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Showing content from https://en.cppreference.com/w/cpp/algorithm/../symbol_index/../algorithm/ranges/is_heap_until.html below:

std::ranges::is_heap_until - cppreference.com

Call signature

Within the specified range, finds the longest range which starting from the beginning of the specified range and represents a heap with respect to comp and proj.

1) The specified range is [first, last).

2) The specified range is r.

The function-like entities described on this page are algorithm function objects (informally known as 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.

The last iterator iter in the specified range for which:

1) The range [first, iter) is a heap with respect to comp and proj.

The range

is a heap with respect to

and

.

\(\scriptsize O(N) \)O(N) applications of comp and proj, where \(\scriptsize N \)N is:

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{};

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

#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)
{
    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);
}
 
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);
}

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

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