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64-bit power of 2 calculation

Posted on 2007-11-25
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Last Modified: 2012-08-13
I found online a nice algorithm for calculating the next highest power of 2.  However, the algorithm takes a 32-bit integer.  It seems to work just as well on 64-bit integers, but I'm not sure if it's guaranteed to always work.  Will the following algorithm always return a 64-bit integer that is the next greatest power of 2?

uint64_t nextp2(uint64_t n)
{
      --n;
       n |= n >> 16;
       n |= n >> 8;
       n |= n >> 4;
       n |= n >> 2;
       n |= n >> 1;
       ++n;
       return n;
}
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Question by:chsalvia
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6 Comments
 
LVL 55

Expert Comment

by:Jaime Olivares
ID: 20347861
the order is important, it should be:

n--;
n |= n >> 1;
n |= n >> 2;
n |= n >> 4;
n |= n >> 8;
n |= n >> 16;
n |= n >> 32;
n++;
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Author Comment

by:chsalvia
ID: 20347886
That doesn't seem to work.

For example, that yields 0 for the value 100
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LVL 55

Accepted Solution

by:
Jaime Olivares earned 125 total points
ID: 20347942
Using my calculator:

n--;    // gives 99
n |= n >> 1;   // gives 115
n |= n >> 2;   // gives 127
n |= n >> 4;   // gives 127  
n |= n >> 8;   // gives 127
n |= n >> 16;   // gives 127
n |= n >> 32;   // gives 127
n++;  // gives 128

so, it works perfectly with 100


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Author Comment

by:chsalvia
ID: 20347974
You're right.  I was screwing up the return value.  Thanks.
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LVL 84

Expert Comment

by:ozo
ID: 20348480
The order is not important
       n |= n >> 16;
       n |= n >> 8;
       n |= n >> 4;
       n |= n >> 2;
       n |= n >> 1;
       n |= n >> 32;
works just as well
but if the number is already a power of two, you get the same number back, not the "next" power of two
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LVL 53

Expert Comment

by:Infinity08
ID: 20349257
>> The order is not important

Indeed. You just needed to add :

       n |= n >> 32;

to make it 64bit integer compatible.
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