text: /* fft/c_pass_n.c
   * 
   * Copyright (C) 1996, 1997, 1998, 1999, 2000 Brian Gough
   * 
   * This program is free software; you can redistribute it and/or modify
   * it under the terms of the GNU General Public License as published by
   * the Free Software Foundation; either version 2 of the License, or (at
   * your option) any later version.
   * 
   * This program is distributed in the hope that it will be useful, but
   * WITHOUT ANY WARRANTY; without even the implied warranty of
   * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
   * General Public License for more details.
   * 
   * You should have received a copy of the GNU General Public License
   * along with this program; if not, write to the Free Software
   * Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.
   */
  
  static int
  FUNCTION(fft_complex,pass_n) (BASE in[],
  			      const size_t istride,
  			      BASE out[],
  			      const size_t ostride,
  			      const gsl_fft_direction sign,
  			      const size_t factor,
  			      const size_t product,
  			      const size_t n,
  			      const TYPE(gsl_complex) twiddle[])
  {
    size_t i = 0, j = 0;
    size_t k, k1;
  
    const size_t m = n / factor;
    const size_t q = n / product;
    const size_t p_1 = product / factor;
    const size_t jump = (factor - 1) * p_1;
  
    size_t e, e1;
  
    for (i = 0; i < m; i++)
      {
        REAL(out,ostride,i) = REAL(in,istride,i);
        IMAG(out,ostride,i) = IMAG(in,istride,i);
      }
  
    for (e = 1; e < (factor - 1) / 2 + 1; e++)
      {
        for (i = 0; i < m; i++)
  	{
  	  const size_t idx = i + e * m;
  	  const size_t idxc = i + (factor - e) * m;
  	  REAL(out,ostride,idx) = REAL(in,istride,idx) + REAL(in,istride,idxc);
  	  IMAG(out,ostride,idx) = IMAG(in,istride,idx) + IMAG(in,istride,idxc);
  	  REAL(out,ostride,idxc) = REAL(in,istride,idx) - REAL(in,istride,idxc);
  	  IMAG(out,ostride,idxc) = IMAG(in,istride,idx) - IMAG(in,istride,idxc);
  	}
      }
  
    /* e = 0 */
  
    for (i=0 ; i<m; i++) 
      {
        REAL(in,istride,i) = REAL(out,ostride,i);
        IMAG(in,istride,i) = IMAG(out,ostride,i);
      }
  
    for (e1 = 1; e1 < (factor - 1) / 2 + 1; e1++)
      {
        for (i = 0; i < m; i++)
  	{
  	  REAL(in,istride,i) += REAL(out,ostride,i + e1*m) ;
  	  IMAG(in,istride,i) += IMAG(out,ostride,i + e1*m) ;
  	}
      }
  
    for (e = 1; e < (factor-1)/2 + 1; e++)
      {
        size_t idx = e*q ;
        const size_t idx_step = e * q ;
        ATOMIC w_real, w_imag ;
  
        const size_t em = e * m ;
        const size_t ecm = (factor - e) * m ;
  
        for (i = 0; i < m; i++) 
  	{
  	  REAL(in,istride,i+em) = REAL(out,ostride,i) ;
  	  IMAG(in,istride,i+em) = IMAG(out,ostride,i) ;
  	  REAL(in,istride,i+ecm) = REAL(out,ostride,i) ;
  	  IMAG(in,istride,i+ecm) = IMAG(out,ostride,i) ;
  	}
  
        for (e1 = 1; e1 < (factor - 1) / 2 + 1; e1++)
  	{
  	  if (idx == 0) {
  	    w_real = 1 ;
  	    w_imag = 0 ;
  	  } else {
  	    if (sign == forward) {
  	      w_real = GSL_REAL(twiddle[idx - 1]) ;
  	      w_imag = GSL_IMAG(twiddle[idx - 1]) ;
  	    } else {
  	      w_real = GSL_REAL(twiddle[idx - 1]) ;
  	      w_imag = -GSL_IMAG(twiddle[idx - 1]) ;
  	    }
  	  }
  
  	  for (i = 0; i < m; i++) 
  	    {
  	      const ATOMIC xp_real = REAL(out,ostride,i + e1 * m);
  	      const ATOMIC xp_imag = IMAG(out,ostride,i + e1 * m);
  	      const ATOMIC xm_real = REAL(out,ostride,i + (factor - e1) *m);
  	      const ATOMIC xm_imag = IMAG(out,ostride,i + (factor - e1) *m);
  	
  	      const ATOMIC ap = w_real * xp_real ;
  	      const ATOMIC am = w_imag * xm_imag ; 
  
  	      ATOMIC sum_real = ap - am;
  	      ATOMIC sumc_real = ap + am;
  
  	      const ATOMIC bp = w_real * xp_imag ;
  	      const ATOMIC bm = w_imag * xm_real ;
  
  	      ATOMIC sum_imag = bp + bm;
  	      ATOMIC sumc_imag = bp - bm;
  
  	      REAL(in,istride,i + em) += sum_real;
  	      IMAG(in,istride,i + em) += sum_imag;
  	      REAL(in,istride,i + ecm) += sumc_real;
  	      IMAG(in,istride,i + ecm) += sumc_imag;
  	    }
  	  idx += idx_step ;
  	  idx %= factor * q ;
  	}
      }
  
    i = 0;
    j = 0;
  
    /* k = 0 */
    for (k1 = 0; k1 < p_1; k1++)
      {
        REAL(out,ostride,k1) = REAL(in,istride,k1);
        IMAG(out,ostride,k1) = IMAG(in,istride,k1);
      }
  
    for (e1 = 1; e1 < factor; e1++)
      {
        for (k1 = 0; k1 < p_1; k1++)
  	{
  	  REAL(out,ostride,k1 + e1 * p_1) = REAL(in,istride,k1 + e1 * m) ;
  	  IMAG(out,ostride,k1 + e1 * p_1) = IMAG(in,istride,k1 + e1 * m) ;
  	}
      }
  
    i = p_1 ;
    j = product ;
  
    for (k = 1; k < q; k++)
      {
        for (k1 = 0; k1 < p_1; k1++)
  	{
  	  REAL(out,ostride,j) = REAL(in,istride,i);
  	  IMAG(out,ostride,j) = IMAG(in,istride,i);
  	  i++;
  	  j++;
  	}
        j += jump;
      }
  
    i = p_1 ;
    j = product ;
  
    for (k = 1; k < q; k++)
      {
        for (k1 = 0; k1 < p_1; k1++)
  	{
  	  for (e1 = 1; e1 < factor; e1++)
  	    {
  	      ATOMIC x_real = REAL(in, istride,i + e1 * m);
  	      ATOMIC x_imag = IMAG(in, istride,i + e1 * m);
  
  	      ATOMIC w_real, w_imag ;
  	      if (sign == forward) {
  		w_real = GSL_REAL(twiddle[(e1-1)*q + k-1]) ;
  		w_imag = GSL_IMAG(twiddle[(e1-1)*q + k-1]) ;
  	      } else {
  		w_real = GSL_REAL(twiddle[(e1-1)*q + k-1]) ;
  		w_imag = -GSL_IMAG(twiddle[(e1-1)*q + k-1]) ; 
  	      }
  
  	      REAL(out,ostride,j + e1 * p_1) = w_real * x_real - w_imag * x_imag;
  	      IMAG(out,ostride,j + e1 * p_1) = w_real * x_imag + w_imag * x_real;
  	    }
  	  i++;
  	  j++;
  	}
        j += jump;
      }
  
    return 0;
  }
  
  