[DFDL-WG] DFDL: TLOG proposal

[Steve Hanson]

As per minutes from last week's call - here is proposal for TLOG after 
more research.

TLOG
The individual fields are a mixture of ASCII strings, proprietary packed 
decimals, and the occasional pure binary data. All fields are delimited by 
a separator. Fields of all types can be fixed length or variable length 
with a maximum. Pure binary data is preceded by a field giving the actual 
length. All lengths in bytes. 

Packed decimals. Like a packed decimal in the IBM sense. These can carry 
negative numbers but use a leading xD sign nibble. No sign nibble if 
positive or unsigned. Odd number of digits (including sign if present) are 
padded with xF nibble. This is best illustrated using examples. 
1234     =>     x12x34 
123      =>     xF1x23 
-1234    =>     xFDx12x34 
-123     =>     xD1x23 

Proposal
1) The 'variable length with a maximum' can not be handled using a 
post-timing assertion, because assertions only apply on parsing. Given 
that IBM MRM allows you to model this case, I think we should allow 
dfdl:length & dfdl:lengthUnits to be specified when dfdl:lengthKind is 
'delimited' or 'pattern'. Extraction during parse is still by scanning. If 
the physical representation exceeds that length after extraction, it is a 
processing error. Similarly when unparsing, if the physical representation 
exceeds that length prior to output, it is a processing error.

2) dfdl:binaryNumberRep is extended with another value 'tlog'. Associated 
property dfdl:binaryDecimalVirtualPoint is applicable. Associated property 
dfdl:binaryPackedSignCodes is not applicable, there being only xD to 
indicate negative numbers, and no distinct nibbles for unsigned or zero.

3) dfdl:lengthKind 'delimited' is permitted for numbers when 
dfdl:representation is 'binary' and dfdl:binaryNumberRep is 'packed' or 
'bcd' or 'tlog' because it is possible to know in advance the range of 
bytes being used, and therefore to choose suitable delimiters.

Regards

Steve Hanson
Programming Model Architect, WebSphere Message Broker,
OGF DFDL WG Co-Chair,
Hursley, UK,
Internet: smh at uk.ibm.com,
Phone (+44)/(0) 1962-815848





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