[natch]

>Do you know htree achieves this space efficiency?

This is a new data structure to me too but if I understand it correctly, one aspect of it, if you do it right, is you don't have to store the tree at all. You just store the data in an array, and use the procedure to look up the item in the array. So the tree is virtual. Anyone who gets it, am I right about this?

You would potentially need a sentinel bit or something at each element of the array to tell you whether there is a value there. Or if all your values were non-zero (say you are storing pointers to other structures) then that is already taken care of.