euler007.clj

; Problem 7
;
; By listing the first six prime numbers: 2, 3, 5, 7, 11,
; and 13, we can see that the 6th prime is 13.
;
; What is the 10001st prime number?

(defn prime? [x]
  (cond (< x 2) false
	(= x 2) true
	(even? x) false
	:else (let [boundary (inc (int (Math/sqrt x)))
		    candidates (range 3 boundary 2)]
		(empty? (drop-while #(pos? (mod x %)) candidates)))))

(defn nth-prime [n] ; get n-th prime number
  (cond (= n 1) 2
	(= n 2) 3
	(pos? n)
	(loop [x 3 count 2]
	  (if (prime? x)
	    (if (= count n)
	      x
	      (recur (+ 2 x) (inc count)))
	    (recur (+ 2 x) count)))))

(def answer (future (nth-prime 10001))) ; Takes long :-)