V LEFT_DIGITS = [
   ‘   ## #’ = 0,
   ‘  ##  #’ = 1,
   ‘  #  ##’ = 2,
   ‘ #### #’ = 3,
   ‘ #   ##’ = 4,
   ‘ ##   #’ = 5,
   ‘ # ####’ = 6,
   ‘ ### ##’ = 7,
   ‘ ## ###’ = 8,
   ‘   # ##’ = 9
]

V RIGHT_DIGITS = Dict(LEFT_DIGITS.items(), (k, v) -> (k.replace(‘ ’, ‘s’).replace(‘#’, ‘ ’).replace(‘s’, ‘#’), v))

V END_SENTINEL = ‘# #’
V MID_SENTINEL = ‘ # # ’

F decodeUPC(input)
   F decode(candidate)
      V pos = 0
      V part = candidate[pos .+ :END_SENTINEL.len]
      I part == :END_SENTINEL
         pos += :END_SENTINEL.len
      E
         R (0B, [Int]())

      [Int] output
      L 6
         part = candidate[pos .+ 7]
         pos += 7

         I part C :LEFT_DIGITS
            output [+]= :LEFT_DIGITS[part]
         E
            R (0B, output)

      part = candidate[pos .+ :MID_SENTINEL.len]
      I part == :MID_SENTINEL
         pos += :MID_SENTINEL.len
      E
         R (0B, output)

      L 6
         part = candidate[pos .+ 7]
         pos += 7

         I part C :RIGHT_DIGITS
            output [+]= :RIGHT_DIGITS[part]
         E
            R (0B, output)

      part = candidate[pos .+ :END_SENTINEL.len]
      I part == :END_SENTINEL
         pos += :END_SENTINEL.len
      E
         R (0B, output)

      V sum = 0
      L(v) output
         I L.index % 2 == 0
            sum += 3 * v
         E
            sum += v
      R (sum % 10 == 0, output)

   V candidate = input.trim(‘ ’)
   V out = decode(candidate)
   I out[0]
      print(out[1])
   E
      out = decode(reversed(candidate))
      I out[0]
         print(out[1]‘ Upside down’)
      E
         I out[1].len == 12
            print(‘Invalid checksum’)
         E
            print(‘Invalid digit(s)’)

V barcodes = [
   ‘         # #   # ##  #  ## #   ## ### ## ### ## #### # # # ## ##  #   #  ##  ## ###  # ##  ## ### #  # #       ’,
   ‘        # # #   ##   ## # #### #   # ## #   ## #   ## # # # ###  # ###  ##  ## ###  # #  ### ###  # # #         ’,
   ‘         # #    # # #  ###  #   #    # #  #   #    # # # # ## #   ## #   ## #   ##   # # #### ### ## # #         ’,
   ‘       # # ##  ## ##  ##   #  #   #  # ###  # ##  ## # # #   ## ##  #  ### ## ## #   # #### ## #   # #        ’,
   ‘         # # ### ## #   ## ## ###  ##  # ##   #   # ## # # ### #  ## ##  #    # ### #  ## ##  #      # #          ’,
   ‘          # #  #   # ##  ##  #   #   #  # ##  ##  #   # # # # #### #  ##  # #### #### # #  ##  # #### # #         ’,
   ‘         # #  #  ##  ##  # #   ## ##   # ### ## ##   # # # #  #   #   #  #  ### # #    ###  # #  #   # #        ’,
   ‘        # # #    # ##  ##   #  # ##  ##  ### #   #  # # # ### ## ## ### ## ### ### ## #  ##  ### ## # #         ’,
   ‘         # # ### ##   ## # # #### #   ## # #### # #### # # #   #  # ###  #    # ###  # #    # ###  # # #       ’,
   ‘        # # # #### ##   # #### # #   ## ## ### #### # # # #  ### # ###  ###  # # ###  #    # #  ### # #         ’
]

L(barcode) barcodes
   decodeUPC(barcode)