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NAME
     rfftopt - compute the length of the closest fast FFT

     INTEGER FUNCTION RFFTOPT(N)

     INTEGER N



     #include <sunperf.h>

     int rfftopt(int len);

PURPOSE
     RFFTOPT computes the length of the closest fast  FFT.   Fast
     Fourier  transform  algorithms, including those used in Per-
     formance Library, work best with  vector  lengths  that  are
     products  of  small  primes.   For example, an FFT of length
     32=2**5 will run faster than  an  FFT  of  prime  length  31
     because  32  is a product of small primes and 31 is not.  If
     your application is such that you can taper or zero pad your
     vector  to  a  larger length then this function may help you
     select a better length and run your FFT faster.

     RFFTOPT will return an integer no  smaller  than  the  input
     argument N that is the closest number that is the product of
     small primes.  RFFTOPT will return 16 for an input  of  N=16
     and return 18=2*3*3 for an input of N=17.

     Note that the length computed here is not guaranteed  to  be
     optimal,  only  to  be a product of small primes.  Also, the
     value returned may change  as  the  underlying  FFTs  become
     capable  of handling larger primes.  For example, passing in
     N=49 today will return 50=2*5*5 rather than  49=7*7  because
     the  FFTs  in  Performance  Library do not have fast radix 7
     code.  In the future, radix 7 code may  be  added  and  then
     N=49 will return 49.

ARGUMENTS
     N         (input) INTEGER
               Length of the vector to be transformed.  N  >=  0.
               RFFTOPT(0) .EQ. 0.

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