/* Program gslffttry: written to demonstrate the problem arising
   in the call to gsl_fft_halfcomplex_unpack:

   !! This is a modification of the example given in the GSL manual
      for real data fft to obtain the half_complex_coefficients
      (CmplxCoeff), assign them to a gsl_complex variable (HCmplxCoeff)
      and to compute the square of thier absolute values (Pwr).

   !! I have commented out the gsl_complex_packed_array CmplxCoeff[2*n]
      below and replaced it with double CmplxCoeff[2*n]. Remove the
      comment to reproduce the "warning" and a resulting error.

   Explanation of the problem:
   > When I use gsl_fft_halfcomplex_unpack, The GSL Ref. Manual (first
   > edition) and also
   > http://sources.redhat.com/gsl/ref/gsl-ref_15.html#SEC240, describes
   > it as int gsl_fft_halfcomplex_unpack (const double
   > halfcomplex_coefficient[], gsl_complex_packed_array
   > complex_coefficient[], size_t stride, size_t n)
   > 
   > When I use it based on the above description, I get the following
   > warning when I compile it (on a Linux PC, with gsl-1.2.90 compiled
   > from sources) Compilation command: gcc -c -Wall -g
   > -I. -I/usr/include -I/home/mow/soft/include analyse_freq.c Warning:
   > analyse_freq.c:190: warning: passing arg 2 of
   > `gsl_fft_halfcomplex_unpack' from incompatible pointer type

   Brian Gough suggested:
   Can you send an example program which reproduces this problem to
   address@hidden, with the output of gcc -v on your system. Thanks.
*/
#include <stdio.h>
#include <stdlib.h>
#include <math.h>
#include <gsl/gsl_errno.h>
#include <gsl/gsl_fft_real.h>
#include <gsl/gsl_fft_halfcomplex.h>
#include <gsl/gsl_complex.h>
#include <gsl/gsl_complex_math.h>

int main()
{
    int i, n=1000;
    double data[1000];
    double Pwr[1000];

    gsl_fft_real_wavetable * real;
    gsl_fft_halfcomplex_wavetable * hc;
    gsl_fft_real_workspace * work;
//  gsl_complex_packed_array CmplxCoeff[2*n];
    double CmplxCoeff[2*n];
    gsl_complex HCmplxCoeff;

/* Creating the square wave */
    for (i=0; i < n; i++)
      {
        data[i] = 0.0;
      }
    for (i = n / 3; i < 2* n / 3; i++)
      {
        data[i] = 1.0;
      }

/* Doing the Fast Fourier transform */
    work = gsl_fft_real_workspace_alloc (n);
    real = gsl_fft_real_wavetable_alloc (n);

    gsl_fft_real_transform (data, 1, n, real, work);
    gsl_fft_real_wavetable_free (real);

/* Deriving the Square of the Complex co-efficients */
    gsl_fft_halfcomplex_unpack (data, CmplxCoeff, 1, n);
    for (i = 0; i < n; i++)
      {
        GSL_SET_COMPLEX(&HCmplxCoeff,CmplxCoeff[i],CmplxCoeff[i+1]);
        Pwr[i] = gsl_complex_abs2(HCmplxCoeff);
        printf("%d %e\n",i,Pwr[i]);
      }

/* Removing contributions from higher frequencies */
    for (i = 11; i < n; i++)
      {
        data[i] = 0.0;
      }

/* Carrying out the inverse transform */
    hc = gsl_fft_halfcomplex_wavetable_alloc (n);
    gsl_fft_halfcomplex_inverse (data, 1, n, hc, work);
    gsl_fft_halfcomplex_wavetable_free (hc);

    for (i = 0; i < n; i++)
      {
         printf("%d %e\n",i, data[i]);
      }

    gsl_fft_real_workspace_free (work);

    exit(0);
}