Python script to evaluate CRC16 CCITT (0xffff)

CRC_CCITT.py

# Computation of CRC16 CCITT (0xFFFC)
# The input parameter for crc16Eval() is a hexString obtained from a bytearray bytes values
# The function returns a string with the hexadecimal value, an int with the low byte value 
# and an int with the high byte value of the crc.
#
# hexArray = bytearray(b'\xe8\x8a\x04\r\x00W\x8b')
# hexString = = "".join(map(chr, hexArray))
# (crcHexStr, crcHighByte, crcLowByte) = crc16Eval(hexString)

class CRC_CCITT:
    def __init__(self): 
        self.tab = 256*[[]] 
        for i in range(256):
            crc = 0
            c = i << 8 
            
            for j in range(8): 
                if (crc ^ c) & 0x8000: 
                    crc = ( crc << 1) ^ 0x1021 
                else:
                    crc = crc << 1 

                c = c << 1 
                crc = crc & 0xffff 

            self.tab[i]=crc 
    
    def update_crc(self, crc, c): 
        short_c=0x00ff & (c % 256) 
        tmp = ((crc >> 8) ^ short_c) & 0xffff 
        crc = (((crc << 8) ^ self.tab[tmp])) & 0xffff 
        return crc 


def crc16Eval(byteStr):
    crc_ccitt = CRC_CCITT()
    crcVal=0xffff

    for c in byteStr: 
        crcVal=crc_ccitt.update_crc(crcVal, ord(c))
        # crcVal=crc_ccitt.update_crc(crcVal, 0)
        
    crcHigh = crcVal >> 8
    crcLow = crcVal & 0x00ff
    return(hex(crcVal), crcHigh, crcLow)