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From: Ben Bacarisse <ben.usenet@bsb.me.uk>
Newsgroups: comp.lang.c
Subject: Re: on an analogy for verifying whether another digit fits (into an unsigned type)
Date: Sat, 29 Jan 2022 02:36:38 +0000
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Meredith Montgomery <mmontgomery@levado.to> writes:

> Ben Bacarisse <ben.usenet@bsb.me.uk> writes:
>
>> Meredith Montgomery <mmontgomery@levado.to> writes:
>>
>>> Bart <bc@freeuk.com> writes:
>>>
>>>> On 16/01/2022 02:27, Meredith Montgomery wrote:
>>>>> uint64_t array_to_uint64(char *s, uint64_t *u)
>>>>> {
>>>>>    uint64_t pos;
>>>>>    uint64_t r;
>>>>>    uint64_t c;
>>>>>    pos = 0; r = 0;
>>>>>    for ( ;; ) {
>>>>>      c = (uint64_t) (unsigned char) (s[pos] - '0');
>>>>>      if (c < 10) {
>>>>>        if( ((UINT64_MAX - c) / 10) >= r)
>>>>
>>>> Dividing by 10 each time is unnecessary (even if usually optimised to
>>>> shifts and multiplies).
>>>>
>>>> You only need to make this check after you've already processed 19
>>>> characters, as it could overflow on the 20th, but not before.
>>>>
>>>> I think when pos >= 18.
>>>
>>> I suppose you're right, but imagine putting such check there.  It would
>>> take even more paragraphs to explain it to myself some time later when
>>> I'm trying to figure out what I wrote a while back.
>>
>> You could just check that the array contains a string of digits
>> lexicographically less than or equal to 18446744073709551615.  If there
>> are fewer digits than this, or, at every position, the digit you have is
>> no greater than the corresponding digit of that number, you are ok.
>
> That seems correct, but if I change the size of my register, then I must
> replace the number too. :-)

Sure.  Just as you'd have to replace UINT64_MAX and so on.  Remember you
don't have to write 18446744073709551615.  You could use snprintf to put
UINT64_MAX into a static buffer.

-- 
Ben.