.NET源代码的内部排序实现

使用JetBrains的DotPeek工具能够方便地查看.net的部分源代码。于是看了一下.NET的内部是怎样实现排序的算法。

在System.Collections.Generic 命名空间下能够看到ArraySortHelper<T>的实现。

public void Sort(T[] keys, int index, int length, IComparer<T> comparer)
    {
      try
      {
        if (comparer == null)
          comparer = (IComparer<T>) Comparer<T>.Default;
        if (BinaryCompatibility.TargetsAtLeast_Desktop_V4_5)
          ArraySortHelper<T>.IntrospectiveSort(keys, index, length, comparer);
        else
          ArraySortHelper<T>.DepthLimitedQuickSort(keys, index, length + index - 1, comparer, 32);
      }
      catch (IndexOutOfRangeException ex)
      {
        IntrospectiveSortUtilities.ThrowOrIgnoreBadComparer((object) comparer);
      }
      catch (Exception ex)
      {
        throw new InvalidOperationException(Environment.GetResourceString("InvalidOperation_IComparerFailed"), ex);
      }
    } 

发如今.NET4.5以上的版本号，開始使用一种叫做 Introspective Sort的排序方法。

 internal static void IntrospectiveSort(T[] keys, int left, int length, IComparer<T> comparer)
    {
      if (length < 2)
        return;
      ArraySortHelper<T>.IntroSort(keys, left, length + left - 1, 2 * IntrospectiveSortUtilities.FloorLog2(keys.Length), comparer);
    }  
 
    private static void IntroSort(T[] keys, int lo, int hi, int depthLimit, IComparer<T> comparer)
    {
      for (; hi > lo; {
        int num;
        hi = num - 1;
      }
      )
      {
        int num = hi - lo + 1;
        if (num <= 16)
        {
          if (num == 1)
            break;
          if (num == 2)
          {
            ArraySortHelper<T>.SwapIfGreater(keys, comparer, lo, hi);
            break;
          }
          else if (num == 3)
          {
            ArraySortHelper<T>.SwapIfGreater(keys, comparer, lo, hi - 1);
            ArraySortHelper<T>.SwapIfGreater(keys, comparer, lo, hi);
            ArraySortHelper<T>.SwapIfGreater(keys, comparer, hi - 1, hi);
            break;
          }
          else
          {
            ArraySortHelper<T>.InsertionSort(keys, lo, hi, comparer);
            break;
          }
        }
        else if (depthLimit == 0)
        {
          ArraySortHelper<T>.Heapsort(keys, lo, hi, comparer);
          break;
        }
        else
        {
          --depthLimit;
          num = ArraySortHelper<T>.PickPivotAndPartition(keys, lo, hi, comparer);
          ArraySortHelper<T>.IntroSort(keys, num + 1, hi, depthLimit, comparer);
        }
      }
    }  
 
    private static int PickPivotAndPartition(T[] keys, int lo, int hi, IComparer<T> comparer)
    {
      int index = lo + (hi - lo) / 2;
      ArraySortHelper<T>.SwapIfGreater(keys, comparer, lo, index);
      ArraySortHelper<T>.SwapIfGreater(keys, comparer, lo, hi);
      ArraySortHelper<T>.SwapIfGreater(keys, comparer, index, hi);
      T obj = keys[index];
      ArraySortHelper<T>.Swap(keys, index, hi - 1);
      int i = lo;
      int j = hi - 1;
      while (i < j)
      {
        do
          ;
        while (comparer.Compare(keys[++i], obj) < 0);
        do
          ;
        while (comparer.Compare(obj, keys[--j]) < 0);
        if (i < j)
          ArraySortHelper<T>.Swap(keys, i, j);
        else
          break;
      }
      ArraySortHelper<T>.Swap(keys, i, hi - 1);
      return i;
    }  

而.NET4.5下面使用的是还有一种排序的方案。

在排序的数字小于16个的时候，直接使用插入排序。

private static void InsertionSort(T[] keys, int lo, int hi, IComparer<T> comparer)
    {
      for (int index1 = lo; index1 < hi; ++index1)
      {
        int index2 = index1;
        T x;
        for (x = keys[index1 + 1]; index2 >= lo && comparer.Compare(x, keys[index2]) < 0; --index2)
          keys[index2 + 1] = keys[index2];
        keys[index2 + 1] = x;
      }
    }  

而假设大于16个的时候，且当递归深度在32次之内的话（也就是数字小于4GB的数量时），使用高速排序。

internal static void DepthLimitedQuickSort(T[] keys, int left, int right, IComparer<T> comparer, int depthLimit)
    {
      while (depthLimit != 0)
      {
        int index1 = left;
        int index2 = right;
        int index3 = index1 + (index2 - index1 >> 1);
        ArraySortHelper<T>.SwapIfGreater(keys, comparer, index1, index3);
        ArraySortHelper<T>.SwapIfGreater(keys, comparer, index1, index2);
        ArraySortHelper<T>.SwapIfGreater(keys, comparer, index3, index2);
        T obj1 = keys[index3];
        do
        {
          while (comparer.Compare(keys[index1], obj1) < 0)
            ++index1;
          while (comparer.Compare(obj1, keys[index2]) < 0)
            --index2;
          if (index1 <= index2)
          {
            if (index1 < index2)
            {
              T obj2 = keys[index1];
              keys[index1] = keys[index2];
              keys[index2] = obj2;
            }
            ++index1;
            --index2;
          }
          else
            break;
        }
        while (index1 <= index2);
        --depthLimit;
        if (index2 - left <= right - index1)
        {
          if (left < index2)
            ArraySortHelper<T>.DepthLimitedQuickSort(keys, left, index2, comparer, depthLimit);
          left = index1;
        }
        else
        {
          if (index1 < right)
            ArraySortHelper<T>.DepthLimitedQuickSort(keys, index1, right, comparer, depthLimit);
          right = index2;
        }
        if (left >= right)
          return;
      }
      ArraySortHelper<T>.Heapsort(keys, left, right, comparer);
    }  

而假设大于4GB的数量时，使用堆排序。

private static void Heapsort(T[] keys, int lo, int hi, IComparer<T> comparer)
    {
      int n = hi - lo + 1;
      for (int i = n / 2; i >= 1; --i)
        ArraySortHelper<T>.DownHeap(keys, i, n, lo, comparer);
      for (int index = n; index > 1; --index)
      {
        ArraySortHelper<T>.Swap(keys, lo, lo + index - 1);
        ArraySortHelper<T>.DownHeap(keys, 1, index - 1, lo, comparer);
      }
    }  
 
    private static void DownHeap(T[] keys, int i, int n, int lo, IComparer<T> comparer)
    {
      T x = keys[lo + i - 1];
      for (; i <= n / 2; {
        int num;
        i = num;
      }
      )
      {
        num = 2 * i;
        if (num < n && comparer.Compare(keys[lo + num - 1], keys[lo + num]) < 0)
          ++num;
        if (comparer.Compare(x, keys[lo + num - 1]) < 0)
          keys[lo + i - 1] = keys[lo + num - 1];
        else
          break;
      }
      keys[lo + i - 1] = x;
    }  

最后，附上swap函数的实现：

private static void SwapIfGreater(T[] keys, IComparer<T> comparer, int a, int b)
    {
      if (a == b || comparer.Compare(keys[a], keys[b]) <= 0)
        return;
      T obj = keys[a];
      keys[a] = keys[b];
      keys[b] = obj;
    }  
 
    private static void Swap(T[] a, int i, int j)
    {
      if (i == j)
        return;
      T obj = a[i];
      a[i] = a[j];
      a[j] = obj;
    }