[DFDL-WG] Unparsing and choices - (related - Re: Fw: Action 233 (deferred) - "byte order not sufficient..." - draft document on experience with binary format MIL-STD-2045

[Mike Beckerle]

There has to be a way to get zero-length on parsing, but an outputValueCalc
string on unparsing.

<defineVariable name="dummyLength" defaultValue="0" external='true'/>

<element name="outputRaw" type="xs:string"
    dfdl:lengthKind="explicit" dfdl:length="{ $tns:dummyLength }"
    dfdl:outputValueCalc="{.....}" />

Would this work? The length expression isn't used when unparsing I believe
in that some other elements are supposed to be storing the length
information. That's the case here though the length value information isn't
being stored, what is being stored is a way to determine the length.

Maybe there's something simpler?



On Mon, Jul 14, 2014 at 1:42 PM, Steve Hanson <smh at uk.ibm.com> wrote:

> Mike
>
> Section 17 of the spec says "It is a schema definition error if
> dfdl:inputValueCalc and dfdl:outputValueCalc are specified on the same
> element. "  So outputRaw will give an SDE.
>
> Regards
>
> Steve Hanson
> Architect, *IBM DFDL*
> <http://www.ibm.com/developerworks/library/se-dfdl/index.html>
> Co-Chair, *OGF DFDL Working Group* <http://www.ogf.org/dfdl/>
> IBM SWG, Hursley, UK
> *smh at uk.ibm.com* <smh at uk.ibm.com>
> tel:+44-1962-815848
>
>
>
> From:        Mike Beckerle <mbeckerle.dfdl at gmail.com>
> To:        Steve Hanson/UK/IBM at IBMGB,
> Cc:        "dfdl-wg at ogf.org" <dfdl-wg at ogf.org>
> Date:        14/07/2014 16:43
> Subject:        Unparsing and choices - (related - Re: [DFDL-WG] Fw:
> Action 233 (deferred) - "byte order not sufficient..." - draft document on
> experience with binary format MIL-STD-2045
> ------------------------------
>
>
>
> Breaking out this one issue for a separate discussion thread.
>
> Consider the choice below. There is a choice of two different elements
> named raw1 and raw2, then a value element which is computed based on one or
> the other of the alternatives.
>
> <choice>
>        <!-- length kind pattern is needed to bound length to max of 99 -->
>        <element name="raw1" type="xs:string"
>            dfdl:lengthKind='pattern'
>            dfdl:lengthPattern="[^\x7F]{0,99}"
>            dfdl:terminator="%DEL;"/>
>        <element name="raw2" type="xs:string"
>             dfdl:lengthKind="explicit"
>             dfdl:length="100"/>
> </choice>
> <element name='value' type='xs:string'
>      dfdl:inputValueCalc='{ if (fn:exists( ../raw1 ) then ../raw1 else
> ../raw2 }'/>
>
> Now, given the statement below:
>
> SMH: There is no way to make a choice branch fail when unparsing. (The
> only 'backtracking' when unparsing a choice is when the infoset contains no
> branch at all then the spec states that each branch is examined in turn
> until one is found that successfully applies defaults. But that's not
> really backtracking, as you can statically deduce the branch from the
> schema alone, so the 'default' branch to use can be computed up front).
>
> So is there any way to fix this and make the above choice work?
>
> This is the only thing I can think of, which ends up with entirely
> separate elements for parsing and unparsing.
>
> I am not sure this works or should work.
>
> <group name="hidG1">
> <sequence>
> <!--
>    raw1 won't exist on unparsing
>   -->
> <element name="raw1" type="xs:string"
>            dfdl:lengthKind='pattern'
>            dfdl:lengthPattern="[^\x7F]{0,99}"
>            dfdl:terminator="%DEL;"
>            minOccurs='0' dfdl:occursCountKind='implicit'/>
> <!--
>    raw2 will only occur parsing if raw1 doesn't
>    and doesn't exist on unparsing
>    -->
> <element name="raw2" type="xs:string"
>             dfdl:lengthKind="explicit"
>             dfdl:length="100"
>             minOccurs='0' dfdl:occursCountKind='expression'
>             dfdl:occursCount="{ if (fn:exists(../raw1) then 0 else 1 }"/>
> <!--
>    the outputRaw will only be used when unparsing.
>    On parsing it will just get "" as value, and nobody uses the value.
>   -->
> <element name="outputRaw" type="xs:string"
>        dfdl:lengthKind="explicit" dfdl:length="{ 0 }"
>        dfdl:outputValueCalc="{ if (fn:string-length(../value) eq 100 then
> ../value else fn:concat(../value, '&#x7F;') }" />
> <sequence>
> </group>
>
> <!--
>     to use, we hide the cruft in a hidden group
>    -->
> <sequence dfdl:hiddenGroupRef="tns:hidG1"/>
> <element name='value' type='xs:string'
>      dfdl:inputValueCalc='{ if (fn:exists( ../raw1 ) then ../raw1 else
> ../raw2 }'/>
>
>
>
>
>
>
>
> 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
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