[DFDL-WG] Spec section 12.3.7.2 - binary reps and zero length

[Steve Hanson]

Section 9 of the specification talks about empty etc representations, 
where the meanings of a length of zero are defined.  However section 
12.3.7.2 has sub-sections for certain binary reps where it is illegal to 
have a length of zero.  Is this deliberate?  The cases where 0 is allowed 
are also those which can have lengthKind 'delimited' so that might be the 
reason. Note that disallowing length of zero prevents use of a default 
value.

12.3.7.2.1: Binary numbers. Table 22 has minimum length in bits => 0 not 
allowed. 

12.3.7.2.2: Floating point numbers: Says length must be exactly 4 bytes or 
8 bytes => 0 not allowed.

12.3.7.2.3: Packed numbers. No minimum length stated => 0 allowed.

12.3.7.2.4: Binary booleans. Refers to 12.3.7.2.1 => 0 not allowed.

12.3.7.2.5: Binary calendars. Says length must be exactly 4 bytes or 8 
bytes => 0 not allowed.

12.3.7.2.6: Packed calendars. No minimum length stated => 0 allowed.

12.3.7.2.7: Opaque binary. No minimum length stated => 0 allowed.

Noticed this when modelling an example of a header with user-defined 
information at the end with the length given by a binary count. 

    <xs:complexType name="Type_Header">
      <xs:sequence>
        <xs:element dfdl:length="75" name="Filler" type="xs:hexBinary"/>
        <xs:element dfdl:length="2" name="HeaderLength" 
type="xs:nonNegativeInteger"/>
        <xs:element dfdl:length="30" name="Filler2" type="xs:hexBinary"/>
         <xs:element dfdl:length="{../HeaderLength - 108}" name="UserInfo" 
type="xs:hexBinary"
                    minOccurs="0" dfdl:occursCountKind="implicit"/> 
      </xs:sequence>
    </xs:complexType>

This could equally be modelled using occursCountKind 'expression' but that 
seems unnecessary if the length is 0:

    <xs:complexType name="Type_Header">
      <xs:sequence>
        <xs:element dfdl:length="75" name="Filler" type="xs:hexBinary"/>
        <xs:element dfdl:length="2" name="HeaderLength" 
type="xs:nonNegativeInteger"/>
        <xs:element dfdl:length="30" name="Filler2" type="xs:hexBinary"/>
         <xs:element dfdl:length="{../HeaderLength - 108}" name="UserInfo" 
type="xs:hexBinary"
                    minOccurs="0" dfdl:occursCountKind="expression" 
dfdl:occursCount="if (../HeaderLength - 108 eq 0) then 0 else 1"/> 
      </xs:sequence>
    </xs:complexType>

Also what do the minimum lengths in Table 22 mean when lengthUnits is 
bytes ?

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
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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