Re: Diff doesn't properly ignore whitespace for this input

Thanks for the reply. It is so neat to see one of the original authors 
update diff with a fix that actually affects me. A nice object lesson in 
production software development for my class, too.

- Tyler

On 7/16/2015 4:45 PM, Jim Meyering wrote:
> On Tue, Jul 14, 2015 at 12:01 PM, Tyler Bletsch <[email protected]> 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