Network Working Group M. Crispin
Internet-Draft University of Washington
Intended status: Proposed Standard August 8, 2007
Expires: February 8, 2008
Document: internet-drafts/draft-crispin-collation-unicasemap-06.txt
i;unicode-casemap - Simple Unicode Collation Algorithm
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Abstract
This document describes "i;unicode-casemap", a simple
case-insensitive collation for Unicode strings. It provides
equality, substring and ordering operations.
Introduction
The "i;ascii-casemap" collation described in [COMPARATOR] is quite
simple to implement and provides case-independent comparisons for the
26 Latin alphabetics. It is specified as the default and/or baseline
comparator in some application protocols, e.g., [IMAP-SORT].
However, the "i;ascii-casemap" collation does not produce
satisfactory results with non-ASCII characters. It is possible, with
a modest extension, to provide a more sophisticated collation with
greater multilingual applicability than "i;ascii-casemap". This
extension provides case-independent comparisons for a much greater
number of characters. It also collates characters with diacriticals
with the non-diacritical character forms.
This collation, "i;unicode-casemap", is intended to be an alternative
to, and preferred over, "i;ascii-casemap". It does not replace the
"i;basic" collation described in [BASIC].
1. Unicode Casemap Collation Description
The "i;unicode-casemap" collation is a simple collation which is
case-insensitive in its treatment of characters. It provides
equality, substring and ordering operations. The validity test
operation returns "valid" for any input.
This collation allows strings in arbitrary (and mixed) character
sets, as long as the character set for each string is identified and
it is possible to convert the string to Unicode. Strings which have
an unidentified character set and/or can not be converted to Unicode
are not rejected, but are treated as binary.
Each input string is prepared by converting it to a "titlecased
canonicalized UTF-8" string according to the following steps, using
using UnicodeData.txt ([UNICODE-DATA]):
(1) A Unicode codepoint is obtained from the input string.
(a) If the input string is in a known charset that can be
converted to Unicode, a sequence in the string's charset
is read and checked for validity according to the rules of
that charset. If the sequence is valid, it is converted
to a Unicode codepoint. Note that for input strings in
UTF-8, the UTF-8 sequence must be valid according to the
rules of [UTF-8]; e.g., overlong UTF-8 sequences are
invalid.
(b) If the input string is in an unknown charset, or an
invalid sequence occurs in step (1)(a), conversion ceases.
No further preparation is performed, and any partial
preparation results are discarded. The original string is
used unchanged with the i;octet comparator.
(2) The following steps, using UnicodeData.txt ([UNICODE-DATA]),
are performed on the resulting codepoint from step (1)(a).
(a) If the codepoint has a titlecase property in
UnicodeData.txt (this is normally the same as the
uppercase property), the codepoint is converted to the
codepoints in the titlecase property.
(b) If the resulting codepoint from (2)(a) has a decomposition
property of any type in UnicodeData.txt, the codepoint is
converted to the codepoints in the decomposition property.
This step is recursively applied to each of the resulting
codepoints until no more decomposition is possible
(effectively Normalization Form KD).
Example: codepoint U+01C4 (LATIN CAPITAL LETTER DZ WITH CARON)
has a titlecase property of U+01C5 (LATIN CAPITAL LETTER D
WITH SMALL LETTER Z WITH CARON). Codepoint U+01C5 has a
decomposition property of U+0044 (LATIN CAPITAL LETTER D)
U+017E (LATIN SMALL LETTER Z WITH CARON). U+017E has a
decomposition property of U+007A (LATIN SMALL LETTER Z) U+030c
(COMBINING CARON). Neither U+0044, U+007A, nor U+030C have
any decomposition properties. Therefore, U+01C4 is converted
to U+0044 U+007A U+030C by this step.
(3) The resulting codepoint(s) from step (2) is/are appended, in
UTF-8 format, to the "titledcased canonicalized UTF-8" string.
(4) Repeat from step (1) until there is no more data in the input
string.
Following the above preparation process on each string, the equality,
ordering and substring operations are as for i;octet.
It is permitted to use an alternative implementation of the above
preparation process if it produces the same results. For example, it
may be more convenient for an implementation to convert all input
strings to a sequence of UTF-16 or UTF-32 values prior to performing
any of the step (2) actions. Similarly, if all input strings are (or
are convertable to) Unicode, it may be possible to use UTF-32 as an
alternative to UTF-8 in step (3).
Note: UTF-16 is unsuitable as an alternative to UTF-8 in step (3),
because UTF-16 surrogates will cause i;octet to collate codepoints
U+E0000 through U+FFFF after non-BMP codepoints.
This collation is not locale sensitive. Consequently, care should be
taken when using OS-supplied functions to implement this collation.
Functions such as strcasecmp and toupper are sometimes locale
sensitive and may inconsistently casemap letters.
The i;unicode-casemap collation is well suited to use with many
Internet protocols and computer languages. Use with natural language
is often inappropriate; even though the collation apparently supports
languages such as Swahili and English, in real-world use it tends to
mis-sort a number of types of string:
o people and place names containing scripts that are not collated
according to "alphabetical order".
o words with characters that have diacriticals. However,
i;unicode-casemap generally does a better job than i;ascii-casemap
for most (but not all) languages. For example, German umlaut
letters will sort correctly, but some Scandinavian letters will
not.
o names such as "Lloyd" (which in Welsh sorts after "Lyon", unlike
in English),
o strings containing other non-letter symbols; e.g., euro and pound
sterling symbols, quotation marks other than '"', dashes/hyphens,
etc.
2. Unicode Casemap Collation Registration
<?xml version='1.0'?>
<!DOCTYPE collation SYSTEM 'collationreg.dtd'>
<collation rfc="XXXX" scope="global" intendedUse="common">
<identifier>i;unicode-casemap</identifier>
<title>Unicode Casemap</title>
<operations>equality order substring</operations>
<specification>RFC XXXX</specification>
<owner>IETF</owner>
<submitter>mrc@cac.washington.edu</submitter>
</collation>
3. Security Considerations
The security considerations for [UTF-8], [STRINGPREP] and
[UNICODE-SECURITY] apply and are normative to this specification.
4. IANA Considerations
The i;unicode-casemap collation defined in section 2 should be added
to the registry of collations defined in [COMPARATOR].
5. Normative References
The following documents are normative to this document:
[COMPARATOR] Newman, C., "Internet Appplication Protocol
Collation Registry", RFC 4790, February 2007.
[STRINGPREP] Hoffman, P. and M. Blanchet, "Preparation of
Internationalized Strings ("stringprep")",
RFC 3454, December 2002.
[UTF-8] Yergeau, F., "UTF-8, a transformation format
of ISO 10646", STD 63, RFC 3629, November 2003.
[UNICODE-DATA] <http://www.unicode.org/Public/UNIDATA/
UnicodeData.txt>
Although the UnicodeData.txt file referenced
here is part of the Unicode standard, it is
subject to change as new characters are added
to Unicode and errors are corrected in Unicode
revisions. As a result, it may be less stable
than might otherwise be implied by the
standards status of this specification.
[UNICODE-SECURITY] Davis, M. and M. Suignard, "Unicode Security
Considerations", February 2006,
<http://www.unicode.org/reports/tr36/>.
6. Informative References:
[BASIC] Newman, C., Duerst, M., and Gulbrandsen, A.,
"i;basic - the Unicode Collation Algorithm",
draft-gulbrandsen-collation-basic, Work in
Progress.
[IMAP-SORT] Crispin, M. "Internet Message Access Protocol -
SORT and THREAD Extensions",
draft-ietf-imapext-sort, Work in Progress (in
RFC Editor queue).
Appendices
Author's Address
Mark R. Crispin
Networks and Distributed Computing
University of Washington
4545 15th Avenue NE
Seattle, WA 98105-4527
Phone: +1 (206) 543-5762
EMail: MRC@CAC.Washington.EDU
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