diffcore-rename: compute dir_rename_guess from dir_rename_counts

dir_rename_counts has a mapping of a mapping, in particular, it has
   old_dir => { new_dir => count }
We want a simple mapping of
   old_dir => new_dir
based on which new_dir had the highest count for a given old_dir.
Compute this and store it in dir_rename_guess.

This is the final piece of the puzzle needed to make our guesses at
which directory files have been moved to when basenames aren't unique.

For the testcases mentioned in commit 557ac03 ("merge-ort: begin
performance work; instrument with trace2_region_* calls", 2020-10-28),
this change improves the performance as follows:

                            Before                  After
    no-renames:       12.775 s ±  0.062 s    12.596 s ±  0.061 s
    mega-renames:    188.754 s ±  0.284 s   130.465 s ±  0.259 s
    just-one-mega:     5.599 s ±  0.019 s     3.958 s ±  0.010 s

Reviewed-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

diffcore-rename.c

@@ -389,6 +389,24 @@ static void dirname_munge(char *filename)
 	*slash = '\0';
 }
 
+static const char *get_highest_rename_path(struct strintmap *counts)
+{
+	int highest_count = 0;
+	const char *highest_destination_dir = NULL;
+	struct hashmap_iter iter;
+	struct strmap_entry *entry;
+
+	strintmap_for_each_entry(counts, &iter, entry) {
+		const char *destination_dir = entry->key;
+		intptr_t count = (intptr_t)entry->value;
+		if (count > highest_count) {
+			highest_count = count;
+			highest_destination_dir = destination_dir;
+		}
+	}
+	return highest_destination_dir;
+}
+
 static void increment_count(struct dir_rename_info *info,
 			    char *old_dir,
 			    char *new_dir)
@@ -512,6 +530,8 @@ static void initialize_dir_rename_info(struct dir_rename_info *info,
 				       struct strset *dirs_removed,
 				       struct strmap *dir_rename_count)
 {
+	struct hashmap_iter iter;
+	struct strmap_entry *entry;
 	int i;
 
 	if (!dirs_removed) {
@@ -558,6 +578,23 @@ static void initialize_dir_rename_info(struct dir_rename_info *info,
 					 rename_dst[i].p->one->path,
 					 rename_dst[i].p->two->path);
 	}
+
+	/*
+	 * Now we collapse
+	 *    dir_rename_count: old_directory -> {new_directory -> count}
+	 * down to
+	 *    dir_rename_guess: old_directory -> best_new_directory
+	 * where best_new_directory is the one with the highest count.
+	 */
+	strmap_for_each_entry(info->dir_rename_count, &iter, entry) {
+		/* entry->key is source_dir */
+		struct strintmap *counts = entry->value;
+		char *best_newdir;
+
+		best_newdir = xstrdup(get_highest_rename_path(counts));
+		strmap_put(&info->dir_rename_guess, entry->key,
+			   best_newdir);
+	}
 }
 
 void partial_clear_dir_rename_count(struct strmap *dir_rename_count)
@@ -682,10 +719,10 @@ static int idx_possible_rename(char *filename, struct dir_rename_info *info)
 	 *       rename.
 	 *
 	 * This function, idx_possible_rename(), is only responsible for (4).
-	 * The conditions/steps in (1)-(3) will be handled via setting up
-	 * dir_rename_count and dir_rename_guess in a future
-	 * initialize_dir_rename_info() function.  Steps (0) and (5) are
-	 * handled by the caller of this function.
+	 * The conditions/steps in (1)-(3) are handled via setting up
+	 * dir_rename_count and dir_rename_guess in
+	 * initialize_dir_rename_info().  Steps (0) and (5) are handled by
+	 * the caller of this function.
 	 */
 	char *old_dir, *new_dir;
 	struct strbuf new_path = STRBUF_INIT;