Re: [PATCH 0/4] MR384: winsock: Support fallback to ICMP over SOCK_DGRAM on Linux

[Jinoh Kang (＠iamahuman)]

Jinoh Kang (@iamahuman) commented about dlls/ws2_32/tests/sock.c:
> +        sum += s;
> +        carry = s > sum;
> +        count -= 2;
> +    }
> +    sum += carry; /* This won't produce another carry */
> +    sum = (sum & 0xffff) + (sum >> 16);
> +
> +    if (count) sum += *data; /* LE-only */
> +
> +    sum = (sum & 0xffff) + (sum >> 16);
> +    /* fold in any carry */
> +    sum = (sum & 0xffff) + (sum >> 16);
> +
> +    check = ~sum;
> +    return check;
> +}
> It's much simpler to implement 1's complement checksum without the use of an explicit `carry` variable.  How about the following:

```suggestion:-25+0
static UINT16 chksum(BYTE *data, unsigned int count)
{
    UINT16 *ptr = (UINT16 *)data, *end = ptr + count / 2;
    unsigned int sum = 0;

    while (ptr != end)
    {
        sum += *ptr++;
        sum = (sum & 0xffff) + (sum >> 16);
    }

    if (count % 2)
    {
        sum += *(BYTE *)ptr; /* LE-only */
        sum = (sum & 0xffff) + (sum >> 16);
    }

    return ~sum;
}
```

Note that `sum` is in range `[0, 0xfffe]` at the start of each iteration.  After addition, the range extends to `[0, 0x1fffd]`; however, since `0xfffd + 0x1 = 0xfffe`, the range folds back to `[0, 0xfffe]`.

Ditto for tests in ntdll.

-- 
https://gitlab.winehq.org/wine/wine/-/merge_requests/384#note_3810