# vat.py - functions for handling United Kingdom VAT numbers
#
# Copyright (C) 2012 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.

>>> compact('GB 980 7806 84')
'980780684'
>>> is_valid('980780684')
True
>>> is_valid('802311781')  # invalid check digit
False
>>> format('980780684')
'980 7806 84'
"""

from stdnum.util import clean


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(weights[i] * int(n) for i, n in enumerate(number)) % 97


def is_valid(number):
    """Checks to see if the number provided is a valid VAT number. This
    checks the length, formatting and check digit."""
    try:
        number = compact(number)
    except:
        return False
    if len(number) == 5 and number.startswith('GD') and number[2:].isdigit():
        # government department
        return int(number[2:]) < 500
    if len(number) == 5 and number.startswith('HA') and number[2:].isdigit():
        # health authority
        return int(number[2:]) >= 500
    if len(number) == 12 and number.isdigit():
        # branch trader: nnn nnnn nn nnn (ignore the last thee digits)
        return checksum(number[:-3]) in (0, 42)
    if len(number) == 9 and number.isdigit():
        # standard number: nnn nnnn nn
        return checksum(number) in (0, 42)
    return False


def format(number):
    """Reformat the passed number to the standard 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:]