I found a solution that seems to work. See below. Any suggestions for improving
it is welcome.
<xs:element name="test">
<xs:complexType>
<xs:sequence>
<!--
The decimal value of the first bit of the input should be
wrapped in an <A> tag if the decimal value of the next 4 bits
equals 7 and the decimal value of the 4 bits after that equals
9,
or the decimal value of the next 4 bits equals 7 and the decimal
value of the 4 bits after that equals 10, otherwise the first
bit
of the input should be wrapped in a <B> tag.
The below is not looking ahead. Assume the input is ACD. If C
is not
equal to 7 and D is not equal to 9 or 10, then we realize that
we made an error, back up, and try the next branch of the
choice.
-->
<xs:choice>
<xs:sequence>
<xs:element name='A' type="unsignedint1"/>
<xs:element name='C' type="unsignedint4">
<xs:annotation>
<xs:appinfo source="http://www.ogf.org/dfdl/";>
<dfdl:assert>{. eq 7}</dfdl:assert>
</xs:appinfo>
</xs:annotation>
</xs:element>
<xs:element name='D' type="unsignedint4">
<xs:annotation>
<xs:appinfo source="http://www.ogf.org/dfdl/";>
<dfdl:assert>{(. eq 9) or (. eq
10)}</dfdl:assert>
</xs:appinfo>
</xs:annotation>
</xs:element>
</xs:sequence>
<xs:sequence>
<xs:element name='B' type="unsignedint1"/>
<xs:element name='C' type="unsignedint4"/>
<xs:element name='D' type="unsignedint4"/>
</xs:sequence>
</xs:choice>
</xs:sequence>
</xs:complexType>
</xs:element>
-----Original Message-----
From: Roger L Costello <coste...@mitre.org>
Sent: Thursday, June 13, 2024 7:10 AM
To: users@daffodil.apache.org
Subject: Warning: Counterintuitive placement detected
Hi Folks,
I need my DFDL to look-ahead:
The decimal value of the first bit of the input should be wrapped in an <A> tag
if the decimal value of the next 4 bits equals 7 and the decimal value of the 4
bits after that equals 9, or the decimal value of the next 4 bits equals 7 and
the decimal value of the 4 bits after that equals 10, otherwise the first bit
of the input should be wrapped in a <B> tag.
Below is my attempt at implementing this. Daffodil gives this warning message:
Schema Definition Warning: Counterintuitive placement detected. Wrap the
discriminator or assert in an empty sequence to evaluate before the contents.
(id: discouragedDiscriminatorPlacement)
Here is my DFDL:
<xs:element name="test">
<xs:complexType>
<xs:sequence>
<xs:choice>
<xs:sequence>
<xs:annotation>
<xs:appinfo source="http://www.ogf.org/dfdl/";>
<dfdl:discriminator testKind="pattern"
testPattern="(.x7x9)|(.x7xA)" />
</xs:appinfo>
</xs:annotation>
<xs:element name='A' type="unsignedint1"/>
</xs:sequence>
<xs:sequence>
<xs:element name='B' type="unsignedint1"/>
</xs:sequence>
</xs:choice>
<xs:element name='C' type="unsignedint4"/>
<xs:element name='D' type="unsignedint4"/>
</xs:sequence>
</xs:complexType>
</xs:element>
