97 lines
2.1 KiB
C
97 lines
2.1 KiB
C
/* Fast Fourier Transformation (FFT)
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*
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* (C) 2016 by Andreas Eversberg <jolly@eversberg.eu>
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* All Rights Reserved
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*
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* This program is free software: you can redistribute it and/or modify
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* it under the terms of the GNU General Public License as published by
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* the Free Software Foundation, either version 3 of the License, or
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* (at your option) any later version.
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*
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* This program is distributed in the hope that it will be useful,
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* but WITHOUT ANY WARRANTY; without even the implied warranty of
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* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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* GNU General Public License for more details.
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*
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* You should have received a copy of the GNU General Public License
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* along with this program. If not, see <http://www.gnu.org/licenses/>.
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*/
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#include <math.h>
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#include <string.h>
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#include "fft.h"
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/*
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* Code based closely to work by Paul Bourke
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*
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* This computes an in-place complex-to-complex FFT
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* x and y are the real and imaginary arrays of 2^m points.
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* dir = 1 gives forward transform
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* dir = -1 gives reverse transform
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*/
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void fft_process(int dir, int m, double *x, double *y)
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{
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int n, i, i1, j, k, i2, l, l1, l2;
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double c1, c2, tx, ty, t1, t2, u1, u2, z;
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/* Calculate the number of points */
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n = 1 << m;
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/* Do the bit reversal */
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i2 = n >> 1;
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j = 0;
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for (i = 0; i < n - 1; i++) {
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if (i < j) {
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tx = x[i];
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ty = y[i];
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x[i] = x[j];
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y[i] = y[j];
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x[j] = tx;
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y[j] = ty;
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}
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k = i2;
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while (k <= j) {
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j -= k;
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k >>= 1;
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}
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j += k;
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}
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/* Compute the FFT */
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c1 = -1.0;
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c2 = 0.0;
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l2 = 1;
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for (l = 0; l < m; l++) {
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l1 = l2;
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l2 <<= 1;
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u1 = 1.0;
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u2 = 0.0;
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for (j = 0; j < l1; j++) {
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for (i = j; i < n; i += l2) {
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i1 = i + l1;
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t1 = u1 * x[i1] - u2 * y[i1];
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t2 = u1 * y[i1] + u2 * x[i1];
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x[i1] = x[i] - t1;
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y[i1] = y[i] - t2;
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x[i] += t1;
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y[i] += t2;
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}
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z = u1 * c1 - u2 * c2;
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u2 = u1 * c2 + u2 * c1;
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u1 = z;
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}
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c2 = sqrt((1.0 - c1) / 2.0);
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if (dir == 1)
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c2 = -c2;
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c1 = sqrt((1.0 + c1) / 2.0);
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}
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/* Scaling for forward transform */
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if (dir == 1) {
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for (i = 0; i < n; i++) {
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x[i] /= n;
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y[i] /= n;
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}
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}
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}
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