subroutine dzfftb(n, r, azero, a, b, wsave)
use fftpack_kind, only: dp => rk
implicit none
integer, intent(in) :: n
real(dp), intent(out) :: r(*)
real(dp), intent(inout) :: wsave(*)
real(dp), intent(in) :: azero, a(*), b(*)
integer :: i, ns2
if (n < 2) then
r(1) = azero
return
elseif (n == 2) then
r(1) = azero + a(1)
r(2) = azero - a(1)
return
else
ns2 = (n - 1)/2
do i = 1, ns2
r(2*i) = 0.5_dp*a(i)
r(2*i + 1) = -0.5_dp*b(i)
end do
r(1) = azero
if (mod(n, 2) == 0) r(n) = a(ns2 + 1)
call dfftb(n, r, wsave(n + 1))
end if
end subroutine dzfftb
