Add the function to make a frequency string from a map.

src/cipher_analytical_machine/symbols/frequencies.clj

@@ -1,7 +1,17 @@
 (ns cipher-analytical-machine.symbols.frequencies
   (:gen-class))
 
-(def english-letter-frequences {
+(defn map-to-string
+  "Convert the map of frequencies to the frequency string. Letters are sorted by frequency in descending order."
+  [frequency-map]
+  (->> frequency-map
+       (reduce (fn [acc el] (conj acc el)) [])
+       (sort-by last)
+       (map first)
+       (reverse)
+       (apply str)))
+
+(def english-letter-frequencies {
    \a 0.0804
    \b 0.0154
    \c 0.0306
@@ -30,7 +40,7 @@
    \z 0.0009
 })
 
-(def ukrainian-letter-frequences {
+(def ukrainian-letter-frequencies {
    \а 0.064
    \б 0.013
    \в 0.046

test/cipher_analytical_machine/symbols/frequencies_test.clj

@@ -0,0 +1,11 @@
+(ns cipher-analytical-machine.symbols.frequencies-test
+  (:require
+   [clojure.test :refer :all]
+   [cipher-analytical-machine.symbols.frequencies :refer :all]))
+
+(deftest calculate-char-index-test
+  (let [symbol-map english-letter-frequencies]
+    (testing "The map must be converted to 'etaoinsrhldcumfpgwybvkxjqz'"
+      (is (= "etaoinsrhldcumfpgwybvkxjqz"
+             (map-to-string symbol-map))))))
+