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https://issues.apache.org/jira/browse/DAFFODIL-2559?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17468875#comment-17468875
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Mike Beckerle commented on DAFFODIL-2559:
-----------------------------------------
Priority lowered as it is unclear what to do about this issue.
> forward-lookahead enhancement dfdlx:distance
> --------------------------------------------
>
> Key: DAFFODIL-2559
> URL: https://issues.apache.org/jira/browse/DAFFODIL-2559
> Project: Daffodil
> Issue Type: New Feature
> Components: Back End, Front End
> Affects Versions: 3.1.0
> Reporter: Mike Beckerle
> Priority: Minor
>
> 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.,
>
> {code:java}
> dfdl:choiceDispatchKey='{
> xs:string(
> dfdlx:lookAhead(
> dfdlx:distance(., ../flag, "bits"),
> 8)
> )
> }'
> {code}
> 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.
>
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