;; 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 as add!,lookup above.
;;
(define-struct dict (add-fn! lookup-fn id))

  ; add!: ANY ANY --> ??
  ; SIDE-EFFECT: change the contents.

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


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

(define new-dict
  (local {(define dict-count 0)}
    (lambda ()
      (local {;; a local list-of-entries,
              ;; used only by the dictionary we're creating now:
              ;;
              (define contents empty)
              
              ;; Contracts as per Dictionary, above.

              ; Return the list-of-entries, for debugging as much as anything.
              ;
              (define (add! k val)
                (begin (set! contents (cons (make-entry k val) contents))
                       contents))
              
              
              (define (lookup key) (lookup-helper key contents))
              
              ;; lookup-helper: key, list-of-entries --> entry or false
              ;; Does the key occur inside "entries"?
              ;; If so, return that entry, else return the sentinel false.
              ;;
              (define (lookup-helper key entries)
                (cond [(empty? entries) false]
                      [(cons?  entries)
                       (cond [(equal? key (entry-key (first entries)))
                              (first entries)]
                             [else (lookup-helper key (rest entries))])]))
              ;
              ; NB this function can be of course easily re-written as a list visitor.
              }
        (begin
          (set! dict-count (add1 dict-count))
          (make-dict add! lookup dict-count))))))

;;;;;; Tests

; Okay, actually both german and french nouns
; *should* be represented by a structure which includes a gender field.

(define english->german (new-dict))
(= 1 (dict-id english->german))
(boolean=? false ((dict-lookup-fn english->german) 'health))
((dict-add-fn!   english->german) 'health 'gesundheit)
((dict-lookup-fn english->german) 'health)
((dict-add-fn!   english->german) 'science 'wissenschaft)
((dict-lookup-fn english->german) 'health)

(define english->french (new-dict))
(= 1 (dict-id english->german))
(= 2 (dict-id english->french))
(boolean=? false
           ((dict-lookup-fn english->french) 'health))
((dict-add-fn!   english->french) 'health 'sante)
((dict-add-fn!   english->french) 'computer 'ordinateur)
((dict-add-fn!   english->french) 'weekend  'weekend)
((dict-lookup-fn english->french) 'health)


((dict-add-fn!   english->german) 'computer-science 'informatik)
((dict-lookup-fn english->german) 'science)
((dict-lookup-fn english->german) 'health)
(boolean=? false
           ((dict-lookup-fn english->german) 'gymnasium))

(new-dict)
(new-dict)
(new-dict)
(= 6 (dict-id (new-dict)))