;;;;
;; Even Fibonacci numbers
;; Problem 2
;;
;; Each new term in the Fibonacci sequence is generated by adding the previous two terms. By
;; starting with 1 and 2, the first 10 terms will be:
;; 1, 2, 3, 5, 8, 13, 21, 34, 55, 89, ...
;; By considering the terms in the Fibonacci sequence whose values do not exceed four
;; million, find the sum of the even-valued terms.
;;;;

(def fib
  (memoize (fn [n]
             (case n
               0 1
               1 1
               (+' (fib (-' n 1)) (fib (-' n 2)))))))

(def +top+ 4000000)

(def result
  (reduce +
    (for [x (range 1 1000)
          :let [f (fib x)]
          :when (= (mod f 2) 0)
          :while (< f +top+)]
      f)))

(println result)