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<xs:choice dfdl:choiceDispatchKey="{ dfdldaf: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 dfdl daf:lookAhead function to DPath.

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daf:lookAhead

• dfdldaf: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
  • Error if looks past EOF
  • Undefined behavior if looks past document boundery when in streaming mode.

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="{ dfdldaf:lookAhead(0,3) }" />

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

<xs:choice dfdl:choiceDispatchKey="{ dfdldaf: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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