ziplist.ss

#lang scheme

(define new-list '(() ()))

(define (left l) (car l))
(define (right l) (car (cdr l)))

(define (insert x l)
  (list (left l) (cons x (right l))))

(define (delete l)
  (list (left l) (cdr (right l))))

(define (move-right l)
  (list (cons (car (right l)) (left l)) (cdr (right l))))

(define (move-left l)
  (list (cdr (left l)) (cons (car (left l)) (right l))))

(define (update x l)
  (list (left l) (cons x (cdr (right l)))))

(define (get l)
  (car (right l)))

(define (rewind x)
  (if (eq? (left x) '()) x
      (rewind (list (cdr (left x)) (cons (car (left x)) (right x))))))

(define (index x)
  (length (left x)))

(define (move-to x l)
  (if (> x (index l)) (move-to x (move-right l))
      (if (= x (index l)) l
          (move-to x (move-left l)))))