[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
Steve Hanson
smh at uk.ibm.com
Wed Jul 16 04:44:32 EDT 2014
I don't think it will work. Firstly A has minOccurs="0" which means there
is the potential for it not to appear in the data, which will cause the
variable to be set the wrong way. Secondly the dfdl:length expression for
B is forward referencing to C which is not allowed.
Regards
Steve Hanson
Architect, IBM DFDL
Co-Chair, OGF DFDL Working Group
IBM SWG, Hursley, UK
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 21:31
Subject: Re: 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
We'll I am a bit concerned if we cannot generally implement an asymetric
parser and unparser inside the same schema.
That is, if you have calculated element named C, then I think one should
be able to have hidden element A which is used only when parsing (C
computed from A), and hidden element B which is used only when unparsing,
(B computed from C), where B is non-existing on parse, and A is
non-existing on unparse.
I think this trick may work:
<!-- hidden. Found when parsing, not in infoset when unparsing -->
<element name="A" type="xs:string" minOccurs="0" dfdl:occursCountKind=
..whatever.. />
<!-- boolean. Tells whether we're parsing or unparsing based on whether A
is in infoset -->
<dfdl:newVariableInstance ref="areParsing" value ="{fn:exists(../A)}"/>
<!-- hidden. Length 0 when parsing. Unparsing gets length and value from C
-->
<element name="B" type="xs:string"
dfdl:lengthKind="explicit"
dfdl:length="{ if ($areParsing) then 0 else fn:string-length(../C) }"
default=""
dfdl:outputValueCalc="{ ../C }" />
<!-- not hidden -->
<element name="C" type="xs:string" dfdl:inputValueCalc="{ ../A }" />
On Mon, Jul 14, 2014 at 3:23 PM, Steve Hanson <smh at uk.ibm.com> wrote:
The proposal for action 261 is that the dfdl:length expression is
evaluated, so that will stop what you propose from working.
For the MIL-STD-2045 'fixedOrTerminated' case then the sequence suggestion
in the other email is much simpler.
Regards
Steve Hanson
Architect, IBM DFDL
Co-Chair, OGF DFDL Working Group
IBM SWG, Hursley, UK
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 19:51
Subject: Re: 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
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
Co-Chair, OGF DFDL Working Group
IBM SWG, Hursley, UK
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, '') }" />
<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
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
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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