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RE: PLease define 'collation'

From: Igor Hersht <igorh@ca.ibm.com>
Date: Tue, 8 Jun 2004 19:29:37 -0400
To: mhk@mhk.me.uk
Cc: public-qt-comments@w3.org, ashokmalhotra@alum.mit.edu, Stephen.Buxton@oracle.com
Message-ID: <OFE7BB8E71.332DBF13-ON85256EAD.0080345A-85256EAD.00810CA7@ca.ibm.com>





>A collation is a mapping from strings to sequences of integers,
>referred to as collation units. This mapping can be described as a
 >function C(xs:string)->xs:integer*.

>Two strings are considered equal if they map to the same
>sequence of collation units.

Problems.

The definition does not correspond the Unicode Collation
Algorithm (see
http://www.unicode.org/unicode/reports/tr10/#Main_Algorithm).
The algorithm consist of 3 steps
Step 1. Produce a normalized form of each input string.
Step 2. The collation element array is built
Step 3. The sort key is formed.
String comprising and matching based on the calculated values of
the sort key.

As far as I understand  the term ?collation unit? should corresponds
the Unicode term ?collation element?.  The step 3 is missing in the
collation
definition . Doing comprising based only on the collation
element is incorrect. The main problem is that collation elements
represent just part of the string. The sort key represents whole string.

Some of the problems (e.g. ignoring punctuation) could be fixed relatively
easy. ?a-? corresponds 2 collation elements, ?a? ? just one.
Using relatively simple algorithm we can find that the second collation
element is ignorable and not to include it in the final sequence.

Some of the problems (e.g. Contextual Sensitivity see 1.3 Contextual
Sensitivity)
would require very serious rework of the existing collation implementations
(the rework could be more time consuming than the rest of XSLT2.0 and XPath
functionality). Just an example from this chapter (which works fine with
the
ICU Unicode Collation Algorithm based implementation).
?In French and a few other languages, however, it is the last accent
difference
that determines the order?
Normal Accent Ordering  {'c','o','t', 0x00EA}  < {'c',0x00F4,'t', 'e'}
French Accent Ordering  {'c','o','t', 0x00EA}  > {'c',0x00F4,'t', 'e'}

There is another simple solution which is to create  sort key from the
collation
elements and map the key to an integer sequence. I don?t think that the
collation
definition make sense here.


Solution

2. Common collation definitions
A collation is a parameter for 2 functions fn:compare and fn:match.

fn:compare( $arg1  as xs:string?, $arg2  as xs:string?,
$collation  as xs:string) as xs:integer
Two strings (arg1 and arg2) are considered equal if fn:compare returns 0.
A string arg1 is considered greater than a string arg2 if fn:compare()
return is more than 0.

fn:match( $arg1  as xs:string?, $arg2  as xs:string?, $collation  as
xs:string) as xs:integer*
fn:match returns 2 integer sequence  (S(start,end)) or an empty ESsequence.

Definitions of fn:contains, fn:starts-with, fn:ends-with,
fn:substring-before and fn:substring-after are quiet obvious in terms
of fn:match.

fn:contains return true if fn:match returns not empty sequence.
fn:starts-with return true if fn:match returns not empty sequence
and start = 0.
fn:ends-with return true if fn:match returns not empty sequence
and end = arg1.length() -1.
fn:substring-before an empty string if fn:match returns an empty sequence.
Otherwise fn:substring-before returns arg1.substring(0,start).
fn:substring-afret an empty string if fn:match returns an empty sequence.
Otherwise fn:substring-before returns
arg1.substring(end, arg1.length()).

2.1 Specific collations
Specific collations should be defined as definitions for
fn:compare and fn:match functions.


2.1.1 Unicode collation
fn:compare and fn:match for the Unicode collation.
defined in http://www.unicode.org/unicode/reports/tr10.
The Unicode collation define different kinds of matching.
I think we can use an ambiguous -minimal one the(see 8 Searching and
Matching).
It could be good idea to define the maximal one (seems to corresponds a
common
sense). The match is maximal if for all positive i and j, there is no match
at Q[s-i,e+j].

2.1.2 The Unicode codepoint collation
fn:compare and fn:match is obvious here and corresponds and corresponds to
the
Michael Kay's one.

2.1.3  Other collations.
Other collations are implementation defined.



Igor Hersht
XSLT Development
IBM Canada Ltd., 8200 Warden Avenue, Markham, Ontario L6G 1C7
Office D2-260, Phone (905)413-3240 ; FAX  (905)413-4839
Received on Tuesday, 8 June 2004 19:30:24 UTC

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