fix_convergence_is…

fft_utils.cc
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			/**
			* file fft_utils.cc
			*
			* @author Till Junge <till.junge@altermail.ch>
			*
			* @date 11 Dec 2017
			*
			* @brief implementation of fft utilities
			*
			* @section LICENCE
			*
			* Copyright © 2017 Till Junge
			*
			* µSpectre 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, or (at
			* your option) any later version.
			*
			* µSpectre 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 GNU Emacs; see the file COPYING. If not, write to the
			* Free Software Foundation, Inc., 59 Temple Place - Suite 330,
			* Boston, MA 02111-1307, USA.
			*/

			#include "fft/fft_utils.hh"


			namespace muSpectre {

			/* ---------------------------------------------------------------------- */
			std::valarray<Real> fft_freqs(size_t nb_samples) {
			std::valarray<Real> retval(nb_samples);
			Int N = (nb_samples-1)/2 + 1; // needs to be signed int for neg freqs
			for (Int i = 0; i < N; ++i) {
			retval[i] = i;
			}
			for (Int i = N; i < Int(nb_samples); ++i) {
			retval[i] = -Int(nb_samples)/2+i-N;
			}
			return retval;
			}

			/* ---------------------------------------------------------------------- */
			std::valarray<Real> fft_freqs(size_t nb_samples, Real length) {
			return fft_freqs(nb_samples)/length;
			}


			} // muSpectre