;; An entry holds a key/value pair
;; for use in Dictionaries.
;;
(define-struct entry (key value))

;; A Dictionary is a structure with two "methods" (fields which are functions):
;; add-fn! and lookup-fn are functions which signatures below.
;;
(define-struct dict (add-fn! lookup-fn))

;; add-fn!: ANY ANY --> ??
;; SIDE-EFFECT: change the contents.
;; We return (the updated) contents, primarily for debugging purposes.

;; lookup: key --> entry or false
;; Does the key occur inside "contents"?
;; If so, return that entry, else return the sentinel false.



;;;;;;;;;;;;;;;;;;

(define (new-dict/vec max-key)
  (local {; Our local datastore: a vector.
          (define data (make-vector max-key false))
          
          ;; All contracts as per Dictionary above,
          ;; except that we are assuming the key is an integer
          ;; in [0,max-key).

          (define (add! k v) (begin (vector-set! data k v)
                                    data))
          (define (lookup k) (vector-ref  data k))
          }
    (make-dict add! lookup)))




;;;;;; Tests

(define fr (new-dict/vec  5))
(define de (new-dict/vec 10))

(boolean=? false ((dict-lookup-fn fr) 3))
((dict-add-fn! fr) 3 'trois)
((dict-lookup-fn fr) 3)

((dict-add-fn! de) 2 'zwei)
(boolean=? false ((dict-lookup-fn de) 3))

"An error:"
((dict-lookup-fn fr) 5)