[DFDL-WG] Action 242: valueLength of complexType - contentLength and valueLength functions

[Mike Beckerle]

(There are a few separate email threads related to Action 242. They have
different subject lines, but all mention Action 242.)

Experience with the Daffodil unparser implementation and
dfdl:outputValueCalc has led us to a number of conclusions:

1) we need the concept of valueLength of a complexType which excludes the
ElementUnused region, so that contentLength of a complexType can include
the ElementUnused region.

The example that motivates this is from PCAP where there are length fields
that aren't the exact length while unparsing but contain other factors:

<element name="len" type="xs:int" dfdl:outputValueCalc="{
fn:max(dfdl:valueLength(../ctElem, 'bytes'), 60) }" .../>
...
<element name="ctElem" dfdl:length="{ ../len }" .../>

In the above, we have a complex type element that has a minimum length of
60. So when unparsing if the actual length of the choice/sequence that is
the model group of the ctElem is of length less than 60, we need to fill
out the ElementUnused region of the DFDL grammar so as to achieve the min
length of 60 bytes.

If the dfdl:outputValueCalc of the len element was to call
dfdl:contentLength, then we would have a circularity.

Erratum 5.18 clarifies that length expressions are evaluated when unparsing
to produce the target length that is used for pad/fill.

This concept must be extended to complex types so that it also addresses
the ElementUnused region that can follow the model group of a complex type.

To clarify all this I suggest the DFDL data grammar must be changed as
follows:

ComplexNormalRep = LeftFraming PrefixLength ComplexContent RightFraming #
Unchanged. Just here for context
ComplexContent = ComplexValue ElementUnused
ComplexValue = Sequence | Choice

The above change basically moves ELementUnused out of the ComplexNormalRep
production into a separate ComplexContent production, and distinguishes
ComplexValue from ComplexContent.

With the above grammar rule changes, I believe the spec as modified for
Erratum 5.18 will be correct. (Everyplace where lengthKind explicit is
lumped in with prefixed etc must be broken out, as it is a different case.
)

2) Spec says escapeCharacter, escapeBlockStart, and excapeBlockEnd
contribute to the content length of an element. (Note:
escapeEscapeCharacter is missing this stipulation and should be made
consistent.)

However, I believe this is incorrect. These all contribute to the *value*
length, that is to say, they appear within the "SimpleValue |
NilLogicalValue" region of the grammar.

3) Definition of dfdl:valueLength must be changed to specify that it
returns the length of the ComplexValue region for complex types, and
excludes any filling of the ElementUnused region.

The definition of dfdl:contentLength is acceptable as is.

However, both require clarification as to when "characters" and "bytes" are
allowed relative to the lengthUnits of the element that is the argument.

(This length units topic is a separate matter and will be a separate
message.)




Mike Beckerle | OGF DFDL Workgroup Co-Chair | Tresys Technology |
www.tresys.com
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