using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading;namespace WBasedRobin
{/// <summary>/// 用于计算WeightRobin的数据结构/// </summary>public class WeightedRobin{private readonly int _weight;private int _count;/// <summary>/// 命中次数（累加）/// </summary>public int ChoosenCount{get{return ++_count;}}/// <summary>/// 权重/// </summary>public int Weight{get{return _weight;}}/// <summary>/// 输出当前的权重/// </summary>public override string ToString(){return "Weight:" + Weight.ToString() + "\tCount:" + _count.ToString();}/// <summary>/// 初始化每一个Server的内部值/// </summary>public WeightedRobin(int weight, int count = 0){_weight = weight;_count = count;}}public class WeightRobinRule{private List<WeightedRobin> _servers = null;private volatile int _index = -1;private volatile int _currentWeight = 0;private volatile bool _isServerChanging = false;private volatile int _maxWeight = 0;private volatile int _gcdWeight = 0;private int GetMaxWeight(IEnumerable<WeightedRobin> weights){return weights.Max(w => w.Weight);}private int GetGCDWeight(int big, int small){if (big < small){big ^= small;small ^= big;big ^= small;}if (big % small == 0){return small;}return GetGCDWeight(small, big % small);}private int GetTotalGCD(){int gcd = GetGCDWeight(_servers[0].Weight, _servers[1].Weight);for (int i = 2; i < _servers.Count; ++i){gcd = GetGCDWeight(gcd, _servers[i].Weight);}return gcd;}/// <summary>/// 初始化权重服务器，至少2台服务器。/// </summary>public WeightRobinRule(int totalServers = 2){Random r = new Random();_servers = new List<WeightedRobin>(totalServers);for (int i = 0; i < totalServers; i++){_servers.Add(new WeightedRobin(r.Next(2, totalServers+1),0));}_maxWeight = GetMaxWeight(_servers);_gcdWeight = GetTotalGCD();}public void DoRolling(){int copyIndex = 0;int copyIndexNext = 0;int copycw = 0;//当服务器数量发生变化的时候，锁住该服务直到完毕。reloop:   while (_isServerChanging) ;for (;;){//拷贝本地的index，用做同步copyIndex = _index;//计算轮询的时候下一个的值copyIndexNext = (copyIndex + 1) % _servers.Count;//同步作用copycw = _currentWeight;//假定轮询后的Next=0，说明完成一轮轮询，权重减去最大公约数if (copyIndexNext == 0){copycw -= _gcdWeight;//如果权重已经扣完，重新从大的开始if (copycw <= 0){copycw = _maxWeight;}}//如果copyIndex和_index相同，说明是同一个线程抢到的，那么直接用本地的替换index进行替换if (Interlocked.CompareExchange(ref _index, copyIndexNext, copyIndex) == copyIndex){_currentWeight = copycw;try{//如果轮询的权重大于等于本地权重，选中它即可。if (_servers[copyIndexNext].Weight >= copycw){int t =  _servers[copyIndexNext].ChoosenCount;break;}}//如果是Index溢出，那么说明服务器数量肯定发生变化了，所以跳过此次轮询，等下一轮，不处理。catch (IndexOutOfRangeException){goto reloop;}}}}/// <summary>/// 移除指定的服务器/// </summary>public WeightedRobin RemoveByIndex(int index){_isServerChanging = true;var removedServer = _servers[index];_servers.RemoveAt(index);_gcdWeight = GetTotalGCD();_maxWeight = GetMaxWeight(_servers);_isServerChanging = false;return removedServer;}/// <summary>/// 增加新的服务器/// </summary>public void AddNewServer(int weight){_isServerChanging = true;_servers.Add(new WeightedRobin(weight, 0));_gcdWeight = GetTotalGCD();_maxWeight = GetMaxWeight(_servers);_isServerChanging = false;}/// <summary>/// 格式化输出结果/// </summary>public override string ToString(){StringBuilder sbu = new StringBuilder(10);foreach (WeightedRobin wr in _servers){sbu.AppendLine(wr.ToString() + Environment.NewLine);}return sbu.ToString();}}
}

using System;
using System.Threading;
using System.Threading.Tasks;namespace WBasedRobin
{class Program{static Random r = new Random();static void Rounding(WeightRobinRule wr){wr.DoRolling();}static void Main(string[] args){WeightRobinRule wr = new WeightRobinRule(5);Timer t = new Timer((j) => { var removedS = wr.RemoveByIndex(0); Console.WriteLine("移除了服务器："+removedS);  }, null, 2050, Timeout.Infinite);t = new Timer((o) => { wr.AddNewServer(6); Console.WriteLine("新增加服务器了。"); }, null, 3000, Timeout.Infinite);Parallel.For(1, 1001, (num) => {Thread.Sleep(r.Next(100, 500));Rounding(wr);});Console.WriteLine(wr);Console.ReadLine();}}
}