Add Chi-squared Statistic function to analyze texts.

src/cipher_analytical_machine/cipher_analyzers.clj

@@ -0,0 +1,38 @@
+(ns cipher-analytical-machine.cipher_analyzers
+  (:require [clojure.string :as cs])
+  (:gen-class))
+
+(defn count-characters
+  "Count the letters and return map."
+  [text]
+  (reduce
+   (fn [acc val]
+     (assoc acc val
+            (inc (get acc val 0))))
+   {} text))
+
+(defn chi-squared-letter-statistic
+  "To calculate the frequency distribution of a letter."
+  [letter-count letter-frequence length-text]
+  (let [letter-frequence (* length-text letter-frequence)]
+    (/ (Math/pow
+        (- letter-count letter-frequence) 2)
+       letter-frequence)))
+
+(defn chi-squared-statistic
+  "To calculate the frequency distribution of a language. More detail about the algorithm here: http://practicalcryptography.com/cryptanalysis/text-characterisation/chi-squared-statistic/"
+  [text letter-frequences]
+  (let [length-text (count text)
+        letter-counts (count-characters (cs/lower-case text))]
+    ;letter-counts
+    (reduce
+     (fn [acc [char count]]
+       (+ acc
+          (if (contains? letter-frequences char)
+            (chi-squared-letter-statistic count
+                                          (get letter-frequences char)
+                                          length-text)
+            0)))
+            0
+            letter-counts)))
+