Add exercise 2 from chapter 22

Author: Bernard Pietraga

chapter_22/protocols_2.exs

@@ -0,0 +1,68 @@
+defprotocol CaesarSalad do
+  def encrypt(string, shift)
+  def rot13(string)
+end
+
+defimpl CaesarSalad, for: List do
+  def encrypt(list, shift), do:
+    list |> Enum.map(&encrypt_char(&1, shift))
+
+  def rot13(list), do:
+    encrypt(list, 13)
+
+  def encrypt_char(char, shift) when char in 97..122, do:
+    97 + rem(char + shift - 97, 26)
+  def encrypt_char(char, shift) when char in 65..90, do:
+    65 + rem(char + shift - 65, 26)
+  def encrypt_char(32, _shift), do:
+    32
+  # Question mark for not matched char
+  def encrypt_char(_, _shift) do
+    63
+  end
+end
+
+defimpl CaesarSalad, for: BitString do
+  def encrypt(string, shift) do
+    String.to_char_list(string)
+    |> CaesarSalad.List.encrypt(shift)
+    |> List.to_string
+  end
+
+  def rot13(string), do:
+    encrypt(string, 13)
+end
+
+defmodule CaesarMacChecker do
+  def run do
+    mac_words = macintosh_dictionary()
+    inspect_words(mac_words, sorted_words(mac_words))
+  end
+
+  def inspect_words(mac_words, words_by_size) do
+    Enum.each(mac_words, &match_word(&1, words_by_size))
+  end
+
+  def match_word(word, words_by_size) do
+    encrypted = CaesarSalad.rot13(word)
+    case MapSet.member?(words_by_size[byte_size(word)], encrypted) do
+      true ->
+         IO.puts "#{word} is equal #{encrypted}"
+      _ ->
+        nil
+    end
+  end
+
+  def macintosh_dictionary do
+    File.stream!("/usr/share/dict/words", [:read, :utf8], :line)
+    |> Enum.map(&String.rstrip(&1) |> String.downcase)
+  end
+
+  def sorted_words(words) do
+    for {length, word_group} <- Enum.group_by(words, &byte_size/1), into: Map.new do
+      {length, Enum.into(word_group, MapSet.new)}
+    end
+  end
+end
+
+CaesarMacChecker.run