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Layering - Data Source/Target Indirection

(See concrete Proposal)

Often one needs multiple passes. The value of some element, which might be a string, a hexBinary, or an array of bytes, wants to be used as the input for more parsing.

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Users have requested a way to describe data structures with pointers linking objects to other objects. E.g. the arbitrary link structure a typical *NIX file system image can contain, with its hard links. This is related to the next topic about offsets to data.

Offsets to Data

(See Discussion/Proposal)

Some data formats contain header information including a table of offsets in the file to later parts of the data. The ability to directly express offsets within the data (absolute, or relative to some anchor, such as the end of the table of offsets) would make describing these kinds of data files much more direct.

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Very often one wants dfdl:lengthKind='delimited' or dfdl:lengthKind='explicit' for simple types, but dfdl:lengthKind="implicit" for complex types. Separating the dfdl:lengthKind into two properties, or having the ability to specify either way, would simplify many schemas that otherwise have a error-prone need to have a dfdl:ref='complex' format reference on every element of complex type to override the default dfdl:lengthKind. That or you have to split the schema and put all simple types in one file (and use only those simple types), and all complex types in another.

Table and Range Lookups

(see concrete Proposal)

Often one has a representation containing enumerations - integer values - which have symbolic meanings. The parsed result from such data wants to contain strings so the logical infoset is readable and understandable. A means is needed to specify a table of integer constants and their corresponding strings, to be used for parsing, and unparsing. Ranges are a generalization where a symbolic string is used to name all the integers that fall in a range.