#lang racket

(require sicp)

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

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

  (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)))))
  (init! l)

  (define (make-dispatch t b)
    (let ([top-ptr t] [back-ptr b] [disp #f])
      (define (rotation-left)
        (make-dispatch (cdr top-ptr) back-ptr))

      (define (rotation-right)
        (make-dispatch (cdar top-ptr) back-ptr))

      (define (rotation-right!)
        (set! top-ptr (cdar top-ptr))
        disp)

      (define (rotation-left!)
        (set! top-ptr (cdr top-ptr))
        disp)

      (define (dispatch m . v)
        (case m
          ['top-ptr top-ptr]
          ['back-ptr back-ptr]
          ['top (caar top-ptr)]
          ['rotation-left (rotation-left)]
          ['rotation-right (rotation-right)]
          ['rotation-right! (rotation-right!)])
        )
      (set! disp dispatch)
      disp))



  (make-dispatch top-ptr back-ptr)))