/* fft/real_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 void FUNCTION(fft_real,pass_2) (const BASE in[], const size_t istride, BASE out[], const size_t ostride, const size_t product, const size_t n, const TYPE(gsl_complex) twiddle[]) { 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; for (k1 = 0; k1 < q; k1++) { const size_t from0 = k1 * product_1; const size_t from1 = from0 + m; const ATOMIC r0 = VECTOR(in,istride,from0); const ATOMIC r1 = VECTOR(in,istride,from1); const ATOMIC s0 = r0 + r1; const ATOMIC s1 = r0 - r1; const size_t to0 = product * k1; const size_t to1 = to0 + product - 1; VECTOR(out,ostride,to0) = s0; VECTOR(out,ostride,to1) = s1; } if (product_1 == 1) return; for (k = 1; k < (product_1 + 1) / 2; k++) { /* forward real transform: w -> conjugate(w) */ const ATOMIC w_real = GSL_REAL(twiddle[k - 1]); const ATOMIC w_imag = -GSL_IMAG(twiddle[k - 1]); for (k1 = 0; k1 < q; k1++) { const size_t from0 = k1 * product_1 + 2 * k - 1; const size_t from1 = from0 + m; const ATOMIC f0_real = VECTOR(in,istride,from0); const ATOMIC f0_imag = VECTOR(in,istride,from0 + 1); const ATOMIC f1_real = VECTOR(in,istride,from1); const ATOMIC f1_imag = VECTOR(in,istride,from1 + 1); const ATOMIC z0_real = f0_real; const ATOMIC z0_imag = f0_imag; const ATOMIC z1_real = w_real * f1_real - w_imag * f1_imag; const ATOMIC z1_imag = w_real * f1_imag + w_imag * f1_real; /* 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; const size_t to0 = k1 * product + 2 * k - 1; const size_t to1 = k1 * product + product - 2 * k - 1; VECTOR(out,ostride,to0) = x0_real; VECTOR(out,ostride,to0 + 1) = x0_imag; /* stored in conjugate location */ VECTOR(out,ostride,to1) = x1_real; VECTOR(out,ostride,to1 + 1) = -x1_imag; } } if (product_1 % 2 == 1) return; for (k1 = 0; k1 < q; k1++) { const size_t from0 = k1 * product_1 + product_1 - 1; const size_t from1 = from0 + m; const size_t to0 = k1 * product + product_1 - 1; VECTOR(out,ostride,to0) = VECTOR(in,istride,from0); VECTOR(out,ostride,to0 + 1) = -VECTOR(in,istride,from1); } return; }