mrben.co.uk

def calculate_grid_check_digit(grid):
    """
    Assumes `grid` is 17 characters long
    and missing the check digit.
    """
    def _mod_36(i):
        """
        A modified modulus function is used
        by the algorithm.
        """
        m = i % 36
        return 36 if m == 0 else m

    grid = grid.upper()
    lookup_values = ['0','1','2','3','4','5','6','7','8','9',
                     'A','B','C','D','E','F','G','H','I','J',
                     'K','L','M','N','O','P','Q','R','S','T',
                     'U','V','W','X','Y','Z']
    p_list = [36]
    s_list = []
    
    for j, c in enumerate(grid):
        c_val = lookup_values.index(c)
        s_list.append(p_list[j]%37+int(c_val))
        p_list.append(_mod_36(s_list[j]) * 2)
    
    i = 0
    while 1:
        if _mod_36(p_list[-1]%37 + i) == 1:
            break;
        i += 1
    return lookup_values[i]

import math

def get_check_digit(b):
    """
    Returns the check digit for a barcode.
    
    It is assumed that the check digit is missing from the input.
    """
    sum = 0
    for x, c in enumerate(b[::-1]):
        if (x + 1) % 2:
            sum += int(c) * 3
        else:
            sum += int(c)
    return str(int(math.ceil(sum / 10.0) * 10) - sum)

def is_valid_barcode(b):
    """
    Checks the last digit of a barcode matches the calculated 
    check digit.
    """
    return get_check_digit(b[0:-1]) == b[-1]

test_data = (
    # Valid barcodes
    ('9771473968012', True),
    ('0123456789012', True),
    ('1234567890128', True),
    # Invalid barcodes
    ('9771473968011', False),
    ('0123456789019', False),
    ('1234567890127', False),
)

for t in test_data:
    print t[1] == is_valid_barcode(t[0])