计算校验和的问题：将int转换为signed int32 [英] Problem in calculating checksum : casting int to signed int32

问题描述

我需要将以下c代码（计算文件的校验和）转换为python。我写了，对应的代码在python，但结果不匹配c版本。问题是，当溢出发生时，python自动地将int调长，这会导致错误的校验和。

I need to convert the following c code (to calculate checksum for a file) to python. I had written, the corresponding code in python but the result didn't match the c version. The problem was that python autmatically promotes int to long whenever overflow occurs and this results in wrong checksums.

任何想法如何克服这个问题？或者有一个python函数将long转换为signed int32？

Any idea how to overcome this problem ? or is there a python function that converts long to signed int32 ?

感谢

int calcChecksum(const guchar *data, gsize len)
{ 

    const guchar *p = data;
    int checksum = 0, g, i = len;

    while(i--) {
            checksum = (checksum << 4) + *p++;

            if((g = (checksum & 0xf0000000)) != 0)
                    checksum ^= g >> 23;

            checksum &= ~g;
    }
    return checksum;
}

解决方案：

感谢所有的帮助。这里是为我工作的函数 -

Thanks for all the help. Here's the function that worked for me -

 def int32(x):
    x = 0xffffffff & x
    if x > 0x7fffffff :
        return - ( ~(x - 1) & 0xffffffff )
    else : return x 

推荐答案

使用 numpy.int32 或 numpy.uint32 如果你需要限制范围。或者通过 1<< 32 在可能溢出的操作后。

Use numpy.int32 or numpy.uint32 if you need to restrict the range. Or mod it by 1 << 32 after operations that could "overflow".

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