swift - Swift 为其标准库实现了什么排序算法？

Question

我想知道 Swift 的 sort 函数是如何实现的。它使用哪种排序算法——是归并排序、快速排序还是完全不同的算法？此函数提供的时间/复杂性保证是什么？

我在网上或官方文档中都找不到任何关于它是如何实现的说明。

Answer 1

更新 2: 正如我们在 Sort.swift 中看到的那样, sort()现在在 Swift 5 中使用“修改后的 timsort”。Timsort是一个

... hybrid stable sorting algorithm, derived from merge sort and insertion sort ...

In the worst case, Timsort takes O(n log n) comparisons to sort an array of n elements. In the best case, which occurs when the input is already sorted, it runs in linear time, meaning that it is an adaptive sorting algorithm.

这意味着sort()恰好是 Swift 5 中的稳定排序，但这仍然是一个实现细节。 MutableCollection.sort 文档指出

The sorting algorithm is not guaranteed to be stable. A stable sort preserves the relative order of elements that compare equal.

另见 Is sort() stable in Swift 5?在 Swift 论坛中:

The algorithm was only recently made stable in preparation for a proposal to make it guaranteed as such.

---

更新:Swift 现在是开源的，并且在

• https://github.com/apple/swift/blob/master/stdlib/public/core/Sort.swift

可以看到对集合进行排序是使用 introsort 完成的最大递归深度为 2*floor(log_2(N))。对于少于 20 个元素的分区，它切换到插入排序，或者堆排序如果达到递归深度。

---

旧答案:定义自定义 Comparable < 中断点的结构和设置:

struct MyStruct : Comparable {
    let val : Int
}

func ==(x: MyStruct, y: MyStruct) -> Bool {
    println("\(x.val) ==  \(y.val)")
    return x.val == y.val
}
func <(x: MyStruct, y: MyStruct) -> Bool {
    println("\(x.val) < \(y.val)")
    return x.val < y.val // <--- SET BREAKPOINT HERE
}

var array = [MyStruct]()
for _ in 1 ... 30 {
    array.append(MyStruct(val: Int(arc4random_uniform(1000))))
}
sort(&array)

显示以下堆栈回溯:

(lldb) bt* thread #1: tid = 0x5a00, 0x00000001001cb806 sort`sort. Swift.Bool + 454 at main.swift:22, queue = 'com.apple.main-thread', stop reason = breakpoint 1.1  * frame #0: 0x00000001001cb806 sort`sort. Swift.Bool + 454 at main.swift:22    frame #1: 0x00000001001cb62b sort`protocol witness for Swift._Comparable.(Swift._Comparable.Self.Type)(Swift._Comparable.Self, Swift._Comparable.Self) -> Swift.Bool in conformance sort.MyStruct : Swift._Comparable + 27 at main.swift:20    frame #2: 0x00000001000f5a98 sort`Swift._partition (inout A, Swift.Range) -> A.Index + 3224    frame #3: 0x00000001000f756a sort`Swift._introSortImpl (inout A, Swift.Range, Swift.Int) -> () + 2138    frame #4: 0x00000001000f6c01 sort`Swift._introSort (inout A, Swift.Range) -> () + 1233    frame #5: 0x00000001000fc47f sort`Swift.sort (inout A) -> () + 607    frame #6: 0x000000010013ea77 sort`partial apply forwarder for Swift.(sort (inout Swift.Array) -> ()).(closure #1) + 183    frame #7: 0x000000010013eaf8 sort`partial apply forwarder for reabstraction thunk helper  from @callee_owned (@inout Swift.UnsafeMutableBufferPointer) -> (@unowned ()) to @callee_owned (@inout Swift.UnsafeMutableBufferPointer) -> (@out ()) + 56    frame #8: 0x0000000100046c4b sort`Swift.Array.withUnsafeMutableBufferPointer (inout Swift.Array)((inout Swift.UnsafeMutableBufferPointer) -> B) -> B + 475    frame #9: 0x00000001000fc5ad sort`Swift.sort (inout Swift.Array) -> () + 157    frame #10: 0x00000001001cb465 sort`top_level_code + 1237 at main.swift:29    frame #11: 0x00000001001cbdca sort`main + 42 at main.swift:0    frame #12: 0x00007fff8aa9a5fd libdyld.dylib`start + 1

以后

(lldb) bt* thread #1: tid = 0x5a00, 0x00000001001cb806 sort`sort. Swift.Bool + 454 at main.swift:22, queue = 'com.apple.main-thread', stop reason = breakpoint 1.1  * frame #0: 0x00000001001cb806 sort`sort. Swift.Bool + 454 at main.swift:22    frame #1: 0x00000001001cb62b sort`protocol witness for Swift._Comparable.(Swift._Comparable.Self.Type)(Swift._Comparable.Self, Swift._Comparable.Self) -> Swift.Bool in conformance sort.MyStruct : Swift._Comparable + 27 at main.swift:20    frame #2: 0x00000001000f449e sort`Swift._insertionSort (inout A, Swift.Range) -> () + 2958    frame #3: 0x00000001000f730e sort`Swift._introSortImpl (inout A, Swift.Range, Swift.Int) -> () + 1534    frame #4: 0x00000001000f797d sort`Swift._introSortImpl (inout A, Swift.Range, Swift.Int) -> () + 3181    frame #5: 0x00000001000f6c01 sort`Swift._introSort (inout A, Swift.Range) -> () + 1233    frame #6: 0x00000001000fc47f sort`Swift.sort (inout A) -> () + 607    frame #7: 0x000000010013ea77 sort`partial apply forwarder for Swift.(sort (inout Swift.Array) -> ()).(closure #1) + 183    frame #8: 0x000000010013eaf8 sort`partial apply forwarder for reabstraction thunk helper  from @callee_owned (@inout Swift.UnsafeMutableBufferPointer) -> (@unowned ()) to @callee_owned (@inout Swift.UnsafeMutableBufferPointer) -> (@out ()) + 56    frame #9: 0x0000000100046c4b sort`Swift.Array.withUnsafeMutableBufferPointer (inout Swift.Array)((inout Swift.UnsafeMutableBufferPointer) -> B) -> B + 475    frame #10: 0x00000001000fc5ad sort`Swift.sort (inout Swift.Array) -> () + 157    frame #11: 0x00000001001cb465 sort`top_level_code + 1237 at main.swift:29    frame #12: 0x00000001001cbdca sort`main + 42 at main.swift:0    frame #13: 0x00007fff8aa9a5fd libdyld.dylib`start + 1

这证实了Airspeed's answer的猜想使用了introsort与用于较小范围的插入排序相结合。

如果数组少于 20 个元素，则似乎只使用插入排序。这可能表明从 introsort 切换到插入排序是 20。

当然， future 实现可能会发生变化。