.NET 排序 Array.SortT 实现分析

System.Array.Sort<T> 是.NET内置的排序方法, 灵活且高效, 大家都学过一些排序算法，比如冒泡排序,插入排序,堆排序等，不过你知道这个方法背后使用了什么排序算法吗?

先说结果, 实际上 Array.Sort 不止使用了一种排序算法, 为了保证不同的数据量的排序场景，都能有一个高性能的表现，实现中包括了插入排序,堆排序和快速排序, 接下来从通过源码看看它都做了哪些事情。

Array.Sort

https://source.dot.net/#System.Private.CoreLib/Array.cs,ec5718fae85b7640

public static void Sort<T>(T[] array)
{if (array == null)ThrowHelper.ThrowArgumentNullException(ExceptionArgument.array);if (array.Length > 1){var span = new Span<T>(ref MemoryMarshal.GetArrayDataReference(array), array.Length);ArraySortHelper<T>.Default.Sort(span, null);}
}

这里我们对 int 数组进行排序, 先看一下这个Sort方法, 当数组的长度大于1时, 会先把数组转成 Span 列表, 然后调用了内部的ArraySortHelper的Default对象的Sort方法。

ArraySortHelper

[TypeDependency("System.Collections.Generic.GenericArraySortHelper`1")]
internal sealed partial class ArraySortHelper<T>: IArraySortHelper<T>
{private static readonly IArraySortHelper<T> s_defaultArraySortHelper = CreateArraySortHelper();public static IArraySortHelper<T> Default => s_defaultArraySortHelper;[DynamicDependency("#ctor", typeof(GenericArraySortHelper<>))]private static IArraySortHelper<T> CreateArraySortHelper(){IArraySortHelper<T> defaultArraySortHelper;if (typeof(IComparable<T>).IsAssignableFrom(typeof(T))){defaultArraySortHelper = (IArraySortHelper<T>)RuntimeTypeHandle.CreateInstanceForAnotherGenericParameter((RuntimeType)typeof(GenericArraySortHelper<string>), (RuntimeType)typeof(T));}else{defaultArraySortHelper = new ArraySortHelper<T>();}return defaultArraySortHelper;}
}

Default 会根据是否实现了 IComparable<T> 接口来创建不同的 ArraySortHelper, 因为上面我对int数组进行排序, 所以调用的是 GenericArraySortHelper 的Sort方法。

GenericArraySortHelper

https://source.dot.net/#System.Private.CoreLib/ArraySortHelper.cs,280

internal sealed partial class GenericArraySortHelper<T>where T : IComparable<T>{// Do not add a constructor to this class because ArraySortHelper<T>.CreateSortHelper will not execute it#region IArraySortHelper<T> Memberspublic void Sort(Span<T> keys, IComparer<T>? comparer){try{if (comparer == null || comparer == Comparer<T>.Default){if (keys.Length > 1){// For floating-point, do a pre-pass to move all NaNs to the beginning// so that we can do an optimized comparison as part of the actual sort// on the remainder of the values.if (typeof(T) == typeof(double) ||typeof(T) == typeof(float) ||typeof(T) == typeof(Half)){int nanLeft = SortUtils.MoveNansToFront(keys, default(Span<byte>));if (nanLeft == keys.Length){return;}keys = keys.Slice(nanLeft);}IntroSort(keys, 2 * (BitOperations.Log2((uint)keys.Length) + 1));}}else{ArraySortHelper<T>.IntrospectiveSort(keys, comparer.Compare);}}catch (IndexOutOfRangeException){ThrowHelper.ThrowArgumentException_BadComparer(comparer);}catch (Exception e){ThrowHelper.ThrowInvalidOperationException(ExceptionResource.InvalidOperation_IComparerFailed, e);}}

首先会判断排序的类型是否是浮点型, 如果是的会做一些排序的调整优化，然后调用了 IntroSort 方法，并传入了两个参数，第一个Keys就是数组的Span列表，那第二个是什么呢? 它是一个int类型的depthLimit参数，这里简单点理解就是算出数组的深度，因为后边会根据这个值进行递归操作，然后进入到 IntroSort 方法。

IntroSort

到这个方法这里就清晰很多了, 这是Array.Sort<T> 排序的主要内容，接着往下看

https://source.dot.net/#System.Private.CoreLib/ArraySortHelper.cs,404

private static void IntroSort(Span<T> keys, int depthLimit)
{Debug.Assert(!keys.IsEmpty);Debug.Assert(depthLimit >= 0);int partitionSize = keys.Length;while (partitionSize > 1){if (partitionSize <= Array.IntrosortSizeThreshold){if (partitionSize == 2){SwapIfGreater(ref keys[0], ref keys[1]);return;}if (partitionSize == 3){ref T hiRef = ref keys[2];ref T him1Ref = ref keys[1];ref T loRef = ref keys[0];SwapIfGreater(ref loRef, ref him1Ref);SwapIfGreater(ref loRef, ref hiRef);SwapIfGreater(ref him1Ref, ref hiRef);return;}InsertionSort(keys.Slice(0, partitionSize));return;}if (depthLimit == 0){HeapSort(keys.Slice(0, partitionSize));return;}depthLimit--;int p = PickPivotAndPartition(keys.Slice(0, partitionSize));// Note we've already partitioned around the pivot and do not have to move the pivot again.IntroSort(keys[(p+1)..partitionSize], depthLimit);partitionSize = p;}
}

第一次进入方法时，partitionSize 就是数组的长度, 这里有一个判断条件，如下, IntrosortSizeThreshold 是一个值为16的常量，它是一个阈值, 如果数组的长度小于等于16, 那么使用的就是插入排序(InsertionSort), 为什么是16呢？这里通过注释了解到, 从经验上来看, 16及以下的数组长度使用插入排序的效率是比较高的。

if (partitionSize <= Array.IntrosortSizeThreshold)
{if (partitionSize == 2){SwapIfGreater(ref keys[0], ref keys[1]);return;}if (partitionSize == 3){ref T hiRef = ref keys[2];ref T him1Ref = ref keys[1];ref T loRef = ref keys[0];SwapIfGreater(ref loRef, ref him1Ref);SwapIfGreater(ref loRef, ref hiRef);SwapIfGreater(ref him1Ref, ref hiRef);return;}InsertionSort(keys.Slice(0, partitionSize));return;
}

InsertionSort

如果数组的长度小于等于3时, 直接进行对比交换, 如果长度大约3并且小于等于16的话, 使用插入排序(InsertionSort), 方法内容如下:

https://source.dot.net/#System.Private.CoreLib/ArraySortHelper.cs,537

private static void InsertionSort(Span<T> keys)
{for (int i = 0; i < keys.Length - 1; i++){T t = Unsafe.Add(ref MemoryMarshal.GetReference(keys), i + 1);int j = i;while (j >= 0 && (t == null || LessThan(ref t, ref Unsafe.Add(ref MemoryMarshal.GetReference(keys), j)))){Unsafe.Add(ref MemoryMarshal.GetReference(keys), j + 1) = Unsafe.Add(ref MemoryMarshal.GetReference(keys), j);j--;}Unsafe.Add(ref MemoryMarshal.GetReference(keys), j + 1) = t!;}
}

HeapSort

if (depthLimit == 0)
{HeapSort(keys.Slice(0, partitionSize));return;
}
depthLimit--;

因为后边是递归操作，所以每次 depthLimit 都会减1, 当深度为0排序还没有完成的时候，就会直接使用堆排序(HeapSort)，方法内容如下：

https://source.dot.net/#System.Private.CoreLib/ArraySortHelper.cs,990

private static void HeapSort(Span<TKey> keys, Span<TValue> values)
{Debug.Assert(!keys.IsEmpty);int n = keys.Length;for (int i = n >> 1; i >= 1; i--){DownHeap(keys, values, i, n);}for (int i = n; i > 1; i--){Swap(keys, values, 0, i - 1);DownHeap(keys, values, 1, i - 1);}
}private static void DownHeap(Span<TKey> keys, Span<TValue> values, int i, int n)
{TKey d = keys[i - 1];TValue dValue = values[i - 1];while (i <= n >> 1){int child = 2 * i;if (child < n && (keys[child - 1] == null || LessThan(ref keys[child - 1], ref keys[child]))){child++;}if (keys[child - 1] == null || !LessThan(ref d, ref keys[child - 1]))break;keys[i - 1] = keys[child - 1];values[i - 1] = values[child - 1];i = child;}keys[i - 1] = d;values[i - 1] = dValue;
}

QuickSort

int p = PickPivotAndPartition(keys.Slice(0, partitionSize), values.Slice(0, partitionSize));IntroSort(keys[(p+1)..partitionSize], values[(p+1)..partitionSize], depthLimit);
partitionSize = p;

这里调用了另外一个方法 PickPivotAndPartition, Pivot 基准, Partition 分区, 这就是快速排序呀！而且还是使用了尾递归的快速排序，其中也使用了三数取中法，方法内容如下

https://source.dot.net/#System.Private.CoreLib/ArraySortHelper.cs,945

private static int PickPivotAndPartition(Span<TKey> keys, Span<TValue> values)
{Debug.Assert(keys.Length >= Array.IntrosortSizeThreshold);int hi = keys.Length - 1;// Compute median-of-three.  But also partition them, since we've done the comparison.int middle = hi >> 1;// Sort lo, mid and hi appropriately, then pick mid as the pivot.SwapIfGreaterWithValues(keys, values, 0, middle);  // swap the low with the mid pointSwapIfGreaterWithValues(keys, values, 0, hi);   // swap the low with the highSwapIfGreaterWithValues(keys, values, middle, hi); // swap the middle with the highTKey pivot = keys[middle];Swap(keys, values, middle, hi - 1);int left = 0, right = hi - 1;  // We already partitioned lo and hi and put the pivot in hi - 1.  And we pre-increment & decrement below.while (left < right){if (pivot == null){while (left < (hi - 1) && keys[++left] == null) ;while (right > 0 && keys[--right] != null) ;}else{while (GreaterThan(ref pivot, ref keys[++left])) ;while (LessThan(ref pivot, ref keys[--right])) ;}if (left >= right)break;Swap(keys, values, left, right);}// Put pivot in the right location.if (left != hi - 1){Swap(keys, values, left, hi - 1);}return left;
}