Add exercise 1 from chapter 22

Author: Bernard Pietraga

.gitignore

@@ -1 +1,2 @@
 .DS_Store
+erl_crash.dump

chapter_22/protocols_1.exs

@@ -0,0 +1,46 @@
+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(char = 32, _shift), do:
+    char
+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
+
+ExUnit.start
+
+defmodule CaesarSaladTest do
+  use ExUnit.Case
+
+  test "list implementation" do
+    expected = CaesarSalad.rot13("Space rocket")
+    assert expected == "Fcnpr ebpxrg"
+  end
+
+  test "string implementation" do
+    expected = CaesarSalad.rot13("Elixir rocks")
+    assert expected == "Ryvkve ebpxf"
+  end
+end