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From: Jim Meyering <jim@meyering.net>
Newsgroups: gnu.utils.bug
Subject: Re: Diff doesn't properly ignore whitespace for this input
Date: Thu, 16 Jul 2015 13:45:55 -0700
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To: Tyler Bletsch <tkbletsc@ncsu.edu>
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On Tue, Jul 14, 2015 at 12:01 PM, Tyler Bletsch <tkbletsc@ncsu.edu> wrote:
> I believe I've found a bug in diff's handling of "ignore whitespace" mode. I
> have two test files that differ only in whitespace and newlines; I've
> verified this using a separate tool (WinMerge) plus doing a diff on the
> files after doing s/\s*/ / on the whole file. When I ask for the diff using
> "-wb", it reports a spurious difference only in whitespace if I give the
> arguments in one order, but correctly reports no differences if I give it
> the reverse order.  Further, I get consistently correct behavior if I add
> the "-d" option.
>
> Example:
>
> $ diff -wB in1.txt in2.txt
> 3946c4201,4203
> < Exits:
> ---
>>
>>
>> Exits:
> $ diff -wB in2.txt in1.txt
> $ diff -dwB in1.txt in2.txt
> $ diff -dwB in2.txt in1.txt
>
> This came up while using diff to automatically grade a text adventure I'm
> having students do in my class -- this is the ONLY file pair out of over
> 3000 that appears to exhibit the problem. This leads me to believe that it
> must be a fairly rare issue. I'm fixing it on my end by always using -d, but
> I think this should be classified as a bug, because it reports a
> non-whitespace difference in files where none exists.
>
> I'm not sure if this mailing list allows attachments, so I've put the files
> in question here:
>
> https://dl.dropboxusercontent.com/u/68643317/diff-bug-test-files.zip
>
> I tried paring the files down to just demonstrate the bug and nothing else,
> but the behavior would seemingly go away at random as I removed content from
> the files. Therefore, I'm including the files in their original form. The
> files represent test output of the text adventure, specifically navigation
> of the default world from the ROM 2.4b6 MUD (after having been converted to
> a format for my class's assignment). This content is safe to share.
>
> I've confirmed that this behavior is present in the following builds of
> diff:
> - diff (GNU diffutils) 2.8.1 on Red Hat Enterprise Linux Server release 6.5
> (Santiago)
> - diff (GNU diffutils) 3.2 on Ubuntu 12.04.4 LTS
> - diff (GNU diffutils) 2.9 on Cygwin 32-bit (Windows 7 x64)

Thank you for the report.
I confirm that it also affects diff-3.3, but found that with the very
latest from diff.git (v3.3-30-g29e8de4), the problem does not arise.
I.e., comparing your two files like this produces no output:

  $ src/diff -wBu /t/in{1,2}.txt | wc -c
  0

I suspect that it was fixed via this change by Paul Eggert:

  http://git.savannah.gnu.org/cgit/diffutils.git/commit/?id=9b48bf3d3ed002e32fad
  http://bugs.gnu.org/16848