Java BigInteger to bigIntegerToBytes(BigInteger b, int numBytes)

Description

The regular java.math.BigInteger#toByteArray() method isn't quite what we often need: it appends a leading zero to indicate that the number is positive and may need padding.

License

Open Source License

Parameter

Parameter	Description
b	the integer to format into a byte array
numBytes	the desired size of the resulting byte array

Return

numBytes byte long array.

Declaration

public static byte[] bigIntegerToBytes(BigInteger b, int numBytes)

Method Source Code

//package com.java2s;
/*//from  w w  w  . ja  v  a  2s  . com
 * This file is part of RskJ
 * Copyright (C) 2017 RSK Labs Ltd.
 * (derived from ethereumJ library, Copyright (c) 2016 <ether.camp>)
 *
 * This program is free software: you can redistribute it and/or modify
 * it under the terms of the GNU Lesser 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 Lesser General Public License for more details.
 *
 * You should have received a copy of the GNU Lesser General Public License
 * along with this program. If not, see <http://www.gnu.org/licenses/>.
 */

import java.math.BigInteger;

public class Main {
    public static final byte[] EMPTY_BYTE_ARRAY = new byte[0];

    /**
     * The regular {@link java.math.BigInteger#toByteArray()} method isn't quite what we often need:
     * it appends a leading zero to indicate that the number is positive and may need padding.
     *
     * @param b the integer to format into a byte array
     * @param numBytes the desired size of the resulting byte array
     * @return numBytes byte long array.
     */
    public static byte[] bigIntegerToBytes(BigInteger b, int numBytes) {
        if (b == null) {
            return EMPTY_BYTE_ARRAY;
        }
        byte[] bytes = new byte[numBytes];
        byte[] biBytes = b.toByteArray();
        int start = (biBytes.length == numBytes + 1) ? 1 : 0;
        int length = Math.min(biBytes.length, numBytes);
        System.arraycopy(biBytes, start, bytes, numBytes - length, length);
        return bytes;
    }

    /**
     * Omitting sign indication byte.
     * <br><br>
     * Instead of {@link org.bouncycastle.util.BigIntegers#asUnsignedByteArray(BigInteger)}
     * <br>we use this custom method to avoid an empty array in case of BigInteger.ZERO
     *
     * @param value - any big integer number. A <code>null</code>-value will return <code>null</code>
     * @return A byte array without a leading zero byte if present in the signed encoding.
     *      BigInteger.ZERO will return an array with length 1 and byte-value 0.
     */
    public static byte[] bigIntegerToBytes(BigInteger value) {
        if (value == null) {
            return EMPTY_BYTE_ARRAY;
        }

        byte[] data = value.toByteArray();

        if (data.length != 1 && data[0] == 0) {
            byte[] tmp = new byte[data.length - 1];
            System.arraycopy(data, 1, tmp, 0, tmp.length);
            data = tmp;
        }
        return data;
    }
}