FFT.java

package org.protocoderrunner.utils;


public class FFT {
	int n, m;

	// Lookup tables. Only need to recompute when size of FFT changes.
	double[] cos;
	double[] sin;

	public FFT(int n) {
		this.n = n;
		this.m = (int) (Math.log(n) / Math.log(2));

		// Make sure n is a power of 2
		if (n != (1 << m)) {
			throw new RuntimeException("FFT length must be power of 2");
		}

		// precompute tables
		cos = new double[n / 2];
		sin = new double[n / 2];

		for (int i = 0; i < n / 2; i++) {
			cos[i] = Math.cos(-2 * Math.PI * i / n);
			sin[i] = Math.sin(-2 * Math.PI * i / n);
		}
	}

	/***************************************************************
	 * fft.c Douglas L. Jones University of Illinois at Urbana-Champaign January
	 * 19, 1992 http://cnx.rice.edu/content/m12016/latest/
	 * 
	 * fft: in-place radix-2 DIT DFT of a complex input
	 * 
	 * input: n: length of FFT: must be a power of two m: n = 2**m input/output
	 * x: double array of length n with real part of data y: double array of
	 * length n with imag part of data
	 * 
	 * Permission to copy and use this program is granted as long as this header
	 * is included.
	 ****************************************************************/
	public void fft(double[] re, double[] im) {
		int i, j, k, n1, n2, a;
		double c, s, t1, t2;

		// Bit-reverse
		j = 0;
		n2 = n / 2;
		for (i = 1; i < n - 1; i++) {
			n1 = n2;
			while (j >= n1) {
				j = j - n1;
				n1 = n1 / 2;
			}
			j = j + n1;

			if (i < j) {
				t1 = re[i];
				re[i] = re[j];
				re[j] = t1;
				t1 = im[i];
				im[i] = im[j];
				im[j] = t1;
			}
		}

		// FFT
		n1 = 0;
		n2 = 1;

		for (i = 0; i < m; i++) {
			n1 = n2;
			n2 = n2 + n2;
			a = 0;

			for (j = 0; j < n1; j++) {
				c = cos[a];
				s = sin[a];
				a += 1 << (m - i - 1);

				for (k = j; k < n; k = k + n2) {
					t1 = c * re[k + n1] - s * im[k + n1];
					t2 = s * re[k + n1] + c * im[k + n1];
					re[k + n1] = re[k] - t1;
					im[k + n1] = im[k] - t2;
					re[k] = re[k] + t1;
					im[k] = im[k] + t2;
				}
			}
		}
	}

}