HASkipList

A transactional version of the deterministic k-(2k+1) top-down operated skip list as described in T. Papadakis, Skip Lists and Probabilistic Analysis of Algorithms. Ch. 4 (Deterministic Skip Lists), pp. 55--78. Waterloo (CA) 1993

It uses the horizontal array technique with a parameter for k (minimum gap size). It uses a modified top-down removal algorithm that avoids the need for a second pass as in the original algorithm, and is careful about object creations, so that it will be able to persist the data structure without any unnecessary reads or writes to the store.

Three implementation notes: (1) We treat the nodes as immutable at the moment, storing them directly in the S#Val child pointers of their parents. While this currently seems to have a performance advantage (?), we could try to avoid this by using S#Refs for the child pointers, making the nodes become mutable. We could avoid copying the arrays for each insertion or deletion, at the cost of more space, but maybe better performance.

(2) The special treatment of isRight kind of sucks. Since now that information is also persisted, we might just have two types of branches and leaves, and avoid passing around this flag.

(3) Since there is a bug with the top-down one-pass removal, we might end up removing the creation of instances of virtual branches altogether again when replacing the current algorithm by a two-pass one.

TODO: nodes or at least leaves should be horizontally connected for a faster iterator and fast pair (interval) search