//===----------------------------------------------------------------------===//
//
// 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
//
//===----------------------------------------------------------------------===//
// <algorithm>
// UNSUPPORTED: c++03, c++11, c++14, c++17
// template<bidirectional_iterator I, sentinel_for<I> S, class Comp = ranges::less,
// class Proj = identity>
// requires sortable<I, Comp, Proj>
// constexpr ranges::prev_permutation_result<I>
// ranges::prev_permutation(I first, S last, Comp comp = {}, Proj proj = {});
// template<bidirectional_range R, class Comp = ranges::less,
// class Proj = identity>
// requires sortable<iterator_t<R>, Comp, Proj>
// constexpr ranges::prev_permutation_result<borrowed_iterator_t<R>>
// ranges::prev_permutation(R&& r, Comp comp = {}, Proj proj = {});
#include <algorithm>
#include <array>
#include <cassert>
#include <cstddef>
#include <ranges>
#include "almost_satisfies_types.h"
#include "test_iterators.h"
template <class Iter, class Sent = sentinel_wrapper<Iter>>
concept HasPrevPermutationIt = requires(Iter first, Sent last) { std::ranges::prev_permutation(first, last); };
static_assert(HasPrevPermutationIt<int*>);
static_assert(!HasPrevPermutationIt<BidirectionalIteratorNotDerivedFrom>);
static_assert(!HasPrevPermutationIt<BidirectionalIteratorNotDecrementable>);
static_assert(!HasPrevPermutationIt<int*, SentinelForNotSemiregular>);
static_assert(!HasPrevPermutationIt<int*, SentinelForNotWeaklyEqualityComparableWith>);
static_assert(!HasPrevPermutationIt<const int*>); // not sortable
template <class Range>
concept HasPrevPermutationR = requires(Range range) { std::ranges::prev_permutation(range); };
static_assert(HasPrevPermutationR<UncheckedRange<int*>>);
static_assert(!HasPrevPermutationR<BidirectionalRangeNotDerivedFrom>);
static_assert(!HasPrevPermutationR<BidirectionalRangeNotDecrementable>);
static_assert(!HasPrevPermutationR<BidirectionalRangeNotSentinelSemiregular>);
static_assert(!HasPrevPermutationR<BidirectionalRangeNotSentinelWeaklyEqualityComparableWith>);
static_assert(!HasPrevPermutationR<UncheckedRange<const int*>>); // not sortable
constexpr std::size_t factorial(size_t i) {
std::array memoized = {1, 1, 2, 6, 24, 120, 720, 5040, 40320};
return memoized[i];
}
template <class Iter, class Range, class Func>
constexpr bool run_prev_permutation(Func call_prev_permutation, Range permuted, Range previous) {
using Result = std::ranges::prev_permutation_result<Iter>;
std::same_as<Result> decltype(auto) ret = call_prev_permutation(permuted);
assert(ret.in == permuted.end());
bool next_found = ret.found;
if (std::ranges::distance(permuted) > 1) {
if (next_found) {
assert(std::ranges::lexicographical_compare(permuted, previous));
} else {
assert(std::ranges::lexicographical_compare(previous, permuted));
assert(std::ranges::is_sorted(permuted, std::ranges::greater{}));
}
}
return next_found;
}
template <class Iter, class Sent, class Func>
constexpr void test_prev_permutations(Func call_prev_permutation) {
std::array input = {4, 3, 2, 1};
auto current_permutation = input;
auto previous_permutation = current_permutation;
// For all subarrays of `input` from `[0, 0]` to `[0, N - 1]`, call `prev_permutation` until no previous permutation
// exists.
// The number of permutations must equal `N!`. `run_prev_permutation` checks that each previous permutation is
// lexicographically less than the previous. If these two conditions hold (the number of permutations is `N!`, and
// each permutation is lexicographically less than the previous one), it follows that the `ranges::prev_permutation`
// algorithm works correctly.
for (std::size_t i = 0; i <= current_permutation.size(); ++i) {
std::size_t count = 0;
bool next_found = true;
while (next_found) {
++count;
previous_permutation = current_permutation;
auto current_subrange = std::ranges::subrange(
Iter(current_permutation.data()), Sent(Iter(current_permutation.data() + i)));
auto previous_subrange = std::ranges::subrange(
Iter(previous_permutation.data()), Sent(Iter(previous_permutation.data() + i)));
next_found = run_prev_permutation<Iter>(call_prev_permutation, current_subrange, previous_subrange);
}
assert(count == factorial(i));
}
}
template <class Iter, class Sent>
constexpr void test_all_permutations() {
test_prev_permutations<Iter, Sent>([](auto&& range) {
return std::ranges::prev_permutation(range.begin(), range.end());
});
test_prev_permutations<Iter, Sent>([](auto&& range) {
return std::ranges::prev_permutation(range);
});
}
template <class Iter, class Sent, int N>
constexpr void test_one(const std::array<int, N> input, bool expected_found, std::array<int, N> expected) {
using Result = std::ranges::prev_permutation_result<Iter>;
{ // (iterator, sentinel) overload.
auto in = input;
auto begin = Iter(in.data());
auto end = Sent(Iter(in.data() + in.size()));
std::same_as<Result> decltype(auto) result = std::ranges::prev_permutation(begin, end);
assert(result.found == expected_found);
assert(result.in == end);
assert(in == expected);
}
{ // (range) overload.
auto in = input;
auto begin = Iter(in.data());
auto end = Sent(Iter(in.data() + in.size()));
auto range = std::ranges::subrange(begin, end);
std::same_as<Result> decltype(auto) result = std::ranges::prev_permutation(range);
assert(result.found == expected_found);
assert(result.in == end);
assert(in == expected);
}
}
template <class Iter, class Sent>
constexpr void test_iter_sent() {
test_all_permutations<Iter, Sent>();
// Empty range.
test_one<Iter, Sent, 0>({}, false, {});
// 1-element range.
test_one<Iter, Sent, 1>({1}, false, {1});
// 2-element range.
test_one<Iter, Sent, 2>({1, 2}, false, {2, 1});
test_one<Iter, Sent, 2>({2, 1}, true, {1, 2});
// Longer sequence.
test_one<Iter, Sent, 8>({8, 7, 6, 5, 4, 3, 2, 1}, true, {8, 7, 6, 5, 4, 3, 1, 2});
// Longer sequence, permutations exhausted.
test_one<Iter, Sent, 8>({1, 2, 3, 4, 5, 6, 7, 8}, false, {8, 7, 6, 5, 4, 3, 2, 1});
}
template <class Iter>
constexpr void test_iter() {
test_iter_sent<Iter, Iter>();
test_iter_sent<Iter, sentinel_wrapper<Iter>>();
test_iter_sent<Iter, sized_sentinel<Iter>>();
}
constexpr void test_iterators() {
test_iter<bidirectional_iterator<int*>>();
test_iter<random_access_iterator<int*>>();
test_iter<contiguous_iterator<int*>>();
test_iter<int*>();
}
constexpr bool test() {
test_iterators();
{ // A custom predicate works.
struct A {
int i;
constexpr bool comp(const A& rhs) const { return i > rhs.i; }
constexpr bool operator==(const A&) const = default;
};
const std::array input = {A{5}, A{4}, A{3}, A{2}, A{1}};
std::array expected = {A{1}, A{2}, A{3}, A{4}, A{5}};
{ // (iterator, sentinel) overload.
auto in = input;
auto result = std::ranges::prev_permutation(in.begin(), in.end(), &A::comp);
assert(result.found == false);
assert(result.in == in.end());
assert(in == expected);
}
{ // (range) overload.
auto in = input;
auto result = std::ranges::prev_permutation(in, &A::comp);
assert(result.found == false);
assert(result.in == in.end());
assert(in == expected);
}
}
{ // A custom projection works.
struct A {
int i;
constexpr A negate() const { return A{i * -1}; }
constexpr auto operator<=>(const A&) const = default;
};
const std::array input = {A{5}, A{4}, A{3}, A{2}, A{1}};
std::array expected = {A{1}, A{2}, A{3}, A{4}, A{5}};
{ // (iterator, sentinel) overload.
auto in = input;
auto result = std::ranges::prev_permutation(in.begin(), in.end(), {}, &A::negate);
assert(result.found == false);
assert(result.in == in.end());
assert(in == expected);
}
{ // (range) overload.
auto in = input;
auto result = std::ranges::prev_permutation(in, {}, &A::negate);
assert(result.found == false);
assert(result.in == in.end());
assert(in == expected);
}
}
{ // Complexity: At most `(last - first) / 2` swaps.
const std::array input = {6, 5, 4, 3, 2, 1};
{ // (iterator, sentinel) overload.
auto in = input;
int swaps_count = 0;
auto begin = adl::Iterator::TrackSwaps(in.data(), swaps_count);
auto end = adl::Iterator::TrackSwaps(in.data() + in.size(), swaps_count);
std::ranges::prev_permutation(begin, end);
assert(swaps_count <= (base(end) - base(begin) + 1) / 2);
}
{ // (range) overload.
auto in = input;
int swaps_count = 0;
auto begin = adl::Iterator::TrackSwaps(in.data(), swaps_count);
auto end = adl::Iterator::TrackSwaps(in.data() + in.size(), swaps_count);
std::ranges::prev_permutation(std::ranges::subrange(begin, end));
assert(swaps_count <= (base(end) - base(begin) + 1) / 2);
}
}
return true;
}
int main(int, char**) {
test();
static_assert(test());
return 0;
}
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