std::is_permutation(3)

💖 std::is_permutation — C++ Standard Libary 💖

okay so you opened the std::is_permutation(3) manual page, and honestly? iconic. This is the C++ Standard Libary corner, and we're about to do a whole vibe check on whether two ranges are permutations of each other. no cap, it's giving algorithm bestie energy. 🦋

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✨ NAME ✨

std::is_permutation — the function template named std::is_permutation, here to check if one range is a permutation of another. periodt. 💅

🌸 Synopsis 🌸

Defined in header <algorithm>.

Overload (1) — since C++11, and constexpr since C++20:

template< class ForwardIt1, class ForwardIt2 >
bool is_permutation( ForwardIt1 first1, ForwardIt1 last1,
                     ForwardIt2 first2 );

Overload (2) — since C++11, and constexpr since C++20:

template< class ForwardIt1, class ForwardIt2,
          class BinaryPredicate >
bool is_permutation( ForwardIt1 first1, ForwardIt1 last1,
                     ForwardIt2 first2, BinaryPredicate p );

Overload (3) — since C++14, and constexpr since C++20:

template< class ForwardIt1, class ForwardIt2 >
bool is_permutation( ForwardIt1 first1, ForwardIt1 last1,
                     ForwardIt2 first2, ForwardIt2 last2 );

Overload (4) — since C++14, and constexpr since C++20:

template< class ForwardIt1, class ForwardIt2,
          class BinaryPredicate >
bool is_permutation( ForwardIt1 first1, ForwardIt1 last1,
                     ForwardIt2 first2, ForwardIt2 last2,
                     BinaryPredicate p );

Checks whether [first1, last1) is a permutation of a range starting from first2:

• For overloads (1,2), the second range has std::distance(first1, last1) elements.
• For overloads (3,4), the second range is [first2, last2).

Then the comparison tea ☕:

• 1,3) Elements are compared using operator==.
• 2,4) Elements are compared using the given binary predicate p.

If ForwardIt1 and ForwardIt2 have different value types, the program is ill-formed. 🫠 If the comparison function is not an equivalence relation, the behavior is undefined. 💀

🎀 Parameters 🎀

• first1, last1 — the range of elements to compare.
• first2, last2 — the second range to compare.
• p — a binary predicate which returns true if the elements should be treated as equal.

The signature of the predicate function should be equivalent to the following:

bool pred(const Type1 &a, const Type2 &b);

While the signature does not need to have const &, the function must not modify the objects passed to it and must be able to accept all values of type (possibly const) Type1 and Type2 regardless of value category (thus, Type1 & is not allowed, nor is Type1 unless for Type1 a move is equivalent to a copy (since C++11)). The types Type1 and Type2 must be such that objects of types InputIt1 and InputIt2 can be dereferenced and then implicitly converted to Type1 and Type2 respectively. 🧚

🦋 Type requirements 🦋

• ForwardIt1, ForwardIt2 must meet the requirements of LegacyForwardIterator.

👑 Return value 👑

true if the range [first1, last1) is a permutation of the range [first2, last2), false otherwise. slay. ✨

🔥 Complexity 🔥

Given N as std::distance(first1, last1):

• 1) Exactly N comparisons using operator== if the two ranges are equal, otherwise O(N^2) comparisons in the worst case.
• 2) Exactly N applications of the predicate p if the two ranges are equal, otherwise O(N^2) applications in the worst case.
• 3,4) If ForwardIt1 and ForwardIt2 are both LegacyRandomAccessIterator, and last1 - first1 != last2 - first2 is true, no comparison will be made. Otherwise:
  • 3) Exactly N comparisons using operator== if the two ranges are equal, otherwise O(N^2) comparisons in the worst case.
  • 4) Exactly N applications of the predicate p if the two ranges are equal, otherwise O(N^2) applications in the worst case.

🌈 Possible implementation 🌈

template<class ForwardIt1, class ForwardIt2>
bool is_permutation(ForwardIt1 first, ForwardIt1 last,
                    ForwardIt2 d_first)
{
    // skip common prefix
    std::tie(first, d_first) = std::mismatch(first, last, d_first);

    // iterate over the rest, counting how many times each element
    // from [first, last) appears in [d_first, d_last)
    if (first != last)
    {
        ForwardIt2 d_last = std::next(d_first, std::distance(first, last));
        for (ForwardIt1 i = first; i != last; ++i)
        {
            if (i != std::find(first, i, *i))
                continue; // this *i has been checked

            auto m = std::count(d_first, d_last, *i);
            if (m == 0 || std::count(i, last, *i) != m)
                return false;
        }
    }
    return true;
}

💜 Note 💜

The std::is_permutation can be used in testing, namely to check the correctness of rearranging algorithms (e.g. sorting, shuffling, partitioning). If x is an original range and y is a permuted range then std::is_permutation(x, y) == true means that y consist of "the same" elements, maybe staying at other positions. iykyk. 💅

🍓 Example 🍓

// Run this code

#include <algorithm>
#include <iostream>

template<typename Os, typename V>
Os& operator<<(Os& os, const V& v)
{
    os << "{ ";
    for (const auto& e : v)
        os << e << ' ';
    return os << '}';
}

int main()
{
    static constexpr auto v1 = {1, 2, 3, 4, 5};
    static constexpr auto v2 = {3, 5, 4, 1, 2};
    static constexpr auto v3 = {3, 5, 4, 1, 1};

    std::cout << v2 << " is a permutation of " << v1 << ": " << std::boolalpha
              << std::is_permutation(v1.begin(), v1.end(), v2.begin()) << '\n'
              << v3 << " is a permutation of " << v1 << ": "
              << std::is_permutation(v1.begin(), v1.end(), v3.begin()) << '\n';
}

🌟 Output 🌟

{ 3 5 4 1 2 } is a permutation of { 1 2 3 4 5 }: true
{ 3 5 4 1 1 } is a permutation of { 1 2 3 4 5 }: false

🌟 See also 🌟

• next_permutation — generates the next greater lexicographic permutation of a range of elements (function template).
• prev_permutation — generates the next smaller lexicographic permutation of a range of elements (function template).
• equivalence_relation (C++20) — specifies that a relation imposes an equivalence relation (concept).
• ranges::is_permutation (C++20) — determines if a sequence is a permutation of another sequence (niebloid).

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xoxo bestie 💖 — this came from http://cppreference.com, dated 2024.06.10, and it's the std::is_permutation(3) page. bye 💅✨