Checksum for diablo 2 files .d2s in python (todo insert the checksum into the file)

d2s_checksum.py

import numpy as np

"""
code to replicate:

static int d2charsave_checksum(unsigned char const *data, unsigned int len, unsigned int offset)
{
    int checksum;
    unsigned int i;
    unsigned int ch;

    if (!data) return 0;
    checksum=0;
    for (i=0; i<len; i++) {
        ch=data[i];
        if (i>=offset && i<offset+sizeof(int)) ch=0;
        ch+=(checksum<0);
        checksum=2*checksum+ch;
    }
    return checksum;
}

"""


def d2charsave_checksum(filename, offset=12):
    def checksum(data, start_value=0):
        acc = np.int32(start_value)

        for value in data:
            acc = np.int32((acc << 1) + value + (acc<0))

        return np.int32(acc)


    data = np.fromfile(filename, dtype=np.uint8)

    checksum_byte_length = np.dtype("int32").itemsize

    pre_data, post_data = data[:offset], data[offset + checksum_byte_length:]

    pre_checksum = checksum(
        pre_data,
        start_value=0
    )

    on_checksum = checksum(
        np.zeros(checksum_byte_length, dtype=np.uint8),
        start_value=pre_checksum
    )

    post_checksum = checksum(
        post_data,
        start_value=on_checksum
    )

    return post_checksum


def main():
    checksum = d2charsave_checksum("items.d2s")
    print((checksum, 343162958))