BigInteger如何在内部存储值？

Question

BigInteger类有一个返回字节数组的方法。这是否表示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纵二进制数据，这一点很重要。例如，如果这个类是使用Int64数组，那么类似的数组通过任何调用函数来处理原始数据会更有效率。

作为一个例子，我调用ToByteArray方法来遍历查找特定二进制模式的字节。

Answer 1

根据the reference source看来，它存储的信息为数据的uint[]和符号的int。

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; 

这里是它执行执行ToByteArray()

// 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; 
    }