(defparameter *ALPHA* "ABCDEFGHIJKLMNOPQRSTUVWXYZ")

;; rechercher un caractere dans une chaine
;;

(defun rechercher-element (item s)
  (dotimes (i (length s))
    (if (equal (aref s i) item)
      (return-from rechercher-element i)
      )
    )
  -1
  )



;; CESAR

(defun coder-cesar (M K)
  (let ((C (make-string (length M) :initial-element #\space))
        )
    (dotimes (i (length M))
      (setf (aref C i)
            (aref *ALPHA* (mod (+ (rechercher-element (aref M i) *ALPHA*) K) (length *ALPHA*)))
            )
      
      )
    C
    )
  )

;; (coder-cesar "BONJOUR" 3) --> "ERQMRXU"

(defun decoder-cesar (C K)
  (coder-cesar C (- K))
  )

;; (decoder-cesar "ERQMRXU" 3) --> "BONJOUR"


(defun coder-subst (M K)
  (let ((C (make-string (length M) :initial-element #\space))
        )
    (dotimes (i (length M))
      (setf (aref C i)
            (aref K (rechercher-element (aref M i) *ALPHA*))
            )
      
      )
    C
    )
  )

;; (coder-subst "BONJOUR" (genere-cle-subst "MAITRECORBEAUSURUNARBREPERCHE" *ALPHA*)) --> "EHPVHJC"




(defun genere-cle-subst (texte alpha)
  (let ((K (make-string (length alpha) :initial-element #\space))
        (ik 0)
        )
    (dotimes (i (length texte))
      (when (< (rechercher-element (aref texte i) K) 0)
        (print (aref texte i))
        (setf (aref K ik) (aref texte i))
        (incf ik)
        )
      )

    (dotimes (i (length alpha))
      (when (< (rechercher-element (aref alpha i) K) 0)
        (print (aref alpha i))
        (setf (aref K ik) (aref alpha i))
        (incf ik)
        )
      )
    
    K
    )
  )

;; (genere-cle-subst "LEZEBUBOITAVIDEMENTLEZEPHYR" *ALPHA*)
   


(defun coder-vig (M K)
  (let ((C (make-string (length M) :initial-element #\space))
        (ik 0)
        posc posk)
    (dotimes (i (length M))
      (setf posc (rechercher-element (aref M i) *ALPHA*))
      (setf posk (rechercher-element (aref K ik) *ALPHA*))
      (setf (aref C i) (aref *ALPHA* (mod (+ posc posk) (length *ALPHA*))))
      (setf ik (mod (+ ik 1) (length K)))
      )
    C
    )
  )

;; (coder-vig "APPELERNORDTROUPESVILLE" "ROUGE") --> "RDJKPVFHUVUHLUYGSMBMCZY"

(defun decoder-vig (M K)
  (let ((C (make-string (length M) :initial-element #\space))
        (ik 0)
        posc posk)
    (dotimes (i (length M))
      (setf posc (rechercher-element (aref M i) *ALPHA*))
      (setf posk (rechercher-element (aref K ik) *ALPHA*))
      (setf (aref C i) (aref *ALPHA* (mod (- posc posk) (length *ALPHA*))))
      (setf ik (mod (+ ik 1) (length K)))
      )
    C
    )
  )

;; (decoder-vig "RDJKPVFHUVUHLUYGSMBMCZY" "ROUGE") --> "APPELERNORDTROUPESVILLE"


(coder-vig "ROUGE" "ROUGE") --> "ICOMI"
(coder-vig "ICOMI" "ROUGE") --> "ZQISM"
(coder-vig "QECYQ" "ROUGE")