Re: [RFH] xdiff shows trivially redundant diff.

[Davide Libenzi <davidel@xmailserver.org>]

On Sun, 2 Apr 2006, Linus Torvalds wrote:

>
>
> On Sun, 2 Apr 2006, Davide Libenzi wrote:
>>
>> Yes, it does even vanilla libxdiff ;) It's not a problem though, since it is
>> created in xdl_cleanup_records() that tries to do a fast pass over the records
>> to try to simplify the real diff operation. In trying to be fast, only hashes
>> are compared, and it happens that the hash for "'')" collides with another one
>> (try to replace one of the "'')" chars with another one). Why is this not a
>> problem? Because what this lead to is only lines to be marked as changed, with
>> a probability of about N/2^(8 * sizeof(long) - 1), even though they are not.
>> And this happens only during sequential groups of lines changed, that is when
>> the hash-colliding line is either at the begin or the end of the run.
>
> Hmm. It's still ugly, though. No possibility to have a "clean up identical
> initial and final lines" stage to get rid of extraneous bogus diffs?

It does ;) If you make the second hunk (the one with the '') line) to be 
the first, the shrink-initial-and-final lines optimizations will make it 
eat the '') line.


> I look at diffs a lot, and while this may be rare, if I were to end up
> having to wonder what the difference is and it turns out that it's just
> due to a libxdelta thing, I'd be a bit irritated and wish it gave me a
> proper diff..

Tomorrow I'll take a look at it.


- Davide