Project Euler Problem12 Fast

euler-12-fast.lisp

(defun sum-n (n)
  "(+ 1 2 3 ... n)"
  (* n (1+ n) 1/2))

(defun check-div (num)
  "return number of factors of num"
  (labels ((check-div-r (n k)
			 (if (= k 1)
				 1
				 (if (integerp (/ n k))
					 (1+ (check-div-r n (1- k)))
					 (check-div-r n (1- k))))))
	(multiple-value-bind (int-unused f) (round (sqrt num))
	  (if (zerop f)
		  (1+ (* 2 (check-div-r num (floor (sqrt num)))))
		  (* 2 (check-div-r num (floor (sqrt num)))))
	  )))

(defun prob-12 (num)
  (do ((i 1 (1+ i)) (result 0))
	  ((> result num) (format t  "ans: ~A~%" (sum-n (1- i))))
	(setq result (check-div (sum-n i)))
	))

(time (prob-12 500))