func: static void rdft_calc_c(RDFTContext *s, FFTSample *data)
  
  {
  
      int i, i1, i2;
  
      FFTComplex ev, od;
  
      const int n = 1 << s->nbits;
  
      const float k1 = 0.5;
  
      const float k2 = 0.5 - s->inverse;
  
      const FFTSample *tcos = s->tcos;
  
      const FFTSample *tsin = s->tsin;
  
  
  
      if (!s->inverse) {
  
          s->fft.fft_permute(&s->fft, (FFTComplex*)data);
  
          s->fft.fft_calc(&s->fft, (FFTComplex*)data);
  
      }
  
      /* i=0 is a special case because of packing, the DC term is real, so we
  
         are going to throw the N/2 term (also real) in with it. */
  
      ev.re = data[0];
  
      data[0] = ev.re+data[1];
  
      data[1] = ev.re-data[1];
  
      for (i = 1; i < (n>>2); i++) {
  
          i1 = 2*i;
  
          i2 = n-i1;
  
          /* Separate even and odd FFTs */
  
          ev.re =  k1*(data[i1  ]+data[i2  ]);
  
          od.im = -k2*(data[i1  ]-data[i2  ]);
  
          ev.im =  k1*(data[i1+1]-data[i2+1]);
  
          od.re =  k2*(data[i1+1]+data[i2+1]);
  
          /* Apply twiddle factors to the odd FFT and add to the even FFT */
  
          data[i1  ] =  ev.re + od.re*tcos[i] - od.im*tsin[i];
  
          data[i1+1] =  ev.im + od.im*tcos[i] + od.re*tsin[i];
  
          data[i2  ] =  ev.re - od.re*tcos[i] + od.im*tsin[i];
  
          data[i2+1] = -ev.im + od.im*tcos[i] + od.re*tsin[i];
  
      }
  
      data[2*i+1]=s->sign_convention*data[2*i+1];
  
      if (s->inverse) {
  
          data[0] *= k1;
  
          data[1] *= k1;
  
          s->fft.fft_permute(&s->fft, (FFTComplex*)data);
  
          s->fft.fft_calc(&s->fft, (FFTComplex*)data);
  
      }
  
  }
  