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/***** 作者 _yuming *************/
好犀利的傅里葉C算法！！！膜拜
- #include "math.h"
- void kfft(pr,pi,n,k,fr,fi,l,il)
- int n,k,l,il;
- double pr[],pi[],fr[],fi[];
- { int it,m,is,i,j,nv,l0;
- double p,q,s,vr,vi,poddr,poddi;
- for (it=0; it<=n-1; it++)
- { m=it; is=0;
- for (i=0; i<=k-1; i++)
- { j=m/2; is=2*is+(m-2*j); m=j;}
- fr[it]=pr[is]; fi[it]=pi[is];
- }
- pr[0]=1.0; pi[0]=0.0;
- p=6.283185306/(1.0*n);
- pr[1]=cos(p); pi[1]=-sin(p);
- if (l!=0) pi[1]=-pi[1];
- for (i=2; i<=n-1; i++)
- { p=pr[i-1]*pr[1]; q=pi[i-1]*pi[1];
- s=(pr[i-1]+pi[i-1])*(pr[1]+pi[1]);
- pr[i]=p-q; pi[i]=s-p-q;
- }
- for (it=0; it<=n-2; it=it+2)
- { vr=fr[it]; vi=fi[it];
- fr[it]=vr+fr[it+1]; fi[it]=vi+fi[it+1];
- fr[it+1]=vr-fr[it+1]; fi[it+1]=vi-fi[it+1];
- }
- m=n/2; nv=2;
- for (l0=k-2; l0>=0; l0--)
- { m=m/2; nv=2*nv;
- for (it=0; it<=(m-1)*nv; it=it+nv)
- for (j=0; j<=(nv/2)-1; j++)
- { p=pr[m*j]*fr[it+j+nv/2];
- q=pi[m*j]*fi[it+j+nv/2];
- s=pr[m*j]+pi[m*j];
- s=s*(fr[it+j+nv/2]+fi[it+j+nv/2]);
- poddr=p-q; poddi=s-p-q;
- fr[it+j+nv/2]=fr[it+j]-poddr;
- fi[it+j+nv/2]=fi[it+j]-poddi;
- fr[it+j]=fr[it+j]+poddr;
- fi[it+j]=fi[it+j]+poddi;
- }
- }
- if (l!=0)
- for (i=0; i<=n-1; i++)
- { fr[i]=fr[i]/(1.0*n);
- fi[i]=fi[i]/(1.0*n);
- }
- if (il!=0)
- for (i=0; i<=n-1; i++)
- { pr[i]=sqrt(fr[i]*fr[i]+fi[i]*fi[i]);
- if (fabs(fr[i])<0.000001*fabs(fi[i]))
- { if ((fi[i]*fr[i])>0) pi[i]=90.0;
- else pi[i]=-90.0;
- }
- else
- pi[i]=atan(fi[i]/fr[i])*360.0/6.283185306;
- }
- return;
- }
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