Off topic 'C' question

* Mike Mascari <mascarm@mascari.com> [000729 18:40] 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?

Thanks for any info,

Think "binary search".

-Alfred

Alfred Perlstein wrote:

* Mike Mascari <mascarm@mascari.com> [000729 18:40] 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?

Thanks for any info,

Think "binary search".

-Alfred

Yeah. Start with 2^16, check if larger. If so, check 2^8, etc. In
Graphics Gems II, there's an algorithm by Ken Shoemake for
finding the lowest 1-bit set. You take the bit-wise AND of the
word and its negative -- that easy. I was hoping something
similar existed for finding the highest bit set. If so, I could
just shift the result left one. If not, if there's a way to flip
the bits in an unsigned integer without barrel shifting, then all
I would have to do is:

flip
take bit-wise AND with negative
flip
shift left 1

The binary search, is of course, better then brute force, but can
be worse than linear for low values. For example, a search for
2^5 would yield:

2^16
2^0
2^8
2^4
2^6
2^5

or 6 iterations instead of 5, plus the actual shifting of the
search value. I guess I was hoping for some "bit-magic" out
there.

Cheers,

Mike Mascari

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

--

#======================================================================#
# It's easier to get forgiveness for being wrong than for being right. #
# Let's break this rule - forgive me. #
#================================================== JanWieck@Yahoo.com #

On Sat, Jul 29, 2000 at 09:38:33PM -0400, 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?

You could use:

pow(x, ((int)(log(x)/log(2)) + 1));

--
Louis-David Mitterrand - ldm@apartia.org - http://www.apartia.org

"Poor girl looks as confused as a blind lesbian in a fish market." -
Simon R. Green

On Fri, Aug 11, 2000 at 11:18:23PM +0200, Louis-David Mitterrand wrote:

On Sat, Jul 29, 2000 at 09:38:33PM -0400, 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?

You could use:

pow(x, ((int)(log(x)/log(2)) + 1));

Sorry the correct way is:

#include <math.h>
#include <stdio.h>
#include <stdlib.h>

int main(int argc, char ** argv) {
int r = atoi(argv[1]);
printf("result is %g\n", pow(2, (int)((log(r)/log(2)) + 1)));
return 0;
}

This works for any power, simply replace 2 by the desired exponent.

--
Louis-David Mitterrand - ldm@apartia.org - http://www.apartia.org