[DFDL-WG] DFDL: output and choices/discriminators

[Alan Powell]

Mike

I think there is an easier solution to your example using an expression 
for the dfdl:length property of the length field instead of the choice.


       <xs:element name="OneOrfourByte" type="xs:int" 
                   dfdl:alignment="1" 
                   dfdl:length='{ if ( ../logicalLength > 127 ) then "31" 
else "7"  }'
                    dfdl:outputValueCalc="{ ../logicalLength }" />
 

For a simple infoset element which can have multiple physical 
representations is really support for union which we have excluded from 
V1. 


Alan Powell

 MP 211, IBM UK Labs, Hursley,  Winchester, SO21 2JN, England
 Notes Id: Alan Powell/UK/IBM     email: alan_powell at uk.ibm.com 
 Tel: +44 (0)1962 815073                  Fax: +44 (0)1962 816898




From:
"Mike Beckerle" <mbeckerle.dfdl at gmail.com>
To:
<dfdl-wg at ogf.org>
Date:
09/06/2008 17:42
Subject:
[DFDL-WG] DFDL: output and choices/discriminators



I?d like to discuss this example in email and/or on our call this week. It 
illustrates that discriminators must be evaluated both on output and on 
input. We proposed at the F2F that assertions are only about parsing, but 
the same cannot be said of discriminators. 
 
Because email often line-wraps in ways that break things I?ve also 
attached the same example as a file.
 
<?xml version="1.0" encoding="iso-8859-1"?>
<!-- 80 column ruler 
2345678901234567890123456789012345678901234567890123456 -->
 
<!-- 
***************************************************************************
********************************************************************************
 
Illustration of use of layering with choices and discrimintors
 
Lessons from this example:
 
Discriminators must be evaluated on output as well as input in order to 
decide
choices.
 
********************************************************************************
*************************************************************************** 
--> 
 
<xs:schema targetNamespace="http://dataformat.org/tests" 
           elementFormDefault="qualified" 
           xsi:schemaLocation=
               "http://www.ogf.org/dfdl/dfdl-0.1 ../../xsd/dfdl.xsd
                http://www.w3.org/2001/XMLSchema ../../xsd/XMLSchema.xsd" 
           xmlns:xs="http://www.w3.org/2001/XMLSchema" 
           xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" 
           xmlns="http://www.ogf.org/dfdl/tests" 
           xmlns:dfdl="http://www.ogf.org/dfdl/dfdl-0.1">
 
<!-- 
*************************************************************************
 
Our default format will be binary, without delimiters, with bits for 
alignment
units, and implicit length kind which we'll override where needed.
 
************************************************************************* 
-->
 
<xs:annotation><xs:appinfo source="http://www.ogf.org/dfdl/dfdl-0.1">
 
<dfdl:defineFormat name="default">
   <dfdl:format representation="binary" 
                lengthKind="implcit" initiator="" separator="" 
terminator=""
                alignmentUnits="bits"
                 />
</dfdl:defineFormat>
 
</xs:appinfo></xs:annotation>
 
 
<!-- 
*************************************************************************
 
Example: a string with smart one byte or four byte length preceding it. 
 
Modeled as a single bit flag, followed by a 7 bit or 31 bit integer after 
it.
 
************************************************************************* 
-->
 
<xs:complexType name="smartLengthString" dfdl:ref="default" 
                dfdl:lengthKind="explicit" dfdl:lengthUnits="bits">
  <xs:sequence> 
 
     <xs:annotation><xs:appinfo source="...">
     <dfdl:hidden>
 
     <xs:element name="lengthFlag" type="xs:byte" 
                 dfdl:length="1" 
                 dfdl:alignment="8" 
                 dfdl:outputValueCalc='{ 
                    if ( ../logicalLength > 127 ) then "1" else "0" 
                 }'
                 />
 
     <xs:choice dfdl:choiceKind='variable' dfdl:choiceResolvable="true">
 
       <!-- First choice alternative: one byte -->
       <xs:sequence>
          <xs:annotation><xs:appinfo source="
http://www.ogf.org/dfdl/dfdl-0.1">
              <dfdl:discriminator test="{ ../lengthFlag != '1' }" />
          </xs:appinfo></xs:annotation>
 
          <xs:element name="oneByte" type="xs:byte" 
                      dfdl:alignment="1" 
                      dfdl:length="7" 
                      dfdl:outputValueCalc="{ ../logicalLength }" />
 
       </xs:sequence>
 
       <!-- Second choice alternative: one byte -->
       <xs:element name="fourByte" type="xs:int" 
                   dfdl:alignment="1" 
                   dfdl:length="31" 
                   dfdl:outputValueCalc="{ ../logicalLength }" />
 
     </xs:choice>
 
     <!-- 
         this logicalLength element below isn't strictly speaking needed 
in this
         example. It's here to illustrate something having both input and 
output
         value calculation, and makes things a bit more readable.
     -->
     <xs:element name="logicalLength" type="xs:int"
                 dfdl:inputValueCalc="{ 
                   if (../lengthFlag = '1') then ../fourByte else 
../oneByte 
                 }"
                 dfdl:outputValueCalc= "{ 
                    dfdl:length(../str, 'characters') 
                 }" 
      />
 
     </dfdl:hidden>
     </xs:appinfo></xs:annotation>
     <xs:annotation><xs:appinfo source="...">
 
     <xs:element name="str" type="xs:string"
                 dfdl:length="{ ../logicalLength }"
                 dfdl:lengthUnits="characters" />
 
  </xs:sequence>
</xs:complexType>
 
 
</xs:schema>
 
 
 
 
 
 
 [attachment "choice-discriminator-example.xml" deleted by Alan 
Powell/UK/IBM] --
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