Minor Description
In ticket DAFFODIL-641, an experimental forward-lookahead feature is added to Daffodil to enable use of choice flags that come a fixed distance ahead in the data.
This is to handle cases from the format NATO STANAG 5516 (link16), where as a particular message format was extended to add new fields, some earlier fields in the message were no longer needed and were repurposed, resulting in fields earlier in the message the format of which depends on a flag that appears after them.
Ticket DAFFODIL-641 details two things. A lookahead feature, and a second aspect that was not implemented, which is a dfdlx:choiceDispatchKeyPolicy which can be "immediate" (the default) or "deferred", meaning a lookahead behavior is needed as the choiceDispatchKey expression refers forward.
This capability is possibly more than is needed, and introduces substantial complexity. For example the schema compiler must prove that the distance to the flag is constant for all possible parses.
Alternatives are possible such as providing a function in the expression language which computes a constant distance between two sibling elements. E.g.,
dfdl:choiceDispatchKey='{
xs:string(
dfdlx:lookAhead(
dfdlx:distance(., ../flag, "bits"),
8)
)
}'
In the above we see an extension function dfdlx:distance which is intended to compute a constant, and fails with a schema definition error if the distance between the two sibling elements is non-constant. The intention here is that this measures the distance from the end of the first argument to the start of the second argument.
This function could be quite restrictive to only sibling elements within the same element parent so as to avoid a lot of complexity in Daffodil's schema compiler for this use case.
Note that this same function might; however, be generalized to help compute things like dfdl:choiceLength, when the dfdl:choiceLengthKind is 'explicit'. This is another case where the schema author must compute an exact distance in the data requiring knowledge of the details of the format that would better be left to the schema compiler to figure out.