Thanks for the input Mike. I had definitely been misinterpreting how dfdl was applying the leastSignificantBitFirst property to the data stream but I think that makes sense now. I appreciate the help.
Brett ________________________________ From: Steve Lawrence <[email protected]> Sent: Thursday, September 13, 2018 5:23:11 AM To: [email protected]; Mike Beckerle Subject: Re: DFDL Schema help Good call, Mike. That's going to be more efficient and probably is the right representation of the data. Here is a schema gist of what Mike describes: https://gist.github.com/stevedlawrence/1404e03a313ff63cd0bad8c79d0ae267 - Steve On 09/12/2018 09:13 PM, Mike Beckerle wrote: > 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 > > > > >
