subroutine dzfftf(n, r, azero, a, b, wsave)
! version 3 june 1979
use fftpack_kind, only: dp => rk
implicit none
integer, intent(in) :: n
real(dp), intent(in) :: r(*)
real(dp), intent(out) :: azero, a(*), b(*)
real(dp), intent(inout) :: wsave(*)
real(dp) :: cf, cfm
integer :: i, ns2, ns2m
if (n < 2) then
azero = r(1)
return
elseif (n == 2) then
azero = 0.5_dp*(r(1) + r(2))
a(1) = 0.5_dp*(r(1) - r(2))
return
else
do i = 1, n
wsave(i) = r(i)
end do
call dfftf(n, wsave, wsave(n + 1))
cf = 2.0_dp/real(n, dp)
cfm = -cf
azero = 0.5_dp*cf*wsave(1)
ns2 = (n + 1)/2
ns2m = ns2 - 1
do i = 1, ns2m
a(i) = cf*wsave(2*i)
b(i) = cfm*wsave(2*i + 1)
end do
if (mod(n, 2) == 1) return
a(ns2) = 0.5_dp*cf*wsave(n)
b(ns2) = 0.0_dp
end if
end subroutine dzfftf
