有哪位大虾能帮我看看这段 FFT 的程序,我感觉好难啊! 这个好象是一种新的快速傅里叶变换算法,跟以前的我看过的一种算法不同,明显要难多了!
主要是 i2rev 和i3rev 这两个参数不知道是用来做什么的? 谢拉!
BOOL fourn(double * data, unsigned long nn[], int ndim, int isign)
{
int idim;
unsigned long i1,i2,i3,i2rev,i3rev,ip1,ip2,ip3,ifp1,ifp2;
unsigned long ibit,k1,k2,n,nprev,nrem,ntot;
double tempi,tempr;
double theta,wi,wpi,wpr,wr,wtemp;
for (ntot=1,idim=1;idim<=ndim;idim++)
ntot *= nn[idim];
nprev=1;
for (idim=ndim;idim>=1;idim--) {
n=nn[idim];
nrem=ntot/(n*nprev);
ip1=nprev << 1;
ip2=ip1*n;
ip3=ip2*nrem;
i2rev=1;
for (i2=1;i2<=ip2;i2+=ip1) {
if (i2 < i2rev) {
for (i1=i2;i1<=i2+ip1-2;i1+=2) {
for (i3=i1;i3<=ip3;i3+=ip2) {
i3rev=i2rev+i3-i2;
SWAP(data[i3],data[i3rev]);
SWAP(data[i3+1],data[i3rev+1]);
}
}
}
ibit=ip2 >> 1;
while (ibit >= ip1 && i2rev > ibit) {
i2rev -= ibit;
ibit >>= 1;
}
i2rev += ibit;
}
ifp1=ip1;
while (ifp1 < ip2) {
ifp2=ifp1 << 1;
theta=isign*6.28318530717959/(ifp2/ip1);
wtemp=sin(0.5*theta);
wpr = -2.0*wtemp*wtemp;
wpi=sin(theta);
wr=1.0;
wi=0.0;
for (i3=1;i3<=ifp1;i3+=ip1) {
for (i1=i3;i1<=i3+ip1-2;i1+=2) {
for (i2=i1;i2<=ip3;i2+=ifp2) {
k1=i2;
k2=k1+ifp1;
tempr=wr*data[k2]-wi*data[k2+1];
tempi=wr*data[k2+1]+wi*data[k2];
data[k2]=data[k1]-tempr;
data[k2+1]=data[k1+1]-tempi;
data[k1] += tempr;
data[k1+1] += tempi;
}
}
wr=(wtemp=wr)*wpr-wi*wpi+wr;
wi=wi*wpr+wtemp*wpi+wi;
}
ifp1=ifp2;
}
nprev *= n;
}
return true;
}