对 python 字节类型执行校验和计算

Question

我第一次需要处理原始数据（具有不同的字节顺序、2 的补码……），因此终于弄清楚了如何使用字节类型。我需要实现以下校验和算法。我了解 C 代码，但想知道如何在 Python3 中优雅地执行此操作... 我确定我可以想出一些可行的方法，但效率低下或不可靠

The checksum algorithm used is the 8-bit Fletcher algorithm. This algorithm works as follows:

Buffer[N] is an array of bytes that contains the data over which the checksum is to be calculated.

The two CK_A and CK_A values are 8-bit unsigned integers, only! If implementing with larger- sized integer values, make sure to mask both CK_A and CK_B with the value 0xff after both operations in the loop.

After the loop, the two U1 values contain the checksum, transmitted after the message payload, which concludes the frame.
CK_A = 0, CK_B = 0 For (I = 0; I < N; I++)
    {
    CK_A = CK_A + Buffer[I]
    CK_B = CK_B + CK_A
    } ```

我的数据结构如下：

source = b'\xb5b\x01<@\x00\x01\x00\x00\x00hUX\x17\xdd\xff\xff\xff^\xff\xff\xff\xff\xff\xff\xff\xa6\x00\x00\x00F\xee\x88\x01\x00\x00\x00\x00\xa5\xf5\xd1\x05d\x00\x00\x00d\x00\x00\x00j\x00\x00\x00d\x00\x00\x00\xcb\x86\x00\x00\x00\x00\x00\x007\x01\x00\x00\xcd\xa2'

我想出了几个关于如何做到这一点的想法，但有问题。以下是我现在的位置，我已经添加了关于我认为它会如何工作（但没有）的评论。

for b in source[5:-2]:

    # The following results in "TypeError("can't concat int to bytes")"
    # So I take one element of a byte, then I would expect to get a single byte.
    # However, I get an int.
    # Should I convert the left part of the operation to an int first?
    # I suppose I could get this done in a couple of steps but it seems this can't be the "correct" way...

    CK_A[-1:] += b

    # I hoped the following would work as a bitmask, 
    # (by keeping only the last byte) thus "emulating" an uint8_t
    # Might not be the correct/best assumption...

    CK_A = CK_A[-1:] 
    CK_B[-1:] += CK_A
    CK_B = CK_B[-1:]
ret = CK_A + CK_B

显然，我没有完全理解这种字节类型 works/should 是如何使用的。

Answer 1

看来我把事情搞得太难了...

    CK_A = 0
    CK_B = 0

    for b in source:
        CK_A += b
        CK_B += CK_A

    CK_A %= 0x100
    CK_B %= 0x100
        
    ret = bytes()
    ret = int.to_bytes(CK_A,1, 'big') + int.to_bytes(CK_B,1,'big')

%=0x100 用作位掩码，仅保留 8 LSB...

对 python 字节类型执行校验和计算

Performing checksum calculation on python bytes type

byte

python-3.8