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- /* fft/c_pass_2.c
- *
- * Copyright (C) 1996, 1997, 1998, 1999, 2000, 2007 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 3 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., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.
- */
- static int
- FUNCTION(fft_complex,pass_2) (const BASE in[],
- const size_t istride,
- BASE out[],
- const size_t ostride,
- const gsl_fft_direction sign,
- 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 factor = 2;
- const size_t m = n / factor;
- const size_t q = n / product;
- const size_t product_1 = product / factor;
- const size_t jump = (factor - 1) * product_1;
- for (k = 0; k < q; k++)
- {
- ATOMIC w_real, w_imag;
- if (k == 0)
- {
- w_real = 1.0;
- w_imag = 0.0;
- }
- else
- {
- if (sign == gsl_fft_forward)
- {
- /* forward tranform */
- w_real = GSL_REAL(twiddle[k - 1]);
- w_imag = GSL_IMAG(twiddle[k - 1]);
- }
- else
- {
- /* backward tranform: w -> conjugate(w) */
- w_real = GSL_REAL(twiddle[k - 1]);
- w_imag = -GSL_IMAG(twiddle[k - 1]);
- }
- }
- for (k1 = 0; k1 < product_1; k1++)
- {
- const ATOMIC z0_real = REAL(in,istride,i);
- const ATOMIC z0_imag = IMAG(in,istride,i);
- const ATOMIC z1_real = REAL(in,istride,i+m);
- const ATOMIC z1_imag = IMAG(in,istride,i+m);
- /* compute x = W(2) z */
- /* x0 = z0 + z1 */
- const ATOMIC x0_real = z0_real + z1_real;
- const ATOMIC x0_imag = z0_imag + z1_imag;
- /* x1 = z0 - z1 */
- const ATOMIC x1_real = z0_real - z1_real;
- const ATOMIC x1_imag = z0_imag - z1_imag;
- /* apply twiddle factors */
-
- /* out0 = 1 * x0 */
- REAL(out,ostride,j) = x0_real;
- IMAG(out,ostride,j) = x0_imag;
-
- /* out1 = w * x1 */
- REAL(out,ostride,j+product_1) = w_real * x1_real - w_imag * x1_imag;
- IMAG(out,ostride,j+product_1) = w_real * x1_imag + w_imag * x1_real;
-
- i++;
- j++;
- }
- j += jump;
- }
- return 0;
- }
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