Converts a BigInteger, e.g. - Java java.math

Java examples for java.math:BigInteger

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Description

Converts a BigInteger, e.g.

Demo Code

/**//from w  w  w . j  av a 2  s .c o m
 * 
 * Copyright (C) 2004-2008 FhG Fokus
 *
 * This file is part of the FhG Fokus UPnP stack - an open source UPnP implementation
 * with some additional features
 *
 * You can redistribute the FhG Fokus UPnP stack and/or modify it
 * under the terms of the GNU General Public License Version 3 as published by
 * the Free Software Foundation.
 *
 * For a license to use the FhG Fokus UPnP stack software under conditions
 * other than those described here, or to purchase support for this
 * software, please contact Fraunhofer FOKUS by e-mail at the following
 * addresses:
 *   upnpstack@fokus.fraunhofer.de
 *
 * The FhG Fokus UPnP stack 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/>
 * or write to the Free Software Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA
 *
 */
//package com.java2s;

import java.math.BigInteger;

public class Main {
    public static void main(String[] argv) throws Exception {
        BigInteger value = new BigInteger("1234");
        System.out.println(bigIntegerToBase64(value));
    }

    /** Converts a BigInteger, e.g. a RSA modulus, to a Base64 encoded string */
    public static String bigIntegerToBase64(BigInteger value) {
        byte[] data = value.toByteArray();
        String result = "";
        // add leading zero if necessary
        if ((data[0] & 0x80) != 0) {
            result = "0";
        }

        for (int i = 0; i < data.length; i++) {
            result += (char) data[i];
        }

        return stringToBase64(result);
    }

    /** Converts a string to a Base64 encoded string */
    public static String stringToBase64(String data) {
        String result = "";
        byte[] dataFragment = new byte[3];
        // ceil(data.length/3)
        for (int i = 0; i < (data.length() + 2) / 3; i++) {
            int fragmentLength = Math.min(3, data.length() - i * 3);
            // copy current data to fragment
            for (int j = 0; j < fragmentLength; j++) {
                dataFragment[j] = (byte) data.charAt(i * 3 + j);
            }
            result += bytesToBase64(dataFragment, fragmentLength);
        }
        return result;
    }

    /** Converts a maximum of three bytes to 4 Base64 characters */
    private static String bytesToBase64(byte[] bytes, int length) {
        String result = "";
        if (length == 3) {
            result += bitsToBase64((byte) ((bytes[0] & 0xFC) >> 2));
            result += bitsToBase64((byte) ((bytes[0] & 0x03) << 4 | (bytes[1] & 0xF0) >> 4));
            result += bitsToBase64((byte) ((bytes[1] & 0x0F) << 2 | (bytes[2] & 0xC0) >> 6));
            result += bitsToBase64((byte) (bytes[2] & 0x3F));
        }
        if (length == 2) {
            result += bitsToBase64((byte) ((bytes[0] & 0xFC) >> 2));
            result += bitsToBase64((byte) ((bytes[0] & 0x03) << 4 | (bytes[1] & 0xF0) >> 4));
            result += bitsToBase64((byte) ((bytes[1] & 0x0F) << 2));
            result += '=';
        }
        if (length == 1) {
            result += bitsToBase64((byte) ((bytes[0] & 0xFC) >> 2));
            result += bitsToBase64((byte) ((bytes[0] & 0x03) << 4));
            result += '=';
            result += '=';
        }
        return result;
    }

    /** Converts a 6-bit value to the corresponding Base64 char */
    private static char bitsToBase64(byte data) {
        if (data >= 0 && data < 26) {
            return (char) (data + 'A');
        }
        if (data >= 26 && data < 52) {
            return (char) (data - 26 + 'a');
        }
        if (data >= 52 && data < 62) {
            return (char) (data - 52 + '0');
        }
        if (data == 62) {
            return '+';
        }
        if (data == 63) {
            return '/';
        }
        return (char) 0;
    }
}

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