One more try. This time using the int-map contrib library. (Thanks to Zack
Tellman.)
Warning: I'm assuming that the int-set seq is automagically sorted so I can
reliably turn it back into a vector without sorting. It seems to be stable at
least. I wanted a vector for fast nth access. (Nth and count are constant
time on vectors.) I thought it would be slow to make the vector every round,
but it runs pretty fast. For small values of max, it was necessary to guard
the nth access with the not-found nil.
(require '[clojure.data.int-map :as im])
(defn lucky3
([max] (lucky3 1 (im/dense-int-set (range 1 max 2))))
([i iset]
(let [v (vec (seq iset))
n (nth v i nil)]
(if (and n (<= n (count v)))
(recur (inc i) (reduce (fn [sss m] (disj sss (nth v m)))
iset
(range (dec n) (count v) n)))
(sequence iset)))))
user=> (time (count (lucky3 1e6)))
"Elapsed time: 48905.491951 msecs"
71918
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