#lang racket

(require sicp)

(define (top r) (r 'top))

(define (make-ring . l)
(let ([top-ptr '()] [back-ptr '()])

  (define (insert-back! x)
    (let ([new-ptr (cons (cons x '()) '())])
      (case top-ptr
        ('() (set! top-ptr new-ptr)
             (set! back-ptr new-ptr))
        (else (set-cdr! (car new-ptr) back-ptr)
              (set-cdr! back-ptr new-ptr)
              (set! back-ptr new-ptr)))))

  (define (init! l)
    (if (null? l)
        (begin (set-cdr! back-ptr top-ptr)
               (set-cdr! (car top-ptr) back-ptr))
        (begin (insert-back! (car l))
               (init! (cdr l)))))
  (define (dispatch m . v)
   (case m
     ['top-ptr top-ptr]
     ['back-ptr back-ptr]
     ['top (caar top-ptr)]
     ['rotation-left (rotation-left)]))

  (define (_make-ring t b)
    (let ([top-ptr t] [back-ptr b])
      (define (dispatch m . v)
   (case m
     ['top-ptr top-ptr]
     ['back-ptr back-ptr]
     ['top (caar top-ptr)]
     ['rotation-left (rotation-left)]))
      dispatch))

  (define (rotation-left)
    (_make-ring (cdr top-ptr) back-ptr))

  (init! l)



  dispatch))