I am trying to get the frequencies from a sound wave using fft. I have found an FFT method, but I am having trouble understanding what it wants me to put in, how to get information out of it, and how to use what I get out of it. I have tried entering the value at index of the buffer as y, with the buffer position as x, but I get weird results. int m seems to be something to do with samples, but I have no clue how to choose what I want to input there. But it effects how much is stored into the x and y double. Luckily dir, is rather easy to understand, it's basically acting like a bool for whether you FFT forward or not.
public static void FFT(short dir, int m, double[] x, double[] y)
{
int n, i, i1, j, k, i2, l, l1, l2;
double c1, c2, tx, ty, t1, t2, u1, u2, z;
// Calculate the number of points
n = 1;
for (i = 0; i < m; i++)
n *= 2;
// Do the bit reversal
i2 = n >> 1;
j = 0;
for (i = 0; i < n  1; i++)
{
if (i < j)
{
tx = x[i];
ty = y[i];
x[i] = x[j];
y[i] = y[j];
x[j] = tx;
y[j] = ty;
}
k = i2;
while (k <= j)
{
j = k;
k >>= 1;
}
j += k;
}
// Compute the FFT
c1 = 1.0;
c2 = 0.0;
l2 = 1;
for (l = 0; l < m; l++)
{
l1 = l2;
l2 <<= 1;
u1 = 1.0;
u2 = 0.0;
for (j = 0; j < l1; j++)
{
for (i = j; i < n; i += l2)
{
i1 = i + l1;
t1 = u1 * x[i1]  u2 * y[i1];
t2 = u1 * y[i1] + u2 * x[i1];
x[i1] = x[i]  t1;
y[i1] = y[i]  t2;
x[i] += t1;
y[i] += t2;
}
z = u1 * c1  u2 * c2;
u2 = u1 * c2 + u2 * c1;
u1 = z;
}
c2 = Math.Sqrt((1.0  c1) / 2.0);
if (dir == 1)
c2 = c2;
c1 = Math.Sqrt((1.0 + c1) / 2.0);
}
// Scaling for forward transform
if (dir == 1)
{
for (i = 0; i < n; i++)
{
x[i] /= n;
y[i] /= n;
}
}
