/* 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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