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Add an explicit stable_adapter<verge_sorter> specializations
An likewise for stable_adapter<verge_adapter>. Instead of simply relying on make_stable, the new specializations use a variant of vergesort that detects strictly descending runs instead of non-ascending ones, and wraps the fallback sorter in stable_t. The technique of detecting non-descending and strictly descending runs is a trick borrowed from timsort in order to preserve stability in natural mergesort algorithms: descending runs are reversed in-place, so equivalent elements wouldn't retain their original order when reversed in such runs. While verge_adapter does not handle bidirectional iterators, the underlying code was changed so that it will be easier to do in the future, but currently this involves some dirty tricks with a stripped down implementation of a sized iterator. This will make the transition to C++20 ranges and standard sized iterators and sentinels easier in the long term. Meanwhile it is just an implementation detail. This commit addresses external issue Morwenn/vergesort#11 and somehow Morwenn/vergesort#7 too. cpp-sort is a better recipient for those variations on vergesort than the standalone project, but some cross-project documentation will be needed anyway.
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| Original file line number | Diff line number | Diff line change |
|---|---|---|
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| /* | ||
| * Copyright (c) 2020 Morwenn | ||
| * SPDX-License-Identifier: MIT | ||
| */ | ||
| #ifndef CPPSORT_DETAIL_SIZED_ITERATOR_H_ | ||
| #define CPPSORT_DETAIL_SIZED_ITERATOR_H_ | ||
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| //////////////////////////////////////////////////////////// | ||
| // Headers | ||
| //////////////////////////////////////////////////////////// | ||
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| namespace cppsort | ||
| { | ||
| namespace detail | ||
| { | ||
| //////////////////////////////////////////////////////////// | ||
| // Mostly a hack to avoid some gratuitous performance loss | ||
| // by passing bidirectional iterators + size to a function | ||
| // accepting a pair of iterators. It is worse than the | ||
| // equivalent C++20 features, but should be good enough for | ||
| // the internal use we make of it. | ||
| // | ||
| // NOTE: the full iterator features are not provided, this | ||
| // is intentional to avoid unintentional uses of the | ||
| // class in the library's internals. | ||
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| template<typename Iterator> | ||
| class sized_iterator | ||
| { | ||
| public: | ||
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| //////////////////////////////////////////////////////////// | ||
| // Public types | ||
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| using iterator_category = iterator_category_t<Iterator>; | ||
| using iterator_type = Iterator; | ||
| using value_type = value_type_t<Iterator>; | ||
| using difference_type = difference_type_t<Iterator>; | ||
| using pointer = pointer_t<Iterator>; | ||
| using reference = reference_t<Iterator>; | ||
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| //////////////////////////////////////////////////////////// | ||
| // Constructors | ||
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| sized_iterator() = default; | ||
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| constexpr sized_iterator(Iterator it, difference_type size): | ||
| _it(std::move(it)), | ||
| _size(size) | ||
| {} | ||
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| //////////////////////////////////////////////////////////// | ||
| // Members access | ||
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| auto base() const | ||
| -> iterator_type | ||
| { | ||
| return _it; | ||
| } | ||
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| auto size() const | ||
| -> difference_type | ||
| { | ||
| return _size; | ||
| } | ||
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| //////////////////////////////////////////////////////////// | ||
| // Element access | ||
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| auto operator*() const | ||
| -> reference | ||
| { | ||
| return *_it; | ||
| } | ||
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| auto operator->() const | ||
| -> pointer | ||
| { | ||
| return &(operator*()); | ||
| } | ||
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| private: | ||
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| Iterator _it; | ||
| difference_type _size; | ||
| }; | ||
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| // Alternative to std::distance meant to be picked up by ADL in | ||
| // specific places, uses the size of the *second* iterator | ||
| template<typename Iterator> | ||
| constexpr auto distance(sized_iterator<Iterator>, sized_iterator<Iterator> last) | ||
| -> difference_type_t<Iterator> | ||
| { | ||
| return last.size(); | ||
| } | ||
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| template<typename Iterator> | ||
| auto make_sized_iterator(Iterator it, difference_type_t<Iterator> size) | ||
| -> sized_iterator<Iterator> | ||
| { | ||
| return { it, size }; | ||
| } | ||
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| }} | ||
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| #endif // CPPSORT_DETAIL_SIZED_ITERATOR_H_ |
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