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(gsl_fft_real,radix2_transform) (BASE data[], const size_t stride, const size_t n)
29 if (n == 1) /* identity operation */
34 /* make sure that n is a power of 2 */
36 result = fft_binary_logn(n) ;
40 GSL_ERROR ("n is not a power of 2", GSL_EINVAL);
47 /* bit reverse the ordering of input data for decimation in time algorithm */
49 status = FUNCTION(fft_real,bitreverse_order)(data, stride, n, logn) ;
51 /* apply fft recursion */
55 for (i = 1; i <= logn; i++)
65 for (b = 0; b < q; b++)
67 ATOMIC t0_real = VECTOR(data,stride,b*p) + VECTOR(data,stride,b*p + p_1) ;
68 ATOMIC t1_real = VECTOR(data,stride,b*p) - VECTOR(data,stride,b*p + p_1) ;
70 VECTOR(data,stride,b*p) = t0_real ;
71 VECTOR(data,stride,b*p + p_1) = t1_real ;
74 /* a = 1 ... p_{i-1}/2 - 1 */
80 const double theta = - 2.0 * M_PI / p;
82 const ATOMIC s = sin (theta);
83 const ATOMIC t = sin (theta / 2.0);
84 const ATOMIC s2 = 2.0 * t * t;
86 for (a = 1; a < (p_1)/2; a++)
88 /* trignometric recurrence for w-> exp(i theta) w */
91 const ATOMIC tmp_real = w_real - s * w_imag - s2 * w_real;
92 const ATOMIC tmp_imag = w_imag + s * w_real - s2 * w_imag;
97 for (b = 0; b < q; b++)
99 ATOMIC z0_real = VECTOR(data,stride,b*p + a) ;
100 ATOMIC z0_imag = VECTOR(data,stride,b*p + p_1 - a) ;
101 ATOMIC z1_real = VECTOR(data,stride,b*p + p_1 + a) ;
102 ATOMIC z1_imag = VECTOR(data,stride,b*p + p - a) ;
104 /* t0 = z0 + w * z1 */
106 ATOMIC t0_real = z0_real + w_real * z1_real - w_imag * z1_imag;
107 ATOMIC t0_imag = z0_imag + w_real * z1_imag + w_imag * z1_real;
109 /* t1 = z0 - w * z1 */
111 ATOMIC t1_real = z0_real - w_real * z1_real + w_imag * z1_imag;
112 ATOMIC t1_imag = z0_imag - w_real * z1_imag - w_imag * z1_real;
114 VECTOR(data,stride,b*p + a) = t0_real ;
115 VECTOR(data,stride,b*p + p - a) = t0_imag ;
117 VECTOR(data,stride,b*p + p_1 - a) = t1_real ;
118 VECTOR(data,stride,b*p + p_1 + a) = -t1_imag ;
125 for (b = 0; b < q; b++)
129 VECTOR(data,stride,b*p + p - p_1/2) *= -1 ;