euler02.cl

; Project Euler Problem 2
; Solution by nathan dotz - nathan (period) dotz (at sign) gmail (period) com
;
; 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, ...
; 
; Find the sum of all the even-valued terms in the sequence
; which do not exceed four million.
;

(defun fib (n)
  (defun fib-worker (n stack)
	  (if (<= n (car stack))
		(cdr stack)
		(fib-worker n (cons (+ (first stack) (second stack)) stack))))
  (fib-worker n '(1 1))
  )

(defun even-small-fib (small-fib)
  (remove-if-not 'evenp small-fib)
  )
 
(princ (apply '+ (even-small-fib (fib 4000000)) ) )