euler002

euler.clj

(ns euler002.euler)

(defn- fib-fast 
  [i a b limit]
  (if 
    (< i limit)
    (fib-fast (inc i) b (+ a b) limit)
    b))

(defn fib 
  [n]
  (if
    (= n 0)
    0
    (fib-fast (bigint 1) (bigint 0) (bigint 1) (bigint n))))

(defn- even-fibs-less-than
 [max-fib]
  (for [n (range)
      :let [f (fib n)]
      :when (even? f)
      :while (< f max-fib)]
  f))

(defn sum-even-fibs-less-than
  [max-fib]
  (reduce + (even-fibs-less-than max-fib)))

euler_test.clj

(ns euler002.euler-test
  (:use midje.sweet
        euler002.euler))

(fact
  (fib 0) => 0
  (fib 1) => 1
  (fib 2) => 1
  (fib 3) => 2
  (fib 4) => 3
  (fib 12) => 144) 

(fact "fib can handle even bigint-large numbers, just for fun"
  (fib 92) => 7540113804746346429 
  (fib 93) => 12200160415121876738) 

(fact "The Project Euler problem 2 answer"
  (sum-even-fibs-less-than 4000000) => 4613732)

Cred till @petterik för fib-fast ;D