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fftfreq.py
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			# -- coding: utf-8 --
			#
			# Copyright (©) 2016-2021 EPFL (École Polytechnique Fédérale de Lausanne),
			# Laboratory (LSMS - Laboratoire de Simulation en Mécanique des Solides)
			#
			# This program is free software: you can redistribute it and/or modify
			# it under the terms of the GNU Affero General Public License as published
			# by the Free Software Foundation, either version 3 of the License, or
			# (at your option) any later version.
			#
			# This program 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 Affero General Public License for more details.
			#
			# You should have received a copy of the GNU Affero General Public License
			# along with this program. If not, see <https://www.gnu.org/licenses/>.

			from __future__ import division

			__author__ = "Lucas Frérot"

			import numpy as np

			from numpy.fft import fftfreq


			def frequencies1D(frequencies):
			n = len(frequencies)
			frequencies[:] = n * fftfreq(n)


			def assign2DFrequencies(qx, qy, frequencies):
			qx, qy = np.meshgrid(qx, qy, indexing='ij')

			frequencies[:, :, 0] = qx
			frequencies[:, :, 1] = qy


			def frequencies2D(frequencies):
			n = frequencies.shape
			qx = fftfreq(n[0]) * n[0]
			qy = fftfreq(n[1]) * n[1]
			assign2DFrequencies(qx, qy, frequencies)


			def hfrequencies2D(frequencies):
			n = frequencies.shape
			qx = fftfreq(n[0]) * n[0]
			qy = np.arange(n[1])
			assign2DFrequencies(qx, qy, frequencies)