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;
unsignedint 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
whenwe have 256 bit integers as standard (in approx. 5-6 years :-) it'll finish with 7 iterations.
Jan
--
#======================================================================#
# It's easier to get forgiveness for being wrong than for being right. #
# Let's break this rule - forgive me. #
#================================================== JanWieck@Yahoo.com #
