我的排序有什么问题？

Question

首先,我必须指定我在Unity 5.3中工作,而新的MonoDevelop不允许我进行调试.团结只是崩溃:(

所以我有一个"目标"列表,我需要根据3个标准​​进行排序:

1. 首先应列出"积极"目标
2. 然后按难度等级排序
3. 最后随机进入同一级别的目标

这是我的代码:

public class Goal {
    public int ID;
    public int Level;
    public bool Active;
}

...

List<Goal> goals;

goals.Sort((a, b) => {
    // first chooses the Active ones (if any)
    var sort = b.Active.CompareTo(a.Active);
    if (sort == 0) {
        // then sort by level
        sort = (a.Level).CompareTo(b.Level);
        // if same level, randomize. Returns -1, 0 or 1
        return sort == 0 ? UnityEngine.Random.Range(-1, 2) : sort;
    } else {
        return sort;
    }
});

当我运行此代码时,有时我会在非活动目标之后获得一个或多个活动目标,但我不明白为什么.

Answer 1

为了正确工作，排序算法不应依赖于变异状态。这里解释了为什么在比较值时使用随机生成器不是一个好主意。

该问题可以通过两种方式解决：

选项 1：预先计算随机数

    var tmp = goals.Select( g=> new {goal = g, weight = rnd.NextDouble()})
        .OrderByDescending(t=>t.goal.Active) // Active first
        .ThenBy(t=>t.goal.Level)
        .ThenBy(t=>t.weight)
        .Select(t=>t.goal)
        .ToList();



    goals.Clear();
    goals.AddRange(tmp);

工作样本

选项 2：排序然后打乱顺序

Random rnd = new Random();

Comparison<Goal> comparison =  (a, b) => {
// first chooses the Active ones (if any)
var sort = b.Active.CompareTo(a.Active);

if (sort == 0) {
// then sort by level
    return sort = (a.Level).CompareTo(b.Level);
  } else 
{
   return sort;
}
};



int startIndex = 0;
int endIndex = 0;

goals.Sort(comparison);

while (startIndex < goals.Count)
{
    for (endIndex = startIndex + 1; endIndex < goals.Count; ++endIndex)
    {
       if (comparison(goals[startIndex], goals[endIndex]) != 0)
       {
          //End of tie
          break;
       }
    }

    if (endIndex - startIndex > 1)
    {
       // Shuffle goals of the same level
       ShuffleRange(goals, startIndex, endIndex - startIndex, rnd);
    }

    startIndex = endIndex;
}   

static void ShuffleRange<T>(List<T> list, int startIndex, int count, Random rnd)
{
     int n = startIndex + count;  
     while (n > startIndex + 1) 
     {  
        int k = rnd.Next(startIndex, n--);  
        T value = list[k];  
        list[k] = list[n];  
        list[n] = value;  
    }  
}           

工作样本

Shuffle算法借鉴自这里