hoge.scm

(define (fib-pathological n)
  (define (go m prev tag acc)
    (cond
      ((= m (+ n 1))
        acc)
      ((< m 2)
        (if (even? tag)
          (go (+ m 1) m (quotient tag 2) (+ acc 1))
          (go (+ m 2) m (quotient tag 2) (+ acc 1))))
      ((= prev (- m 2))
        (if (even? tag)
          (go (+ m 1) m (quotient tag 2) acc)
          (go (+ m 2) m (quotient tag 2) acc)))
      ((= prev (- m 1))
        (go (- m 2) m (+ (* tag 2) 1) acc))
      (else
        (go (- m 1) m (* tag 2) acc))))
  (go n (+ n 1) 0 0))

(display (fib-pathological 20))