Minor suggestion. Your second assert should be a discriminator.
This would be a more precise expression of exactly the criteria for choosing this sequence as the data format. DFDL can use that to improve diagnostic behavior if both "assertions" hold, but something is wrong with the data for elements B, C, and D. There is even a small possibility of an incorrect false-positive parse happening that the discriminator rules out. On Thu, Jun 13, 2024 at 7:52 AM Roger L Costello <coste...@mitre.org> wrote: > 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> >
