prog/elisp/projecteuler/2.el

;; 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.

(defvar x 1)
(defvar y 2)

;; we are setting the sum equal to the sum of the first 3 fibonacci numbers: 1, 1, 2
(defvar sum 4) ;; sum = 1 + 1 + 2

(defun next-fib-number ()
  "finds the next fibonacci number"
  (cond
   ((< x y)
    (setq x (+ x y)))
   ((< y x)
    (setq y (+ x y)))))

(next-fib-number)

(defvar i 0)
(while (< i 4000000)
  (setq sum (+ sum (next-fib-number)))
  (setq i (+ i 1)))

(print sum)


(provide '2)
;;; 2.el ends here