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sinc.c
1 /*
2 * Copyright (c) 2002, 2017 Jens Keiner, Stefan Kunis, Daniel Potts
3 *
4 * This program is free software; you can redistribute it and/or modify it under
5 * the terms of the GNU General Public License as published by the Free Software
6 * Foundation; either version 2 of the License, or (at your option) any later
7 * version.
8 *
9 * This program is distributed in the hope that it will be useful, but WITHOUT
10 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS
11 * FOR A PARTICULAR PURPOSE. See the GNU General Public License for more
12 * details.
13 *
14 * You should have received a copy of the GNU General Public License along with
15 * this program; if not, write to the Free Software Foundation, Inc., 51
16 * Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.
17 */
18
19 #include "infft.h"
20
21 R Y(sinc)(const R x)
22 {
23 /* Based on sinc function from Boost C++ library. */
24 const R b = NFFT_EPSILON;
25 const R bs = SQRT(b);
26 const R bs2 = SQRT(bs);
27
28 if (FABS(x) >= bs2)
29 return SIN(x)/x;
30 else
31 {
32 R r = K(1.0);
33
34 if (FABS(x) >= b)
35 {
36 const R x2 = x * x;
37 r -= x2 / K(6.0);
38
39 if (FABS(x) >= bs)
40 r += (x2 * x2) / K(120.0);
41 }
42
43 return r;
44 }
45 }