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;; ============================================================
;; R4RS-Compatible Dice Code System (Proper Lists Only)
;; Two user-facing functions:
;;   (encode->dice "ABC123")
;;   (dice->binary '(D1 D2 D13))
;; ============================================================

;; ------------------------------------------------------------
;; 1. Proper list: character → dice code
;;    Replace these mappings with your real dice code.
;; ------------------------------------------------------------

(define char->dice
  '((#\A D1)
    (#\B D2)
    (#\C D3)
    (#\D D4)
    (#\E D5)
    (#\F D6)
    (#\0 D10)
    (#\1 D11)
    (#\2 D12)
    (#\3 D13)
    (#\4 D14)
    (#\5 D15)
    (#\6 D16)
    (#\7 D17)
    (#\8 D18)
    (#\9 D19)))

;; ------------------------------------------------------------
;; 2. Proper list: dice code → binary string
;;    Replace these with your reversible binary encoding.
;; ------------------------------------------------------------

(define dice->bin
  '((D1  "000001")
    (D2  "000010")
    (D3  "000011")
    (D4  "000100")
    (D5  "000101")
    (D6  "000110")
    (D10 "001010")
    (D11 "001011")
    (D12 "001100")
    (D13 "001101")
    (D14 "001110")
    (D15 "001111")
    (D16 "010000")
    (D17 "010001")
    (D18 "010010")
    (D19 "010011")))

;; ------------------------------------------------------------
;; Utility: lookup in a proper-list association list
;; Each entry is (key value), not (key . value)
;; ------------------------------------------------------------

(define (lookup key alist)
  (let ((p (assoc key alist)))
    (if p
        (cadr p)   ;; second element of the list
        (error "No mapping for key" key))))

;; ------------------------------------------------------------
;; USER-FACING FUNCTION 1:
;; Convert letters/digits → dice code list
;; ------------------------------------------------------------

(define (encode->dice str)
  (map (lambda (ch)
         (lookup ch char->dice))
       (string->list str)))

;; ------------------------------------------------------------
;; USER-FACING FUNCTION 2:
;; Convert dice code list → binary string
;; ------------------------------------------------------------

(define (dice->binary dice-list)
  (apply string-append
         (map (lambda (d)
                (lookup d dice->bin))
              dice-list)))

;; ------------------------------------------------------------
;; Example:
;; (encode->dice "AB3")
;; → (D1 D2 D13)
;;
;; (dice->binary '(D1 D2 D13))
;; → "000001000010001101"
;; ------------------------------------------------------------