Add a implementation of Caesar cipher.

src/cipher_analytical_machine/caesar.clj

@@ -0,0 +1,37 @@
+(ns cipher-analytical-machine.caesar
+  (:require [clojure.string :as cs])
+  (:gen-class))
+
+(defn calculate-char-index
+  "Calculate the index of a char."
+  [char symbols]
+  (cs/index-of symbols char))
+
+(defn encrypt-index
+  "Calculates new index of a character. Uses its index, the key and max-index (exclusive)."
+  [index key max-index]
+  (-> index
+       (+ key)
+       (mod max-index)))
+
+(defn encrypt-char
+  "Calculates new index of a character. Uses a char from symbols, the key and max-index (exclusive)."
+  [char key symbols max-index]
+  (get symbols
+       (-> char
+           (calculate-char-index symbols)
+           (encrypt-index key max-index))))
+
+(defn encrypt-message
+  "Encrypt a message using the Caesar cipher."
+  [message key symbols]
+  (let [max-index (count symbols)]
+    (cs/join
+     (map (fn [char] (encrypt-char char key symbols max-index))
+          message))))
+
+(defn decrypt-message
+  "Decrypt the ciphtext using the Caesar cipher."
+  [ciphertext key symbols]
+  (encrypt-message ciphertext (- key) symbols))
+