2006马上就要过去了，blog也长草了。贴上一点代码，意思一下
前段时间在C＃里需要用到random shuffle功能，一时没找到合适的代码，就按自己的理解写了一段，所得到结果也能满足自己的需要。值得注意的一点是随机数生成器的选择。直接以Random做为随机数生成器因为时钟精度问题，在一个小的时间段内会得到同样的伪随机数序列，你shuffle后会得到同一个结果。.net提供了RNGCryptoServiceProvider可以避免这种情况，下面是几种用法的示例
1 /// <summary>
2 /// RandomShuffle
3 /// WuErPing 2006/12/07
4 /// </summary>
5 public sealed class RandomShuffle
6 {
7 private RandomShuffle() { }
8
9 // pseudo-random number generator, using a time-dependent default seed value.
10 static public List<int> Shuffle(int size)
11 {
12 List<int> list = new List<int>(size);
13 for (int i = 0; i < size; ++i)
14 {
15 list.Insert(i, i);
16 }
17 System.Random random = new Random();
18 for (int i = 0; i < list.Count; ++i)
19 {
20 int var = random.Next(0, list.Count);
21 int temp = list[i];
22 list[i] = list[var];
23 list[var] = temp;
24 }
25
26 return list;
27 }
28
29 // using a RNGCryptoServiceProvider().GetHashCode() seed value
30 static public List<int> ShuffleEx(int size)
31 {
32 List<int> list = new List<int>(size);
33 for (int i = 0; i < size; ++i)
34 {
35 list.Insert(i, i);
36 }
37 System.Random random = new Random(new RNGCryptoServiceProvider().GetHashCode());
38 for (int i = 0; i < list.Count; ++i)
39 {
40 int var = random.Next(0, list.Count);
41 int temp = list[i];
42 list[i] = list[var];
43 list[var] = temp;
44 }
45
46 return list;
47 }
48
49 // Cryptographic random number generators create cryptographically strong random values
50 static public List<int> ShufflePro(int size)
51 {
52 List<int> list = new List<int>(size);
53 for (int i = 0; i < size; ++i)
54 {
55 list.Insert(i, i);
56 }
57 byte[] randomBytes = new Byte[4];
58 RNGCryptoServiceProvider rng = new RNGCryptoServiceProvider();
59 for (int i = 0; i < list.Count; ++i)
60 {
61 rng.GetNonZeroBytes(randomBytes);
62 int randomSeed = (randomBytes[0] << 24) | (randomBytes[1] << 16) | (randomBytes[2] << 8) | randomBytes[3];
63 int var = randomSeed % list.Count;
64 //var = System.Math.Abs(var);
65 if (var < 0) var *= -1;
66 int temp = list[i];
67 list[i] = list[var];
68 list[var] = temp;
69 }
70 return list;
71 }
72 }
注：如果要深究random shuffle算法，可以看标准C＋＋的random_shuffle的实现。根据SGI的文档http://www.sgi.com/tech/stl/random_shuffle.html，算法来自
[1] This algorithm is described in section 3.4.2 of Knuth (D. E. Knuth, The Art of Computer Programming. Volume 2: Seminumerical Algorithms, second edition. Addison-Wesley, 1981). Knuth credits Moses and Oakford (1963) and Durstenfeld (1964).
附：vc8的random_shuffle的实现
1 // TEMPLATE FUNCTION random_shuffle
2 template<class _RanIt,
3 class _Diff> inline
4 void _Random_shuffle(_RanIt _First, _RanIt _Last, _Diff *)
5 { // shuffle [_First, _Last)
6 _DEBUG_RANGE(_First, _Last);
7 const int _RANDOM_BITS = 15; // minimum random bits from rand()
8 const int _RANDOM_MAX = (1U << _RANDOM_BITS) - 1;
9
10 _RanIt _Next = _First;
11 for (unsigned long _Index = 2; ++_Next != _Last; ++_Index)
12 { // assume unsigned long big enough for _Diff count
13 unsigned long _Rm = _RANDOM_MAX;
14 unsigned long _Rn = ::rand() & _RANDOM_MAX;
15 for (; _Rm < _Index && _Rm != ~0UL;
16 _Rm = _Rm << _RANDOM_BITS | _RANDOM_MAX)
17 _Rn = _Rn << _RANDOM_BITS
18 | (::rand() & _RANDOM_MAX); // build random value
19
20 std::iter_swap(_Next, _First + _Diff(_Rn % _Index)); // swap a pair
21 }
22 }
23
http://www.cnblogs.com/WuErPIng/archive/2006/12/31/609224.html
