python CRC32-mpeg2校验

之前在做STM32的IAP程序，为了提高可靠性，采用了CRC32校验。STM32有硬件CRC，采用的校验方式为CRC32-mpeg2。上位机是用python写的，在计算CRC校验值时，最初查到了binascii.crc32，但是发现它并不是CRC32-mpeg2。

google了一下，找到了某位前辈写的代码。考虑到STM32采用的是小端模式，对此进行了处理。但是不能进行反复计算，所以又对程序进行了改进。

def generate_crc32_table(_poly):

    global custom_crc_table

    for i in range(256):
        c = i << 24

        for j in range(8):
            c = (c << 1) ^ _poly if (c & 0x80000000) else c << 1

        custom_crc_table[i] = c & 0xffffffff


def crc32_stm(bytes_arr, *args):

    length = len(bytes_arr)


    k = 0
    if(args):
        crc = args[0]
    else:
        crc = 0xffffffff

    while length >= 4:

        v = ((bytes_arr[k] << 24) & 0xFF000000) | ((bytes_arr[k+1] << 16) & 0xFF0000) | \
        ((bytes_arr[k+2] << 8) & 0xFF00) | (bytes_arr[k+3] & 0xFF)

        crc = ((crc << 8) & 0xffffffff) ^ custom_crc_table[0xFF & ((crc >> 24) ^ v)]
        crc = ((crc << 8) & 0xffffffff) ^ custom_crc_table[0xFF & ((crc >> 24) ^ (v >> 8))]
        crc = ((crc << 8) & 0xffffffff) ^ custom_crc_table[0xFF & ((crc >> 24) ^ (v >> 16))]
        crc = ((crc << 8) & 0xffffffff) ^ custom_crc_table[0xFF & ((crc >> 24) ^ (v >> 24))]

        k += 4
        length -= 4

    if length > 0:
        v = 0

        for i in range(length):
            v |= (bytes_arr[k+i] << 24-i*8)

        if length == 1:
            v &= 0xFF000000

        elif length == 2:
            v &= 0xFFFF0000

        elif length == 3:
            v &= 0xFFFFFF00

        crc = (( crc << 8 ) & 0xffffffff) ^ custom_crc_table[0xFF & ( (crc >> 24) ^ (v ) )];
        crc = (( crc << 8 ) & 0xffffffff) ^ custom_crc_table[0xFF & ( (crc >> 24) ^ (v >> 8) )];
        crc = (( crc << 8 ) & 0xffffffff) ^ custom_crc_table[0xFF & ( (crc >> 24) ^ (v >> 16) )];
        crc = (( crc << 8 ) & 0xffffffff) ^ custom_crc_table[0xFF & ( (crc >> 24) ^ (v >> 24) )];

    return crc
if __name__ == "__main__":
    poly = 0x04C11DB7
    buf = [0x31,0x32,0x33,0x34,0x35,0x36,0x37,0x38,0x39,0x30,0x31,0x32]
    buf_s = [0x00,0x00,0x00,0x00]

    generate_crc32_table(poly)
    crc_stm = crc32_stm(bytearray(buf),0x19a38afe)

    print ("%x"%crc_stm)