[DFDL-WG] Action 292 - Write up of hexBinary with lengthUnits 'bits'

[Steve Hanson]

Mike

Comments in-line.

Regards
 
Steve Hanson
IBM Hybrid Integration, Hursley, UK
Architect, IBM DFDL
Co-Chair, OGF DFDL Working Group
smh at uk.ibm.com
tel:+44-1962-815848
mob:+44-7717-378890
Note: I work Tuesday to Friday 



From:   Mike Beckerle <mbeckerle.dfdl at gmail.com>
To:     dfdl-wg at ogf.org
Date:   15/05/2018 22:10
Subject:        [DFDL-WG] Action 292 - Write up of hexBinary with 
lengthUnits 'bits'
Sent by:        "dfdl-wg" <dfdl-wg-bounces at ogf.org>




The proposed change is to allow lengthUnits 'bits' for hexBinary data. 
This turned out to be more complex to describe than I originally suspected 
because of the need to deal with XSD minLength and maxLength facets, which 
are always measured in bytes. Those in conjunction with 
dfdl:lengthKind='explicit' and dfdl:lengthUnits='bits' create some minor 
complexities. 


The below changes match the Daffodil implementation of this proposed 
feature. 

These sentences in the description of dfdl:lengthUnits in Section 12.3 
must change.

'bits' may only be used for xs:boolean, xs:byte, xs:short, xs:int, 
xs:long, xs:unsignedByte, xs:unsignedShort, xs:unsignedInt, and 
xs:unsignedLong simple types with binary representation.
'bytes' must be used for type xs:hexBinary.

The text should read

'bits' may only be used for xs:hexBinary and for xs:boolean, xs:byte, 
xs:short, xs:int, xs:long, xs:unsignedByte, xs:unsignedShort, 
xs:unsignedInt, and xs:unsignedLong simple types with binary 
representation.


Later in the section 12.3.2, the paragraph:

When unparsing a simple element with binary representation, then for 
hexBinary the length is the number of bytes in the infoset value padded to 
the XSD minLength facet value using dfdl:fillByte, and for the other types 
the length is the minimum number of bytes to represent the value and any 
sign.

Must change to:
When unparsing a simple element with binary representation, then for types 
other than hexBinary the length is the minimum number of bytes to 
represent the value and any sign. 
For type hexBinary when the dfdl:lengthUnits is 'bytes' then the length is 
the number of bytes in the infoset value padded to the XSD minLength facet 
value using dfdl:fillByte. 
For type hexBinary when the dfdl:lengthUnits is 'bits':
First the data is padded to XSD minLength bytes as if the dfdl:lengthUnits 
was 'bytes'.
When dfdl:lengthKind is other than 'explicit', the length in bits is the 
number of bytes times 8.
When the dfdl:lengthKind is 'explicit' then the value is further padded or 
truncated to fit the target length, in bits.
if the data does not have sufficient bytes to supply the target length in 
bits it is a processing error.  <--- no, see 12.3.7.2.7 and you just said 
you padded it
if the data is longer than the minimum number of bytes needed to supply 
the target length in bits, it is a processing error.
If the explicit length in bits is not a multiple of 8, then the final byte 
is only partially unparsed according to the current dfdl:byteOrder  <--- 
you mean bitOrder  


>>SMH: Section 12.3.2 is about dfdl:lengthKind 'delimited', so discussion 
should be limited to delimited only, and detail for other length kinds 
moved to their sections or 12.3.7.2.7 for stuff common to specified 
lengths. 

>>SMH: Need to consider the impact of this for lengthKinds implicit, 
explicit, prefixed, as they all use lengthUnits. And for explicit, need to 
cover when dfdl:length is an expression (so variable length on output).

Section 12.3.7.2.7   Length of Binary Opaque Elements, the first sentence 
must be modified from:

"The dfdl:lengthUnits property must be 'bytes'. It is a schema definition 
error otherwise."

to 

"The dfdl:lengthUnits property must be 'bytes' or 'bits'. It is a schema 
definition error otherwise. Note that even when the dfdl:lengthUnits 
property is 'bits', the values of the XSD minLength and XSD maxLength 
facets are still always interpreted as constraining the length in units of 
bytes. 

>>SMH: Earlier in 12.3.7.2 it says "The dfdl:lengthUnits can be 'bytes' or 
'bits' unless otherwise stated. It is schema definition error if 
dfdl:lengthUnits is 'characters'. " so the first two sentences can just be 
removed. 

That's the end of the actual proposed language.


Note about alternatives: I considered the alternative to make it a schema 
definition error when dfdl:lengthUnits is 'bits' type is 'xs:hexBinary', 
and the XSD:minLength or XSD:maxLength facets are defined. I decided to go 
with the description above to support the use case where a data item is, 
in hex, some number of bytes long for a valid XML infoset, but the 
representation is explicitly a partial byte smaller. E.g., in a hexBinary 
with 8 bytes, but fewer than 64 bits in the representation. Ex: 63 bits as 
the explicit target length, so 1 bit will be unused from the xs:hexBinary 
logical value, but the above rules insure no more than 7 bits go unused 
from the final byte of the hexBinary logical value. 

This is trying to be consistent with the notion that we do not truncate 
data to fit into the available length except for xs:string when properties 
explicitly allow it. 
It is also trying to be consistent with our treatment of binary integers 
where a xs:long value may be output into an element having room for any 
number of bits, and any extra bits in the logical value are ignored. 



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