[Cif2-encoding] Drafting issues

[James Hester]

Dear Group,

As I think we have reached a consensus in principle, and are now moving into
discussion of precise definitions, let us have wording arguments only once
(that is, for a single document).  I think that our base document must be
the one that the DDLm group agreed on - the link once again is
http://www.iucr.org/__data/assets/pdf_file/0017/41426/cif2_syntax_changes_jrh20100705.pdf-
simply because it will be unnecessarily confusing for the DDLm group
to
deal with two documents at once, and the 'Changes' document is admirably
precise.  I reiterate once again that I am happy with the motion that
Herbert presented, with the proviso that one paragraph is rewritten as I
have recently proposed.  Herbert - if you would like to negotiate that
paragraph with me by Skype, I'm happy to do that too.

I have appended a text version of what I consider to be the relevant
sections of the 'changes' document to this message.  I am happy to provide
the complete document in OpenOffice format to anybody who would like it.
Herbert - if you think any of the non-encoding discussion in your motion is
not already covered in the 'Changes' document, please advise.

I will be posting my own suggestion, largely based on parts of the motion
that Herbert and I drafted yesterday, in a reply to this email.

  CIF - Changes to the specification 05 July 2010

This document specifies changes to the *syntax *of CIF. We refer to the
current syntax specification of CIF as CIF1, and the new specification as
CIF2. To date all archival CIFs are CIF1.

The changes to syntax are necessitated by the adoption of new dictionary
functionalities that introduce several extensions, including new data types,
and method definitions using dREL.

It is assumed the reader has a thorough understanding of the CIF1
specification.
TERMINOLOGY

Reference to *character(s)* means abstract characters assigned code points
by *Unicode*. Specific characters are referenced according to Unicode
convention, U⁠+⁠*xxxx*[*x*[*x*]], where *xxxx*[*x*[*x*]]* *is the four- to
six-digit hexadecimal representation of the assigned code point. The
designated character encoding for CIF2 is UTF-8.

Reference to *ASCII *characters means characters U⁠+⁠0000* *through* *
U⁠+⁠007F,* *or, equivalently the first 128 characters of the *ISO 8859 1* (*
LATIN 1*)* *character set.

Reference to *newline *or *\n *means the sequence that conventionally
terminates a line record (which is environment dependent). * See Change 3.*

Reference to *whitespace *means the characters ASCII space (U⁠+⁠0020), ASCII
horizontal tab (U⁠+⁠0009) and the *newline *characters. Without regard to
local convention, the various other characters that Unicode classifies as
whitespace (character categories Zs and Zp) do not constitute *whitespace *for
the purposes of CIF2.
PREAMBLE

CIF2 significantly extends CIF1 functionality, primarily through new
dictionary features. CIF2 is not fully backwards-compatible with CIF1: many
files compliant with CIF1 are also compliant with CIF2, but some are not
(see especially change 5, below). The CIF1 standard will continue to operate
for the foreseeable future in parallel with CIF2.
CHANGE 1 ‒ NEW (MAGIC CODE)

A CIF2 file is uniquely identified by a required magic code at the beginning
of its first line. The code is,

#\#CIF_2.0

followed immediately by *whitespace*.
CHANGE 2 ‒ NEW (CHARACTER SET)

CIF2 files are standard variable length text files, which for compatibility
with older processing systems will have a maximum line length of 2048
characters. As discussed above and below, however, there are some
restrictions on the character set for token delimiters, separators and data
names.

In keeping with XML restrictions we allow the characters

U⁠+⁠0009 U⁠+⁠000A U⁠+⁠000D
U⁠+⁠0020 – U+007E
U+00A0 - U⁠+⁠D7FF
U⁠+⁠E000 – U+FDCF
U⁠+⁠FDF0 - U+FFFD
U⁠+⁠10000 - U⁠+⁠10FFFD

In addition, character U+FEFF and characters U+xFFFE or U+xFFFF where x is
any hexadecimal digit are disallowed. Unicode reserves the code points E000 –
F8FF for private use. The IUCr and only the IUCr may specify what characters
are assigned to these code points in the context of CIF2.

*Reasoning: There is growing demand for the wider character set afforded by
Unicode to be made available in applications, especially those where
internationalisation is an issue.*

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