WarpPI/core/src/main/java/org/nevec/rjm/EulerPhi.java

package org.nevec.rjm;

import java.math.BigInteger;

/**
 * Euler totient function.
 *
 * @see <a href="http://oeis.org/A000010">A000010</a> in the OEIS.
 * @since 2008-10-14
 * @since 2012-03-04 Adapted to new Ifactor representation.
 * @author Richard J. Mathar
 */
public class EulerPhi {
	/**
	 * Default constructor.
	 * Does nothing().
	 */
	public EulerPhi() {}

	/**
	 * Compute phi(n).
	 *
	 * @param n
	 *            The positive argument of the function.
	 * @return phi(n)
	 */
	public BigInteger at(final int n) {
		return at(new BigInteger("" + n));
	} /* at */

	/**
	 * Compute phi(n).
	 *
	 * @param n
	 *            The positive argument of the function.
	 * @return phi(n)
	 */
	public BigInteger at(final BigInteger n) {
		if (n.compareTo(BigInteger.ZERO) <= 0) {
			throw new ArithmeticException("negative argument " + n + " of EulerPhi");
		}
		final Ifactor prFact = new Ifactor(n);
		BigInteger phi = n;
		if (n.compareTo(BigInteger.ONE) > 0) {
			for (int i = 0; i < prFact.primeexp.size(); i += 2) {
				final BigInteger p = new BigInteger(prFact.primeexp.elementAt(i).toString());
				final BigInteger p_1 = p.subtract(BigInteger.ONE);
				phi = phi.multiply(p_1).divide(p);
			}
		}
		return phi;
	} /* at */

	/**
	 * Test program.
	 * It takes one argument n and prints the value phi(n).<br>
	 * java -cp . org.nevec.rjm.EulerPhi n<br>
	 *
	 * @since 2006-08-14
	 */
	public static void main(final String[] args) throws ArithmeticException {
		final EulerPhi a = new EulerPhi();
		final int n = new Integer(args[0]).intValue();
		System.out.println("phi(" + n + ") = " + a.at(n));
	}
} /* EulerPhi */