merge-recursive: clean up get_renamed_dir_portion()

Dscho noted a few things making this function hard to follow.
Restructure it a bit and add comments to make it easier to follow.  The
restructurings include:

  * There was a special case if-check at the end of the function
    checking whether someone just renamed a file within its original
    directory, meaning that there could be no directory rename involved.
    That check was slightly convoluted; it could be done in a more
    straightforward fashion earlier in the function, and can be done
    more cheaply too (no call to strncmp).

  * The conditions for advancing end_of_old and end_of_new before
    calling strchr were both confusing and unnecessary.  If either
    points at a '/', then they need to be advanced in order to find the
    next '/'.  If either doesn't point at a '/', then advancing them one
    char before calling strchr() doesn't hurt.  So, just rip out the
    if conditions and advance both before calling strchr().

Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

diff --git a/merge-recursive.c b/merge-recursive.c
index 22a12cfeba381d184db376301ac7b19d66f80e85..f80e48f623b98a86c5a0d014431aeb92e3de62f7 100644 (file)
--- a/merge-recursive.c
+++ b/merge-recursive.c
@@ -1943,8 +1943,8 @@ 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;
-       int old_len, new_len;
 
+       /* Default return values: NULL, meaning no rename */
        *old_dir = NULL;
        *new_dir = NULL;
 
@@ -1955,43 +1955,55 @@ static void get_renamed_dir_portion(const char *old_path, const char *new_path,
         *    "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.
-        *
-        * Also, if the basename of the file changed, we don't care.  We
-        * want to know which portion of the directory, if any, changed.
+        */
+
+       /*
+        * 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 == NULL || end_of_new == NULL)
-               return;
+               return; /* We haven't modified *old_dir or *new_dir yet. */
+
+       /* 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 */
+
        /*
-        * We've found the first non-matching character in the directory
-        * paths.  That means the current directory we were comparing
-        * represents the rename.  Move end_of_old and end_of_new back
-        * to the full directory name.
+        * 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 == '/')
-               end_of_old++;
-       if (*end_of_old != '/')
-               end_of_new++;
-       end_of_old = strchr(end_of_old, '/');
-       end_of_new = strchr(end_of_new, '/');
+       if (end_of_old == old_path && end_of_new == new_path &&
+           *end_of_old == *end_of_new)
+               return; /* We haven't modified *old_dir or *new_dir yet. */
 
        /*
-        * It may have been the case that old_path and new_path were the same
-        * directory all along.  Don't claim a rename if they're the same.
+        * 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/.
         */
-       old_len = end_of_old - old_path;
-       new_len = end_of_new - new_path;
+       end_of_old = strchr(++end_of_old, '/');
+       end_of_new = strchr(++end_of_new, '/');
 
-       if (old_len != new_len || strncmp(old_path, new_path, old_len)) {
-               *old_dir = xstrndup(old_path, old_len);
-               *new_dir = xstrndup(new_path, new_len);
-       }
+       /* 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 remove_hashmap_entries(struct hashmap *dir_renames,