upc.rb

# based on http://code.activestate.com/recipes/528911-barcodes-convert-upc-e-to-upc-a/
class Upc
  def self.convert_upc_e_to_upc_a(upce=nil)
    return upce if upce.nil? or upce.length < 6 or upce.length > 8 # not a UPC-E

    upce = case upce.length
    when 6
      upce
    when 7
      upce[0,6]
    when 8
      upce[1,6]
    else
      return nil
    end

    val = "0" + case upce[5,1] # 6th digit
    when "0", "1", "2"
      upce[0,2] + upce[5,1] + "0000" + upce[2,3] ## 12600 00345
    when "3"
      upce[0,3] + "00000" + upce[3,2]            ## 12300 00045
    when "4"
      upce[0,4] + "00000" + upce[4,1]            ## 12340 00005
    else
      upce[0,5] + "0000" + upce[5,1]             ## 12345 00006
    end
    val + calculate_check_digit(val).to_s
  end
  
  private

    def self.calculate_check_digit(v)
      chk = 0
      v.split(//).each_with_index do |s, i|
        chk += s.to_i * (i%2==0 ? 1 : 3)
      end
      (10 - (chk % 10)) % 10
    end
end

in case you ever have to do this too. Converts from things like "01210806" to "012000001086"