_and_process_renames(struct merge_options *opt,
 	ALLOC_GROW(combined.queue,
 		   renames->pairs[1].nr + renames->pairs[2].nr,
 		   combined.alloc);
-	clean &= collect_renames(opt, &combined, MERGE_SIDE1);
-	clean &= collect_renames(opt, &combined, MERGE_SIDE2);
+	clean &= collect_renames(opt, &combined, MERGE_SIDE1,
+				 &renames->dir_renames[2],
+				 &renames->dir_renames[1]);
+	clean &= collect_renames(opt, &combined, MERGE_SIDE2,
+				 &renames->dir_renames[1],
+				 &renames->dir_renames[2]);
 	QSORT(combined.queue, combined.nr, compare_pairs);
 
 	clean &= process_renames(opt, &combined);

@@ -1139,7 +1139,6 @@ static int handle_content_merge(struct merge_options *opt,
 
 /*** Function Grouping: functions related to directory rename detection ***/
 
-MAYBE_UNUSED
 static void get_renamed_dir_portion(const char *old_path, const char *new_path,
 				    char **old_dir, char **new_dir)
 {

@@ -1243,11 +1242,61 @@ static void get_renamed_dir_portion(const char *old_path, const char *new_path,
 	*new_dir = xstrndup(new_path, end_of_new - new_path);
 }
 
+static void increment_count(struct strmap *dir_rename_count,
+			    char *old_dir,
+			    char *new_dir)
+{
+	struct strintmap *counts;
+	struct strmap_entry *e;
+
+	/* Get the {new_dirs -> counts} mapping using old_dir */
+	e = strmap_get_entry(dir_rename_count, old_dir);
+	if (e) {
+		counts = e->value;
+	} else {
+		counts = xmalloc(sizeof(*counts));
+		strintmap_init_with_options(counts, 0, NULL, 1);
+		strmap_put(dir_rename_count, old_dir, counts);
+	}
+
+	/* Increment the count for new_dir */
+	strintmap_incr(counts, new_dir, 1);
+}
+
 static void compute_rename_counts(struct diff_queue_struct *pairs,
 				  struct strmap *dir_rename_count,
 				  struct strset *dirs_removed)
 {
-	die("Not yet implemented!");
+	int i;
+
+	for (i = 0; i < pairs->nr; ++i) {
+		char *old_dir, *new_dir;
+		struct diff_filepair *pair = pairs->queue[i];
+
+		if (pair->status != 'R')
+			continue;
+
+		/* Get the old and new directory names */
+		get_renamed_dir_portion(pair->one->path, pair->two->path,
+					&old_dir,        &new_dir);
+		if (!old_dir)
+			/* Directory didn't change at all; ignore this one. */
+			continue;
+
+		/*
+		 * Make dir_rename_count contain a map of a map:
+		 *   old_directory -> {new_directory -> count}
+		 * In other words, for every pair look at the directories for
+		 * the old filename and the new filename and count how many
+		 * times that pairing occurs.
+		 */
+		if (strset_contains(dirs_removed, old_dir))
+			increment_count(dir_rename_count, old_dir, new_dir);
+
+		/* Free resources we don't need anymore */
+		free(old_dir);
+		free(new_dir);
+	}
 }
 
 static void get_provisional_directory_renames(struct merge_options *opt,