//
// Listing 1.4: The Discrete Fast Fourier Transform (FFT): 
//

#define M_PI 3.14159265358979323846

void smbFft(float *fftBuffer, long fftFrameSize, long sign) 
/*
 
 FFT routine, (C)1996 S.M.Bernsee. Sign = -1 is FFT, 1 is iFFT (inverse)
 
 Fills fftBuffer[0...2*fftFrameSize-1] with the Fourier transform of the time 
 domain data in fftBuffer[0...2*fftFrameSize-1]. The FFT array takes and returns
 the cosine and sine parts in an interleaved manner, ie. 
 fftBuffer[0] = cosPart[0], fftBuffer[1] = sinPart[0], asf. fftFrameSize must 
 be a power of 2. It expects a complex input signal (see footnote 2), ie. when 
 working with 'common' audio signals our input signal has to be passed as 
 {in[0],0.,in[1],0.,in[2],0.,...} asf. In that case, the transform of the 
 frequencies of interest is in fftBuffer[0...fftFrameSize].
 
 */
{
    float wr, wi, arg, *p1, *p2, temp;
    float tr, ti, ur, ui, *p1r, *p1i, *p2r, *p2i;
    long i, bitm, j, le, le2, k, logN;
    logN = (long)(log(fftFrameSize)/log(2.)+.5);
	
    for (i = 2; i < 2*fftFrameSize-2; i += 2) {
        for (bitm = 2, j = 0; bitm < 2*fftFrameSize; bitm <<= 1) {
            if (i & bitm) j++;
            j <<= 1;
		}
		if (i < j) {
			p1 = fftBuffer+i; p2 = fftBuffer+j;
			temp = *p1; *(p1++) = *p2;
			*(p2++) = temp; temp = *p1;
			*p1 = *p2; *p2 = temp;
		}
	}
	
	for (k = 0, le = 2; k < logN; k++) {
		le <<= 1;
		le2 = le>>1;
		ur = 1.0;
		ui = 0.0;
		arg = M_PI / (le2>>1);
		wr = cos(arg);
		wi = sign*sin(arg);
		for (j = 0; j < le2; j += 2) {
			p1r = fftBuffer+j; p1i = p1r+1;
			p2r = p1r+le2; p2i = p2r+1;
			for (i = j; i < 2*fftFrameSize; i += le) {
				tr = *p2r * ur - *p2i * ui;
				ti = *p2r * ui + *p2i * ur;
				*p2r = *p1r - tr; *p2i = *p1i - ti;
				*p1r += tr; *p1i += ti;
				p1r += le; p1i += le;
				p2r += le; p2i += le;
			}
			tr = ur*wr - ui*wi;
			ui = ur*wi + ui*wr;
			ur = tr;
		}
	}
}