I was just wondering if there are other people with an interest in
developing an FFT class.
I have a simple Cooley-Tukey FFT implementation to start on, and
perhaps we could work on implementing other variations.
Now obviously this implementation is very crude, and needs many
improvements, but I believe that an FFT library could be a valuable
addition to the commons. Please let me know what your opinions on
this are.
import org.apache.commons.math.complex.ComplexUtils;
import org.apache.commons.math.complex.Complex;
public class FFT {
public static Complex[] fft(Complex[] input) {
Complex[] output = new Complex[input.length];
if (input.length == 1) {
output[0] = input[0];
} else {
Complex[] even = new Complex[input.length/2];
Complex[] odd = new Complex[input.length/2];
for (int i = 0; i < input.length/2; i++) {
even[i] = input[i*2];
odd[i] = input[i*2 + 1];
}
Complex[] E = fft(even);
Complex[] O = fft(odd);
for (int i = 0; i < input.length; i++) {
if (i < input.length/2) {
output[i] = E[i].add(ComplexUtils.exp(new Complex(0,
-2*Math.PI/input.length*i)).multiply(O[i]));
} else {
output[i] = E[i - input.length/2].subtract(ComplexUtils.exp(
new Complex(0, -2*Math.PI/input.length*(i
- input.length/2))).multiply(O[i - input.length/2])
);
}
}
}
return output;
}
public static Complex[] ifft(Complex[] input) {
Complex[] postInput = new Complex[input.length];
for (int i = 0; i < input.length; i++) {
postInput[i] = input[i].conjugate();
}
Complex[] output = fft(postInput);
for (int i = 0; i < input.length; i++) {
output[i] = output[i].conjugate().divide(new Complex(input.length,
0));
}
return output;
}
}
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