FFTW test

fftw_test.f90

program fftw_test
  use, intrinsic :: iso_c_binding
  implicit none
  include 'fftw3.f03'

  integer(c_int), parameter :: n = 16, k = 2
  integer(c_size_t) :: plan_forward, plan_backward
  integer(c_int) :: i
  real(c_double) :: pi
  real(c_double), dimension(n) :: gin, gout
  complex(c_double_complex), dimension(n/2) :: w

  call dfftw_plan_dft_r2c_1d(plan_forward, n, gin, w, FFTW_ESTIMATE)
  call dfftw_plan_dft_c2r_1d(plan_backward, n, w, gout, FFTW_ESTIMATE)

  pi = acos(-1.0_c_double)
  print *, "init"
  do i = 1, n
    gin(i) = sin(2 * pi * k * (i - 1) / n)
    print *, i, gin(i)
  end do

  call dfftw_execute_dft_r2c(plan_forward, gin, w)
  print *, "analysis"
  do i = 1, n/2
    print *, i-1, w(i)
  end do

  call dfftw_execute_dft_c2r(plan_backward, w, gout)
  gout(:) = gout(:) / n
  print *, "synthesis"
  do i = 1, n
    print *, i, gout(i), gout(i) - gin(i)
  end do

  call dfftw_destroy_plan(plan_forward)
  call dfftw_destroy_plan(plan_backward)

end program fftw_test

Makefile

.SUFFIXES:
.SUFFIXES: .o .f90
FC = gfortran12
LDLIBS = -lfftw3
LDFLAGS = -L/usr/local/lib
FFLAGS = -O2 -I/usr/local/include
SRCS = $(TARGET).f90
OBJS = $(SRCS:.f90=.o)
TARGET = fftw_test

$(TARGET) : $(OBJS)
	$(FC) $(LDFLAGS) -o $@ $? $(LDLIBS)

clean:
	rm -f $(OBJS) $(TARGET)

.f90.o:
	$(FC) $(FFLAGS) -c $<