BigInteger 如何在内部存储值？

Question

BigInteger 类有一个返回字节数组的方法。这是否表明该类在内部也使用字节数组来存储数字？

为了选择正确的数据类型来操作二进制数据，了解这一点很重要。例如，如果该类使用 Int64 数组，则类似的数组可以更有效地通过任何调用函数来操作原始数据。

例如，我调用 ToByteArray 方法来遍历字节寻找特定的二进制模式。

Answer 1

根据the reference source，它似乎将其信息存储为uint[] 用于数据和一个int 用于符号。

namespace System.Numerics
{
    /// <summary>Represents an arbitrarily large signed integer.</summary>
    [Serializable]
    public struct BigInteger : IFormattable, IComparable, IComparable<BigInteger>, IEquatable<BigInteger>
    {

    // For values int.MinValue < n <= int.MaxValue, the value is stored in sign
    // and _bits is null. For all other values, sign is +1 or -1 and the bits are in _bits
    internal int _sign;
    internal uint[] _bits;

这是它执行ToByteArray()的代码

    // Return the value of this BigInteger as a little-endian twos-complement
    // byte array, using the fewest number of bytes possible. If the value is zero,
    // return an array of one byte whose element is 0x00.
    public byte[] ToByteArray() {
        if (_bits == null && _sign == 0)
            return new byte[] { 0 };

        // We could probably make this more efficient by eliminating one of the passes.
        // The current code does one pass for uint array -> byte array conversion,
        // and then another pass to remove unneeded bytes at the top of the array.
        uint[] dwords;
        byte highByte;

        if (_bits == null) {
            dwords = new uint[] { (uint)_sign };
            highByte = (byte)((_sign < 0) ? 0xff : 0x00);
        }
        else if(_sign == -1) {
            dwords = (uint[])_bits.Clone();
            NumericsHelpers.DangerousMakeTwosComplement(dwords);  // mutates dwords
            highByte = 0xff;
        } else {
            dwords = _bits;
            highByte = 0x00;
        }

        byte[] bytes = new byte[checked(4 * dwords.Length)];
        int curByte = 0;
        uint dword;
        for (int i = 0; i < dwords.Length; i++) {
            dword = dwords[i];
            for (int j = 0; j < 4; j++) {
                bytes[curByte++] = (byte)(dword & 0xff);
                dword >>= 8;
            }
        }

        // find highest significant byte
        int msb;
        for (msb = bytes.Length - 1; msb > 0; msb--) {
            if (bytes[msb] != highByte) break;
        }
        // ensure high bit is 0 if positive, 1 if negative
        bool needExtraByte = (bytes[msb] & 0x80) != (highByte & 0x80);

        byte[] trimmedBytes = new byte[msb + 1 + (needExtraByte ? 1 : 0)];
        Array.Copy(bytes, trimmedBytes, msb + 1);

        if (needExtraByte) trimmedBytes[trimmedBytes.Length - 1] = highByte;
        return trimmedBytes;
    }