Java BigInteger Calculate getBytes(BigInteger big, int bitlen)

License

Parameter

Return

Declaration

static final byte[] getBytes(BigInteger big, int bitlen) 

Method Source Code

//package com.java2s;
//License from project: Apache License 

import java.math.BigInteger;

public class Main {
    /**//from   ww  w  .  j  a  v  a  2  s.com
     * Returns a byte-array representation of a <code>{@link BigInteger}<code>.
     * No sign-bit is output.
     *
     * <b>N.B.:</B> <code>{@link BigInteger}<code>'s toByteArray
     * returns eventually longer arrays because of the leading sign-bit.
     *
     * @param big <code>BigInteger<code> to be converted
     * @param bitlen <code>int<code> the desired length in bits of the representation
     * @return a byte array with <code>bitlen</code> bits of <code>big</code>
     */
    static final byte[] getBytes(BigInteger big, int bitlen) {

        //round bitlen
        bitlen = ((bitlen + 7) >> 3) << 3;

        if (bitlen < big.bitLength()) {
            throw new IllegalArgumentException("IllegalBitlength");
        }

        byte[] bigBytes = big.toByteArray();

        if (((big.bitLength() % 8) != 0) && (((big.bitLength() / 8) + 1) == (bitlen / 8))) {
            return bigBytes;
        }

        // some copying needed
        int startSrc = 0; // no need to skip anything
        int bigLen = bigBytes.length; //valid length of the string

        if ((big.bitLength() % 8) == 0) { // correct values
            startSrc = 1; // skip sign bit

            bigLen--; // valid length of the string
        }

        int startDst = bitlen / 8 - bigLen; //pad with leading nulls
        byte[] resizedBytes = new byte[bitlen / 8];

        System.arraycopy(bigBytes, startSrc, resizedBytes, startDst, bigLen);

        return resizedBytes;
    }
}

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