Layering will work, but this problem is simpler than that.
Pretty sure this is just dfdl:bitOrder='leastSignificantBitFirst' with
dfdl:byteOrder="littleEndian" data.
However, the order of the fields is reversed from the way you are thinking. The
first field is the 12 bit field, the second field is the 16 bit field, the
third is the 4 bit field. This should illustrate what I mean:
Byte 4 Byte 3 Byte 2 Byte 1
Hex: 9 0 0 0 2 0 0 1
Bits 1001 0000 0000 0000 0010 0000 0000 0001
f1 xxxx xxxx xxxx
f2 yyyy yyyy yyyy yyyy
f3 zzzz
...mike beckerle
Tresys
________________________________
From: Gedvilas, Brett L2 <[email protected]>
Sent: Wednesday, September 12, 2018 2:14 PM
To: [email protected]
Subject: DFDL Schema help
Hi everyone,
I am a new daffodil user and I was looking for input on a DFDL schema
definition I'm trying to create. I'm working with some binary physics data, the
format of which can loosely be described as fields that are aggregated together
and packed into a single 32-bit integer before being written to memory. The
gist of the issue is that because not all fields fall nicely on 1-byte
divisions, different pieces of a field will get jumbled if you read the data as
a linear stream from memory. This is best illustrated by a simple example:
Consider the following 32-bit hex value: 0x90 00 20 01
The problem arises because the values that have meaning in context are 0x9
(consisting of 4 bits), 0x0002 (16 bits), and finally 0x001 (12 bits).
When this value gets stored in memory on a little endian architecture we see
the following: 0x01 20 00 90. Trying to read those bit sequences as a stream
from memory will yield 0x0, 0x1200, and 0x090, which are clearly incorrect.
The simplest approach I can envision is to read in the value as an entire
32-bit value and then perform some processing via masks/bit shift in order to
extract the correct values. Is there a more straightforward solution to this
problem? or does anyone have experience or insights solving this issue using
daffodil?
Thanks!
Brett
