std::ranges::search() algorithm

od C++20

Simplified

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

// (2)
constexpr ranges::borrowed_subrange_t<R1>
    search( R1&& r1, R2&& r2, Pred pred = {}, Proj1 proj1 = {}, Proj2 proj2 = {} );

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

• I1, I2 - std::forward_iterator
• S1, S2 - std::sentinel_for<I1>, std::sentinel_for<I2>
• Pred - (none)
• Proj1, Proj2 - (none)
• (2) - R1, R2 - std::ranges::forward_range

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::forward_iterator I1,
  std::sentinel_for<I1> S1,
  std::forward_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 ranges::subrange<I1>
    search( I1 first1, S1 last1, I2 first2, S2 last2, Pred pred = {},
            Proj1 proj1 = {}, Proj2 proj2 = {} );

// (2)
template<
  ranges::forward_range R1,
  ranges::forward_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::borrowed_subrange_t<R1>
    search( R1&& r1, R2&& r2, Pred pred = {}, Proj1 proj1 = {}, Proj2 proj2 = {} );

Searches for the first occurence of a range of elements inside another range.

• (1) Searches for the first occurrence of the sequence of elements [first2; last2) in the range [first1; last1). Elements are compared using binary predicate pred after being projected with proj2 and proj1, respectively.

• (2) Same as (1), but uses r1 as the first source range and r2 as the second source 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.

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

Parameters

first1 last1	The range of elements to examine.
first2 last2	The range of elements to search for.
r1	The range of elements to examine.
r2	The range of elements to search for.
pred	Binary predicate to compare the elements with.
proj1	Projection to apply to the elements in the first range.
proj2	Projection to apply to the elements in the second range.

Return value

• (1) Value of type ranges::subrange<I1> initialized as follows:

  {
    i,
    i + (i == last1 ? 0 : ranges::distance(first2, last2))
  }

  that denotes the first occurrence of the sequence [first2; last2) (aka needle) in range [first1; last1) (aka haystack) (after projections with proj1 and proj2 and binary predicate pred after that).

  If [first2; last2) is empty or if no such sequence is found, the return value is effectively initialized with { last1, last1 }.

• (2) Same as (1), except that the return type is ranges::borrowed_subrange_t<R1>.

Complexity

• (1) Given S as ranges::distance(first2, last2) and N as ranges::distance(first1, last1)
• (2) Given S as ranges::distance(r2) and N as ranges::distance(r1)

At most S * N applications of predicate and each projection.

Exceptions

(none)

Possible implementation

search(1)

struct search_fn
{
    template<std::forward_iterator I1, std::sentinel_for<I1> S1,
             std::forward_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 ranges::subrange<I1>
        operator()(I1 first1, S1 last1, I2 first2, S2 last2, Pred pred = {},
                   Proj1 proj1 = {}, Proj2 proj2 = {}) const
    {
        for (;; ++first1)
        {
            I1 it1 = first1;
            for (I2 it2 = first2;; ++it1, ++it2)
            {
                if (it2 == last2)
                    return {first1, it1};
                if (it1 == last1)
                    return {it1, it1};
                if (!std::invoke(pred, std::invoke(proj1, *it1), std::invoke(proj2, *it2)))
                    break;
            }
        }
    }

    template<ranges::forward_range R1, ranges::forward_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::borrowed_subrange_t<R1>
        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::move(pred), std::move(proj1), std::move(proj2));
    }
};

inline constexpr search_fn search {};

Examples

Main.cpp

#include <algorithm>
#include <cctype>
#include <iostream>
#include <iterator>
#include <string_view>

using namespace std::literals;

void print(int id, const auto& haystack, const auto& needle, const auto& found)
{
    std::cout << id << ") search(\"" << haystack << "\", \"" << needle << "\"); ";
    const auto first = std::distance(haystack.begin(), found.begin());
    const auto last = std::distance(haystack.begin(), found.end());
    if (found.empty())
        std::cout << "not found;";
    else
    {
        std::cout << "found: \"";
        for (const auto x : found)
            std::cout << x;
        std::cout << "\";";
    }
    std::cout << " subrange: {" << first << ", " << last << "}\n";
}

int main()
{
    constexpr auto haystack {"abcd abcd"sv};
    constexpr auto needle {"bcd"sv};

    // the search uses iterator pairs begin()/end():
    constexpr auto found1 = std::ranges::search(
        haystack.begin(), haystack.end(),
        needle.begin(), needle.end());
    print(1, haystack, needle, found1);

    // the search uses ranges r1, r2:
    constexpr auto found2 = std::ranges::search(haystack, needle);
    print(2, haystack, needle, found2);

    // 'needle' range is empty:
    constexpr auto none {""sv};
    constexpr auto found3 = std::ranges::search(haystack, none);
    print(3, haystack, none, found3);

    // 'needle' will not be found:
    constexpr auto awl {"efg"sv};
    constexpr auto found4 = std::ranges::search(haystack, awl);
    print(4, haystack, awl, found4);

    // the search uses custom comparator and projections:
    constexpr auto bodkin {"234"sv};
    auto found5 = std::ranges::search(haystack, bodkin,
        [](const int x, const int y) { return x == y; }, // pred
        [](const int x) { return std::toupper(x); }, // proj1
        [](const int y) { return y + 'A' - '1'; }); // proj2
    print(5, haystack, bodkin, found5);
}

Output

1) search("abcd abcd", "bcd"); found: "bcd"; subrange: {1, 4}
2) search("abcd abcd", "bcd"); found: "bcd"; subrange: {1, 4}
3) search("abcd abcd", ""); not found; subrange: {0, 0}
4) search("abcd abcd", "efg"); not found; subrange: {9, 9}
5) search("abcd abcd", "234"); found: "bcd"; subrange: {1, 4}