Background for discussion on DFDL WG call today. Part of deferred action 242.

Regards

Steve Hanson
IBM Integration Bus, Hursley, UK
Architect, IBM DFDL
Co-Chair, OGF DFDL Working Group
smh@uk.ibm.com
tel:+44-1962-815848
mob:+44-7717-378890
----- Forwarded by Steve Hanson/UK/IBM on 24/05/2016 11:58 -----

From: Steve Hanson/UK/IBM
To: Mike Beckerle <mbeckerle.dfdl@gmail.com>
Cc: "dfdl-wg@ogf.org" <dfdl-wg@ogf.org>, dfdl-wg-bounces@ogf.org
Date: 28/04/2014 11:10
Subject: Re: [DFDL-WG] dfdl:contentLength and complex types with interior alignmentFill

Mike

This doesn't solve the problem fully, because it assumes an alignment of 0 and in practice the alignment might be some odd number that forces the first child with alignment 2, 4 or 8 onto a different offset, and hence the length is different. So I'm not sure whether the additional calculation helps much.

Regards

Steve Hanson
Architect, IBM DFDL
Co-Chair, OGF DFDL Working Group
IBM SWG, Hursley, UK
smh@uk.ibm.com
tel:+44-1962-815848

From: Mike Beckerle <mbeckerle.dfdl@gmail.com>
To: "dfdl-wg@ogf.org" <dfdl-wg@ogf.org>,
Date: 24/04/2014 22:36
Subject: [DFDL-WG] dfdl:contentLength and complex types with interior alignmentFill
Sent by: dfdl-wg-bounces@ogf.org

dfdl:contentLength interacts pretty badly with alignment because alignmentFill regions mean you cannot know the size of a complex type element without knowing its starting position.

It is very likely that one will have outputValueCalc that wants to evalutate dfdl:contentLength on an infoset item before the starting position for that item is known or knowable.

To fix this we say that dfdl:contentLength(e) on complex-type infoset item e, is computed as if element e's alignment was first put into place along with the rest of the LeftFraming, and then the content length computed as if the complex content was unparsed starting from after that LeftFraming up to the final RightFraming In other words, the entire element has to be unparsed, not just the complex content of it, and then the part of that unparsed data that corresponds to the complex content is used as the return value from dfdl:contentLength.

This works because element e's alignment (LeftFraming generally) is not within the complex content for element e. It is before the complex content of the element, so if the schema puts an explicit alignment on element e, then it will clearly be aligned that way when it is really unparsed, but the dfdl:contentLength function will also compute a length from the infoset value as if that same alignment were in place.

I think that is about the best we can do.

Thoughts?

Mike Beckerle | OGF DFDL Workgroup Co-Chair | Tresys Technology | www.tresys.com
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Unless stated otherwise above:
IBM United Kingdom Limited - Registered in England and Wales with number 741598.
Registered office: PO Box 41, North Harbour, Portsmouth, Hampshire PO6 3AU