DFDL-1634 provides support for simpleType uinons in Daffodil.

However, a union is still validated (in "limited" validation mode, or when dfdl:checkConstraints is called) by sequentially walking through the union members one by one.

When the union consists of enumerations, or ranges, or mixtures of only those two, then a faster mechanism is needed.

E.g., for a union that is just enumerations, the validation should take constant time, by using a hash/table lookup.

For a union of min/max ranges, some sort of decision tree that rapidly determines validity is required.

combinations of these are also possible. A common situation would be for the integer 0 to mean "No Statement", 1-25 are valid values that have some mathematical meaning, and 26-31 are illegal.

We want to express this like

<simpleType>

   <union memberTypes="ex:noStatement ex:allowed ex:illegal"/>

</simpleType>

<simpleType name="noStatement">

  <restriction base="xs:int">

     <enumeration value="0"/>

 </restriction>

</simpleType>

<simpleType name="valid">

   <union memberTypes="ex:car ex:train ex:plane.... ex:bicycle"/

</simpleType>

<simpleType name="car"><restriction base="xs:int"><enumeration value="1"/></restriction></simpleType>

<simpleType name="train"><restriction base="xs:int"><enumeration value="2"/></restriction></simpleType>

<simpleType name="plane"><restriction base="xs:int"><enumeration value="3"/></restriction></simpleType>

... 21 more ...

<simpleType name="bicycle"><restriction base="xs:int"><enumeration value="25"/></restriction></simpleType>

<simpleType name="illegal">

   <restriction base="xs:int">

       <minInclusive value="26"/>

      <maxInclusive value="31"/>

   </restriction>

</simpleType>

In the above, the union containing the 25 enumeration simple types will be iterated with a loop that will go through all 25 possibles one by one. This needs to be improved to be a constant time dispatch.