02-euler.clj

;; http://projecteuler.net/problem=2

(def fib
  (memoize
   (fn [n]
     (cond
      (= n 0) 1
      (= n 1) 2
      true (+ (fib (- n 1)) (fib (- n 2)))))))

(defn lazy-seq-fibo
  ([]
     (concat [0 1] (lazy-seq-fibo 0N 1N)))
  ([a b]
     (let [n (+ a b)]
       (lazy-seq
        (cons n (lazy-seq-fibo b n))))))

(defn ignore-head [n col]
  (if (= n 0)
    col
    (recur (- n 1) (rest col))))

(println (->> (lazy-seq-fibo)
              (ignore-head 2)
              (filter even?)
              (take-while (fn [n] (<= n 4000000N)))
              (reduce + 0)))