Java BigInteger is Prime isPrime(BigInteger value)

Description

Checks if value is a prime number,

License

Open Source License

Parameter

Parameter	Description
value	BigInteger to check

Return

true if value is prime false otherwise

Declaration

public static boolean isPrime(BigInteger value) 

Method Source Code

//package com.java2s;
/*// www  .  j a  v  a 2  s .  c o m
 * Utility class to simplify the use of BigDecimals
 * Copyright (C) 2012 Martin Absmeier, IT Consulting Services
 *
 * This program is free software: you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation, either version 3 of the License, or
 * (at your option) any later version.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with this program.  If not, see <http://www.gnu.org/licenses/>.
 */

import java.math.BigDecimal;
import java.math.BigInteger;
import java.math.MathContext;
import java.math.RoundingMode;

public class Main {
    /**
     * Checks if <code>value</code> is a prime number,
     * 
     * @param value
     *            BigInteger to check
     * @return true if value is prime false otherwise
     */
    public static boolean isPrime(BigInteger value) {
        if (value.signum() < 0) {
            throw new IllegalArgumentException("Only positive values permitted.");
        }

        BigInteger two = new BigInteger("2");

        // If value < 2
        if (value.compareTo(two) < 0) {
            return false;
        }
        // If value == 2
        if (value.equals(two)) {
            return true;
        }
        // If value % 2 == 0      
        if (value.remainder(two).equals(BigInteger.ZERO)) {
            return false;
        }

        BigDecimal sqrt = sqrt(new BigDecimal(value), 0);
        for (long i = 3; i <= sqrt.longValue(); i += 2) {
            if (value.remainder(BigInteger.valueOf(i)).equals(BigInteger.ZERO)) {
                return false;
            }
        }

        return true;
    }

    /**
     * 
     * @param value
     * @param decimalPlaces
     * @return
     */
    public static BigDecimal sqrt(BigDecimal value, int decimalPlaces) {
        BigDecimal zero = BigDecimal.ZERO.setScale(decimalPlaces + 10);
        BigDecimal one = BigDecimal.ONE.setScale(decimalPlaces + 10);
        BigDecimal two = new BigDecimal("2").setScale(decimalPlaces + 10);
        BigDecimal maxerr = one.movePointLeft(decimalPlaces);
        BigDecimal lower = zero;
        BigDecimal upper = value.compareTo(one) <= 0 ? one : value;
        BigDecimal mid = BigDecimal.ZERO;

        while (true) {
            mid = lower.add(upper).divide(two);
            // mid = lower.add(upper).divide(two, RoundingMode.HALF_UP);
            BigDecimal sqr = mid.multiply(mid);
            BigDecimal error = value.subtract(sqr).abs();
            if (error.compareTo(maxerr) <= 0) {
                break;
            }
            if (sqr.compareTo(value) < 0) {
                lower = mid;
            } else {
                upper = mid;
            }
        }

        return mid.round(getMathContext(decimalPlaces));
    }

    private static MathContext getMathContext(int decimalPlaces) {
        return new MathContext(decimalPlaces, RoundingMode.HALF_UP);
    }
}

Related

1. fnvHash(final byte[] data, final BigInteger fnvOffs, final BigInteger fnvPrime, final BigInteger mod)
2. hasSqrtModPrime(BigInteger r, BigInteger p)
3. isBigPrime(BigInteger number)
4. isCoprime(BigInteger a, BigInteger b)
5. isFermatPrime(BigInteger f)
6. primeProcessPart(BigInteger from)
7. sqrtModPrime(BigInteger rSquare, BigInteger p)