Better expose the fftpack interface

example/bench02_zfft.f90

@@ -1,4 +1,4 @@
-program main
+program bench2
 use fftpack, only: zffti, zfftf, zfftb, fft, ifft
 use fftpack_kind, only: rk
 

example/bench03_dfft.f90

@@ -1,4 +1,4 @@
-program main
+program bench3
 use fftpack, only: dffti, dfftf, dfftb, rfft, irfft
 use fftpack_kind, only: rk
 

example/real-forward-transform.f90

@@ -0,0 +1,41 @@
+program forward_transform_of_real_function
+  !! This program computes the forward transform of a real function and constructs
+  !! the complex result (re)organized to match the array subscripting to the common
+  !! analytical form [1].  Which form one uses in practice requires balancing the
+  !! need for speed versus clarity. 
+  !!
+  !! [1] https://docs.scipy.org/doc/scipy/reference/generated/scipy.fftpack.rfft.html#scipy.fftpack.rfft
+  use fftpack, only: rfft, irfft
+  implicit none
+  integer j, k
+  integer, parameter :: N = 8
+  double precision, parameter :: two_pi = 2.D0*acos(-1.D0), tolerance = 1.0D-06, f_avg = 3.D0, zero=0.D0
+  double precision, parameter :: f(0:N-1) = f_avg + [(cos(two_pi*dble(j)/dble(N)), j=0,N-1)]
+  double precision, dimension(0:N-1) :: f_round_trip, rfft_f
+  integer, parameter :: rk = kind(two_pi)
+  complex(rk) f_hat(0:N/2)
+
+  call assert(mod(N,2)==0, "the algorithm below requires even N")
+
+  rfft_f(:) = rfft(f)/dble(N)
+  f_hat(:) = [ cmplx(rfft_f(0),zero), [( cmplx(k,k+1),  k=lbound(rfft_f,1)+1,ubound(rfft_f,1)-1,2)], cmplx(zero,rfft_f(N-1)) ]
+  f_round_trip(:) = dble(irfft(rfft_f))
+  !call assert(any(abs(f_round_trip - f) < tolerance), "inverse of forward FFT must yield the original function")
+
+  print *, "f = ", f
+  print *, "f_hat = ", f_hat
+  print *, "f_round_trip = ", f_round_trip
+
+  !print '(3(10x,a,10x))',"f", "f_round_trip", "rfft_f"
+  !do m = 1, size(f)
+  !  print *, f(m), f_round_trip(m), rfft_f(m)
+  !end do
+  !print *
+
+contains
+  pure subroutine assert(assertion, description)
+    logical, intent(in) :: assertion
+    character(len=*), intent(in) :: description
+    if (.not. assertion) error stop description
+  end subroutine
+end program

fpm.toml

@@ -15,11 +15,6 @@ implicit-typing=true
 implicit-external=true
 source-form="default"
 
-[build]
-auto-executables = false
-auto-tests = false
-auto-examples = false
-
 [dev-dependencies]
 test-drive = { git = "https://github.com/fortran-lang/test-drive", tag = "v0.4.0" }
 

src/CMakeLists.txt

@@ -1,66 +1,67 @@
-# Locate the source directory
+# Locate the source directories
 set(dir ${CMAKE_CURRENT_SOURCE_DIR})
+set(subdir "${dir}/fftpack")
 
 # The source files
 set(FFTPACK_SOURCES
-    ${dir}/cfftb1.f90
-    ${dir}/cfftf1.f90
-    ${dir}/cffti1.f90
-    ${dir}/cosqb1.f90
-    ${dir}/cosqf1.f90
-    ${dir}/dcosqb.f90
-    ${dir}/dcosqf.f90
-    ${dir}/dcosqi.f90
-    ${dir}/dcost.f90
-    ${dir}/dcosti.f90
-    ${dir}/dfftb.f90
-    ${dir}/dfftf.f90
-    ${dir}/dffti.f90
-    ${dir}/dsinqb.f90
-    ${dir}/dsinqf.f90
-    ${dir}/dsinqi.f90
-    ${dir}/dsint.f90
-    ${dir}/dsinti.f90
-    ${dir}/dzfftb.f90
-    ${dir}/dzfftf.f90
-    ${dir}/dzffti.f90
-    ${dir}/ezfft1.f90
+    ${subdir}/cfftb1.f90
+    ${subdir}/cfftf1.f90
+    ${subdir}/cffti1.f90
+    ${subdir}/cosqb1.f90
+    ${subdir}/cosqf1.f90
+    ${subdir}/dcosqb.f90
+    ${subdir}/dcosqf.f90
+    ${subdir}/dcosqi.f90
+    ${subdir}/dcost.f90
+    ${subdir}/dcosti.f90
+    ${subdir}/dfftb.f90
+    ${subdir}/dfftf.f90
+    ${subdir}/dffti.f90
+    ${subdir}/dsinqb.f90
+    ${subdir}/dsinqf.f90
+    ${subdir}/dsinqi.f90
+    ${subdir}/dsint.f90
+    ${subdir}/dsinti.f90
+    ${subdir}/dzfftb.f90
+    ${subdir}/dzfftf.f90
+    ${subdir}/dzffti.f90
+    ${subdir}/ezfft1.f90
     ${dir}/fftpack.f90
-    ${dir}/fftpack_dct.f90
-    ${dir}/fftpack_fft.f90
-    ${dir}/fftpack_fftshift.f90
-    ${dir}/fftpack_ifft.f90
-    ${dir}/fftpack_ifftshift.f90
-    ${dir}/fftpack_irfft.f90
-    ${dir}/fftpack_rfft.f90
-    ${dir}/fftpack_utils.f90
-    ${dir}/passb.f90
-    ${dir}/passb2.f90
-    ${dir}/passb3.f90
-    ${dir}/passb4.f90
-    ${dir}/passb5.f90
-    ${dir}/passf.f90
-    ${dir}/passf2.f90
-    ${dir}/passf3.f90
-    ${dir}/passf4.f90
-    ${dir}/passf5.f90
-    ${dir}/radb2.f90
-    ${dir}/radb3.f90
-    ${dir}/radb4.f90
-    ${dir}/radb5.f90
-    ${dir}/radbg.f90
-    ${dir}/radf2.f90
-    ${dir}/radf3.f90
-    ${dir}/radf4.f90
-    ${dir}/radf5.f90
-    ${dir}/radfg.f90
-    ${dir}/rfftb1.f90
-    ${dir}/rfftf1.f90
-    ${dir}/rffti1.f90
-    ${dir}/rk.f90
-    ${dir}/sint1.f90
-    ${dir}/zfftb.f90
-    ${dir}/zfftf.f90
-    ${dir}/zffti.f90
+    ${subdir}/fftpack_dct.f90
+    ${subdir}/fftpack_fft.f90
+    ${subdir}/fftpack_fftshift.f90
+    ${subdir}/fftpack_ifft.f90
+    ${subdir}/fftpack_ifftshift.f90
+    ${subdir}/fftpack_irfft.f90
+    ${subdir}/fftpack_rfft.f90
+    ${subdir}/fftpack_utils.f90
+    ${subdir}/passb.f90
+    ${subdir}/passb2.f90
+    ${subdir}/passb3.f90
+    ${subdir}/passb4.f90
+    ${subdir}/passb5.f90
+    ${subdir}/passf.f90
+    ${subdir}/passf2.f90
+    ${subdir}/passf3.f90
+    ${subdir}/passf4.f90
+    ${subdir}/passf5.f90
+    ${subdir}/radb2.f90
+    ${subdir}/radb3.f90
+    ${subdir}/radb4.f90
+    ${subdir}/radb5.f90
+    ${subdir}/radbg.f90
+    ${subdir}/radf2.f90
+    ${subdir}/radf3.f90
+    ${subdir}/radf4.f90
+    ${subdir}/radf5.f90
+    ${subdir}/radfg.f90
+    ${subdir}/rfftb1.f90
+    ${subdir}/rfftf1.f90
+    ${subdir}/rffti1.f90
+    ${subdir}/rk.f90
+    ${subdir}/sint1.f90
+    ${subdir}/zfftb.f90
+    ${subdir}/zfftf.f90
+    ${subdir}/zffti.f90
 )
 set(FFTPACK_SOURCES ${FFTPACK_SOURCES} PARENT_SCOPE)