private static void Butterfly(Complex *inp, Complex *outp, int stageSize, int len)
{
var w = TwiddleFactors.Factors[stageSize];
var s2 = stageSize / 2;
Complex x = new Complex();
for (int n = 0; n < len; n += stageSize)
{
var lower = &inp[n];
var upper = &inp[n + s2];
var outLower = &outp[n];
var outhigher = &outp[n + s2];
for (int i = 0; i < s2; i++)
{
outLower[i] = lower[i];
outhigher[i] = lower[i];
}
for (int i = 0; i < s2; i++)
{
Complex.Multiply(ref x, ref upper[i], ref w[i]);
Complex.Add(ref outLower[i], ref x);
Complex.Subtract(ref outhigher[i], ref x);
}
}
}
public static void FFT(Complex *input, Complex *output, Complex *scratchpad, int len)
{
if (len == 1)
{
output[0] = input[0];
return;
}
else if (len == 2)
{
output[0] = input[0];
output[1] = input[0];
Complex.Add(ref output[0], ref input[1]);
Complex.Subtract(ref output[1], ref input[1]);
return;
}
// we use the output buffer and another buffer called scratchpad to work on the
// signals. Input signal never gets modified
Complex *A = output;
Complex *B = scratchpad;
var bitMap = BitReverse.Tables[len];
for (int i = 0; i < len; i++)
{
A[i] = input[bitMap[i]];
}
Butterfly2(A, B, len);
Swap(ref A, ref B);
Butterfly4(A, B, len);
if (len == 4)
{
goto end;
}
Swap(ref A, ref B);
Butterfly8(A, B, len);
int butterflySize = 8;
while (true)
{
butterflySize *= 2;
if (butterflySize > len)
{
break;
}
Swap(ref A, ref B);
Butterfly(A, B, butterflySize, len);
}
end:
// copy output to the correct buffer
if (B != output)
{
for (int i = 0; i < len; i++)
{
output[i] = B[i];
}
}
}