std::ranges::shuffle() algorithm

od C++20

Simplified

// (1)
I shuffle( I first, S last, Gen&& gen );

// (2)
ranges::borrowed_iterator_t<R> shuffle( R&& r, Gen&& gen );

The type of arguments are generic and have the following constraints:

• I - std::random_access_iterator
• S - std::sentinel_for<I>
• R - std::ranges::random_access_range
• G - (none)

Additionally, each overload has the following constraints:

• (1) - permutable<I> && uniform_random_bit_generator<remove_reference_t<Gen>>
• (2) - permutable<ranges::iterator_t<R>> && uniform_random_bit_generator<remove_reference_t<Gen>>

(The std:: namespace was omitted for readability)

Detailed

// (1)
template<
  std::random_access_iterator I,
  std::sentinel_for<I> S,
  class Gen
>
requires std::permutable<I> &&
         std::uniform_random_bit_generator<std::remove_reference_t<Gen>>
I shuffle( I first, S last, Gen&& gen );

// (2)
template<
  ranges::random_access_range R,
  class Gen
>
requires std::permutable<ranges::iterator_t<R>> &&
         std::uniform_random_bit_generator<std::remove_reference_t<Gen>>
ranges::borrowed_iterator_t<R> shuffle( R&& r, Gen&& gen );

• (1) Reorders the elements in the given range [first; last) such that each possible permutation of those elements has equal probability of appearance.

• (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.

Parameters

first last	The range of elements to shuffle.
r	The range of elements to shuffle.
gen	The random number generator.

Return value

An iterator equal to last.

Complexity

Exactly (last - first) - 1 swaps.

Exceptions

(none)

Possible implementation

shuffle(1) and shuffle(2)

struct shuffle_fn
{
    template<std::random_access_iterator I, std::sentinel_for<I> S, class Gen>
    requires std::permutable<I> &&
             std::uniform_random_bit_generator<std::remove_reference_t<Gen>>
    I operator()(I first, S last, Gen&& gen) const
    {
        using diff_t = std::iter_difference_t<I>;
        using distr_t = std::uniform_int_distribution<diff_t>;
        using param_t = typename distr_t::param_type;
        distr_t D;
        const auto n {last - first};
        for (diff_t i {1}; i < n; ++i)
            ranges::iter_swap(first + i, first + D(gen, param_t(0, i)));
        return ranges::next(first, last);
    }

    template<ranges::random_access_range R, class Gen>
    requires std::permutable<ranges::iterator_t<R>> &&
             std::uniform_random_bit_generator<std::remove_reference_t<Gen>>
    ranges::borrowed_iterator_t<R> operator()(R&& r, Gen&& gen) const
    {
        return (*this)(ranges::begin(r), ranges::end(r), std::forward<Gen>(gen));
    }
};

inline constexpr shuffle_fn shuffle {};

Examples

Main.cpp

#include <algorithm>
#include <array>
#include <iostream>
#include <random>

void print(const auto& a)
{
    for (const auto e : a)
        std::cout << e << ' ';
    std::cout << '\n';
}

int main()
{
    std::array a {'A', 'B', 'C', 'D', 'E', 'F'};
    print(a);

    std::random_device rd;
    std::mt19937 gen {rd()};

    for (int i {}; i != 3; ++i)
    {
        std::ranges::shuffle(a, gen);
        print(a);
    }
}

Possible Output

A B C D E F
F E A C D B
E C B F A D
B A E C F D