I concur. The prefix is effectively a different element containing an integer. Specifically a specially hidden integer element. There is no problem if the encodings vary any more than there is a problem if the prefix is binary data, but the data itself text in some encoding.

There are many knids of things like this that can be expressed in DFDL, which don't exist anywhere in real data, but which DFDL has to be powerful enough to express, otherwise DFDL would be too limited in other cases.

So, nobody *should* ever create data where the prefix is still text, but in a different encoding than the data, but that said, it is no more difficult to implement than two adjacent elements, where there is a transition from one encoding to another. I've never seen alternating fields in different encodings, but I have seen files with mixtures of ascii, binary, and ebcdic data.


On Tue, Aug 27, 2013 at 12:44 PM, Steve Hanson <smh@uk.ibm.com> wrote:
Assuming that the prefix contains the length in characters then I think this works ok when the encoding is different.  The parser will first parse the prefix according to prefixLengthType to get the prefix value, which is always of known length. If prefixIncludesPrefixLength is 'yes' then it subtracts this known length from the prefix value, giving the length of the data, which might be in a different encoding.

I think we should continue to allow this.  In the past we have talked about a DFDL 2.0 feature that allowed the initiator and terminator to be specified using a simple type, precisely to cover the (rare) cases where the characteristics of these delimiters are different to the data itself.  Doing it this way prevents a property explosion on the element itself.  I view prefixLengthType as the first example of this principle.

Regards

Steve Hanson
Architect, IBM Data Format Description Language (DFDL)
Co-Chair, OGF DFDL Working Group
IBM SWG, Hursley, UK
smh@uk.ibm.com
tel:+44-1962-815848
----- Forwarded by Steve Hanson/UK/IBM on 23/08/2013 16:20 -----

From:        Alex Wood1/UK/IBM
To:        Steve Hanson/UK/IBM@IBMGB, Mark Frost/UK/IBM@IBMGB, dfdl-wg@ogf.org,
Date:        23/08/2013 14:58
Subject:        dfdl:lengthKind='prefixed' , different encodings for prefix and content where prefixIncludesPrefixLength is ‘yes.


Hi All,

Considering a case similar to that excluded by errata 2.76. An element with lengthKind 'prefixed' and  prefixIncludesPrefixLength 'true' but where the prefix type and the element both have lengthUnits 'characters' but have different encodings (or specifically encodings with different lengths of characters).

I believe the issue that 2.76 is trying to avoid is the issue of determining the length value in say characters when the prefix contains no characters.

I am wondering if there is also a slightly subtler issue when we are calculating a length in characters but where a part of the length is in a different encoding from the other.
For example the prefix contains 2 UTF16 (2 byte) characters and the content contains 2 UTF32 (4 byte) characters..
Do we just quote a length in characters regardless of encoding. eg. 4 characters.  Or is this confusing ....



2.76
.
Section 12.3.4

. When property prefixIncludesPrefixLength is ‘yes’there are some restrictions that need to be added to enable reliable lengths to be calculated:
o  If the prefix type is lengthKind 'implicit' or 'explicit' then the lengthUnits properties of
both the prefix type and the element must be the same.


Kind Regards,

- Alex

Alex Wood -
Software Engineer -
WebSphere Message Broker Development
IBM DFDL Development

MP 211, IBM UK Labs, Hursley Park, Winchester, Hants. SO21 2JN.
Tel: Internal 246272, External 01962 816272
Notes: Alex Wood1/UK/IBM@IBMGB
e-mail: wooda@uk.ibm.com

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