std::ranges::mismatch,std::ranges::mismatch_result(3)

NAME
std::ranges::mismatch,std::ranges::mismatch_result - std::ranges::mismatch,std::ranges::mismatch_result

Synopsis
   Defined in header <algorithm>
   Call signature
   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 > (1) (since C++20)
   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 = {} );
   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, (2) (since C++20)
   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 = {} );

Helper types
   template<class I1, class I2> (3) (since C++20)
   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, that is:
     * Explicit template argument lists may not be specified when calling any of them.
     * None of them is visible to argument-dependent lookup.
     * When one of them is found by normal unqualified lookup for the name to the left
       of the function-call operator, it inhibits argument-dependent lookup.
   In practice, they may be implemented as function objects, or with special compiler
   extensions.

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

Possible implementation
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 (!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;

Example
   This program determines the longest substring that is simultaneously found at the
   very beginning of the given string and at the very end of it, in reverse order
   (possibly overlapping) // Run this code
 #include <algorithm>
 #include <cstddef>
 #include <iostream>
 #include <ranges>
 #include <string_view>
 constexpr std::string_view mirror_ends(const std::string_view in)
 {
     const auto end = std::ranges::mismatch(in, in | std::views::reverse).in1;
     const std::size_t length = std::ranges::distance(in.begin(), end);
     return { in.cbegin(), length };
 }
 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

See also
   ranges::equal determines if two sets of elements are the same
   (C++20) (niebloid)
   ranges::find
   ranges::find_if
   ranges::find_if_not finds the first element satisfying specific criteria
   (C++20) (niebloid)
   (C++20)
   (C++20)
   ranges::lexicographical_compare returns true if one range is lexicographically less
   (C++20) than another
                        (niebloid)
   ranges::search searches for a range of elements
   (C++20) (niebloid)
   mismatch finds the first position where two ranges differ
                        (function template)