encrypt_decrypt_zipcode.rb

def invert(code)
  "#{code[1].chr}#{code[0].chr}"
end

def encrypt(zipcode)
  codes = {
    "00" => "AL",
    "01" => "UG",
    "10" => "CN",
    "11" => "X8",
    "20" => "EP",
    "21" => "Z0",
    "30" => "GR",
    "31" => "1B",
    "40" => "ID",
    "41" => "3D",
    "50" => "JS",
    "51" => "4C",
    "60" => "HQ",
    "61" => "2A",
    "70" => "FO",
    "71" => "09",
    "80" => "DM",
    "81" => "Y7",
    "90" => "BK",
    "91" => "V5"
  }

  encrypted_zipcode = ''
  index = 0

  zipcode.each_char do |char|         # 2
    code_1 = codes[char + '1']        # Z0
    inverted_code_1 = invert(code_1)  # 0Z
    code_0 = codes[char + '0']        # EP
    inverted_code_0 = invert(code_0)  # PE

    if index.even?
      partial = code_1
      partial = inverted_code_1 if index > 3 && !encrypted_zipcode.include?(inverted_code_1)
    else
      partial = code_0
      partial = inverted_code_0 if index > 3 && !encrypted_zipcode.include?(inverted_code_0)
    end

    encrypted_zipcode << partial
    index += 1
  end

  encrypted_zipcode
end

def decrypt(encrypted_zipcode)
  codes = {
    "AL" => "0", "LA" => "0",
    "UG" => "0", "GU" => "0",
    "CN" => "1", "NC" => "1",
    "X8" => "1", "8X" => "1",
    "EP" => "2", "PE" => "2",
    "Z0" => "2", "0Z" => "2",
    "GR" => "3", "RG" => "3",
    "1B" => "3", "B1" => "3",
    "ID" => "4", "DI" => "4",
    "3D" => "4", "D3" => "4",
    "JS" => "5", "SJ" => "5",
    "4C" => "5", "C4" => "5",
    "HQ" => "6", "QH" => "6",
    "2A" => "6", "A2" => "6",
    "FO" => "7", "OF" => "7",
    "09" => "7", "90" => "7",
    "DM" => "8", "MD" => "8",
    "Y7" => "8", "7Y" => "8",
    "BK" => "9", "KB" => "9",
    "V5" => "9", "5V" => "9"
  }

  zipcode = ''

  (0...encrypted_zipcode.length).step(2) do |i|
    code = "#{encrypted_zipcode[i].chr}#{encrypted_zipcode[i + 1].chr}" 
    zipcode << codes[code]
  end

  zipcode
end