rft.f90

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subroutine rft(f, ffnul, ffcos, ffsin, jpts, mharm)                      
!-----------------------------------------------------------------------  
!  calculates fourier cosine and sine coefficients ffcos and              
!  ffsin of the array ff corresponding to the  function                   
!  f(t)=.5*ffnul+sum(ffcos(m)*cos(m*t)+ffsin(m)*sin(m*t))                 
!  where mharm.le.jpts/2-1, ffnul=ff(0) and t=2*pi*(j-1)/jpts.            
!  the input array f(j) is not destroyed by calling rftcos.               
!  typical use is for mharm much smaller than jpts/2-1, so that           
!  rft2 cannot be used directly.                                          
!-----------------------------------------------------------------------  

  use itm_types

  implicit none

  integer(itm_i4), intent(in) :: jpts, mharm
  real(r8), dimension(jpts), intent(in) :: f
  real(r8), intent(out) :: ffnul
  real(r8), dimension(mharm), intent(out) :: ffcos, ffsin

  real(r8), dimension(jpts + 2) :: fstore
  real(r8) :: fac
  integer(itm_i4) :: j, m

  do j = 1, jpts
    fstore(j) = f(j)
  end do
  call rft2(fstore, jpts, 1)
  fac = 2._r8 / jpts
  ffnul = fstore(1) * fac

  do m =1, mharm
    ffcos(m) = fstore(2 * m + 1) * fac
    ffsin(m) = - fstore(2 * m + 2) * fac
  end do

  return
end subroutine rft