da_find_fft_factors.inc

1 subroutine da_find_fft_factors(n, n_ok, fft_factors)
2 
3    !---------------------------------------------------------------------------
4    ! Purpose: Calculates prime factors of input number.
5    !---------------------------------------------------------------------------
6 
7    implicit none
8 
9    integer, intent(in)  :: n
10    logical, intent(out) :: n_ok
11    integer, intent(out) :: fft_factors(:)
12 
13    integer :: i, k, l
14    integer :: nfax, nu, ifac
15    integer :: jfax(num_fft_factors)
16    integer :: lfax(7)
17 
18    data lfax /6,8,5,4,3,2,1/
19 
20    ! in da_control
21    !if (trace_use) call da_trace_entry("da_find_fft_factors")
22 
23    !---------------------------------------------------------------------------
24    ! [1.0] Find factors of vector size (8,6,5,4,3,2; only one 8 allowed):
25    !---------------------------------------------------------------------------
26 
27    n_ok = .false.
28    fft_factors(:) = 0
29 
30    ! look for sixes first, store factors in descending order
31    nu=n
32    ifac=6
33    k=0
34    l=1
35 
36 20 continue
37 
38    if (mod(nu,ifac).ne.0) goto 30
39    
40    ! 6 is a factor:
41    k=k+1
42    jfax(k)=ifac
43    if (ifac.ne.8) goto 25
44    if (k.eq.1) goto 25
45    jfax(1)=8
46    jfax(k)=6
47 
48 25 continue
49    nu=nu/ifac
50    if (nu.eq.1) goto 50
51    if (ifac.ne.8) goto 20
52 
53 30 continue
54    l=l+1
55    ifac=lfax(l)
56    if (ifac .gt. 1) goto 20
57 
58    ! illegal factors:
59    ! write (unit=message(1),fmt='(a,i4,a)') 'n = ', n, ' contains illegal factors.'
60    ! call da_warning(__file__,__line__,message(1:1))
61    
62    goto 9
63 
64    ! now reverse order of factors
65 50 continue
66    nfax=k
67    fft_factors(1)=nfax
68    do i=1,nfax
69       fft_factors(nfax+2-i)=jfax(i)
70    end do
71    
72    n_ok = .true.
73       
74 9  continue
75 
76    !if (trace_use) call da_trace_exit("da_find_fft_factors")
77 
78 end subroutine da_find_fft_factors
79 
80