007alt.clj

; lazy prime generator - very useful

(defn lazy-primes []
  (letfn [(enqueue [sieve n step]
            (let [m (+ n step)]
              (if (sieve m)
                (recur sieve m step)
                (assoc sieve m step))))
          (next-sieve [sieve n]
            (if-let [step (sieve n)]
              (-> sieve
                  (dissoc n)
                  (enqueue n step))
              (enqueue sieve n (+ n n))))
          (next-primes [sieve n]
            (if (sieve n)
              (recur (next-sieve sieve n) (+ n 2))
              (cons n (lazy-seq (next-primes (next-sieve sieve n) (+ n 2))))))]
    (cons 2 (lazy-seq (next-primes {} 3)))))

(time (println (nth (lazy-primes) 1000000)))