[DFDL-WG] suggest: need hexBinary with lengthUnits 'bits' with length not a multiple of 8.

[Steve Hanson]

If allowing lengthUnits 'bits' for a new logical/physical combination has 
no effect on the infoset then that should be ok. 

'binarySecond' & 'binaryMilliseconds'. These were designed to correspond 
to C data types and are always treated as signed. Allowing 'bits' should 
be ok as long as the same rules for signed 'int' & 'long' respectively are 
used.

'hexBinary' as you note causes a problem as the XSD type must be a 
multiple of 8 bits. That's why it has the restriction of 'bytes' only 
today. If we allow 'bits', then on parsing DFDL would have to pad either 
using 0 bits or the corresponding bits of dfdl:fillByte, and on unparsing 
DFDL would have to trim off the excess as long as it matched 0 bits or the 
corresponding bits of dfdl:fillByte. Today fillByte is never used for 
trimming.

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



From:   Mike Beckerle <mbeckerle.dfdl at gmail.com>
To:     "dfdl-wg at ogf.org" <dfdl-wg at ogf.org>
Date:   26/01/2017 05:00
Subject:        [DFDL-WG] suggest: need hexBinary with lengthUnits 'bits' 
with length not a multiple of 8.
Sent by:        "dfdl-wg" <dfdl-wg-bounces at ogf.org>



We have users who have binary blobs the size of which is given in bits, 
and these blobs are not a multiple of 8 long.

Today the DFDL spec doesn't allow hexBinary to have lengthUnits 'bits'. 

I am wondering if this restriction should be lifted. 

XSD constrains hexBinary to always have an even number of Hex digits, so 
we would have to do the same. 

So for an example, a 17 bit long hexBinary containing all 1 bits would be 
FFFF80 

Erratum 5.15 extends the types that are allowed to have length in bits to 
include packed calendars. So there is precedent for opening this 
restriction up if need arises.

I claim we need to 

(a) allow length units bits for all types
(b) restrict the length to have to be 32-bits or 64-bits only, for types 
xs:float and xs:double when representation 'binary'
(c) restrict packed decimal to have lengths be a multiple of 4 bits (when 
specified in units 'bits')

All other restrictions should be lifted as those restrictions just cause 
problems in some formats.

For example 12.3.7.2.5 Specifies that binary calendars must be 4 bytes or 
8 bytes exactly, and cannot be specified in units 'bits'. This is just a 
mistake in DFDL.  I have even seen binary calendars with 33 bits length. 
(seconds since 1-1-1970 representation aka binarySeconds) That additional 
bit extends the end time substantially.  

These restrictions were put into DFDL because our experience of many 
bit-granularity formats was limited.

What we've found is that there are plenty of data formats where the notion 
of a "byte" is simply absent. Nothing uses multiples of 8 bits for 
anything, and nothing is measured in those units. It's always measured in 
bits.  Even for things like float and double, which have impliicit lengths 
of 4 and 8 bytes respectively, many specifications will express those as 
32 bits or 64 bits. Having to divide by 8 just makes the DFDL schema 
awkward. Similarly in these formats strings are given length in bits. 448 
bits worth of 7-bit packed ascii characters is 64 characters, occupying 56 
bytes, but the spec uses 448. 

These changes are all backward-compatible. They make legal property 
settings that previously had no meaning and caused SDEs.

Discussion?




Mike Beckerle | OGF DFDL Workgroup Co-Chair | Tresys Technology | 
www.tresys.com
Please note: Contributions to the DFDL Workgroup's email discussions are 
subject to the OGF Intellectual Property Policy
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