gistfile1.txt

module Caracal
  class DecodeProductId

    def initialize(digital_product_id)
      @digital_product_id = digital_product_id
    end

    def self.run(digital_product_id)
      new(digital_product_id).decode
    end

    def decode
      extracted_encoded_product_key = extract_encoded_product_key

      key, i = "", 24

      while i >= 0
        index = 0
        j = 14
        while j >= 0
          index = (index << 8) | extracted_encoded_product_key[j].to_i
          extracted_encoded_product_key[j] = (index / 24)
          index = (index % 24)

          j -= 1
        end

        key = valid_chars_in_product_key[index].chr + key
        key = ("-" + key) if ((i % 5 == 0) and i > 0)

        i -= 1
      end

      key
    end

    private
    def valid_chars_in_product_key
      "BCDFGHJKMPQRTVWXY2346789".scan(/./).map(&:ord)
    end

    def extract_encoded_product_key
      digital_product_id_as_array.slice(52, 15)
    end

    def digital_product_id_as_array
      @digital_product_id.unpack('H*').first.gsub(/(..)/, '\1 ').split(' ').map { |x| x.to_i(16) }
    end
  end
end