Engima1.0.rkt

#lang racket
(define alph (string->list "ABCDEFGHIJKLMNOPQRSTUVWXYZ"))
(define r1 (string->list "EKMFLGDQVZNTOWYHXUSPAIBRCJ"))
(define r2 (string->list "AJDKSIRUXBLHWTMCQGZNPYFVOE"))
(define r3 (string->list "BDFHJLCPRTXVZNYEIWGAKMUSQO"))
(define r4 (string->list "ESOVPZJAYQUIRHXLNFTGKDCMWB"))
(define r5 (string->list "VZBRGITYUPSDNHLXAWMJQOFECK"))
(define refB (string->list "YRUHQSLDPXNGOKMIEBFZCWVJAT"))
(define refC (string->list "FVPJIAOYEDRZXWGCTKUQSBNMHL"))

;; letter must be a char. Alphabet is a list from a string (list of chars)
;; gets the index of a specific letter in a sequence of letters
(define (get-idx-of letter alphabet)
  (define (iter count a)
    (if (equal? (car a) letter)
        count
        (iter (+ 1 count) (cdr a))))
  (iter 0 alphabet))

;; gets specific index in a sequence of letters
(define (get idx alphabet)
  (define (iter count a)
    (if (= 0 count) (car a) (iter (- count 1) (cdr a))))
  (iter idx alphabet))

(define (pass-through-rotor letter rotor)
  (get (get-idx-of letter alph) rotor))

(define (reflect-through-rotor letter rotor)
  (get (get-idx-of letter rotor) alph))

;; rotating the rotor
(define (rotate-rotor times rotor)
  ;; shift it all up one
  (define (iter times to-shift rotor )
    (if (= 0 times) (append rotor to-shift)
        (iter (- times 1) (append to-shift (list (car rotor))) (cdr rotor))))
  (iter times (list) rotor))

(define (pass-letter letter rotor1 rotor2 rotor3 reflector)
  ;; letter passes through r1, r2, r3, hits reflector board and goes back
  ;; through r3, r2, r1
  (define in (pass-through-rotor
              (pass-through-rotor
               (pass-through-rotor letter rotor1)
               rotor2)
              rotor3))
  (reflect-through-rotor
   (reflect-through-rotor
    (reflect-through-rotor
     (reflect-through-rotor in reflector)
     rotor3)
    rotor2)
   rotor1))

(define (get-next-rotor rotor rotor-prev notch)
  (if (equal? (car rotor-prev) notch) (rotate-rotor 1 rotor) rotor))

(define (encrypt-text text rotor1 rotor2 rotor3 reflector plugs notches)
  (define notch1 (car notches))
  (define notch2 (cadr notches))
  (define notch3 (caddr notches))

  (define (iter text encrypted r1 r2 r3 count)
    (if (empty? text) (list->string encrypted)
        (iter (cdr text) (append encrypted (list
                                            (pass-letter (car text)
                                                             r1 r2 r3 reflector)))
              (rotate-rotor 1 r1)
              (if (= 0 (modulo count 26)) (rotate-rotor 1 r2) r2)
              (if (= 0 (modulo count 676)) (rotate-rotor 1 r3) r3)
              (+ count 1))))
  (iter (string->list text) (list) rotor1 rotor2 rotor3 1))

(provide get-idx-of get pass-through-rotor reflect-through-rotor
         rotate-rotor pass-letter encrypt-text)