Factorize.java

package com.isti.traceview.jnt.FFT;

/**
 * Supplies static methods for factoring integers needed by various FFT classes.
 * 
 * @author Bruce R. Miller bruce.miller@nist.gov
 *  Contribution of the National Institute of Standards and Technology,
 *  not subject to copyright.
 *  Derived from GSL (Gnu Scientific Library)
 *  GSL's FFT Code by Brian Gough bjg@vvv.lanl.gov
 *  Since GSL is released under
 *  <a href="http://www.gnu.org/copyleft/gpl.html">GPL</a>,
 *  this package must also be.
 */
public class Factorize {

	/**
	 * Return the prime factors of n. The method first extracts any factors in
	 * fromfactors, in order (which needn't actually be prime). Remaining
	 * factors in increasing order follow.
	 */
	public static int[] factor(int n, int fromfactors[]) {
		int factors[] = new int[64]; // Cant be more than 64 factors.
		int nf = 0;
		int ntest = n;
		int factor;

		if (n <= 0) // Error case
			throw new Error("Number (" + n + ") must be positive integer");

		/* deal with the preferred factors first */
		for (int i = 0; i < fromfactors.length && ntest != 1; i++) {
			factor = fromfactors[i];
			while ((ntest % factor) == 0) {
				ntest /= factor;
				factors[nf++] = factor;
			}
		}

		/* deal with any other even prime factors (there is only one) */
		factor = 2;
		while ((ntest % factor) == 0 && (ntest != 1)) {
			ntest /= factor;
			factors[nf++] = factor;
		}

		/* deal with any other odd prime factors */
		factor = 3;
		while (ntest != 1) {
			while ((ntest % factor) != 0) {
				factor += 2;
			}
			ntest /= factor;
			factors[nf++] = factor;
		}

		/* check that the factorization is correct */
		int product = 1;
		for (int i = 0; i < nf; i++) {
			product *= factors[i];
		}
		if (product != n)
			throw new Error("factorization failed for " + n);

		/* Now, make an array of the right length containing the factors... */
		int f[] = new int[nf];
		System.arraycopy(factors, 0, f, 0, nf);
		return f;
	}

	/**
	 * Return the integer log, base 2, of n, or -1 if n is not an integral power
	 * of 2.
	 */
	public static int log2(int n) {
		int log = 0;

		for (int k = 1; k < n; k *= 2, log++)
			;

		if (n != (1 << log))
			return -1; /* n is not a power of 2 */
		return log;
	}
}