Binary period

gistfile1.scm

(define (>> n i) (bitwise-arithmetic-shift n (- i)))
(define << bitwise-arithmetic-shift-left)
(define & bitwise-and)

(define (split n p)
  (let* ((bc (bitwise-length n))
         (mid (div bc 2)))
    (define (get-bit-seq s c)
      (let ((rs (- bc s c))
            (m  (- (<< 1 c) 1)))
        (& (>> n rs) m)))
    (and  (<= p mid)
          (let-values (((d m) (div-and-mod bc p)))
            (let f ((i 0)(a '()))
              (if (>= i d)
                  (and (apply = a)
                       (= (>> (car a) (- p m)) (& (- (<< 1 m) 1) n)))
                  (f (+ i 1) 
                     (cons (get-bit-seq (* i p) p) a))))))))

(define (bit-period n)
  (let ((q (div (bitwise-length n) 2)))
    (let f ((i 1))
      (if (> i q)
          -1
          (let ((r (split n i)))
            (if r 
                i 
                (f (+ i 1))))))))
         
(map bit-period '(955 102 #b10101010101 #b1111111111111110 #b111011011101101))