[DFDL-WG] Action 088 - define semantics of choiceKind 'fixedLength'

[Tim Kimber]

More work to do on this, but no time before the meeting. Let's discuss the 
rules below, and refine afterwards:

If dfdl:choiceKind='fixedLength' then there must be at least one branch of 
the choice for which all of the content has a calculable length.

Definition : A DFDL region is a region of the data stream described by the 
DFDL grammar. 
Note : we need to define this term - we use the term 'region' without 
definition throughout the specification. Sometimes we call it a 'grammar 
region'.

Definition: A DFDL region has a calculable length if the length of its 
representation in units of bytes can be calculated from the DFDL 
description alone.

The leading alignment region has a calculable length if the alignment of 
the component is the same as the alignment of the parent. 
The trailing alignment region has a calculable length if the alignment of 
the component is the same as the alignment of the parent.
Q: Is this rule necessary?

The initiator region has a calculable length if all of the possible 
initiator values have a calculable length.
An initiator value is a DFDL string literal. It has a calculable length if 
the DFDL string literal does not contain any variable-length parts.

The terminator region has a calculable length if all of the possible 
terminator values have a calculable length.
A terminator value is a DFDL string literal. It has a calculable length if 
the DFDL string literal does not contain any variable-length parts.

A variable-length part  is any of the following
- a character string in a variable-width encoding
- a DFLD entity in a variable-width encoding
- a DFDL generic entity which can match a variable number of characters ( 
NL, WSP+, WSP* )

The simple content region has a calculable length if both of the following 
are true:
- either the encoding is a fixed-width encoding or the representation does 
not contain characters.
- either lengthKind is 'explicit' and the length is not a DFDL expression 
or lengthKind is 'implicit' and there is an implicit length for the simple 
type and its dfdl:representation

The complex content region has a calculable length if all of the following 
are true:
- either the encoding is a fixed-width encoding or the representation ( 
recursively ) does not contain characters.
- the element has lengthKind='explicit' and dfdl:length is not a DFDL 
expression
- all group members, recursively, are required ( minOccurs=maxOccurs )

regards,

Tim Kimber, Common Transformation Team,
Hursley, UK
Internet:  kimbert at uk.ibm.com
Tel. 01962-816742 
Internal tel. 246742






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