115 lines
4.9 KiB
Markdown
115 lines
4.9 KiB
Markdown
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---
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title: "C++ 的 sort 实现"
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date: 2023-07-29T15:09:46+08:00
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tags: []
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categories: []
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weight: 50
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show_comments: true
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draft: false
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---
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目前 C++ 中 std::sort 的实现使用的是 [内省排序 （Introsort）][1]、和插入排序共用的方式。当总元素数量小于一个阈值时（通常是 16），那么 sort 将只会使用插入排序。在内省排序中，当到达一定深度时（深度为元素数量的对数值），排序将转为堆排序。
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<!--more-->
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```cpp
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/// This is a helper function...
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template<typename _RandomAccessIterator, typename _Compare>
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_GLIBCXX20_CONSTEXPR
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inline _RandomAccessIterator
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__unguarded_partition_pivot(_RandomAccessIterator __first,
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_RandomAccessIterator __last, _Compare __comp)
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{
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_RandomAccessIterator __mid = __first + (__last - __first) / 2;
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std::__move_median_to_first(__first, __first + 1, __mid, __last - 1,
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__comp);
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return std::__unguarded_partition(__first + 1, __last, __first, __comp);
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}
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template<typename _RandomAccessIterator, typename _Compare>
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_GLIBCXX20_CONSTEXPR
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inline void
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__partial_sort(_RandomAccessIterator __first,
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_RandomAccessIterator __middle,
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_RandomAccessIterator __last,
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_Compare __comp)
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{
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std::__heap_select(__first, __middle, __last, __comp);
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std::__sort_heap(__first, __middle, __comp);
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}
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/// This is a helper function for the sort routine.
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template<typename _RandomAccessIterator, typename _Size, typename _Compare>
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_GLIBCXX20_CONSTEXPR
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void
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__introsort_loop(_RandomAccessIterator __first,
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_RandomAccessIterator __last,
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_Size __depth_limit, _Compare __comp)
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{
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while (__last - __first > int(_S_threshold))
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{
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if (__depth_limit == 0)
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{
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std::__partial_sort(__first, __last, __last, __comp);
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return;
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}
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--__depth_limit;
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_RandomAccessIterator __cut =
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std::__unguarded_partition_pivot(__first, __last, __comp);
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std::__introsort_loop(__cut, __last, __depth_limit, __comp);
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__last = __cut;
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}
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}
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template<typename _RandomAccessIterator, typename _Compare>
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_GLIBCXX20_CONSTEXPR
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inline void
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__sort(_RandomAccessIterator __first, _RandomAccessIterator __last,
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_Compare __comp)
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{
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if (__first != __last)
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{
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std::__introsort_loop(__first, __last,
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std::__lg(__last - __first) * 2,
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__comp);
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std::__final_insertion_sort(__first, __last, __comp);
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}
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}
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/**
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* @brief Sort the elements of a sequence.
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* @ingroup sorting_algorithms
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* @param __first An iterator.
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* @param __last Another iterator.
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* @return Nothing.
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*
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* Sorts the elements in the range `[__first, __last)` in ascending order,
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* such that for each iterator `i` in the range `[__first, __last - 1)`,
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* `*(i+1) < *i` is false.
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*
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* The relative ordering of equivalent elements is not preserved, use
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* `stable_sort()` if this is needed.
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*/
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template<typename _RandomAccessIterator>
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_GLIBCXX20_CONSTEXPR
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inline void
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sort(_RandomAccessIterator __first, _RandomAccessIterator __last)
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{
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// concept requirements
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__glibcxx_function_requires(_Mutable_RandomAccessIteratorConcept<
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_RandomAccessIterator>)
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__glibcxx_function_requires(_LessThanComparableConcept<
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typename iterator_traits<_RandomAccessIterator>::value_type>)
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__glibcxx_requires_valid_range(__first, __last);
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__glibcxx_requires_irreflexive(__first, __last);
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std::__sort(__first, __last, __gnu_cxx::__ops::__iter_less_iter());
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}
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```
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这里的函数**从下往上**看，外部调用的 `std::sort` 是下面第一个函数，它经过一些断言以后直接使用了 `std::__sort`，也就是倒数第二个函数。在 `std::__sort` 中，当容器内部不为空的时候，固定先使用内省排序再使用插入排序。在内省排序（`std::__introsort_loop`）中，若容器元素数量小于阈值就什么也不做，就相当于在数量较小时只使用插入排序，当达到深度限制以后，将转为堆排序（即 `std::__partial_sort` 的实现），否则（`std::__unguarded_partition_pivot`）将选取处于容器中间位置的元素作为基准，将小于基准的移动到左侧，大于基准的移动到右侧，并递归调用，即快速排序的思路。
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关于深度限制为 2 倍的容器元素个数的对数，在理想情况下容器每次被二等分时是永远到达不了这个限制的，但需要注意基准的选择是直接选取位于中间位置的元素，也因此极大概率是无法二等分的，此时就能够达到容器深度的限制了。
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[1]: https://zh.wikipedia.org/wiki/%E5%86%85%E7%9C%81%E6%8E%92%E5%BA%8F
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