Re: portable way to get highest bit set?

candycanearter07 <[email protected]> Wed, 11 Oct 2023 19:26:21 -0500

Newsgroups	comp.lang.c,alt.comp.lang.c
Organization	A noiseless patient Spider
Message-ID	<[email protected]>

On 10/11/23 12:03, Ian C wrote:
> On 11/10/2023 17:19, candycanearter07 wrote:
>> On 10/11/23 02:27, Anton Shepelev wrote:
>>> candycanearter07:
>>>
>>>> What is the best/most portable way to get the highest bit
>>>> set?
>>>>
>>>> ie. 011010001
>>>> to  010000000
>>>
>>> What is your best attempt, even if unsuccessful?
>>>
>>
>> int out;
>> for(out = 0x100000000; out; out >> 1)
>>      if(out & input) break;
>>
>> I'm trying to find something size independent, though.
>> Or at least able to be swapped out with a #define
> 
> If you want the highest bit, 0x800000000 may be a better start :-)
> 
> If it was me I'd do it in reverse.  Assuming it's unsigned, shift your 
> input to the right until it hits zero, counting the number of shifts 
> which will be your bit number.  Assuming my head is currently working.
> 
> 
> Ian C

Actually yeah, I think that could work.
-- 
user <candycane> is generated from /dev/urandom