Re: Off topic 'C' question

Mike Mascari wrote:
> I have a quick question. What is the quickest way to determine
> the next highest power of two which is greater than a given
> integer in 'C'. For example, given the number 7, I would like to
> return 8. Given the number 13, I would like to return 16, etc. Is
> there a gem to do this without shifting a bit value from 1 left
> up to a maximum of 32 (or 64) iterations?

Binary search.

I assumed you really mean greater than, so that
next_power2(4096) is 8192.

For 32 bit values, the function

unsigned int next_power2_32 (unsigned int value)
{
unsigned int comp = 1 << 16;
unsigned int off = 8;

if (value == 0)
return 1;

while (off > 0 && comp != value)
{
if (comp > value)
comp >>= off;
else
comp <<= off;

off >>= 1;
}

if (comp <= value)
comp <<= 1;
return comp;
}

is guaranteed to have at maximum 4 loop iterations for any
value you want. Should be polished up a little for values >=
(1 << 31), but I leave that to you. Obviuosly, looking for 64
bit numbers, the loop max would be 5, and when we have 256
bit integers as standard (in approx. 5-6 years :-) it'll
finish with 7 iterations.

Jan

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