James,

The checkConstraints function is just a convenience that saves you having to duplicate constraints in an assert or discriminator. For now, just duplicate the constraint as a discriminator. This works fine as long as you can express the constraint as a DFDL expression, which with your example you can.

I've tested your xsd exactly as you supplied below (without the terminator) on my latest MBTK and it parses 'abc' fine. I don't see the infinite loop error. We did have some bugs in that area where the check was being applied too strictly which we fixed.

I then tried with 'cba' which parsed without error, except of course that the values ended up in the wrong elements. So I added discriminators to check that the elements matched their fixed value, and 'cba' then parsed into the correct elements.

<xsd:element dfdl:length="1" dfdl:lengthKind="explicit" dfdl:occursCountKind="implicit" fixed="b" minOccurs="0" name="b" type="xsd:string">
<xsd:annotation>
<xsd:appinfo source="http://www.ogf.org/dfdl/">
<dfdl:discriminator>{. eq 'b'}</dfdl:discriminator>
</xsd:appinfo>
</xsd:annotation>
</xsd:element>

I then tried with more complex strings, such as 'cbabaccba', and they all parsed ok.

To make the infoset more symmetric, with one child per array occurrence, you can use a choice instead of a sequence.

Making that change then results in:

Here's the xsd with discriminators and choices. See if it works with your MBTK.

If you are still hitting the infinite loop error then add the %NL; terminator to the array element. This will parse data of the form:

c
b
a
b
a
c
c
b
a

Regards

Steve Hanson
Architect, Data Format Description Language (DFDL)
Co-Chair, OGF DFDL Working Group
IBM SWG, Hursley, UK
smh@uk.ibm.com
tel:+44-1962-815848

From: "Garriss Jr., James P." <jgarriss@mitre.org>
To: "dfdl-wg@ogf.org" <dfdl-wg@ogf.org>,
Date: 05/03/2013 19:15
Subject: Re: [DFDL-WG] unordered sequence with constrained occurrences
Sent by: dfdl-wg-bounces@ogf.org

> The error message is because you don't make forward progress through the data with potentially unbounded occurrences.

I think you just said, “MBTK prevents an infinite loop.” That makes sense.

> If there are delimiters then model those and you might not get the error.

I think you just said, “To let MBTK know when it should stop checking, you need a terminator of some sort.” That also makes sense. So I added a terminator (%NL;) here:

Good news: That fixed the problem, so long as my input is “abc”.

Bad news: This breaks if the input is any other legal value, such as “abbc” or “cba” or “b”.

The problem for all of these is that my dear friend, checkConstraints, is not implemented yet, thus I can’t prevent the parser from slurping up the wrong character. I don’t know how anyone can build a non-trivial DFDL schema that involves any sort of choice without this method; I swear, it must be the single most important thing you guys have created for DFDL.

Until checkConstraints is implemented, I’m not really able to test this schema with MBTK.

Thanks so much for your help answering my questions, Steve!

From: Steve Hanson [mailto:smh@uk.ibm.com]
Sent: Tuesday, March 05, 2013 1:46 PM
To: Garriss Jr., James P.
Cc: dfdl-wg@ogf.org; dfdl-wg-bounces@ogf.org
Subject: Re: [DFDL-WG] unordered sequence with constrained occurrences

James,

The error message is because you don't make forward progress through the data with potentially unbounded occurrences. Is this because you are using a cut-down schema? If there are delimiters then model those and you might not get the error.

Once you have processed the array you can use asserts to check the count. However IBM DFDL does not implement the count functions yet.

Give me a couple of days to look at this more closely. I have a customer visit tomorrow hence the delay.

Regards

Steve Hanson
Architect, Data Format Description Language (DFDL)
Co-Chair, OGF DFDL Working Group
IBM SWG, Hursley, UK
smh@uk.ibm.com
tel:+44-1962-815848

From: "Garriss Jr., James P." <jgarriss@mitre.org>
To: "dfdl-wg@ogf.org" <dfdl-wg@ogf.org>,
Date: 05/03/2013 16:19
Subject: Re: [DFDL-WG] unordered sequence with constrained occurrences
Sent by: dfdl-wg-bounces@ogf.org

Hmmm, maybe not. I said:

> The unordered sequence can be modeled with a data array

Yet when implemented in MBTK, it throws a fatal error:

fatal: CTDP3148E: Infinite loop at offset 3: The DFDL parser cannot process array element 'ABCarray' because maxOccurs is unbounded and the length of the previous occurrence was zero.

I think what happens is that on the last pass through the array, it doesn’t find a, b, or c, so it throws a fatal error.

So is this a bug in MBTK? Or can DFDL not model an unordered sequence? Or am I just doing it wrong?

Here’s a sample DFDL schemas that illustrates the point:

<?xml version="1.0" encoding="UTF-8"?>
<xsd:schema xmlns:dfdl="http://www.ogf.org/dfdl/dfdl-1.0/"
xmlns:fmt="http://www.ibm.com/dfdl/GeneralPurposeFormat"
xmlns:ibmSchExtn="http://www.ibm.com/schema/extensions" xmlns:xsd="http://www.w3.org/2001/XMLSchema">
<xsd:import namespace="http://www.ibm.com/dfdl/GeneralPurposeFormat"
schemaLocation="IBMdefined/GeneralPurposeFormat.xsd" />
<xsd:element ibmSchExtn:docRoot="true" name="ABC">
<xsd:complexType>
<xsd:sequence dfdl:separator="">
<xsd:annotation>
<xsd:appinfo source="http://www.ogf.org/dfdl/">
<dfdl:sequence />
</xsd:appinfo>
</xsd:annotation>
<xsd:element dfdl:occursCountKind="implicit" maxOccurs="unbounded"
minOccurs="1" name="ABCarray">
<xsd:complexType>
<xsd:sequence dfdl:separator="">
<xsd:element dfdl:length="1" dfdl:lengthKind="explicit"
dfdl:occursCountKind="implicit" fixed="a" minOccurs="0" name="a"
type="xsd:string" />
<xsd:element dfdl:length="1" dfdl:lengthKind="explicit"
dfdl:occursCountKind="implicit" fixed="b" minOccurs="0" name="b"
type="xsd:string" />
<xsd:element dfdl:length="1" dfdl:lengthKind="explicit"
dfdl:occursCountKind="implicit" fixed="c" minOccurs="0" name="c"
type="xsd:string" />
</xsd:sequence>
</xsd:complexType>
</xsd:element>
</xsd:sequence>
</xsd:complexType>
</xsd:element>
<xsd:annotation>
<xsd:appinfo source="http://www.ogf.org/dfdl/">
<dfdl:format ref="fmt:GeneralPurposeFormat" />
</xsd:appinfo>
</xsd:annotation>
</xsd:schema>

Test with “abc” as sample input.

From: Garriss Jr., James P.
Sent: Tuesday, March 05, 2013 8:43 AM
To: dfdl-wg@ogf.org
Subject: unordered sequence with constrained occurrences

Suppose text data has 3 constructs: a, b, and c.

· a must occur 1 time
· b can occur 0 or 1 time
· c can occur any number of times, 0 or more

These 3 constructs can appear in any order.

So these are valid inputs:

abc
a
bcccca

But these are not:

ccbcc
abbc
abcabc

Can data like this be modeled with DFDL?

The unordered sequence can be modeled with a data array, like this:

Array (0 to unbounded)
Sequence
a (0 to 1)
b (0 to 1)
c (0 to 1)
/Sequence
/Array

But I don’t know how to constrain the total number of occurrences.

Appreciate any ideas!--
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Unless stated otherwise above:
IBM United Kingdom Limited - Registered in England and Wales with number 741598.
Registered office: PO Box 41, North Harbour, Portsmouth, Hampshire PO6 3AU