bsloane1650 commented on a change in pull request #225: Implement daf:lookAhead
URL: https://github.com/apache/incubator-daffodil/pull/225#discussion_r290338037
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File path:
daffodil-runtime1/src/main/scala/org/apache/daffodil/dpath/DAFFunctions.scala
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@@ -72,3 +78,63 @@ case object DAFError extends RecipeOp {
}
}
}
+
+case class DAFLookAhead(recipes: List[CompiledDPath])
+ extends FNTwoArgs(recipes) {
+
+ def computeValue(arg1: AnyRef, arg2: AnyRef, dstate: DState): AnyRef = {
+ val offset = arg1.asInstanceOf[JLong]
+ val lBitSize = arg2.asInstanceOf[JLong]
+
+ /*
+ * Since daf:lookAhead is defined to take unsigned arguements, the DPath
interperater
+ * will error out on the cast if a negative arguement is supplied, so we
do not need to SDE here.
+ */
+
+ Assert.invariant(offset >= 0)
+ Assert.invariant(lBitSize >= 0)
+
+ val totalLookahead = offset + lBitSize
+ if (totalLookahead > 512) {
Review comment:
My main motivation for this limit was to be nice to future implementations
where infinite lookahead might be less trivial. An added benifit is that it
prevents people from shooting themselves in the foot with a long lookahead on
streamed input.
Adding the tunable maxDAFLookaheadBits
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