//===----------------------------------------------------------------------===//
//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See https://llvm.org/LICENSE.txt for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
//
//===----------------------------------------------------------------------===//
// UNSUPPORTED: c++03, c++11, c++14, c++17
// <algorithm>
// template<random_access_iterator I, sentinel_for<I> S, class Comp = ranges::less,
// class Proj = identity>
// requires sortable<I, Comp, Proj>
// constexpr I
// ranges::nth_element(I first, I nth, S last, Comp comp = {}, Proj proj = {}); // since C++20
//
// template<random_access_range R, class Comp = ranges::less, class Proj = identity>
// requires sortable<iterator_t<R>, Comp, Proj>
// constexpr borrowed_iterator_t<R>
// ranges::nth_element(R&& r, iterator_t<R> nth, Comp comp = {}, Proj proj = {}); // since C++20
#include <algorithm>
#include <array>
#include <concepts>
#include <functional>
#include <iterator>
#include <optional>
#include <ranges>
#include "almost_satisfies_types.h"
#include "test_iterators.h"
// SFINAE tests.
using BadComparator = ComparatorNotCopyable<int*>;
static_assert(!std::sortable<int*, BadComparator>);
template <class Iter, class Sent = sentinel_wrapper<Iter>, class Comp = std::ranges::less>
concept HasNthElementIt = requires(Iter first, Iter nth, Sent last, Comp comp) {
std::ranges::nth_element(first, nth, last, comp);
};
static_assert(HasNthElementIt<int*>);
static_assert(!HasNthElementIt<RandomAccessIteratorNotDerivedFrom>);
static_assert(!HasNthElementIt<RandomAccessIteratorBadIndex>);
static_assert(!HasNthElementIt<int*, SentinelForNotSemiregular>);
static_assert(!HasNthElementIt<int*, SentinelForNotWeaklyEqualityComparableWith>);
static_assert(!HasNthElementIt<int*, int*, BadComparator>);
static_assert(!HasNthElementIt<const int*>); // Doesn't satisfy `sortable`.
template <class Range, class Comp = std::ranges::less>
concept HasNthElementR = requires(Range range, std::ranges::iterator_t<Range> nth, Comp comp) {
std::ranges::nth_element(range, nth, comp);
};
static_assert(HasNthElementR<UncheckedRange<int*>>);
static_assert(!HasNthElementR<RandomAccessRangeNotDerivedFrom>);
static_assert(!HasNthElementR<RandomAccessRangeBadIndex>);
static_assert(!HasNthElementR<UncheckedRange<int*, SentinelForNotSemiregular>>);
static_assert(!HasNthElementR<UncheckedRange<int*, SentinelForNotWeaklyEqualityComparableWith>>);
static_assert(!HasNthElementR<UncheckedRange<int*>, BadComparator>);
static_assert(!HasNthElementR<UncheckedRange<const int*>>); // Doesn't satisfy `sortable`.
template <std::size_t N, class T, class Iter>
constexpr void verify_nth(const std::array<T, N>& partially_sorted, std::size_t nth_index, Iter last, T expected_nth) {
// Note that the exact output of `nth_element` is unspecified and may vary between implementations.
assert(base(last) == partially_sorted.data() + partially_sorted.size());
auto b = partially_sorted.begin();
auto nth = b + nth_index;
auto e = partially_sorted.end();
if (nth == e)
return;
assert(*nth == expected_nth);
// All elements on the left are <= nth.
assert(std::all_of(b, nth, [&](const auto& v) { return v <= *nth; }));
// All elements on the right are >= nth.
assert(std::all_of(nth, e, [&](const auto& v) { return v >= *nth; }));
{
auto sorted = partially_sorted;
std::ranges::sort(sorted);
// The element at index `n` is the same as if the range were fully sorted.
assert(sorted[nth_index] == *nth);
}
}
template <class Iter, class Sent, std::size_t N>
constexpr void test_one(std::array<int, N> input, std::size_t nth_index, std::optional<int> expected_nth = {}) {
assert(expected_nth || nth_index == N);
{ // (iterator, sentinel) overload.
auto partially_sorted = input;
auto b = Iter(partially_sorted.data());
auto nth = b + nth_index;
auto e = Sent(Iter(partially_sorted.data() + partially_sorted.size()));
std::same_as<Iter> decltype(auto) last = std::ranges::nth_element(b, nth, e);
if (nth_index != N) {
verify_nth(partially_sorted, nth_index, last, *expected_nth);
} else {
assert(partially_sorted == input);
}
}
{ // (range) overload.
auto partially_sorted = input;
auto b = Iter(partially_sorted.data());
auto nth = b + nth_index;
auto e = Sent(Iter(partially_sorted.data() + partially_sorted.size()));
auto range = std::ranges::subrange(b, e);
std::same_as<Iter> decltype(auto) last = std::ranges::nth_element(range, nth);
if (nth_index != N) {
verify_nth(partially_sorted, nth_index, last, *expected_nth);
} else {
assert(partially_sorted == input);
}
}
}
template <class Iter, class Sent, std::size_t N>
constexpr void test_all_cases(std::array<int, N> input) {
auto sorted = input;
std::sort(sorted.begin(), sorted.end());
for (int n = 0; n != N; ++n) {
test_one<Iter, Sent, N>(input, n, sorted[n]);
}
test_one<Iter, Sent, N>(input, N);
}
constexpr void test_iterators() {
auto check = []<class Iter, class Sent> {
// Empty sequence.
test_one<Iter, Sent, 0>({}, 0);
// 1-element sequence.
test_all_cases<Iter, Sent>(std::array{1});
// 2-element sequence.
test_all_cases<Iter, Sent>(std::array{2, 1});
// 3-element sequence.
test_all_cases<Iter, Sent>(std::array{2, 1, 3});
// Longer sequence.
test_all_cases<Iter, Sent>(std::array{2, 1, 3, 6, 8, 4, 11, 5});
// Longer sequence with duplicates.
test_all_cases<Iter, Sent>(std::array{2, 1, 3, 6, 2, 8, 6});
// All elements are the same.
test_all_cases<Iter, Sent>(std::array{1, 1, 1, 1});
{ // nth element is in the right place.
std::array input = {6, 5, 3, 1, 4, 2};
constexpr std::size_t N = input.size();
test_one<Iter, Sent, N>(input, 2, /*expected_nth=*/3);
}
// Already sorted.
test_all_cases<Iter, Sent>(std::array{1, 2, 3, 4, 5, 6});
// Descending.
test_all_cases<Iter, Sent>(std::array{6, 5, 4, 3, 2, 1});
// Repeating pattern.
test_all_cases<Iter, Sent>(std::array{2, 1, 2, 1, 2, 1});
};
check.operator()<random_access_iterator<int*>, random_access_iterator<int*>>();
check.operator()<random_access_iterator<int*>, sentinel_wrapper<random_access_iterator<int*>>>();
check.operator()<contiguous_iterator<int*>, contiguous_iterator<int*>>();
check.operator()<contiguous_iterator<int*>, sentinel_wrapper<contiguous_iterator<int*>>>();
check.operator()<int*, int*>();
check.operator()<int*, sentinel_wrapper<int*>>();
}
constexpr bool test() {
test_iterators();
{ // A custom comparator works.
const std::array input = {1, 2, 3, 4, 5};
std::ranges::greater comp;
{
auto in = input;
auto last = std::ranges::nth_element(in.begin(), in.begin() + 1, in.end(), comp);
assert(in[1] == 4);
assert(last == in.end());
}
{
auto in = input;
auto last = std::ranges::nth_element(in, in.begin() + 1, comp);
assert(in[1] == 4);
assert(last == in.end());
}
}
{ // A custom projection works.
struct A {
int a;
constexpr bool operator==(const A&) const = default;
};
const std::array input = {A{2}, A{1}, A{3}};
{
auto in = input;
auto last = std::ranges::nth_element(in.begin(), in.begin() + 1, in.end(), {}, &A::a);
assert(in[1] == A{2});
assert(last == in.end());
}
{
auto in = input;
auto last = std::ranges::nth_element(in, in.begin() + 1, {}, &A::a);
assert(in[1] == A{2});
assert(last == in.end());
}
}
{ // `std::invoke` is used in the implementation.
struct S {
int i;
constexpr S(int i_) : i(i_) {}
constexpr bool comparator(const S& rhs) const { return i < rhs.i; }
constexpr const S& projection() const { return *this; }
constexpr bool operator==(const S&) const = default;
};
const std::array input = {S{2}, S{1}, S{3}};
{
auto in = input;
auto last = std::ranges::nth_element(in.begin(), in.begin() + 1, in.end(), &S::comparator, &S::projection);
assert(in[1] == S{2});
assert(last == in.end());
}
{
auto in = input;
auto last = std::ranges::nth_element(in, in.begin() + 1, &S::comparator, &S::projection);
assert(in[1] == S{2});
assert(last == in.end());
}
}
{ // `std::ranges::dangling` is returned.
std::array in{1, 2, 3};
[[maybe_unused]] std::same_as<std::ranges::dangling> decltype(auto) result =
std::ranges::nth_element(std::move(in), in.begin());
}
return true;
}
int main(int, char**) {
test();
static_assert(test());
return 0;
}
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