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/*
* Licensed to the Apache Software Foundation (ASF) under one or more
* 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.numbers.primes;
import java.text.MessageFormat;
import java.util.List;
/**
* Methods related to prime numbers in the range of int
.
*
* - primality test
* - prime number generation
* - factorization
*
*/
public final class Primes {
/** Exception message format when an argument is too small. */
static final String NUMBER_TOO_SMALL = "{0} is smaller than the minimum ({1})";
/**
* Utility class.
*/
private Primes() {}
/**
* Primality test: tells if the argument is a (provable) prime or not.
*
* It uses the Miller-Rabin probabilistic test in such a way that a result is guaranteed:
* it uses the firsts prime numbers as successive base (see Handbook of applied cryptography
* by Menezes, table 4.1).
*
* @param n Number to test.
* @return true if {@code n} is prime. All numbers < 2 return false.
*/
public static boolean isPrime(int n) {
if (n < 2) {
return false;
}
for (final int p : SmallPrimes.PRIMES) {
if (0 == (n % p)) {
return n == p;
}
}
return SmallPrimes.millerRabinPrimeTest(n);
}
/**
* Return the smallest prime greater than or equal to n.
*
* @param n Positive number.
* @return the smallest prime greater than or equal to {@code n}.
* @throws IllegalArgumentException if n < 0.
*/
public static int nextPrime(int n) {
if (n < 0) {
throw new IllegalArgumentException(MessageFormat.format(NUMBER_TOO_SMALL, n, 0));
}
if (n == 2) {
return 2;
}
n |= 1; // make sure n is odd
if (n == 1) {
return 2;
}
if (isPrime(n)) {
return n;
}
// prepare entry in the +2, +4 loop:
// n should not be a multiple of 3
final int rem = n % 3;
if (0 == rem) { // if n % 3 == 0
n += 2; // n % 3 == 2
} else if (1 == rem) { // if n % 3 == 1
// if (isPrime(n)) return n;
n += 4; // n % 3 == 2
}
while (true) { // this loop skips all multiple of 3
if (isPrime(n)) {
return n;
}
n += 2; // n % 3 == 1
if (isPrime(n)) {
return n;
}
n += 4; // n % 3 == 2
}
}
/**
* Prime factors decomposition.
*
* @param n Number to factorize: must be ≥ 2.
* @return the list of prime factors of {@code n}.
* @throws IllegalArgumentException if n < 2.
*/
public static List primeFactors(int n) {
if (n < 2) {
throw new IllegalArgumentException(MessageFormat.format(NUMBER_TOO_SMALL, n, 2));
}
return SmallPrimes.trialDivision(n);
}
}