3 * Copyright (C) 1996, 1997, 1998, 1999, 2000, 2007 Brian Gough
5 * This program is free software; you can redistribute it and/or modify
6 * it under the terms of the GNU General Public License as published by
7 * the Free Software Foundation; either version 3 of the License, or (at
8 * your option) any later version.
10 * This program is distributed in the hope that it will be useful, but
11 * WITHOUT ANY WARRANTY; without even the implied warranty of
12 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
13 * General Public License for more details.
15 * You should have received a copy of the GNU General Public License
16 * along with this program; if not, write to the Free Software
17 * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.
21 FUNCTION(fft_real,pass_2) (const BASE in[],
27 const TYPE(gsl_complex) twiddle[])
31 const size_t factor = 2;
32 const size_t m = n / factor;
33 const size_t q = n / product;
34 const size_t product_1 = product / factor;
36 for (k1 = 0; k1 < q; k1++)
38 const size_t from0 = k1 * product_1;
39 const size_t from1 = from0 + m;
41 const ATOMIC r0 = VECTOR(in,istride,from0);
42 const ATOMIC r1 = VECTOR(in,istride,from1);
44 const ATOMIC s0 = r0 + r1;
45 const ATOMIC s1 = r0 - r1;
47 const size_t to0 = product * k1;
48 const size_t to1 = to0 + product - 1;
50 VECTOR(out,ostride,to0) = s0;
51 VECTOR(out,ostride,to1) = s1;
57 for (k = 1; k < (product_1 + 1) / 2; k++)
60 /* forward real transform: w -> conjugate(w) */
61 const ATOMIC w_real = GSL_REAL(twiddle[k - 1]);
62 const ATOMIC w_imag = -GSL_IMAG(twiddle[k - 1]);
64 for (k1 = 0; k1 < q; k1++)
66 const size_t from0 = k1 * product_1 + 2 * k - 1;
67 const size_t from1 = from0 + m;
69 const ATOMIC f0_real = VECTOR(in,istride,from0);
70 const ATOMIC f0_imag = VECTOR(in,istride,from0 + 1);
72 const ATOMIC f1_real = VECTOR(in,istride,from1);
73 const ATOMIC f1_imag = VECTOR(in,istride,from1 + 1);
75 const ATOMIC z0_real = f0_real;
76 const ATOMIC z0_imag = f0_imag;
78 const ATOMIC z1_real = w_real * f1_real - w_imag * f1_imag;
79 const ATOMIC z1_imag = w_real * f1_imag + w_imag * f1_real;
81 /* compute x = W(2) z */
84 const ATOMIC x0_real = z0_real + z1_real;
85 const ATOMIC x0_imag = z0_imag + z1_imag;
88 const ATOMIC x1_real = z0_real - z1_real;
89 const ATOMIC x1_imag = z0_imag - z1_imag;
91 const size_t to0 = k1 * product + 2 * k - 1;
92 const size_t to1 = k1 * product + product - 2 * k - 1;
94 VECTOR(out,ostride,to0) = x0_real;
95 VECTOR(out,ostride,to0 + 1) = x0_imag;
97 /* stored in conjugate location */
98 VECTOR(out,ostride,to1) = x1_real;
99 VECTOR(out,ostride,to1 + 1) = -x1_imag;
103 if (product_1 % 2 == 1)
106 for (k1 = 0; k1 < q; k1++)
108 const size_t from0 = k1 * product_1 + product_1 - 1;
109 const size_t from1 = from0 + m;
110 const size_t to0 = k1 * product + product_1 - 1;
112 VECTOR(out,ostride,to0) = VECTOR(in,istride,from0);
113 VECTOR(out,ostride,to0 + 1) = -VECTOR(in,istride,from1);