def c_checksum
      sum = 0
      checksum_values.each_with_index do |value, index|
        sum += ((index % 20) + 1) * value
      end
      sum % 47
    end
def c_checksum
      sum = 0
      checksum_values.each_with_index do |value, index|
        sum += ((index % 20) + 1) * value
      end
      sum % 47
    end
Calculates the C checksum based on checksum_values