NFFT  3.3.2
sinc.c
00001 /*
00002  * Copyright (c) 2002, 2016 Jens Keiner, Stefan Kunis, Daniel Potts
00003  *
00004  * This program is free software; you can redistribute it and/or modify it under
00005  * the terms of the GNU General Public License as published by the Free Software
00006  * Foundation; either version 2 of the License, or (at your option) any later
00007  * version.
00008  *
00009  * This program is distributed in the hope that it will be useful, but WITHOUT
00010  * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS
00011  * FOR A PARTICULAR PURPOSE.  See the GNU General Public License for more
00012  * details.
00013  *
00014  * You should have received a copy of the GNU General Public License along with
00015  * this program; if not, write to the Free Software Foundation, Inc., 51
00016  * Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.
00017  */
00018 
00019 #include "infft.h"
00020 
00021 R Y(sinc)(const R x)
00022 {
00023   /* Based on sinc function from Boost C++ library. */
00024   const R b =  EPSILON;
00025   const R bs = SQRT(b);
00026   const R bs2 = SQRT(bs);
00027 
00028   if (FABS(x) >= bs2)
00029     return SIN(x)/x;
00030   else
00031   {
00032     R r = K(1.0);
00033 
00034     if (FABS(x) >= b)
00035     {
00036       const R x2 = x * x;
00037       r -= x2 / K(6.0);
00038 
00039       if (FABS(x) >= bs)
00040         r += (x2 * x2) / K(120.0);
00041     }
00042 
00043     return r;
00044   }
00045 }