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<xs:choice dfdl:choiceDispatchKey="{ tag }">
  <xs:element name="a" type="xs:int" dfdl:length="16" dfdl:choiceBranchKey="1"/>
  <xs:element name="b" type="xs:int" dfdl:length="16" dfdl:choiceBranchKey="2"/>
</xs:choice>
<xs:element name="tag" type="xs:int" dfdl:length="8" />

An existing proposal (Proposal: DFDL base +plus offset feature - Enables describing TIFF) would allow for this, by making it possible to put the <tag> element first in the infoset, despite it occuring later in the bitstream. However, such a proposal imposes unnessasary complexity for such a usecase. In particular, the schema must specify explicitly to jump forward and backward in the bitstream. Further, the full generallity of the schema involves considering additional issues surrounding unparsing (such as overlapping data).

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<xs:choice dfdl:choiceDispatchKey="{ dafdfdlx:lookAhead(16,8) }">
  <xs:element name="a" type="xs:int" dfdl:length="16" dfdl:choiceBranchKey="1"/>
  <xs:element name="b" type="xs:int" dfdl:length="16" dfdl:choiceBranchKey="2"/>
</xs:choice>
<xs:element name="tag" type="xs:int" dfdl:length="8" />

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This proposal is to add the daf dfdlx:lookAhead function to DPath.

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dfdlx:lookAhead

• dafdfdlx:lookAhead(distance, bitSize) 
  • read bitSize bits, where the first bit is located at an offset of distance from the current location
• Restrictions
  • distance >=0
  • bitSize >= 0
  • distance + bitSize <= Implementation defined limit no less than 512 bits
  • Cannot be called during unparse
  • ParseError if looks past EOF
  • Undefined behavior if looks past document boundery when in streaming mode.
  • bitOrder and byteOrder are determined by the current location. Changes between the current location and the location containing the data being read will not be respected.

Examples

The following two elements are equivalent:

• <xs:element name="a" type="xs:unsignedInt" dfdl:length="3" dfdl:lengthUnits="bits" />
• <xs:element name="a" type="xs:unsignedInt" dfdl:length="3" dfdl:lengthUnits="bits" dfdl:inputValueCalc="{ dafdfdlx:lookAhead(0,3) }" />

The following example demonstrates using lookAhead to branch based on a field in the future:

<xs:choice dfdl:choiceDispatchKey="{ dafdfdlx:lookAhead(16,8) }">
  <xs:element name="a" type="xs:int" dfdl:length="16" dfdl:choiceBranchKey="1"/>
  <xs:element name="b" type="xs:int" dfdl:length="16" dfdl:choiceBranchKey="2"/>
</xs:choice>
<xs:element name="tag" type="xs:int" dfdl:length="8" />

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