class CRC(object):

    '''Implementation of direct table algorithm

    Reference: http://www.ross.net/crc/download/crc_v3.txt
    '''

    def __init__(self, width=4, poly=0x04C11DB7, reverse=False, initial=0):
        self.bits = width * 8
        self.initial = initial
        self.table = []
        topbit = self.bitmask(self.bits - 1)
        for index in xrange(256):
            if reverse:
                index = self.reflect(index, 8)
            r = index << (self.bits - 8)
            for i in xrange(8):
                if r & topbit:
                    r = (r << 1) ^ poly
                else:
                    r <<= 1
            if reverse:
                r = self.reflect(r, self.bits)
            self.table.append(r & ((((1 << (self.bits - 1)) - 1) << 1) | 1))

    def checksum(self, *args):
        clear = 2 ** self.bits - 1
        r = self.initial
        for data in args:
            for byte in data:
                r = ((r << 8) ^ self.table[(r >> 24) ^ ord(byte)]) & clear
        return r

    @classmethod
    def reflect(cls, v, b):
        t = v
        for i in xrange(b):
            if t & 1:
                v |= cls.bitmask((b - 1) - i)
            else:
                v &= ~cls.bitmask((b - 1) - i)
            t >>= 1
        return v

    @staticmethod
    def bitmask(x):
        return 1 << x