add atbash

atbash-cipher/README.md

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+# Atbash Cipher
+
+Create an implementation of the atbash cipher, an ancient encryption system created in the Middle East.
+
+The Atbash cipher is a simple substitution cipher that relies on
+transposing all the letters in the alphabet such that the resulting
+alphabet is backwards. The first letter is replaced with the last
+letter, the second with the second-last, and so on.
+
+An Atbash cipher for the Latin alphabet would be as follows:
+
+```plain
+Plain:  abcdefghijklmnopqrstuvwxyz
+Cipher: zyxwvutsrqponmlkjihgfedcba
+```
+
+It is a very weak cipher because it only has one possible key, and it is
+a simple monoalphabetic substitution cipher. However, this may not have
+been an issue in the cipher's time.
+
+Ciphertext is written out in groups of fixed length, the traditional group size
+being 5 letters, and punctuation is excluded. This is to make it harder to guess
+things based on word boundaries.
+
+## Examples
+- Encoding `test` gives `gvhg`
+- Decoding `gvhg` gives `test`
+- Decoding `gsvjf rxpyi ldmul cqfnk hlevi gsvoz abwlt` gives `thequickbrownfoxjumpsoverthelazydog`
+## Source
+
+Wikipedia [http://en.wikipedia.org/wiki/Atbash](http://en.wikipedia.org/wiki/Atbash)
+
+## Submitting Incomplete Solutions
+It's possible to submit an incomplete solution so you can see how others have completed the exercise.

atbash-cipher/atbash-cipher-test.el

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+;;; atbash-cipher-test.el --- Tests for Atbash Cipher (exercism)
+
+;;; Commentary:
+
+;;; Code:
+
+(require 'cl)
+
+(load-file "atbash-cipher.el")
+
+(ert-deftest encode-no ()
+  (should (equal "ml" (encode "no"))))
+
+(ert-deftest encode-yes ()
+  (should (equal "bvh" (encode "yes"))))
+
+(ert-deftest encode-OMG ()
+  (should (equal "lnt" (encode "OMG"))))
+
+(ert-deftest encode-O-M-G ()
+  (should (equal "lnt" (encode "O M G"))))
+
+(ert-deftest encode-long-word ()
+  (should (equal "nrmwy oldrm tob"
+                 (encode "mindblowingly"))))
+
+(ert-deftest encode-numbers ()
+  (should (equal "gvhgr mt123 gvhgr mt"
+                 (encode "Testing, 1 2 3, testing."))))
+
+(ert-deftest encode-sentence ()
+  (should (equal "gifgs rhurx grlm"
+                 (encode "Truth is fiction."))))
+
+(ert-deftest encode-all-the-things ()
+  (let ((plaintext "The quick brown fox jumps over the lazy dog.")
+        (ciphertext "gsvjf rxpyi ldmul cqfnk hlevi gsvoz abwlt"))
+    (should (equal ciphertext
+                   (encode plaintext)))))
+
+(provide 'atbash-cipher-test)
+;;; atbash-cipher-test.el ends here

atbash-cipher/atbash-cipher.el

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+;;; atbash-cipher.el --- Atbash-Cipher (exercism)
+
+;;; Commentary:
+
+
+
+
+;;; Code:
+
+(require 'subr-x)
+(defconst alphabet "abcdefghijklmnopqrstuvwxyz")
+(defconst atbash
+  (let ((idx 0)
+         (reverse-alph-list (reverse alphabet)))
+    (mapcar #'(lambda (arg)
+                (let* ((reverse-char (substring reverse-alph-list idx (1+ idx))))
+                  (incf idx)
+                  (list arg (string-to-char reverse-char)))) alphabet)))
+
+(defun encode (plaintext)
+  "Encode PLAINTEXT to atbash-cipher encoding."
+  (let* ((prepared-string (downcase (replace-regexp-in-string "[^[:alnum:]_-]" ""  plaintext)))
+         (encoded-string (mapconcat #'(lambda(arg)
+                                        (char-to-string (or (car (alist-get arg atbash)) arg))) prepared-string ""))
+         (result "")
+         (counter 0))
+    (while (not (= counter (length encoded-string)))
+      (if (= 0 (% (1+ counter) 5))
+          (setq result (concat result (substring encoded-string counter (1+ counter)) " "))
+        (setq result (concat result (substring encoded-string counter (1+ counter)))))
+      (setq counter (1+ counter)))
+    (string-trim result)))
+
+
+(provide 'atbash-cipher)
+;;; atbash-cipher.el ends here