FFTW3 multiply

	#include <stdio.h>
	#include <stdlib.h>
	#include <math.h>
	#include <fftw3.h>

	int main(int argc, char* argv[])
	{
	  // 231 x 11
	  float x[] = {2.0f, 3.0f, 1.0f, 0.0f, 0.0f, 0.0f, 0.0f, 0.0f,};
	  float y[] = {1.0f, 1.0f, 0.0f, 0.0f, 0.0f, 0.0f, 0.0f, 0.0f,};
	  int n = sizeof(x)/sizeof(float);
	  int m = sizeof(y)/sizeof(float);
	  {
	    fftwf_plan p, q;
	    p = fftwf_plan_r2r_1d(n, x, x, FFTW_R2HC, FFTW_ESTIMATE);
	    q = fftwf_plan_r2r_1d(n, y, y, FFTW_R2HC, FFTW_ESTIMATE);
	    fftwf_execute(p);
	    fftwf_execute(q);
	    fftwf_destroy_plan(p);
	    fftwf_destroy_plan(q);
	  }

	  x[0] *= y[0];
	  x[n/2] *= y[n/2];
	  for (int j = 1; j < n/2; j++)
	    {
	      float e = x[j];
	      float d = x[n - j];
	      float f = y[j];
	      float g = y[n - j];
	      x[n - j] = e*g+f*d;
	      x[j] = e*f-d*g;
	    }

	  {
	    fftwf_plan p;
	    p = fftwf_plan_r2r_1d(n, x, x, FFTW_HC2R, FFTW_ESTIMATE);
	    fftwf_execute(p);
	    fftwf_destroy_plan(p);
	  }

	  for (int j = 0; j < n; j++)
	    { 
	      x[j] = x[j]/n;
	    }

	  float xx = x[0];

	  for(int j = 0;j < n;j ++)
	    {
	      // 桁の処理は省略
	      fprintf(stderr, "[%4d]%f\n", j, x[j]);
	    }
	  return 0;
	}

	cc -lfftw3f -lm

	[   0]2.000000
	[   1]5.000000
	[   2]4.000000
	[   3]1.000000
	[   4]-0.000000
	[   5]0.000000
	[   6]0.000000
	[   7]0.000000

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[Dazjorz](http://dazjorz.com/) (17-September-2009)

    -- Dazjorz
    -- Chat with Shlomi Fish

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FFTW3 多倍長乗算