Data commit

Task/Substitution-cipher/Racket/substitution-cipher.rkt

@@ -0,0 +1,55 @@
+#lang racket/base
+(require racket/list racket/function racket/file)
+
+(define abc "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz")
+
+;; Used to generate my-key for examples
+(define (random-key (alphabet abc))
+  (list->string (shuffle (string->list alphabet))))
+
+(define (cypher/decypher key (alphabet abc))
+  ;; alist is fine, hashes are better over 40 chars... so alist for
+  ;; abc, hash for ASCII.
+  (define ab-chars (string->list alphabet))
+  (define ky-chars (string->list key))
+  (define cypher-alist (map cons ab-chars ky-chars))
+  (define decypher-alist (map cons ky-chars ab-chars))
+  (define ((subst-map alist) str)
+    (list->string (map (lambda (c) (cond [(assoc c alist) => cdr] [else c]))
+                       (string->list str))))
+  (values (subst-map cypher-alist)
+          (subst-map decypher-alist)))
+
+(define (cypher/decypher-files key (alphabet abc))
+  (define-values (cypher decypher) (cypher/decypher key alphabet))
+  (define ((convert-file f) in out #:exists (exists-flag 'error))
+     (curry with-output-to-file out #:exists exists-flag
+            (lambda () (display (f (file->string in))))))
+  (values (convert-file cypher)
+          (convert-file decypher)))
+
+(module+ test
+  (require rackunit)
+  (define my-key "LXRWzUrIYPJiVQyMwKudbAaDjSEefvhlqmOkGcBZCFsNpxHTgton")
+
+  (define-values (cypher decypher) (cypher/decypher my-key abc))
+
+  (define in-text #<<T
+The quick brown fox...
+.. jumped over
+the lazy dog!
+T
+    )
+  (define cypher-text (cypher in-text))
+
+  (define plain-text (decypher cypher-text))
+  (displayln cypher-text)
+  (check-equal? plain-text in-text)
+
+  (define-values (file-cypher file-decypher) (cypher/decypher-files my-key abc))
+  (file-cypher "data/substitution.in.txt" "data/substitution.crypt.txt" #:exists 'replace)
+  (file-decypher "data/substitution.crypt.txt" "data/substitution.plain.txt" #:exists 'replace)
+  (displayln "---")
+  (displayln (file->string "data/substitution.crypt.txt"))
+  (check-equal? (file->string "data/substitution.in.txt")
+                (file->string "data/substitution.plain.txt")))