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#undef DEBUG
#include <math.h>
#include <stdlib.h>
#include <stdio.h>
#define PI 3.14159265358979323846
extern void dfft(double * xr, double * xi, int np);
double * xr, * xi;
#ifdef DEBUG
void trace()
{
int i;
for (i=0; i<=15; i++) printf("%d %g %g\n",i,xr[i],xi[i]);
printf("-----------\n");
}
void print_int(int n) { printf("%d\n", n); }
void print_float(double x) { printf("%g\n", x); }
#endif
int main(int argc, char ** argv)
{
int n, np, npm, n2, i, j;
double enp, t, y, z, zr, zi, zm, a;
double * pxr, * pxi;
if (argc >= 2) n = atoi(argv[1]); else n = 12;
np = 1 << n;
enp = np;
npm = np / 2 - 1;
t = PI / enp;
xr = calloc(np, sizeof(double));
xi = calloc(np, sizeof(double));
pxr = xr;
pxi = xi;
*pxr = (enp - 1.0) * 0.5;
*pxi = 0.0;
n2 = np / 2;
*(pxr+n2) = -0.5;
*(pxi+n2) = 0.0;
for (i = 1; i <= npm; i++) {
j = np - i;
*(pxr+i) = -0.5;
*(pxr+j) = -0.5;
z = t * (double)i;
y = -0.5*(cos(z)/sin(z));
*(pxi+i) = y;
*(pxi+j) = -y;
}
#ifdef DEBUG
trace();
#endif
dfft(xr,xi,np);
#ifdef DEBUG
trace();
#endif
zr = 0.0;
zi = 0.0;
npm = np-1;
for (i = 0; i <= npm; i++ ) {
a = fabs(pxr[i] - i);
if (zr < a) zr = a;
a = fabs(pxi[i]);
if (zi < a) zi = a;
}
zm = zr;
if (zr < zi) zm = zi;
printf("%d points, error %g\n", np, zm);
return 0;
}
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