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> Members public 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及以下得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 point SwapIfGreaterWithValues(keys, values, 0, hi); // swap the low with the high SwapIfGreaterWithValues(keys, values, middle, hi); // swap the middle with the high TKey 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; }
总结
本文主要介绍了System.Array.Sort<T> 排序的内部实现, 发现它使用了插入排序,堆排序和快速排序,大家有兴趣可以看一下Java或者Golang的排序实现,希望对您有用。
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