Re: [PATCH 1/2] merge-recursive: clean up get_renamed_dir_portion()

[Johannes Schindelin <Johannes.Schindelin@gmx.de>]

Hi Elijah,

On Fri, 11 Oct 2019, Elijah Newren via GitGitGadget wrote:

> From: Elijah Newren <newren@gmail.com>
>
> 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>

This commit message, as well as the patch, make a lot of sense to me.
Thank you for doing this!

Ciao,
Dscho

> ---
>  merge-recursive.c | 60 ++++++++++++++++++++++++++++-------------------
>  1 file changed, 36 insertions(+), 24 deletions(-)
>
> diff --git a/merge-recursive.c b/merge-recursive.c
> index 22a12cfeba..f80e48f623 100644
> --- 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,
> --
> gitgitgadget
>
>