std::ranges::set_intersection() algorithm

od C++20

Simplified

// (1)
constexpr set_intersection_result<I1, I2, O>
    set_intersection( I1 first1, S1 last1, I2 first2, S2 last2, O result,
                    Comp comp = {}, Proj1 proj1 = {}, Proj2 proj2 = {} );

// (2)
constexpr set_intersection_result<ranges::borrowed_iterator_t<R1>,
                                  ranges::borrowed_iterator_t<R2>, O>
    set_intersection( R1&& r1, R2&& r2, O result, Comp comp = {},
                    Proj1 proj1 = {}, Proj2 proj2 = {} );

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

• I1, I2 - std::input_iterator
• S1, S2 - std::sentinel_for<I1>, std::sentinel_for<I2>
• R1, R2 - std::ranges::input_range
• O - std::weakly_incrementable
• Comp - (none)
• Proj1, Proj2 - (none)

The Proj and Comp template arguments have the following default types: std::identity, ranges::less for all overloads.

Additionaly, each overload has the following constraints:

• (1) - mergeable<I1, I2, O, Comp, Proj1, Proj2>
• (2) - mergeable<ranges::iterator_t<R1>, ranges::iterator_t<R2>, O, Comp, Proj1, Proj2>

(The std:: namespace was ommited here for readability)

Detailed

// (1)
template<
  std::input_iterator I1,
  std::sentinel_for<I1> S1,
  std::input_iterator I2,
  std::sentinel_for<I2> S2,
  std::weakly_incrementable O,
  class Comp = ranges::less,
  class Proj1 = std::identity,
  class Proj2 = std::identity
>
  requires std::mergeable<I1, I2, O, Comp, Proj1, Proj2>
constexpr set_intersection_result<I1, I2, O>
    set_intersection( I1 first1, S1 last1, I2 first2, S2 last2, O result,
                    Comp comp = {}, Proj1 proj1 = {}, Proj2 proj2 = {} );

// (2)
template<
  ranges::input_range R1,
  ranges::input_range R2,
  std::weakly_incrementable O,
  class Comp = ranges::less,
  class Proj1 = std::identity,
  class Proj2 = std::identity
>
  requires std::mergeable<ranges::iterator_t<R1>, ranges::iterator_t<R2>,
                          O, Comp, Proj1, Proj2>
constexpr set_intersection_result<ranges::borrowed_iterator_t<R1>,
                                  ranges::borrowed_iterator_t<R2>, O>
    set_difference( R1&& r1, R2&& r2, O result, Comp comp = {},
                    Proj1 proj1 = {}, Proj2 proj2 = {} );

With the helper types defined as follows:

template< class I1, class I2, class O >
using set_intersection_result = ranges::in_in_out_result<I1, I2, O>;

Constructs a sorted range beginning at result consisting of elements that are found in both sorted input ranges [first1; last1) and [first2; last2).

If some element is found m times in [first1; last1) and n times in [first2; last2), the first min(m, n) elements will be copied from the first range to result.

The order of equivalent elements is preserved.

• (1) Elements are compared using the given binary comparison function comp.
• (2) Same as (1), but uses r1 as the first range and r2 as the second range, as if using ranges::begin(r1) as first1, ranges::end(r1) as last1, ranges::begin(r2) as first2, and ranges::end(r2) as last2.

Undefined Behaviour

The behavior is undefined

if:

• The input ranges are not sorted with respect to comp and proj1 or proj2, respectively
• Or the resulting range overlaps with either of the input ranges.

The function-like entities described on this page are niebloids.

Parameters

first1 last1	The first sorted input range.
r r1	The first sorted input range.
first2 last2	The second sorted input range.
r2	The second sorted input range.
result	The beginning of the destination range.
comp	Comparator to apply to the projected elements.
proj1	Projection to apply to the elements in the first range.
proj2	Projection to apply to the elements in the second range.

Return value

A value of type ranges::set_intersection_result initialized as follows:

{
  last1,
  last2,
  result_last
}

Where result_last is the end of the constructed range.

Complexity

Given N1 as ranges::distance(first1, last1) and N2 as ranges::distance(first2, last12):

2 * (N1 + N2) − 1 comparisons and applications of each projection.

Exceptions

(none)

Possible implementation

set_intersection(1) and set_intersection(2)

struct set_intersection_fn
{
    template<std::input_iterator I1, std::sentinel_for<I1> S1,
             std::input_iterator I2, std::sentinel_for<I2> S2,
             std::weakly_incrementable O, class Comp = ranges::less,
             class Proj1 = std::identity, class Proj2 = std::identity>
    requires std::mergeable<I1, I2, O, Comp, Proj1, Proj2>
    constexpr ranges::set_union_result<I1, I2, O>
        operator()(I1 first1, S1 last1, I2 first2, S2 last2,
                   O result, Comp comp = {},
                   Proj1 proj1 = {}, Proj2 proj2 = {}) const
    {
        while (!(first1 == last1 or first2 == last2))
        {
            if (std::invoke(comp, std::invoke(proj1, *first1), std::invoke(proj2, *first2)))
                ++first1;
            else if (std::invoke(comp, std::invoke(proj2, *first2), std::invoke(proj1, *first1)))
                ++first2;
            else
                *result = *first1, ++first1, ++first2, ++result;
        }
        return {ranges::next(std::move(first1), std::move(last1)),
                ranges::next(std::move(first2), std::move(last2)),
                std::move(result)};
    }

    template<ranges::input_range R1, ranges::input_range R2,
             std::weakly_incrementable O, class Comp = ranges::less,
             class Proj1 = std::identity, class Proj2 = std::identity>
    requires std::mergeable<ranges::iterator_t<R1>, ranges::iterator_t<R2>,
                            O, Comp, Proj1, Proj2>
    constexpr ranges::set_intersection_result<ranges::borrowed_iterator_t<R1>,
                                              ranges::borrowed_iterator_t<R2>, O>
        operator()(R1&& r1, R2&& r2, O result, Comp comp = {},
                   Proj1 proj1 = {}, Proj2 proj2 = {}) const
    {
        return (*this)(ranges::begin(r1), ranges::end(r1),
                       ranges::begin(r2), ranges::end(r2),
                       std::move(result), std::move(comp),
                       std::move(proj1), std::move(proj2));
    }
};

inline constexpr set_intersection_fn set_intersection {};

Examples

Main.cpp

#include <algorithm>
#include <iostream>
#include <iterator>
#include <vector>

void print(const auto& v, const auto& rem)
{
    std::cout << "{ ";
    for (const auto& e : v)
        std::cout << e << ' ';
    std::cout << '}' << rem;
}

int main()
{
    const auto in1 = {1, 2, 2, 3, 4, 5, 6};
    const auto in2 = {2, 2, 3, 3, 5, 7};
    std::vector<int> out {};

    std::ranges::set_intersection(in1, in2, std::back_inserter(out));

    print(in1, " ∩ "), print(in2, " = "), print(out, "\n");
}

Output

{ 1 2 2 3 4 5 6 } ∩ { 2 2 3 3 5 7 } = { 2 2 3 5 }