Brian Hurt wrote:
> I was just looking at the map.ml implementation, and noticed that the
> logic for when to do a rotation was:
>       if hl > hr + 2 then begin
> Isn't this supposed to be:
>    if hl >= hr + 2 then begin

No, it was a conscious decision to use height-balanced binary trees
with an height imbalance of at most 2, rather than at most 1 as in
standard AVL trees.  As you note, log(N) access times are still
guaranteed, and it's a tradeoff between lookup time vs. rebalancing time.
Light experimentation suggested that imbalance <= 2 is globally more
efficient than imbalance <= 1.  Didn't try with larger imbalance bounds.

This said, red-black trees would probably work faster anyway, but I'll
let the algorithm experts on this list comment.

- Xavier Leroy