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Re: portable way to get highest bit set?

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Ian C <deathstation9000@gmail.com> writes:

> 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.

Wouldn't you first write a test function to be sure the
value returned is correct?  ;)

I found it harder to write a correct test function than
to write a function that just returns a correct value.
Interesting, eh?

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portable way to get highest bit set? candycanearter07 <no@thanks.net> - 2023-10-11 01:56 -0500
  Re: portable way to get highest bit set? Anton Shepelev <anton.txt@g{oogle}mail.com> - 2023-10-11 10:27 +0300
    Re: portable way to get highest bit set? David Brown <david.brown@hesbynett.no> - 2023-10-11 11:31 +0200
      Re: portable way to get highest bit set? Anton Shepelev <anton.txt@g{oogle}mail.com> - 2023-10-11 12:33 +0300
        Re: portable way to get highest bit set? David Brown <david.brown@hesbynett.no> - 2023-10-11 13:58 +0200
          Re: portable way to get highest bit set? Anton Shepelev <anton.txt@g{oogle}mail.com> - 2023-10-11 15:17 +0300
            Re: portable way to get highest bit set? David Brown <david.brown@hesbynett.no> - 2023-10-11 16:22 +0200
              Re: portable way to get highest bit set? Anton Shepelev <anton.txt@g{oogle}mail.com> - 2023-10-11 18:08 +0300
              Re: portable way to get highest bit set? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2023-10-11 17:20 +0100
                Re: portable way to get highest bit set? David Brown <david.brown@hesbynett.no> - 2023-10-11 18:28 +0200
    Re: portable way to get highest bit set? candycanearter07 <no@thanks.net> - 2023-10-11 11:19 -0500
      Re: portable way to get highest bit set? David Brown <david.brown@hesbynett.no> - 2023-10-11 18:45 +0200
      Re: portable way to get highest bit set? Ian C <deathstation9000@gmail.com> - 2023-10-11 18:03 +0100
        Re: portable way to get highest bit set? Tim Rentsch <tr.17687@z991.linuxsc.com> - 2023-10-11 10:17 -0700
      Re: portable way to get highest bit set? Tim Rentsch <tr.17687@z991.linuxsc.com> - 2023-10-11 10:34 -0700
      Re: portable way to get highest bit set? Richard Harnden <richard.nospam@gmail.invalid> - 2023-10-11 19:02 +0100
  Re: portable way to get highest bit set? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2023-10-11 13:56 +0100
  Re: portable way to get highest bit set? Lew Pitcher <lew.pitcher@digitalfreehold.ca> - 2023-10-11 14:54 +0000
  Re: portable way to get highest bit set? Kaz Kylheku <864-117-4973@kylheku.com> - 2023-10-11 17:23 +0000

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