subroutine cfftb1(n,c,Ch,Wa,Ifac)
use fftpack_kind
implicit none
real(rk) :: c , Ch , Wa
integer :: i , idl1 , ido , idot , Ifac , ip , iw , ix2 , ix3 , ix4, &
k1 , l1 , l2 , n , n2 , na , nac , nf
dimension Ch(*) , c(*) , Wa(*) , Ifac(*)
nf = Ifac(2)
na = 0
l1 = 1
iw = 1
do k1 = 1 , nf
ip = Ifac(k1+2)
l2 = ip*l1
ido = n/l2
idot = ido + ido
idl1 = idot*l1
if ( ip==4 ) then
ix2 = iw + idot
ix3 = ix2 + idot
if ( na/=0 ) then
call passb4(idot,l1,Ch,c,Wa(iw),Wa(ix2),Wa(ix3))
else
call passb4(idot,l1,c,Ch,Wa(iw),Wa(ix2),Wa(ix3))
endif
na = 1 - na
elseif ( ip==2 ) then
if ( na/=0 ) then
call passb2(idot,l1,Ch,c,Wa(iw))
else
call passb2(idot,l1,c,Ch,Wa(iw))
endif
na = 1 - na
elseif ( ip==3 ) then
ix2 = iw + idot
if ( na/=0 ) then
call passb3(idot,l1,Ch,c,Wa(iw),Wa(ix2))
else
call passb3(idot,l1,c,Ch,Wa(iw),Wa(ix2))
endif
na = 1 - na
elseif ( ip/=5 ) then
if ( na/=0 ) then
call passb(nac,idot,ip,l1,idl1,Ch,Ch,Ch,c,c,Wa(iw))
else
call passb(nac,idot,ip,l1,idl1,c,c,c,Ch,Ch,Wa(iw))
endif
if ( nac/=0 ) na = 1 - na
else
ix2 = iw + idot
ix3 = ix2 + idot
ix4 = ix3 + idot
if ( na/=0 ) then
call passb5(idot,l1,Ch,c,Wa(iw),Wa(ix2),Wa(ix3),Wa(ix4))
else
call passb5(idot,l1,c,Ch,Wa(iw),Wa(ix2),Wa(ix3),Wa(ix4))
endif
na = 1 - na
endif
l1 = l2
iw = iw + (ip-1)*idot
enddo
if ( na==0 ) return
n2 = n + n
do i = 1 , n2
c(i) = Ch(i)
enddo
end subroutine cfftb1
