std::ranges::mismatch() algorithm

od C++20

Simplified

// (1)
constexpr mismatch_result<I1, I2>
    mismatch( I1 first1, S1 last1, I2 first2, S2 last2,
              Pred pred = {}, Proj1 proj1 = {}, Proj2 proj2 = {} );

// (2)
constexpr mismatch_result<ranges::borrowed_iterator_t<R1>,
                          ranges::borrowed_iterator_t<R2>>
    mismatch( R1&& r1, R2&& r2, Pred pred = {},
              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<I>, std::sentinel_for<I1>, std::sentinel_for<I2>
• R1, R2 - std::ranges::input_range
• Pred - (none)
• Proj1, Proj2 - (none)

The Pred template arguments has the following default type of std::equal_to for all overloads.
The Proj1 and Proj2 template arguments have a default type of std::identity for all overloads.

Additionally, each overload has the following constraints:

• (1) - indirectly_comparable<I1, I2, Pred, Proj1, Proj2>
• (2) - indirectly_comparable< ranges::iterator_t<R1>, ranges::iterator_t<R2>, Pred, Proj1, Proj2>

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

Detailed

// (1)
template<
  std::input_iterator I1,
  std::sentinel_for<I1> S1,
  std::input_iterator I2,
  std::sentinel_for<I2> S2,
  class Pred = ranges::equal_to,
  class Proj1 = std::identity,
  class Proj2 = std::identity
>
  requires std::indirectly_comparable<I1, I2, Pred, Proj1, Proj2>
constexpr mismatch_result<I1, I2>
    mismatch( I1 first1, S1 last1, I2 first2, S2 last2,
              Pred pred = {}, Proj1 proj1 = {}, Proj2 proj2 = {} );

// (2)
template<
  ranges::input_range R1,
  ranges::input_range R2,
  class Pred = ranges::equal_to,
  class Proj1 = std::identity,
  class Proj2 = std::identity
>
  requires std::indirectly_comparable<
             ranges::iterator_t<R1>, ranges::iterator_t<R2>, Pred, Proj1, Proj2>
constexpr mismatch_result<ranges::borrowed_iterator_t<R1>,
                          ranges::borrowed_iterator_t<R2>>
    mismatch( R1&& r1, R2&& r2, Pred pred = {},
              Proj1 proj1 = {}, Proj2 proj2 = {} );

With the helper types defined as follows:

template< class I1, class I2 >
using mismatch_result = ranges::in_in_result<I1, I2>;

Returns the first mismatching pair of projected elements from two ranges: one defined by [first1; last1) or r1 and another defined by [first2; last2) or r2.

• (1) Elements are compared using the given binary predicate p.

• (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.

Parameters

first1 last1	An iterator-sentinel pair denoting the first range of the elements to compare.
r1	The first range of the elements to compare.
first2 last2	An iterator-sentinel pair denoting the second range of the elements to compare.
r2	The second range of the elements to compare.
pred	Predicate to apply to the projected elements.
proj1	Projection to apply to the first range of elements.
proj2	Projection to apply to the second range of elements.

Return value

ranges::mismatch_result with iterators to the first two non-equal elements.

If no mismatches are found when the comparison reaches last1 or last2, whichever happens first, the object holds the end iterator and the corresponding iterator from the other range.

Complexity

At most std::min(last1 - first1, last2 - first2) applications of the predicate and corresponding projections.

Exceptions

(none)

Possible implementation

mismatch(1) and mismatch(2)

struct mismatch_fn
{
    template<std::input_iterator I1, std::sentinel_for<I1> S1,
             std::input_iterator I2, std::sentinel_for<I2> S2,
             class Pred = ranges::equal_to,
             class Proj1 = std::identity, class Proj2 = std::identity>
    requires std::indirectly_comparable<I1, I2, Pred, Proj1, Proj2>
    constexpr std::mismatch_result<I1, I2>
        operator()(I1 first1, S1 last1, I2 first2, S2 last2,
                   Pred pred = {}, Proj1 proj1 = {}, Proj2 proj2 = {}) const
    {
        for (; first1 != last1 && first2 != last2; ++first1, (void)++first2)
            if (not std::invoke(pred, std::invoke(proj1, *first1),
                                      std::invoke(proj2, *first2)))
                break;

        return {first1, first2};
    }

    template<ranges::input_range R1, ranges::input_range R2,
             class Pred = ranges::equal_to,
             class Proj1 = std::identity, class Proj2 = std::identity>
    requires std::indirectly_comparable<ranges::iterator_t<R1>, ranges::iterator_t<R2>,
                                        Pred, Proj1, Proj2>
    constexpr ranges::mismatch_result<ranges::borrowed_iterator_t<R1>,
                                      ranges::borrowed_iterator_t<R2>>
        operator()(R1&& r1, R2&& r2, Pred pred = {}, Proj1 proj1 = {}, Proj2 proj2 = {}) const
    {
        return (*this)(ranges::begin(r1), ranges::end(r1),
                       ranges::begin(r2), ranges::end(r2),
                       std::ref(pred), std::ref(proj1), std::ref(proj2));
    }
};

inline constexpr mismatch_fn mismatch;

Examples

This program determines the longest substring that is simultaneously found at the very beginning and at the very end of the given string, in reverse order (possibly overlapping).

Main.cpp

#include <algorithm>
#include <iostream>
#include <ranges>
#include <string_view>

[[nodiscard]]
constexpr std::string_view mirror_ends(const std::string_view in)
{
    const auto end = std::ranges::mismatch(in, in | std::views::reverse).in1;
    return {in.cbegin(), end};
}

int main()
{
    std::cout << mirror_ends("abXYZba") << '\n'
              << mirror_ends("abca") << '\n'
              << mirror_ends("ABBA") << '\n'
              << mirror_ends("level") << '\n';

    using namespace std::literals::string_view_literals;

    static_assert("123"sv == mirror_ends("123!@#321"));
    static_assert("radar"sv == mirror_ends("radar"));
}

Output

ab
a
ABBA
level