Hi Mike

If I understand this correctly, it is just a way of being able to encode negative numbers. For a given precision the offset is always fixed, so in your example no of bits is 3 so offset is 4. So the lengthKind must be 'explicit' or 'implicit'.

If so then the table in 13.7 becomes:

Logical Type	Permitted Value
Decimal, Integer, NonNegativeInteger	packed, bcd, binary, ibm4690Packed
Long, Int, Short, Byte,	packed, binary, offsetBinary, ibm4690Packed (but not bcd)
UnsignedLong, Unsignedint, UnsignedShort, UnsignedByte	packed, bcd, binary, ibm4690Packed

Any other subtleties?

Regards

Steve Hanson

IBM Hybrid Integration, Hursley, UK
Architect, IBM DFDL
Co-Chair, OGF DFDL Working Group
smh@uk.ibm.com
tel:+44-1962-815848
mob:+44-7717-378890

From: Mike Beckerle <mbeckerle.dfdl@gmail.com>
To: "dfdl-wg@ogf.org" <dfdl-wg@ogf.org>
Date: 14/09/2017 01:35
Subject: [DFDL-WG] propose new dfdl:binaryNumberRep "offsetBinary"
Sent by: "dfdl-wg" <dfdl-wg-bounces@ogf.org>

We have found a number of places that use offset-binary numeric representation. This is also called excess-K, or biased, but I think offset binary is a better description of it.

In this representation you take an unsigned binary, and just subtract an offset. E.g., for a 3-bit number, mostSignificantBitFirst:

bits    unsigned twos-comp offsetBinary

000       0         0          -4
001       1         1          -3
010       2         2          -2
011       3         3          -1
100       4        -4           0
101       5        -3           1
110       6        -2           2
111       7        -1           3

At the moment, users have to work around this in Daffodil using inputValueCalc and outputValueCalc. This is feasible, but really awkward for such a simple concept.

So we suggest that the next revision of DFDL include dfdl:binaryNumberRep="offsetBinary"

Thoughts?

...mikeb

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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