I was reading the docs for std.algorithm, when I came across topN. This
is, of course, a highly useful problem, with several solutions; I was a
bit surprised to see the claim that it runs in linear time. As far as I
know, the only ways of achieving that would be (1) using the
super-elegant, but highly inefficient, algorithm of Blum, Floyd, Pratt,
Rivest and Tarjan, often known as Select, or (2) using soft heaps. (The
latter, I know less about.)
Checking the source, I found that -- as I suspected -- it uses the more
common Randomized-Select (without actual randomization here, though),
which only has an *expected* (or average-case) linear running time. It
suffers the same worst-case problems as Quicksort.
I'm not objecting to the use of algorithm -- it's a good choice in
practice -- but the docs should probably specify that the linear
guarantee does not hold in the worst case?
--
Magnus Lie Hetland
http://hetland.org
