如何获取任意长度的 BigInteger 的 2 的补码值

Question

BigInteger中有获取2的补码的方法吗？例如：如果有一个负值的 BigInteger

BigInteger a = new BigInteger("-173B8EC504479C3E95DEB0460411962F9EF2ECE0D3AACD749BE39E1006FC87B8", 16);

然后我想得到 BigInteger 形式的 2 的补码

BigInteger b = E8C4713AFBB863C16A214FB9FBEE69D0610D131F2C55328B641D61EFF9037848

我可以从 0xFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFF 减去第一个BigInteger，但是对于任何长度的BigInteger 是否有通用的方法来计算它？

Answer 1

为了使这个值成为它的二进制补码，你将不得不对内容进行操作。那当然是不可能的，所以你先把内容拿出来，操作一下，然后把它们放到一个新的BigInteger:

public static BigInteger twosComplement(BigInteger original)
{
    // for negative BigInteger, top byte is negative  
    byte[] contents = original.toByteArray();

    // prepend byte of opposite sign
    byte[] result = new byte[contents.length + 1];
    System.arraycopy(contents, 0, result, 1, contents.length);
    result[0] = (contents[0] < 0) ? 0 : (byte)-1;

    // this will be two's complement
    return new BigInteger(result);
}

public static void main(String[] args)
{
    BigInteger a = new BigInteger("-173B8EC504479C3E95DEB0460411962F9EF2ECE0D3AACD749BE39E1006FC87B8", 16);
    BigInteger b = twosComplement(a);

    System.out.println(a.toString(16).toUpperCase());
    System.out.println(b.toString(16).toUpperCase());

    // for comparison, from question:
    System.out.println("E8C4713AFBB863C16A214FB9FBEE69D0610D131F2C55328B641D61EFF9037848");
}

输出：

-173B8EC504479C3E95DEB0460411962F9EF2ECE0D3AACD749BE39E1006FC87B8
E8C4713AFBB863C16A214FB9FBEE69D0610D131F2C55328B641C61EFF9037848
E8C4713AFBB863C16A214FB9FBEE69D0610D131F2C55328B641D61EFF9037848

而这个新的 BigInteger 确实是二进制补码，而不仅仅是对位的重新解释。

如何获取任意长度的 BigInteger 的 2 的补码值

How to get the 2's complement value of a BigInteger of arbitrary length

java

android

biginteger

twos-complement

negative-number