## The PollardRho.java Java example source code

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* contributor license agreements. See the NOTICE file distributed with
* this work for additional information regarding copyright ownership.
* The ASF licenses this file to You under the Apache License, Version 2.0
* (the "License"); you may not use this file except in compliance with
* the License. You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
package org.apache.commons.math3.primes;
import java.util.ArrayList;
import java.util.List;
import org.apache.commons.math3.util.FastMath;
/**
* Implementation of the Pollard's rho factorization algorithm.
* @since 3.2
*/
class PollardRho {
/**
* Hide utility class.
*/
private PollardRho() {
}
/**
* Factorization using Pollard's rho algorithm.
* @param n number to factors, must be > 0
* @return the list of prime factors of n.
*/
public static List<Integer> primeFactors(int n) {
final List<Integer> factors = new ArrayList();
n = SmallPrimes.smallTrialDivision(n, factors);
if (1 == n) {
return factors;
}
if (SmallPrimes.millerRabinPrimeTest(n)) {
factors.add(n);
return factors;
}
int divisor = rhoBrent(n);
factors.add(divisor);
factors.add(n / divisor);
return factors;
}
/**
* Implementation of the Pollard's rho factorization algorithm.
* <p>
* This implementation follows the paper "An improved Monte Carlo factorization algorithm"
* by Richard P. Brent. This avoids the triple computation of f(x) typically found in Pollard's
* rho implementations. It also batches several gcd computation into 1.
* <p>
* The backtracking is not implemented as we deal only with semi-primes.
*
* @param n number to factor, must be semi-prime.
* @return a prime factor of n.
*/
static int rhoBrent(final int n) {
final int x0 = 2;
final int m = 25;
int cst = SmallPrimes.PRIMES_LAST;
int y = x0;
int r = 1;
do {
int x = y;
for (int i = 0; i < r; i++) {
final long y2 = ((long) y) * y;
y = (int) ((y2 + cst) % n);
}
int k = 0;
do {
final int bound = FastMath.min(m, r - k);
int q = 1;
for (int i = -3; i < bound; i++) { //start at -3 to ensure we enter this loop at least 3 times
final long y2 = ((long) y) * y;
y = (int) ((y2 + cst) % n);
final long divisor = FastMath.abs(x - y);
if (0 == divisor) {
cst += SmallPrimes.PRIMES_LAST;
k = -m;
y = x0;
r = 1;
break;
}
final long prod = divisor * q;
q = (int) (prod % n);
if (0 == q) {
return gcdPositive(FastMath.abs((int) divisor), n);
}
}
final int out = gcdPositive(FastMath.abs(q), n);
if (1 != out) {
return out;
}
k += m;
} while (k < r);
r = 2 * r;
} while (true);
}
/**
* Gcd between two positive numbers.
* <p>
* Gets the greatest common divisor of two numbers, using the "binary gcd" method,
* which avoids division and modulo operations. See Knuth 4.5.2 algorithm B.
* This algorithm is due to Josef Stein (1961).
* </p>
* Special cases:
* <ul>
* <li>The result of {@code gcd(x, x)}, {@code gcd(0, x)} and {@code gcd(x, 0)} is the value of {@code x}.
* <li>The invocation {@code gcd(0, 0)} is the only one which returns {@code 0}.
* </ul>
*
* @param a first number, must be ≥ 0
* @param b second number, must be ≥ 0
* @return gcd(a,b)
*/
static int gcdPositive(int a, int b){
// both a and b must be positive, it is not checked here
// gdc(a,0) = a
if (a == 0) {
return b;
} else if (b == 0) {
return a;
}
// make a and b odd, keep in mind the common power of twos
final int aTwos = Integer.numberOfTrailingZeros(a);
a >>= aTwos;
final int bTwos = Integer.numberOfTrailingZeros(b);
b >>= bTwos;
final int shift = FastMath.min(aTwos, bTwos);
// a and b >0
// if a > b then gdc(a,b) = gcd(a-b,b)
// if a < b then gcd(a,b) = gcd(b-a,a)
// so next a is the absolute difference and next b is the minimum of current values
while (a != b) {
final int delta = a - b;
b = FastMath.min(a, b);
a = FastMath.abs(delta);
// for speed optimization:
// remove any power of two in a as b is guaranteed to be odd throughout all iterations
a >>= Integer.numberOfTrailingZeros(a);
}
// gcd(a,a) = a, just "add" the common power of twos
return a << shift;
}
}

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