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Re: [ddlm-group] UTF-8 BOM

To run through the alternatives and some of the arguments so far:

(i) treating an embedded BOM as an ordinary character runs against the Unicode recommendations.  If we wish our standard to be respected, I think we should at least respect other standards and the thinking that has gone into them

(ii) treating an embedded BOM as whitespace is OK with the Unicode standard, but means that a non-ASCII character now has syntactic meaning in the CIF.  I think this would be completely inconsistent on our part, as an invisible character (when displayed) can actually be used to delimit strings.  This is my least preferred solution, as it goes against the human-readability expected of CIFs

(iii) ignoring embedded BOMs is bad because they can be a 'tip off to a serious problem'.

(iv) treating embedded BOMs as syntax errors will cause issues when CIF2 files are naively concatenated

I think the only viable alternatives are to choose (iii) or (iv).

So: why exactly is ignoring a BOM a problem?  If the embedded BOM is the leading BOM from a UTF16 file that has been naively concatenated, it will have bytes 0xFE 0xFF.  This byte sequence (and the reverse) is not acceptable UTF8, leading to a decoding error from the UTF8 decoding step.  The subsequent bytes will be UTF16, which should cause a decoding failure in any case.   So I deduce that we are simply discussing how to treat a UTF8 BOM, which can only find its way into a CIF file by naive concatenation of UTF8-encoded files written by certain programs.

If the embedded BOM is a UTF-8 BOM, then ignoring it would be OK, as I don't see that it is indicative of any problems beyond misguided choice of text editor.

So I would advocate ignoring (and removing) UTF8-BOMs in the input stream, and treating all other BOMs as syntax errors.  Individual applications may wish to give users the option of interpreting U+FEFF as the deprecated ZWNBP (and translating to the correct character) on the understanding that if this occurs outside a delimited string it will cause a syntax error.


PS am I the only one who thinks it unlikely that Wordpad users would choose to use 'cat' to join file fragments together?

On Wed, May 19, 2010 at 3:46 AM, Herbert J. Bernstein <yaya@bernstein-plus-sons.com> wrote:
Allow me to clarify my position, so there is no misunderstanding:

I believe that we will be dealing with a world with at least UTF-8
and UCS-2/UTF-16 encodings for many years to come.  I have no
objection to CIF2 being specified solely in terms of UTF-8 for
simplicity and consistency, but if we are to write software that
people can use, we must have a reasonable position with respect
to the encodings people use, and that means that, at the very
least, we need to accept and process UTF-8 BOMs as harmless
additional text.  Some of us will also be supporting UCS-2/UTF-16
directly in our applications.  I don't mind if other applications
are only going to support UTF-8, but inasmuch as, as long as
we have java and web browsers, we are going to encounter UCS-2/UTF-16,
we should do something sensible when a UCS-2/UTF-16 BOM pops up,
either doing the internal translation if we so choose, or, if that
is not handled by a particular application, issuing a polite warning
suggesting the used of an external translator if the application does
not wish to handle UCS-2/UTF-16.

BOMS will almost always appear in modern UCS-2/UTF-16 files, and when
they are converted to UTF-8 that will give us yet another source of
UTF-8 BOMs.  I believe the sensible thing to so it to recognize BOMs.

 Herbert J. Bernstein, Professor of Computer Science
   Dowling College, Kramer Science Center, KSC 121
        Idle Hour Blvd, Oakdale, NY, 11769


On Tue, 18 May 2010, Bollinger, John C wrote:

> Herbert Bernstein wrote:
>> Let me see if I understand this correctly -- a user takes 2 perfectly good
>> CIF2 files, edits each to clean up, say, some comments to keep straight where
>> one begins and one ends, using a well-designed modern text editor that
>> happens to put a BOM at the start of each file, concatenates the two files
>> with cat to ship them into the IUCr, and suddenly they have a syntax error
>> caused by a character that they cannot see!!!
>> To me this seems pointless when it is trivial for software to recognize the
>> character and handle it sensibly.
> And that is my principal rationale for preferring that embedded U+FEFF be recognized as CIF whitespace.  With that approach, the concatenation of two well-formed CIF2 files is always a well-formed CIF2 file, regardless of the presence or absence of BOMs in the original files.  Note, too, that such concatenation cannot produce a mixed-encoding file because files encoded in UTF-16[BE|LE], UTF-32[BE|LE], or any other encoding that can be distinguished from UTF-8 are not well-formed CIF2 files to start.  The file concatenation scenario thus does not provide a use case for the CIF2 *specification* to recognize embedded U+FEFF as an encoding marker.
> On the other hand, I again feel compelled to distinguish program behaviors from the CIF2 format specification.  None of the above would prevent a CIF processor from recognizing and handling CIF-like character streams encoded via schemes other than UTF-8, nor from recognizing embedded U+FEFF code sequences in various encodings as encoding switches, thereby handling mixed-encoding files.  Indeed, such a program or library would be invaluable for correcting encoding-related errors.  That does not, however, mean that such files must be considered well-formed CIF2, no matter how likely they may (or may not) be to arise.
> James Hester wrote:
>> I would be happy to call an embedded BOM a syntax error.
> In light of the possibility of U+FEFF appearing in a data value (for example, from cutting text from a Unicode manuscript and pasting it into a CIF), I need to refine my earlier blanket alternative of treating embedded U+FEFF as a syntax error.  I now think it would be ok to treat U+FEFF as a syntax error *provided* that it appears outside a delimited string.  That's still not my preference, though, and I feel confident that Herb will still disagree.
> Regards,
> John
> --
> John C. Bollinger, Ph.D.
> Computing and X-Ray Scientist
> Department of Structural Biology
> St. Jude Children's Research Hospital
> John.Bollinger@StJude.org
> (901) 595-3166 [office]
> www.stjude.org
> Email Disclaimer:  www.stjude.org/emaildisclaimer
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