[DFDL-WG] lengthKind = prefixed and length in bits when serializing...

[Steve Hanson]

Alex

What you suggest makes sense. There is an implicit limitation with 
"prefixed" in that I can parse an element with an odd number of bits (say 
13) but be unable to serialize exactly what had been parsed. I don't think 
there's anything you can do about that, other than use "explicit" with an 
expression as Tim says.

However, I think what you have noted is an issue for all "prefixed" binary 
rep data, not just lengthUnits "bits".  Take a look at Table 16 in section 
12.3.5. This was added to handle the same issue for lengthKind "pattern" 
where all we have is the infoset data. The rightmost column says what to 
output for binary reps (in practice, it is virtually identical to Table 15 
for "implicit" in section 12.3.3).  We need this for "prefixed" too.

Further, some of the rows in Table 16 are needed for "delimited". 
Delimited binary data is permitted under a couple of circumstances - when 
the binary rep is "packed" or "BCD".  Section 12.3.2 does not cover what 
to output for binary reps.

Note that errata 3.9 effectively removes Table 16 as "pattern" no longer 
supports binary rep data. The information in the text column is still 
needed though, but a closer examination says it is incorrect, as it should 
take into account dfdl:textOutputMinLength or xs:minLength (see wording in 
12.3.2 for "delimited" and 12.3.4 for "prefixed").
 
Let's discuss on next DFDL WG call.
Regards

Steve Hanson
Architect, Data Format Description Language (DFDL)
Co-Chair, OGF DFDL Working Group
IBM SWG, Hursley, UK
smh at uk.ibm.com
tel:+44-1962-815848



From:   Tim Kimber/UK/IBM
To:     Alex Wood1/UK/IBM at IBMGB
Cc:     Steve Hanson/UK/IBM at IBMGB
Date:   23/03/2012 21:55
Subject:        Re: lengthKind = prefixed and length in bits when 
serializing... 


Fair question. Off the cuff response is 
- follow lengthKind=implicit rules for lengthKind=prefixed
- if that's not what the modeller wants then they can always use 
lengthKind=explicit with an expression and model the prefix length as a 
separate element. Then they can use an outputValueCalc expression on the 
length element to produce whatever result they require.

regards,

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





From:   Alex Wood1/UK/IBM
To:     Steve Hanson/UK/IBM at IBMGB, Tim Kimber/UK/IBM at IBMGB
Date:   23/03/2012 19:33
Subject:        lengthKind = prefixed and length in bits when 
serializing... 


Hi Steve, Tim.

Looking at one of Neil's defects for lengthKind = prefixed
Case where lengthUnits = Bits

How to we determine what length (in bits) to serialize from the value and 
logical type ?
I could not find a clear definitive answer in the specification. 

On parsing it is clear that any length is OK as long as it is less than 
the natural length of the logical type. 
So the same rules as for lengthKind = explicit, only the length was parsed 
out of the prefix. 

On serializing, lengthKind = explicit and implicit have specific lengths 
to serialize. 

However lengthKind = prefixed does not have a length to work with. 
That's ok for lengthUnits = bytes or characters where we know the length 
of data we are putting in the bitstream.

But for lengthKind = bits it's not so obvious.

For example an integer value of 1 for an unsignedInt could be serialized 
in a length anything between 1 and 32 bits. 

One way would be to find the minimum number of bits we could use to 
serialized all significant bits of the value. 

However my instinct is that it ought to follow the lengthKind =  implicit 
rules and serialize the number of bits for the logical types natural 
length.
UnsignedLong    64
UnsignedInt     32
UnsignedShort   16
UnsignedByte    8
Boolean         32


Thoughts ?

Kind Regards,

- Alex

Alex Wood - 
Software Engineer - 
WebSphere Message Broker Development
Lab Advocate - PMI Mortage 

MP 211, IBM UK Labs, Hursley Park, Winchester, Hants. SO21 2JN.
Tel: Internal 246272, External 01962 816272
Notes: Alex Wood1/UK/IBM at IBMGB
e-mail: wooda at uk.ibm.com







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