;;;;
;; 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.
;;;;

(load "utils.scm")

(define +top+ 4000000)

(define (fib n)
  (case n
    ((0) 1)
    ((1) 1)
    (else (+ (fib (- n 1))
             (fib (- n 2))))))

(define result
  (sum
    (filter (lambda (x) (= 0 (modulo x 2)))
            (do ((x 1 (+ x 1))
                 (f 1 (fib x))
                 (fs '()))
              ((>= f +top+) fs)
              (set! fs (cons f fs))))))

(display result)