;; A Chain is a class with two methods
;; that represents an immutable chain of objects.
;; getLength: --> number, which returns the length of the chain
;; getNext: --> Chain, which returns a new Chain that is one longer 
;; and linked to this Chain.
(define-struct Chain (getLength getNext))

;; A zero length Chain
(define chain0
  (local
      [;; chainFactory: Chain --> Chain
       ;; An internal factory to enable recursive self-generation
       ;; parent is the previous Chain the new Chain links to.
       ;; This is the inductive generation process
       (define (chainFactory parent)
         (local
             [(define newChain  ;; for self reference
                (make-Chain
                 (lambda ()   ;; recursively calculate the length
                   (+ 1 ((Chain-getLength parent))))
                 (lambda ()  ;; recursively self generate using the factory
                   (chainFactory newChain))))]
           newChain))  ;; return the new chain
       (define this  ;; for self reference
         (make-Chain
          (lambda ()   ;; base case chain has length zero
            0) 
          (lambda ()   ;; recursively self-generate using the factory
            (chainFactory this))))]
    this))  ;; return the base case chain


"Chain test cases:"

(= 0 ((Chain-getLength chain0)))

(define chain1 ((Chain-getNext chain0)))
(= 1 ((Chain-getLength chain1)))

(define chain2 ((Chain-getNext chain1)))
(= 2 ((Chain-getLength chain2)))

(define chain3 ((Chain-getNext chain2)))
(= 3 ((Chain-getLength chain3)))


;;addNChain: Chain natnum --> Chain
;; Adds n chains to a-chain.
(define (addNChain a-chain n)
  (cond
    [(zero? n) a-chain]
    [(< 0 n) (addNChain ((Chain-getNext a-chain)) (sub1 n))]))

"AddNChain test cases:"
(define chain25 (addNChain chain0 25))
(= 25 ((Chain-getLength chain25)))

(= 40 ((Chain-getLength (addNChain chain25 15))))