You are thinking along the lines I was; however, the
challenge is that I cannot find a way to do this using multilayer so I'm
uncomfortable suggesting that it's possible at all anymore. Here's some
reasoning why.
In particular, it's the intersection of the induction
across the items with the first, middle*, last thing, and the spanning that
seems to defy my efforts to cut it up into progressive transformation layer by
layer. In some conversations I've referred to this problem as the
"non-conforming trees" problem. The fundamental shapes of the trees are not
compatible, and expressing the transformation between them isn't easily done via
induction of any kind on one or the other of the trees.
To me the First, Middle*, Last thing is very problematic.
It's effectively a little regular language (in the formal sense) that has to be
recognized. Generally this requires a finite-state-machine, and what makes FSMs
interesting and complex is always the way you diagnose malformed data in
addition to recognizing correct data.
Now, a finite-state-machine is, to my mind, the ultimate
procedural abstraction, the quintessential opposite of "declarative" expression.
To be declarative about a FSM you end up saying "recognize this regular
language", and providing a description of the regular language, which is of
course, just begging the question of how it actually works.
(And for us, we're not really talking about a regular
language of character text, but a pattern of usage in the binary data layout
that obeys the pattern of a regular language. So it's not like having a little
regular expression thing for validating text strings helps with this
problem.)
I guess I'm arguing that a black box approach to this is
not only acceptable, but is highly likely to be the only "good" way to do it. In
light of this I've suggested a rep property called "streamFormat" (perhaps
should be renamed "recordFormat"), which gets values from the set VS, V, VBS,
FB, FBS, etc. etc. all these well-defined legacy data formats (there are 19 of
them I think). In additon, one should be able to extend this by
introduction of a blackbox transformation.
And ... here's the rub...if that's true for this case, then
other "hard" examples like run-length encoding seem also in this
category.
There's several "leaps of faith" just made in these
arguments, so i'd still like people to take this "XML challenge" and see if
there's some magic I'm overlooking.
...mikeb
Without digging too much into
the details, I'd say this is an example where multi-layer comes in. The DFDL
would describe a hidden layer in which the first, middle, last data elements
would be identified and put into a list, and then that hidden list would be
used as the input to create items in the output
layer.
I think this is
conceptually similar to one of our run-length encoding examples (more complex
of course). If you read a sequence if ints and then a sequence of floats and
need to output a sequence of floats with int[i] repeats of float[i], it would
be easiest to create a hidden layer representing the int and float sequences
and to then produce output from that. If you don't think about a layer, even
this example gets painful - I need to read an int, skip forward somewhere to
find a float, skip back to get the next int, etc.
Mike's full
example, not starting with the XML-ized version, might be something that
requires more than one layer - read the original into something with with XML
schema Mike defines, then a layer making a sequence of data elements, and then
something that has the desired logical output.
I guess I would
claim that this would not be too bad a way to describe a fairly complex format
in terms of a fairly different logical structure. Whether one *should* do this
in DFDL, or whether it would make more sense to a) write a black box parser to
get to items, or b) use DFDL to get to the initial schema Mike wrote and use
XSLT afterwards to convert to the desired logical structure. I think there are
enough cases where we need the multilayer functionality in DFDL that are
relatively simple that we have to have it, which means it will then be
possible to deal with complex transformations in DFDL even if not
simple/practical.
Jim
-----Original
Message-----
From: owner-dfdl-wg@ggf.org
[mailto:owner-dfdl-wg@ggf.org] On Behalf Of
mike.beckerle@ascentialsoftware.com
Sent: Thursday, November 18,
2004 9:53 PM
To: dfdl-wg@gridforum.org
Subject: [dfdl-wg]
simple way to study hard DFDL example problem - IBM Format VS rec ords as
XML
I've come up with a way to articulate the difficulties I'm having
with DFDL for complex file formats.
This problem may not be that hard for someone with more XML, XPath or
XQuery experience, so I'd apprecate it if you could look it over and if
necessary even run it by your resident XML experts.
In
case the emailer mangles all the line lengths, I've also attached the below
as a file.
<!-- Logically, our data is this: -->
<ITEM>The first item</ITEM>
<ITEM>This is the
second item</ITEM>
<ITEM>The
third</ITEM>
<!-- That is, data having this "logical" schema
-->
<sequence>
<element name="ITEM" type="string"
minOccurs="0"
maxOccurs="unbounded"/>
</sequence>
<!-- But the below is the input data were starting from. What you
see below simulates
the structural issues of IBM
Format-VS, but converting the problem into an XML to
XML
transformation problem
-->
<BLOCK>
<SEGMENT>
<WHOLE/> <!-- a WHOLE segment holds a whole item (Duh!). This
element is really a type tag. -->
<DATA>The
first item</DATA>
</SEGMENT>
</BLOCK>
<BLOCK>
<SEGMENT>
<FIRST/> <!-- a FIRST segment holds the first part of an item.
-->
<DATA>Thi</DATA>
</SEGMENT>
</BLOCK>
<BLOCK>
<SEGMENT>
<MIDDLE/> <!-- a MIDDLE segment holds data from the center of an
item -->
<DATA>s is t</DATA>
</SEGMENT>
</BLOCK>
<BLOCK>
<SEGMENT>
<MIDDLE/>
<DATA>he
sec</DATA>
</SEGMENT>
</BLOCK>
<BLOCK>
<SEGMENT>
<LAST/> <!-- a LAST segment holds data from the end of the
item. -->
<DATA>ond
item</DATA>
</SEGMENT>
<SEGMENT>
<WHOLE/><!-- This second
segment in this block is a WHOLE segment. However
in general the 2nd segment of a block could be a WHOLE or the
FIRST segment of another multi-segment multi-block spanning item
-->
<DATA>Third item</DATA>
</SEGMENT>
</BLOCK>
<!-- Some observations: -->
<!-- Data is organized into
BLOCKs -->
<!-- Each block contains 1 or 2 SEGMENTs
-->
<!-- Each SEGMENT is either a WHOLE item, or the item spans 2
or more SEGMENTs -->
<!-- Spanning data is broken on arbitrary
boundaries across segments it spans -->
<!-- Spanning involves a
FIRST, MIDDLE*, LAST segment structure. -->
<!-- MIDDLE* means zero
or more MIDDLE segments. -->
<!-- The question: how can we express the transformation into the
desired logical form?
Or is this beyond the call
of duty for DFDL?
Goals include to be as
declarative as possible, and ideally, do it as a set
of
XML Schema annotations in the GGF DFDL
style. -->
<!-- here's an XSD (untested) for the input data structure
-->
<complexType
name="Format_VS_t">
<sequence>
<element
name="BLOCK" type="Block_t" minOccurs="0"
maxOccurs="unbounded"/>
</sequence>
</complexType>
<complexType name="Block_t">
<sequence>
<element name="SEGMENT" type="Segment_t" minOccurs="1"
maxOccurs="2"/>
</sequence>
</complexType>
<complexType
name="Segment_t">
<sequence>
<choice>
<element
name="WHOLE">
</element>
<element
name="FIRST">
</element>
<element
name="LAST">
</element>
<element name="MIDDLE">
</element>
</choice>
<element name="DATA"
type="string"/>
</sequence>
</complexType>