text: # vat.py - functions for handling United Kingdom VAT numbers
  #
  # Copyright (C) 2012-2015 Arthur de Jong
  #
  # This library is free software; you can redistribute it and/or
  # modify it under the terms of the GNU Lesser General Public
  # License as published by the Free Software Foundation; either
  # version 2.1 of the License, or (at your option) any later version.
  #
  # This library is distributed in the hope that it will be useful,
  # but WITHOUT ANY WARRANTY; without even the implied warranty of
  # MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
  # Lesser General Public License for more details.
  #
  # You should have received a copy of the GNU Lesser General Public
  # License along with this library; if not, write to the Free Software
  # Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA
  # 02110-1301 USA
  
  """VAT (United Kingdom (and Isle of Man) VAT registration number).
  
  The VAT number can either be a 9-digit standard number, a 12-digit standard
  number followed by a 3-digit branch identifier, a 5-digit number for
  government departments (first two digits are GD) or a 5-digit number for
  health authorities (first two digits are HA). The 9-digit variants use a
  weighted checksum.
  
  >>> validate('GB 980 7806 84')
  '980780684'
  >>> validate('802311781')  # invalid check digit
  Traceback (most recent call last):
      ...
  InvalidChecksum: ...
  >>> format('980780684')
  '980 7806 84'
  """
  
  from stdnum.exceptions import *
  from stdnum.util import clean, isdigits
  
  
  def compact(number):
      """Convert the number to the minimal representation. This strips the
      number of any valid separators and removes surrounding whitespace."""
      number = clean(number, ' -.').upper().strip()
      if number.startswith('GB'):
          number = number[2:]
      return number
  
  
  def checksum(number):
      """Calculate the checksum. The checksum is only used for the 9 digits
      of the number and the result can either be 0 or 42."""
      weights = (8, 7, 6, 5, 4, 3, 2, 10, 1)
      return sum(w * int(n) for w, n in zip(weights, number)) % 97
  
  
  def validate(number):
      """Check if the number is a valid VAT number. This checks the length,
      formatting and check digit."""
      number = compact(number)
      if len(number) == 5:
          if not isdigits(number[2:]):
              raise InvalidFormat()
          if number.startswith('GD') and int(number[2:]) < 500:
              # government department
              pass
          elif number.startswith('HA') and int(number[2:]) >= 500:
              # health authority
              pass
          else:
              raise InvalidComponent()
      elif len(number) == 11 and number[0:6] in ('GD8888', 'HA8888'):
          if not isdigits(number[6:]):
              raise InvalidFormat()
          if number.startswith('GD') and int(number[6:9]) < 500:
              # government department
              pass
          elif number.startswith('HA') and int(number[6:9]) >= 500:
              # health authority
              pass
          else:
              raise InvalidComponent()
          if int(number[6:9]) % 97 != int(number[9:11]):
              raise InvalidChecksum()
      elif len(number) in (9, 12):
          if not isdigits(number):
              raise InvalidFormat()
          # standard number: nnn nnnn nn
          # branch trader: nnn nnnn nn nnn (ignore the last thee digits)
          # restarting: 100 nnnn nn
          if int(number[:3]) >= 100:
              if checksum(number[:9]) not in (0, 42, 55):
                  raise InvalidChecksum()
          else:
              if checksum(number[:9]) != 0:
                  raise InvalidChecksum()
      else:
          raise InvalidLength()
      return number
  
  
  def is_valid(number):
      """Check if the number is a valid VAT number."""
      try:
          return bool(validate(number))
      except ValidationError:
          return False
  
  
  def format(number):
      """Reformat the number to the standard presentation format."""
      number = compact(number)
      if len(number) == 5:
          # government department or health authority
          return number
      if len(number) == 12:
          # includes branch number
          return number[:3] + ' ' + number[3:7] + ' ' + number[7:9] + ' ' + number[9:]
      # standard number: nnn nnnn nn
      return number[:3] + ' ' + number[3:7] + ' ' + number[7:]
  