Subject: Re: [GSoC PATCH 1/1] diff: improve scaling of filenames in diffstat to handle UTF-8 chars

From: Junio C Hamano <[email-masked]>
Date: Wed, 14 Jan 2026 14:50:02 -0800

LorenzoPegorari <[email-masked]> writes:

> The `show_stats()` function tries to scale the filenames in the diffstat to
> ensure they don't exceed the given `name-width`. It does so by calculating
> the "display width" of the characters to be dropped, but then advances the
> filename pointer by that number of bytes.
>
> However, the "display width" of a character is not always equal to its byte
> count. The result is that sometimes, when displaying UTF-8 characters,
> filenames exceed the given `name-width`, and frequently the bytes of the
> UTF-8 characters are truncated.
>
> The following is an example of the issue, where the 2 files are "HelloHi" and
> "Hello你好", and `name-width=6`:
>
>     ...oHi | 0
>     ...<BD><A0>好 | 0
>
> Make the filename pointer move by the actual number of bytes of the
> characters to drop from the filename, rather than their display width, using
> the `utf8_width()` function.
>
> Signed-off-by: LorenzoPegorari <[email-masked]>
> ---
>  diff.c | 15 ++++-----------
>  1 file changed, 4 insertions(+), 11 deletions(-)

Two comments and a half.

 * The change needed for this is surprisingly simple.

 * You already know about samples that may exhibit the issue you are
   addressing.  Can we add it as a test case somewhere in t/
   directory?

 * The NEEDSWORK item addressed by this patch is one of the two
   NEEDSWORK items added by ce8529b2 (diff: leave NEEDWORK notes in
   show_stats() function, 2022-10-21).  Makes me wonder how involved
   the changes would need to be to solve the other one?

Thanks.

> diff --git a/diff.c b/diff.c
> index a68ddd2168..271ace5728 100644
> --- a/diff.c
> +++ b/diff.c
> @@ -2859,17 +2859,10 @@ static void show_stats(struct diffstat_t *data, struct diff_options *options)
>  			char *slash;
>  			prefix = "...";
>  			len -= 3;
> -			/*
> -			 * NEEDSWORK: (name_len - len) counts the display
> -			 * width, which would be shorter than the byte
> -			 * length of the corresponding substring.
> -			 * Advancing "name" by that number of bytes does
> -			 * *NOT* skip over that many columns, so it is
> -			 * very likely that chomping the pathname at the
> -			 * slash we will find starting from "name" will
> -			 * leave the resulting string still too long.
> -			 */
> -			name += name_len - len;
> +
> +			while (name_len > len)
> +				name_len -= utf8_width((const char**)&name, NULL);
> +
>  			slash = strchr(name, '/');
>  			if (slash)
>  				name = slash;