Discriminators/asserts annotated on sequences are evaluated at the end of the
sequence after the content has been evaluated. This is usually not what is
intended so Daffodil outputs this warning. To get it to evaluate before the
content (like what you intend in this case) you need to wrap the
discriminator/assert in a sequence, for example:
<xs:sequence>
<xs:sequence>
<xs:annotation>
<xs:appinfo source="http://www.ogf.org/dfdl/";>
<dfdl:discriminator ... />
</xs:appinfo>
</xs:annotation>
</xs:sequence>
<xs:element name='A' type="unsignedint1"/>
</xs:sequence>
Tha should resolve the warning.
Also, note that lengthKind="pattern" is character based, so might not work as
expected since your data is bit-based. In order for a pattern discriminator to
work you need to decode 4-bit characters (since both fields you care about are 4
bits) after skipping the first bit. We have the X-DFDL-HEX-MSBF encoding that
can decode 4-bit characters to a hex string, but there is no way to skip the
first bit. You could move the discriminator sequence to after the A element,
which works since at that point A is already parsed and the first bit is
skipped, e.g.
<xs:sequence>
<xs:element name='A' type="unsignedint1"/>
<xs:sequence dfdl:encoding="X-DFDL-HEX-MSBF">
<xs:annotation>
<xs:appinfo source="http://www.ogf.org/dfdl/";>
<dfdl:discriminator testKind="pattern" testPattern="(79)|(7A)" />
</xs:appinfo>
</xs:annotation>
</xs:sequence>
</xs:sequence>
But that feels a bit complex and unclear to me.
I would suggest instead using the dfdlx:lookAhead extension function which was
added exactly for this kind of thing. For example, your discriminator would look
something like this:
<dfdl:discriminator testKind="expression" test="{
((dfdlx:lookAhead(1, 4) eq 7) and (dfdlx:lookAhead(5, 4) eq 9)) or
((dfdlx:lookAhead(1, 4) eq 7) and (dfdlx:lookAhead(5, 4) eq 10))
}" />
It's a bit longer, but I think the intention is much ore clear. It's also
probably going to be significantly more efficient since it doesn't have to
decode characters and then run a regex on them.
On 2024-06-13 07:09 AM, Roger L Costello wrote:
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>
