(defun square-p (d)
  (eq (isqrt d) (sqrt d)))

;;
;; ripped from http://www.cs.jhu.edu/~jason/software/fractions/fractions.lisp
;;
(defun true-sqrt-cf (d)
  (assert (not (square-p d)))
  (loop with (a b c) = (list 0 1 1)
        with first-partial-quotient = (isqrt d)
        with gcd

        for partial-quotient = (truncate (+ a (isqrt (* b b d))) c)

        collect partial-quotient
        
        do (decf a (* c partial-quotient))   ;; subtract partial-quotient from x
           (psetf a (* -1 a c)               ;; take reciprocal via parallel assignment
                  b (* b c)
                  c (- (* b b d) (* a a)))
           (setf gcd (gcd a b c)             ;; put into lowest terms
                 a (/ a gcd)
                 b (/ b gcd)
                 c (/ c gcd))

        until (= partial-quotient (* 2 first-partial-quotient))))  ;; about to repeat

;; answer problem 64
(defvar *count* 0)
(loop for i from 1 to 10000 do
  (if (not (square-p i))
    (if (evenp (length (true-sqrt-cf i)))
      (incf *count*))))

(format t "~A~%" *count*)