Add a function to decrypt a message with the gamma cipher.

src/cipher_analytical_machine/ciphers/gamma.clj

@@ -65,3 +65,14 @@
          (map vector array)
          (map (fn [[a g]] (add-mod a g module))))))
 
+(defn decrypt-message
+  "Decrypt a message using the gamma cipher. The function is case-insensitive. The key is an array of three elements [a b c]. The array contains integer in range [0, module)."
+  [key symbols message]
+  (let [module (count symbols)
+        table (ss/generate-sorted-substitution-table symbols)]
+    (->> message
+         (ss/decrypt-by-table (set/map-invert table))
+         (decrypt-array key module)
+         (ss/decrypt-by-table table)
+         (cs/join))))
+

test/cipher_analytical_machine/ciphers/gamma_test.clj

@@ -53,3 +53,10 @@
       [4 32 15] 33 [6 28 2 8 3 5 13 16 21 1] [3 14 1 4 18 19 23 8 27 17]
       [1 2  3 ] 3  [0 1 2 2 0 2 1 1 0]       [0 2 1 1 0 1 2 2 0])))
 
+(deftest decrypt-message-test
+  (testing "Checking that the message is decrypted as expected"
+    (are [key symbols message expected]
+        (= expected (decrypt-message key symbols message))
+      [4 32 15] "абвгґдеєжзиіїйклмнопрстуфхцчшщьюя" "ешвжгдймсб" "гкбґопужчн"
+      [1 2  3 ] "абв" "абввавбба" "авббабвва")))
+