Mike Beckerle created DAFFODIL-2689:
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Summary: Asterix format uses DFDL disallowed 4th combination of
byte order and bit order
Key: DAFFODIL-2689
URL: https://issues.apache.org/jira/browse/DAFFODIL-2689
Project: Daffodil
Issue Type: New Feature
Components: Back End, Front End
Affects Versions: 3.3.0
Reporter: Mike Beckerle
Attachments: image-2022-04-13-11-10-28-938.png
The Asterix CAT 034 message uses byteOrder bigEndian but bit order
leastSignificantBitFirst for numbers that span bytes. For example they describe
an 11-bit integer in this way:
!image-2022-04-13-11-10-28-938.png!
In this picture you can see they are numbering the bits from the right,
leastSignificantBitFirst starting from 1. But the COUNTER field spans all of
Octet 3, but also includes the least significant 3 bits of Octet 2. In those
least significant 3 bits are the most significant (numerically) 3 bits of the
11 bit COUNTER integer, so this is bigEndian byte order, but
leastSignificantBitFirst bit order.
The DFDL standard does not allow this combination of bitOrder and byteOrder, as
there was no known documented use case at the time the new bitOrder property
was added to the DFDL specification. It was however, proposed that it was
likely this combination would be needed, and here is a well-documented use
case.
So Daffodil should lift this restriction and allow this combination.
The workaround in the mean time is to parse this counter as two separate
integers, one 3 bits and one 8 bits, and use an inputValueCalc expression to
compute a field with the expected value.
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