Add the simple substitution cipher.

src/cipher_analytical_machine/ciphers/simple_substitution.clj

@@ -0,0 +1,47 @@
+(ns cipher-analytical-machine.ciphers.simple-substitution
+  (:require [clojure.string :as cs]
+            [clojure.set :as set]
+            [cipher-analytical-machine.ciphers.caesar :as caesar])
+  (:gen-class))
+
+(defn shuffled-numbers
+  "Generate the shuffled order for integer list."
+  [size]
+  (-> size
+      (take (range))
+      (shuffle)))
+
+(defn generate-substitution-table
+  "Generate the map (char, int) for the substittion."
+  [symbols]
+  (->> (count symbols)
+       (shuffled-numbers)
+       (zipmap symbols)))
+
+(defn find-value-in-table
+  "It uses the substitution table to find the value of a char or a number."
+  [char substitution-table symbols]
+  (get substitution-table char))
+
+(defn encrypt-message
+  "Encrypt a message using the simple substitution cipher. The function is case-insensitive. If a symbol isn't in the symbols list, then it will be removed."
+  [message key substitution-table symbols]
+  (let [max-index (count symbols)]
+    (->> message
+         (cs/lower-case)
+         (caesar/encrypt-message key symbols)
+         (map (fn [char] (find-value-in-table char substitution-table symbols)))
+         (cs/join \,))))
+
+(defn decrypt-message
+  "Decrypt a message using the simple substitution cipher. The function is case-insensitive."
+  [message key substitution-table symbols]
+  (let [substitution-table (set/map-invert substitution-table)
+        max-index (count symbols)
+        message (cs/split message #",")]
+      (->> message
+           (map #(Integer/parseInt %))
+           (map (fn [char] (find-value-in-table char substitution-table symbols)))
+           (caesar/encrypt-message key symbols)
+           (cs/join))))
+