1//===----------------------------------------------------------------------===//
2//
3// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
4// See https://llvm.org/LICENSE.txt for license information.
5// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
6//
7//===----------------------------------------------------------------------===//
8
9// <algorithm>
10
11// UNSUPPORTED: c++03, c++11, c++14, c++17
12
13// template<bidirectional_iterator I, sentinel_for<I> S, class Comp = ranges::less,
14// class Proj = identity>
15// requires sortable<I, Comp, Proj>
16// constexpr ranges::prev_permutation_result<I>
17// ranges::prev_permutation(I first, S last, Comp comp = {}, Proj proj = {});
18// template<bidirectional_range R, class Comp = ranges::less,
19// class Proj = identity>
20// requires sortable<iterator_t<R>, Comp, Proj>
21// constexpr ranges::prev_permutation_result<borrowed_iterator_t<R>>
22// ranges::prev_permutation(R&& r, Comp comp = {}, Proj proj = {});
23
24#include <algorithm>
25#include <array>
26#include <cassert>
27#include <cstddef>
28#include <ranges>
29
30#include "almost_satisfies_types.h"
31#include "test_iterators.h"
32
33template <class Iter, class Sent = sentinel_wrapper<Iter>>
34concept HasPrevPermutationIt = requires(Iter first, Sent last) { std::ranges::prev_permutation(first, last); };
35
36static_assert(HasPrevPermutationIt<int*>);
37static_assert(!HasPrevPermutationIt<BidirectionalIteratorNotDerivedFrom>);
38static_assert(!HasPrevPermutationIt<BidirectionalIteratorNotDecrementable>);
39static_assert(!HasPrevPermutationIt<int*, SentinelForNotSemiregular>);
40static_assert(!HasPrevPermutationIt<int*, SentinelForNotWeaklyEqualityComparableWith>);
41static_assert(!HasPrevPermutationIt<const int*>); // not sortable
42
43template <class Range>
44concept HasPrevPermutationR = requires(Range range) { std::ranges::prev_permutation(range); };
45
46static_assert(HasPrevPermutationR<UncheckedRange<int*>>);
47static_assert(!HasPrevPermutationR<BidirectionalRangeNotDerivedFrom>);
48static_assert(!HasPrevPermutationR<BidirectionalRangeNotDecrementable>);
49static_assert(!HasPrevPermutationR<BidirectionalRangeNotSentinelSemiregular>);
50static_assert(!HasPrevPermutationR<BidirectionalRangeNotSentinelWeaklyEqualityComparableWith>);
51static_assert(!HasPrevPermutationR<UncheckedRange<const int*>>); // not sortable
52
53constexpr std::size_t factorial(size_t i) {
54 std::array memoized = {1, 1, 2, 6, 24, 120, 720, 5040, 40320};
55 return memoized[i];
56}
57
58template <class Iter, class Range, class Func>
59constexpr bool run_prev_permutation(Func call_prev_permutation, Range permuted, Range previous) {
60 using Result = std::ranges::prev_permutation_result<Iter>;
61
62 std::same_as<Result> decltype(auto) ret = call_prev_permutation(permuted);
63 assert(ret.in == permuted.end());
64 bool next_found = ret.found;
65
66 if (std::ranges::distance(permuted) > 1) {
67 if (next_found) {
68 assert(std::ranges::lexicographical_compare(permuted, previous));
69 } else {
70 assert(std::ranges::lexicographical_compare(previous, permuted));
71 assert(std::ranges::is_sorted(permuted, std::ranges::greater{}));
72 }
73 }
74
75 return next_found;
76}
77
78template <class Iter, class Sent, class Func>
79constexpr void test_prev_permutations(Func call_prev_permutation) {
80 std::array input = {4, 3, 2, 1};
81 auto current_permutation = input;
82 auto previous_permutation = current_permutation;
83
84 // For all subarrays of `input` from `[0, 0]` to `[0, N - 1]`, call `prev_permutation` until no previous permutation
85 // exists.
86 // The number of permutations must equal `N!`. `run_prev_permutation` checks that each previous permutation is
87 // lexicographically less than the previous. If these two conditions hold (the number of permutations is `N!`, and
88 // each permutation is lexicographically less than the previous one), it follows that the `ranges::prev_permutation`
89 // algorithm works correctly.
90 for (std::size_t i = 0; i <= current_permutation.size(); ++i) {
91 std::size_t count = 0;
92 bool next_found = true;
93
94 while (next_found) {
95 ++count;
96 previous_permutation = current_permutation;
97
98 auto current_subrange = std::ranges::subrange(
99 Iter(current_permutation.data()), Sent(Iter(current_permutation.data() + i)));
100 auto previous_subrange = std::ranges::subrange(
101 Iter(previous_permutation.data()), Sent(Iter(previous_permutation.data() + i)));
102
103 next_found = run_prev_permutation<Iter>(call_prev_permutation, current_subrange, previous_subrange);
104 }
105
106 assert(count == factorial(i));
107 }
108}
109
110template <class Iter, class Sent>
111constexpr void test_all_permutations() {
112 test_prev_permutations<Iter, Sent>([](auto&& range) {
113 return std::ranges::prev_permutation(range.begin(), range.end());
114 });
115
116 test_prev_permutations<Iter, Sent>([](auto&& range) {
117 return std::ranges::prev_permutation(range);
118 });
119}
120
121template <class Iter, class Sent, int N>
122constexpr void test_one(const std::array<int, N> input, bool expected_found, std::array<int, N> expected) {
123 using Result = std::ranges::prev_permutation_result<Iter>;
124
125 { // (iterator, sentinel) overload.
126 auto in = input;
127 auto begin = Iter(in.data());
128 auto end = Sent(Iter(in.data() + in.size()));
129
130 std::same_as<Result> decltype(auto) result = std::ranges::prev_permutation(begin, end);
131 assert(result.found == expected_found);
132 assert(result.in == end);
133 assert(in == expected);
134 }
135
136 { // (range) overload.
137 auto in = input;
138 auto begin = Iter(in.data());
139 auto end = Sent(Iter(in.data() + in.size()));
140 auto range = std::ranges::subrange(begin, end);
141
142 std::same_as<Result> decltype(auto) result = std::ranges::prev_permutation(range);
143 assert(result.found == expected_found);
144 assert(result.in == end);
145 assert(in == expected);
146 }
147}
148
149template <class Iter, class Sent>
150constexpr void test_iter_sent() {
151 test_all_permutations<Iter, Sent>();
152
153 // Empty range.
154 test_one<Iter, Sent, 0>({}, false, {});
155 // 1-element range.
156 test_one<Iter, Sent, 1>({1}, false, {1});
157 // 2-element range.
158 test_one<Iter, Sent, 2>({1, 2}, false, {2, 1});
159 test_one<Iter, Sent, 2>({2, 1}, true, {1, 2});
160 // Longer sequence.
161 test_one<Iter, Sent, 8>({8, 7, 6, 5, 4, 3, 2, 1}, true, {8, 7, 6, 5, 4, 3, 1, 2});
162 // Longer sequence, permutations exhausted.
163 test_one<Iter, Sent, 8>({1, 2, 3, 4, 5, 6, 7, 8}, false, {8, 7, 6, 5, 4, 3, 2, 1});
164}
165
166template <class Iter>
167constexpr void test_iter() {
168 test_iter_sent<Iter, Iter>();
169 test_iter_sent<Iter, sentinel_wrapper<Iter>>();
170 test_iter_sent<Iter, sized_sentinel<Iter>>();
171}
172
173constexpr void test_iterators() {
174 test_iter<bidirectional_iterator<int*>>();
175 test_iter<random_access_iterator<int*>>();
176 test_iter<contiguous_iterator<int*>>();
177 test_iter<int*>();
178}
179
180constexpr bool test() {
181 test_iterators();
182
183 { // A custom predicate works.
184 struct A {
185 int i;
186 constexpr bool comp(const A& rhs) const { return i > rhs.i; }
187 constexpr bool operator==(const A&) const = default;
188 };
189 const std::array input = {A{.i: 5}, A{.i: 4}, A{.i: 3}, A{.i: 2}, A{.i: 1}};
190 std::array expected = {A{.i: 1}, A{.i: 2}, A{.i: 3}, A{.i: 4}, A{.i: 5}};
191
192 { // (iterator, sentinel) overload.
193 auto in = input;
194 auto result = std::ranges::prev_permutation(in.begin(), in.end(), &A::comp);
195
196 assert(result.found == false);
197 assert(result.in == in.end());
198 assert(in == expected);
199 }
200
201 { // (range) overload.
202 auto in = input;
203 auto result = std::ranges::prev_permutation(in, &A::comp);
204
205 assert(result.found == false);
206 assert(result.in == in.end());
207 assert(in == expected);
208 }
209 }
210
211 { // A custom projection works.
212 struct A {
213 int i;
214 constexpr A negate() const { return A{.i: i * -1}; }
215 constexpr auto operator<=>(const A&) const = default;
216 };
217 const std::array input = {A{.i: 5}, A{.i: 4}, A{.i: 3}, A{.i: 2}, A{.i: 1}};
218 std::array expected = {A{.i: 1}, A{.i: 2}, A{.i: 3}, A{.i: 4}, A{.i: 5}};
219
220 { // (iterator, sentinel) overload.
221 auto in = input;
222 auto result = std::ranges::prev_permutation(in.begin(), in.end(), {}, &A::negate);
223
224 assert(result.found == false);
225 assert(result.in == in.end());
226 assert(in == expected);
227 }
228
229 { // (range) overload.
230 auto in = input;
231 auto result = std::ranges::prev_permutation(in, {}, &A::negate);
232
233 assert(result.found == false);
234 assert(result.in == in.end());
235 assert(in == expected);
236 }
237 }
238
239 { // Complexity: At most `(last - first) / 2` swaps.
240 const std::array input = {6, 5, 4, 3, 2, 1};
241
242 { // (iterator, sentinel) overload.
243 auto in = input;
244 int swaps_count = 0;
245 auto begin = adl::Iterator::TrackSwaps(in.data(), swaps_count);
246 auto end = adl::Iterator::TrackSwaps(in.data() + in.size(), swaps_count);
247
248 std::ranges::prev_permutation(begin, end);
249 assert(swaps_count <= (base(end) - base(begin) + 1) / 2);
250 }
251
252 { // (range) overload.
253 auto in = input;
254 int swaps_count = 0;
255 auto begin = adl::Iterator::TrackSwaps(in.data(), swaps_count);
256 auto end = adl::Iterator::TrackSwaps(in.data() + in.size(), swaps_count);
257
258 std::ranges::prev_permutation(std::ranges::subrange(begin, end));
259 assert(swaps_count <= (base(end) - base(begin) + 1) / 2);
260 }
261 }
262
263 return true;
264}
265
266int main(int, char**) {
267 test();
268 static_assert(test());
269
270 return 0;
271}
272

source code of libcxx/test/std/algorithms/alg.sorting/alg.permutation.generators/ranges.prev_permutation.pass.cpp