[SciPy-dev] fft code for testing
Pearu Peterson
pearu at scipy.org
Sat May 31 12:49:25 CDT 2003
On Sat, 31 May 2003, Chuck Harris wrote:
> > Btw, does anybody know an efficient way how to test if
> > an integer is a power-of-two in C? Currently djbfft wrapper
> > uses
> > switch (n) {
> > case 2:;case 4:;... case 8192: ..
> > }
> > but there must be a better way.
>
> Fooling around a bit, I think the construction
>
> int foo(int n)
> {
> int ok = 1;
>
> if ( n == 1 ||
> n == 2 ||
> n == 4 ||
> n == 8 );
> else
> ok = 0;
> return ok;
> }
>
> produces the best looking assembly here.
There are 31 C int's that are power of two (assuming 32 bit
machines). Though, may be only the first 24 or so are used in real
applications; for instance, the size of double complex array of length
2**24 is 256MB.
So, when n is not a power-of-two, then at least 31 C int comparisons are
required. I wonder if this is the lowest bound of #operations or
can we do better?
Pearu
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