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Subject: Re: Bug in collation functions?
To: cygwin AT cygwin DOT com
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From: Ken Brown <kbrown AT cornell DOT edu>
Message-ID: <5634F6BA.7070301@cornell.edu>
Date: Sat, 31 Oct 2015 13:13:30 -0400
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On 10/30/2015 10:07 AM, Ken Brown wrote:
> Hi Corinna,
>
> On 10/30/2015 8:03 AM, Corinna Vinschen wrote:
>> On Oct 29 18:21, Ken Brown wrote:
>>> The fallback I had in mind is to return the shorter string if they have
>>> different lengths and otherwise to revert to wcscmp.
>  >
>> I had a longer look into this suggestion and the below code and it took
>> me some time to find out what bugged me with it:
>>
>> What about str/wcsxfrm?
>>
>> Per POSIX, calling strcmp on the result of strxfrm is equivalent to
>> calling strcoll (analogue with wcs*).  If you extend *coll to perform an
>> extra check on the length, you will have cases in which the above rule
>> fails.  You can't perform the length test on the result of *xfrm and
>> expect the same result as in *coll.
>>
>> In fact, when calling LCMapStringW with NORM_IGNORESYMOLS (you would
>> have to do this anyway if we add this flag in *coll), the resulting
>> transformed strings created from the input strings "11" and "1.1" would
>> be identical, so a length test on the xfrm string is not meaningful at
>> all.
>>
>> The bottom line is, afaics, we must make sure that CompareStringW and
>> LCMapStringW are called the same way, and their result/output has to be
>> returned to the caller.  Performing an extra check in *coll which can't
>> be reliably performed in *xfrm is not feasible.
>>
>> Does that make sense?
>
> Yes, I see the problem, and I don't see a good way around it.  So I
> think we probably have to leave things as they are and live with the
> fact that we can't do comparisons that ignore whitespace and punctuation.
>
> The alternative of allowing str/wcscoll to return 0 on unequal strings
> doesn't seem feasible in view of Eric's comments.

I have one other idea.  What would you think of defining a function 
cygwin_strcoll that's like strcoll but with an extra bool parameter 
'ignoresymbols'?  If ignoresymbols = false, this would be the same as 
strcoll.  If ignoresymbols = true, this would use NORM_IGNORESYMBOLS 
with the fallback I suggested.

That way applications that prefer to be more glibc-compatible and don't 
need strxfrm could do something like

   #define strcoll(A,B) cygwin_strcoll ((A), (B), true)

If you think this is reasonable, I'll submit a patch.  If not, no problem.

Ken


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