Difference between revisions of "cpp/algorithm/ranges/inplace merge"

Revision as of 10:27, 10 July 2021

Defined in header `<algorithm>`
Call signature
template< std::bidirectional_iterator I, std::sentinel_for<I> S, class Comp = ranges::less, class Proj = std::identity > requires std::sortable<I, Comp, Proj> I inplace_merge( I first, I middle, S last, Comp comp = {}, Proj proj = {} );	(1)	(since C++20)
template< ranges::bidirectional_range R, class Comp = ranges::less, class Proj = std::identity > requires std::sortable<ranges::iterator_t<R>, Comp, Proj> ranges::borrowed_iterator_t<R> inplace_merge( R&& r, ranges::iterator_t<R> middle, Comp comp = {}, Proj proj = {} );	(2)	(since C++20)

Merges two consecutive sorted ranges [first, middle) and [middle, last) into one sorted range [first, last).

A sequence is said to be sorted with respect to the comparator comp and projection proj if for any iterator it pointing to the sequence and any non-negative integer n such that it + n is a valid iterator pointing to an element of the sequence, std::invoke(comp, std::invoke(proj, *(it + n)), std::invoke(proj, *it))) evaluates to false.

This merge function is stable, which means that for equivalent elements in the original two ranges, the elements from the first range (preserving their original order) precede the elements from the second range (preserving their original order).

1) Elements are compared using the given binary comparison function comp and projection object proj, and the ranges must be sorted with respect to the same.

2) Same as (1), but uses r as the range, as if using ranges::begin(r) as first, and ranges::end(r) as last.

The function-like entities described on this page are niebloids, that is:

Explicit template argument lists cannot be specified when calling any of them.
None of them are visible to argument-dependent lookup.
When any of them are found by normal unqualified lookup as the name to the left of the function-call operator, argument-dependent lookup is inhibited.

In practice, they may be implemented as function objects, or with special compiler extensions.

struct inplace_merge_fn {
    template<std::bidirectional_iterator I, std::sentinel_for<I> S,
             class Comp = ranges::less, class Proj = std::identity>
        requires std::sortable<I, Comp, Proj>
    I operator()(I first, I middle, S last, Comp comp = {}, Proj proj = {}) const {
        I last_it = ranges::next(middle, last);
        inplace_merge_slow(first, middle, last_it,
                           ranges::distance(first, middle),
                           ranges::distance(middle, last_it),
                           std::ref(comp), std::ref(proj));
        return last_it;
    }
 
    template<ranges::bidirectional_range R, class Comp = ranges::less,
             class Proj = std::identity>
        requires std::sortable<ranges::iterator_t<R>, Comp, Proj>
    ranges::borrowed_iterator_t<R>
    operator()(R&& r, ranges::iterator_t<R> middle,
               Comp comp = {}, Proj proj = {}) const {
        return (*this)(ranges::begin(r), std::move(middle), ranges::end(r),
                       std::move(comp), std::move(proj));
    }
 
private:
    template<class I, class Comp, class Proj>
    static void inplace_merge_slow(I first, I middle, I last, std::iter_difference_t<I> n1,
                                   std::iter_difference_t<I> n2, Comp comp, Proj proj) {
        if (n1 == 0 || n2 == 0)
            return;
        if (n1 + n2 == 2 && comp(proj(*middle), proj(*first))) {
            ranges::iter_swap(first, middle);
            return;
        }
 
        I cut1 = first, cut2 = middle;
        std::iter_difference_t<I> d1{}, d2{};
 
        if (n1 > n2) {
            d1 = n1 / 2;
            ranges::advance(cut1, d1);
            cut2 = ranges::lower_bound(middle, last, *cut1,
                                       std::ref(comp), std::ref(proj));
            d2 = ranges::distance(middle, cut2);
        }
        else {
            d2 = n2 / 2;
            ranges::advance(cut2, d2);
            cut1 = ranges::upper_bound(first, middle, *cut2,
                                       std::ref(comp), std::ref(proj));
            d1 = ranges::distance(first, cut1);
        }
 
        I new_middle = ranges::rotate(cut1, middle, cut2);
        inplace_merge_slow(first, cut1, new_middle, d1, d2,
                           std::ref(comp), std::ref(proj));
        inplace_merge_slow(new_middle, cut2, last, n1 - d1, n2 - d2,
                           std::ref(comp), std::ref(proj));
    }
};
 
inline constexpr inplace_merge_fn inplace_merge{};

Example

Run this code

#include <algorithm>
#include <complex>
#include <functional>
#include <iostream>
#include <iterator>
#include <vector>
 
void print(auto const& v, auto const& rem, int middle = -1)
{
    for (int i{}; auto n : v)
        std::cout << (i++ == middle ? "│ " : "") << n << ' ';
    std::cout << rem << '\n';
}
 
template <std::random_access_iterator I, std::sentinel_for<I> S>
requires std::sortable<I>
void merge_sort(I first, S last)
{
    if (last - first > 1) {
        I middle {first + (last - first) / 2};
        merge_sort(first, middle);
        merge_sort(middle, last);
        std::ranges::inplace_merge(first, middle, last);
    }
}
 
int main()
{
    // custom merge-sort demo
    std::vector v {8, 2, 0, 4, 9, 8, 1, 7, 3};
    print(v, ": before sort");
    merge_sort(v.begin(), v.end());
    print(v, ": after sort\n");
 
    // merging with comparison function object and projection
    using CI = std::complex<int>;
    std::vector<CI> r { {0,1}, {0,2}, {0,3}, {1,1}, {1,2} };
    const auto middle { std::ranges::next(r.begin(), 3) };
    auto comp { std::ranges::less{} };
    auto proj { [](CI z) { return z.imag(); } };
 
    print(r, ": before merge", middle - r.begin());
    std::ranges::inplace_merge(r, middle, comp, proj);
    print(r, ": after merge");
}

Output:

8 2 0 4 9 8 1 7 3 : before sort
0 1 2 3 4 7 8 8 9 : after sort
 
(0,1) (0,2) (0,3) │ (1,1) (1,2) : before merge
(0,1) (1,1) (0,2) (1,2) (0,3) : after merge

Compiler support
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Language
Standard library
Standard library headers
Named requirements
Feature test macros (C++20)
Language support library
Concepts library (C++20)
Metaprogramming library (C++11)
Diagnostics library
General utilities library
Strings library
Containers library
Iterators library
Ranges library (C++20)
Algorithms library
Numerics library
Localizations library
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Filesystem library (C++17)
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Concurrency support library (C++11)
Execution support library (C++26)
Technical specifications
Symbols index
External libraries

first	-	the beginning of the first sorted range
middle	-	the end of the first range and the beginning of the second range
last	-	the end of the second sorted range
r	-	the range of elements to merge inplace
comp	-	comparison to apply to the projected elements
proj	-	projection to apply to the elements in the range.

ranges::merge (C++20)	merges two sorted ranges (niebloid)[edit]
ranges::set_union (C++20)	computes the union of two sets (niebloid)[edit]
ranges::is_sorted (C++20)	checks whether a range is sorted into ascending order (niebloid)[edit]
ranges::sort (C++20)	sorts a range into ascending order (niebloid)[edit]
inplace_merge	merges two ordered ranges in-place (function template) [edit]

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Difference between revisions of "cpp/algorithm/ranges/inplace merge"

Revision as of 10:27, 10 July 2021

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Return value

Complexity

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Possible implementation

Example

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