Yes, I was thinking that for a general-purpose component having sets that only contain smaller ranges are easily done with polymorphism (different representations for different cases), but it should also efficiently handle when the character to test is most-often in the ASCII/ANSI subrange, too.

My favorite doodle right now is a class with 4 pointer members: small_low points to a 16-byte bitmap for 0..127, likewise small_high for 128..255. large points to an array of arrays that handle 16-bit characters, and huge points to a deeper chain of arrays of arrays for 31-bit characters.

For testing, the small_low and small_high bitmaps are one pointer away, even if the large bitmap is populated. It doesn't have to chace down multiple levels after seeing that the high byte is zero.

The previous favorite design is a list of numbers. Even entries are starting points of a range (inclusive) and odd entries are ending points (exclusive). Do a binary search in the array and see if it's part of an "on" range or an "off" range.

That is good for large collections with few distinct runs, but uses a different algorithm than the ASCII case. The first one I mentioned is the same throughout, just different tree depths.

—John