(ns cipher-analytical-machine.analyzers.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-frequencies]
  (let [length-text (count text)
        letter-counts (count-characters (cs/lower-case text))]
    ;letter-counts
    (reduce
     (fn [acc [char count]]
       (+ acc
          (if (contains? letter-frequencies char)
            (chi-squared-letter-statistic count
                                          (get letter-frequencies char)
                                          length-text)
            0)))
            0
            letter-counts)))