Re: git rename/moved status unreliable in ruby

From: Jeff King <peff@peff.net>
Date: May 4, 2026, 10:00 UTC
Message-ID: <20260504100056.GB599780@coredump.intra.peff.net>
In-Reply-To: <IC7a4NnSKMdvXlVyaSDYEtU7iRlKdJGzCwrXNCFKrtFfnBJTMrwY522rHF8PfzYxFs43huo0KFGrqB6f4IQjmvYi2B8Ehh0cwfjHHOYW_RU=@proton.me>

On Sat, May 02, 2026 at 09:34:18AM +0000, sebastien.stettler wrote:

> Has there been explorations of ignoring white space for the similarity checker, i would 
> assume that majority of white space movements across many languages would result in a 
> semantically similar document in most cases.

I don't think anybody has ever looked into it. We do have "-w" and friends for diffs, and it makes sense that there might be some mode to soften renames in the same way (especially if you are doing a "-w" diff, or a merge that ignores whitespace).

The line you need to touch is probably this:

diff --git a/diffcore-delta.c b/diffcore-delta.c
index 2b7db39983..379f6010d3 100644
--- a/diffcore-delta.c
+++ b/diffcore-delta.c
@@ -147,6 +147,8 @@ static struct spanhash_top *hash_chars(struct repository *r,
 		/* Ignore CR in CRLF sequence if text */
 		if (is_text && c == '\r' && sz && *buf == '\n')
 			continue;
+		if (is_text && (c == ' ' || c == '\t'))
+			continue;
 
 		accum1 = (accum1 << 7) ^ (accum2 >> 25);
 		accum2 = (accum2 << 7) ^ (old_1 >> 25);

but:

  1. The option to ignore whitespace would need to be plumbed through
     the rest of the diffcore code.

  2. This concept probably throws off some other rename heuristics.
     E.g., I think we do a rough check that the sizes of the objects are
     not too far apart before even looking at the content. So you could
     construct a pathological case where the line "a\n" was changed to
     have a million spaces, and the files would look like they couldn't
     possibly be similar, even though they are identical when ignoring
     whitespace. I think in practice you could just ignore this, as sane
     cases would tend to have a reasonable ratio of content to
     whitespace changes.

-Peff