create an efficient(-ish) CRC16 computation from an arbitrary taps polynomial

crc16.py

'''
See 16 bit values from here:
http://en.wikipedia.org/wiki/Cyclic_redundancy_check#Commonly_used_and_standardized_CRCs
for some commonly used values for taps.
Since this implementation uses right shifting, take the values from the 'reversed' column.
Some examples: 0xA001, 0x8408, 0xEDD1, 0xA6BC, 0x91A0, 0xD405
These are all maximal taps; that is, the permutation over integers mod 2**16 that they define
has one giant cycle rather than multiple small cycles.
'''
class CRC16(object):
    def __init__(self, taps):
        table = []
        for i in range(0xFF):
            val = i << 8
            for j in range(16):
                val = (val >> 1) ^ ( -(val&1) & taps)
            table.append(val)
        self.table = tuple(table)

    def crc16(self, data):
        table = self.table
        crc = 0
        for byte in data:
            crc = (crc>>8) ^ table[(crc ^ ord(byte)) & 0xFF]
        return crc

    def add_crc(self, data):
        crc = self.crc16(data)
        return data + chr(crc & 0xFF) + chr(crc>>8)

    def check_crc(self, data):
        return self.crc16(data) == 0