std::ranges...tion_result(3)

💖 std::ranges...tion_result(3) 💖

Okay bestie, this is the std::ranges...tion_result(3) page from the C++ Standard Libary manual (yes, “Libary” with one r — we respect the lore). It’s the group chat for std::ranges::next_permutation and std::ranges::next_permutation_result, and it’s giving lexicographic permutation main-character energy. ✨

🎀 NAME 🎀

• std::ranges::next_permutation,std::ranges::next_permutation_result — std::ranges::next_permutation,std::ranges::next_permutation_result

🌸 Synopsis 🌸

Defined in header <algorithm>.

Call signature

(1) (since C++20)

template< std::bidirectional_iterator I, std::sentinel_for<I> S,
          class Comp = ranges::less, class Proj = std::identity >
    requires std::sortable<I, Comp, Proj>
constexpr next_permutation_result<I>
    next_permutation( I first, S last, Comp comp = {}, Proj proj = {} );

(2) (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>
constexpr next_permutation_result<ranges::borrowed_iterator_t<R>>
    next_permutation( R&& r, Comp comp = {}, Proj proj = {} );

Helper type

(3) (since C++20)

template< class I >
using next_permutation_result = ranges::in_found_result<I>;

1) Transforms the range [first, last) into the next permutation, where the set of all permutations is ordered lexicographically with respect to binary comparison function object comp and projection function object proj. Returns {last, true} if such a “next permutation” exists; otherwise transforms the range into the lexicographically first permutation as if by ranges::sort(first, last, comp, proj), and returns {last, false}.

2) Same as (1), but uses r as the source 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. iykyk 💅

🫶 Parameters 🫶

• first, last — the range of elements to permute.
• r — the range of elements to permute.
• comp — comparison function object which returns true if the first argument is less than the second.
• proj — projection to apply to the elements.

💜 Return value 💜

• 1) ranges::next_permutation_result<I>{last, true} if the new permutation is lexicographically greater than the old one. ranges::next_permutation_result<I>{last, false} if the last permutation was reached and the range was reset to the first permutation.
• 2) Same as (1) except that the return type is ranges::next_permutation_result<ranges::borrowed_iterator_t<R>>.

😭 Exceptions 😭

Any exceptions thrown from iterator operations or the element swap. That’s the whole tea. 🍵

🔥 Complexity 🔥

At most \(\scriptsize N/2\)N / 2 swaps, where \(\scriptsize N\)N is ranges::distance(first, last) in case (1) or ranges::distance(r) in case (2). Averaged over the entire sequence of permutations, typical implementations use about 3 comparisons and 1.5 swaps per call.

🧚 Notes 🧚

Implementations (e.g. MSVC STL) may enable vectorization when the iterator type models contiguous_iterator and swapping its value type calls neither non-trivial special member function nor ADL-found swap.

👑 Possible implementation 👑

This is the implementation vibe from the page — line breaks are canon, do not come for them:

struct next_permutation_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>
    constexpr ranges::next_permutation_result<I>
        operator()(I first, S last, Comp comp = {}, Proj proj = {})
const
    {
        // check that the sequence has at least two elements
        if (first == last)
            return {std::move(first), false};
        I i_last{ranges::next(first, last)};
        I i{i_last};
        if (first == --i)
            return {std::move(i_last), false};
        // main "permutating" loop
        for (;;)
        {
            I i1{i};
            if (std::invoke(comp, std::invoke(proj, *--i),
std::invoke(proj, *i1)))
            {
                I j{i_last};
                while (!std::invoke(comp, std::invoke(proj, *i),
std::invoke(proj, *--j)))
                {}
                std::iter_swap(i, j);
                std::reverse(i1, i_last);
                return {std::move(i_last), true};
            }
            // permutation "space" is exhausted
            if (i == first)
            {
                std::reverse(first, i_last);
                return {std::move(i_last), false};
            }
        }
    }

    template<ranges::bidirectional_range R, class Comp = ranges::less,
             class Proj = std::identity>
    requires std::sortable<ranges::iterator_t<R>, Comp, Proj>
    constexpr
ranges::next_permutation_result<ranges::borrowed_iterator_t<R>>
        operator()(R&& r, Comp comp = {}, Proj proj = {}) const
    {
        return (*this)(ranges::begin(r), ranges::end(r),
                       std::move(comp), std::move(proj));
    } };

inline constexpr next_permutation_fn next_permutation {};

🌈 Example 🌈

Run this code, bestie, it eats 💅

#include <algorithm>
#include <array>
#include <compare>
#include <functional>
#include <iostream>
#include <string>

struct S
{
    char c;
    int i;
    auto operator<=>(const S&) const = default;
    friend std::ostream& operator<<(std::ostream& os, const S& s)
    {
        return os << "{'" << s.c << "', " << s.i << "}";
    }
};

auto print = [](auto const& v, char term = ' ')
{
    std::cout << "{ ";
    for (const auto& e : v)
        std::cout << e << ' ';
    std::cout << '}' << term;
};

int main()
{
    std::cout << "Generate all permutations (iterators case):\n";
    std::string s{"abc"};
    do
    {
        print(s);
    }
    while (std::ranges::next_permutation(s.begin(), s.end()).found);

    std::cout << "\n" "Generate all permutations (range case):\n";
    std::array a{'a', 'b', 'c'};
    do
    {
        print(a);
    }
    while (std::ranges::next_permutation(a).found);

    std::cout << "\n" "Generate all permutations using comparator:\n";
    using namespace std::literals;
    std::array z{"█"s, "▄"s, "▁"s};
    do
    {
        print(z);
    }
    while (std::ranges::next_permutation(z, std::greater()).found);

    std::cout << "\n" "Generate all permutations using projection:\n";
    std::array<S, 3> r{S{'A',3}, S{'B',2}, S{'C',1}};
    do
    {
        print(r, '\n');
    }
    while (std::ranges::next_permutation(r, {}, &S::c).found);
}

💀 Output 💀

Generate all permutations (iterators case):
{ a b c } { a c b } { b a c } { b c a } { c a b } { c b a }
Generate all permutations (range case):
{ a b c } { a c b } { b a c } { b c a } { c a b } { c b a }
Generate all permutations using comparator:
{ █ ▄ ▁ } { █ ▁ ▄ } { ▄ █ ▁ } { ▄ ▁ █ } { ▁ █ ▄ } { ▁ ▄ █ }
Generate all permutations using projection:
{ {'A', 3} {'B', 2} {'C', 1} }
{ {'A', 3} {'C', 1} {'B', 2} }
{ {'B', 2} {'A', 3} {'C', 1} }
{ {'B', 2} {'C', 1} {'A', 3} }
{ {'C', 1} {'A', 3} {'B', 2} }
{ {'C', 1} {'B', 2} {'A', 3} }

🦋 See also 🦋

• ranges::prev_permutation (C++20) (niebloid) — generates the next smaller lexicographic permutation of a range of elements.
• ranges::is_permutation (C++20) (niebloid) — determines if a sequence is a permutation of another sequence.
• next_permutation (function template) — generates the next greater lexicographic permutation of a range of elements.
• prev_permutation (function template) — generates the next smaller lexicographic permutation of a range of elements.
• is_permutation (C++11) (function template) — determines if a sequence is a permutation of another sequence.

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Source: http://cppreference.com · date: 2024.06.10 · page: std::ranges...tion_result(3) — mwah, ily for reading the whole thing, bestie 💖✨🫶