CRC_basic.py

def crc(msg, div, code="000"):
    msg += code

    msg = list(msg)
    div = list(div)

    for i in range(len(msg) - len(code)):
        if msg[i] == "1":
            for j in range(len(div)):
                msg[i+j] = f"{int(msg[i+j]) ^ int(div[j])}"
                # msg[i+j] = f"{(int(msg[i+j])+int(div[j])) % 2}"
                    


    return ''.join(msg[-len(code):]) 
    # suppose msg = ["1", "2", "3", "4"]
    # and length(div) = 2
    # o/p of ''.join(msg[-len(code):]) -> "34"


div = "1011"
msg = "10111011101011"

remainder0 = crc(msg, div) 
print(remainder0)

# introducing error on any bit of msg
# to inverse the bit do xor

msg = list(msg) # convert string to list for chaning bit
msg[6] = str(int(msg[6]) ^ 1) # int(msg[6], 2) will convert string at index 6 in binary form
msg = "".join(msg) # convert list to string

remainder1 = crc(msg, div, remainder0)
print(remainder1)

# at sender side: remainder0 there will be some remainder
# add the msg and the remainder 
# at reciver side if no noise occurs in communication remainder1 should be 0