# std::ranges::is_heap_until

From cppreference.com
< cpp‎ | algorithm‎ | ranges

Defined in header `<algorithm>` |  |
Call signature |  |
template< std::random_access_iterator I, std::sentinel_for<I> S, class Proj = std::identity, std::indirect_strict_weak_order< std::projected<I, Proj>> Comp = ranges::less > constexpr I is_heap_until( I first, S last, Comp comp = {}, Proj proj = {} ); | (1) | (since C++20)
template< ranges::random_access_range R, class Proj = std::identity, std::indirect_strict_weak_order<std::projected<ranges::iterator_t<R>, Proj>> Comp = ranges::less > constexpr ranges::borrowed_iterator_t<R> is_heap_until( R&& r, Comp comp = {}, Proj proj = {} ); | (2) | (since C++20)

Examines the range `[`first`,` last`)` and finds the largest range beginning at first which is a max heap.

1) Elements are compared using the given binary comparison function comp and projection object proj.
2) Same as (1), but uses r as the 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 cannot be specified when calling any of them.
- None of them are visible to argument-dependent lookup.
- When any of them are found by normal unqualified lookup as the name to the left of the function-call operator, argument-dependent lookup is inhibited.

In practice, they may be implemented as function objects, or with special compiler extensions.

### Parameters

first, last | - | the range of elements to examine
r | - | the range of elements to examine
pred | - | predicate to apply to the projected elements
proj | - | projection to apply to the elements

### Return value

The upper bound of the largest range beginning at first which is a max heap. That is, the last iterator `it` for which range `[`first`,` it`)` is a max heap with respect to comp and proj.

### Complexity

Linear in the distance between first and last.

### Notes

A max heap is a range of elements `[`f`,` l`)`, arranged with respect to comparator comp and projection proj, that has the following properties:

  - With `N = l - f`, `p = f[(i - 1) / 2]`, and `q = f[i]`, for all `0 < i < N`, the expression std::invoke(comp, std::invoke(proj, p), std::invoke(proj, q)) evaluates to false.
  - A new element can be added using ranges::push_heap, in \(\scriptsize \mathcal{O}(\log N)\)𝓞(log N) time.
  - The first element can be removed using ranges::pop_heap, in \(\scriptsize \mathcal{O}(\log N)\)𝓞(log N) time.

### Possible implementation

    struct is_heap_until_fn
    {
        template<std::random_access_iterator I, std::sentinel_for<I> S,
                 class Proj = std::identity, std::indirect_strict_weak_order<
                 std::projected<I, Proj>> Comp = ranges::less>
        constexpr I
            operator()(I first, S last, Comp comp = {}, Proj proj = {}) const
        {
            std::iter_difference_t<I> n {ranges::distance(first, last)}, dad {0}, son {1};
            for (; son != n; ++son)
            {
                if (std::invoke(comp, std::invoke(proj, *(first + dad)),
                                      std::invoke(proj, *(first + son))))
                    return first + son;
                else if ((son % 2) == 0)
                    ++dad;
            }
            return first + n;
        }

        template<ranges::random_access_range R, class Proj = std::identity,
                 std::indirect_strict_weak_order<std::projected<ranges::iterator_t<R>, Proj>>
                 Comp = ranges::less>
        constexpr ranges::borrowed_iterator_t<R>
            operator()(R&& r, Comp comp = {}, Proj proj = {}) const
        {
            return (*this)(ranges::begin(r), ranges::end(r), std::move(comp), std::move(proj));
        }
    };

    inline constexpr is_heap_until_fn is_heap_until {};

### Example

The example renders a given vector as a (balanced) Binary tree.

Run this code

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

    void out(const auto& what, int n = 1)
    {
        while (n-- > 0)
            std::cout << what;
    }

    void draw_bin_tree(auto first, auto last);

    int main()
    {
        std::vector<int> v {3, 1, 4, 1, 5, 9};
        std::ranges::make_heap(v);

        // probably mess up the heap
        v.push_back(2);
        v.push_back(6);

        out("v after make_heap and push_back: \n");
        draw_bin_tree(v.begin(), v.end());

        out("the max-heap prefix of v: \n");
        const auto heap_end = std::ranges::is_heap_until(v);
        draw_bin_tree(v.begin(), heap_end);
    }

    void draw_bin_tree(auto first, auto last)
    {
        auto bails = [](int n, int w)
        {
            auto b = [](int w) { out("┌"), out("─", w), out("┴"), out("─", w), out("┐"); };
            n /= 2;
            if (!n)
                return;
            for (out(' ', w); n-- > 0; )
                b(w), out(' ', w + w + 1);
            out('\n');
        };
        auto data = [](int n, int w, auto& first, auto last)
        {
            for(out(' ', w); n-- > 0 && first != last; ++first)
                out(*first), out(' ', w + w + 1);
            out('\n');
        };
        auto tier = [&](int t, int m, auto& first, auto last)
        {
            const int n {1 << t};
            const int w {(1 << (m - t - 1)) - 1};
            bails(n, w), data(n, w, first, last);
        };
        const auto size {std::ranges::distance(first, last)};
        const int m {static_cast<int>(std::ceil(std::log2(1 + size)))};
        for (int i {}; i != m; ++i)
            tier(i, m, first, last);
    }

Output:

    v after make_heap and push_back:
           9
       ┌───┴───┐
       5       4
     ┌─┴─┐   ┌─┴─┐
     1   1   3   2
    ┌┴┐ ┌┴┐ ┌┴┐ ┌┴┐
    6
    the max-heap prefix of v:
       9
     ┌─┴─┐
     5   4
    ┌┴┐ ┌┴┐
    1 1 3 2

### See also

ranges::is_heap (C++20) | checks if the given range is a max heap (niebloid)
ranges::make_heap (C++20) | creates a max heap out of a range of elements (niebloid)
ranges::push_heap (C++20) | adds an element to a max heap (niebloid)
ranges::pop_heap (C++20) | removes the largest element from a max heap (niebloid)
ranges::sort_heap (C++20) | turns a max heap into a range of elements sorted in ascending order (niebloid)
is_heap_until (C++11) | finds the largest subrange that is a max heap (function template)

Retrieved from "https://en.cppreference.com/mwiki/index.php?title=cpp/algorithm/ranges/is_heap_until&oldid=150672"